ANCIENT MULTIRING BASINS ON THE MOON REVEALED BY CLEMENTINE LASER ALTIMETRY

Analysis of laser altimetry data from Clementine has confirmed and ext ended our knowledge of nearly obliterated multiring basins on the moon . These basins were formed during the early bombardment phase of lunar history, have been filled to varying degrees by mare lavas and region al ejecta blankets, and have been degraded by the superposition of lar ge impact craters. The Mendel-Rydberg Basin, a degraded three-ring fea ture over 600 kilometers in diameter on the lunar western limb, is abo ut 6 kilometers deep from rim to floor, only slightly less deep than t he nearby younger and much better preserved Orientale Basin (8 kilomet ers deep). The South Pole-Aitken Basin, the oldest discernible impact feature on the moon, is revealed as a basin 2500 kilometers in diamete r with an average depth of more than 13 kilometers, rim crest to floor . This feature is the largest, deepest impact crater yet discovered in the solar system. Several additional depressions seen in the data may represent previously unmapped ancient impact basins.