Cassini First Radio Science Observations of Titan's Atmosphere and Surface

Details Cassini First Radio Science Observations of Titan's Atmosphere and Surface Cassini First Radio Science Observations of Titan's Atmosphere and Surface

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.3602m&link_type=abstract

American Astronomical Society, DPS meeting #38, #36.02; Bulletin of the American Astronomical Society, Vol. 38, p.548

Astronomy and Astrophysics

Astronomy

Scientific paper

The first two Cassini radio occultations of Titan's atmosphere occurred on March 18 (T12) and May 20 (T14), 2006. The atmosphere was probed on the ingress and egress sides, yielding observations at four mid-southern latitudes. Titan's surface was also probed using bistatic-scattering during the inbound period on T12 and the inbound and outbound periods on T14. In all cases, quasi-monochromatic S-, X-, and Ka-bands RCP signals (13, 3.6, and 0.94 cm-wavelength, respectively) were transmitted from Cassini. Both the RCP and LCP signal components were observed at multiple ground receiving stations of the NASA/DSN. Demanding spacecraft maneuvers to point the Cassini high-gain antenna to virtual Earth during the occultations, and to track the specular region on Titan's surface during the bistatic observations were successfully implemented. For the first time ever, quasi-specular bistatic scattering surface echo is detected on both the inbound and outbound T14 observations. Although weak, an X-band RCP and LCP reflected spectral components are clearly detectable. Their total power ratio determines the refractive index of the regions probed and its likely nature (liquid vs solid). The echo appears consistent with reflection from localized hydrocarbon liquid regions embedded in mostly nonspecularly reflecting terrain. The atmospheric refracted S and X signals were tracked down to Titan's surface. The Ka signal was consistently extinguished by atmospheric absorption at about 10 km above the surface. Observed changes of signal frequency is used to recover the refractivity profiles of the neutral atmosphere, hence determine the corresponding temperature-pressure profiles assuming 100% N2 composition. Changes of signal strength, corrected to remove refractive defocusing, reveals both small-scale and large-scale effects. The former is likely due to gravity waves, turbulence, and layers. The latter exhibits remarkable wavelength dependence and is likely caused by dispersive N2-N2 collision-induced gaseous absorption, although additional extinction mechanisms may also be responsible.

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