by Paul Murdin
In 2005 the Deep Impact probe was crashed onto Comet Tempel 1 and excavated a crater to expose its interior. Most of the comet’s ice lies below the surface in accumulations that feed jets of vaporized water that spurt out from the comet. Giotto witnessed how jets on Halley’s Comet threw out 3 tonnes of comet material per second. Rosetta witnessed jets being shot from Comet 67P. The gas includes vaporized tarry substances, gases of carbon compounds that make comets some of the smelliest places in the Solar System. Most of the ejected grains of dust are no larger than specks of cigarette smoke. The largest grain detected by Giotto was 40 milligrams, but the large particle that damaged the spacecraft was perhaps as heavy as 1 gram.
Comets also have a second, faint, straight tail of gas, which is pushed back by the solar wind. In 1985, NASA’s International Cometary Explorer (ICE) probe explored the gas tail of Comet 21P/Giacobini–Zinner. It is made of plasma, gas that has been ionized by the collision of solar particles.
Comets lose material in their coma and tail at every passage past the Sun, and refreeze once they have left the Sun behind. After a number of passages there is no more loose material and the crusty surface is thick, keeping the comet’s icy material trapped below: the comet becomes inactive or extinct. It becomes an asteroid, of the sort known as a ‘Centaur’, which has signs of fading cometary activity – a very faint tail, an intermittent coma.
It is thought that comets formed when the Sun formed, 4.5 billion years ago, from interstellar ices condensing onto grains of interstellar dust. They were originally planetesimals that congealed from scraps of dust and gas in the pre-solar nebula. Since then, they remained almost unaltered in two cold, outer regions of the Solar System, until they fell towards the Sun, ultimately doomed to melt like snowmen when the Sun rises. Short-period comets come from the Kuiper Belt, which is located in the outer Solar System beyond the orbit of Neptune. The source of the long-period or sporadic comets is thought to be the Oort Cloud, a spherical reservoir of comets surrounding the Solar System. Dutch astronomer Jan Oort discovered in 1950 that many long-period comets fall towards the Sun from a distance of between 20,000 and 200,000 times the distance from the Sun to Earth. Comets that formed inside Neptune’s orbit were ejected into distant orbits during encounters with giant planets, and formed the Oort Cloud and the Kuiper Belt. Occasional encounters with each other, or with passing stars or giant clouds of interstellar material, reinject some comets from the Oort Cloud and Kuiper Belt back into the inner Solar System.
It is likely that, early in the history of the Solar System, there were frequent collisions of comets with Earth. Some of our ocean water may have been brought to Earth by comets, although, when Rosetta measured some of the features of the composition of water on Comet 67P, it proved to be dissimilar to the water in Earth’s oceans. In addition to water, complex organic molecules (and especially ‘prebiotic’ organic molecules) could also have acted as seeds for the development of life on Earth.
Astronomical Cycles
The Earth’s climate, seasons and the weather
Nature is an endless combination and repetition of a very few laws. She hums the old well-known air through innumerable variations.
Ralph Waldo Emerson, ‘History’, in Essays, 1841
As the Sun warms the atmosphere, the land and the oceans, it sets in motion the cycles that generate the Earth’s weather. But what triggers the catastrophic chill of an Ice Age? A Serbian mathematics teacher, Milutin Milankovič, suspected that Ice Ages were caused by cyclical changes in the Earth’s orbit. Deep below the ocean floor and the Antarctic ice, the proof of Milankovič’s theory was waiting to be uncovered.
The Sun warms the Earth most directly in the regions around the Equator. There, sunlight radiates through the atmosphere to warm the ground and surface of the sea. As they warm, the layer of air at the base of the atmosphere becomes less dense and it rises (a process called ‘convection’). Cold air is drawn in underneath the rising warm air. At high altitude, the warm air cools and flows away from the Equator, eventually beginning to sink again at about latitude 30º and returning to the equator at ground level. In 1686, Edmond Halley discovered that this cycle was the engine for the major wind systems of the world.
Although this convection cycle creates the wind pattern, the wind does not blow just north and south from the equator. In the days of sailing ships, sea travel was heavily dependent on the ‘trade winds’, which flow strongly and consistently from the east in the areas around the Equator. Why do the trade winds blow at right angles to what you would expect? George Hadley, an English lawyer and amateur meteorologist, explained the phenomenon in 1735; the closed cell of air cycling from the Equator to latitude 30º and back is consequently called a ‘Hadley cell’. Hadley realized the important effect of the rotation of the Earth on wind motion. As air blowing over the Earth’s surface moves from a higher latitude region, where the wind’s eastward velocity is lower, into a region of higher eastward velocity (at the Equator), the wind picks up a westward motion.
Hadley’s intuitive explanation was given a sound physical basis in 1835 by the French physicist Gaspard-Gustave de Coriolis, who applied Newton’s fundamental mathematical theories to the problem. Because of the rotation of the Earth, winds deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is called the Coriolis Effect. In 1856 Hadley cells and the Coriolis Effect were brought together in a unified theory by the American meteorologist William Ferrel.
The position and the strength of the winds determine the weather, which therefore ultimately depends on the strength of solar radiation. The warmest zone on the Earth is in general the Equator, but more precisely the sub-solar latitude (that is, the area of the Earth that lies directly ‘below’ the Sun). Because the Earth is tilted at 23.5º to its orbital plane around the Sun, the sub-solar latitude changes position throughout the year, moving from the Tropic of Cancer at 23.5º N in June to the Tropic of Capricorn at 23.5º S in December. This produces the annual cycle of the seasons, which are influenced locally by factors such as latitude, altitude, terrain, vegetation and proximity to oceans, and may be characterized by particular weather phenomena such as a rainy season or a persistent wind like the mistral.
Additionally, the eccentricity of the Earth’s orbit around the Sun has a small but noticeable effect on the annual weather cycle. The Earth does not orbit the Sun in a perfect circle, but in a slight ellipse, which causes the distance between the Earth and the Sun to fluctuate at different times of the year. The Earth is closest to the Sun in the first week of January and furthest in the first week of July. This magnifies the effect of the Earth’s tilt on the seasons, so in December and January the summer solar radiation is stronger in the Southern Hemisphere than it is in June and July in the Northern Hemisphere, which is why summers tend to be hotter in the Southern Hemisphere.
If the Earth’s tilt and its eccentric orbit stayed the same forever, these annual cycles of the seasons would remain the same. However, long-term cycles of change in the Earth’s position and orientation cause changes in the seasons over time. The Earth’s axis does not always point in the same direction, but precesses in a cone over a period of 26,000 years, which causes the seasons to shift within that timescale. Moreover, the tilt does not remain constant at 23.5º, but nods between 21.5º and 24.5º over a period of 41,000 years. The larger the tilt, the greater the variation of the seasons.
The eccentricity of the Earth’s orbit (which produces the hotter summers in the Southern Hemisphere) is also variable, changing between almost 0 and 7%, on a timescale of 100,000 years. At present, the Earth is midway through that cycle, with a difference of 3.4% between January and July.
The total effect of all these orbital cycles on the weather is complicated. Throughout the 1920s and 1930s the Serbian civil engineer and geophysicist Milutin Milankovič, in his second career as a mathematics teacher, devoted himself to studying their effects on climate change. For this reason, they are called Milankovič (or Mila
nkovitch) cycles. He attributed the Ice Ages (periods when there are extensive ice sheets, like the one in Antarctica) to these cycles, discovering that recent cold periods have occurred approximately every 100,000 years, when all the Earth’s different orbital cycles coincided to produce maximum cooling. His discovery was verified after his death by analysis of ocean sediments and Antarctic ice cores, which show isotopic variations in different layers that were caused by temperature differences when the layers were deposited. In the USA, ice cores are stored at the US National Ice Core Laboratory in a building in Denver, Colorado. It has over 14,000 metres of ice cores from thirty-four drill sites in Greenland, Antarctica, and high mountain glaciers in the western United States. The cores are kept at a temperature of -35 °C, with four levels of backups and safety systems.
These ice cores show that the present Ice Age (defined by glaciologists as a period in which there are extensive ice sheets, like the one in Antarctica) began 40 million years ago. It grew colder during the Pliocene and Pleistocene periods, starting around 3 million years ago, with the spread of ice sheets across the Northern Hemisphere. Since then, glaciers have advanced and retreated every 40,000 to 100,000 years. The most recent retreat of the glaciers ended about 10,000 years ago.
However, Milankovič cycles are not severe enough to alter global temperature by the amounts that are recorded in ocean sediments and ice cores on their own, so there must be other processes, like the greenhouse effect, that amplify the effects of fluctuations in the amount of solar radiation reaching the Earth. Terrestrial volcanism, continental drift and changes in the composition of the Earth’s atmosphere all play a part. The recent increase in human-generated carbon dioxide and other greenhouse gases has started to cause changes that are quicker and more extreme than the Milankovič cycles, increasing the Earth’s temperature at an unprecedented rate.
Asteroid Impacts
How the Earth developed
We have learned now that we cannot regard this planet as being fenced in and a secure abiding place for Man; we can never anticipate the unseen good or evil that may come upon us suddenly out of space.
H. G. Wells, The War of the Worlds, 1898
A miner’s all-consuming but futile quest for riches under a strange hill in Arizona unearthed treasure of a different kind: evidence of meteorite impacts on Earth. From a monstrous fireball in Siberia to mysterious sparkling stones in the wall of a medieval church, the evidence of meteorite impacts has transformed our understanding of evolution and geologic change.
US Route 40 runs east from Flagstaff, Arizona, across the dry Colorado Plateau and passes close to what from a distance looks like a low hill. This is Coon Butte. It is a complete surprise to reach the top of the hill and find oneself looking out over a large, deep, circular crater. The crater is 1,200 metres in diameter and 170 metres deep, and its rim rises 45 metres above the surrounding plains. Its interior walls have been weathered since it was formed, and the wall material as well as wind-driven dust from the surrounding plain have raised the floor, so it was originally deeper and narrower than it is now.
In 1891 the crater was mapped by Grove Karl Gilbert of the US Geological Survey. In 1895 Gilbert considered the possibility that the crater had been made by a meteorite impact, but rejected this explanation on two grounds. First, the amount of ejected material in the crater walls and on the surrounding plain was no larger than the hole in the ground. Overestimating how big a meteor would have to be in order to produce a crater of the size of Coon Butte, Gilbert did not find the large quantity of extra material he expected. Secondly, when he searched the crater floor with a magnetometer for a large mass of iron, Gilbert found nothing: there was no sign that an intact meteorite had buried itself in the ground. Gilbert therefore concluded that the crater was volcanic, like the nearby San Francisco mountains.
Nevertheless, in 1902, on a hotel veranda in Tucson, a Philadelphia mining engineer named Daniel Moreau Barringer heard local gossip from a government agent that the crater was meteoritic. His imagination was fired by the mention of meteors, and the businessman in him was attracted by the possibility that buried under the crater was a large iron-nickel mass, which he could mine and sell – the nickel was particularly valuable. Samples of rocks found nearby on the surface of the plateau contained 5% nickel and traces of iron, mixed with the ejecta from the crater; the iron and nickel fragments were, therefore, evidently coeval with the crater’s formation, and Barrington concluded that Coon Butte had been formed by a meteor.
Barringer and a partner, Benjamin Chew Tilghman, bought mining rights to the area containing the crater and began to search for the meteor mass below its centre. The crater was so nearly circular that it seemed logical to assume that the direction of impact had been straight down. By 1908 the pair had drilled twenty-eight holes in the crater floor, but found no meteorite.
Tilghman then noticed that the ejecta littering the surrounding plain was asymmetrical, strewn to the south. Moreover, the southern rim of the crater was raised, as if the meteor had burrowed under it. Barringer experimented by firing projectiles into earth. The craters he produced were always circular, even if the projectile impacted at an angle, but the ejecta from the crater continued the forward momentum of the projectile. On the basis of this experiment, the two men shifted the focus of their search, and drilled a mine shaft into the interior south wall of the crater. Yet they still did not find the mass of iron-nickel. In nearly every hole their drill encountered an isolated hard ‘obstruction’, which could well have been a fragment of the meteorite, but they were fixated on discovering the motherlode and paid no attention to these small pieces.
Undeterred by the repeated failures of their mining strategy, Barringer and Tilghman presented their theory that Coon Butte was a meteor crater to the Philadelphia Academy of Sciences in 1906 and to geologists at Princeton University in 1909. Barringer’s style of argument was belligerent. He heaped scorn on Gilbert, who was a well-respected geologist, and accused established scientists of blind prejudice. This did not endear him to academic audiences and, on most occasions, they could not stop from tittering.
Meanwhile, Barringer’s investors were becoming increasingly alienated by his intemperate remarks, and demoralized by the continuing expense of the fruitless search. When the geologist George Merrill theorized that the Coon Butte meteorite would have shattered into small pieces on impact, leaving no single mass of iron-nickel to be found, they gradually withdrew from the project, abuse flung at their departing backs. Disillusioned by Barringer’s demand to continue drilling, Tilghman also pulled out.
Barringer found new backers, but by 1928, nervous at the money that they had spent, they consulted the astronomer Forest Ray Moulton at the University of Chicago. Moulton estimated the mass of the Coon Butte meteorite at 300,000 tons compared to Barringer’s estimate of 10 million, which reduced the investors’ potential return by 97%; the site would be even less profitable if the meteorite had fragmented into a large number of small pieces that would be impractical to collect. In an attempt to reassure them, Barringer sought a second opinion from Princeton astronomer Henry Norris Russell, who confirmed Moulton’s calculations. Having spent a fortune of about $1,000,000 (more than ten times that amount at today’s value) on his doomed hunt for the meteorite, Barringer died in 1929.
Barringer’s discovery that the Coon Butte crater was meteoritic was potentially of greater value than the meteorite itself. Scientists began using Coon Butte as a model to explain similar craters in the Solar System, such as those on the Moon. Barringer’s explanation received dramatic support when, in 1908, the region near the Tunguska river in Siberia was rocked by explosions from a monstrous fireball. It was not until 1921 and 1922 that the Russian scientist Leonid Kulik was able to visit the area and found the new impact crater, which had clearly been caused by a meteor and was surrounded by flattened trees. Kulik’s work became known in the West by 1928. Nevertheless, the scientific establishment continued to assert that both Coon Butte and t
he craters on the Moon were volcanic.
It was only between 1957 and 1963 that Barringer’s theory about the origin of Coon Butte was decisively confirmed by astrogeologist Gene Shoemaker. He discovered and described the tell-tale geological clues that indicate a crater has been made by a meteorite impact rather than a volcanic explosion. The key sign is the presence of shocked quartz minerals such as coesite and stishovite, which are fused at much higher temperatures and pressures than those generated by volcanic action, and were first identified in craters formed by nuclear weapons testing. Using these criteria, Shoemaker and his wife Caroline identified many of the 160 other impact craters known around the world. One of these was the Ries crater, which surrounds the village of Nördlingen in Bavaria. The walls of St George’s church in Nördlingen are made of a rare and beautiful sparkling mineral called suevite, a strongly shocked material generated by the meteorite impact.
Scientists now estimate that Coon Butte crater was formed about 50,000 years ago, when mammoths, sloths, bison and camels roamed the Colorado Plateau. Animals as far as 25 kilometres away would have been killed or injured by the blast. An impact of this size occurs on average only once in a thousand years on Earth. But once every million years or so the Earth is struck by a meteor large enough to devastate a continent. The extinction of the dinosaurs was caused by a meteor impact that, 66 million years ago, created the Chicxulub crater in Mexico, which was discovered in 1978 by oil geophysicist Glen Penfield.
The realization that meteor impacts shape the landscape of the Earth resolved a 250-year dispute among geologists and evolutionary scientists. At the end of the eighteenth century, years before Darwin formulated his theory of evolution, the French palaeontologist Georges Cuvier proposed that individual catastrophes were the major engines of change in the Earth’s natural history, causing phenomena such as continental drift and the extinction of species. This theory was widely accepted at the time because it was consistent with creationist biblical interpretations (for instance, those relating to Noah’s Flood). The Russian-Jewish psychiatrist Immanuel Velikovsky revived similar views in the 1950s, knitting mythology, archaeology and pseudo-science into fantastic but very popular theories about floods and cosmic fires. Scientists feel instinctively uncomfortable with this worldview, called ‘catastrophism’, because it suggests that everything on Earth – its ecosystems, weather, geology and geography – is subject to arbitrary events, a ‘tale told by an idiot’, and therefore cannot be analysed or modelled using scientific methods. Reacting against this approach, geologists James Hutton in the eighteenth century and Charles Lyell in the nineteenth promoted the theories of ‘uniformitarianism’ and ‘gradualism’, insisting that geological change occurs slowly over long periods of time, more along the lines of Darwinian evolution. The Scottish geologist Archibald Geikie paraphrased Hutton in 1905, saying ‘The present is the key to the past.’