Titanʹs surface from reconciled Cassini microwave reflectivity and emissivity observations

The surface of Titan, Saturnʹs largest moon, appears to consist of solid hydrocarbons or CO2 with considerable small-scale structure, according to a new electromagnetic scattering model that uses both Cassini radar reflectivity and emissivity measurements. Our model resolves a well-known ambiguity in active and passive observations of surfaces, namely that the dielectric constant retrieved from radiometric data is usually less than that obtained from radar backscatter analysis. We present here parameters of Titan derived from radar and radiometric responses using a single set of surface physical descriptors. The observed dielectric constant ranges from 1.75–2.5, consistent with the solids above, but also with unconsolidated water or water/ammonia ice. Inferred rms slopes of the surface range from 10°–16° at Cassiniʹs 2.2 cm-λ, with radar albedos of about 30%. Emissivities of the surface are quite high, on the order of 90%. We find that the majority of the backscattered radar energy results from volume, rather than surface, scattering processes. Thus large-scale variations in radar brightness are mainly diagnostic of composition and structure within Titanʹs near subsurface. The high level of volume scatter (about 15% of incident energy) and consequent high radar albedo suggest significant wavelength-scale structure and multiple scatter within the medium. This is most consistent with a surface composed either of ubiquitous mm-size bubbles within the “ice,” analogous to a vesicular cryolava, or structures similar to ice pipes and lenses found in terrestrial ice sheets. The cryovolcanic hypothesis suggests planet-wide resurfacing by active volcanism to explain the large-scale uniformity of observed backscatter. If the lens/pipe mechanism pertains, it can imply percolation of precipitating liquid or surface melt through the Titan “firn.”