The Moon after Apollo

The Apollo programme left behind a mountain of data and over a third of a tonne of samples, most of which were carefully documented by the crews as they gathered them. To repeat the publicist's mantra, this treasure trove really did keep scientists 'busy for years' and has formed the bedrock on which theories of planetary formation and evolution have been built. Prior to the space age, planetary science had been in the doldrums, with only blurred photographic evidence to feed the creativity of scientists. With Apollo's scientific harvest, planetary science was taken out of the doldrums into an age when ground truth - actual rocks gathered in situ -could inform new theories and help to sort the wheat from the chaff.

Our current understanding of how the Moon was formed first gained acceptance at a conference of lunar scientists in 1984 on the west coast of Hawaii. This idea, chiefly proposed by William Hartmann and Alistair Cameron, has yet to be toppled. It is a story of birth rising out of incomprehensible violence.

Our solar system formed about 4,600 million years ago out of a coalescing cloud of dust and gas known as the solar nebula. Most material ended up in the Sun, but some formed a disk out of which the planets gradually grew, or accreted - a process whereby gravity causes loose material in space to gradually gather into ever larger bodies. The light pressure and solar wind from the new star tended to push lighter substances out to the further reaches of the system while heavier substances tended to stay in the Sun's vicinity. This created predominantly rocky planets near the Sun, gaseous giants further out, and frozen worlds beyond the point at which even gases become liquid or solid.

About 40 million years after the solar system's birth, two nascent planets were orbiting the new Sun at similar distances and it was only a matter of time before they met. The larger body, our proto-Earth, received an off-centre impact by a body half its diameter in a cataclysm that defies the imagination. The iron cores of the two worlds merged but a large amount of mantle material had been thrown into a giant cloud of debris surrounding the new Earth.

In a relatively short time, some accounts suggest within only a year, this ejected material had itself coalesced to form a new, smaller world - the Moon. As it did so, the huge energy of its fast accretion melted its outer layer to form an ocean of molten rock, or magma, that lasted long enough for its components to fractionate - like a salad dressing that has been left too long in a cupboard. As the lighter components rose to the top, they cooled and crystallised to form the rocks of the Moon's new crust. They were typically light-coloured and rich in aluminium. Below the crust, in the Moon's mantle, the rocks were heavier and richer in iron. The regions that were last to solidify gathered up those elements that had difficulty fitting into the crystal lattice, leading to them being described as KREEPy.

The solar system was still a mass of debris for the first 800 million years of its existence and large impacts were commonplace on all the planets. The Moon retains the scars of this early bombardment all over its lighter-toned surface where large craters abound, often overlapping one another. During this time, it sustained a particularly large collision when an object gouged out the South Pole-Aitken Basin, a 2,500-kilometre depression across the Moon's far side. About 4 billion years ago, the impact of large objects seems to have peaked before tailing off suddenly. The dark patches we now see on the Moon's near side were mostly formed within large circular basins that were formed by these giant impact events. Of particular interest to the lunar science community was the Imbrium Basin, which was dated to 3.91 billion years ago from Apollo samples. As noted, as this basin was formed, rock from deep within the Moon that had the KREEP characteristic was excavated.

About half a billion years later, prodigious quantities of lava, rich in iron and magnesium, were expressed through the fractured crust. It filled the basins and other low-lying areas to form enormous smooth basalt plains to which we applied romantic names like Mare Tranquillitatis, Mare Serenitatis and Oceanus Procel-larum. The last gasps of this activity probably died out 'only' about a billion years ago but most of it has been dated to around 3.3 billion years ago. Since then, little has changed on the Moon. Every few tens of millions of years, there is a very large impact that produces a spectacular fresh crater and sprays the landscape with a new layer of rubble and dust. Apart from that, only a slow but incessant barrage of hypervelocity grains of dust, and the occasional larger object, sandblasts the top layer of the surface. Throughout the eons, the topography is eventually rounded off and the landscape is draped with a blanket of ground-up soil.

As our probes have extended our reach into the depths of the solar system, their new data serves to elaborate on the knowledge gleaned a generation ago when men explored a new world and could select samples quickly and intelligently.

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