Meridional distribution of CH3C2H and C4H2 in Saturn's stratosphere from CIRS/Cassini limb and nadir observations
Résumé
Limb and nadir spectra acquired by Cassini/CIRS (Composite InfraRed Spectrometer) are analyzed in order to derive, for the first time, the meridional variations of diacetylene (C 4H 2) and methylacetylene (CH 3C 2H) mixing ratios in Saturn's stratosphere, from 5 hPa up to 0.05 hPa and 80°S to 45°N. We find that the C 4H 2 and CH 3C 2H meridional distributions mimic that of acetylene (C 2H 2), exhibiting small-scale variations that are not present in photochemical model predictions. The most striking feature of the meridional distribution of both molecules is an asymmetry between mid-southern and mid-northern latitudes. The mid-southern latitudes are found depleted in hydrocarbons relative to their northern counterparts. In contrast, photochemical models predict similar abundances at north and south mid-latitudes. We favor a dynamical explanation for this asymmetry, with upwelling in the south and downwelling in the north, the latter coinciding with the region undergoing ring shadowing. The depletion in hydrocarbons at mid-southern latitudes could also result from chemical reactions with oxygen-bearing molecules. Poleward of 60°S, at 0.1 and 0.05 hPa, we find that the CH 3C 2H and C 4H 2 abundances increase dramatically. This behavior is in sharp contradiction with photochemical model predictions, which exhibit a strong decrease towards the south pole. Several processes could explain our observations, such as subsidence, a large vertical eddy diffusion coefficient at high altitudes, auroral chemistry that enhances CH 3C 2H and C 4H 2 production, or shielding from photolysis by aerosols or molecules produced from auroral chemistry. However, problems remain with all these hypotheses, including the lack of similar behavior at lower altitudes. Our derived mean mixing ratios at 0.5 hPa of (2.4 ± 0.3) × 10 -10 for C 4H 2 and of (1.1 ± 0.3) × 10 -9 for CH 3C 2H are compatible with the analysis of global-average ISO observations performed by Moses et al. (Moses, J.I., Bézard, B., Lellouch, E., Gladstone, G.R., Feuchtgruber, H., Allen, M. [2000a]. Icarus 143, 244-298). Finally, we provide values for the ratios [CH 3C 2H]/[C 2H 2] and [C 4H 2]/[C 2H 2] that can constrain the coupled chemistry of these hydrocarbons.