Vertical and meridional distribution of ethane, acetylene and propane in Saturn's stratosphere from CIRS/Cassini limb observations
Résumé
Measuring the spatial distribution of chemical compounds in Saturn's stratosphere is critical to better understand the planet's photochemistry and dynamics. Here we present an analysis of infrared spectra in the range 600-1400 cm acquired in limb geometry by the Cassini spacecraft between March 2005 and January 2008. We first determine the vertical temperature profiles from 3 to 0.01 hPa, at latitudes ranging from 70 °N to 80 °S. We infer a similar meridional temperature gradient at 1-2 hPa as in recent previous studies (Fletcher et al., 2007; Howett et al., 2007). We then retrieve the vertical profiles of C H and C H from 3 to 0.01 hPa and of C H around 1 hPa. At 1 hPa, the meridional variation of C H is found to follow the yearly averaged solar insolation, except for a strong equatorial mole fraction of 8×10, nearly two times higher than expected. This enhancement in abundance can be explained by the descent of hydrocarbon-rich air, with a vertical wind speed at the equator of 0.25 ±0.1 mm/s at 1 hPa and 0.4 ±0.15 mm/s at 0.1 hPa. The ethane distribution is relatively uniform at 1 hPa, with only a moderate 25% increase from 35 °S to 80 °S. Propane is found to increase from north to south by a factor of 1.9, suggesting that its lifetime may be shorter than Saturn's year at 1 hPa. At high altitudes (1 Pa), C H and C H abundances depart significantly from the photochemical model predictions of Moses and Greathouse (2005), except at high southern latitudes (62, 70 and 80 °S) and near the equator. The observed abundances are found strongly depleted in the 20-40 °S region and enhanced in the 20-30 °N region, the latter coinciding with the ring's shadow. We favor a dynamical explanation for these anomalies.
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