[meteorite-list] Remnants of the 1994 Comet Crash in Jupiter

[Ron Baalke]

Observatoire de Paris
Paris, France

Contact:
Bruno Bézard, Observatoire de Paris, LESIA
Tél: 33 1 45 07 77 17
Fax: 33 1 45 34 76 83
E-mail: Bruno.BÉZARD at obspm.fr

Emmanuel Lellouch, Observatoire de Paris, LESIA
Tél: 33 1 45 07 76 72
Fax: 33 1 45 07 71 10
E-mail: Emmanuel.LELLOUCH at obspm.fr

3 September 2004

Remnants of the 1994 comet crash in Jupiter

In July 1994, more than 20 fragments of Comet Shoemaker-Levy struck Jupiter at a 
latitude of about 45 deg S (see Figure 1). High-temperature chemical reactions 
that occurred during the impacts created a suite of new compounds, such as 
hydrogen cyanide (HCN) and carbon monoxide (CO). The comet also deposited water 
vapor, which in presence of CO and sunlight is expected to be converted 
gradually into carbon dioxide (CO2). Since the collision, these species have 
been slowly spread in latitude but are still visible ten years later. A study to 
which were associated researchers from Paris Observatory, and published in the 
online version of the journal Science, shows that unexpectedly the latitudinal 
distributions of HCN and CO2 are markedly different (Fig. 2). This result, 
derived from measurements by the Cassini spacecraft as it encountered Jupiter in 
December 2000, is presently difficult to understand.

The Cassini spacecraft, now orbiting Saturn, swung by Jupiter in December 2000. 
The infrared spectrometer CIRS observed the planet in the spectral range 10 - 
1400 cm-1 (7 mm - 1 mm) at a spectral resolution up to 0.5 cm-1 and a spatial 
resolution of 0.02 of the planetary diameter at closest approach. The data 
collected during the encounter allowed mapping the abundances of HCN and CO2 in 
the Jovian stratosphere (Kunde et al. 2004).

HCN peaks near the impact latitude (45 deg S) and has a broad distribution (see 
Figure 2). It decreases smoothly toward the north up to approximately 50 deg N. 
Beyond 50 deg 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.

In this framework, the distribution of CO2 is quite surprising, with a maximum 
concentration southward of 60 deg S, three times higher than at the impact 
latitude. It decreases abruptly northward of 50 deg S and is only marginally 
detected northward of 30 deg 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. À paraître dans Science (10 septembre 2004). Online 
version accessible at http://www.sciencexpress.org (19 august 2004)

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.

IMAGE CAPTIONS:

[Figure 1:
http://www.obspm.fr/actual/nouvelle/sep04/jupiter-f1.jpg (44KB)]
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].

[Figure 2:
http://www.obspm.fr/actual/nouvelle/sep04/jupiter-f2.gif (23KB)]
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 deg S. 
The differences in their latitudinal variations are thus unexpected and not 
clearly understood.