The Moon

by Oliver Morton

  Which is all to say that you would miss it, were it gone. But not that you would wish for it if it had never been. What madness it would be, to imagine the waxing, waning, night-lighting Moon in an unMooned world—to dream of a sky-thing which could slide seamless over the Sun. And that poses a further question. What might this world lack that is as hard to imagine as the Moon would be on an unMooned Earth? Of what absences are we unaware?

  As well as an absence, the Moonless world would be marked by difference. Folklore, nocturnal action and assignation, marine and maritime life: all would be otherwise. Also, perhaps, science. In an essay published in 1972 Isaac Asimov argued that, for all that its craters and earthshine played a role in the acceptance of Copernicanism in the 17th century, in the greater scheme of things the existence of the Moon had thwarted the progress of astronomy, slowing the necessary move from a picture of the universe centred on the Earth to one centred on the Sun, for the simple reason that, although the other planets and the Sun do not orbit the Earth, the Moon really does.

  The Sun’s apparent path through the sky can be explained equally well by the Sun going round the Earth and the Earth going round the Sun. The same is true for the Moon, though, as far as I know, no one has ever actually put forward a lunocentric theory of the Earth’s movement. But there is no way to explain the Moon’s path through the sky in terms of the Moon orbiting the Sun. So either the Moon moves round the Earth and the Earth moves round the Sun, and the universe has more than one axis for things to turn on, or the Sun and Moon both move around the Earth. The second is obviously much simpler, and also satisfies the strong intuition that the Earth is stationary. A geocentric solar system made explaining the paths of the other planets through the sky a little tricky, but there were ways around that, if you were imaginative enough.

  If there had been no Moon, Asimov argued, astronomers from Babylon onwards would have realized that having everything move around the Sun would be just as simple as having everything move round the Earth—and would have made understanding all the other planetary orbits entirely straightforward. If this truth had been known throughout history, he went on, there would have been much less conflict between science and religion—indeed the former might have wholeheartedly supported the latter, something he thought devoutly to be wished. The mechanistic, gravitational revolution associated with the period from Copernicus to Newton might have come about centuries or millennia earlier.

  What was more, he speculated, a non-geocentric theory of the universe would have encouraged a less anthropocentric attitude to the living world, and thus no environmental crisis of the sort which, by that point, was a deep concern to him. Without the Moon, in short, 20th-century science might have been millennia in advance of where it was, and a Galactic Empire well on its way. With it, the world of the 1970s was close to a total ecological collapse. He called the essay “The Tragedy of the Moon”.

  The gloomy ideas are, in themselves, entertaining trifles. A historiography in which science simply springs up if not suppressed, and a philosophy in which astronomy has enough access to the human heart to govern its reverence for the world, hardly seem compelling. But that such thoughts came to one of America’s greatest science fiction writers when, at the time of Apollo, he looked out across the early morning New York skyline at the setting Moon speaks of a pessimism worth noting. And there is also the question Asimov does not examine. The Copernican revolution might have come about more quickly if deprived of the Moon’s empty landscape in the sky, but would it have been as profound? If the Earth had been treated as just another planet since the birth of astronomy, then how, and why, would the other Sun-circling points of light have become worlds?

  THE CIRCUMSTANCES OF THE IMPACT THAT FORMED THE MOON may or may not have been unlikely, which may or may not have implications for life elsewhere. A catastrophic rain of later impacts, though, must most definitely have happened. It can be read on the Moon’s face. But how long, and hard, was that rain?

  The astrogeological effort that Shoemaker got under way in the 1960s created a relative time scale for lunar landscapes: the Copernican sat on top of the Eratosthenian, on top of the Imbrian, and so on. Providing dates for the transitions, though, was hard. The best tool to hand was, again, provided by impacts. Younger surfaces will in general have fewer craters on them than older ones. If you had a model of the rate at which things hit the Moon today, and estimates of how that rate might have changed over time, you could translate the frequency of craters into estimates of length of time the surface studded by those craters had been exposed to impacts. On this basis Bill Hartmann calculated in the mid-1960s that the lunar maria were about 3.6bn years old. When maria basalts brought back by the Apollo missions were precisely dated in the lab, they showed a range of ages pleasingly close to Hartmann’s estimate. Unsurprisingly, the rocks associated with the impacts which had created the basins in which those basalts sat were older.

  What came as a shock was that those impact-basin ages seemed to be highly concentrated: the impacts that had formed the basins seemed to have happened rat-a-tat-tat almost half a billion years after the Earth and Moon had formed. This led to the notion of a “Late Heavy Bombardment”—that some 500m years after the formation of the solar system the rate of impacts, which was in long-term decline, suddenly peaked back up for some reason. As astrobiology got going in the 1990s, this phenomenon began to look like a very interesting connection between the history of life and the history of the solar system.

  If the bombardment was responsible for most of the visible damage to the surface of the Moon, it would have treated the Earth even worse; the Earth’s greater size and stronger gravity mean that a given population of incoming rocks will hit it more often and harder. If there were 30 to 40 basin-forming impacts on the Moon, there would have been 100 or more on the Earth. The biggest of them would have been bigger than any on the Moon, capable not just of burning the land but also of boiling the sea: the water from all the oceans would have been turned into a thick atmosphere of superheated steam, and the crust sterilised down to a depth of a kilometre or more.

  The earliest universally accepted evidence of life on Earth is 3.5bn years old. But there are rocks that date back 3.8bn years that are taken to carry strong chemical hints of the presence of life. If a whole spate of potentially planet-sterilising impacts took place 3.9bn years ago, that 3.8bn-year-old evidence suggests either that life got started very quickly or that life has a remarkable resilience. Both possibilities are interesting to astrobiologists.

  If you think it took the whole early history of the Earth—in effect, all 500-odd-million years of the Hadean—for life to get going, then you will tend to think it a more unlikely process than if you think it got going in just 100m years; things that take a long time to come right seem intrinsically less likely than things which come off quickly. If life could get going in just 100m years, people argued, life might be quite an easy trick for a planet to pull off.

  Perhaps more intriguing was the idea of resilience. In 1998 Norm Sleep, a professor at Stanford, and Kevin Zahnle, a researcher at NASA Ames—half of the Chaotian quartet—published a paper noting that if the Earth was subjected to ocean-boiling impacts, then one of the best ways for life to survive them was to follow the advice that protagonists in gangster films always ignore and get out of town till things cool down. Big impacts throw smaller rocks nearby out into space. A few travel to other planets, as the presence of meteorites from Mars on the Earth demonstrates. Most, though, eventually fall back whence they came. But they may spend hundreds, or thousands, or hundreds of thousands, of years in space before they do so—long enough for even the effects of an all-out land-burning ocean-boiler of an impact to have worn off.

  Satellites brought back to the Earth from orbit have demonstrated that the inert spores formed by some bacteria can survive for years in space. Lodged into the pores of a rock they might survive for millennia. And if the orbital-refuge idea is true, this might be related to the fact that
their most distant ancestors evolved to do so. Boiling the oceans represents quite the evolutionary bottleneck. If everything that can’t survive a few thousand years in space is wiped out, then what repopulates the Earth will be space-ready by definition. Tsiolkovsky’s notion of life evolving towards space would be given a twist; to leave the planet would be to revisit its childhood survival strategy.

  Or even, perhaps, its birthplace. If primitive life can survive occasional episodes of spaceflight, then the origin of life need not be on the planet where that life takes root. The arguments that apply to terrestrial microbes surviving superheated atmospheres by sitting them out in space apply to Martian microbes, too, should there have been any. If the conditions of the early solar system encouraged life to develop the capability to survive spaceflight, in doing so they also provided it with the ability to pass from planet to planet.

  In the early solar system, with lots of big impacts going on, the rate at which meteorites from one planet arrived at another was surprisingly high. The orbital refuge paper calculates that, back then, thousands of meteorites from Mars rained down on the Earth every year; some of those rocks would have been in transit for less than a decade. The flow of rocks from Earth to Mars would have been considerably smaller—it is harder to launch a meteorite off the heavier Earth, especially if you need to get it into space at a speed that will take it all the way to Mars. But it would still have been appreciable.

  If life originated on Earth, it could have spread to Mars through this sort of “transpermia”; if on Mars, it might quite likely have fallen to Earth.*

  Even while becoming part of the increasingly cosmic context in which people thought about early life, though, the Late Heavy Bombardment was not universally accepted: Hartmann, among others, never liked it. He thought that to the extent the effect was real, the lack of evidence for basins more than 4bn years ago simply showed that such evidence got written over, not that such impacts never happened. There was not an uptick in impacts around 3.9bn years ago; it was just that later impacts had overwritten the evidence of the earlier ones. This is an increasingly widely held view. Arguments that the bombardment might have been caused by Jupiter and Saturn swinging in towards the Sun and out again thanks to a peculiar orbital resonance, which seemed to provide a mechanism whereby the solar system would have been filled with asteroids and comets thrown out of earlier, stable orbits around the relevant time, look less convincing now than when they were first made a decade or so ago.

  As well as finding itself without an explanation, these days the rock record of the Late Heavy Bombardment looks a little more dubious, too. Recent studies suggest that the Apollo rocks seemed to offer evidence for impacts clustered around the same time simply because most, maybe all, of the rocks in question actually came from the same impact: Imbrium. Its debris is estimated to have covered about a fifth of the Moon’s nearside. Its remnants can be found in rocks from almost every Apollo landing site. Rocks taken to have come from other impacts may have simply been Imbrium ejecta mischaracterized.

  About ten years ago, when America’s National Academy of Sciences put together a report on what science needed to be done on the Moon, sorting out the timing and severity of the Late Heavy Bombardment, if any, was top of the list: “Science goal 1a”. That report concluded, as has almost everyone else taking an interest, that this means getting some more Moonrocks, looking at the isotopes that date them and thus establishing in absolute, rather than relative, terms when various impacts happened. And the place to start is with the biggest impact of them all (bar the one at the beginning), the one which created the South Pole-Aitken basin. As the name suggests, it stretches up all the way from the South Pole to the farside crater Aitken—just 17° south of the equator. That makes it 2,500km across: a whopper by any planet’s standards. You could fit India and Argentina into it and have room left over. If you were willing to leave off the autonomous regions of Guangxi, Inner Mongolia, Tibet and Xinjiang you could get all the rest of China into it.

  Unlike the biggest nearside basins, South Pole-Aitken has no smooth sea of maria basalt at its base. But it is still distinctly darker than the surrounding highlands, probably because it has dug deeper into the crust than any other basin. It is 13km deep; the Leibniz Mountains, which define its north-eastern rim, are from foot to peak the highest on the Moon. Stratigraphers have decided it is the oldest distinctly identifiable feature the Moon has to offer. They have also identified places in it where there may be rocks which were melted in the impact itself and which would provide a precise date for it.

  Whether or not impacts ticked up just before and around the creation of the Imbrium basin, they may have dropped off quite quickly afterwards. And so, in the spirit of continuing an interplanetary trend in geological time scales, I suggest the following. Given that, as yet, there is no specific marker for the end of the Hadean on Earth, might it not, at least as a temporary measure, be fair to use Imbrium to date the end of the Hadean too? Whether or not there was a spasm towards the end, basin formation really was the dominant geological process on the Moon and a very important one on the early Earth. In the 3.8bn years afterwards they really did diverge—until humans began to mess around with both of them in the very recent past. Whether or not one agrees that Earth and Moon should share an Anthropocene, it seems reasonable that the end of their distinct but twinned childhoods should be marked by a single event—and if it is to be one of which a clear record remains, that has to be an event on the Moon. The event responsible for the most distinctively rimmed of the Moon’s seas, the arc of its half-encircling mountains visible to all, is surely as good a one as any.

  IMPACTS HAVE CONTINUED FOR THE REST OF LUNAR HISTORY, the only real exception to Robert Heinlein’s tongue-in-cheek dictum that “nothing ever happens on the moon”. And they brought the Moon at least two things that humans might treasure.

  The first is water. Many asteroids are made of minerals that contain a bit of water; those known as “carbonaceous chondrites” can be over 20% water by mass. Comets are a good bit wetter still. When a body of either sort hits the Moon, the water it contains is vaporised and much of that vapour is immediately lost back to space. But some sticks around. On the hot, sunlit side of the Moon’s night-edge, it forms a tenuous atmosphere; on the dark, cold side, an all but undetectable frost. As the night-edge sweeps round the planet, the volatiles move from ice below to vapour above and back again on a monthly basis.

  In time, most of this asymmetric atmosphere is lost—the Moon is too small to keep such a wrapping around it. The Sun’s ultraviolet light ionises the volatile molecules, after which the charged particles of the solar wind strip them away. But some of them remain as frost in perpetuity—because some of the Moon never sees the light of day.

  The Moon has a low obliquity; it sits almost straight up with respect to the ecliptic. This means that the Moon’s poles are lit tangentially, with the Sun never rising far above the horizon. The shadows are long—so long that some of them never end. In craters at the poles there are places where the horizon-hugging Sun cannot shine. It may rise high enough to light the inner rim of a crater, creating the morning-lit side which Galileo, when first convincing people that the craters were craters, compared to the western side of an Alpine valley. And as the Moon slowly turns, the part of the inner rim that is lit changes, too, as if being broiled on a sluggish rotisserie. But though most of the rim is illuminated at some time or other, the floor never is. The only light it sees is the secondary light reflected from the rim.

  And some of the crater’s interior does not even see that—because there are craters within craters, and from those inner craters the rim of the outer one often is invisible. The depths of such craters see the Sun neither directly nor indirectly.

  Most of the craters which contain this perpetual darkness are around the South Pole: the crater named after Gene Shoemaker is one. Being in the depths of the South Pole-Aitken basin gives the region a head start when it comes to avoiding sunlight. Bu
t there are pools of perpetual darkness in the north, too. And at both poles the darkness is phenomenally cold—colder, remarkably, than the surface of Pluto, which is 30 times farther from the Sun. Pluto may get sunlight a thousand times weaker than that which bathes the Moon, but every square metre of it gets some of that light some of the time. Go without sunlight at all for a few billion years and you can get really cold: the floors of the sunless craters are at about minus 238°C, 35 degrees above absolute zero.

  If vapours produced by impacts or possibly from other sources rime these craters with frost and nothing subsequently re-vaporises it, it is fair to imagine that that frost accumulates. Such accumulation would make the creeping growth of glaciers look whip-tip fast; but it has had billions of years in which to play out. And so something very slightly like a glacier growing into the sky could take form: a laminate of dust-dirty ice, growing a few millimetres every million years if it’s lucky, lit only by the stars towards which it is so very slowly reaching.

  At least, that was the case for the past few billion years. Around the 25th year of Grinspoon’s Anthropocene, though, other radiations began to impinge on the sunless craters. First radar, then lasers, shone down from orbit to probe their depths. Other instruments, rather brilliantly, made use of the stars themselves, picking up reflections of ultraviolet starlight from the craters’ interiors. Together these and later studies provide strong evidence that layers of ice really do exist in the craters’ depths.

  This has made those enthusiastic about the Return to the Moon very happy. Layers of ice at the poles could be used to provide a research base, or indeed a permanent settlement, with water and oxygen, greatly cutting down on the need to bring supplies up from Earth. And splitting water into hydrogen and oxygen gives you high-quality rocket fuel and the perfect stuff to burn it with.