5. The Moon: stepping stone to the planets
The Moon's history
The age of the oldest rocks - Moon, Earth and meteorites
The time at which different features on the Moon originated can be determined from rocks returned from them. These relics have remained unaffected by the erosion that removed the primordial record from most terrestrial rocks. The ages of the lunar rocks can be determined by examining unstable radioactive elements and their stable decay products.
The daughter isotopes must be trapped in the rock and not escape or the estimated age will be too short. In fact, the daughters can escape quite easily when the rock is molten; only when it cools and solidifies do the daughters start to accumulate. For this reason, the ages determined for the rocks are really the times since the rock became solid. And if the rock is re-melted, say by the impact of a meteorite, its radioactive clock is reset, and the age will measure the time since the last solidification.
Formation of the highlands and maria
The retrieved lunar rocks have taken us back into time, to the formative stages of the Moon. They record events from the earliest history of the solar system that have been erased on Earth by water, wind and geologic activity. Radioactive dating of Moon rocks and primitive meteorites indicates, for example, that the Moon was assembled a mere 50 million years after the solar system itself was born 4.6 billion years ago.
As the external cratering rate was declining rapidly, internal processes set to work. The radioactive decay of long-lived unstable elements, such as uranium and thorium, produced heat that gradually warmed up the interior. There followed an era of volcanism, lasting for 700 million years, from 3.9 to 3.2 billion years ago. The outer zone of solid rock gradually cooled from the outside in, becoming thicker, and lava worked its way from deeper and deeper in the Moon. The magma flow may have stopped 3.2 billion years ago, since the youngest lunar samples are that old, but some mare basalts could be as young as 1 billion years old.
As molten basaltic rock welled up from the interior, it penetrated the thin crust beneath the great impact basins on near side of the Moon, flooding them with lava and producing the dark circular maria that can be seen today. Successive lava flows set their marks in some maria, showing that they were not formed in a single quick pulse of volcanism, but by repeated outpourings that gradually filled the near-side basins. The lava inundated all craters in its path, wiping the slate clean of previous impacts. preparing a fresh surface to record new impacts which, by this time, had greatly diminished in intensity. Thus the maria are relatively unscarred and most of their craters are small and relatively young.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University