The Moons Largest Impact Basin
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The Moon's Largest Impact Basin

November 30, 2012
Stratigraphic relationships show that SPA is the oldest impact basin on the moon, but scientists are intensely interested in just how old it is. Lunar samples suggest that most of the major basins on the moon formed around 3.9 billion years ago in a period called the late heavy bombardment. By this time most of the large debris within the solar system should have already accreted to form the planets, so such a large number of big impacts occurring at nearly the same time may have been due to unusual gravitational dynamics in the early Solar System. Was the impact that caused the SPA basin also a part of some cataclysmic event that occurred 3.9 billion years ago? If so, that impact is strong evidence for an extreme event that would have affected all of the terrestrial planets, including Earth at a time when life was just beginning. If the basin is much older, that may suggest that instead of a spike in the impact rate at 3.9 billion years, the number of impacts simply trailed off from a peak earlier on. How can we find out just how old the SPA impact basin is? The best way would be to sample materials from the interior of the basin and use radiometric age-dating techniques to determine when they were last molten, as heat from the impact would have melted a large volume of material, resetting radiometric clocks. But the basin is so old that its surface has been cratered many times over, meaning that some of the rocks would have had their radiometric ages reset by these subsequent impacts. So it may be difficult to find rocks with ages that truly reflect the SPA event without careful consideration of the local geology. A preliminary monochrome mosaic of the moon from the LROC Wide Angle Camera (WAC), centered in the middle of the South Pole-Aitken basin. Arrow points to the South Pole and an X marks the region of interest within the basin and the location of the NAC detail above [NASA/GSFC/Arizona State University].

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