Dark and Bright Albedo Changes in the Wake of a Titan Rainstorm

Abstract

Turtle et al. (2011) previously announced large-scale surface changes in Titan's tropics following a 2010 September cloudburst event. Those changes were areas that had darkened, and the darkening was attributed to surface wetting by rain. Here we will discuss the results of continued monitoring of the darkened areas by Cassini VIMS and ISS. These new observations show that instead of reverting to their previous state, the rain-darkened areas instead brightened beyond their original albedos starting a few months after the cloudburst event. The brightening was unexpected, and spectra show that it occurs in each of Titan's atmospheric wavelength windows. The brightened spectra show some similarity to he heretofore unique signature of Xanadu. The areas slowly revert to their original spectra over a period of a year. The two hypotheses that we have not eliminated involve (1) volatile frosts resulting from evaporative cooling of rain-derived surface methane that later sublime, (2) deposition of a thin surface layer of very-fine-grained particles (similar to terrestrial playa lakes) that degrade due to aeolean erosion. Future calculations and observations will serve to constrain the mechanism that drives the brightening.