I attended the conference all day today. The morning began with several presentations on the dust environment at Pluto. Much of this work was motivated by the concern about the hazard that dust might pose to New Horizons, and the study has generally allayed the team's concerns. From a science standpoint, the most interesting question is: does Pluto have any rings? Simulations of what happens to dust that arises from impacts onto Pluto's small satellites suggest that it could, but that the dust would not survive in the system for very long before being swept up by Pluto or Charon. Several people reported attempts to detect rings at Pluto either by direct imaging or by stellar occultations, none of which succeeded in detecting anything. But that doesn't eliminate the possibility that rings exist. My impression is that the question of whether Pluto has rings will not be settled until we get lookback images from New Horizons after the flyby. With that geometry, tenuous, dusty rings will scatter light forward to New Horizons and should be detectable if New Horizons is commanded to look at the right spots along the little moons' orbits.

Dust talks were followed by talks about the compositions of the surfaces of Pluto and Charon. The take-home point of this set of presentations, led off by Dale Cruikshank, is that the two worlds are very different. Although Pluto must have a substantial quantity of water ice in its interior, there's no evidence for water ice on its surface. Instead we see lots of methane and nitrogen ice; some of the methane is dissolved in transparent, crystalline nitrogen. There is also carbon monoxide. Charon is totally different. Its surface is made of water ice and ammonia hydrate.

By analogy to Triton, there should be hydrogen cyanide and carbon dioxide on Pluto, but neither has been detected yet. There was group puzzlement about the non-detection of carbon dioxide on the surface of Pluto: irradiation of carbon monoxide should produce carbon dioxide. There was debate about what New Horizons will actually be able to add to the surface composition discussion, because what's really needed to get at the most interesting not-yet-detected possible compounds on Pluto -- namely, heavier hydrocarbons -- is mid-infrared spectrometry. New Horizons can't do that; the James Webb Space Telescope should be able to. But Alan Stern expressed the expectation that heterogeneity of Pluto's surface will mean local concentrations of different materials, and if things like hydrogen cyanide are locally concentrated enough, New Horizons' spectral data will be able to detect and identify them.

Will Grundy gave a cool presentation showing how the abundance of carbon monoxide and nitrogen ices on Pluto vary with longitude but vary together -- the two ices, he said, are totally miscible. Methane ice abundance also varies with longitude, but its peak is 90 degrees of longitude away from the peak in the other two ices. He also showed that there has been a decline in carbon monoxide and nitrogen abundance with time (or at least in the strength of their spectral absorptions), and that the decline is accelerating with time. Pluto's surface is dynamic, and New Horizons should see surfaces that have changed recently!

Marc Buie opened the afternoon session with a review of Charon science, spending the first five minutes of his talk complaining about how long it took to convince Brian Marsden of the Minor Planet Center that Charon actually existed. He talked a lot about the mutual event observations performed between 1985 and 1990 (that's when the Pluto-Charon orbital plane was seen edge-on from Earth, so the two alternately transited and eclipsed each other). He argued that these data sets are still the strongest constraint we have on Pluto's diameter, and that the Pluto diameter derived from these mutual event observations is systematically smaller than that derived from stellar occultations, which are dependent on how you model Pluto's atmosphere. New Horizons' high-resolution imaging and radio occultations will settle the question of its diameter once and for all, thank goodness!

One of the most surprising things in Marc's presentation was a comparison of Charon's infrared reflectance spectrum to that of Saturn's moon Tethys. (The two are similar in size, too.) This is very strange because the two are in radically different environments; Tethys' color, for instance, is affected by the way that Saturn's magnetic field interacts with E-ring particles. Like the nondetection of carbon dioxide on Pluto, this match caused great puzzlement among the meeting attendees.