The Moon

by Oliver Morton

  Apparently superimposed on the picture’s lower half, a bright horizontal field, textured more than featured. In 1969, well after the imagery of “Earthrise” entered the culture, Mark Rothko used the same bisected construction, black above, textured grey below, in one of his last untitled paintings. It was a painting, he said, about death. Is it in the grey, the black or their juxtaposition that death is to be found? He did not say.

  The third element of “Earthrise” is within its black upper half, a second brightness. That light in the darkness is the Earth waning gibbous, slightly more than half of its face in the Sun, its night-edge a relaxed bow over the limb of the grey-lost Moon. This is not death: this is not nothing; this is life, proud and sharp.

  Just above the night-edge, almost central to the disk (and thus more or less the closest bit of the Earth to the camera) sits Ascension Island, the invisible-at-such-distance speck of volcanic rock where, at the very moment the picture was taken, the antenna at a tracking station known as the Devil’s Ashpit was monitoring Apollo 8’s radio transmissions.

  People had imagined such sights before. But when artists had drawn them, they had almost all got them revealingly wrong. The Earth seen from the Moon in such prefigurings was almost always a school-room globe, dominated by the outlines of familiar continents: the world as mapped by humans and known by humans. Seen in itself, though, it was not a world but a planet, strange and changing, its features barely recognisable, its character unmistakeable. Not a representation, but a presence.

  In “Earthrise” the North Pole is to the right of the disk, below the night-edge, invisible in its midwinter solstice. The South Pole blazes midsummer bright on the limb’s upper left. Indeed, all the limb shines white. The cloud which sits over the seas and hides the coast of Brazil is as bright as the ice of the Antarctic. The contrast with the blackness beyond is absolute.

  Within that unbroken white border, the disk’s most obvious features are its curving weather fronts. They wrap themselves clockwise over the Southern Ocean, widdershins over the Atlantic to the north, their ceaseless movement expressed in the straining of their curves. More permanent features are harder to make out. To the left, just above the night-edge, is the sunlit coast of the Namib Desert, though you would be hard put to spot it without being told what to look for; to the right, more prominent, are the bright sands of the western Sahara. The only clearly identifiable geographical feature is a discrete inflection of sharply seen North African coast called Ras Nouadhibou. In 1441—at the beginning of the European voyages of exploration which the Apollo missions sought, in their way, to emulate—Nuno Tristão, a mariner from the court of Henry the Navigator, became the first of his countrymen to sail past that cape. On the same voyage he became the first European to take slaves from the shores of West Africa. Today the bay behind the cape is a graveyard for abandoned ships.*

  The land is darker than the cloud, the ocean, contrary to Leonardo’s expectation, darker still—except for one central spot, a bit more than half of the distance between the night-edge and the limb, a spot which shines in a manner all its own. It is the part of the South Atlantic in which, at this particular moment, for this particular geometry, the sea’s surface really has become the mulberry mirror Leonardo believed it to be; the afternoon sunlight is hitting it at just the right angle to bounce straight towards the Moon rising in the eastern sky. This wave-bounced highlight, known in optics as a specular (meaning mirrorlike) reflection, has a different quality to that of the cloud tops. It is burnished: bright like metal, not like snow.

  But it is all the same light. The men of Apollo 8 were too far away to see any human lights from the cities of Africa, which would then have been the dimmest of embers; all of Nigeria, at that time, consumed less electricity than a small city in America. They saw no fires, no volcanoes, no lightning. Everything they saw, on Earth and Moon alike, they saw by sunlight.

  Yet how different the two equally sunlit bodies look in that picture: one complex and fine-featured, dynamic and contained, its stillness purely a function of the shutter, like a snapshot of a dancer in mid-air; the other a partial expanse of greys that might go on forever, uneven but subdued, slightly lacquered-looking, still. It seems unfinished yet long abandoned, in need of something but incapable of anything.

  The Moon reflects just 12% of the light it receives from the Sun. The rest it absorbs, like tarmac in a desert. The rocks close to its surface heat up to more than 100°C during their 340-hour day. In the 340-hour night they cool as the energy the Sun imparted to them slips back into the cosmos as heat; by late in the night their temperature is down to minus 150°C. But this huge thermal swing achieves almost nothing. The Sun’s heat penetrates only a metre or so down into the dust, rubble and rock; lower than that, nothing changes. Energy comes down from the sky, energy goes back out to the sky, energy makes no difference worth the telling in between.

  The Earth absorbs 20% less energy from the Sun, per square metre, than the Moon does. But it is able to do infinitely more with it.

  The flow of energy which, on the Moon, just heats up a thin skin of rock, drives perpetual change on the surface of the Earth. Every second, 16m tonnes of water evaporated by the Sun at the surface rise into the sky. In the cooler heights that water vapour condenses into clouds in smooth, soft layers, clouds in tall, stormy towers, clouds like hawks and handsaws and whales, into tiny clouddrops and fattened raindrops, hard-falling hail, high-floating ice and every other lightness and darkness, softness and hardness of the sky. The condensing of the water vapour releases to the atmosphere the energy that evaporating the water took from the sunlit surface, fashioning gradients of temperature and pressure that all but ceaselessly stir the air.

  The oceans, too, move heat, shipping it in bulk from the tropics to the poles, the journey twisted by the same Coriolis force as those curls of “Earthrise” cloud which go one way in the north, the other in the south. These polewards flows redistribute about 5% of the energy the Sun delivers to the Earth, the heat of their waters, the play of their currents and the Sun-driven stirring of the air above forming storms and winds and monstrous waves—as well as moments of sudden sharp stillness, nights of clarity as brittle as ice and fogs that settle motionless for days. And all this happens simply because it can. Simply because the presence of an ocean and an atmosphere allows it to—in fact, requires it to.

  And there is more. Life has furnished the Earth with leaves as great in their aggregate surface area as the continents themselves. Along with the less familiar, but just as vital, photosynthetic membranes of algae and bacteria, those leaves take up about a thousandth of the incoming sunlight and use it to turn some of the air’s carbon dioxide and some of the water that rises and falls from the skies into oxygen and biomass.

  This is the transformation that keeps the atmosphere off-kilter; this is the transformation that is basic to the Earth as a living planet. Almost everything on Earth that lives does so thanks to that transformation; all the energy that is ever taken from eating another living thing comes ultimately from sunlight. Every twitch of a muscle and every spark of a nerve is sunlight, too.

  Few who look at “Earthrise” appreciate these climatic, oceanic and biogeochemical dynamics in any detail. But almost all appreciate what they mean: the orb they are seeing varies and is various, it is changing, dynamic, a living world above the softly bleak unworld below. The twofold message of “Earthrise” is simple: that the Earth is there, in the sky, alive; and that someone alive, there in the sky, has seen it.

  THE EARTHLIGHT OBSERVATIONS MADE AT THOSE OBSERVATORIES in Provence and Arizona in the early 2000s were a recapitulation of “Earthrise”. Using what returns from the Moon to get another new view of life on Earth, they and subsequent studies have captured the same sense of the planet’s off-balance dynamism, though in numbers instead of pictures.

  Here are the features of the Earth the ashen light of the Moon can reveal. The chemical disequilibrium that James Lovelock first appreciated as a sign
of life can be seen in the spectrum of the atmosphere; the presence of oxygen and methane, gases eager to react with one another, requires there to be continuous supplies of both—supplies we know to be driven by life. The presence of oceans can be inferred from glints like those the crew of Apollo 8 saw in the South Atlantic—such specular reflections polarise light, and the polarisation is measurable at a distance.

  It is not just the presence of seas and life that the earthlight reveals. Because oceans are dark and continents light, the distribution of land and sea can be worked out from regular changes in brightness. Obviously, the regular 24-hour period of those fluctuations in itself reveals the length of the day; more subtly, the copious noise in those fluctuations provides a sense of how much cloud cover the planet enjoys and how much that cover varies over time. Indeed, it was to measure the Earth’s total amount of cloud cover, and to look for long-term changes in it as a result of global warming, that scientists started in the 1990s to systematically measure the earthlight from the Moon.

  The presence of plants can be seen, too, through an intriguing feature called the “red edge”. The pigments in plants absorb almost all the visible wavelengths of light to power the great transformation of photosynthesis. The only visible wavelengths most of them do without are the green ones, which they reflect; that is why leaves look green. Beyond the visible wavelengths, though, they exhibit another colour. Leaves reflect infrared radiation, which has wavelengths a bit longer than those of visible light, very efficiently indeed. This is not a matter of happenstance; it is an evolutionary necessity. If leaves absorbed all the infrared energy that hit them, as well as most of the visible light, they would get too hot. As a result, when we look down on forest canopies from the mountaintops they are dark to our eyes, but the right camera sees them as dazzling bright with cast-off infrared.

  Leaves are common enough on Earth’s surface that the light of the Earth as a whole shows this effect. As you go from detecting the red wavelengths of light to the longer infrared ones, the planet brightens sharply: the spectrum has a “red edge”. Such a feature, it is argued, could not be explained by mere minerals. Only a surface that has evolved to make optimal use of some wavelengths of light while prudently rejecting others could provide such a threshold. Seeing such an edge in the reflected starlight of an exoplanet would be an indicator of similar evolution there, as well.

  Thus earthlight has taught astronomers a lot about what is now widely taken to be the greatest near-term challenge of their trade: how to find evidence that distant exoplanets are, like the Earth, alive. By using the passive, lifeless mirror provided by a body that, for the most part, they scorn, they have found out what can be learned about the Earth’s true doubles, light years away.

  The irony goes deeper than that. The appreciation of the dim earthshine on the Moon by Galileo, Kepler and their contemporaries was a key part of the Copernican revolution—part of the discovery that the Earth was not the centre of the universe, that it was not so special, that it was a planet among many orbiting a star among many. Ever since, astronomy has had something of the humblebrag about it: look how skilfully and powerfully, the astronomers boast, we can show just how not-special we are. As their science spread its horizons wider, studying whole galaxies and clusters of galaxies, reaching back to the Big Bang itself, its observations were studiously framed so as to drive home the cosmically diminished significance of the species doing the observing.

  But since, and possibly because of, “Earthrise”, it has been possible to observe this trend reversing. The Earth has not been restored to the centre of the astronomers’ universe. But a sense that what is special about the Earth—life—is somehow central to the universe, too, has grown. That, as with Apollo 8, looking out matters most as a way of looking back. That it is the point of departure, not the destination, that matters.

  Astronomy is thus now increasingly treated, especially by its popularisers, not as a way of understanding the vast and inhuman universe but as a way of understanding, through that universe, humans and the Earth. One of the ways this is done is by concentrating on origins. That the origin of the universe is “where we come from” is treated not as a truism—where else would we come from?—but as a genuine addition to humankind’s self-knowledge, thus rendering the search for further information about the quantum fluctuations of the Big Bang oddly personal. Rather than revealing human insignificance, such studies of the ineffably vast and ancient are seen as, or perhaps sold as, deepening our connection to the cosmos.

  The same sense of connection stretches, as the prospectus for a new space telescope recently put it, “From Cosmic Birth to Living Earths”. It is now a commonplace in lay accounts of astronomy that the search for other planets which carry life is a priority second only to understanding the origins of the universe—and perhaps surpassing it. Astronomy as encountered by the public (and, increasingly, as funded by them) is ever less about stars, ever more about exoplanets. The great thrust of the discipline is no longer to find things ever farther away that show the Earth to be ever less significant. It is to sift through the endless streams of starlight for something that looks like earthlight—to find something, somewhere in the galaxy, as special as the Earth. Something which might look back at us.

  Just as the earthshine visible on the Moon had a walk-on part in the great decentring of the Earth, now it has a walk-on part in this anti-Copernican astrobiological recentring of the heavens on life and its Earth homes. It is a way in which the unworld Moon teaches the world of Earth what life is, and how to see it.

  There may be other such lessons. To live there, or to imagine living there, and find how strange it really is to take life from the planet that has shaped it and which it has shaped in its turn. To step outside the endless flows through which that reciprocal shaping had been achieved, leaving them behind you in the sky, and to come to terms with a life purely technological, purely human, constantly kept from death only by thin, manufactured walls. To learn to what extent such a future can be a continuation of the history that has come before and to what extent must it be a break with it—or a dead end. To stand on a grey, foreshortened plain, your shadow cast, in an alien night, by the light of the world that gave you birth.

  The Moon still has new reflections to offer.

  * A member of Congress unhappy about funding what he saw as the caprice of scientists noted that “So useless has the [Geological] Survey become that one of its most distinguished members has no better way to employ his time than to sit up all night gaping at the Moon.”

  * Bouncing lasers off the Apollo retro-reflectors, though, is not a plausible pastime for amateurs; the episode of “The Big Bang Theory” called “The Lunar Excitation” is misleading in this respect.

  * According to Brian Eno, whose album “Apollo”, the soundtrack to the film “For All Mankind”, is a sublime meditation on the lunar experience, all but one of the astronauts who took tapes to the Moon took some country and western music; it is part of the reason why his piece makes use of steel guitar.

  * The different resonances of the orientations come in part from the syntax of cinema. Drama tends to move horizontally—movement across the screen takes you forward in time. The left-to-right reveal of the Death Star (a space station easily confused with a moon) as it moves out from behind the planet Yavin is inherently plot-y: Lucas uses it to build narrative tension. Moving the camera vertically signifies a pause, or a timelessness—a looking up, a heightening of tone. Thus Kubrick’s alignments of Earth, Moon and Sun seem to transcend plot or drama, taking on the more sublime sense of a cosmos defined only by vastness and viewpoint.

  * It is also, in an odd coincidence, one of the best places in the world to buy a bit of moonrock. None of the rocks brought back by later Apollo missions are, officially, in private hands. But about 1 in 1,000 of the meteorites that land on Earth comes, originally, from the Moon, and in general meteorites are most easily found in deserts: the dryness stops them from becoming weathered, and thu
s less distinctive; the open landscape allows them to stand out; and desert dwellers tend to be sharp-eyed and attentive. The markets of Nouadhibou are among the best places to buy meteorites, including lunar ones, gathered by Saharan nomads.

  ITS SIZE AND APPEARANCE

  ITS MASS IS AROUND 73 MILLION TRILLION TONNES, 50 TIMES THE mass of the Earth’s oceans, but just 1.2% of the mass of the Earth as a whole. If you were to slice straight through the Earth at 55°S—the parallel that passes through the southern tip of South America—the end-of-an-egg cap you would cut off, 840km deep and 7,000km across, would have roughly the same mass as the Moon.

  This is less than the mass of any of the solar system’s planets; it is a tenth the mass of Mars. Three of Jupiter’s moons—Ganymede, Io and Callisto—are more massive than the Moon. So is Titan, Saturn’s largest moon. But those moons represent only a tiny fraction of the mass of the mighty planets they orbit—about a five-thousandth—as opposed to the Moon’s eightieth of the Earth’s.

  The Moon is more than five times the mass of the dwarf planet Pluto and some 25 times the mass of all the asteroids in the asteroid belt put together.

  At least 95% of this mass is rock. A small amount of it forms a crust, which is about 40km thick, on average; the rest of it forms an underlying mantle. The Moon’s iron core, if it has one, is less than a twentieth of the mass of all that rock. It is no more than 300km across and mostly, perhaps entirely, solid. Unlike the Earth’s iron core, which represents 30% of the planet’s mass and is mostly molten, whatever core the Moon may have produces nothing noticeable by way of a magnetic field.

  The Moon’s atmosphere would, if warmed up and pressurised to the conditions at the Earth’s surface, barely fill a parish church. For almost all practical purposes, it has no atmosphere at all.