TITANS SURFACE REVIEWED - THE NATURE OF BRIGHT AND DARK TERRAIN

A number of recent observational results concerning Titan's surface ar e synthesized in order to constrain possible surface physical states a nd compositions. With the remote sensing data as a guide, the surface is divided into two types of terrain, ''bright'' (extending from 60-16 0deg Titan longitude) and ''dark'' (encompassing the rest of the objec t). Although substantial discrepancies exist in published near-infrare d albedo estimates, which may be at least in part explained by Titan's aspect and seasonally-varying haze the bright region has a surface al bedo consistently 0.1 higher than the dark region. Red (673 nm) albedo s of 0.45 and 0.35 are deduced for the bright and dark regions. Recent ly-revised radar data are reviewed, which show Titan to be very differ ent from the Galilean satellites, in emissivity, backscatter cross sec tion, and polarization. The data rule out ''clean'' water ice, and thi ck and widespread organic deposits. Titan's terrains exhibit radar pro perties resembling silicate-rich surfaces, with the ''bright'' terrain more ''icy'', but other surfaces are possible, and we note the likeli hood of small-scale surface heterogeneity. Evidence for a bright north pole is also reviewed, and it is speculated ,that tbe poles may have more surface liquids than lower latitudes. Although remote sensing dat a sets will increase in quantity and quality over the coming five year s, it is argued that a final resolution of the nature of Titan's surfa ce must await the completion of the Cassini/Huygens mission, set to ar rive at Saturn in 2004. (C) 1997 Elsevier Science Ltd.