BASALT THICKNESS IN MARE HUMORUM - THE CRATER EXCAVATION METHOD

Basalt thicknesses in mare basins have been determined using assumptio ns about the premare topography of partly buried craters and by compar ison to the Orientale basin. Differences in those assumptions have led to a factor of 4 difference in mare thickness estimates. Further, kno wledge of thickness is restricted to areas in which buried craters are present. Using mixing models applied to multispectral images acquired by the Clementine spacecraft, we have shown that craters in the mare sometimes excavate highland material from below the mare cover. Using such craters and assumptions about their depth of excavation, we obtai n independent estimates of basalt thickness. Our results are in agreem ent with the lowest of previous thickness estimates. We derive a volum e of 40,000 km(3) for basalts in Mare Humorum, considerably less than the similar to 110,000 km(3) from previous estimates. We also confirm a diameter of 425 km for the Humorum multiring basin based on the Clem entine gridded global topography, assuming an original morphology like Orientale.