Physics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.p42b..08c&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #P42B-08
Physics
0933 Remote Sensing, 5464 Remote Sensing, 5470 Surface Materials And Properties
Scientific paper
The Arecibo 12.6 cm radar system was used to obtain echo spectra for Titan in late 2001 and late 2002. A circularly polarized signal was transmitted with the Arecibo 305 m antenna and the echo was received in both the OC (expected for a mirror like reflection) and SC senses of circular polarization. For most of the observations Arecibo was used to receive the echo but for one observation in 2001 and for most of the 2002 observations the new 100 m Green Bank Telescope (GBT) was also used. Arecibo's limited tracking time combined with the 2 hr 15 min round-trip-light time to the Saturn system meant that it could only receive the echo for about 30 min corresponding to 0.5 deg of Titan rotation. For echo reception with the GBT the signal-to-noise ratio is lower but the echo can be received for the full round-trip-time corresponding to over 2 deg of motion of the sub-earth point on Titan. Sixtenn spectra were obtained in 2001 at sub-earth points uniformly spaced over the full range of longitudes. The 9 Arecibo/Arecibo and 15 Arecibo/GBT observations in 2002 did not provide uniform coverage. The latitudes of the sub-earth tracks in 2001 and 2002 were 25.9S and 26.2S, respectively. Most of the echo power is contained in a broad diffuse component. However, about 70% of the spectra show a weak specular echo that varies in width and amplitude with sub-earth longitude. The disk averaged normalized radar backscatter cross section varies from about 0.12 near 270 deg to about 0.19 at 90 deg, the longitude of the high near-IR albedo feature. The errors on these values are dominated by systematic errors of about 30%. The circular polarization ratio varies from about 0.4 to 0.6. The OC cross sections as a function of sub-earth longitude correlate very strongly with the disk integrated 2 micron albedoes from Griffith et al (1998). Fits of combined diffuse and specular scattering laws to the 16 2001 spectra resulted in 12 statistically significant specular echoes with the normalized cross sections ranging from 0.007 to 0.04 and rms slopes from 0.5 deg to 3.5 deg. These values for the cross sections are generally consistent with those expected for reflections from liquid hydrocarbons. A mix of liquid ethane, methane and nitrogen at Titan's surface temperature has a dielectric constant in the range of 1.65 to 1.81 (Thompson and Squyres, 1990) corresponding to radar cross sections between 0.016 to 0.022. The measured cross sections are also consistent with reflections from a higher dielectric constant surface of which only a fraction is smooth enough to give a specular echo. References: Griffith, C.A., T. Owen, G.A. Miller and T. Geballe, Nature, 395, 575-578, 1998. Thompson, W.R. and S.W. Squyres, Icarus, 86, 336-354, 1990.
Black Gregory J.
Campbell Don B.
Carter Lynn Marie
Ostro Steven J.
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