From the depths of the Earth
The planet Theia had, like the Earth, formed early, from the mass of dust and rock-melt droplets of the accretionary disc. Theia is calculated to have been about the size of Mars, yet it was to have nothing like that planet’s longevity. Its orbit was close enough to that of Earth for collision to be certain, sooner or later. The two planets came together at something of the order of 40,000 kilometres per hour. Theia lost its separate identity over a few tumultuous minutes, and the Earth was smashed, like a grapefruit hit hard with a hammer. In that conflagration the material of the two planets, having instantly converted into boiling magma and vapour, simply merged. Theia’s core sank to join the Earth’s. Some of the outer layer of both planets splashed out into a cloud of plasma that encircled the suddenly re-formed Earth, and that cloud condensed to form a new companion to our planet—the Moon. It is a fine story, this, of the Moon’s creation through a spectacular planetary collision. It is likely true, too: though it is not certainly so, simply being the hypothesis that now best explains the character of our Earth and its satellite. Like many such stories in science, it is currently the one that best fits the evidence. It has been calculated, on the basis of these two bodies’ mass, momentum and orbit, that it would have been extraordinarily difficult for the Earth to have captured intact a stray planetary body of this size. However, holding onto a mass of ejecta flung out by impact, kept in balance by the twin, opposing forces of gravity and centrifugal force, is a more plausible means of having formed the Moon. More, this concept explains the remarkable isotopic similarity of these two bodies, generated by the intense mixing associated with impact. Mars, by contrast, has very different proportions of, say, the different isotopes of oxygen, because it formed in a different part of the Solar System, where the atoms of the original accretionary disk had been shuffled into different combinations.