Correlations between Cassini VIMS spectra and RADAR SAR images: Implications for Titan's surface composition and the character of the Huygens probe landing site

Research areas:
  • Laurence A. Soderblom
  • Randolph L. Kirk
  • Jonathan I. Lunine
  • Jeffrey A. Anderson
  • Kevin H. Baines
  • Jason W. Barnes
  • Janet M. Barrett
  • Robert H. Brown
  • Bonnie J. Buratti
  • Roger N. Clark
  • Dale P. Crulkshank
  • Charles Elachi
  • Michael A. Janssen
  • Ralf Jaumann
  • Erich Karkoschka
  • Stéphane Le Mouélic
  • Rosaly M. Lopes
  • Ralph D. Lorenz
  • Thomas B. McCord
  • Philip D. Nicholson
  • Jani Radebaugh
  • Bashar Rizk
  • Christophe Sotin
  • Ellen R. Stofan
  • Tracie L. Sucharski
  • Martin G. Tomasko
  • Stephen D. Wall
Titan's vast equatorial fields of RADAR-dark longitudinal dunes seen in
Cassini RADAR synthetic aperture images correlate with one of two dark
surface units discriminated as ``brown{''} and ``blue{''} in Visible and
Infrared Mapping Spectrometer (VIMS) color composites of
short-wavelength infrared spectral cubes (RGB as 2.0, 1.6, 1.3 mu m). In
such composites bluer materials exhibit higher reflectance at 1.3 pm and
lower at 1.6 and 2.0 pm. The dark brown unit is highly correlated with
the RADAR-dark dunes. The dark brown unit shows less evidence of water
ice suggesting that the saltating grains of the dunes are largely
composed of hydrocarbons and/or nitriles. In general, the bright units
also show less evidence of absorption due to water ice and are inferred
to consist of deposits of bright fine precipitating tholin aerosol dust.
Some set of chemical/mechanical processes may be converting the bright
fine-grained aerosol deposits into the dark saltating hydrocarbon and/or
nitrile grains. Alternatively the dark dune materials may be derived
from a different type of air aerosol photochemical product than are the
bright materials. In our model, both the bright aerosol and dark
hydrocarbon dune deposits mantle the VIMS dark blue water ice-rich
substrate. We postulate that the bright mantles are effectively
invisible (transparent) in RADAR synthetic aperture. radar (SAR) images
leading to lack of correlation in the RADAR images with optically bright
mantling units. RADAR images mostly show only dark dunes and the water
ice substrate that varies in roughness, fracturing, and porosity. If the
rate of deposition of bright aerosol is 0.001-0.01 mu m/yr, the surface
would be coated (to optical instruments) in hundreds-to-thousands of
years unless cleansing processes are active. The dark dunes must-be
mobile on this very short timescale to prevent the accumulation of
bright coatings. Huygens landed in a region of the VIMS bright and dark
blue materials and about 30 km south of the nearest occurrence of dunes
visible in the RADAR SAR images. Fluvial/pluvial processes, every few
centuries or millennia, must be cleansing the dark floors of the incised
channels and scouring the dark plains at the Huygens landing site both
imaged by Descent Imager/Spectral Radiometer (DISR). (c) 2007 Elsevier
Ltd. All rights reserved.