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.
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