On the HCN and CO2 abundance and distribution in Jupiterʹs stratosphere

Observations of Jupiter by Cassini/CIRS, acquired during the December 2000 flyby, provide the latitudinal distribution of HCN and CO2 in Jupiterʹs stratosphere with unprecedented spatial resolution and coverage. Following up on a preliminary study by Kunde et al. [Kunde, V.G., and 41 colleagues, 2004. Science 305, 1582–1587], the analysis of these observations leads to two unexpected results (i) the total HCN mass in Jupiterʹs stratosphere in 2000 was ( 6.0 ± 1.5 ) × 10 13   g , i.e., at least three times larger than measured immediately after the Shoemaker–Levy 9 (SL9) impacts in July 1994 and (ii) the latitudinal distributions of HCN and CO2 are strikingly different: while HCN exhibits a maximum at 45° S and a sharp decrease towards high Southern latitudes, the CO2 column densities peak over the South Pole. The total CO2 mass is ( 2.9 ± 1.2 ) × 10 13   g . A possible cause for the HCN mass increase is its production from the photolysis of NH3, although a problem remains because, while millimeter-wave observations clearly indicate that HCN is currently restricted to submillibar ( ∼ 0.3   mbar ) levels, immediate post-impact infrared observations have suggested that most of the ammonia was present in the lower stratosphere near 20 mbar. HCN appears to be a good atmospheric tracer, with negligible chemical losses. Based on 1-dimensional (latitude) transport models, the HCN distribution is best interpreted as resulting from the combination of a sharp decrease (over an order of magnitude in K y y ) of wave-induced eddy mixing poleward of 40° and an equatorward transport with ∼ 7   cm s −1 velocity. The CO2 distribution was investigated by coupling the transport model with an elementary chemical model, in which CO2 is produced from the conversion of water originating either from SL9 or from auroral input. The auroral source does not appear adequate to reproduce the CO2 peak over the South Pole, as required fluxes are unrealistically high and the shape of the CO2 bulge is not properly matched. In contrast, the CO2 distribution can be fit by invoking poleward transport with a ∼ 30   cm s −1 velocity and vigorous eddy mixing ( K y y = 2 × 10 11   cm 2 s −1 ). While the vertical distribution of CO2 is not measured, the combined HCN and CO2 results imply that the two species reside at different stratospheric levels. Comparing with the circulation regimes predicted by earlier radiative-dynamical models of Jupiterʹs stratosphere, and with inferences from the ethane and acetylene stratospheric latitudinal distribution, we suggest that CO2 lies in the middle stratosphere near or below the 5-mbar level.