Retrieving Saturn's Atmospheric Temperatures from Radio-Occultation Soundings
Abstract
Inverting radio-occultation soundings to retrieve vertical profiles of density, pressure, and temperature vs. altitude generally requires some assumption of symmetry. The assumption of spherical symmetry, at least locally, suffices for many atmospheres, but not for a rapidly rotating oblate planet like Saturn. In this case, one can assume that the atmosphere is axisymmetric and its flow barotropic, i.e., that its differential rotation (including its zonal winds) are functions of the cylindrical distance from the planetary axis of rotation. The barotropic assumption, which guarantees that the surfaces of constant geopotential height, pressure, and temperature coincide, need only apply in the atmospheric shell probed by the occultations. This allows the use of a geopotential incorporating both gravity and differential rotation to determine the shape of the atmosphere, which is needed to correctly compute the path of the refracted radio signal. Typically one assumes a meridional wind profile obtained from cloud-tracking studies to define the geopotential surfaces. Comparison of the retrieved shape of the isobaric surfaces to that assumed in the ray tracing calculations for the range of latitudes probed by the Cassini occultations of Saturn (70° N to 70° S) indicates deviations in isobaric altitudes of ½ scale height or less. Changing the assumed wind profile alters the shape of the geopotential surfaces, the refraction of the rays traversing the atmosphere, and the retrieved atmospheric profiles. Using winds obtained from Voyager (Sanchez-Lavega et al., 2000) and from Cassini at two levels in the upper troposphere and tropopause region (García-Melendo et al., 2011), we compare retrieved temperatures in Saturn's atmosphere at latitudes within 10° of the equator, where differences among the wind profiles are greatest. Over much of this interval, the Cassini winds are significantly weaker, and result in retrieved atmospheric temperatures that are typically 1 K warmer than the case using the Voyager-era winds. We discuss the implications of these shifts for the retrieval of the helium abundance using infrared spectra and occultation soundings.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.P31D..07F
- Keywords:
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- 5704 Atmospheres;
- PLANETARY SCIENCES: FLUID PLANETSDE: 5706 Aurorae;
- PLANETARY SCIENCES: FLUID PLANETSDE: 5724 Interiors;
- PLANETARY SCIENCES: FLUID PLANETSDE: 5737 Magnetospheres;
- PLANETARY SCIENCES: FLUID PLANETS