Shoemaker-Levy 9 lives on in the atmosphere of Jupiter

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Figure 1. A typical aspect of Jupiter's disk during the past six months. North is up, East is left. This sketch includes the comet impact sites visible on July 20 1994 at 3:00 UT. For comparison the beam of the JCMT at 354 GHz is 14", rather less than the semi-diameter.

As it was the cometary fragments came into Jupiter's atmosphere one after another like beads on a wire, impacting in much the same place relative to the Earth-Jupiter line-of-sight, just behind the south-eastern limb. Within an hour of impact each site was clearly visible as the planet rotated. Even before this, optical and infrared telescopes were able to see the fireballs peeking over the limb, as hot gases were ejected into the stratosphere of the planet from the tunnel driven deep into the lower layers of the atmosphere by each fragment. However, while most of the visually exciting effects were recorded by CCD's and on film, Jupiter soon after became a day-time object, leaving the field to the radio telescopes. And now, as Jupiter once again can be seen at night, and the impact sites are much less visible, it also turns out that the most long-lasting effects are likely also to be those which can be seen by radio telescopes.

For orientation purposes, the sketch in Figure 1 shows a typical face of Jupiter as presented to Earth during the July 1994 observations and, with only slight changes, throughout the follow-up observations described here. The apparent diameter of Jupiter is somewhat smaller than it was last July, now typically 33". The collisions of the comet fragments took place just behind the south-east limb (lower left in Figure 1), and 'rose' shortly afterward. Thus gas from the collisions was observed approaching the Earth (blue- shifted with respect to the mean velocity of Jupiter) in the south-east. Some eight hours later a given point 'sets' at the south-west limb, with a corresponding receding velocity. The relative red- and blue-shifts are about ± 8 km/sec respectively.

From the beginning of the international Jupiter/SL-9 campaign the decision was taken to use the JCMT to concentrate on observations of the HCN molecule, mostly of the J=4-3 ground-state rotational transition at 354 GHz. Under truly desperate atmospheric conditions we retreated to the J=3-2 line at 265 GHz. Observations of CO were to be carried out by IRAM, since HCN could not be covered by IRAM's receivers, and this seemed like a fair division of the labour. Nobody knew what to expect actually; no molecules had been detected in the radio-mm-submm regions in the atmosphere of Jupiter hitherto. However, many molecules, such as H3+, CH4+ and other hydrocarbons, NH3 and PH3, were already well known in the near and middle infrared.

So it was an occasion for rejoicing (as far as sleep-deprived astronomers will allow it) when a clear HCN(4-3) line was seen in emission on July 16. Nobody knew what it meant, but we have not looked back since that time. At IRAM both CO and CS were also detected. On the premise that such molecules were being thrown into the stratosphere, and that there should be something to learn about chemistry and the origin of the species being observed, we continued afterwards sporadically to monitor the HCN lines. In fact the emission faded quickly, and when we next looked in August, there was nothing to be seen. Also, at this time, and throughout the fall the sky was mostly poor, and the quality of the data were not very good as a result. Nevertheless, observations were continued, making use of the kind efforts of various visiting observers and local staff staying after formal shift end. The Director JCMT gave his go-ahead to pre-empt other daytime observations at weekends.

Figure 2. A typical (i.e. good) spectrum of the J=4-3 HCN line in absorption against the disk of Jupiter. This spectrum was taken on 21 December 1994 towards the impact site of one of the largest fragments (G), about an hour before the site's transit. A baselevel has been fitted to put the line profile at zero level; typically the true baselevel in such spectra is close to 100 K Ta* due to the background from Jupiter.

When we started looking at the data we saw that HCN was now in absorption. The implication was clear: that the hot gas which had been seen in emission had cooled enough that it was now colder than the background of Jupiter, and was now residing in a stratospheric haze layer. CO and CS could also be seen in absorption by IRAM. In fact the first tentative evidence of HCN absorption is in late August-early September, in data where the lines had been confused with baseline ripples. In between-times the lines must be very weak, and it is no surprise, given the sky conditions, that they could not be seen.

Since that time a more concerted effort to make use of late mornings to observe Jupiter has been made. We were (and still are) interested to know (a) how long HCN molecules could last before being destroyed, (b) how quickly they would migrate away the specific impact sites in longitude, (c) whether they would disperse in latitude, and (d) whether other species could be found in the haze. As far as we know the Jupiter-SL9 'event' (if it happened to Earth we might be less clinical about it, and we would need a bigger word, like 'catastrophe', or 'apocalypse') was unique. However, sightings of visual blemishes on Jupiter going back into early telescopic times has to make one wonder how often this does happen.

As this issue of the Newsletter goes to press, we can say that HCN is still there. However, it appears that CO has faded away, according to IRAM. We can also say that of the other species we have searched for, including H2S, HNC and H2CO, none have been detected. We know that there has been no appreciable north-south migration of HCN - we have done quite a few measurements of the 'anti-site' positions as controls. Although the beam of the JCMT (14" at 354 GHz) is a significant fraction of the size of Jupiter, it is possible to set limits on the width of the stratospheric band in latitude; it seems to be rather less than 7". As well, it looks as though the stratospheric haze is uniform in longitude on the scale of the beam; we are able to point at any part of the latitude zone and see HCN clearly within 5 minutes under decent sky conditions.

For a time in late 1994 the JCMT was the only telescope capable of observing Jupiter in the HCN lines, because of the planet's position in the daytime sky. In fact on December 17 the planet passed within about 20 arcmin of the limb of the Sun, and the following day very nice observations of HCN were made by Fred Baas just before local noon. As it has for solar observations, the fact that the JCMT has a membrane transparent to sub-mm waves in place during observing allows one to avoid damage, and to maintain excellent pointing and focus control.

Figure 3. Observations of HCN(4-3) towards the East limb (top), West limb (middle) and middle of the southern impact zone (bottom) of Jupiter on 24 December. Towards the Eastern limb gas at and beyond the limb is approaching the observer; at the same time some absorption is picked up from less blueshifted seen against the disk of the planet. The two components partially cancel each other, and lead to apparently narrow line components. The opposite effect is seen on the (receding) western limb. On the meridian the usual absorption line is visible. Spectra intensities are normalised to a Jupiter background of 100 K.

Just when the monitoring of HCN was starting to become mundane, on the morning of December 21 Fred Baas obtained a rather strange result: a 'P-Cygni' profile towards the eastern limb. For the first time since July it seemed we were seeing emission in HCN, and first instincts said that this was an instrumental artefact. However, further thought suggested the following explanation: that while the absorption was as usual, cold gas seen against the disk of Jupiter, the emission was due to gas in the Jovian stratosphere seen projected against colder sky beyond the limb. The sense of the emission and absorption was consistent with this idea, and suggested that if one observed the other limb, the profile should be reversed.

An opportunity to test this naive explanation presented itself on Christmas eve, and Fred was again there as we obtained exactly the expected result, as shown in Figure 3.

Since this result, which continues to consistently repeat (for the HCN 3-2 line also), the same effect has been seen by IRAM in the CS(5-4) transition, and, looking more closely at our own data, it would seem to be present even as far back as September 2, when poor baselines were judged to be the cause. As for a serious model, we have some doubts about the naive explanation being the full story. The extreme velocities are not quite as we would expect. There is plenty to chew on in this material, and I expect it will take us some time to unravel it. Some insights should be gained at the DPS meeting in Baltimore in May.

For the moment however there are a few things we can say. First, the line widths of HCN (typically 8-10 km/s to half-power) are much greater than the thermal line widths expected anywhere in Jupiter's observable atmosphere. If the line width is due then to pressure broadening it fixes the altitude at which HCN is situated. Second, the observed cooling of the molecules would imply a substantial cooling of Jupiter's stratosphere due to material tossed up by the explosive impacts. Third, it seems likely that HCN was formed by shock chemistry in the impacts. Fourth, having lived for six months, HCN is probably shielded against photolysis by solar radiation by methane, and we have to look for other means for the eventual demise of HCN. We expect HCN to last for at least several months, and perhaps years. In the meantime it will be important to continue looking at HCN in Jupiter with the JCMT.

Reference:

"The Collision of the Comet Shoemaker-Levy 9 with Jupiter: Detection and Evolution of HCN in the Stratosphere of the Planet"; A. Marten, D. Gautier, T. Owen, M.J. Griffin, H.E. Matthews, D. Bocklee-Morvan, P. Colom, J. Crovisier, E. Lellouch, D.A. Naylor, G.R. Davis, G. Orton, I de Pater, S. Atreya, B. Han, D.B. Sanders, D. Strobel; submitted to Geophysical Research Letter (1995).