Your query was: herrick

HR: 1340h
AN: P33C-0261
TI: Hypothesis for the possible absence of mid-sized craters on Titan
AU: * Herrick, R R
EM: rherrick@gi.alaska.edu
AF: Geophysical Institute, University of Alaska Fairbanks, 903 Koyukuk Dr, Fairbanks, AK 99775-7320 United States
AB: Extant imagery of Titan has revealed two well-preserved impact craters estimated at 80 and 440 km in diameter, and there are no obvious smaller craters. Atmospheric filtering cannot account for a lack of craters tens of kilometers in diameter. It is difficult to imagine an erosional process that completely wipes out mid-sized craters but nicely preserves large ones. It would be very fortuitous if the 440-km crater was both young and located within the small percentage of Titan covered by high-resolution imagery. I suggest instead that this apparently unusual cratering record is the result of Titan having a thin, extremely weak layer overlying a "normal" ice or ice-rock layer. This thin layer behaves as a fluid under impact conditions so that the effect is like a terrestrial impact into shallow water. No impact structure is preserved until the structure is large enough to uplift an impermeable rim of the lower-layer material that rises above the level of the initial ground level. As crater diameter increases the weak layer becomes proportionally less significant, so that the largest impact structures look similar to their counterparts on the terrestrial planets. In this scenario Titan could be tectonically and cryovolcanically nearly inactive but with erosional processes occurring within this thin active layer. The smallest preserved craters will have little more than a rim and a flat floor, but larger craters will have well-developed ring structures. The size-frequency distribution may have a constant exponential dependence at larger diameters with an abrupt lower cutoff. The lower size range of craters may have a depth-diameter ratio that actually increases with crater diameter, as larger craters are more able to maintain a floor depth below the hydrostatic level. Our current understanding of possible near-surface materials on Titan does not suggest any potential candidates for a thin surface layer that is slightly colder than its melting point. However, some pre-Cassini research suggests the possible presence of a near-surface regolith saturated with liquid methane or ethane. Perhaps such a layer acts as a mud-like slurry that is able to sustain modest surface relief and geologic structures under normal conditions and becomes completely strengthless under impact conditions. Scaling arguments suggest that the saturated regolith layer could be just a few hundred meters thick to prevent craters smaller than several tens of kilometers in diameter from leaving a permanent structure.
DE: 5420 Impact phenomena, cratering (6022, 8136)
DE: 5470 Surface materials and properties
DE: 6281 Titan
SC: Planetary Sciences [P]
MN: Fall Meeting 2005