There is a huge variety of types of surfaces on Pluto. That variety relates, in part, to a variety in surface materials on Pluto. The main materials on Pluto are water ice, carbon monoxide ice, nitrogen ice, methane ice, and tholins. At the poster session, I asked geophysicist Bill McKinnon about the properties of these ices. He explained that at the 40-kelvin temperatures that prevail on Pluto, water ice is as strong and solid as rock is on Earth, but the other ices are weaker. Nitrogen and carbon monoxide ices will act very similarly on the surface (and are miscible in each other). The main difference between nitrogen and carbon monoxide on Pluto is that nitrogen is more volatile, and will turn to vapor when heated more quickly than carbon monoxide will. Both have much less strength than water ice, so should flow more readily, much as glacial ice does on Earth. Note that even though they can flow, they are still solid, and the flow we're talking about is relatively slow, in the neighborhood of several centimeters per year. Methane ice can mix into nitrogen and carbon monoxide ices as well, it's less volatile, and may be more rigid. Nitrogen and carbon monoxide ices are both denser than water ice at 40 kelvins, while methane has only half the density of water ice. Tholins are organic solids produced when solar radiation bombards these ices, and are likely the agent that produces reddish stains in various places on Pluto.

In a poster presentation, John Spencer showed that the color of the reddish pole of Pluto is spectrally quite different to the darker reddish terrain near Pluto's equator. He argued from spectral data that the stuff at the pole very likely represents brand-new tholin material, newly concocted by irradiation of ices that accumulated at the north pole during the previous winter. It is quite distinct from the equatorial tholin material, which has probably been exposed to the sun for a very, very long time.

Pluto has fascinating topography. Because everything but water ice flows on Pluto, anywhere you see steep topography, it has to be built of water. It was surprising that Pluto had so much topography, and also surprising how varied it was. Let's begin with the mountains northwest of Sputnik planum: