When the investigator, having under consideration a fact or group of facts whose origin or cause is unknown, seeks to discover their origin, his first step is to make a guess.
Grove Karl Gilbert, ‘The Origin of Hypotheses’
‘Craters? Why didn’t we think of craters?’
Isaac Asimov to Frederik Pohl, on first seeing the
images of Mars from Mariner 4
If you care for impressive and beguiling landscapes, Flagstaff, Arizona has a lot to recommend it. The San Francisco peaks – remnants of a shattered volcano similar in scale to Mount St Helens – loom over a town scarcely a hundred years old, wrapped in the forests that attracted its founders. To the south the beautiful canyons of Sedona, carved into the rocks of the Colorado Plateau by water draining from beneath the forests; to the east the spectacular Painted Desert; to the north the Grand Canyon itself, more than a billion years deep, more gazed at and photographed than any other hole the world has to offer. Around the San Francisco peaks sit lower cinder cones like giant black molehills, weirdly fresh. Some are intact, some thoroughly quarried: their ash grits the roads in winter. One of them, remarkably, is in the process of being turned into a vast meditation on earth and sky, light and stone, by the artist James Turrell, earth movers his chisels.
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