Mapping Mars: Science, Imagination and the Birth of a World

скачать книгу бесплатно


‘A Little Daft on the Subject of the Moon’ (#ulink_110dd06d-dbf6-5605-a585-cb11cf3f08df)

We pride ourselves upon being men of the world, forgetting that this is but objectionable singularity unless we are, in some wise, men of more worlds than one.

Percival Lowell, Mars

The story of how Meteor Crater came to be understood as the best-preserved earthly exemplar of the ancient landscapes of Mars comes in two parts. In the first part a great geologist got it wrong, but in doing so showed how geology could, in principle, tackle subjects beyond the earth. In the second part a great geologist got it right and used his insights to turn the geological mapping of other planets, including Mars, into a practical concern. Both men were geologists with the US Geological Survey and both were filled with the romance of the American West – a romance that both science and the popular imagination have projected on to Mars for more than a century.

Grove Karl Gilbert was one of the happy generation of American geologists which, in the second part of the nineteenth century, took its impressive beards and intellects to every corner of the American West. They were part of a worldwide phenomenon that the historian William Goetzmann has called the second age of exploration – the period between Cook’s voyages in the late eighteenth century and Amundsen’s trek to the south pole in the early twentieth during which Europeans moved beyond the coastlines of other continents and across their hearts. The centres of Africa, Asia and Australia were all explored at this time.

In the American West Gilbert and his peers – John Wesley Powell, Clarence King, Clarence Dutton, William Davis – encountered a world that spoke to them of the archaic and at the same time cried out for the modern, an awe-inspiring natural world that could only be opened to civilisation through the technologies of electricity, irrigation and the railroad. Its forbidding landscapes – often wonderfully captured by the artists and photographers who accompanied the various expeditions – were utterly unlike those the scientists were familiar with back east; plateaux dissected by massive erosion, the strangely faulted terrains of the basin and range province, all manner of volcanic dramas. The explorers measured the landscapes, mapped them, developed new language to describe them: ‘laccolith’, ‘isostasy’, ‘gradation’. Dutton, in particular, was a literary gent (as well as a soldier, a chemist and a theology school drop-out) who styled himself ‘omnibiblical’; his writing overflows with energy. His descriptions were evocative, grandiose and sometimes extremely funny, continuously aware of his audience back east and the novelty he was bringing to it. In his memoir of the Grand Canyon, the Survey’s first publication, he wrote in self-justification:

I have in many places departed from the severe ascetic style which has become conventional in scientific monographs. Perhaps an apology is called for. Under ordinary circumstances the ascetic discipline is necessary. Give the imagination an inch and it is apt to take an ell, and the fundamental requirement of the scientific method – accuracy of statement – is imperiled. But in the Grand Cañon district there is no such danger. The stimulants which are demoralizing elsewhere are necessary here to exalt the mind sufficiently to comprehend the sublimity of the subjects. Their sublimity has in fact been hitherto underrated. Great as is the fame of the Grand Cañon of the Colorado, the half remains to be told.

Dutton, Gilbert and their peers did not just find new language with which to express themselves; they came up with new theories about how the earth might work, theories which allowed for far greater violence and more sudden novelty than the sedate forms of geology practised by their European forebears and contemporaries. It was the need to explain the landscapes of the West, and the mineral wealth they might hold, that led to American geology becoming a nationally distinct enterprise quicker than any of the country’s other sciences.

By the time he came to Meteor Crater in 1891 Gilbert had spent twenty years, as he put it, ‘aboard the occidental mule’, trying to understand the processes that had shaped the landscapes around him. He was a precise man, mathematically orientated, but he also had a zest for the experiences that would help him explain how the landscapes he carefully measured had come to be. It was, he wrote, ‘the natural and legitimate ambition of a properly constituted geologist to see a glacier, witness an eruption and feel an earthquake’. When he achieved the last of those ambitions in 1906, it was with ‘unalloyed pleasure’; woken by the shocks of the San Francisco earthquake he set to timing them and measuring their direction. He brought the same precision to his other work, closely harnessed to a love of physical and mechanical analogy.

His love for the orderly and mathematically tractable led him to study the stars as well as the earth. Travelling down the Grand Canyon on one of the first expeditions to do so, he had made a point of observing Venus from its depths. He was by his own admission ‘a little daft on the subject of the moon’, and in Washington DC he made use of the Naval Observatory’s telescopes to observe it in detail, prompting ridicule from congressmen who affected to think that if distinguished members of the US Geological Survey had nothing better to do than look at the heavens, the Survey should clearly be disbanded, its earthly work complete. In Gilbert’s thought, though, geology and astronomy belonged together; together they could explain not just rocks but entire planets.

In the summer of 1891 a Dr Foote reported to the American Association for the Advancement of Science that he had found significant amounts of meteoritic iron at Canyon Diablo in the Painted Desert, near the crater at Coon Butte. Gilbert was intrigued. He thought matter falling from the sky might shed light on what he saw as one of the great planetary problems: why the earth’s crust is systematically denser in ocean basins than under continents. Gilbert thought this heterogeneity might be due to the fact that the earth had been assembled from smaller objects, which later theorists would call planetesimals: dense crust marked the contributions of dense planetesimals. Gilbert wondered whether the large crater in this field of meteoritic iron marked the spot where a ‘small star’ had been ‘added to the earth’ relatively recently. Always ready to head west when possible, he arrived at Meteor Crater that October.

Gilbert saw two possible types of explanation for the crater: it could have been formed by something coming in – an impact – or by something coming out – a volcanic explosion. The best argument for a falling star was the meteoritic iron littering the surrounding desert. Gilbert calculated the odds of a crater forming in such a dense meteor field purely by chance as 800 to 1. If the crater had been clearly volcanic, then this might not have mattered. But though there were volcanoes nearby, the crater’s walls and floors were sedimentary rock, the same strata of sandstone and limestone from which the rest of the Colorado Plateau is built.

In a typically methodical manner Gilbert set out to test the alternatives through their implications. If there were a ‘star’ buried beneath the crater somewhere, then like Archimedes in his bath it would have displaced material that was there before. If so, there would be more material in the crater’s raised rim and its surrounding blanket of ejecta than was needed to refill the crater itself. But when, through painstaking surveying, Gilbert and his assistants compared the volume of the crater’s cavity with the volume of the rock that had been excavated in the catastrophe, they found that if the rim and ejecta were put back into the crater they would almost exactly fill it up; thus there was no evidence for the bulk of an added meteor below the crater floor. What was more, if a large iron meteorite did lie buried there it should have had a quite discernible magnetic field. But no such field was found. So Gilbert decided the crater had been formed by an explosion of steam, set off when deep volcanic activity had penetrated a subterranean aquifer; he placed Coon Butte in the family of anomalous volcanic craters called ‘maars’ (no relation). This hypothesis sat well with the natural inclination of the area’s uneducated shepherds: that the crater looked as though it had been formed by something exploding out of the earth, not by something falling into it.

Disappointed as he may have been – a maar is an interesting thing, but hardly a star – Gilbert still put his observations to good use. In his 1895 address as president of the Geological Society of Washington, published as ‘On the Origin of Hypotheses’, he presented the story of Coon Butte as a sort of moral fable on the correct way of approaching geology. To explain a novel feature, the geologist should first reason by analogy: what sort of thing is it like? The analogy might seem a distant one – a gaping crater in a desert is not very like the ‘raindrop falling on soft ooze’ to which Gilbert compared Coon Butte – but that need not matter. What matters is that there be a number of analogies, that they have different physical implications, and that those implications then be tested. This was Gilbert’s highly influential encapsulation of what was becoming the pragmatic cornerstone of geological science in America: a method of ‘multiple working hypotheses’ in which contradictory explanations were to be entertained simultaneously.

One of the disappointments for Gilbert in finding Meteor Crater to have been produced from within and not without was that he had hoped to use it as an analogy with which to bolster his theories about the moon. Everyone who wrote on the moon explained it by analogy to the earth; the problem lay in choosing the right analogy. In 1874 James Carpenter, a Greenwich astronomer, and James Nasmyth, an engineer whose father had been a landscape artist and whose own pictures of the moon had caught the eye of Prince Albert, published a wonderful illustrated book called The Moon: Considered as a Planet, a World and a Satellite.

(#litres_trial_promo) Inside, spectacular photographs and prints of the moon are compared with similarly lit photographs of a range of other objects – an old man’s wrinkled hand, a desiccated apple, a cracked sphere of glass. The idea is to teach the reader’s eye new ways of seeing the moon and give his mind new analogies by which to understand it. (Their influence was long-lasting. Lowell used the desiccated apple in his books on Mars to demonstrate what happens when a planet dries up; the first post-Mariner textbook on Martian geology has very Nasmyth-like cracked glass spheres in it to demonstrate stress patterns.)

To make their case for the volcanic origin of the moon’s craters, Nasmyth and Carpenter created a scale model of what Vesuvius and the bay of Naples must look like from above and compared it with similar models of the lunar surface. Other lunar analogies on offer suggested that the dark expanses of the moon called ‘seas’ were in fact made of ice, or that they were the dried beds of seas now vanished. Charles Babbage, the pioneer of mechanical computing, elaborated on this idea with the notion that craters in these dried seas were in fact coral atolls like those studied by Darwin.

Gilbert rejected all these analogies, seeing the craters and larger basins and ‘seas’ as the marks left by planetesimals. His idea was that once the earth had been ringed by planetesimals – much as Saturn is ringed today – and that these had then coalesced into the moon; the last ones in had left the surface scarred. Lacking a natural earthly analogue for such cratering, Gilbert experimented with crater making himself, firing various projectiles into clay: he called the hobby ‘his knitting’ and found the results satisfactorily lunar. To those who objected to a geologist trespassing in the realms of astronomy, he defended his speculations in terms that could serve as the credo for astrogeology to this day: ‘The problem is largely a problem of the interpretation of form, and is therefore not inappropriate to one who has given much thought to the origin of terrestrial topography.’

Gene Shoemaker’s thinking on terrestrial topography, which would find application in the interpretation of form on the moon and beyond, took place in large part on the Colorado Plateau which Gilbert had known so well (indeed, he had given it its name). In 1948, twenty years old and with a Caltech degree in geology already behind him, Shoemaker joined the US Geological Survey and found himself working in southern Colorado. He discovered that he loved the landscapes of the south-west. He loved the pines, he loved the open spaces, he loved the great, vaulting skies. He stared up at the desert moon with wonder.

In the field, he did not have much contact with the rest of the world. But he did get the Caltech alumni newspaper, which revealed that experiments with captured V2 rockets elsewhere in New Mexico were reaching the very edge of the atmosphere. It was a revelation. ‘Why, we’re going to explore space,’ he later remembered thinking, ‘and I want to be part of it! The moon is made of rock, so geologists are the logical ones to go there – me, for example.’

Shoemaker kept his wild dream to himself – a decade before Sputnik there was little call for space age geology. The atomic age, though, needed geologists badly. Cold War strategy required that America develop reliable domestic sources of uranium, and the Colorado Plateau was thought likely to hold the reserves required. So in his first years with the USGS Shoemaker joined in the last great American mining boom; at the same time he started work on a Ph.D at Princeton and got married. He criss-crossed the Colorado Plateau from site to site, ‘half man, half jeep’, according to his wife, Carolyn, who often accompanied him. It wasn’t normal for geologists’ wives to come along on field trips, but the Shoemakers didn’t care. When they had children, the children came too.

All the while, Shoemaker kept thinking about the moon. He read everything there was to read on the subject, including Grove Karl Gilbert; he tailored his fieldwork to suit his extraterrestrial interests. It was this which led him to map diatremes in the Painted Desert’s Hopi Buttes. Diatremes are volcanic features, chimneys of magma that rise to the surface causing explosions, which throw out a lot of normally well-buried rock and comparatively little lava; they can create the low-lying craters called maars to whose number Gilbert had added Meteor Crater. As a uranium prospector, Shoemaker was interested in diatremes because the rocks they threw out when they cleared their throats might be from uranium-bearing strata. As a would-be lunar geologist, he was interested in them because their associated craters often occur in families laid out along a straight line; many lunar craters show a similar linear tendency. Diatremes might thus be analogies by which to understand some forms of volcanism on the moon.

Shoemaker caught his first fleeting glimpse of Meteor Crater in the late summer of 1952. One afternoon, driving past the town of Winslow, he convinced his wife and a colleague that the site to which the great Grove Karl Gilbert had devoted his time might be worth a look. They didn’t have the entrance fee required for the public viewing platform on the north edge of the crater, so they had to take an indirect approach via a dirt track and then scramble up the rim. By the time they got to the top, the sun was setting and most of the great bowl was already in twilight. They stayed only a few minutes and Shoemaker saw nothing to contradict Gilbert’s assessment. In a few years, though, he would. A landscape that was only then being brought into existence gave Shoemaker the analogy he needed to understand Meteor Crater.

By the mid-1950s the American uranium rush had uncovered deposits of the stuff large enough to meet any plausible need. Plutonium, though, was another matter. Natural plutonium exists in only the most tiny of quantities, on the order of a kilogram per planet or so. If you want to make bombs of plutonium – which has a number of advantages over the use of uranium – you first have to produce the stuff. This is normally done with the help of a nuclear reactor which breeds plutonium from uranium. However, in the mid-1950s a quicker alternative started to be discussed: simply wrapping nuclear weapons in now plentiful uranium and setting them off somewhere where their blasts would be contained. Neutrons given off in the explosion would turn some of the mantling uranium into plutonium, which could then be scraped up and put into more bombs.

A necessary part of this alarming scheme was finding places where you could set off nuclear bombs without the debris being scattered to kingdom come. One possibility was salt caverns; since Shoemaker’s Princeton work dealt with salt structures in southern Utah, he was called on to look into the question. As a result, he saw what few other geologists had had the chance to see; a pair of craters at the Atomic Energy Commission’s Nevada test site that had been formed by underground nuclear explosions, craters called Teapot Ess and Jangle U. The shapes of the craters, he learned, were determined by the interplay of various sets of shock waves, some heading out from the bomb, some bouncing back. The nuclear-testing fraternity had a keen understanding of shock waves; a correct calculation of the behaviour of the shock waves inside their bombs was the key to getting them to go off properly in the first place. So Shoemaker learned of the power of shock waves both from the physicists and from the craters themselves, their edges deformed by enormous pressures, the sand around them fused to glass.

When Shoemaker went back to Meteor Crater in 1957 it was not directly because of his new experience in matters nuclear (he advised the bomb makers, incidentally, that keeping explosions contained underground was not going to work). But that new experience changed the way he saw the crater; now it looked like the aftermath of something like a nuclear explosion, something formed by shock waves in a matter of seconds. He set about making a systematic study – a process which required that he make friends with its owners, the Barringer family. In the early decades of the twentieth century Daniel Barringer, a lawyer and mining engineer, spent a lot of time trying to convince people that the crater had been formed by an impact, and a lot of money trying to mine its floor for the huge and valuable lump of pure iron he expected to find there. His failure to convince the world that it was even worth looking for such iron was in part due to the fact that no less an authority than G. K. Gilbert had disagreed with the idea. Barringer’s heirs had inherited both a largely useless crater and a dislike of geologists from the Survey. Shoemaker, though, became their friend and ally, in part because he was introduced to them by their old schoolmaster.

Shoemaker’s work at the crater in the late 1950s both vindicated Daniel Barringer’s insight and revealed his dream to have been illusory. Shoemaker found places where heat had turned sandstones into glass; he mapped inverted strata on the crater’s rim where the lip had been folded back on itself just as the rim of Jangle U had. Most crucially, he and a colleague found that the sandstones in the crater contained coesite, a very dense mineral created when quartz is subjected to extreme pressures. Only transient shock waves of immense power could create such pressures on the scale required: coesite was frozen smoke from the impact’s gun. Meteor Crater was indeed caused by an impact, as Barringer had thought. But the rebounding shock waves that formed the crater had completely exploded the incoming lump of iron he had hoped to profit from. As Gilbert’s volumetric assessments had shown, there was no extraterrestrial motherlode beneath the floor.

While Shoemaker worked to forge this causal link between heavens and earth, a matching connection was being set up in the opposite direction; the first earthly objects were being sent beyond the planet. Exploration of the moon suddenly seemed a practical possibility and Shoemaker was keen that the USGS should grasp it – just as Gilbert would have been. The Survey was keen, too. It had expanded a lot during the search for uranium, and now that that search was over, and the Atomic Energy Commission had withdrawn funding, it was left with more geologists than it had money or jobs for. Shoemaker’s astrogeological dreams might take up the slack – if funding could be found.