The planets’ crusts were torn into fragments of every size, from mountain to Mozambique, some pushed out and aside, some crushed in the crunch of the mantles. As those disrupted mantles began to melt and flow, the planets’ iron cores, themselves distorted by shock, found a new freedom. The cores were not headed straight for each other; Theia did not hit Tellus straight on, but at a tangent. Its core passed by that of Tellus, losing energy as it ploughed through the tortured mantle, the resistance of the rock—now partially molten, partially vaporised—stretching and streamlining and warping its iron from smooth sphere to mangled tear. It did not have the energy, though, to pass right through the rock and out the other side. It slowed and curled, turned and fell towards the core of Tellus. Within an hour the hammer hit the anvil, and sank into it. At the centre of the newborn Earth, the two cores became one.
The mantles did not quite coalesce as completely. As Theia struck its glancing blow part of its mantle sloughed off into that of the planet it had struck, but some drove on past, pushing a layer of Tellurian mantle ahead of it like mud on the blade of a bulldozer. Still travelling with more than half of Theia’s original speed, blade and burden rose together back out into space. Much fell back. Much did not. Some escaped completely to form a short-lived ring around the Sun. But a lot stayed in orbit around the wrecked, reforming planet below. It was from that fiery orbital aftermath that the Moon was to grow.
The Earth writhes molten beneath its molten sky. Welcome to the Hadean.
WHAT COULD HAVE LED TO A SCENARIO SO EXTRAVAGANT BECOMING the most widely accepted account of the Moon’s origin—albeit one which still has some big questions to answer? Its rise, which dates back to the 1970s and 1980s, was mostly due to the knowledge brought back from Apollo. Oxygen comes in three different isotopes. Apollo samples quickly showed that the ratios between these three isotopes in rocks from the Moon were very like those in rocks from the Earth—and unlike those of asteroids or of rocks from Mars (which sometimes fall to the Earth as meteorites, having been blasted off their home planet by much larger impacts). Such isotope ratios are taken to show in which zone of the great Chaotian disk the various rocks formed. Identical isotope ratios seemed to mean that the Earth and Moon formed in the same zone.
Analysis of the Apollo samples also revealed that moonrocks were very low in volatile compounds—water, carbon monoxide, nitrogen, sulphur and other light elements. Data from the seismographs the astronauts installed on the surface and from measurements of the Moon’s gravity field made in orbit showed that it had only a very small iron core, if it has one at all. But if it formed where the Earth had formed—and thus presumably by the same mechanism and of the same stuff—how could that be? Why would it have so few of the volatiles with which the Earth was well stocked? Why had it not formed a big strapping core like the Earth’s? Mars and Venus have done so. Mercury, the smallest but densest planet, has a core that takes up more than half its volume.
In short, in terms of its make-up, the Moon didn’t really look like a planet in its own right. It looked like a dollop of the Earth’s mantle which had somehow scooped itself out and placed itself into orbit, no core attached.
The idea of such a fission was first proposed by George Darwin, Charles Darwin’s son. Darwin fils was interested in the drag on the Earth’s rotation caused by the tidal bulges raised by the Moon. Turning beneath a tidal bulge that stays put, driving tides in and out of shallow seas and across great oceans, means that the Earth continually loses energy to friction—a loss which slows the Earth’s rotation and lengthens the leash on which it holds the Moon.
This results from the conservation of angular momentum—a property of bodies, or systems, which depends on how their mass is distributed and how fast they are spinning. Move mass closer to something’s spin axis and, if angular momentum is conserved, it will spin faster; move it farther out and things will slow down. It is an underappreciated side benefit of scientific progress that figure skating, originally developed on a frozen fen near Cambridge as a way of demonstrating this phenomenon, has gone on to become a very popular sport.*
You can change angular momentum only with a torque—a force applied off-centre so as to change the spin. With no torque applied to a system from outside, its angular momentum must stay the same.
This principle applies to the Earth-Moon system, tied together as it is by gravity. When the skater’s arm extends, her body spins more slowly. The energy dissipated by the tides thus means both that the Earth’s days are getting longer and that the Moon must have been closer in the past. By calculating the rate of its recession, Darwin found that, some 54m years ago, the two bodies would have been one. From this he derived the idea of a single body, spinning very fast, splitting into two. Long before it was explained by plate tectonics, devotees of Darwin’s idea claimed that the great quasi-circular hole currently filled by the waters of the Pacific marked the divot from which the Moon had been thus ejected.† But no one could really explain why the planet would have come asunder in the first place.
The giant-impact theory, as the story of Tellus and Theia is known, seemed to provide an extraterrestrial precipitating event for the Moon’s recession as well as explain everything else that other theories could not. It got a chunk of Earthish mantle, with its telltale oxygen-isotope ratios, into orbit without an iron core. It stretched out and melted that chunk, baking the volatiles out so as to ensure a desiccated end product. It even explained why the Earth-Moon system had a high angular momentum in the first place. Theia’s off-centre impact would have applied a massive torque to Tellus, producing a planet that spun very rapidly and the Moon that would, over billions of years of tidal braking, slow it back down.
Put forth after Apollo by, among others, Bill Hartmann—the man who first appreciated the ubiquity of ringed impact basins—and Don Davis, who helped guide Apollo 13 safely back to Earth, the giant-impact theory gained widespread credence in the mid-1980s. Early supercomputer models, some using code written to explore the effects of nuclear weapons, were able to sketch out what might have happened, an endeavour which seemed both sexy and confirmatory. But at the heart of the theory’s success were the twin virtues of a great deal of explanatory power and no serious rivals. The idea of the Moon happening to pass by and being pulled into the Earth’s orbit—the capture hypothesis—could not be made to work, then or now, without immense special pleading. Nor did it explain the similarities between the bodies. Co-accretion, in which the two simply formed together, explained the similarities, but not the differences—the Moon’s lack of volatiles and core. Nor did it explain where all that angular momentum came from. The fission hypothesis lacked any sort of mechanism by which one planet might split in two.
What is more, the giant-impact theory helped explain one of the fundamental discoveries made by the Apollo missions. Whereas the dark plains of the maria were made of basalt, the brighter highlands were made of anorthosite, a rock composed mostly of calcium plagioclase, a mineral from the family called feldspars which are most familiar, I suspect, as the light-coloured non-quartz bits of granite kitchen worktops. If you take hot magma made from the Earth’s mantle and let it cool under lowish pressures, calcium plagioclase is the first mineral to crystallise out as a solid.*
If formed from the orbital debris of a giant impact, the Moon would have started life covered by an ocean of magma—a hot layer of liquid rock hundreds of kilometres deep. (The post-impact Earth would have had such a magma ocean, too, but maybe only a tenth of the depth and possibly not over all of its surface.) As the ocean cooled, it did not freeze from the top down, as Nasmyth had argued it would. Minerals began to crystallise out at depth, the first of them plagioclases. Being lighter than the surrounding magma, they would have floated to the top. The magma ocean would thus have grown a crust composed mostly of calcium plagioclase.
Since the Moon, small and quick to cool, never developed any mechanism for recycling its crust, this primordial crust stayed put, except when blasted away by im
pacts or covered by later, darker basalts. One of the samples of almost-entirely-plagioclase highland rock brought back by the Apollo astronauts was 4.46bn years old—less than 100m years younger than the Earth and the Moon.
But for all its explanatory value—not to mention drama—the giant-impact theory has run into problems over the past decade. Further studies of moonrocks, using more and more delicate techniques to tease out finer and finer isotopic details, have found that they are not simply broadly similar to Earth rocks. In some respects, they are effectively identical. At the same time, more detailed computer models of the impact find that most of the stuff which would end up in orbit would have come from Theia, not Tellus. To reconcile this with identical oxygen-isotope ratios—and, now, some very detailed and similar measurements of the isotopes of other elements, too—would require Theia to be made of raw material remarkably similar to that of Tellus. If the two were identical to begin with, the explanatory edge gained by mixing them is blunted.
Few are ready to abandon the giant-impact theory because of this problem. That said, there is no widespread consensus on how to fix it. Some postulate a Theia very similar in composition to Tellus. Others prefer to make Theia either bigger or faster; that gets more energy into the system and gets more of the Tellurian mantle mixed in with Theia’s and up into orbit.
In the early days of the giant-impact theory, higher energies were frowned upon, because unless you allowed a lot of special pleading they left the Earth-Moon system spinning too fast. Recently, though, mechanisms have been suggested whereby a torque applied by the Sun could bleed quite a lot of this excess angular momentum out of the Earth-Moon system quite quickly. The calculations on which this idea is based are not yet rock-solid; it is conceivable that, because it has the useful effect of allowing a wider range of impacts, the idea is getting a comparatively easy ride. For the moment, though, it has served to put high-energy impacts onto the agenda.
More energy means more mass in orbit, more heat, more angular momentum to stir things up with, more magma and a larger, hotter atmosphere of vaporised rock around the Earth. Indeed, the distinction between atmosphere and the mass in orbit could, to some extent, break down, creating an orbiting torus of molten and vaporised mantle much larger than the planet proper. Some of its proponents have started to refer to such a high-energy outcome as a “synestia”, a thick doughnut-like disk dimpled in the middle. The Earth is in the dense middle of the dimple; the Moon will form out of the distended doughnut, which is a thoroughly stirred-up mix of both partners’ mantles. Much of what does not end up in the Moon will return to Earth.
Whether you can hold such a cosmic doughnut together long enough to freeze a small planet out of it is an open question. The physics and chemistry that result when you pump a star-day of energy into a planet-sized rock are bound to be more complex than early models can grasp. But some way of getting more stuff into orbit and mixing it up more thoroughly seems, at the moment, a promising way to go.
And such strangeness is easier to think of now than it was in the early post-Apollo days, when a straightforward collision still seemed a touch outré. The discovery of thousands of planets beyond the solar system has stretched scientists’ sense of what a planet can be. Some are so hot as to have their atmospheres permanently swollen, some locked so close to their star that one side is always almost melting. One star has a ring of hot rock around it that some have seen as the short-lived by-product of a collision quite as powerful as that of Theia and Tellus. The universe offers a far richer array of planetary possibilities than the bimodal distribution of small rocky inner and large gassy outer objects seen around the Sun today.
ACCEPTING, FOR THE TIME BEING, THAT THE MOON WAS BORN IN some sort of giant impact, was the fact that two planets hit each other in a way that would form a large moon unlikely? In some ways, such questions do not matter. It happened, or it didn’t; look at the evidence, make the models, get new data and deal with it.* But from another point of view, it might be rather significant to know the answer, in a counter-Copernican sort of way.
The Earth has life—indeed, it has intelligent life. It also has a large moon. Is it possible that the two are related? If they are, then if a large moon is unlikely, planets with intelligent life may be, too. The Earth may be rare. These are the sorts of questions that keep astrobiologists up at night, often in bars.
In “Rare Earth” (2000), Donald Brownlee, an astronomer, and Peter Ward, a palaeontologist, make a vigorous and influential case that the Earth is unusual, and thus that humankind is too. Although microbial life might develop quite easily on many planets, they argue, the evolution of complex life had, on Earth, depended on various aspects of both its home planet and its home solar system being just so. The Moon is part of their argument.
The idea that the Moon had a relevance to life that goes beyond nocturnal illumination was not new. Some had argued that, without the Moon, the Earth would have a stiflingly thick atmosphere like that of Venus. Others had suggested that lunar tides—much bigger in the Earth’s early days because the Moon was still much closer—were crucial to life’s origins. By sloshing seawater into tidal pools from which it would then evaporate, they provided a way to concentrate the chemicals life would need. This is not an idea many people are interested in at the moment—recent fashion has been to look for life’s origin in deep-ocean hydrothermal vents where the tug of the tides does not matter. But ideas on the subject have changed before, and may change again; hypotheses, like tidal pools, come and go.
Brownlee and Ward, though, plump for another lunar effect—a damping down of the Earth’s wobbling. Planets do not sit up straight in their orbits: they lean over. The Earth’s axis of rotation is currently at an angle of 23.4° to the vertical, as measured with respect to the plane of the ecliptic. It is slowly in the process of sitting up straighter; but once it reaches about 22.1°, in a bit more than 10,000 years, it will start to lean back over again. Its obliquity nods between 22.1° and 24.5° every 41,000 years. The effect this nodding has on the intensity of the planet’s seasons is one of the things which sets the rhythm of the ice ages that mark out the Quaternary.
On more-or-less-moonless Mars, shifts in obliquity are both bigger and less regular. Sometimes Mars sits bolt upright, with no seasons worth mentioning. At other times it reclines as far as 60°—a posture in which its inhabitants, if there were any, would experience extraordinarily hot and cold hyper-seasons, with the midnight Sun seen far into the tropics at midsummer.
In the 1990s Jacques Laskar, one of the astronomers who discovered the role that chaos plays in the seemingly stable solar system, showed that the difference between the Earth’s gentle nodding and Mars’s wild oscillations could be accounted for by the Moon. A constant lunar tug on the Earth’s equatorial bulge—a paunchy distortion of the planet’s sphericity caused by its spin—keeps it sitting up pretty straight. Take the Moon away, and the Earth’s obliquity becomes even less stable than Mars’s, swinging as high as 85°—a planet flat on its back. Having the poles point almost straight at the Sun during summers and almost directly away from it in winters would remove all temperate zones from the planet.
In “Rare Earth”, Brownlee and Ward argue that these sometimes extraordinary obliquities would give a moonless Earth a climate so catastrophe-prone that complex life would be very hard put to flourish. Subsequently, though, the story has been shown to be a bit more complicated than that.
Changes in a planet’s obliquity depend on the gravitational influences of the other planets in the solar system. The slower that planet rotates, up to a point, the more sensitive it is to these chaos-inducing tugs. The Earth and Mars are in somewhat similar orbits and currently have days of very similar length. That is why Laskar found that the axis of a moonless Earth jerks back and forth.
But unlike moonless Mars, which may have had pretty much the same length of day for all its history, the Earth has not. The Moon may be stabilizing the Earth’s obliquity now—but as George Darwin po
inted out, it is also responsible for the Earth having a sufficiently long day for chaotic obliquity changes to be a risk in the first place. If the Earth had started with a ten-hour day and no Moon, it would still have a ten-hour day, more or less, and its obliquity would have been stable all the while.
It is still possible to make a case that complex life is a lot more likely on an Earth-like planet if it has a big moon. David Waltham, a British astrobiologist, suggests in his book “Lucky Planet” (2016) that complex life needs both a pretty stable obliquity and a fairly long day—a combination the Earth would not have were it not for the Moon. On planets with significantly shorter days, he argues, the transfer of heat from equator to poles would be less efficient. The winds and currents responsible for that transfer are diverted from the direct equator-to-pole trajectory that you might expect into the looping swirls of brilliant white seen in “Earthrise” by the Coriolis effect, which swings them to the east or west. The faster a planet spins, the stronger that effect will be—and the harder it will be to get warmth to the poles. Dr Waltham argues that the Moon is just the right size to allow the Earth both a stable obliquity and poles warm enough to keep most ice ages relatively minor. It is a cunning argument, but not a compelling one. It may be that making significant progress on the question of the Moon’s importance for life will have to wait until the presence of complex life is—or is not—discovered by inspecting the earthlight-like light from distant planetary systems.
If complex life can indeed develop on a world without a large moon, a further question arises: what would it have been for humans to play out their history under a unMooned sky? Something, surely, would have been lost: but what? Moonlessness is not in itself an untoward experience; it is, after all, a monthly phenomenon. New Moon and no Moon seem hardly different. But the dynamics of the world would change. Night would be a deeper thing, always dark, unchanging; the sea gentler, its tides low and metronomic, never spring and never neap. Nothing would wax or wane; the kneeling-god-drama of eclipse would be unknown: the seasons would be the only cycles, the constellations, the only permanent features of the sky.
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