Figure 1 In July 1994, more than 20 fragments of Comet Shoemaker-Levy 9 (SL9) collided with Jupiter. New gas compounds, produced through shock chemistry along with dark solid particles, were then deposited in the stratosphere. Some species, such as HCN and CO2, are still detectable today. [Credit NASA/Hubble Space Telescope Comet Science Team]. Click on the image to enlarge it
HCN peaks near the impact latitude (45°S) and has a broad distribution. It decreases smoothly toward the north up to approximately 50°N. Beyond 50° N or S, the abundance falls off abruptly. Once produced by shock chemistry during the SL9 impacts, HCN is stable, so that it is a tracer of atmospheric motions. The location of peak abundance still being around the impact latitude indicates that equatorial spreading occurred mostly by wave-induced diffusion rather than meridional winds. The decrease at high latitudes could result from strong circumpolar winds (vortices), which dynamically isolate polar regions from lower latitudes. This effect is analogous to the polar vortex that produces a confinement vessel for the Antarctic ozone hole in Earth’s stratosphere.
Latitudinal distribution of HCN and CO2 as determined from measurements by the CIRS instrument aboard Cassini in December 2000. The plotted intensities are proportional to the column abundance of the species. The original source of these two compounds is thought to be the SL9 impact in 1994, around 45°S. The differences in their latitudinal variations are thus unexpected and not clearly understood. Click on the image to enlarge it In this framework, the distribution of CO2 is quite surprising, with a maximum concentration southward of 60°S, three times higher than at the impact latitude. It decreases abruptly northward of 50°S and is only marginally detected northward of 30°S. If, as admitted up to now, HCN and CO2 are both products of the SL9 collision (Griffith et al. 2004 ; Lellouch et al. 2002) and are similarly distributed in altitude, this is extremely surprising and difficult to understand. Perhaps the two species got distributed at different altitudes and are therefore transported by different atmospheric currents. An alternative interpretation is that some non-SL9 or post SL9 chemistry is involved. Maybe the formation of carbon dioxide is more complex than we thought. In fact, the precipitation of oxygen ions from the Jovian magnetosphere in the auroral regions may lead to the formation of water vapor and OH radicals. These radicals could then react with the CO from SL9 and form, at high southern latitudes, the CO2 observed by Cassini/CIRS. It not clear however if the oxygen influx required to reproduce the observations is consistent or not with the loading rate of the magnetosphere from the Galilean satellites (mostly Io). These observations clearly give valuable insights into the dynamics and chemistry of the upper atmosphere of Jupiter. We still need to work on the above, or perhaps others, scenarios to really understand what the observations mean !
References
- Kunde, V.G., Flasar, F.M., Jennings, D.E., Bézard, B., Strobel, D.F. et al. 2004. Jupiter’s atmospheric composition from the Cassini thermal infrared spectroscopy experiment. Griffith, C.A., Bézard, B., Greathouse, T., Lellouch, E., Lacy, J., Kelly, D., Richter, M.J. 2004. Meridional transport of HCN from SL9 impacts on Jupiter. Icarus 170, 58-69 Lellouch, E., Bézard, B., Moses, J.I., Drossart, P., Feuchtgruber, H., Bergin, E.A., Moreno, R., Encrenaz, T. 2002. The origin of water vapor and carbon dioxide in Jupiter’s stratosphere. Icarus 159, 112-131 Several co-investigators from LESIA are participating to the analysis of the CIRS data.