The American Astronomical Society's Division for Planetary Sciences met in Maryland this week. Many of the scientists gathered to discuss results from last summer's crash between Jupiter and Comet Shoemaker-Levy 9. In fact, the meeting featured more than 100 papers on the Great Crash. One interesting finding was that many of the comet fragments created fireballs of the same height, regardless of the size of the incoming fragments. Also, water was detected with certainty in at least one of the plumes. That water may have come from the comet, or from Jupiter itself, which is thought to have a cloud layer of water deep in its troposphere. But it's also possible that the water was synthesized in the hot fireballs from oxygen-bearing compounds carried by the comet and hydrogen present in Jupiter's atmosphere.
Astronomers also announced new results about the asteroid 243 Ida and its tiny moon Dactyl. The double nature of Ida was discovered in August 1993, when the Galileo spacecraft encountered it, but only recently have all the flyby data been radioed to Earth for analysis. According to Galileo scientists, Dactyl's orbit allowed them to determine the mass of Ida and thus its density. That value is somewhere between 2.2 and 2.9 grams per cubic cm, a loose range because Dactyl's orbit is only crudely known. But that's good enough to rule out the possibility that Ida is a stony-iron body, even though that's what its spectrum suggests. Instead, Ida could well have a composition like that of ordinary chondrite meteorites, which are primitive and largely unaltered. Interestingly, the spectra of Ida and Dactyl are similar but different. It's thought that the binary system formed during the collision and breakup that created a family of asteroids with very similar orbits, to which Ida belongs.
And observers using the Hubble Space Telescope showed remarkable views of Titan at the meeting and what appears to be the surface of Saturn's largest satellite. Titan is enveloped in a dense haze that even the Voyager cameras could not peer through. But in the near infrared the haze becomes more transparent, and HST's pictures suggest that a huge bright "continent" exists on the hemisphere of Titan that faces forward in its orbit. The planet-size world is though to have considerable liquid on its surface, or perhaps oceans, composed of liquid hydrocarbons. The new Hubble results don't prove that the seas exist, however, only that it has large bright and dark regions on its surface.
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