The Moon could have been derived from a well-mixed disk of rock vapour that was produced after the early Earth collided with another planet. This persuasive idea offers a fresh perspective on the history of both bodies.
“It is the very error of the moon; She comes more nearer earth than she was wont; And makes men mad.” Thus spoke Othello, who was talking of a murderous madness. For 30 years scientists have been gripped, albeit with less deadly consequences, by a maddening paradox to do with the Moon: how near its oxygen-isotope composition is to that of Earth.
It is generally agreed that the Moon formed in a giant impact between Earth and another, smaller body. But previous simulations of that event show that, dynamically, the Moon should be largely derived from the smaller body, in which case its oxygen isotopic composition should be different from that of Earth. Writing in Earth and Planetary Science Letters, Pahlevan and Stevenson1 put forward an explanation for the oxygen-isotope paradox: they propose that the giant impact produced a disk of rock vapour within which the atoms were able to mix before the lunar component condensed out.
The scientific context of this story goes back to the discovery of large variations in the isotopic composition of oxygen in meteorites2, which occurred at about the same time as lunar samples were being returned by Apollo astronauts. These variations are so large that they can readily be used to identify which meteorites come from the same planet or asteroid. Lunar samples were found to have a terrestrial isotopic composition2, which was taken as evidence that the Moon was derived from Earth3. But the evidence from dynamic simulations is that most of the mass of the Moon came from another planet, Theia, that hit the early Earth with a glancing blow4.
The isotopic composition of every element studied so far is the same in lunar samples as it is in samples from Earth, apart from elements that would change because of radioactive decay, or through the effects of cosmic rays or the charged particles of the solar wind. But the background Solar System heterogeneity is far bigger for oxygen than for most other elements2. So, with the development of more precise techniques, a further analysis of lunar samples aimed to resolve an Earth–Moon oxygen-isotope difference that it was thought must surely exist at some finite level5. Far from doing that, however, samples from the two bodies were found to be identical to within better than five parts per million. One explanation was that Theia must have formed from the same mix of material as Earth.
Pahlevan and Stevenson's dynamic simulations1 of Solar System formation build on previous work in assuming that, based on the measured difference between Earth and Mars, an oxygen-isotopic gradient existed within the early planetesimals of the circumstellar disk. This is a reasonable assumption, but might not be easy to test at the required precision until samples are returned from sizeable bodies such as Mercury and Venus. The obvious test at present involves a class of meteorites that were probably derived from the asteroid 4 Vesta. The oxygen-isotope data for these meteorites do not fit with the proposed radial-gradient model. But this can be explained if, as seems likely, Vesta has migrated outwards from the innermost Solar System.
Based on this putative gradient, the new calculations1 show that it is unlikely for two bodies such as Theia and Earth to have had the same bulk isotopic composition. Indeed, it is hard to imagine how they could have formed from the same mix of primordial matter unless they grew at exactly the same distance from the Sun, which in turn raises the question of why it took them so long to collide. Tungsten-isotope measurements indicate that the Moon did not form until about 4.5 billion years ago, more than 40 million years after the birth of the Solar System6.
The energy released when sizeable worlds collide is astonishing by geological standards. The giant impact would almost certainly have melted most of the Earth and produced a magma ocean7. Much of the Earth and Theia may have been vaporized. Pahlevan and Stevenson1 calculate that over the time required for the disk of vapour to cool and condense, the atoms of the silicate portion of Earth — that is, everything but the metal core — would have been vigorously and repeatedly stirred, mixing with the atoms of the disk and eliminating isotopic variations (Fig. 1).
Support for Pahlevan and Stevenson's theory comes from a study8 of silicon isotopic compositions, which differ in meteorites and samples from the silicate Earth. The latter are fractionated to heavier isotopic compositions, and this is thought to have been caused by equilibration with silicon that was incorporated into Earth's core under very high pressures and temperatures. Theia was a small (Mars-sized) planet that should have had a silicon isotopic composition like that of meteorites and Mars. However, the Moon's composition is heavy, like that of the silicate Earth8 — which is hard to explain unless Pahlevan and Stevenson are correct and there was large-scale equilibration.
If they are indeed right, it means that certain features of the Moon's composition reflect those of the early Earth. So, since the first lunar samples were returned, we have, without realizing it, been analysing a unique archive about our own planet — a body that has lost almost all traces of its early development with 4.5 billion years of subsequent bombardment, mantle convection and geological reprocessing.
The new theory raises lots of questions. For example, it is unclear whether all isotopic systems would have been efficiently mixed. Mixing is related to volatilization, so highly refractory elements might still show differences. Exploring this possibility will probably require improvements in mass spectrometry to resolve tiny isotopic differences — unlike oxygen, most elements in the periodic table have an almost identical proportion of isotopes, whether measured on a lump of the Empire State Building or a meteorite from the asteroid belt.
Another issue is that, although the Moon is isotopically identical to Earth, it is chemically different. In particular, it is far more depleted in volatile elements such as alkalis, even though these elements should have mixed especially efficiently. This can be explained if volatiles were somehow lost from the hot disk from which the Moon condensed. But other differences are harder to account for. The iron content of lunar basalt rocks is higher than that of Earth's and more like those of Mars' and Vesta's. This is thought to reflect a higher iron content in the lunar mantle. The lower iron content of Earth's mantle was thought to reflect an early, less-oxidized stage in the growth of the Earth9. However, if the Moon is essentially a sample of Earth at the time of the giant impact, it may indicate that Earth's mantle originally had a higher iron content that was subsequently depleted by formation of the core.
The ramifications of Pahlevan and Stevenson's model1 extend well beyond these issues. Testing and developing this exciting new theory will be priorities over the coming years — the maddening paradox may well be about to generate a new wave of highly creative science.
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Journal of Geophysical Research (2010)