The Spectral Nature of Titan's Major Geomorphological Units: Constraints on Surface Composition

Thèmes de recherche:
Année:
2018
Auteurs:
  • A. Solomonidou
  • A. Coustenis
  • R. M. C. Lopes
  • M. J. Malaska
  • S. Rodriguez
  • P. Drossart
  • C. Elachi
  • B. Schmitt
  • S. Philippe
  • M. Janssen
  • M. Hirtzig
  • S. Wall
  • Christophe Sotin
  • K. Lawrence
  • N. Altobelli
  • E. Bratsolis
  • J. Radebaugh
  • K. Stephan
  • R. H. Brown
  • Stéphane Le Mouélic
  • A. Le Gall
  • E. V. Villanueva
  • J. F. Brossier
  • A. A. Bloom
  • O. Witasse
  • C. Matsoukas
  • A. Schoenfeld
Journal:
JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS
Volume:
123
Numéro:
2
Pages:
489-507
Mois:
Février
ISSN:
2169-9097
Résumé:
We investigate Titan's low-latitude and midlatitude surface using
spectro-imaging near-infrared data from Cassini/Visual and Infrared
Mapping Spectrometer. We use a radiative transfer code to first evaluate
atmospheric contributions and then extract the haze and the surface
albedo values of major geomorphological units identified in Cassini
Synthetic Aperture Radar data, which exhibit quite similar spectral
response to the Visual and Infrared Mapping Spectrometer data. We have
identified three main categories of albedo values and spectral shapes,
indicating significant differences in the composition among the various
areas. We compare with linear mixtures of three components (water ice,
tholin-like, and a dark material) at different grain sizes. Due to the
limited spectral information available, we use a simplified model, with
which we find that each albedo category of regions of interest can be
approximately fitted with simulations composed essentially by one of the
three surface candidates. Our fits of the data are overall successful,
except in some cases at 0.94, 2.03, and 2.79m, indicative of the
limitations of our simplistic compositional model and the need for
additional components to reproduce Titan's complex surface. Our results
show a latitudinal dependence of Titan's surface composition, with water
ice being the major constituent at latitudes beyond 30 degrees N and 30
degrees S, while Titan's equatorial region appears to be dominated
partly by a tholin-like or by a very dark unknown material. The albedo
differences and similarities among the various geomorphological units
give insights on the geological processes affecting Titan's surface and,
by implication, its interior. We discuss our results in terms of origin
and evolution theories.
Plain Language Summary Titan, Saturn's moon, has been investigated by
the Cassini mission for almost 13 years, unveiling an exotic world with
many features similar to Earth. One of the mysteries that still has not
been resolved even after that many years of exploration is the nature of
its surface composition. Titan is a very complex world with
multivariable geology and a very thick and hazy atmosphere that shields
the surface from remote sensing observations, prohibiting direct
evaluation of its composition. In our study we analyze spectro-imaging
data from the Cassini visual and infrared spectrometer. We first infer
the atmospheric contribution and then extract true surface properties.
We study major geomorphological regions on Titan, which include among
other mountains, plains, craters, and dunes. We derive their surface
albedo values and shapes that reveal the brightness of the surface and
compare them with materials that we expect to find on Titan's surface,
such as water ice, tholins (atmospheric products), and a very dark
unknown component. The results from this analysis show that Titan
presents a pattern in its surface composition distribution with its
equator being dominated by organic materials from the atmosphere and a
very dark unknown material, while higher latitudes contain more water
ice.