Pluto on the Eve of Exploration by New Horizons: Small moons, dust, surfaces, interiors

I arrived late yesterday in Laurel, Maryland, just in time for the first poster session at the "Pluto on the Eve of Exploration" conference. However, I must admit I didn't look at a single poster; I exchanged my drink ticket for a beer and immediately engaged in the hallway conversation that makes science meetings worth attending. I asked people what they'd seen on this day of the meeting that had most intrigued them, and nearly everybody mentioned Mark Showalter's presentation, which concerned the two larger of Pluto's small satellites, Nix and Hydra. He argued that they rotate chaotically, like only one other known moon in the solar system (Saturn's Hyperion). That means their rotation rates and spin orientations can vary widely and unpredictably. I didn't see the talk, because I was on the plane, but I learned more about it through discussion.

Chaotic rotation would be a byproduct of the very unusual gravitational environment that these moons are in: they are in near-resonant orbits around a close binary. They are constantly getting nudged and destabilized by their changing distances to Pluto and the lower-mass but still quite large and much closer Charon. That was the theoretical part of Showalter's presentation. He also showed observational data. He and others have been trying to get good lightcurves on Nix and Hydra to determine their rotation states. Usually getting a rotation rate from a light curve is pretty straightforward -- you look for a periodically repeating variation in brightness, and the wavelength of that repeat is either the rotation rate or an integer multiple of the rotation rate. But nobody's lightcurve data makes any sense.

What was funny about the hallway discussion about Showalter's presentation is that I talked to at least one scientist who was convinced by the theoretical part but not the observational part, and another for whom that was reversed, and a third (Bill McKinnon) who wasn't convinced by either one. Convinced or not, everyone was talking about it!

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.

There were several talks about attempting to produce and study Pluto-like ices in the laboratory. With the presence of nitrogen and methane in an atmosphere and as surface ices and with ultraviolet irradiation from the Sun, there may actually be some pretty complex chemistry going on, producing large hydrocarbon molecules. Just like on Titan.

Finally, several talks concerned the interiors of Pluto and Charon. These were necessarily theoretical, but Francis Nimmo in particular talked about how different scenarios for Pluto's internal structure and evolutionary history would result in different things observable by New Horizons, so that New Horizons should be able to settle some of the questions about Pluto's interior and history. For instance, it's possible that Pluto once had a subsurface ocean. It may still have one. Or it may never have had one. Each case will produce different observable tectonic features on the surface, and could affect the current shape of Pluto (as in, how much it's flattened).

It was a long day of talks -- it started at 8:00 and finished at 5:30 -- and it seemed to me that there tended to be quite a bit of overlap from talk to talk. I wished there'd been less presentation and more opportunity for discussion -- the few times that talks inspired conversation among attendees were the most interesting moments, I thought. This is a general problem at scientific meetings, and I think it's an error. What's the point of getting a hundred people together and then not providing time for them to actually talk with each other? Fewer talks, more posters, more time to talk science with smart people.

Tomorrow will be equally packed, with talks on geology in the morning and on surface-atmosphere interactions in the afternoon.

I'm not the only one reporting from this meeting. In particular, deputy project scientist Kimberly Ennico Smith is producing tons of blog entries with more detail on the talks. And Julie Rathbun is taking notes into Twitter, paying much closer attention to each one than I've been able to do. Many other people are tweeting using the #PlutoSci hashtag. Stay tuned!