Early analysis of the stellar and radio occultations they performed with Charon to try to detect its atmosphere resulted in a non-detection. Now the team can quantify that: if Charon does have any atmosphere, it is less dense than our Moon's atmosphere. To put it another way, physically speaking, there have to be some molecules and ions floating around above any solid surface in a vacuum, but there are way fewer of those molecules above a given area of Charon's surface than there even are above the Moon's surface.

Charon's surface is dominated by water ice, but the spectroscopists have located outcrops of ammonium or ammonium hydrates. The geologist in me keeps balking at typing "outcrops of ammonium," but at Pluto's cold temperatures, these materials do function as rocks do on Earth, so "outcrops" they are. Stern also mentioned the intriguing presence of sublimation pits in Charon's surface -- intriguing because nothing would be sublimating there today.

Anne Verbiscer talked about the phase curves of Pluto and Charon. A phase curve is a study of how strongly a surface reflects light depending on the angle at which light travels from the light source, from the surface, to your eye (or your camera). Many surfaces in the solar system display a strong "opposition surge," in which the surface appears to suddenly brighten when you view it with the light source directly behind you. This is a property of rough and/or porous surfaces. From Earth, with Hubble, we can only see Pluto and Charon at phase angles ranging up to 2 degrees, because we're so much closer to the Sun that we almost always see them nearly at opposition. New Horizons got a much wider range of phase angles (though not lowest phase), and Verbiscer showed phase curves, with Charon having a much stronger opposition surge than Pluto. Charon, she said, has a highly porous surface of opaque water ice particles, and Hydra and Nix are similar. Pluto has a more compact surface, with transparent grains of methane and nitrogen. Overall, Pluto's global scattering properties are similar to those of Neptune's moon Triton.

Looking a bit more closely at the way that Pluto's surface reflects light, Bonnie Buratti showed the results of some work she's been doing on quantitative albedo mapping. She reminded the audience that most of the variations in intensity within a spacecraft image are not intrinsic, but are rather due to changes in viewing geometry. So they processed New Horizons' images to make a map showing what the surface would look like if incidence, emission, and phase angles were all zero. She found that Pluto's bright region (Sputnik) is almost as bright as Enceladus, and its darkest regions are under 10% reflective. Of all the other worlds in the outer solar system, only Iapetus has so much contrast; Iapetus is overall darker than Pluto but has a similar factor-of-10 range in its albedo. Buratti went on to speculate that Eris, with its Enceladus-like extraordinarily high albedo, must be active, because a surface so bright is hard to maintain with any amount of contaminants on the surface.