by Paul Murdin
Following Darwin, uniformitarianism became the dominant paradigm of geology for over a century, until Barringer’s crater, Kulik’s fireball and the Shoemaker’s discoveries made their impact, forcing scientists to combine the two opposing views. Cosmic fireballs are not the stuff of legend, but are meteors, which cause dramatic effects on Earth. Cuvier put it well in 1817 in his great work Of Changes in the Structure of the Earth: ‘Life, therefore, has often been disturbed on this Earth by terrible events – calamities which, at their commencement, have perhaps moved and overturned to a great depth the entire outer crust of the globe…numberless living things have been the victims of these catastrophes….Their races have become extinct.’ Scientists now believe that geological history is both uniform and catastrophic, a process of slow and gradual evolution, punctuated by occasional brief transformational events.
The Origin of the Moon
Neither daughter nor sister of the Earth
In all fairness to those who by training are not prepared to evaluate the fundamental difficulties of going from one planet to another, or even from the earth to the Moon, it must be stated that there is not the slightest possibility of such journeys.
F. R. Moulton, Consider the Heavens, 1935
The Moon is not just the Earth’s nearest neighbour: it is the passive guardian of life on Earth, and its dead landscape, visible to the naked eye, holds the clues to our own planet’s origins. When astronauts first walked on the Moon in 1969, they not only disproved Moulton’s prediction, but also brought back dust and rocks that would reveal lunar secrets.
From time immemorial, people have speculated about the dark patches on the surface of the Moon: in folklore they are said to have the shape of a man, a rabbit or a crone carrying firewood. In the seventeenth century Galileo pointed his telescope at the Moon and discovered mountains and craters in a barren, dry desert. More recently, American astronauts and Russian spacecraft have explored the lunar landscape, and the geology of the rock samples they collected has shed light on the origins of the Moon. Despite its peaceful appearance in the night sky, the Moon has a violent past, having been born in a fiery collision between the Earth and another planet. Yet without this catastrophic event, the Earth would not be stable enough to support life.
The Apollo programme of human lunar exploration arose in response to a challenge issued by President John F. Kennedy at the height of the Cold War in 1961, as the USA and USSR competed for primacy in the ‘Space Race’. The exploration of the Moon was initially driven by nationalist aims and technological innovations, although a scientific research programme was established to operate in tandem with the landings.
The American programme began with a fleet of twenty-two robot spacecraft launched by NASA to the Moon: the Rangers crash-landed; the Surveyors landed softly; and the Lunar Orbiters, Explorers and early Apollos surveyed the Moon from orbit (plate VI). Later Apollos landed with their human passengers.
Early Moon landings were manned by test pilots and military men with only basic training in geology, and the first landing sites were chosen for feasibility rather than scientific interest. Apollo 11 (plate X) landed on a featureless plain in the Sea of Tranquillity, Apollo 12 in the Ocean of Storms. Later lunar missions targeted areas of geological interest in the hopes of gaining a clearer understanding of the origins and composition of the Moon. Apollos 13 and 14 were meant to land at Fra Mauro, a site made of debris thrown from the meteorite impact that created the massive crater of the Mare Imbrium basin, and would therefore include rocks ejected from deep inside the Moon. The Apollo 13 landing had to be aborted when the spacecraft’s service module was damaged by the explosion of an oxygen tank. Apollo 14 was targeted at the same site, landing safely, but the astronauts got lost during the moonwalk, and did not collect the rock samples the geologists wanted. Apollos 15, 16 and 17 were more successful, landing near the Apennine Mountains, the Descartes highlands and the Littrow valley in the Taurus Mountains, sites chosen for their geological significance, with the Taurus-Littrow valley showing signs of recent volcanism. All the Apollo modules that landed on the Moon brought back rock samples for terrestrial analysis – over 300 kilograms altogether. Some small lunar samples from other sites were returned to Earth by three robotic explorers from the Soviet Luna series of spacecraft.
Other rocks from the Moon were found to be made of materials that were melted more recently than the ‘Genesis Rock’, probably by the impact of a meteor that formed a lunar crater. At the Shorty crater near the Apollo 17 landing site, astronaut Harrison Schmitt, a professional geologist, discovered ‘orange soil’ containing volcanic glass from an explosive volcanic fire fountain that erupted 3.6 billion years ago.
Before lunar rock samples were available for analysis, theories of how the Moon was made were very disparate, compromised by insoluble riddles, and – in the words of chemist Harold Urey – ‘tended to prove that the Moon did not exist’. George Darwin (son of Charles Darwin), Don Wise and John O’Keefe suggested that the Moon was the Earth’s ‘daughter’: that it was formed from material that had spun off from the Earth’s mantle. Other planetologists, such as Gerard Kuiper, thought that the Earth and Moon originated as two ‘sister’ planets during the early formation of the Solar System. Yet others held that the Earth captured the Moon after it had formed somewhere else, either drawing it into orbit intact (Harold Urey), or as a kind of ‘Saturn’s ring’ of pieces that later coalesced into a sphere (Ernst Öpik).
At a conference in Kona, Hawaii, in 1984, a dramatic and radically different theory was adopted to explain the origins of the Moon, incorporating the information from the analysis of lunar rocks. This was the ‘collision model’, first mentioned in 1946 by Harvard geologist Reginald Daly and revived in 1975 by planetologists William Hartmann and Donald R. Davis and astrophysicist A. G. W. Cameron. They held that the Moon and the Earth both formed from the glancing collision of two planets: the proto-Earth and a planet the size of Mars called Theia (named after the Titan in Greek mythology who gave birth to the moon-goddess Selene). During the collision, Theia and the proto-Earth broke up and reassembled as two new planets, the Earth and the Moon. Both Theia and the proto-Earth had molten iron cores, which coalesced to form the core of one of the new planets – the Earth – surrounded by a thin mantle. Fragments from the mantles of the two colliders coalesced into the other planet, now the Moon. This explains why some lunar rocks had a similar composition to rocks found on Earth, but others did not. The heat of the collision melted the mantle material that formed the Moon, generating KreeP rocks and encouraging volcanic activity. The high heat also vaporized all the water on the Moon, which is why it is so dry. The rapid rotation of the Earth and the tilt of the Moon’s orbit were also consequences of the collision.
Without this violent freak event, life as we now know it would not exist on Earth. Compared to other satellites, the Moon is unusually large relative to the Earth and consequently its gravity is able to stabilize the Earth’s rotation. As a result, the Earth has seen less drastic changes in the tilt of its rotational axis than other planets and its climate has been unusually stable. This has facilitated the evolution of life on Earth, especially multicellular and mammalian life, which needs aeons of time to evolve. Moreover, the collision produced the large iron core of the Earth, which generates the strong and stable magnetic field that defends the Earth’s atmosphere against erosion by the solar wind. It seems possible that without that collision 4 billion years ago there would be no human beings on Earth.
Mercury
The Late Heavy Bombardment
The Moon He placed in the first circle around the Earth, the Sun in the second above the Earth; and the Morning Star [Venus] and the Star called Sacred to Hermes [Mercury] He placed in those circles which move in an orbit equal to the Sun in velocity, but endowed with a power contrary thereto; whence it is that the Sun and the Star of Hermes and the Morning Star regularly overtake and are overtaken by one another.
Plato, Timaeus, c. 360
BCE
Even though Mercury is not far from the Earth, it is something of an enigma, because its close proximity to the Sun makes it difficult to observe. We do know that Mercury’s surface, like the Moon’s, was heavily bombarded by meteors at an early stage in the planet’s development. But the cause of the bombardment – and what it might have done to the Earth – is still as much a mystery as Mercury itself.
Mercury is the smallest planet, not much bigger than the Moon. It is the closest planet to the Sun, which it orbits in eighty-eight days. Greek astronomers at first had two names for Mercury, as they thought it was two separate planets, calling it ‘Apollo’ when it was to the east of the Sun and ‘Hermes’ when it was to the west. It was reputedly Pythagoras who realized that the two bodies were actually the same planet.
Even though Mercury is relatively near to the Earth, its close proximity to the Sun makes it very difficult to see using conventional telescopes. As Mercury is never more than 28º from the Sun, it is never high in the sky and can only be viewed from the Earth in the twilight. Nor is observation from space any easier. The Hubble Space Telescope is not permitted to view Mercury directly, as the Sun’s heat and light could damage it. For much the same reasons, it is difficult to design a space probe capable of visiting Mercury. A spacecraft will quickly overheat as it approaches so close to the Sun, and will be peppered by storms of solar particles. Furthermore, as the spacecraft drops towards the Sun it will pick up speed as it is drawn down by the Sun’s strong gravitational field, and can overshoot Mercury. This must be countered by burning large amounts of fuel or by approaching Venus in just the right direction, at just the right time in the orbits of both planets, so that Venus’s gravity can be used to slow the spacecraft down.
Because of these constraints Mercury is one of the least studied planets. Radar has proved a useful tool, but its capabilities are limited at such a distance. In 1965 it was used to show that Mercury turns exactly three times on its axis for every two orbits around the Sun. A solar day on Mercury (sunrise to sunrise) therefore lasts two Mercury years, or 176 Earth days.
The first space probe successfully to visit Mercury was Mariner 10 in 1974–75, but it was more than thirty years before the second space visit was accomplished by the Messenger probe in 2008–9. Mariner 10 discovered that Mercury suffers extreme fluctuations of temperature, ranging from 90 K (-183 °C) to 700 K (427 °C), as the atmosphere is too thin to provide an insulating effect. The floors of some deep craters near the poles never see direct sunlight and never warm above 112 K. There are patches in these craters that seem to reflect radar pulses in the same way that ice does, and specific deposits of ice were confirmed by Messenger. It may have been deposited by melting comets: a large comet impact on Mercury’s surface could have generated a temporary steamy atmosphere, which may have condensed and frozen in the dark, cold craters.
The thin ‘atmosphere’ is constantly being lost and replenished and is therefore properly termed an ‘exosphere’ rather than an atmosphere. Mercury’s exosphere consists mainly of hydrogen and helium picked up from the Sun, but also contains less abundant atoms like sodium and silicon that have been knocked off the surface of the planet by the solar wind, a process called ‘sputtering’. A surprise discovery from Messenger was that the exosphere contains water, perhaps derived from the cometary ice at the poles.
Mercury’s surface is heavily cratered, like the Moon’s. In fact, the oldest surface areas of both bodies were cratered at the same time. To estimate the ages of the surfaces of different planets and satellites, astronomers count the number of meteor craters – the younger the surface, the fewer craters it has, especially large ones. These counts suggest that meteor impacts occurred more frequently throughout the Solar System during an early period in its history. This is confirmed by studies of rocks collected from the Moon by the Apollo astronauts and lunar meteorites collected on Earth.
Lunar meteorites are pieces of the Moon that were knocked off its surface by the impact of asteroids. None of them is older than 3.9 billion years. This posed a puzzle: given that the Moon was 4.6 billion years old, why were the oldest meteorites so much younger? Lunar rocks collected by Apollo astronauts showed that the crust of the Moon had been strongly heated 3.9 billion years ago. What had caused this catastrophic heating? Between 1974 and 1976 studies by a number of planetologists, including Fouad Tera, Dimitri Papanastassiou, Gerald Wasserburg and Grenville Turner, suggested that 3.9 billion years ago asteroids and meteors had heavily bombarded the surface of the Moon for a discrete period of time (200 million years) and melted it. They called this event the ‘lunar cataclysm’; it is now known as the Late Heavy Bombardment. The oldest parts of Mercury’s surface were cratered during the same period.
No one knows why the bombardment occurred. There may have been a major collision between planets, or a disturbance in the outer Solar System that caused a rain of asteroids or Kuiper Belt Objects to fall inward towards the Sun. The focusing effect of the Sun’s strong gravity meant that Mercury was pummelled especially heavily by infalling asteroids. A particularly large impact created the Caloris Basin (plate XV), one of the largest craters in the Solar System, with a diameter of 1,550 kilometres. As discovered by Messenger, the impact caused volcanoes to spring up around the rim of the crater. Mariner 10 had already discovered that shock waves from the impact travelled to the other side of the planet to create an unusual hilly region known as the ‘Weird Terrain’. The whole planet would have rung like a bell from the force of the impact, which was on the verge of world-shattering.
Of course, if the Moon and Mercury suffered under the Late Heavy Bombardment, so did the Earth. The Bombardment produced about 1,700 craters on the Moon that were more than 20 kilometres in diameter. Given the larger size of our planet, the bombardment would have generated ten times as many craters on Earth, some of which would have been as large as 1,000 kilometres in diameter. This scenario is supported by the discovery that extraterrestrial isotopes are especially abundant in deep sediments laid down in Greenland and Canada during the time of the Late Heavy Bombardment. It might also be significant that the fossil record of life on Earth seems to have started after 3.9 billion years ago. If life had begun to evolve before then, the Bombardment would have interrupted the process and erased any earlier traces. Alternatively, the Late Heavy Bombardment may actually have triggered the evolution of life, bringing an abundance of organic molecules to the Earth on asteroids or comets.
The Greenhouse Effect
Venus and the Earth
The world scientific community has begun to sound the alarm about the grave dangers posed…by greenhouse warming, and again we’re taking some mitigating steps, but again those steps are too small and too slow.…The runaway greenhouse effect on Venus is a valuable reminder that we must take the increasing greenhouse effect on Earth seriously.
Carl Sagan, Cosmos: Who speaks for Earth?, 1990
The greenhouse effect was discovered on the Earth and on Venus in parallel. It is a property of some of the gases in the atmosphere that keeps the surface of the Earth at a comfortable temperature. The greenhouse effect is essential for life on Earth. However, man-made (anthropogenic) greenhouse gases threaten to upset its benign equilibrium. The surface of Venus is hot enough to melt lead. Could the same thing happen on Earth?
Venus is the second-nearest planet to the Sun, at about three quarters the distance of the Earth. It is Earth-like in its size and physical properties, such as its internal structure, solid surface, atmosphere and magnetic field. Seen through a telescope, Venus seems to have an almost uniform white surface. In 1761 the Russian scientist Mikhail Lomonosov observed a halo around Venus as it passed in front of the Sun. In this way, Lomonosov realized that the halo was scattered light radiating from Venus’s upper atmosphere. The planet’s white ‘surface’ was actually an opaque layer of thick white clouds.
Because it is highly reflective and close to both the Sun and the Earth, Venus is the brightest object in the sky aft
er the Sun and the Moon. It was thus one of the first celestial objects whose spectrum (the bands of visible and invisible light it emits) was photographed. In 1932 Walter Adams and Theodore Dunham, Jr, imaged Venus’s spectrum using the 100-inch telescope at the Mount Wilson Observatory in California and specially made emulsions supplied by C. E. Kenneth Mees of the Eastman Kodak Company. They discovered that previously unknown spectral lines in Venus’s spectrum were generated by carbon dioxide, which proved that this gas was a major constituent of Venus’s atmosphere.
The carbon-dioxide atmosphere of Venus had peculiar properties. Between 1923 and 1928 American astronomers Edison Pettit and Seth Nicholson measured infrared radiation that came from the tops of Venus’s clouds and discovered that the temperature there was cold, ranging from -37 to -42 °C. In 1956 pioneer radio astronomer Cornell H. Mayer and his colleagues at the US Naval Research Laboratory measured the microwave radiation emitted from deep within Venus’s atmosphere, much nearer to the ground. Mayer’s team discovered that the surface of the planet had a very high temperature indeed, over 300 °C, hot enough to melt lead. This was a surprise: the dense and highly reflective atmosphere of Venus should have reduced the amount of solar radiation that was able to reach the planet’s surface. In 1961 Cornell astronomer Carl Sagan put forward the currently accepted explanation for the high temperature: the atmosphere of Venus has a strong greenhouse effect, much stronger than the Earth’s.
