Thursday, May 12, 2005

Titan Science Magazine Special Issue

The Titan Science Special Issue is now out. I have read through the RADAR paper, and have posted an admittedly lengthy summary below. This is, after all, the stuff I am most interested in, so naturally I feel every thing written is important. Once I get to the RPWS results, I might just post one or two comments ;) BTW, I have not read the other papers. I intend to do that during Survivor tonight and will post a summary tomorrow.
  • C. Elachi et al. Cassini Radar Views the Surface of Titan. Science 308, pg. 970-974.
    • Not dissimilar from the LPSC abstracts presented earlier this year
    • Only covers results from the Ta RADAR SAR swath (no results from T3)
    • Don't forget to check out the Supplementary Online Material for this paper. Within the document is a 128 pixel/degree version of the Ta Sar strip. This document also contains maps of backscatter cross-section and brightness temperature variations.
    • Titan's surface appears to be youthful thanks to its relative lack of impact craters and fairly heterogeneous
    • Five surface units observed based on brightness, morphology, and texture
      • Dark Homogeneous unit
        • Regional in scale
        • Covers one-fourth of the Ta Sar Swath
        • Relatively low backscatter
        • Patchy in SAR swath
      • Mottled Unit
        • Regional in scale
        • Covers much of the rest of the Ta Swath
        • Higher backscatter
        • Gradiation at boundaries with Homogeneous unit
      • Bright Lobate unit
        • Local scale unit
        • Variable brightness
        • Gradational and sharp boundaries with both digitate and sheet examples
        • Sometimes somewhat circular
      • Bright, lineated unit
        • Local scale unit
        • Seen east of Ganesa Macula
        • Sharp borders
        • Lineations within, spaced 1 to 2 kilometers apart
        • Maybe akin to the grooved terrain of Ganymede
      • Dark, patchy unit
        • Local scale unit
        • Halloween Cat an example
        • dark patches tens of km across
        • Connected by narrow dark features
        • Topographically controlled?
    • Large, circular feature 180 km across, named Ganesa Macula, could either be a "pancake dome" or relaxed crater
      • 20-km wide central pit, maybe a caldera
      • Channels radiate to the south from the central pit
      • Lobate features along eastern edge might be flows
    • Backscatter cross-section data, from the scatterometry mode, shows several distinct surface units that seem to correlate with albedo in ISS images
      • "Raw" backscatter cross-section data shows the highest backscatter in the central portion of Xanadu, eastern "Sumatra", and near the 80-km wide crater (from SOM)
      • The southwestern portion of Xanadu shows the largest difference between radar backscatter and near-IR albedo, with moderate back scatter even for areas that are brightest in ISS and VIMS
    • Radiometry data shows disk-averaged brightness temperatures of 85-86K. This yields a surface emissivity of 0.9 based on an actual surface temperature of 94K
      • Dielectric constant between 2.1 in the northern mid-latitudes and 1.6 for the southern mid-latitudes
      • Consistent with hydrocarbons or porous ice
      • Disk-resolved observations show significant brightness temperature variations (+/- 10K) with a rough anti-correlation with near-IR albedo and backscatter; due to emissivity variations
        • In other words, darker, smoother material is warmer radiometrically than brighter, rougher material
    • RADAR altimetry and radarclinometry (shape-from-shading) results show that for the small region of Titan looks at in Ta, very little topographic variations is seen, with only 150 meters of variation in the altimetry data over 400 km. Radarclinometry results indicate a few topographic features, such as the small 15 km wide hills seen at the end of the swath. This hills appear to be 100 m tall. A flow-like features was found to be 250 meters tall. A larger flow-like features was found to have a height of 1 km, considered to be too large by the RADAR team. This results may have been hampered by gradations in texture on the surface of the flow.
    • No obvious impact structures found on Ta. Cryovolcanism is the favored method of resurfacing by the RADAR team
    • Based on comparisons between Cassini RADAR and Arecibo data, the largest contributor is bright SAR and scatterometry returns is volume scattering. Volume scattering occurs when a radar beams is scattered multiple times with a material, in this case presumably ice, before returning to the antenna. Pure water ice and other tranparent materials tend to be better volume scatters than ice with impurities like ammonia. So brightness variations in scatterometry and SAR data maybe due to compositional variations, where pure water ice would appear bright not only because of a higher dielectric constant but would appear enhanced due to volume scattering.
    • Alternatively, porous polyacetylene would appear optically bright and radar bright (due to its semiconductive properties) when floating on a lake of methane


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