On Combining Thermal-infrared And Radio-occultation Data Of Saturn's And Titan's Atmospheres

Details On Combining Thermal-infrared And Radio-occultation Data Of Saturn's And Titan's Atmospheres On Combining Thermal-infrared And Radio-occultation Data Of Saturn's And Titan's Atmospheres

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008dps....40.5307f&link_type=abstract

American Astronomical Society, DPS meeting #40, #53.07; Bulletin of the American Astronomical Society, Vol. 40, p.495

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Radio-occultation and thermal-infrared measurements are complementary investigations for sounding planetary atmospheres. Although the vertical resolution afforded by radio occultations is higher, an instrument like CIRS can easily generate global maps of temperature and composition, whereas occultation soundings are usually distributed more sparsely. The starting point for radio-occultation inversions is determining the residual Doppler-shifted frequency, that is the shift in frequency from what it would be in the absence of the atmosphere. Hence the positions and relative velocities of the spacecraft, target atmosphere, and DSN receiving station must be known to high accuracy. It is not surprising that the inversions can be susceptible to sources of systematic errors. Stratospheric temperature profiles on Titan retrieved from Cassini radio occultations were found to be very susceptible to errors in the reconstructed spacecraft velocities ( 1 mm/s). Here the ability to adjust the spacecraft ephemeris so that the profiles matched those retrieved from CIRS limb sounding proved to be critical in mitigating this error. A similar procedure can be used for Saturn, although the sensitivity of its retrieved profiles to this type of error seems to be smaller. One issue that has appeared in inverting the Cassini occultations by Saturn is the uncertainty in its equatorial bulge, that is, the shape in its iso-density surfaces at low latitudes. The recent controversy in the equatorial winds, i.e., whether they changed between the Voyager era and later epochs of Cassini and some Hubble observations, has made it difficult to know the exact shape of the surface, and it leads to uncertainties in the retrieved temperature profiles 1 K for a given He abundance. When combining CIRS thermal-infrared spectra with synthetic spectra computed from retrieved radio-occultation temperature profiles, to determine the volume mixing ratio of He to H2, this propagates into errors in He/H2 0.02 .

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