A History of Geology and Planetary Science

The noted blogger Fjordman is filing this report via Gates of Vienna.
For a complete Fjordman blogography, see The Fjordman Files. There is also a multi-index listing here.

This essay was originally published in three parts. See: Part 1, Part 2, and Part 3.


People have appreciated stones for practical or decorative usages since prehistoric times. Flint has been utilized for tools for tens of thousands of years. Gemstones were valued and traded objects in ancient Mesopotamia, Egypt, China, India, the Americas and elsewhere. One of the most notable and systematic ancient works on what we recognize as mineralogy was On Stones by the Greek natural philosopher Theophrastus, completed just after 300 BC. In the first century AD, the Roman author Pliny the Elder described many minerals and their uses and was among the first observers to correctly identify amber as fossilized resin from trees.

There are those who claim that the history of geology begins in the eleventh century AD with the Persian polymath Avicenna, a view which is not entirely convincing.. In China, the polymath Shen Kuo upon noticing that there were seashells embedded in a sandstone cliff far above sea level inferred that the sandstone must have derived from an ancient beach that had somehow been compressed and elevated. While this insight was correct, it remained an isolated observation and was not followed up by other Chinese or Asian scholars. Moreover, the Greek philosopher Xenophanes of Colophon had observed already in the sixth century BC that seashells occur in mountains far away from where the sea is today. Geology, like modern science in general, was born in Europe after the Scientific Revolution and the Enlightenment.

There were a number of important medieval mines in the mountainous regions of Germany and Eastern Europe. After the introduction of Chinese gunpowder during the Mongol conquests and the independent development of large cannon in Europe, the demand for copper for the manufacture of bronze cannon in the fifteenth century was a stimulus for advances such as the “liquation” process, used in mixed ores to separate silver from copper.

The German scholar Georgius Agricola (1494-1555) was a Renaissance pioneer in the fields of mineralogy and metallurgy. He got a degree from the University of Leipzig and studied medicine in Italy. On his return to Saxony in 1526 he developed a life-long interest in mining and spent much time in Bohemia, probably the richest metal mining district in Europe. His work De Re Metallica, published posthumously in 1556, was a comprehensive summary of all aspects of mining and metal production then known. His work was highly regarded by contemporaries and has stood the test of time well.

Athanasius Kircher (1601-1680), a German Jesuit priest and universal scholar, explored the volcanoes of southern Italy. Mount Vesuvius had a large eruption in 1631. A few years later, Kircher had himself lowered by means of a rope into its crater to study its inner structure. In his work Mundus Subterraneus (Subterranean World) he postulated the existence of a central fire inside of the Earth which was feeding heat to the surface through various channels and fissures. The sources of the combustion were thought to be coal, sulfur and other materials.

Nicolas Steno, or Niels Stensen (1638-1686) from Copenhagen, Denmark, studied medicine and moved to Italy in 1665. In 1666, two fishermen caught a huge shark which Steno dissected. While examining its teeth he was struck by their resemblance to stony objects that were found in certain rocks. He argued that these objects had come from once-living sharks and come to be buried in mud or sand that was now dry land. His English contemporaries Robert Hooke and partly John Ray, too, argued that fossils were the geologically preserved remains of once-living organisms. In Agricola’s time, “fossil” was a term that was applied to virtually any object dug from the ground, be that organic remains such as ammonites and trilobites or regular rocks. Steno is especially famous for his law of superposition. In 1669 he concluded that layers of rock (strata) are arranged in a time sequence with the oldest on the bottom and the youngest on the top, unless later processes have disturbed this arrangement.

The French naturalist Jean-Étienne Guettard (1715-1786) was the first person to recognize the volcanic nature of the Auvergne region in central France. In addition to this he prepared early geological maps and identified heat as the causative factor of change in the Earth’s landforms. Nicolas Desmarest (1725-1815) in the 1760s studied the Auvergne region and found large basalt deposits and traces of flows of lava (magma, or molten rock) from nearby now-extinct volcanoes. Vulcanists such as Desmarest argued that basalts had flowed from volcanoes, and studies of extinct volcanoes confirmed this assertion. The German naturalist Alexander von Humboldt carried our major studies of volcanoes in the early nineteenth century.

The word “geology” as a term for the study of the Earth was popularized in the late eighteenth century by the Swiss (Genevan) naturalists Horace-Bénédict de Saussure (1740-1799), the aristocrat and scholar famous for his many voyages in the Alps, and Jean-André Deluc (1727-1817). Saussure is often considered the founder of alpinism or mountaineering and conquered Mont Blanc (4,810 m) in 1787. At the summit he tested the boiling point of water, the temperature of the snow and the pulse of his guides. Deluc was the son of a clockmaker and spent years climbing the Alps with his brother. He made accurate instruments to measure the height of mountains and in 1773 sought a place in England. He was elected a fellow of the Royal Society in London on the strength of his barometry and his instrumentation skills.

The German scholar Abraham Gottlob Werner (1749-1817) studied law at the University of Leipzig and later got a teaching appointment at the Mining Academy of Freiberg in Saxony, where he stayed for many years. As a talented mineralogist he worked up simple descriptive standards of classification and discovered eight new minerals. Mineralogy gradually diminished from the overarching category for the study of the Earth to a mere subdiscipline. While sometimes wrong, Werner was an influential geologist and the first to work out a comprehensive theory for the history of the Earth’s formation. He believed that all rock was once sediment or precipitate in a universal ocean, a view which became known as Neptunism.

James Hutton (1726-1797) was the leading representative of the rival Plutonist school. Born and educated in Edinburgh, Scotland, during what has become known as the Scottish Enlightenment, Hutton was a Newtonian in natural philosophy and counted among his friends the chemist Joseph Black, the economist Adam Smith and the inventor James Watt. Hutton proposed the uniformitarian view of geological history where all strata could be accounted for in terms of geological forces operating over very long periods of time, such as the slow erosion of rocks. His ideas were popularized by John Playfair (1748-1819) of the University of Edinburgh and picked up by the young Scottish geologist Charles Lyell (1797-1875).

Charles Lyell became fascinated with geology as a young man and took field trips to Continental Europe. Sicily with the active stratovolcano Mount Etna in particular impressed him. As a member of the Geological Society he took part in lively debates and supported the uniformitarian theory. Contrary to catastrophism, which indicated that our planet has been shaped primarily by sudden, violent events, uniformitarianism indicated the past to have been an uninterrupted period of erosion, sediment deposition, volcanic action and earthquakes. These slow and incremental processes, still going on today, could account for great changes if given enough time. This view implied that the Earth had to be many millions of years old.

Lyell’s Principles of Geology, first published in 1830, was very successful and accessible to a wider audience, something which Hutton’s work never had been. It went through many editions and brought the author an income so that he could travel and expand his ideas. Lyell influenced a number of men of science, including the young Charles Darwin. Modern geology can be said to have been born with Charles Lyell’s extension of James Hutton’s theories.

The principles of stratigraphy, the study of the Earth’s strata or layers of sedimentary rock, had been created by Nicolas Steno in the late 1600s and were rapidly extended between 1810 and 1840. Over the next century, geologists filled in the details of the stratigraphic column with ever-greater precision. By the turn of the nineteenth century it was generally accepted among Western European scholars that fossils could be used to identify and correlate strata.

The great naturalist Georges Cuvier (1769-1832), widely considered the founder of paleontology and comparative anatomy, together with fellow French scholar Alexandre Brongniart (1770-1847) produced a pioneering geological map of the Paris region in 1812. Brongniart had studied chemistry under the brilliant chemist Antoine Lavoisier. The fruitful collaboration between these two men established a scientific approach to stratigraphy and demonstrated that geological strata could be recognized by the fossils found within them.

The self-educated English surveyor, canal engineer and geologist William Smith (1769-1839) came from a family of small farmers. He received little formal education, but from an early age took an interest in exploring fossils. Based on careful stratigraphic investigations from canals and quarries he published his Geologic Map of England and Wales with Part of Scotland in 1815, the world’s first nationwide geological map. Partly due to his humble origins and limited education his great contributions were overlooked at first by the scientific community, and Smith suffered from severe financial difficulties for many years. Not until the later part of his life was his valuable work fully appreciated.

Although the marriage between geology and mining took a long time to yield practical results, the frequent claims that dynamic Britain during the Industrial Revolution was exhausting its coal supplies turned out to be false alarms. State-sponsored geological surveys were undertaken throughout Europe and North America after the mid-nineteenth century. This research would greatly benefit the mining industry as well as the emerging petroleum industry. Many geologists in the twentieth century found work in the oil industry, which joined geological surveys and mining as the main sources of non-academic employment.

Roderick Murchison (1792-1871) was born into a wealthy Scottish Highland family. He spent years in the army and became a very active member of the Geological Society of London, associating with Charles Lyell and the Englishman Adam Sedgwick (1785-1873). Murchison’s great work The Silurian System in 1839 established the Silurian geological time period of the Paleozoic Era, followed a year later by the Devonian while he was collaborating with Sedgwick. Murchison’s travels through Russia and Scandinavia after 1840 resulted in the establishment of the Permian period, which ended 250 million years ago with the greatest mass extinction of life on Earth, which wiped out perhaps 90% of all then-existing species.

Adam Sedgwick taught geology at the University of Cambridge. He proposed the Cambrian period, the first part of the Paleozoic, lasting from roughly 540 million to 490 million years ago. Judging from the fossil record this was an age of rapid development of complex life forms which is often referred to as the Cambrian Explosion. The American paleontologist Charles Walcott (1850-1927), then working for the Smithsonian Institution, in 1909 exposed the Burgess Shale, one of the world’s richest fossil fields, in the Canadian Rocky Mountains. It provides us with invaluable insights into what life was like 505 million years ago.

The first vertebrates (animals with a backbone) apparently evolved during the Cambrian and the ensuing Ordovician period. The twentieth century witnessed a greater ability to uncover evidence of Pre-Cambrian fossils of bacteria and primitive life forms such as stromatolites stretching back several billion years before the Cambrian period. Paleontology has been successfully adopted far beyond the Western world. Some very exciting finds of feathered dinosaurs were uncovered in China in the late twentieth and early twenty-first centuries.

The first dinosaurs identified as such, not as “dragon bones” or something of that nature, were named in the 1820s, as was the entire field of paleontology. One of the earliest named was Megalosaurus by the English geologist William Buckland (1784-1856). In 1822 the wife of the influential English paleontologist Gideon Mantell (1790-1852) noticed an object which he recognized as a fossil tooth but was unable to match to any known creature. The respected scholar Georges Cuvier in Paris in an uncharacteristic error suggested that the remains were from a rhinoceros. In London, Mantell was shown the skeleton of an iguana with teeth almost identical to the ancient teeth that he had just found, though much smaller. He realized that he had discovered the remains of an extinct giant reptile which he called Iguanodon. Also in England, Mary Anning (1799-1847) was a professional fossil collector who produced many remarkable finds. Perhaps the most important one was her discovery of the first plesiosaur.
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The English paleontologist Richard Owen (1804-1892) coined the term “dinosaur” in 1842. The name means “terrible lizard” and is not very scientifically accurate, but it stuck. Owen was a quarrelsome man who claimed the discovery of the Iguanodon for himself when it had been done by Gideon Mantell, yet according to Bill Bryson in A Short History of Nearly Everything he contributed to the development of modern museums: “Owen’s plan was to welcome everyone, even to the point of encouraging working men to visit in the evening, and to devote most of the museum’s space to public displays. He even proposed, very radically, to put informative labels on each display so that people could appreciate what they were viewing. In this, somewhat unexpectedly, he was opposed by T. H. Huxley, who believed that museums should be primarily research institutions. By making the Natural History Museum an institution for everyone, Owen transformed our expectations of what museums are for.”

The Scottish geologist James Hall (1761-1832), a friend of James Hutton, founded experimental geology by artificially producing various rock types in the laboratory. He carried out dangerous experiments with limestone heated under pressure and lived to report that it did indeed consolidate under sufficient pressure. In the twentieth century Pentti Eskola (1883-1964), a professor of geology and mineralogy in Helsinki, Finland, applied chemical methods to the study of minerals and metamorphic facies (groups of mineral compositions in metamorphic rocks), thereby laying the foundations of studies in metamorphic petrology.

There are three main rock types: Igneous rocks are formed from the solidification of molten rock (magma). Intrusive igneous rocks such as diorite, gabbro and granite solidify below the Earth’s surface while extrusive igneous rocks such as basalt, obsidian and pumice solidify on or above the surface. Sedimentary rocks are formed by the accumulation of sediments. Some such as conglomerate and sandstone are formed from mechanical weathering debris. Organic sedimentary rocks such as coal form from the accumulation of plant or animal debris.

Metamorphic rocks have been modified by heat, pressure and chemical processes, usually while buried deep below the Earth’s surface. This has altered the mineralogy, texture and chemical composition of the rocks. Examples of this would be marble produced from the metamorphism of limestone or quartzite from the metamorphism of sandstone with quartz.

Scientific studies of mineralogy began with Georgius Agricola in the sixteenth century. The development of chemistry with figures such as Antoine Lavoisier and John Dalton led to a greater emphasis on this discipline when studying minerals. Abraham Gottlob Werner was one of those who favored a chemical classification. There were a few exceptions; the German mineralogist Friedrich Mohs (1773-1839) in 1822 devised the Mohs relative hardness scale where diamond was assigned the number 10. The method of comparing minerals by observing which ones can scratch others was mentioned already by Theophrastus around 300 BC.

The French mineralogist René-Just Häuy (1743-1822) in the early nineteenth century established the science of crystallography. The polarization of light, described by the French mathematician Etienne-Louis Malus in 1809, became a useful tool when combined with improved microscopes. The Encyclopædia Britannica online contains a very comprehensive entry covering all aspects of the Earth sciences. Here is what it says about mineralogy:

“In 1814 Jöns Jacob Berzelius of Sweden published a system of mineralogy offering a comprehensive classification of minerals based on their chemistry. Berzelius recognized silica as an acid and introduced into mineralogy the group known as silicates. At mid-century the American geologist James Dwight Dana’s System of Mineralogy, in its third edition, was reorganized around a chemical classification, which thereafter became standard for handbooks. The development of the polarizing microscope and the technique for grinding sections of rocks so thin as to be virtually transparent came in 1827 from studies of fossilized wood by William Nicol. In 1849 Clifton Sorby showed that minerals viewed in thin section could be identified by their optical properties, and soon afterward improved classifications of rocks were made on the basis of their mineralogic composition. The German geologist Ferdinand Zirkel’s Mikroscopische Beschaffenheit der Mineralien und Gesteine (1873; ‘The Microscopic Nature of Minerals and Rocks’) contains one of the first mineralogic classifications of rocks and marks the emergence of microscopic petrography as an established branch of science.”

The nebular hypothesis was first proposed in 1734 by the Swedish philosopher and theologian Emanuel Swedenborg (1688-1772), who was born in Stockholm and studied at Uppsala University. He wrote on mathematics, chemistry, physics, mineralogy and astronomy and made a sketch of a glider-type aircraft. The German Enlightenment philosopher Immanuel Kant developed this theory further in 1755, and the French astronomer Pierre-Simon Laplace also advanced a nebular hypothesis in 1796. Laplace suggested that our Solar System was created from the cooling and condensation of a large and hot rotating “nebula,” a gassy cloud of particles and dust. This idea strongly influenced scientists in the nineteenth century, and central elements of it have survived to this day. For a long time, geologists preferred the hypothesis that the Earth had cooled and contracted. The work on rates of cooling made by the brilliant French mathematical physicist Joseph Fourier seemed to support this model.

In 1831 the French geologist Élie de Beaumont (1798-1874) suggested that the Earth had cooled from a molten body and that the crust at intervals had buckled under the strain, throwing up mountain ranges. Variants of this contraction theory flourished, culminating in the four-volume Face of the Earth (1883-1904) by Eduard Suess (1831-1914), a professor of geology at the University of Vienna. Throughout the twentieth century, geologists amassed a mass of new data from all corners of the planet and, crucially, from the bottom of the oceans.

Geologists knew that there was evidence of past upheavals, but many still believed these had been caused by the Biblical Flood of Noah. There were a few individuals who believed that glaciation had been more extensive in the past than it is today, for instance because of the presence of huge stones (“erratics”) dumped far away from the strata where they belonged. They included the Norwegian geologist Jens Esmark (1763-1839) writing in the 1820s and the German-Swiss mining engineer and naturalist Jean de Charpentier (1786-1855).

In Norway and the Alps there are still surviving glaciers and the landscape was shaped by previous ones. The Norwegian fjords are valleys carved by glacial activity and now filled with seawater. The ideas of Charpentier and others in Switzerland were taken up and developed further by the Swiss paleontologist, geologist and glaciologist Louis Agassiz (1807-1873).

Louis Agassiz from Môtier, Switzerland studied medicine at Zürich and Heidelberg before moving to Paris, where he was influenced by the ideas of Georges Cuvier. Despite initial skepticism, after personal studies he became an enthusiastic supporter of the glacial model. In 1840 he published a work in two volumes entitled Etudes sur les glaciers (Study on Glaciers), which is considered the first mature scientific work on the existence of a previous Ice Age when glaciers had covered much larger land areas than they do now. Last Glacial Maximum was about 20,000 years ago. Agassiz moved to the United States and became professor of geology at Harvard University in 1847. Later scholars discovered evidence for several ice ages, not just one. Still, this left the unresolved issue of what could cause such ice ages.

The French mathematician Joseph Adhemar (1797-1862) suggested that ice ages were caused by astronomical forces. His theory was modified by the Scottish scientist James Croll (1821-1890) and above all by the gifted Serbian civil engineer and mathematician Milutin Milankovitch (1879-1958). Milankovitch studied at the Institute of Technology in Vienna in Austria-Hungary and later taught mechanics, theoretical physics and astronomy at the University of Belgrade in Serbia. During the turmoil in the Balkans following the collapse of the Ottoman and Austro-Hungarian Empires he served in the Serbian army during World War I. He picked up an obsession with climate and a determination to set up a detailed mathematical explanation of how temperatures change as a result of changes in the eccentricity, axial tilt and precession of the Earth’s orbit around the Sun. His complex work on what has became known as Milankovitch cycles took years to complete and was carried out only with brain power. It was published in a 1920 book that met with widespread acclaim.

The reasons for the periodic ice ages we see in the geological records are not fully understood, but are believed to be at least partly related to cyclic changes in the Earth’s orbit and axial tilt. Other factors such as changes in the composition of the atmosphere or the position of the continents, eruptions of supervolcanoes or cometary impacts may contribute as well.

We currently know a lot more about the surface of other planets such as Mars than about the interior of our own, but what little we think we know to a large extent derives from the study of seismic waves. The Dutch mathematician Willebrord Snell in the seventeenth century in what has become known as Snell ‘s Law described the bending of light, or refraction, which takes place when light travels from one medium to a medium with a different composition and density, for instance from air to water. This effect can be seen by anybody in a small boat who puts an oar into the water and observes how it appears to be “bent.” This phenomenon is caused by the change in velocity that occurs when light waves pass from one medium to another. The same principle applies to other waves, too, for example seismic waves, the shock waves generated by earthquakes or explosions that travel through the Earth’s interior.

The Irish geophysicist Richard Dixon Oldham (1858-1936) discovered that seismic waves travel through the interior of the Earth in different directions and at different speeds. This insight was used by the Croatian seismologist Andrija Mohorovicic (1857-1936), who had studied physics in Prague and taught geophysics at the University of Zagreb.. By analyzing the data from a 1909 earthquake he realized that the velocity of a seismic wave is related to the density of the material that it is moving through. He interpreted the acceleration of seismic waves observed within Earth’s outer shell as a compositional change within the Earth itself.

This Mohorovicic Discontinuity, or “ Moho “ for short, is thought to constitute the boundary between the Earth’s crust and mantle. It can be found at an average depth of 8 kilometers beneath the ocean basin and as much as 32 kilometers beneath the continents.

Beno Gutenberg (1889-1960) was born in Darmstadt, Germany and completed all of his university education at the University of Göttingen. After earning his Ph.D., Gutenberg turned his attention to recent discoveries about the Earth’s interior. In 1913 he became the first person to give a reasonable estimate of the size and properties of the Earth’s core.

In 1930 Gutenberg became a professor of geophysics at the California Institute of Technology in the USA where one of his colleagues was the American seismologist Charles Francis Richter (1900-1985). They collaborated on the development of various scales using seismic waves so that observers could assign magnitudes to earthquakes. In 1935 this work resulted in the creation of a logarithmic magnitude scale that came to be named after Richter alone. Earthquakes below 2.5 or so on the Richter scale are too weak to be noticed by humans. Earthquakes with an intensity of 10.0 or more have so far never been measured, the strongest being one of 9.5 in Chile in 1960. Based on descriptions, the Great Lisbon Earthquake which destroyed the Portuguese capital city in 1755 may have approached a magnitude of 9.0.

The Moment Magnitude scale was introduced in 1979 at the California Institute of Technology by the American Thomas C. Hanks and the Japanese-born seismologist Hiroo Kanamori as a successor to the Richter scale. It is currently the preferred tool by seismologists for comparing the energy released by earthquakes as it is considered to be more precise.

The Danish seismologist Inge Lehmann (1888-1993) studied at the University of Copenhagen in Denmark and later worked on cataloging seismograms from Denmark and the Danish-ruled island of Greenland. In 1929 a large earthquake occurred near New Zealand. Lehmann studied the recorded shock waves, some of which were reflected back. In a 1936 she theorized that the Earth’s center consists of two parts: a solid inner core surrounded by a liquid outer core.

The outer core boundary lies below the mantle almost 2,900 km beneath the Earth’s surface. The inner core begins about 5150 kilometers beneath the Earth’s surface where the temperature is estimated to be around 6000 °C, similar to or maybe even slightly higher than that of the Sun’s surface. In total, the Earth’s core is believed to be more than 7,000 kilometers in diameter, making it roughly comparable in size to the planet Mars.

The French naturalist Georges-Louis Leclerc, Comte de Buffon in the 1770s made one of the first truly scientific attempts to establish the age of the Earth. He assumed that it had gradually cooled from a much hotter state in its early history. Based on experiments with heating balls of iron he estimated that the Earth was at least 75,000 years old. While this is far too young it was nevertheless a lot older than the six thousand or so years that a literal reading of the Bible would indicate. This brought Comte de Buffon condemnations from some Church authorities.

The French mathematical physicist Joseph Fourier (1768-1830) studied the mathematical theory of heat conduction and phenomena related to heat, including what we now know as the greenhouse effect. He established the partial differential equation governing heat diffusion and solved it by using infinite series of trigonometric functions we call Fourier series, which remain in active use today by astronomers and others. Fourier tried to maneuver through the turbulent times of the French Revolution. Having good relations with some of the leading French mathematicians of his day such as Joseph-Louis Lagrange, Pierre-Simon Laplace and Gaspard Monge helped him in this regard. Fourier went with Napoleon Bonaparte on his Egyptian expedition in 1798 and oversaw the publishing of the enormous work Description de l’Egypte afterward. His most famous work on heat began before 1807 but was expanded and finally published as Théorie analytique de la chaleur (The Analytical Theory of Heat) in 1822.

Fourier used his equations about heat flow to see how long it would have taken Earth to cool. His formula gives an estimated age of some 100 million years, vastly greater than any scholar had suggested before. Fourier appears to have been so stunned by his discovery that he never published it. The Earth has indeed cooled since its creation, but its core is still hot due to heat produced from radioactive decay. In 1859 Charles Darwin published an estimate of 300 million years for a piece of rock. Lord Kelvin calculated that it would take the Earth about 100 million years to cool from an assumed primordial molten condition to its present state.

The birth of geophysics as distinct from geology depended upon the discovery of radioactivity by Henri Becquerel and Pierre and Marie Curie in France in the late 1890s. In the early 1900s Ernest Rutherford together with Frederick Soddy discovered that radioactive elements such as uranium and thorium break down into other elements in a predictable sequence. Rutherford suggested that this decay of radioactive elements could be used to measure the age of rocks.

The American physical chemist Bertram Boltwood (1870-1927), a graduate of Yale University, in 1907 reasoned that since he knew the rate at which uranium breaks down (its half-life), he could use the proportion of lead in the uranium ores as a clock. His calculations put the Earth’s age at up to 2.2 billion years. The same idea was utilized by Arthur Holmes (1890-1965) in Britain and Clair Cameron Patterson (1922-1995) in the United States.

According to The Oxford Guide to the History of Physics and Astronomy, “Robert John Strutt and his student, the geologist Arthur Holmes, pursued Rutherford’s idea. By 1911, Holmes had used uranium/lead ratios to estimate the ages of several rocks from the ancient Precambrian period. One appeared to be 1,600 million years old. Many geologists were initially skeptical, but by 1930, largely as a result of the work of Holmes, most accepted radioactive dating as the only reliable means to determine the ages of rocks and of the earth itself. The discovery of isotopes in 1913, and the development of the modern mass spectrometer in the 1930s, greatly facilitated radioactive dating. By the late 1940s, the method produced an estimate of between 4,000 and 5,000 million years for the age of the earth. In 1956, the American geochemist Clair Cameron Patterson compared the isotopes of the earth’s crust with those of five meteorites. On this basis, he decided that the earth and its meteorites had an age of about 4,550 million years. All subsequent estimates of the age of the earth have tended to confirm Patterson’s conclusion.”

Since more than 70 percent of the Earth’s surface is covered by seawater, detailed studies of the oceans were of great importance to science as well as to practical navigation. The first modern text devoted exclusively to marine science was Histoire physique de la mer (1725) by the Italian military man and naturalist Luigi Ferdinando Marsigli (1658-1730), who assembled information about water temperature, salinity, currents, ocean plants and animals. The eighteenth century witnessed an acceleration of this trend as European explorers charted distant lands and the science of chemistry was rapidly maturing in Western Europe itself.

Ocean currents engaged the curiosity of scholars such as Benjamin Franklin who in the 1780s mapped the Gulf Stream, which originates in the Gulf of Mexico and brings vast amounts of warm water across the Atlantic Ocean to northwestern Europe, substantially contributing to the regional climate and the relatively mild winters there. Benjamin Thompson, Count Rumford’s heat experiments led him to attribute ocean circulation to differences in water density, a theory which was accepted after some delay. The English geographer James Rennell (1742-1830) served in the British Royal Navy, where his numerous voyages allowed him to make accurate maps and charts of currents and tides. His final work Currents of the Atlantic Ocean was a landmark study published posthumously by his daughter in 1832.

Matthew Fontaine Maury (1806-1873) of the United States Navy in 1842 became superintendent of the U.S. Depot of Charts and Instruments in Washington D.C. From the study of old ships’ captains logs Maury assembled data on winds, currents and other information and finally published his Physical Geography of the Sea in 1855, widely considered the first extensive textbook of oceanography. The Scots-Canadian oceanographer John Murray (1841-1914) was the first person to use the term “oceanography” and along with the Norwegian marine zoologist Johan Hjort (1869-1948) in 1912 published the influential book The Depths of the Ocean. The Norwegian oceanographer Harald Sverdrup (1888-1957) with associates in the USA developed a comprehensive theory of ocean circulation.

By the early twentieth century, several persons had suggested the existence of some form of continental drift, but it was the German meteorologist and geologist Alfred Wegener (1880-1930) who had the biggest impact. He had studied at the universities of Heidelberg, Innsbruck and Berlin. From 1906 to 1908 he worked as a meteorologist to a Danish expedition to Greenland. In 1910 he had been struck, as others had been before him, of how Africa and South America seemed to “fit together..” It was known that surprisingly similar fossils and landforms could sometimes be found on opposite sides of major oceans. Based on these findings Wegener proposed his theory of continental drift in 1915 in his masterpiece The Origin of Continents and Oceans (Die Entstehung der Kontinente und Ozeane). He suggested that there had once been a giant continent which he named “Pangaea” (“All-Earth”).

Scientists currently hold that Pangaea split up more than 200 million years ago. These ideas are now widely accepted; in fact, it is believed that the major continents have drifted apart and been reunited in supercontinents several times with intervals of some hundreds of millions of years. However, it took generations for this theory to be accepted, in part because scientists lacked sufficient information about the ocean floors and in part because Wegener himself could not properly explain exactly how continental drift happens or what drives it.

In the late nineteenth century the invention of reasonably accurate and compact seismographs spurred the development of seismology into a quantitative discipline. In 1880 the Englishman John Milne (1850-1913), the inventor of the first modern seismograph, as a foreign advisor to the Meiji government founded the Seismological Society of Japan. Scientists in earthquake-ridden Japan soon joined the Germans and the Americans as world leaders in geophysics.

In 1906 the French geophysicist Bernard Brunhes (1867-1910) discovered that the Earth’s magnetic field had changed direction in the geological past, but it took decades before his discovery was accepted by the scientific community. The Japanese geophysicist otonori Matuyama (1884-1958), the son of a Zen abbot, after studies at the Imperial University in Kyoto worked in Chicago with the American geologist Thomas Chamberlin (1843-1928). Matuyama proposed that long periods had existed in the geological past in which the polarity of the Earth’s magnetic poles was the opposite of what it is now. In a geomagnetic reversal the magnetic north and south become interchanged. In the past 4 million years there have been nine such reversals, which leave detectable magnetic imprints in certain rock samples.

The English geologist Arthur Holmes was a proponent of Wegener’s continental drift. His pioneering work on radioactive heat and geological time had led him to a profound understanding of processes in our planet’s interior. He proposed that very slow-moving convection currents in the Earth’s mantle cause continental breakup, seafloor formation and continental drift. The Dutch geophysicist Felix Andries Vening Meinesz (1887-1966) measured gravity anomalies above the ocean floors. Before the 1940s, most geologists had assumed that the sea floor represented the most ancient crust. When samples were finally obtained from the ocean beds it turned out that they were far younger than expected and that the youngest samples were found next to the volcanically active mid-ocean ridges.

The existence of a “mountain range” in the middle of the Atlantic Ocean had been suspected since the laying of the first transatlantic telegraph cable in 1858, but the global system of mid-ocean ridges was mapped only after 1950. Both the United States and the Soviet Union, the latter with less financial resources at their disposal than the former, needed to known more about the ocean environment to navigate with their nuclear submarines. The seas constituted an important frontline in their Cold War superpower rivalry. Research coupled with electronic computers greatly increased the understanding of oceanography and atmospheric physics and facilitated the integration of these various fields into the umbrella discipline of Earth science.

Studies of paleomagnetism (changes in the magnetic field “fossilized” in magnetic minerals) and distribution of fossils triggered a revolution in geology and the emerging Earth sciences. The great American geologist Harry Hammond Hess (1906-1969) while working as an officer in the US Navy during WWII conducted research of the ocean floors using the transport’s sounding gear. In 1960 at Princeton University, Hess put these pieces together and advanced the theory that the Earth’s crust moves laterally from volcanically active oceanic ridges.

“Sea-floor spreading” helped to establish continental drift as scientifically respectable. The Canadian geophysicist John Tuzo Wilson (1908-1993) created the new synthesis which became known as plate tectonics. Other major figures include the American geophysicist Maurice Ewing (1906-1974), the Englishman Dan McKenzie (born 1942) for his work on mantle convection, the French scientist Xavier Le Pichon (born 1937) and Edward Bullard (1907-1980), born into a family in Norwich, England made wealthy from brewing beer.

According to scholars Philip Rehbock and Gary Weir, “Wegener proposed that the present configuration of the continents, and other phenomena from stratigraphy, paleontology, and biogeography, could be accounted for by assuming the gradual movement of the continents horizontally over the face of the globe. The theory gained few adherents until the 1960s, by which time new lines of evidence helped bring about the plate-tectonics revolution. Evidence came from studies of paleomagnetism and polar wandering carried out by P. M. S. Blackett, S. Keith Runcorn, and their colleagues in Britain; heat flow from mid-ocean ridges, by British geophysicist Edward Bullard; seismological activity along mid-ocean ridges, by Americans Maurice Ewing and Bruce Heezen; and gravity anomalies, by the Dutch geophysicist Felix Andries Vening-Meinesz and the American Harry H. Hess. In 1960, Hess proposed the hypothesis, subsequently known as sea-floor spreading, that would explain all of these phenomena. In the mid-1960s, the British geophysicists Frederick Vine and Drummond Matthews confirmed the hypothesis by analyzing patterns of magnetic anomalies around mid-ocean ridges, and the Glomar Challenger drilled directly into the Mid-Atlantic Ridge. J. Tuzo Wilson’s 1965 concept that the earth’s surface consists of several rigid but mobile plates put the finishing touch on plate tectonics.”

If two of these plates are moving away from each other, an ocean ridge or continental ridge will form. If they collide, one may be pushed under the other in a process called subduction. If neither plate can subduct under the other they push up mountain ranges. The Mid-Atlantic Ridge straddled by volcanically active Iceland, famous for its geysers and hot springs, has churned out magma (molten rock) and expanded the Atlantic for millions of years. The Eurasian Plate collides with the African Plate in the Mediterranean region, which is why the eastern Mediterranean is a volcanically and seismologically active area with volcanoes Mount Etna on Sicily and Mount Vesuvius east of Naples. In the Aegean Sea, the massive Thera eruption at the island of Santorini around 1600 BC weakened the Minoan civilization.

Whereas the Alps in Central Europe were born through a collision between the African and European plates the Himalayas in Asia, the planet’s highest mountain range, were created by a collision between the Eurasian Plate and the Indo-Australian Plate. The so-called Pacific Ring of Fire is a very large region plagued by frequent earthquakes and/or volcanic eruptions, from California and the Andes region via Japan and Java to Mount Ruapehu and the volcanoes of New Zealand. Most of this can be explained by the movements of tectonic plates. The western Pacific Ocean contains the deepest trench on the Earth’s crust, the Mariana Trench, which lies at the subduction boundary between two tectonic plates.

While most of the volcanic activity on our planet can be understood by plate tectonics there are some exceptions. According to a theory formulated by the Canadian J. Tuzo Wilson in the 1960s, a hotspot is an area of persistent volcanic activity which originates at unusually hot areas of the mantle-core boundary in the Earth’s interior. Examples of this would be Galápagos, the unique Yellowstone area in the USA and Hawaii. According to this view, the Hawaiian Islands are peaks of an undersea mountain range created by the slow movement of a tectonic plate across a hotspot. The possibility that the Hawaiian Islands become younger to the southeast had been suspected by native Hawaiians based on observed differences in erosion. The nature of hotspots is not fully settled but remains a subject of active debate.

In 2009 the Norwegian geophysicists Rolf Mjelde and Jan Inge Faleide, based on overlapping evidence from Hawaii and Iceland, published a theory that the Earth has a heartbeat in the sense that events at the mantle-core boundary may be dispatching simultaneous plumes of magma towards the surface at these two widely separated regions every 15 million years. “These two are on very different parts of the Earth, so I don’t think the synchrony could be related to something in the mantle,” says Mjelde. “It must relate to the core somehow. I can’t see any other possibility.” Their claims were met with a mixture of fascination and skepticism among scientists as no presently known theory can explain why such a pulse exists, if it does.

The theory of plate tectonics triggered a revolution in the Earth sciences in the late twentieth century. There are those who view it as a paradigm shift like the ones described by the American philosopher Thomas Kuhn (1922-1996) in his influential work The Structure of Scientific Revolutions from 1962. Kuhn broke with several key positivist doctrines held by philosophers of science such as Karl Popper. According to him, science enjoys periods of stable progress punctuated by revolutions when one conceptual world view is replaced by another. Kuhn received some criticism for these views by scholars who claimed that it is more appropriate to describe the development of social ideas than the history of science; some postmodern thinkers have used his ideas to claim that there is no such thing as objective truth.

While the theory of plate tectonics is now almost universally accepted we still don’t know how this process got started in the first place. Scientists estimate that it began at least 2.5 billion years ago and possibly as much as 4 billion years ago. Geologist Vicki Hansen of the University of Minnesota in a controversial hypothesis from 2007 suggested that the impact of a large asteroid or comet in the distant past might have kick-started plate tectonics.

Stones falling from the sky were often viewed by ancient peoples as signs from the gods. In Enlightenment Europe, stories about such events were dismissed by many scholars as common superstition. The German naturalist Ernst Chladni (1756-1827), who is often regarded as the founder of meteoritics, in 1794 published a paper suggesting asserting that masses of iron and rock enter the Earth’s atmosphere from above and produce fireballs when heated by friction with the air. He concluded that meteorites must be extraterrestrial objects. This view was defended by the German astronomer Heinrich Olbers (1758-1840), but ridiculed by those who believed that meteorites were of volcanic, terrestrial origin.

Eyewitness accounts of fireballs were initially dismissed, yet fresh and seemingly reliable reports of stones falling from the sky appeared at the turn of the nineteenth century. The young English chemist Charles Howard (1774-1816) read Chladni’s work and decided to analyze the chemical composition of these rocks. Working with the French mineralogist Jacques-Louis de Bournon he made the first thorough scientific analysis of meteorites. Here is a quote from the book Cosmic Horizons, edited by Neil De Grasse Tyson and Steven Soter:

“The two scientists found that the stones had a dark shiny crust and contained tiny ‘globules’ (now called chondrules) unlike anything seen in terrestrial rocks. All the iron masses contained several percent nickel, as did the grains of iron in the fallen stones. Nothing like this had ever been found in iron from the Earth. Here was compelling evidence that the irons and rocks were of extraterrestrial origin. Howard published these results in 1802. Meanwhile, the first asteroid, Ceres, was discovered in 1801, and many more followed. The existence of these enormous rocks in the solar system suggested a plausible source for the meteorites. Space wasn’t empty after all. Finally, in 1803, villagers in Normandy witnessed a fireball followed by thunderous reverberations and a spectacular shower of several thousand stones. The French government sent the young physicist Jean-Baptist Biot to investigate. Based on extensive interviews with witnesses, Biot established the trajectory of the fireball. He also mapped the area where the stones had landed: it was an ellipse measuring 10 by 4 kilometers, with the long axis parallel to the fireball’s trajectory. Biot’s report persuaded most scientists that rocks from the sky were both real and extraterrestrial.”

The French physicist Jean-Baptiste Biot (1774-1862) also did work on the polarization of light and contributed to electromagnetic theory. He accompanied the great chemist Joseph Gay-Lussac in 1804 on the first balloon flight undertaken for scientific purposes, reaching a height of several thousand meters while doing research on magnetism and the atmosphere. The Montgolfier brothers had performed the first recorded manned balloon flight in France in 1783. The French meteorologist Léon Teisserenc de Bort (1855-1913) later discovered the stratosphere, the layer of the Earth’s atmosphere above the troposphere which contains most of the clouds and weather systems, by using unmanned, instrumented hydrogen balloons.

The French physicist Charles Fabry (1867-1945) discovered the ozone layer in the upper atmosphere in 1913. Ozone (O3) is tri-atomic oxygen that exists in the stratosphere as a gas and protects us from most of the harmful effects of ultraviolet radiation from the Sun. It was described by the German-Swiss chemist Christian Friedrich Schönbein (1799-1868) in 1840.

Unmanned gas balloons are still used for meteorological, scientific and even military purposes. Without a pressurized cabin, manned ballooning becomes dangerous in the upper reaches of the atmosphere due to the cold, the low pressure and particularly the lack of oxygen. The inventor Auguste Piccard (1884-1962), originally from Basel, Switzerland, served as a professor of physics in Brussels, created balloons equipped with pressurized cabins and set a number of flight records in the 1930s, reaching an altitude of 23,000 meters.

His son Jacques Piccard (1922-2008), a Brussels-born Swiss oceanographer, explored the deepest reaches of the oceans when he and the American Don Walsh (born 1931) in 1960 used the bathyscaphe Trieste to travel 10,900 metres down to the bottom of the Challenger Deep in the Mariana Trench in the western Pacific. The pressure exerted by ten meters depth of water roughly equals one atmosphere. At the bottom of the Challenger Deep the pressure is consequently well over one thousand times the standard atmospheric pressure at sea level, yet amazingly Piccard and Walsh spotted certain types of fish living even under these conditions.

The French naval officer, explorer, ecologist, author and prizewinning filmmaker Jacques-Yves Cousteau (1910-1997) invented the aqualung together with the engineer Émile Gagnan (1900-1979) in 1943.. Cousteau was a pioneer in the development of underwater cameras as well. He possessed the rare gift of being able to communicate his love of the natural world to a mainstream audience and did a great deal to popularize knowledge of underwater biology.

The Austrian physicist Victor Francis Hess (1883-1964), educated at the Universities of Graz and Vienna, in a series of balloon ascents in 1911-13 established that radiation increases with altitude. Early explorers of radioactivity studied its intensity from church steeples and tall buildings.. Hess found that radiation declined during the first 1,000 m of ascent but then began to rise again, reaching double that at surface level at 5,000 m. By flying his balloon at night and during a solar eclipse he demonstrated that this radiation comes from outer space but not from the Sun. This high-energy ionizing radiation is now called cosmic rays.

The troposphere contains about 80% of the total mass of the atmosphere, with most of the rest concentrated in the stratosphere. The air temperature in the troposphere drops with altitude. The depth of this layer varies from 8 to 16 kilometers from the cold polar region to the warm tropics. Commercial airliners typically cruise at altitudes of 10 km to optimize jet engine fuel burn and stay above most of the turbulent weather found below. The Sun heats the Earth’s surface creating cycles of warming and rising air which later cools again. Coupled with the rotation of the Earth this moves heat and moisture around and creates weather patterns.

From an altitude of 11 to 50 kilometers above the Earth’s surface we find the stratosphere. The lower portion of this layer is influenced by the polar jet stream and subtropical jet stream, fast uniform winds concentrated in a narrow band. The stratosphere defines a layer in which temperatures rises with increasing altitude. This is caused by the absorption of ultraviolet (UV) radiation from the Sun by the ozone layer. Weather balloons filled with lighter-than-air gases such as hydrogen or helium may reach the stratosphere but cannot explore the layers above it as the decreasing pressure causes the balloons to expand until they disintegrate.

The mesosphere stretches from 50 km to 80 km. Temperatures here drop with increasing altitude to almost -100°C, making this the coldest atmospheric layer. The ionosphere can be found at an altitude of around 80 km, at the border to the thermosphere. Ultraviolet radiation from the Sun here causes ionization. While the temperatures here are very high the air is extremely thin. There is no specific height at which the atmosphere begins or ends, but the Kármán line, named after the Hungarian-born American physicist Theodore von Kármán (1881-1963), at 100 km above sea level is sometimes used to mark the beginning of space.

The International Space Station (ISS), a research laboratory in a microgravity environment and probably the most expensive man-made object ever created, is currently found in a stable Low Earth Orbit within the upper part of the thermosphere, between 320 and 380 kilometers above the Earth’s surface. Artificial satellites are usually not put into orbit at altitudes of less than 2-300 km as this is often considered impractical. While the air here is extremely thin, detectable traces of the atmosphere can nevertheless be found as high as 500 kilometers. With the exosphere, beginning at roughly 500 km, the atmosphere gradually turns into space.

Space is full or rocks of different sizes. A meteoroid is too small to be an asteroid or a comet. Even smaller particles are called cosmic dust. If meteoroids enter our atmosphere they become meteors. Most meteors, which often travel at several times the speed of sound, burn up by the friction, typically in the mesosphere, and create fireballs we see as shooting stars. The few that survive this fiery entry and reach the Earth’s surface are known as meteorites.

Many meteorite s are remnants of the Solar System as it was during its formation, most of them probably fragments of asteroids. There are some periodic meteor showers such as the Perseids and the Leonids when our planet is passing through debris trails left behind from certain comets. Based on their chemical composition, a few dozen meteorites are believed to come from the Moon or the planet Mars. Most likely, impacts from meteorites there fired fragments into space, a few of which eventually reached the Earth. The Martian meteorites in particular are treasured objects of study and worth many times their weight in gold.

There are three main meteorite types: iron, stony-iron and stone meteorites. The stony-iron meteorites, the least abundant of the three main types, are thought to have formed at the core/mantle boundary of their parent bodies. In some iron meteorites the iron-nickel alloys can grow into a complex interlocking crystalline pattern known as the Widmanstätten pattern, named after the Austrian scientist and director of the Imperial Porcelain Works in Vienna Count Alois von Beckh Widmanstätten (1753-1849) who described it in 1808, although the English geologist G. Thomson (1760-1806) had probably done the same independently just before. These beautiful patterns are visible when iron meteorites are cut, polished and etched.

By far the largest group in terms of numbers consists of stone meteorites, which once formed part of the outer crust of a planet or asteroid. According to science writer Geoffrey Notkin, “Some stone meteorites contain small, colorful, grain-like inclusions known as ‘chondrules.’ These tiny grains originated in the solar nebula, and therefore pre-date the formation of our planet and the rest of the solar system, making them the oldest known matter available to us for study. Stone meteorites that contain these chondrules are known as ‘chondrites.’“

The English chemist and physicist Henry Cavendish was an extremely meticulous experimenter. With a combination of many different intricate measurements, in 1797-98 he determined the Earth’s mass to within 1% of the modern estimate. Borrowing an idea from the French naturalist Charles-Augustin de Coulomb who had investigated the electrical force between small charged metal spheres, the English natural philosopher John Michell suggested using a torsion balance to detect the tiny gravitational attraction between metal spheres. Since Michell himself died in 1793, Cavendish carried out the experiment at his private home.

Henry Cavendish arrived at an average density of the Earth of 5.48 times that of water. The modern value for the mean density is almost 5..52 times the density of water. Since the density of the material we know from the Earth’s crust is significantly less than this, denser materials must exist within the Earth’s interior. The Earth’s total mass is 5.9736 × 1024 kg. By the late eighteenth century, European scholars had developed a reasonable understanding of the size of the Solar System, the size and mass of the Earth itself and its distance to the other planets.

The French geochemist and mining engineer Gabriel Daubrée (1814-1896) developed a classification system for meteorites and their composition. In 1866 he presented his theory that the Earth has an iron-nickel core, similar to the alloys we see in many iron meteorites.

In early planetary evolution, iron and other heavy elements separated from lighter ones to form dense cores. In the very young Earth the lightest minerals would consequently have floated to the surface. There is no scientific consensus on exactly when during the first Hadean period the Earth had cooled sufficiently to develop a solid crust, but the oldest terrestrial minerals so far discovered, zircons from Western Australia, are at least 4.4 billion years old. The crust is composed primarily, though not exclusively, of low-density rocks and silicate minerals (formed from oxygen and silicon) such as feldspars and quartz. The core is thought to consist primarily of iron, with some nickel and sulfur and traces of other elements.

As we have seen, the nebular hypothesis was presented in the eighteenth century by Emanuel Swedenborg, Immanuel Kant and Pierre-Simon Laplace. The planets and asteroids all revolve around the Sun in the same direction and roughly the same plane, which strongly indicates that they were created during the same process. In 1905 the geologist Thomas Chamberlin and astronomer Forest Ray Moulton (1872-1952) in the USA developed a theory that smaller objects, planetesimals, during the formation of the Solar System collided to build the larger planets. With certain modifications, some elements of their theory remain in use to this day.

We currently believe that almost 4.6 billion years ago the Solar Nebula, a cloud of interstellar dust and gas, was slowly spinning in a flat rotating disk. Many objects in the Oort cloud and the Kuiper belt may be chunks known as planetesimals left over from this period. The formation of protoplanetary disks can be studied in the Orion Nebula today. Small chunks of matter collided to form kilometer-sized planetesimals, some of which collided further to form planetary embryos and moons. As these entities grew larger and got nontrivial gravity their growth accelerated. It is possible that a protoplanet slammed into the young Earth and created our Moon; the Solar System was a much more turbulent place back then than it is now.

Well over 99 % of the material in the Solar Nebula gathered to form a protostar, until the temperature and pressure at the core was high enough to start hydrogen fusion and give birth to our Sun. It is possible for such clouds of collapsing gas and dust to break up into two or more centers, giving birth to binary star systems consisting of two stars orbiting around their common center of mass. We know from observations that systems of two, three or multiple stars are quite common in the universe, for example our neighbor the Alpha Centauri system.

The Italian astronomer Giuseppe Piazzi (1746-1826) studied at Milan, Turin and Rome, taught philosophy for a time at Genoa and mathematics at the University of Malta. Well-connected in Italy and within the European scientific community he managed to establish a decent observatory at Palermo, Sicily. In 1801 Piazzi discovered Ceres, the first known asteroid. He correctly believed it to lie in the orbital region between Mars and Jupiter but soon fell ill and lost track of the object. Based on the very limited number of observations the brilliant young German mathematician Carl Friedrich Gauss successfully calculated its orbit.

Ceres is by far the most massive body in the asteroid belt. Its size is sufficient to give it a spherical shape and it is therefore considered a “dwarf planet.” It contains a significant quantity of water ice and probably preserves a record of what the Solar System was like when it was first condensing from cosmic dust into planetesimals and larger protoplanets. Many astronomers believe that it was the gravitational influence of the neighboring gas giant Jupiter that prevented the plantesimals of the asteroid belt from forming into a regular planet.

Soon after Piazzi’s discovery the German physician and astronomer Heinrich Olbers found two more asteroids, Pallas and Vesta. Next to his medical practice in Bremen, Olbers conducted astronomical observations and discovered several comets. He is famous for popularizing Olbers’ Paradox, a seemingly simple question which scholars found surprisingly difficult to answer: Why is the sky dark at night? If the universe is eternal and contains an infinite number of stars then presumably it should be much lighter than it is. The preferred answer to this, at least within the framework of the current Big Bang cosmological model, is that the universe is not infinitely old (less than 14 billion years old) and that it is expanding. Partial credit for providing the correct answer to this riddle has been given to the American author Edgar Allan Poe (1809-1849) as well as to the great British physicist Lord Kelvin.

After the introduction of photography, the German astronomer and astrophotographer Max Wolf (1863-1932) from the University of Heidelberg invented a technique for revealing asteroids by the streaks they left on photographic plates, thus finding hundreds of new ones. The discovery of the asteroid belt provided an explanation for many impact craters on various planetary bodies. The asteroid belt separates the four smaller, rocky terrestrial planets of the Inner Solar System, Mercury, Venus, the Earth (also known as Terra or Tellus) and Mars, from the four gas giants of the Outer Solar System, Jupiter, Saturn, Uranus and Neptune.

The discipline of Earth science was invented in the 1960s and 70s when it replaced geology as the major discipline for studying our planet, just as geology had once replaced mineralogy. Geophysicists, geologists, oceanographers and meteorologists began working on related problems using similar techniques and implements. At the same time the first space probes were being sent to investigate other bodies in our Solar System, which meant that geologists could extend the scope of their investigations to the domain formerly dominated by astronomers. This led to the creation of planetary science and branches such as astrobiology.

The American geologist and astronomer Eugene Shoemaker (1928-1997) from Los Angeles, California was one of the pioneers of planetary science and arguably founded astrogeology in the 1960s. In much of his asteroid and comet work he collaborated with his wife Carolyn Shoemaker (born 1929), who has discovered many comets. The Shoemakers were co-discoverers of the comet Shoemaker-Levy 9, whose spectacular collision with Jupiter in 1994 was followed with great interest by astronomers. Eugene Shoemaker did much to bring attention to the significance of impacts from comets and asteroids during the Earth’s history.

“Fluid” or weak bodies such as comets that come within the so-called Roche limit of the large planets, named after the astronomer and mathematician Édouard Roche (1820-1883) from Montpellier, France, will be pulled apart by tidal forces. This is what happened to Shoemaker-Levy 9 before its various fragments collided with Jupiter in 1994..

We know from the fossil record that there have been several mass extinctions of life on our planet previously, but the causes of many of them remain hotly disputed. Many people believe that the extinction which ended the age of the dinosaurs roughly 65 million years ago was at least partly caused by the impact of a large asteroid or comet. The American experimental physicist and Nobel laureate Luis W. Alvarez (1911-1988) from the University of California, Berkeley together with his son Walter Alvarez (born 1940) suggested this theory in 1980.

One key piece of evidence is a clay layer (the K-T boundary) which contains an unusually high concentration of the platinum metal iridium. Iridium is very rare in the Earth’s crust because it is very dense and therefore presumably sank into the core during the differentiation period in the Earth’s molten childhood along with many other (though luckily not all) heavy elements. It is much more abundant in meteorites and by extension probably in asteroids and comets. Since then, a huge impact crater from this geological period has been identified outside of the Yucatán Peninsula, although doubts have been raised by some scholars as to whether this impact was the sole cause of this particular mass extinction. For instance, there was a series of massive volcanic eruptions in the Deccan Traps in India at about this time.

Auroras in the Northern Hemisphere are called northern lights; in the Southern Hemisphere southern lights. They appear as arcs, clouds and streaks which move across the night sky. The most common colors are green and red, although other colors may occur, too. Auroras have been observed on some other planets such Jupiter and Saturn as well, caused by their strong magnetic fields. They become more spectacular the closer you get to the Arctic or Antarctic regions, which is one of the reasons why their nature was finally worked out in Scandinavia.

The Swedish astronomers Anders Celsius and Olof Hiorter (1696-1750) in 1741 studied the aurora borealis and noticed that it disturbed magnetic compass needles, although they could not fully explain why. While in Paris, Celsius had become acquainted with the French naturalist Pierre-Louis de Maupertuis (1698-1759) who supported Isaac Newton’s theory that the shape of the Earth swelled near the equator and slightly flattened near the poles.

To settle the matter, in 1736 the French Geodesic Mission sent one expedition to Scandinavian Lapland under Celsius and Maupertuis and another to Ecuador close to the Equator. Their measurements proved that Newton’s theory was correct. Maupertuis is also remembered for advocating the physical principle of least action and for his work on heredity before the nineteenth century theories of evolution by the naturalist Jean-Baptiste Lamarck in France and those of Charles Darwin and Alfred Russel Wallace in Britain.

At Göttingen in Germany, the physicist Wilhelm Weber (1804-1891) cooperated closely with the brilliant mathematician Carl Friedrich Gauss. The German naturalist and explorer Alexander von Humboldt in 1805 reported that magnetic intensity varies across the Earth’s surface and encouraged the establishment of an international network of magnetic observatories. By the 1830s Gauss and his younger collaborator Weber took over from Humboldt as leaders in geomagnetism. Accurate measurements were emphasized by both of them as they realized that these were crucial for developing and verifying physical laws.

During the Enlightenment it had been established that air can be electrically charged. In 1752 the Danish-Norwegian professor and bishop Erik Pontoppidan (1698-1764) suggested that the aurora borealis is an electrical phenomenon. The Scottish physicist Balfour Stewart (1828-1887), who studied terrestrial magnetism, in the 1880s proposed the existence of electrical currents in a high, conductive layer in the atmosphere to explain geomagnetic variations.

At this time the physical chemist Svante Arrhenius (1859-1927) from Sweden was developing his theory of ionic bonds, formed by the attraction between two ions with opposite charges. One example of this would be common table salt (NaCl). In the early 1800s the great English naturalist Michael Faraday had suggested that charged particles which he termed “ions” were formed by the process of electrolysis, but Arrhenius ‘ work led him to believe that electrolytes contain ions even when they are not exposed to electricity.

Ions are atoms that have acquired positive or negative net electric charge due to losing or gaining one or more electrons. The ionosphere is a layer of ionized air in the upper atmosphere, extending from about 80 km, where radiation from the Sun and to a lesser extent cosmic rays break apart molecules and atoms of air, leaving ions and free-floating electrons.

In 1902 the Italian physicist and radio pioneer Guglielmo Marconi successfully sent radio waves across the Atlantic Ocean. The English physicist Oliver Heaviside (1850-1925) and the American electrical engineer Arthur E. Kennelly (1861-1939) in 1902 independently predicted the existence of a conducting reflective layer that was bouncing radio waves back to the ground over vast distances in spite of the Earth’s curvature. The gifted English physicist Edward Appleton (1892-1965), who had studied under J. J. Thomson and Ernest Rutherford, in 1924 through a series of experiments proved the existence of the layer in the upper atmosphere now called the ionosphere. The ionosphere’s existence was fully established about 1930 by Appleton and Douglas R. Hartree (1897-1958) in Britain. Further studies of this layer were carried out by the English geophysicist Sydney Chapman (1888-1970).

The Norwegian physicist Kristian Birkeland (1867-1917) grew up in Kristiania (Oslo) and at the turn of the twentieth century undertook expeditions to near-Arctic regions to study aurora currents. He hypothesized that they were caused by the interaction of energetic particles from outside of the atmosphere with atoms in the upper atmosphere. He managed to experimentally reproduce the Solar System in miniature in his laboratory. He placed a magnetized sphere, a “ terrella “ representing the Earth, inside a vacuum chamber, aimed a beam of electrons towards it and could see that they were steered by the magnetic field to the vicinity of its magnetic poles. Birkeland’s ideas were nevertheless rejected by most scientists at the time and were only verified by satellites generations later. Even a brilliant man such as Lord Kelvin in 1892 erroneously stated that no matter passes between the Sun and the Earth.

In a drive to finance his often expensive research, Birkeland teamed up with the Norwegian industrialist Samuel Eyde (1866-1940) and invented the first industrial scale method to extract nitrogen-based fertilizers from the air. However, by the 1920s their method was no longer able to compete with the Haber-Bosch process. The German Jewish scholar Fritz Haber (1868-1934) invented a process, further developed by Carl Bosch (1874-1940), for mass production of nitrates, which in turn permits the mass production of fertilizers and explosives. Incidentally, Haber was also one of the developers of chemical warfare during World War I.

One large piece of the puzzle was the discovery of zones of highly energetic charged particles trapped in the Earth’s magnetic field. After the Soviet Union in 1957 launched the world’s first artificial satellite into orbit, the Sputnik 1, the United States launched its own Explorer 1 in 1958 whose Geiger counter detected a powerful radiation belt surrounding the Earth. This was the first major scientific discovery of the Space Age. The Van Allen radiation belts, named after the American space scientist James Van Allen (1914-2006), consist of two distinct parts, one inner and one outer belt, formed by somewhat different physical processes.

In 1959 the astrophysicist Thomas Gold (1920-2004) proposed the name “magnetosphere” to the highly magnetized region of space where a planet’s magnetic field dominates the plasma of the solar wind. The magnetosphere has a teardrop shape because it is compressed on the Sun side while its tail is pushed away from the Sun, similar to comet tails. Studies of comet tails by the German astronomer Ludwig Biermann (1907-1986) and others led to successful predictions of the solar wind and of the hydrogen halos around comets.

Hans Christian Ørsted in Denmark in 1820 found a connection between electrical and magnetic phenomena and opened up the study of electromagnetism. The French mathematical physicist and astronomer François Arago described the generation of magnetism by rotation in the 1820s, and his observations were expanded by Michael Faraday. As we have seen, the existence of a liquid outer core separated from a solid inner core inside the Earth was discovered in 1936 by the seismologist Inge Lehmann from Denmark. The German-born physicist Walter M. Elsasser (1904-1991) in 1946 published his theory that the Earth’s electromagnetic field is generated by an internal dynamo caused by currents in the outer core.

According to our best estimates, the planet Jupiter consists of almost 90% hydrogen and 10% helium by numbers of atoms, or 75/25% by mass, with additional traces of methane, water, ammonia and other chemical substances. This is believed to be close to the composition of the primordial Solar Nebula which existed 4.6 billion years ago. Saturn has a similar composition. The intense radiation surrounding Jupiter would be fatal to humans; its magnetosphere is immense and in volume even bigger than the Sun itself, making it arguably the largest structure in our Solar System. It is thought that Jupiter’s magnetic field is generated by an internal dynamo caused by the circulation of metallic hydrogen in that planet’s outer core.

The English geophysicists Sydney Chapman and Vincent Ferraro (1907-1974) in 1930 proposed that the Sun emits huge clouds of plasma, containing equal numbers of positive ions and electrons. It has since then been established that the Sun emits plasma at great speed at all times, not merely during magnetic storms as Chapman and Ferraro had assumed. This is the solar wind, whose existence was predicted in 1958 by the astrophysicist Eugene Parker (born 1927) at the University of Chicago in the USA against strong opposition. He developed his models when observations of comet tails still provided most of the available data. Parker’s work has greatly increased our understanding of the magnetic fields of the Earth and the Sun..

The American astronomer Fred Whipple (1906-2004) in 1950 proposed the “dirty snowball” model where comets have icy cores inside thin insulating layers of dirt. Whipple believed that jets of material ejected as a result of solar heating were the cause of minor orbital changes for some comets. This model is still held to be predominantly correct. The nucleus contains a mixture of dust and water ice with elements of frozen carbon dioxide, methane and ammonia.

Comets are frozen remains of the nebula that formed our Solar System. As they approach the Sun, heat vaporizes some of the frozen materials so that the comet’s nucleus spews gas and dust particles into space. Around the nucleus, which is normally a few kilometers in diameter, comets develop a cloud of diffuse material called a coma. Comet tails are pushed away by solar radiation and the solar wind and consequently always point away from the Sun. Some scientists believe that comets originally brought to the young Earth some of the water and carbon-based molecules that make up living things, although whether substantial amounts of water were brought from comets to our oceans has been disputed based on chemical analysis.

Sunspots are strongly magnetic and appear dark because they are slightly cooler than the regions that surround them. Based on daily observations between 1826 and 1843, the German amateur astronomer Heinrich Schwabe (1789-1875), a pharmacist living in the town of Dessau, in 1843 announced that sunspots vary in number in a cycle of roughly eleven years. He was originally looking for a yet-unknown planet moving inside the orbit of Mercury. His article caught the eye of Alexander von Humboldt, who in 1851 published Schwabe’s table updated to 1850. After that many scientists became interested in the 11-year sunspot cycle. It has later been established that periods with many sunspots correspond to high solar activity.

The Swiss astronomer Rudolf Wolf (1816-1893) had studied at the universities of Zürich, Vienna and Berlin, where the German astronomer Johann Franz Encke was one of his teachers. Wolf became director of the Bern Observatory in Switzerland in 1847 and in 1848 devised the “Zürich sunspot number” to gauge the number of sunspots.

A gentleman of independent means, the English amateur astronomer Richard Carrington (1826-1875), devoted himself to the study of sunspots. Carrington found by observing their motions that the Sun rotates faster at the equator than near the poles. Another early pioneer in the study of sunspot cycles was the German astronomer Gustav Spörer (1822-1895).

The Anglo-Irish geophysicist Edward Sabine (1788-1883) in 1852 found an association between the sunspot cycle and the occurrence of large magnetic storms. On September 1, 1859, Richard Carrington in England through his telescope, which projected an 11-inch-wide image of the Sun on a screen, observed what we now know was a huge solar flare, a magnetic explosion on the Sun. Only 17 hours later this event triggered a large magnetic storm on the Earth. Just before dawn the next day, auroras occurred even in Cuba and Hawaii. Spark discharges shocked telegraph operators in several regions and set telegraph paper on fire.

Unusual solar activity can cause geomagnetic storms (disturbances in the Earth’s magnetosphere) and interrupt electromagnetic communications, for instance by affecting the ionosphere. A powerful solar flare of the strength observed by Carrington could potentially cause quite serious damage today due to our much more extensive reliance on electromagnetic equipment and communications in the twenty-first century as compared to the mid-nineteenth.

Early estimates of stellar surface temperatures made using Newton’s law of cooling gave far too high temperatures. More accurate values were obtained by using the radiation laws of the Slovenian physicist Joseph Stefan in 1879 and the German physicist Wilhelm Wien in 1896. Stefan calculated the temperature of the Sun’s surface to about 5400 °C, which was the most sensible value by date. Stefan’s Law or the Stefan-Boltzmann Law, named after Stefan and his Austrian student Ludwig Boltzmann, suggests that the amount of radiation given off by a body is proportional to the fourth power of its temperature as measured in Kelvin units.

The part of the Sun that we normally see has a temperature of more than 5500 degrees C, almost 5800 K. Temperatures in the core, where nuclear fusion occurs, reach over 15 million K. The lowest layer of the atmosphere is called the photosphere. The next zone is the chromospheres, where the temperature rises to 20,000 K. The corona, the Sun’s outer atmosphere, is remarkably hot. In the part nearest the surface the temperature is 1 million to 6 million K, but it can reach tens of millions of degrees when a flare occurs. Sunspots are cooler regions where magnetic energy builds up and is often released in solar flares and discharges of charged particles known as coronal mass ejections. These events can trigger space storms that affect the Earth. The flow of coronal gas into space is known as the solar wind. The corona is visible during total solar eclipses as a large halo of white, glowing gas, but the relative rarity of such eclipses present logistical difficulties for detailed observations.

The technical problems associated with producing an artificial eclipse to study the Sun were solved by the French solar physicist Bernard Lyot (1897-1952), an expert in optics who had studied engineering in Paris in addition to mathematics, physics and chemistry. As an astronomer, Lyot found that the lunar surface behaves like volcanic dust and that Mars has sandstorms. In 1930 he invented an instrument dubbed the coronagraph, a telescope equipped with an occulting disk sized in such a way as to block out the solar disk, which is much more difficult than it sounds. By 1931 he was obtaining photographs of the corona. He found new spectral lines in the corona and made the first motion pictures of solar prominences.

In the 1930s, Lyot boldly inferred a coronal temperature of around 600,000 K. This claim was met with skepticism at the time. Acceptance of these very high temperatures came through the spectroscopic work of the German astrophysicist Walter Grotrian (1890-1954) and the Swedish astrophysicist Bengt Edlén (1906-1993) soon after, but an explanation for how the Sun’s upper atmosphere could be so much hotter than its surface took a long time to work out.

The Swedish physicist Hannes Alfvén (1908-1995) was one of the founders of plasma physics and magnetohydrodynamics, the study of plasmas in magnetic fields. Alfvén was born in Norrköping, Sweden. Both his parents were practicing physicians. He studied at Uppsala University and became a research physicist and professor in Stockholm. He made many discoveries in solar and space plasma physics and his work on cosmic rays led him to propose in 1937 the existence of a galactic magnetic field. The one discovery for which he is best known is the magnetohydrodynamic wave commonly called the Alfvén wave, whose existence for decades was difficult to prove. Finally in 2009, pictures taken by a team using the Swedish Solar Telescope in Spain’s Canary Islands revealed that “corkscrew” waves — Alfvén waves — were pushing heat from the Sun’s surface to its outer atmosphere, the corona.

The amount of energy the Sun puts out varies over an 11-year cycle which also governs the appearance of sunspots. While that cycle changes the total amount of solar energy reaching the Earth only by a tiny fraction, perhaps 0.1 percent, this small variation appears to be sufficient to affect our weather patterns; by how much remains a field of active research. We know that a period with very few sunspots called the Maunder Minimum, named after the English astronomer Edward Maunder (1851-1928), began around 1650, at the same time as a period of unusually cold weather called the Little Ice Age. Was this merely a coincidence?

Henrik Svensmark (born 1958) from the Center for Sun-Climate Research at the Danish National Space Center in Copenhagen has proposed that solar activity and cosmic rays are instrumental in determining the warming and cooling of the Earth. He builds on the work of physicist Eigil Fiin-Christensen who with Knud Lassen Fiin in 1991 looked at solar activity over the last century and found a remarkable correlation to temperatures on our own planet.

Cosmic rays are energetic particles, most of them protons, originating from outer space. Galactic cosmic rays are subatomic particles — protons along with some heavy nuclei — accelerated to velocities approaching the speed of light by distant supernova explosions. In addition to being modulated by the Earth’s magnetic field these have to enter the heliosphere, the protective bubble stretching beyond the orbit of Pluto where the solar wind, the plasma of electrons and atomic nuclei constantly ejected from the Sun, dominates interstellar space. When solar activity is strong, the solar wind allows fewer external cosmic rays to reach our Solar System and our planet.

Henrik Svensmark and his colleagues carried out a landmark study of cosmic rays and clouds. They demonstrated that such rays could produce small aerosols, the basic building blocks for cloud condensation nuclei. The condensation of clouds affects the energy balance and by extension the temperature on Earth. They received support for these studies from the Danish Carlsberg Foundation, founded by the beer producer which was an early pioneer in scientific brewing. As Mr. Svensmark puts it in an interview with the science magazine Discover:

“We live in a unique time in history, because this period has the highest solar activity we have had in 1,000 years, and maybe even in 8,000 years. And we know that changes in solar activity have made significant changes in climate. For instance, we had the little ice age about 300 years ago. You had very few sunspots between 1650 and 1715, and for example, in Sweden in 1696, it caused the harvest to go wrong. People were starving — 100,000 people died — and it was very desperate times, all coinciding with this very low solar activity. The last time we had high solar activity was during the medieval warming, which was when all of the cathedrals were built in Europe. And if you go 1,000 years back, you also had high solar activity, and that was when Rome was at its height. So I think there’s good evidence that these are significant changes that are happening naturally. If we are talking about the next century, there might be a human effect on climate change on top of that, but the natural effect from solar effect will be important.”

Far from all scientists agree that there is an intimate link between the alleged global warming going on today and cosmic rays, although the American astrophysicist Eugene Parker, the discoverer of solar wind, takes this hypothesis seriously. Nevertheless, these investigations contribute to an emerging multidisciplinary field of cosmoclimatology, the study of how “space weather” and events outside of the Earth itself may affect the climate on our planet.

According to NASA’s fine website there are at least 100 billion stars in our own Milky Way Galaxy, possibly much more than that, compared to a trillion (million times a million) or so in the huge neighboring Andromeda Galaxy. Our Solar System lies in a spiral arm about 25,000 light-years from the center of our galaxy and needs approximately 225 million years to complete one orbit of it. There are some scientists who speculate whether our position relative to the Milky Way ‘s center can be associated with certain geological time periods on Earth.

Scholars during the past two hundred years have vastly increased our knowledge about the chemistry of life. European chemists in the early nineteenth century made a distinction between inorganic and organic chemistry. They correctly considered the latter to be more complex, but mistakenly believed that organic substances could only be made by living creatures. This changed when the gifted German chemist Friedrich Wöhler (1800-1882), a student of the Swedish scholar Jöns Jakob Berzelius who also collaborated with the leading German chemist Justus von Liebig, in 1828 discovered that urea, an organic compound and one of the constituents of urine, could be synthesized from inorganic materials.

Gradually it became clear that there is no fundamental difference between organic and inorganic chemistry apart from the fact that organic compounds are often complex. They contain carbon atoms, which have the ability to combine with other atoms in numerous different ways. The German chemist Friedrich August Kekule von Stradonitz, or August Kekulé (1829-1896), who taught at the Universities of Heidelberg, Ghent and Bonn, in 1858 established the fact that carbon has a valence (combining power) of four. This insight was of fundamental importance in the evolution of organic chemistry, which is today synonymous with carbon-based chemistry. Kekulé had the idea that carbon atoms could link up in rings as well as chains. This was independently proposed by Archibald Scott Couper (1831-1892) from Scotland as well. In 1865 Kekulé described the ring structure of benzene molecules.

Carbon with atomic number six has physical properties which enable it to form millions of compounds. It has two common allotropes where its atoms are bonded together in different ways: Diamond is the hardest known naturally occurring mineral while graphite is soft and was named by Abraham Gottlob Werner from Greek for “to write” due to its use in pencils.

A more recently discovered class of carbon allotropes are fullerenes, hollow, cagelike molecules composed of at least 60 atoms of carbon. Spherical fullerenes resemble a European-style football and are called “buckyballs” after the American architect Buckminster “Bucky” Fuller (1895-1983), famous for his geodesic domes. C60 fullerene was discovered in 1985 by a team from Rice University in the United States and the University of Sussex in Britain. The English chemist Harold Kroto (born 1939) soon shared a Nobel Prize in Chemistry for the discovery with the Americans Richard Smalley (1943-2005) and Robert Curl (born 1933). Cylindrical fullerenes are known as nanotubes and are exceptionally strong.

The Russian biochemist Alexander Oparin (1894-1980) majored in plant physiology at Moscow State University and was influenced by the ideas of the English naturalist Charles Darwin. He extended Darwin’s theory of evolution backwards in time to explain how simple organic and inorganic materials might have combined into more complex compounds. In 1922 Oparin introduced the concept of a brew of organic compounds and carbon-based molecules, a “primordial soup,” as the origin of life on Earth. The English evolutionary biologist J. B. S. Haldane (1892-1964) independently proposed a closely related, though not entirely identical, hypothesis at roughly the same time.. These ideas initially faced powerful opposition but have since then become accepted in their main outlines. Oparin published the book The Origin of Life and organized the first international meeting on the origin of life in Moscow in 1957.

The Dutch-born astronomer Gerard Kuiper and the American physical chemist Harold Urey (1893-1981) renewed interest in the Solar System. Kuiper discovered the carbon dioxide atmosphere of Mars and contributed to the first phase of space exploration. Urey investigated the distribution of elements in the Solar System in his book The Planets: Their Origin and Development (1952) and helped to develop the field of cosmochemistry or astrochemistry.

Harold Urey in 1921 entered the University of California to work under Gilbert Newton Lewis. He spent the following year at Niels Bohr’s Institute for Theoretical Physics in Copenhagen, Denmark. In 1931 he developed a method for distillation of liquid hydrogen which aided the discovery of deuterium, for which he received a Nobel Prize in 1934. Deuterium is a stable isotope of hydrogen where the nucleus contains one proton and one neutron. Tritium is a radioactive and very rare hydrogen isotope with two neutrons.

Urey moved to Chicago in 1945. One of his doctoral students at the University of Chicago was Stanley Miller (1930-2007), who decided to test the Oparin-Haldane theory experimentally. The famous Miller-Urey experiment from 1953 mixed water, methane, ammonia and hydrogen in a chamber to simulate the Earth’s presumed early atmosphere and used an electric discharge to simulate lightning. After just a week, organic compounds had been formed in the shape of amino acids, the basic building blocks of life as we know it.

There are those who believe that the oceans appeared already within two hundred million years after the Earth was formed (it was too hot at first), while others think this happened later and that primitive life forms developed soon afterward, maybe 3.8 billion to 3.5 billion years ago. The oxygen-rich atmosphere we currently enjoy, which makes complex life forms such as ourselves possible, is the result of several billion years of work by cyanobacteria, also known as blue-green algae, which use water, carbon dioxide and sunlight to produce oxygen.

Around the year 1900, several European scientists rediscovered a neglected research paper on heredity by the Bohemian monk Gregor Mendel, who had conducted breeding experiments with pea plants a few decades earlier. The American geneticist Thomas Hunt Morgan soon demonstrated that chromosomes are key factors in heredity. DNA had been isolated by the Swiss physician Friedrich Miescher already in 1869, but he didn’t grasp the importance of his find. In 1944 the Canadian-born USA-based medical researcher Oswald Avery and his co-workers more or less demonstrated that DNA itself was the unit of genetic inheritance, a fact that was further established through experiments conducted by the Americans Alfred Hershey and Martha Chase in 1952. Finally, James D. Watson and Francis Crick working at Cambridge University in England delineated the double-helix structure of DNA in 1953.

DNA, deoxyribonucleic acid, is the molecule that contains the genetic code for all currently known life forms on Earth except for some RNA-based viruses. Whether viruses constitute life forms is debatable since they have no metabolism and cannot reproduce without infecting a host cell. DNA consists of two long, twisted chains made up of nucleotides. Each nucleotide contains one base, one phosphate molecule and the sugar molecule deoxyribose. A gene is a segment of a DNA molecule that contains information for making a protein. Proteins perform the chemical reactions in our bodies and provide the body’s main building materials, forming the architecture of our cells. Amino acids are the building blocks of proteins. Chromosomes are cellular structures containing genes. Humans normally have 23 pairs of chromosomes.

During sexual reproduction the egg cell of the mother and the sperm cell of the father undergo cell division where the 46 chromosomes are divided in half and the egg and the sperm cells end up with 23 chromosomes each. The baby ends up with a complete set, half of them from each parent. In every cell in the human body there is a nucleus where genetic material is stored in genes grouped in chromosomes. Individuals suffering from the disorder known as Down’s syndrome, named after the English doctor John Langdon Down (1828-1896) who first described it in 1866, have three copies of chromosome number 21. The correct explanation for this was made by the French geneticist Jérôme Lejeune (1926-1994) in 1959.

Evolutionary biologists differ in their views of what came first, genes and then proteins or vice versa; this is the new version of the ancient chicken-or-the-egg debate. Unlike double-stranded DNA, the related ribonucleic acid (RNA) usually comes as a single strand and is quite flexible. According to the website of the National Institute of General Medical Sciences, “Each year, researchers unlock new secrets about RNA. These discoveries reveal that it is truly a remarkable molecule and a multi-talented actor in heredity. Today, many scientists believe that RNA evolved on the Earth long before DNA did. Researchers hypothesize — obviously, no one was around to write this down — that RNA was a major participant in the chemical reactions that ultimately spawned the first signs of life on the planet.”

One of the most important breakthroughs in biology during the twentieth century was the realization from the 1970s and 1980s on that life on our planet is far hardier than scientists had previously suspected. Living organisms have been found in many extremely harsh and difficult environments ranging from the superheated waters of submarine volcanic vents to the ultra-dry bitter cold of the Antarctic Dry Valleys. We can encounter organisms living in boiling water or caves dripping with sulfuric acid. Most of these extremophiles are microbes.

The American research vessel Alvin in 1964 became the first deep-sea submersible capable of carrying a pilot and two scientific observers to a depth of 4,000 meters. It was used in the discovery of black smokers in 1977 as it surveyed the Galapagos Rift. A pioneer in the field of deep-water research and archaeology is Robert Ballard (born 1942), an explorer and oceanographer from the United States. Ballard is mostly remembered among the general public for his leading role in the discovery of the wreck of the famous RMS Titanic in 1985.

The Russian Mir submersibles, built in Finland for the Soviet Union in 1987, were used by director James Cameron (born 1954) for the underwater filming of the Titanic, located at a depth of 3, 820 meters, for his 1997 blockbuster Hollywood film of the same name. The expedition’s leader, the Russian scientist Anatoly Sagalevitch (born 1938), has also led diving expeditions to the unique Lake Baikal in eastern Siberia, the world’s deepest freshwater lake.

Black smokers are chimneylike structures on the ocean floor made up of sulfur-bearing minerals or sulfides. Just as we can find natural hot springs in certain volcanically active regions on land, similar phenomena called deep-sea hydrothermal (hot water) vents can occur under the oceans next to mid-ocean ridges, where molten rock bubbles up from the mantle to the sea floor and forms new oceanic crust. Here we can encounter unusual life forms such as tube worms and giant clams. Most notably, in this environment of perpetual darkness we can find entire ecosystems that exist totally without the aid of sunlight. They are based not on the common photosynthesis but on chemosynthesis, by converting heat, methane and sulfur into food and energy. A few researchers such as the German chemist Günter Wächtershäuser (born 1938) have suggested that life on Earth may have begun in environments similar to these.

Tardigrades, sometimes known as “water bears,” are tiny water-dwelling eight-legged critters that grow to a size of about 1 millimeter. They are able to survive extreme temperatures and live from the highest mountain tops to the bottom of the oceans. In 2008 a box of water bears was launched into orbit aboard the Russian Satellite FOTON-M3 and spent ten days in containers that exposed them to the vacuum, radiation and extreme cold of space. Amazingly, some of them survived this brutal treatment and returned to Earth where they managed to lay healthy eggs. While tiny and simple, tardigrades are nevertheless multicellular organisms technically classified as animals. This was the first time that it had been demonstrated that an animal with a mouth, head, brain, legs, eyes, nerves and muscles has the ability to survive unprotected in space — an ability previously only proved for some lichens and bacteria.

A few scientists support the idea of panspermia (“all seed”), according to which life exist all over the universe, or the more moderate concept of exogenesis (“outside origin”) where life on Earth originated elsewhere, maybe in the form of extraterrestrial microbes brought here with meteorites. These hypotheses are highly controversial and remain the view of a very small minority of scholars, but the fact that a few microscopic terrestrial organisms can survive in space is certainly interesting. Since life on our planet is hardier than we expected this increases the likelihood that it can exist elsewhere, too. This is especially intriguing now that we finally have the technological capability to explore other bodies in our Solar System.

The polymath Mikhail Lomonosov (1711-1765) was a pioneer of modern science in the Russian Empire. Born in a small village, his family were nominally peasants but still enjoyed a degree of freedom not known to serfs in Central Russia. He concealed his identity, as peasants could not attain the prestigious Academy in Moscow, and pretended to be the son of a priest to gain admission. He soon impressed his teachers with his intelligence. In 1735 he was selected to the new Imperial Academy of Sciences in St. Petersburg. Lomonosov studied for several years in Western Europe and picked up a German wife. In 1748 he opened the first modern chemical laboratory in Russia. He also promoted Russian history and language.

According to the book Venus in Transit by Eli Maor, “It was during the 1761 transit that Lomonosov, observing from his home in St. Petersburg, saw a faint, luminous ring around Venus’s black image just as it entered the sun’s face; the sight was repeated at the moment of exit. He immediately interpreted this as due to an atmosphere around Venus, and he predicted that it might even be thicker than Earth’s. Lomonosov reported his finding in a paper which, like most of his written work, was only published many years after his death in 1765. But it was not until 1910, one hundred and fifty years after the transit, that his paper appeared in German translation and became known in the West. Up until then the discovery of Venus’s atmosphere had been credited to William Herschel.”

The Venera 7 probe from the Soviet Union in 1970 became the first space probe to transmit data from the surface of Venus. The Earth’s atmosphere is by volume composed of roughly 78% nitrogen gas (N2), 21% oxygen gas (O2), 0.9% argon (Ar), some water vapor (the gas phase of water, H2O), almost 0.04% carbon dioxide (CO2) and small amounts of a number of other gases such as methane (CH4). While the three latter gases constitute a tiny part of the atmosphere they trap heat from the Sun and warm the Earth through the greenhouse effect.

The mass of the atmosphere of Venus consist of 96.5% carbon dioxide and the planet is surrounded by thick clouds of sulfuric acid. The dense atmosphere produces a run-away greenhouse effect and the temperature at Venus’ surface is more than 460 degrees Celsius, hot enough to melt lead. It is highly unlikely whether life as we can conceive of it can exist in such an inhospitable environment, except possibly in the cooler upper atmosphere. The Martian atmosphere, too, is dominated by carbon dioxide at 95.3 percent plus some additional nitrogen, argon and water vapor, but at the surface the atmospheric pressure is typically 0.7 percent that of the Earth’s surface. In contrast, atmospheric pressure at the surface of Venus is about 92 times that of the Earth and by extension roughly ten thousand times that of Mars.

The astronomer Giovanni Schiaparelli (1835-1910) from the winy Piedmont region of northwestern Italy explained regular meteor showers as the result of the dissolution of comets and proved this for the Perseids, thereby forging a link between comets and certain meteors. He studied Mars and named its “seas” and “continents.” According to the book The Planet Mars: A History of Observation and Discovery by William Sheehan, “In Italian, canali can mean either ‘channels’ or ‘canals.’ It is clear that Schiaparelli had completely natural features in mind—-indeed, he often used the word fiume (river) as a synonym. Strictly speaking, the term channel would have been preferable, but instead it was canal, with all its connotations of artificial waterways, that was adopted in English, with far-reaching consequences.”

Schiaparelli’s alleged observations of Martian canals stimulated the American businessman and astronomer Percival Lowell (1855-1916) to found his observatory in the 1890s and search for intelligent life on Mars. He also predicted the existence of a planet beyond the orbit of Neptune. The young American astronomer Clyde Tombaugh (1906-1997) discovered Pluto in 1930 while working at the Lowell Observatory in the USA. He used photographic plates, which were used in astronomy and particle physics long after they had gone out of popular use, although astronomers like most others later switched to digital cameras. In Roman mythology Pluto, the equivalent of the Greek deity Hades and his abode of the same name, was the god of the dark underworld. “PL” also happens to be the initials of Percival Lowell.

Our seasons are caused by the Earth being tilted on its axis by 23.5 degrees. This axial tilt is not constant but varies very slowly from 22.1 to 24.5 degrees in a cycle of 41,000 years. In June the Northern Hemisphere faces the Sun and receives more direct sunlight, which means that there is summer in the north and winter in the Southern Hemisphere. The exact opposite is the case six months later when the Earth is on the other side of its orbit and is tilted the other way vis-à-vis the Sun. The seasonal daylight differences and the gradual lengthening or shortening of days are subtle in the tropics, but the regions next to the poles will be in total darkness by mid-winter and in turn receive 24 hours of nonstop sunlight (the “midnight Sun”) in the middle of the summer, close to winter solstice and summer solstice, respectively.

Mars has an axial tilt of 25.2 degrees and has seasons just like the Earth, only longer ones since the Martian year — its orbital period around the Sun — is longer with 687 Earth days. One crucial difference is that while the Earth’s orbit is nearly circular, Mars has a significantly more elliptical orbit with a more pronounced orbital eccentricity. This means that the difference between the point when it is closest to the Sun, the perihelion, and the point when it is furthest away from the Sun, the aphelion, is small and climatically insignificant for our own planet but significant in the case of Mars. While the Martian atmosphere is thin it is still dense enough to support a weather system. Huge dust storms can cover almost the entire planet and are often strongest at perihelion when the Sun heats its atmosphere the most.

Mars is named for the ancient Roman god of war. Its reddish color, similar to that of rust (iron oxide), comes from iron-rich minerals in its soil. Large amounts of water probably once flowed on its surface, which contains many channels, gullies and huge valleys. The planet’s seasonal polar caps consist of a mixture of solid carbon dioxide (“dry ice”) and water ice.

Since there are currently no seas on Mars there is no “sea level” to measure from, but by any standard the massive shield volcano Olympus Mons is the largest known mountain in the entire Solar System. Standing about 25 kilometers higher than its surrounding landscape it is almost three times higher than Mount Everest. However, the force of gravity at the Martian surface is only about 38 percent that of the Earth. A mountain of similar size probably wouldn’t have survived on our planet as it would have been crushed under its own weight.

While it remains a possibility that plate tectonics once existed on Mars, astrogeologists believe that plate tectonics processes are no longer active there now. Olympus Mons is thought to be fixed over a hotspot which has allowed repeated eruptions to build it to its present height. On the Earth, hotspots remain stationary while crustal plates move above them. It is virtually certain that our hot sister planet Venus is still volcanically active. It is distinctly possible that cooler Mars is so as well, but we currently possess no proof of this.

Mars has no global magnetic field, which means that its core is probably entirely solid. The Earth’s magnetic field creates a protective bubble against the solar wind and harmful rays from space. Since Mars lacks this and has a thinner atmosphere, the planet’s surface is much more exposed to potentially harmful radiation. The average temperature is about -60 degrees Celsius, but varies considerably seasonally and regionally from a comfortable 20 degrees plus near the equator at summer to a staggering minus 130 degrees Celsius near the poles during the winter. Because of these factors, if evidence of past or present microbial life is ever found on Mars, many astrobiologists suspect that it will be located just below the surface.. There, microbes might be more shielded from cosmic radiation, and heat from the still hot Martian core makes it more likely that non-trivial amounts of water could still exist in liquid form.

Joan Oró Florensa (1923-2004) was a Catalan biochemist who graduated from Barcelona University in 1947 and emigrated from Spain to the USA in 1952. Following the Miller-Urey experiment from 1953, Joan Oró in 1959 demonstrated in a related experiment that adenine, one of the components of DNA, formed in abundance in his “primordial soup.” Oró was one of the first scientists to suggest the possibility that comets could have acted as carriers of organic molecules to the Earth’s early biosphere. From the 1960s he worked with NASA, including the Viking missions to Mars in 1976. While the results were inconclusive back then, it is possible that their equipment was not sensitive enough to detect life forms even if present.

In 2003-2004, American space scientists as well as data from the Mars Express Orbiter by the European Space Agency (ESA) found evidence of methane in the Martian atmosphere. There could be an active source of methane production on the planet. This is not by itself proof of the presence of life as methane can be produced through both biological and non-biological processes, but the discovery is definitely encouraging. Obviously, if we do find extraterrestrial life on Mars or elsewhere it is not at all certain that it will be carbon-based and DNA-based as life is on our own planet. Theoretically speaking, extraterrestrial life could be so different from the life forms we are familiar with that we would find it hard to recognize it as life at all.

From the Age of Exploration until the twentieth century, European explorers and eventually scholars reached almost all corners of the planet, including the polar region. Vitus Bering (1681-1741), a Danish navigator in the service of the Russian Navy, was the first known European to see Alaska. The Bering Strait, which separates Siberia and the Asian continent from North America, is named after him. Across this strait there was a land bridge during the last Ice Age. It is likely that nomadic hunters from Siberia entered Alaska and the Americas from here. There is archaeological evidence of the widespread presence of the Clovis culture in North and South America before 10,000 BC, but it is remains possible that there were several different waves of settlement and that the first one began even earlier than this.

The expeditions of the American explorers Frederick Cook (1865-1940) and Robert Peary (1856-1920) to the North Pole region and those of the Norwegian explorer Roald Amundsen (1872-1928) and the Anglo-Irish explorer Ernest Shackleton (1874-1922) to Antarctica, while greatly fascinating, were not noted primarily for their scientific interest; they were more about glory and adventure in addition to the possibility of discovering new and potentially important sea lanes. Modern polar science began with the first International Polar Year in 1882. This was the brainchild of the officer Carl Weyprecht (1838-1881) of the Austro-Hungarian navy, who had discovered Franz Josef Land in 1874 while searching for the Northeast Passage.

Expeditions to Greenland by the Norwegian explorer and scientist Fridtjof Nansen (1861-1930) in the 1890s and by Alfred Wegener from Germany in the early 1900s surveyed the glaciers and ice sheets there. The same was the case with the British Royal Naval officer Robert Falcon Scott’s (1868-1912) Antarctic expedition of 1901-1904. In recent decades, ice sheets have gained international attention for preserving some of the finest records of climate change over the last hundred thousand years or more. Glaciology, the study of ice, ice formations and glaciers, is of increasing importance to planetary scientists.

Next to Mars, the most promising candidates for primitive life in our Solar System might not be the planets but rather some of their moons. The study of ice on bodies such as Jupiter’s moons Europa, Callisto and Ganymede as well as Saturn’s icy satellites Enceladus with its large water vapor geysers and especially Titan with its dense atmosphere and surface liquid in the form of hydrocarbon lakes is an emerging scientific field. Ganymede is the largest satellite in the Solar System. Like Titan it is larger in diameter than the innermost planet Mercury but has less mass. Mercury is a dense object. Europa may harbor a large liquid water ocean and is therefore a priority target for future space probes. The same could be true of Callisto.

The fourth of Jupiter’s large Galilean moons, Io, is the most volcanically active body in the Solar System, with volcanoes spewing out sulfur and sulfur dioxide to a height of hundreds of kilometers. The heat is caused by massive tidal forces generated by Jupiter and other moons. Volcanism exists on other bodies, though not necessarily in the form of molten rock (lava) as is the case here on Earth. The path-breaking American Voyager 2 spacecraft in 1989 observed cryovolcanoes (ice volcanoes) on Triton, the largest moon of the planet Neptune. The temperature at the surface of Triton is only 34.5 K (-235 C), at least as cold as Pluto. In this extreme cold methane, nitrogen and carbon dioxide all freeze solid. The geysers Voyager 2 observed on Triton are probably nitrogen geysers driven by seasonal heating by the Sun.

A recent advance of tremendous importance is the discovery of the first extrasolar planets, planets orbiting other stars. Hundreds of these have been found during the first generation after 1990 alone. This has led to the establishment of a new branch within planetary science dubbed exoplanetology or exoplanet science. Most of the planets discovered so far have been gas giants detected through indirect means by observing the effects they have on the stars they orbit, but methods are rapidly improving and more Earth-like planets have been identified.

It is highly unlikely that we in the foreseeable future, if ever, will have the technological capability to send robotic probes to explore extrasolar planets, let alone manned missions. Nevertheless, by studying them from a distance we can learn a great deal about planet formation and about how common Earth-like planets are in our galaxy and in the universe.

Unlike most elements lighter than iron the alkali metal lithium with atomic number three is not easily produced in stars. According to current theories, most of it was probably created after the Big Bang. Yet astronomers see a wide range of different lithium levels in Sun-like stars. With the European Southern Observatory’s HARPS spectrograph survey of hundreds of stars, astronomer Garik Israelian of Spain’s Instituto de Astrofisica de Canarias in Tenerife and his colleagues found that those that had an orbiting planetary system had lithium levels similar to the Sun’s while those that did not had higher levels. If this insight is correct it might suggest an easier way to look for undiscovered planetary systems around other stars.