But they are not. Nasmyth, and most of those who came after him, underappreciated the exaggeration that oblique lighting throws on quite modest relief.* The sharp shadows that people had been interpreting since Galileo are cast by soft, rounded things, far less angular than the Andes or the Alps. They are shoulders, not shards. There is barely a slope on the Moon that couldn’t be walked or clambered up, even if the gravity was as strong as the Earth’s. If the Moon sports anything that could be fairly called a cliff, it has yet to be seen.
This is because, despite the lack of wind, rain and ice, the mountains of the Moon are indeed being eroded. They are constantly hit by particles of dust moving at orbital speeds. The bombardment is not intense—1,800 tonnes of dust a year, even moving at over 10km/s, imparts far less energy to the Moon’s surface than a few minutes’ worth of rain does to the Earth’s. But this thinner-than-thistledown, faster-than-bullets onslaught is unremitting. No crag can stand against it over a billion years.
This misapprehension as to erosion speaks to Nasmyth’s greater, deeper mistake. The bombardment of the Moon is not limited to dust. It has included objects up to the size of pretty big asteroids. Such objects can deliver extraordinary amounts of energy, enough to reshape landscapes on the largest of scales in just a couple of seconds.
In 1941, while waiting for a lecture at the Field Museum in Chicago to get under way, a Midwestern industrialist who had been an astrophysicist in his youth, Ralph Baldwin, was struck by curious grooves visible in a photograph of the Moon. These were not superficial rays like those that emanate from Tycho but shadow-casting gouges that showed real relief. It looked, he thought, as though something had moved not into, or out of, the Moon but across it, at great speed and with great force. What process could create such a sculpture?
Intrigued, Baldwin studied other pictures and observed the Moon itself. He found that the gouges that had caught his eye had a radial structure, fanning out from a point in the centre of Mare Imbrium, and concluded that this “Imbrium Sculpture” had been created by debris thrown out by a giant impact. It followed from this that the arcing mountains that defined Imbrium were the rim of a crater—one which, at over a thousand kilometres across, would have dwarfed Tycho or Copernicus. The dark lavas that made Imbrium a sea were the result of eruptions that had come after the impact, filling up part of the great void that it left.
And if the great circle of Imbrium had been formed that way, then surely all the other lesser circles had, too. The craters of the Moon, from small to enormous, had all been formed by impacts.
This idea, which Hooke had decided against in part because he could not imagine what such impactors might be, had been around for a time. Proctor toyed with it in the 19th century. Grove Karl Gilbert, the 19th-century American geologist a little crazy on the subject of the Moon, believed that the Moon’s craters and maria had been created when leftover building blocks of planetary formation, “planetesimals”, had fallen on to it in its early years. In the early 20th century, Ernst Öpik and Charles Gifford, astronomers from Estonia and New Zealand, respectively, independently came to the same conclusion by considering the ever-growing number of asteroids and comets with which careful observation was populating the solar system.
But the idea did not really develop any traction. This may in part have been the result of bad luck and bias. Though Gilbert was famous, his Moon paper was published in a peculiarly obscure journal; astronomers did not look to Estonia and New Zealand for the next big thing, less still to the Oliver Machinery Company of Grand Rapids, Michigan, where Ralph Baldwin was to be found. But there was also a deeper reason for resistance. If the Moon was thus battered, its neighbour the Earth must have a catastrophic past, too.
Geologists, committed to their uniformitarianism, deeply disliked catastrophes. Astronomers didn’t much care for them, either. When, in “Around the Moon”, Barbicane gently mocks Arden for suggesting one of the “much abused comets” as a cause for the bright rays from Tycho, he was echoing Francois Arago, an astronomer friend of Verne and Nasmyth. Arago made it part of his mission in life to convince anyone who would listen that comets were not ill omens and should not be imagined as agents of collisional doom. They should be appreciated for the unthreatening wonders of the sky they were.
The person who changed this, thereby both creating the framework for the modern geological understanding of the Moon and playing a key role in the realisation that, in fact, impact catastrophes do take place on Earth, was a geologist called Gene Shoemaker. Shoemaker had a magnificent scientific imagination, a fierce commitment to expanding geology’s disciplinary reach beyond the Earth and a powerful personality (his colleagues lampooned him in office-party skits as Dream Moonshaker). He also had something else which proved crucial: experience of nuclear explosions.
Studies Shoemaker made of Jangle U and Teapot ESS, two craters in Nevada created by underground nuclear tests, showed them to have distinctive features unlike those of craters formed by volcanism, most obviously curled rims in which the rock layers were folded back on themselves. Working from an unpublished paper by Edward Teller, Shoemaker began to understand the power and behaviour of the shock waves that created these features.
It was with eyes thus trained that he visited, in 1957, another crater: Barringer Crater, also known as Meteor Crater, about 70km east of Flagstaff, Arizona, and a bit more than a kilometre across. Its owners, the Barringers, had believed for generations that it had been formed by a large meteor—in part because many metal meteorites had been found nearby—and had put considerable money and effort into trying to reach the body of meteoric iron they believed must be buried beneath its floor. The Geological Survey had taken the position that it was a “maar”, a peculiar sort of volcanic crater created when subterranean magma vaporises an aquifer. The Barringers blamed the Survey’s mundane explanation for scaring off investment. As an employee of the hated Survey, it took Shoemaker some time to gain their trust.
There was a striking irony in all this. The Survey scientist who had earned the enmity of the Barringers by declaring the crater a maar had been none other than Gilbert himself. Hearing of the crater and its associated meteorites, and believing as he did that lunar craters were created by impacts, Gilbert had been keen to investigate Meteor Crater as a possible Earthly exemplar of that which he could normally only study through a telescope. In Arizona, though, he found no evidence of an impact. There was no evidence of anything buried beneath the floor of the crater. What was more, the sediments that had been thrown out of the crater onto the surrounding plateau seemed, he thought, to have the same volume as the crater itself. Where, then, were the remnants of the impactor? Much as he might have wanted to find an impact crater, he could not convince himself that this actually was one; it was a maar, and the nearby meteorites were a coincidence.
Shoemaker showed that Gilbert had been wrong about the impact for the same reason that the Barringers had been wrong to think there was a motherlode of valuable metal to be found beneath the crater. Meteor Crater had indeed been made by a piece of extraterrestrial metal, but there was more or less none of it left. A metal body just 50m or so across—very small compared to the size of the crater—had hit the plateau at a speed high enough to release about 10,000 times more energy than the small nuclear blasts at Teapot ESS and Jangle U. The energy was in the form of two shock waves: one slammed forward into the plateau’s limestone; the other, backwards into the impactor. The forward shock scooped out Meteor Crater in just the same way that the shock waves from the nuclear detonations at the Nevada test site scooped out theirs, throwing a crater’s-worth of ejecta out onto the surrounding plateau and leaving the lip of the crater turned over in the telltale way Shoemaker had learned to recognise. The reverse shock vaporised the impactor. Daniel Barringer had never found the crater-forming meteorite because it had blown itself up.
Shoemaker was not the first to see the analogy between the destructive power of the bomb and the cratered face of the Moon. Robert H
einlein was, along with Arthur C. Clarke, one of the two great Moon writers of the decades leading up to Apollo. The first of his lunar novels, “Rocket Ship Galileo” (1947) was also the first of the series he wrote to prepare post-war children and teenagers for the post-Hiroshima world. I bought it decades later, at the age of 10 or 11, at a church bookstall. It is the story of a nuclear physicist who recruits three all-American teenage boys to make the first trip to the Moon.
Ross floated face down and stared out at the desolation. They were… approaching the sunrise line of light and darkness. The shadows were long on the barren wastes below them, the mountain peaks and the great gaping craters more horrendous on that account.… “I’m not dead certain I’m glad I came.”
Morrie grasped his arm, to steady himself apparently, but quite as much for the comfort of solid human companionship. “You know what I think, Ross,” he began, as he stared out at the endless miles of craters. “I think I know how it got that way. Those aren’t volcanic craters, that’s certain—and it wasn’t done by meteors. They did it themselves!”
“Huh? Who?”
“The moon people. They did it. They wrecked themselves. They ruined themselves. They had one atomic war too many.”
“Huh? What the—” Ross stared, then looked back at the surface as if to read the grim mystery there.
Nuclear weapons gave Heinlein a way to imagine instantaneous energies creating craters many kilometres across, reshaping landscapes in a fuckflash—the same insight Shoemaker’s study would later confirm. He was wrong to discount meteors. But he was right about the sheer and sudden scale of the destruction. The energy that creates the craters of the Moon is not the energy of the foundry. It is the energy of the bomb.
WHEN HE WAS A BOY IN THE 1950S, IN LOVE WITH BOTH SCIENCE and art, Bill Hartmann, like James Nasmyth, used to make plaster models of the Moon’s craters to try to grasp what it would be like to see them not from above but from the side. In the 1960s, as a graduate student at the University of Arizona in Tucson, he worked on a way of seeing that did the reverse, allowing bits of the Moon normally seen obliquely to be looked on as if from above. It was a way of looking that revealed the Moon to be not just subject to impacts but subject to almost nothing but impacts.
Gerard Kuiper, Bill Hartmann’s boss and a pioneer of planetary science, had procured a white hemisphere about a metre in diameter on to which he and his assistants could project telescopic images of the Moon’s nearside. In general, if you project a picture onto a sphere, you will distort it; it will flow down the flanks like stretched toffee. But if the picture you have is of another sphere, and you get the projection just right, what was distortion becomes rectification. A two-dimensional image of one half of a sphere projected onto a hemispherical screen produces a three-dimensional image.
Kuiper’s technique thus allowed him and his students to see the near side of the Moon from new angles: in left and right profile, as it were. Bill’s job was to take pictures of the projection for a “Rectified Atlas of the Moon” that Kuiper was working on. Examining the Rook Mountains and, to the south of them, Mare Orientale, an obscure, oblique blob on the Moon’s western limb as seen from the Earth, he had what he later called a “Eureka Moment”.* The Rook Mountains were not just a line of peaks. They were one arc of a pair of concentric features that encircled Mare Orientale’s dark basalt heart; and the surrounding landscape was cut by a “complex mass of radial valleys and striations” coming from the centre of the sea like the sculpture which Baldwin, and before him Gilbert, had seen around Mare Imbrium.
That there were big impacts on the Moon had, in the few years since Shoemaker’s breakthrough, become widely accepted. They explained arcuate mountain ranges like the Alps and Apennines around Mare Imbrium. Baldwin’s idea that the dark maria were sheets of basalt that had erupted into the basins formed by large impacts a lot later on—hundreds of millions of years later, it transpired—had become pretty much conventional wisdom.
What Bill discovered was that this explained more than the maria. There were ringed basins that had almost no basalt within them, too. The more Hartmann looked at the rectified projection of the Moon, the more such markings he saw. All the big mare basins had vestiges of such structures around them; and so did large basins that contained no basalt.
Some of what he was seeing had been described piecemeal before; but once Bill had learned how to look for basins, he saw the surface as a whole beginning to fit together in a new way—a new gestalt, as he would later put it. Impacts explained everything, from small bowl-like pits to simple craters a few kilometres across to larger craters with their distinctive central peaks to basins with multiple rings and more complex internal peaks, some of the biggest of which were filled with maria lavas. The maria were not uninteresting, but they were epiphenomena. All the large-scale structure of the Moon was down to impacts.
In subsequent decades “multi-ring impact basins” have proved to be a near universal feature of planetary surfaces. They have been discovered on all the rocky inner planets and most of the larger moons of the outer ones: Valhalla, on Callisto, is a particularly fine example. Venus, with a youngish crust, offers only a few. Mercury is covered with the things. The Earth’s have mostly been erased by erosion and plate tectonics, but the ground-down Vredefort structure in South Africa can still be recognised for what it is—at least it can if you look down from orbit and see the Earth as the Earth sees the Moon.
AS THE EARTH SAW THE MOON.
Now it has seen it differently. It has seen it in close-up from orbit and from the surface—most intimately in the many thousands of exposures captured by the Hasselblads of the Apollo astronauts.
The surface is not, in absolute terms, bright. In its nature tarmac dark, reflecting only about 12% of the sunlight that hits it. When evenly, brilliantly lit, though, it can but look bright.
It is not quite monochrome. Its colours are faint, and they are almost all variations on shades of grey, but there is at least a hint of pigment: something reddish, or something bluish. The slopes, if there are slopes to be seen, have a different tone from the plains below. Those that rise most steeply show the most changed texture; nothing loose clings to them, and they are but rock.
There is something complete about those rising forms. They are unfurrowed; nothing has rasped at them, or for the most part, cut through them. They never double back on themselves. Their subdued relief is not without variation, but its range of expression is restricted. The long heights lack any pattern save that of slow curves.
They cast shadows, though. In the photographs, the shadowed sides of the Moon’s distant hills and rilles look as black as the sky. In reality, they are not—they are lit by the Sun as reflected from nearby sunlit surfaces. The daytime shadows of the Moon are lit by moonshine. The night is lit by earthshine.
As yet, no one has seen that nocturnal illumination from the surface. The Apollo astronauts saw it from orbit, though. Ken Mattingly, who flew Apollo 16’s command module, Casper, told the author Andrew Chaikin that the experience was “like flying over snow-covered terrain on the Earth with a bright moon and totally clear skies. You get this magic terrain—you can see relief. But it has this sameness, this uniformity in color.… [I]t’s just like that—except you can see more detail, because Earthshine is so much brighter than moonshine.”
The astronauts on the surface found distance difficult to read. The idea that, on a smaller world, the horizon is closer is easily understood. In practice it is hard to gauge. Evolution has given humans a strong intuition for how far their eyes can see when they stand on a flat plain. The Apollo astronauts never quite got the hang of overriding that intuition, endlessly thinking features in the landscape were farther away than the maps said they were.
It is not that the landscape in the photographs lacks telling detail; it has features and objects in it, rocks and boulders which sit proud of the rubble and grit, distinct and individual. But the details mostly make no sense. A few do: a boulder at th
e bottom of a slope, having rolled or slid down when dislodged from higher up by a Moonquake, is easily read. Sometimes there is a track down the slope above it that shows its trajectory. In general, though, there is no process to be inferred here. The landscape may have features that move one into another, slopes that become plains, ridges that roll back, but they do not have stories in the way a river’s valley does. It is, after all, just the work of impacts. The Moon’s timescape has no flow; just punctuation.
The exception to the landscape’s resistance to measurement lay in the Earth-objects that many of the photographs capture. Looking back at their landing sites, the astronauts knew how far away they were. When Intrepid, the lunar module for Apollo 12, landed a few hundred metres from Surveyor 4, an extraordinary feat of precision navigation, its crew guessed the distance to the other spacecraft easily. Human detail was legible in a way the lunar landscape was not.
The rest of the rocks are just sitting where they were left. There is a sense of things having been abandoned. It is the opposite of a pause, not a stasis that interrupts a process but a stasis that is the norm.
* The full Moon always rises around sunset. Waxing gibbous moons, which can be seen in the days leading up to the full Moon, rise before sunset. Waning gibbous moons rise after sunset and as a result can be seen in the morning sky after sunrise.
* Though the darkness of the eastern slopes gets less intense the while, thanks to the secondary light from the bright-lit west.
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