Observations Shed New Light On the Comet Collision
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Before the comet impact, there was a great deal of speculation and prediction about whether the 21 nuclei would survive before reaching Jupiter, or were so fragile that gravitational forces would pull them apart into thousands of smaller fragments. HST's high resolution images show that the nuclei, the largest of which were probably a few kilometers across, did not break up catastrophically before plunging into Jupiter's atmosphere. This reinforces the notion that the atmospheric explosions were produced by solid, massive impacting bodies.
Was SL9 a comet or an asteroid?
At present, observations seem to slightly favor a cometary origin, though an asteroidal origin cannot yet be ruled out. The answer isn't easy because comets and asteroids have so much in common: they are small bodies; they are primordial, having formed 4.6 billion years ago along with the planets and their satellites; either type of object can be expected to be found in Jupiter's vicinity. The key difference is that comets are largely icy while the asteroids are virtually devoid of ice because they formed too close to the Sun.
What Is That Dark Stuff Made Of?
The HST Faint Object Spectrograph (FOS) detected many gaseous absorptions associated with the impact sites and followed their evolution over the next month. Most surprising were the strong signatures from sulfur-bearing compounds like diatomic sulfur (S2), carbon disulfide (CS2), and hydrogen sulfide (H2S). Ammonia (NH3) absorption also was detected. The S2 absorptions seemed to fade on timescales of a few days, while the NH3 absorptions at first got stronger with time, and finally started fading after about one month. During observations near the limb of Jupiter, the FOS detected emissions from silicon, magnesium and iron that could only have originated from the impacting bodies, since Jupiter itself normally does not have detectable amounts of these elements. This dark debris is a natural tracer of wind patterns and allows astronomers a better understanding of the physics of the Jovian atmosphere.
The high speed easterly and westerly jets have turned the dark "blobs" originally at the impact sites into striking "curly-cue" features. Although individual impact sites were still visible a month later despite the shearing, the fading of Jupiter's scars has been substantial and it now appears that Jupiter will not suffer any permanent changes from the explosions.
HST's ultraviolet observations show the motion of very fine impact debris particles now suspended high in Jupiter's atmosphere. The debris eventually will diffuse down to lower altitudes. This provides the first information ever obtained about Jupiter's high altitude wind patterns. HST gives astronomers a "three dimensional" perspective showing the wind patterns at high altitudes and how they differ from those at the visible cloudtop level. At lower altitudes, the impact debris follows east-west winds driven by sunlight and Jupiter's own internal heat. By contrast, winds in the high Jovian stratosphere move primarily from the poles toward the equator because they are driven mainly by auroral heating from high energy particles.
New Auroral Activity...
HST detected unusual auroral activity in Jupiter's northern hemisphere just after the impact of the comet's "K" fragment. This impact completely disrupted the radiation belts which have been stable over the last 20 years of radio observations. Aurorae, glowing gases that create the northern and southern lights, are common on Jupiter because energetic charged particles needed to excite the gases are always trapped in Jupiter's magnetosphere. However, this new feature seen by HST was unusual because it was temporarily as bright or brighter than the normal aurora, short-lived, and outside the area where Jovian aurorae are normally found. Astronomers believe the K impact created an electromagnetic disturbance that traveled along magnetic field lines into the radiation belts. X-ray images taken with the ROSAT satellite further bolster the link to the K impact. They reveal unexpectedly bright X-ray emissions that were brightest near the time of the K impact, and then faded.
