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

If the land is wonderful, so is the sky, which seems to expand in sympathy with the majesty below. The air is dry, clean, a little thin – just the sort of place astronomers like to set up shop. Above the town, amid the ponderosa pines of Mars Hill, sits the telescope through which Percival Lowell imagined the landscapes of Mars. You can go up and have a look through it, if you like; at the right time of year you’ll be able to see Mars floating in the eyepiece just as he did, blotchy but beckoning. During my most recent visit it was the wrong time of year, with Mars best seen at about five in the morning, long after Lowell Observatory has closed itself to tourists. But even watched from a motel car park in the pre-dawn glow, Mars seemed closer in Flagstaff than it does in most places, shining clear and bright and true.

To appreciate land and sky together, drive about half an hour east of Flagstaff. A quarter of an hour beyond the line in the landscape where the Coconino forest responds to some subtle cue of altitude or precipitation and gives way to the Painted Desert, you’ll find what used to be called Coon Butte. From a distance it looks not unlike the flattened mesas that sit further off behind it, except for the fact that its heights are a little more crenellated. As you come closer, though, you begin to get the feeling that it is something quite different: smaller, lower, subtly different in form and nature. Rather than sitting on top of the desert like the low flat hills to the south, or puncturing it like the cinder cones behind you, Coon Butte seems to be a bending of the plateau itself, a twisting of the land towards the sky. And so it is.

Coon Butte is one of the places where the sciences of astronomy and geology meet. It marks the spot where, 50,000 years ago, a very small asteroid’s orbit round the sun was cut short by the surface of the earth. Most asteroids are made of stone friable enough that small ones will explode high above the earth’s surface, shattered by the shock of being slowed by the atmosphere. The fifty-metre asteroid that struck the Painted Desert that day was made of sterner stuff: iron. It pierced the atmosphere intact and ploughed on into the planet. Only after it had punched a hole through the surface of the desert did shock waves tear it apart in an underground explosion a thousand times more energetic than that of the Hiroshima bomb, throwing millions of tonnes of the plateau’s rocks back into the sky. The strata of rock surrounding the impact were bent upwards, raising the surface of the desert in a ring and forming a sharp upturned rim to the crater. Some boulders were thrown half the distance back to Flagstaff; within four kilometres the desert was covered with a thick blanket of debris. The hole left behind was about 200 metres deep and 1.2 kilometres across, excavated in seconds. The rim of raised rock stood sixty or seventy metres above the surrounding desert. After 50,000 years, erosion has smoothed it down to fifty metres.

Meteor Crater, as it is now called, is an impressive sight. By the time you reach the observation area on the north side of its rim – the only part to which the public normally has access – you have driven at least eight kilometres out of your way, you have paid for a ticket, you have walked past a gift shop and a well thought-out visitor’s centre; you know what to expect. Even so, to come across this sudden theatre of steep relief in an otherwise flat desert takes you aback. It is a big, dramatic hole, its base smoothed by the dried-out bed of a little lake, the strata of raw bedrock poking out of its sides like piers in an arena to seat a million.

At the same time, by the standards of truly dramatic valleys, canyons and volcanoes – standards the Arizona landscape requires all its tourist features to measure up to – Meteor Crater is not really so terribly large. Its sides are steep and deep, to be sure: if St Paul’s Cathedral were built at the bottom, the great golden cross on top of the dome would be well below your eye level. But the depths are enclosed in a way that almost belittles them. Craters are the most revealing of landscapes; from the rim you can quickly take in all there is to see. And this is not that large a crater. The rim is only four kilometres around. You could walk round it in a couple of hours (the walking is quite hard, for the rim is not regular); your eye runs round it automatically, limiting its scope in the process. Anything you can grasp this easily cannot give a sense of true enormity.

The most striking effect is not to look down from the rim into the crater’s depths, but rather to look straight across. To the south, the circle of the crater’s rim and that of the further horizon lie one upon the other, tangent arcs. Turn your head slowly – pan like a camera – and they become detached. The rim falls away from the true horizon; it twists into the middle distance, banking towards you as the true horizon keeps its distance, becoming a feature within the landscape rather than a limit at the edge of it. Eventually it ends up under your feet, a rampart of rubble dividing the bowl enclosed within from the great desert plain outside. And yet the rim still feels linked to the horizon itself. The great circle of the planet and the ring of the rim seem aspects of the same thing; the great void below echoes the great vault above. The effect has something in common with the old cliché of a straight road, a flat plain and a vanishing point on the horizon. But here there are no points and lines and directions: just circles turning in on themselves over 360°. This sense of a world arranged in nested circles may be something nothing else can offer as well as a deep astronomical impact with a well-preserved rim. And on the earth, there are no other impact craters with rims as well preserved as Meteor Crater’s.

On Mars, by way of contrast, there may be a quarter of a million impact craters the size of Meteor Crater. And there are craters of all other sizes, too. There are great impact basins large enough to put the European Union into; there are craters small enough to use for tennis courts. There are craters that overlap like the circles of an Olympic flag. There are craters on the rims of bigger craters. There are craters within craters within craters. Some are as young as Meteor Crater itself, some even younger. Some are more than 80,000 times older, landscapes more ancient than anything on the earth’s shifting surface except a few tiny zircon crystals preserved by chance.

And those are just the ones you can see in the airbrush maps and the Viking pictures; the ones with clear, well defined rims. One of the discoveries made with the data from Mars Global Surveyor’s MOLA altimeter was that there are hidden craters, too, craters yet more ancient than the visible ones, if only by a little. The MOLA team has developed all sorts of ways of using brightness and colour cues to bring out different aspects of their vast data-set. One of their best tricks is a way of looking at the planet slice by slice on a computer screen. The spectrum of colours that allows the eye to understand what it is seeing is concentrated into a thin range of altitudes – just a few hundred metres, perhaps – with all lower places darkly blue, all higher grimly purple; the highlighted range can be moved up or down at will. Look at a mountain this way and you will see a circular band of rainbow with a dark centre. Toggle the highlighted range upwards and the noose of light will tighten to a solid disc at the summit; lower it and the ring of colour will expand slowly until it smears itself out across the plains at the mountain’s base.

Run this magical palette over the surface of Mars and crater rims will stand proud as thin, hollow crowns. But rimless craters can be found too: solid circles of equal altitude. These are old, eroded craters, craters the unaided eye would never pick up. These shadow craters can be quite big: one of the first to be discovered this way was about 450 kilometres across, giving it an area about the same as Michigan’s or England’s, and definitely putting it in the first division of Martian craters. And they are quite numerous. In the summer of 1999, seventy flat circles of various sizes that looked like ancient impact scars were discovered by one high school student doing an internship with the MOLA geology team.

Discoveries on such a scale mark a peculiarly auspicious beginning to a scientific career. But if the intern was spectacularly successful in how she did her job, she was not particularly distinctive in the job she was doing. Almost every geologist who looks to the skies as well as the earth starts off counting craters; most will still be doing so, now and then, decades later. They are the way that astrogeologists measure time. On the earth, geological time is measured in layers; layering is history and depth is age, as a drive to any edge of the Colorado Plateau will demonstrate. Stratification, though, like embonpoint, is best seen in profile; on planets looked on only from above the study of strata is geometrically challenging, to put it mildly. But craters, too, are the testaments of time; like sediment on a sea floor, they accumulate over the years. Most planets with rocky surfaces are amply supplied with craters: the earth, endlessly reinventing its surface through erosion and plate tectonics, is the great exception. Reading the record of craters has made sense of the geology of the moon, has revealed global cataclysms responsible for remaking the surface of Venus and has provided, at least in outline, the history of the Martian surface from the most recent sharp-edged scar to the most ancient rimless basin.

It was through Meteor Crater mat people first learned how to read such records. It gave them what geologists most need: an analogue through which to understand processes not yet understood in any other way. Analogy sits at the heart of geology; it has long linked the past to the present, and now serves to tie the earthly to the alien. Meteor Crater allowed geologists to understand impacts, and its nested horizons became the door to other worlds and other times. Its role in understanding was not just theoretical. In the 1960s Meteor Crater was one of the sites chosen to train the only men from earth ever to walk anywhere else; strain your eyes and by the lake bed at the bottom you can see the statue of an Apollo astronaut that commemorates them. From his point of view there is no double horizon; beyond his little bowl of a world there is just the great urgent vault of the sky.

‘A Little Daft on the Subject of the Moon’

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.* 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.