Titan’s surface at 2.2-cm wavelength imaged by the Cassini RADAR radiometer: Calibration and first results

M.A. Janssen a,∗, R.D. Lorenzb, R. Westa, F. Paganelli c, R.M. Lopesa, R.L. Kirkd, C. Elachia, S.D. Walla,
W.T.K. Johnson a, Y. Anderson a, R.A. Boehmera, P. Callahan a, Y. Gima, G.A. Hamiltona, K.D. Kelleher a, L. Rotha, B. Stiles a, A. Le Gall a, the Cassini Radar Team
a Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
b Space Department, Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
c European Center of Geodynamics and Seismology, L-7256 Walferdange, Luxembourg
d U.S. Geological Survey, Flagstaff, AZ 86001, USA

a r t i c l e i n f o a b s t r a c t

Article history:

Received 10 April 2008
Revised 4 October 2008
Accepted 18 October 2008
Available online 25 November 2008

Satellites, surfaces
Satellites, composition
Radio observations

The first comprehensive calibration and mapping of the thermal microwave emission from Titan’s surface
is reported based on radiometric data obtained at 2.2-cm wavelength by the passive radiometer included
in the Cassini Radar instrument. The data reported were accumulated from 69 separate observational
segments in Titan passes from Ta (October 2004) through T30 (May 2007) and include emission from
94% of Titan’s surface. They are diverse in the key observing parameters of emission angle, polarization,
and spatial resolution, and their reduction into calibrated global mosaic maps involved several steps.
Analysis of the polarimetry obtained at low to moderate resolution (50+ km) enabled integration of
the radiometry into a single mosaic of the equivalent brightness temperature at normal incidence with
a relative precision of about 1 K. The Huygens probe measurement of Titan’s surface temperature and
radiometry obtained on Titan’s dune fields allowed us to infer an absolute calibration estimated to be
accurate to a level approaching 1 K. The results provide evidence for a surface that is complex and
varied on large scales. The radiometry primarily constrains physical properties of the surface, where we
see strong evidence for subsurface (volume) scattering as a dominant mechanism that determines the
emissivity, with the possibility of a fluffy or graded-density surface layer in many regions. The results are
consistent with, but not necessarily definitive of a surface composition resulting from the slow deposition
and processing of organic compounds from the atmosphere.

© 2008 Elsevier Inc. All rights reserved.