Geologic Processes
Section M.4.
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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

Geologic Processes

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