Titanʹs surface reviewed: the nature of bright and dark terrain

A number of recent observational results concerning Titanʹs surface are synthesized in order to constrain possible surface physical states and compositions. With the remote sensing data as a guide, the surface is divided into two types of terrain, “bright” (extending from 60–160 deg Titan longitude) and “dark” (encompassing the rest of the object). Although substantial discrepancies exist in published near-infrared albedo estimates, which may be at least in part explained by Titanʹs aspect and seasonally-varying haze, the bright region has a surface albedo consistently 0.1 higher than the dark region. Red (673 nm) albedos of 0.45 and 0.35 are deduced for the bright and dark regions. Recently-revised radar data are reviewed, which show Titan to be very different from the Galilean satellites, in emissivity, backscatter cross section, and polarization. The data rule out “clean” water ice, and thick and widespread organic deposits. Titanʹs terrains exhibit radar properties resembling silicate-rich surfaces, with the “bright” terrain more “icy”, but other surfaces are possible, and we note the likelihood of small-scale surface heterogeneity. Evidence for a bright north pole is also reviewed, and it is speculated that the poles may have more surface liquids than lower latitudes. Although remote sensing data sets will increase in quantity and quality over the coming five years, it is argued that a final resolution of the nature of Titanʹs surface must await the completion of the Cassini/Huygens mission, set to arrive at Saturn in 2004.