|
|
The Planetary Society Blog
By Emily Lakdawalla
DPS: Day 4: Pluto and Charon
Sep. 8, 2005 | 06:46 PDT | 13:46 UTC
This morning in Cambridge the day began at Pluto and Charon, with five talks, and they were pretty interesting.
Leslie Young talked first about what we can expect observationally from Pluto over the next decade, between now and when New Horizons arrives. (New Horizons is the flyby mission to Pluto & Charon set to launch this January.) Pluto is now moving toward south polar winter; in the next decade its elliptical orbit will take it 2 AU farther from the Sun, and the subsolar latitude (which is a measure of the season) will change from 34 to 49 degrees latitude. According to some models for Pluto's atmosphere, Young said, the atmospheric pressure should have peaked when Pluto was at perihelion (closer to the Sun), and should have been decreasing since then, going down by a factor of 2 over the next decade. "But nature surprises us," Young said. "If you compare the atmospheric profiles from 1988 to 2002, instead of having an atmosphere that dropped precipitously after perihelion, you have an atmosphere that approximately doubled, at least at the altitude observed in stellar occultations. To some people this was a very large surprise. But if you take a look at the modeling papers by Hansen and Paige, you find a large range of predictions for behaviors of Pluto’s atmosphere. With lower thermal inertia, you can get extremely large excursions of atmospheric pressure over the change in season, and you can get peak pressure well past perihelion."
One problem has been that stellar occultations at Pluto and Charon have been very rare in the past. But Young pointed out that Pluto and Charon will be traversing the Milky Way as seen from the Earth over the next decade, so "occultations should be really good. We're in the unprecedented position of getting to pick which occultations we'll study based on scientific or logistical reasons."
Next, Bill McKinnon gave a talk about "Ice-eleven" in the outer solar system. Anyone who's read Vonnegut's Slaughterhouse V has heard of a fictional new crystal form of ice called "ice-nine," but ice-eleven (ice XI) is a real form of ice that is stable at very low pressures and temperatures, and McKinnon's main point was that we may lready be seeing it in the outer solar system and have to think about that. The difference between ordinary ice, ice-one or ice I, and ice XI is its degree of crystal ordering. "Ice-one is a crystalline lattice of water molecules in hexagonal symmetry. In ordinary water ice the orientation of the hydrogens or protons is flipping around continuously, making a disordered structure. Solid-state physicists have long theorized that if you cooled ice enough you’d get a transition to a purely ordered solid called ice XI. In order to form it, you have to be able to reorient all the molecules. This is a well studied phenomenon. Reorientation times are 20 years to 100s of millions of years. There is a temperature range, 72 to 50 K where this works on geologically reasonable time scales." He went on to say that impurities can accelerate the process of conversion of ice I to ice XI.
So ice XI can be found wherever the temperatures are right -- they must be below 72 Kelvin all the time, but not too cold or the reorientation of the water molecules can't take place. "You find appropriate temperatures in the Uranian satellites, everybody's darling Enceladus, and at Pluto and Charon, at least Pluto’s warmer spots," and Triton as well. So, why do we care? Can we see ice XI, and does its presence mean anything for the geology of icy bodies? McKinnon said that "We don’t have laboratory spectra for ice XI in near infrared wavelengths" but that you'd expect the spectrum of ice XI to look like ice I but with sharper spectral features because of the increased order of the crystal structure. The reason that ice XI is important is because it's expected to have physical properties that differ slightly from ice I, which has implications for anybody doing modeling of the geology and geophysics of these bodies. "In the lab, the thermal conductivity of ice XI is greater than in ice I by about 20%," McKinnon said, and models indicate that it may even be higher. "We are almost certainly already looking at surfaces that are mixtures of ice I and ice XI in the outer solar system. It should affect spectra and it has non-trivial geophysical effects."
Next up was Mark Buie talking about Hubble observations of Pluto. "I had hoped that this would be the final word on the data set," he began, "but in fact it's going to be a progress report." He's working on data acquired with the Advanced Camera for Surveys on Hubble between July 2002 and June 2003 over 12 orbits through two filters. Previous Pluto images came from the Faint Object Camera in 1994. To make a long story short, they have a couple or three hundred images of Pluto, each only a few pixels across, and it's taken so far two years of computer time on a 20-computer parallel cluster to turn those images into maps of the albedo (brightness/darkness) across Pluto. There were "oohs" when he showed his maps though. "It is heartening to see that the general dark areas and bright areas are staying the same since 1994. The high southern latitudes are frustratingly difficult to constrain"--difficult to constrain, because they are on the limb, just barely visible, and frustrating because that's where you'd expect to see the greatest change in seasonal albedo patterns from 1994 to 2003 as the putative freezing of the atmosphere begins.
Then Buie showed a color map, which really got people excited. The albedo patterns in the two color filters are similar but different enough that there are clearly different colored terrains on the surface. The color map "explains one of the longstanding mysteries about Pluto, which is: why, with such strong albedo variations, do you never see a color change" on Pluto as it rotates. Even cameras that can't resolve Pluto's disk from Earth should see its color change as it rotates and these different colored regions come into view. But as Buie showed, "the color variations are intimately intermixed on a hemispherical level, and it's hard to separate them out." His final slide was of the new maps rotating on a Pluto globe. "Frankly," he said, "I'm still at the point of just sittin' and starin' at 'em," he said of his maps, and the audience seemed to agree. I was thinking that it's amazing how much work he's having to do to get these maps to come out -- when, hopefully, New Horizons will blow them all away. But not for 10 years, by which time the season will have changed, and Buie's maps, though low in resolution, will be important for before-and-after comparisons.
Finally, Bruno Sicardy talked about the stellar occultation by Charon which I wrote an article on a little while ago. He showed results from three telescopes, which got three different chords across Charon -- that is, the telescopes were in three different places across South America, so they measured the star traversing three different (but parallel) paths behind Charon. From these observations he got a very highly accurate measurement of the size of Charon: the disk was a circle 602.5 plus or minus 1 kilometer in radius. That's the first I've ever seen such a precise measurement of the diameter of Charon. From that you can calculate a density of 1.73 plus or minus 0.08 grams per cubic centimeter. Someone asked him whether they tried to fit an ellipse to it, but he said that a circle perfectly fits the three chords; they do not detect a departure from a circle. So hopefully soon they'll be able to nail down the diameter of Pluto in the same way.
I have much much more from yesterday and today but now it's time to go back into Titan sessions so I'll wrap it up here.
|
|
[ what is this? ]