CRATERING ON TITAN AND IMPLICATIONS FOR TITANS ATMOSPHERIC HISTORY

A means for putting limits on the history of Titan's atmosphere is proposed by considering the breakup of bolides during atmospheric entry and the resulting modification of the crater size-frequency distribution at the surface, which will be determined by radar during the Cassini-Huygens mission. Bolides entering the atmosphere experience forces that eventually exceed the body's strength and disruption occurs. The size of the bolide which can reach the surface unfragmented depends on the atmospheric pressure. Given observation of the position of the turndown in crater numbers and relative surface age (the surface age is determined through the crater size-frequency distribution for diameters exceeding the minimum) a rough determination of the evolutionary trend of atmospheric pressure in Titan's history is achievable. This is a key scientific issue since Titan's atmosphere went through changes of atmospheric pressure in the past. We show that (a) Cassini-Huygens instruments are potentially capable of obtaining the necessary observations, and (b) for an atmosphere with the current density, crater numbers are predicted to begin to turn down at roughly 40-20 km for icy cometary bolides and stony bolides, respectively. Results for plausible early denser or thinner atmospheres are measurably different.