THE MOON AFTER APOLLO
The Apollo programme left behind an archive of data and over a third of a tonne of samples, which, to repeat the publicist’s mantra, really did keep scientists "busy for years” and they have formed the bedrock on which theories of planetary formation and evolution have been built. Prior to the space age, planetary science was in the doldrums, with only blurred photographic evidence to feed scientific curiosity. With Apollo’s scientific harvest, planetary science entered an age in which 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 eonferenee in Hawaii in 1984. 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 remain 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 tremendous cataclysm. The iron cores of the two worlds merged and a large amount of mantle material was ejected to form a giant cloud of debris around what was now Earth.
In a relatively short Lime, 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 to fractionate – like a salad dressing that has been left in a cupboard for too long. As the lighter components rose to the Lop. they cooled and crystallised to form a solid crust. They were typically light-coloured and rich in aluminium. Below the crust, in the 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 bombardment in the form of large craters, often overlapping one another, all over its lighter-toned surface. During this time, it sustained a particularly large collision when an object gouged out the South Pole-Aitken Basin, a 2.500- kilometre depression on the Moon’s far side. About four billion years ago, the
impact of large objects seems to have peaked before tailing off suddenly. The dark patches wc now see on the Moon’s near side were mostly formed within huge circular basins that were formed by the largest of these 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. Scars from its formation can be traced across much of the near side and therefore its age provides an important benchmark for the relative ages of other superimposed features. As noted, the formation of this basin excavated rock from deep within the Moon that had KREEPy characteristics.
About half a billion years later, prodigious quantities of lava, rich in iron and magnesium, were erupted 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 1 ranquilliiatis, Mare Serenitatis and Oeeanus Proccllar – um. The last gasps of this activity probably died out ‘only’ about a billion years ago but its peak was around 3.3 billion years ago. Since then, little has changed on the Moon. The material from the bottom of the Apollo 15 deep core had lain undisturbed for 500 million years. 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, the occasional large object and a slow but incessant barrage of hypervelocity grains of dust sandblasts the top layer of the surface. Across the eons, the topography becomes rounded off and the landscape is draped with a thickening blanket of ground-up rock, the lunar regolith.
This is the kind of profound knowledge produced by careful, focused exploration. As later generations of probes extended our reach into the depths of the solar system, their new data has elaborated on the story of planetary genesis gleaned by men who explored a new w orld in person and applied the power of human intelligence.