Anomalous radar backscatter from Titan’s surface?

M.A. Janssen a,⇑, A. Le Gall a, L.C. Wyeb
a Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
b Stanford University, Electrical Engineering Department, Stanford, CA 94305, USA

Article history:
Received 21 April 2010
Revised 11 November 2010
Accepted 14 November 2010
Available online 26 November 2010

Satellites, Surfaces
Radio observations
Radar observations

a b s t r a c t

Since Cassini arrived at Saturn in 2004, its moon Titan has been thoroughly mapped by the RADAR instrument
at 2-cm wavelength, in both active and passive modes. Some regions on Titan, including Xanadu
and various bright hummocky bright terrains, contain surfaces that are among the most radar-bright
encountered in the Solar System. This high brightness has been generally attributed to volume scattering
processes in the inhomogeneous, low-loss medium expected for a cold, icy satellite surface. We can test
this assumption now that the emissivity has been obtained from the concurrent radiometric measurements
for nearly all the surface, with unprecedented accuracy (Janssen et al., and the Cassini RADAR
Team [2009]. Icarus 200, 222–239). Kirchhoff’s law of thermal radiation relates the radar and radiometric
properties in a way that has never been fully exploited. In this paper we examine here how this law may
be applied in this case to better understand the nature of Titan’s radar-bright regions. We develop a quantitative
model that, when compared to the observational data, allows us to conclude that either the
reflective characteristics of the putative volume scattering subsurface must be highly constrained, or,
more likely, organized structure on or in the surface is present that enhances the backscatter.
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