Saturn's Helium Abundance from Cassini CIRS and RSS Data
Abstract
The ratio of helium to hydrogen in Saturn's atmosphere provides an important constraint on models of Saturn's formation and evolution, but has been poorly constrained by available data. Measurements combining Voyager infrared and radio data [1] gave a He/H2 mole ratio of 0.034 ± 0.024, far below the protosolar value and requiring either significant helium loss or sequestration in the interior. Prompted by discrepancies between the Voyager infrared/radio and Galileo probe measurements of helium on Jupiter, Conrath and Gautier [2] reevaluated the helium abundance on Saturn from Voyager infrared data only, obtaining a He/H2 mole ratio of 0.135 ± 0.025, inconsistent with the previous results. Because of this discrepancy, estimating Saturn's atmospheric helium abundance has been a major goal of the Cassini mission.We are estimating the He/H2 mole ratio in Saturn's atmosphere using a combination of data from the Cassini Radio Science Subsystem (RSS) and Composite InfraRed Spectrometer (CIRS). Radio occultations, in which a carrier signal is observed as Cassini passed behind Saturn as seen from antennas on Earth, give a vertical profile of atmospheric refractivity, which can be converted to temperature as a function of pressure assuming a mean molecular weight. The assumed molecular weight is adjusted until synthetic thermal infrared spectra match spectra observed by CIRS at the same latitude and time. Preliminary results from a set of eleven near-equatorial occultations taken in 2005 give He/H2 mole ratios varying between ~0.060 and 0.085.[1] Conrath, B. J., et al., 1984, Ap. J., 282:807-815[2] Conrath, B. J. and D. Gautier, 2000, Icarus, 144:124-134
- Publication:
-
AAS/Division for Planetary Sciences Meeting Abstracts #48
- Pub Date:
- October 2016
- Bibcode:
- 2016DPS....4850801A