DPS 2010: Pluto and Charon opposition surges, Nix and Hydra masses, Pluto and Eris compositions

Posted By Emily Lakdawalla

2010/10/25 11:18 CDT

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An awful lot of the talks in the Pluto session on Tuesday morning, October 5, at the Division of Planetary Sciences meeting spent more time focusing on how bad weather conditions were during the astronomers' attempts to view Pluto as it occulted background stars than they did on any measurements or science that came out from the data. I finally got some bloggable notes from Marc Buie's presentation on his work observing the Pluto system with Hubble. His talk was an update on what was announced earlier this year, namely that "something has fundamentally changed on the surface of Pluto" between 2000 and 2002. "Every light curve from 1954 to 2000 showed a flat light curve and dark color," he said; but "now there is significant reddening and longitudinally variable" color.

Something I hadn't appreciated before is that, because of Pluto's inclined orbit, we haven't yet seen it at zero phase. Slowly, as the years pass and Pluto's orbital motion brings it closer to the ecliptic, we on Earth can see it at lower and lower phase. Buie reported that as we are seeing Pluto at lower and lower phase, there is no visible opposition surge, and no difference in this behavior with color. However, Charon is different, and a "real surprise." Its light curve shows a strong opposition surge, with strikingly nonlinear behavior. The surge is color-dependent, "which tells me it is related to coherent backscatter, not self-shadowing," Buie said.

Just for explanation's sake, here's what a striking opposition surge looks like in an older observation of Saturn's rings:

Opposition surge of Saturn's rings

Three views of Saturn captured on different dates with the 2.2-meter telescope at the Calar Alto Observatory in Spain. Phase angles are measured in degrees. On January 13, 2005, Earth crossed the disk of the Sun as seen from Saturn, so Earth-based telescopes saw Saturn and its rings and moons with a "phase angle" (Sun-target-observer angle) of zero degrees. The globe of Saturn does not change much in the three views, but the rings flash into brilliance as the phase angle goes to zero. This effect is called "opposition surge" and was also seen on some of Saturn's moons, especially Enceladus.

Stephen Tegler managed to present both observational and experimental work on Pluto and Eris in one talk. He showed how, in experiments, mixtures of nitrogen and methane form a single frozen ice, a mixture, when they first freeze, but as the temperature in their apparatus was dropped, the methane ceased to be soluble in the nitrogen and began to exsolve, forming distinct grains of nitrogen and methane ice. He also showed spectra of both Pluto and Eris, and that he could explain the shapes of their spectra with a model of 97% nitrogen and 3% methane ices for Pluto, and 91% nitrogen and 9% methane ices for Eris.

Dave Tholen gave a really nice talk on recent work with Hubble to improve our estimates of the sizes and masses of Nix and Hydra, the two recently discovered moons in the Pluto-Charon system. He found their albedoes to be lower, and therefore their masses and diameters larger, than previous work indicates. His results (I hope I copied all the numbers correctly, please take these with a grain of salt):

Body	Mass (kg)	Diameter (km)	Albedo
Pluto	1.304 x 10²²	2294	0.61
Charon	1.516 x 10²¹	1212	0.34
Nix	1 x 10¹⁸ (an increase)	106 (old value was 88)	0.06 (old value was 0.08)
Hydra	4.6 x 10¹⁷ (an increase)	81 (old value was 72)	0.14 (old value was 0.18)

The analysis that Tholen and his coworkers did involved analyzing the positions in the bodies in something like 400 images, finding a best-fit solution to equations of motion involving 22 different free parameters (which I think are the position and velocity vectors for Charon, Nix, and Hydra, and the masses of the four bodies in the system). So the results may yet change as they locate better solutions within their "complex chi-squared hypersurface" (a new statistical term for me).

Pluto and its Moons
Pluto, Charon, Nix, and Hydra, as observed by the Hubble Space Telescope on February 15, 2006 Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the Hubble Space Telescope Pluto Companion Search Team

At this rate, I will finish writing up my notes from the Division of Planetary Sciences meeting just in time for the fall meeting of the American Geophysical Union to begin...sigh...