Titan's Surface Observed With the Cassini RADAR Scatterometer

Details Titan's Surface Observed With the Cassini RADAR Scatterometer Titan's Surface Observed With the Cassini RADAR Scatterometer

May 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agusm.p13a..03w&link_type=abstract

American Geophysical Union, Spring Meeting 2005, abstract #P13A-03

Physics

5410 Composition, 5464 Remote Sensing, 5470 Surface Materials And Properties, 5494 Instruments And Techniques

Scientific paper

The Cassini Titan RADAR, a multibeam Ku-band (13.78 Ghz, λ = 2.17 cm) linearly polarized radar instrument [1], includes an active, real-aperture scatterometer mode in which the central antenna beam is used to measure regional-scale backscatter across large areas of a target's surface. Raster scans permit the study of backscatter variation over diverse incidence angles, helping to constrain surface structure and composition. During the first targeted Titan fly-by (Ta) on October 26, 2004, the scatterometer observed Titan with resolutions around 100 km and incidence angles ranging from 0° to 60°, covering a total of about 107 km2. Backscattered power includes specular and diffuse components, and varies strongly over the surface. Initial modeling of the collected Ta inbound backscatter measurements and, separately, the collected Ta outbound measurements show an average angular dependence that can be described by the sum of a specular Hagfors Law and a diffuse Cosine Law. The best-fit model for the specular term implies rms slopes of a few degrees and bulk dielectric constants between 1 and 2, consistent with those inferred from radiometer measurements. The model for the diffuse component gives estimates of 0.7 (inbound) and 0.4 (outbound) for Titan's disc-integrated albedo in the same-linear polarization (SL). The Ta inbound pass, in particular, shows at least two populations of scatterers with varying reflectivity, both of which are highly correlated with near-IR reflectivity [3]. The albedo estimates are much larger than expected given the Earth-based 13 cm-λ Arecibo [2] and 3.5 cm-λ Goldstone results [4]; however, calibration of the instrument is still somewhat uncertain. Data from the upcoming T3 and T4 Titan fly-bys should be collected by the time of this meeting. With a specular component in Cassini's 2.2-cm echoes and also in Arecibo's 13-cm echoes [2], Titan's radar signature is unique among icy solar system bodies. It requires the existence of extended surface regions that are nearly flat at centimeter scales, suggesting that some smooth terrain is present. The high radar backscatter at larger incidence angles and the observed negative correlation of radar cross-sections and microwave brightness temperatures imply that volume scattering, rather than surface slope or roughness, dominates the echo power. Thus, the surface likely contains a layer of material that is highly transparent at Ku-band. The near-surface structure and composition must be heterogeneous to produce the large backscatter variations observed. References: [1] Elachi C. et al. (2005). Science (submitted); [2] Campbell D. et al. (2003). Science, 302, 241; [3] Smith P. H. et al. (1996). Icarus, 119, 336-349. [4] Muhleman D. O. et al. (1995). Annu. Rev. Earth Planet. Sci. 23, 337-374.

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