Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p44a..03w&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #P44A-03
Astronomy and Astrophysics
Astronomy
5410 Composition (1060, 3672), 5464 Remote Sensing, 5470 Surface Materials And Properties, 6280 Saturnian Satellites, 6949 Radar Astronomy
Scientific paper
The Cassini Titan Radar Mapper has played a significant role in investigating the surface of Titan. Each of the RADAR's four operational modes individually contributes essential information about an area of coverage. When taken together, the different radar data types reveal a more complete story, providing complementary information on the physical and electrical properties of the surface. This paper studies overlapping data collected by the RADAR's Scatterometer and Radiometer over the seven close Titan flybys to-date (Ta-Inbound and Ta-Outbound, October 2004; T3, February 2005; T4, March 2005; T7, September 2005; T8-Inbound and T8-Outbound, October 2005) and attempts to reconcile the implications of the two. The scatterometer uses the central antenna beam at 13.78 GHz (2.17 cm-λ) in its active, real-aperture mode to produce regional-scale backscatter images across large areas of the surface. While the instrument awaits the same linear (SL) polarization echoes, the Radiometer is passively listening, measuring the brightness of thermal emission at the same polarization from the same footprint area on Titan's surface. For each observation, we separately model the backscatter and brightness temperature measurements, and form surface maps from the residuals after correcting for angular effects. We compare these relative surface maps side-by-side and find that most observed features are common to both and are anti-correlated (i.e. radar bright regions are radiometrically cold, radar dark regions are radiometrically warm). To model the observed backscatter, we fit the data to traditional scattering models: a Hagfors' or Gaussian specular term plus a cosine term that accounts for the diffuse, volume scattering within Titan's surface. The best fit model suggests values for the dielectric constant and rms surface slope of the observed area. In modeling the observed brightness temperatures, we assume Titan to be a perfectly emitting blackbody sphere covered with a uniform layer of unknown material and use Fresnel's equations to calculate the power transmitted through the layer and emitted at the surface. We then determine the dielectric constant whose model best-fits the data. The results from the backscatter and emissivity models appear to be in agreement, suggesting dielectric constants that vary between 1 and 2 over the areas observed. Over the course of the six scatterometer observations, there have been several instances of overlap; the Ta-Outbound and T3 passes intersected in a region bounded by -35° to +50° N and 0° to 10° W, and Ta-Inbound and T8-Inbound will intersect in a region bounded by -17° to 0° N and 100° to 225° W. By observing overlapping regions with varying angular and polarization coverage, we are able to further constrain inferences made about surface structure and composition.
Cassini RADAR Team
Edmonds Yvonne
Janssen Michael A.
Lorenz Ralph D.
Ostro Steven J.
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