Lunar Geological Historical Development
The Moon was formed when a Mars-sized body impacted the primordial
Earth. Ejecta from the impact coalesced in orbit and grew into the Moon.
Little terrestrial core material was ejected in the impact, which led to
the high quantities of light elements on the Moon.
Early in the Moon's life, radioactivity, residual heat from the solar
system's formation, solar radiation, and meteorite bombardment melted the
Moon's surface layer to depths in excess of 90 km and in these melted
surfaces of the Moon the different elements separated. The heavier elements
sank to the bottom of the Moon's surface and the lighter ones stayed on the
surface of these melted areas. More than 4 billion
years ago a huge bombardement of meteors created the Mare basins such as
Mare Orientale and Mare Imbrium.
3.1 to 3.8 billion years ago crater escavation and radioisotope decay
produced lava flows from the Moon's interior that filled in the larger
craters and produced superimposed lava, the regions referred to as mares.
The was made possible in part by the much thinner crust on the nearside,
which was breached by the mainly meteoric activity. This period resurfaced
the Mare basins, and changed the look of the Moon dramatically. It also
brought a different group of minerals to the Moon's surface, increasing the
diversity of resources available.
Over the last 3 billion years, since the nearside's lava resurfacing
ended, the Moon's surface has been pitted with craters, which is how we
find it today. Although the level of bombardment does not compare with
the early periods of the Moon's development (meteor populations have
declined since them), there has still been enough activity to return the
Moon to its earlier state of cratered chaos.
-- Marvin Ostrega
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