Based on an email today from New Horizons team member William McKinnon, the APOD text has now been changed to note that the mountains are (likely) composed primarily of water ice. This had also been alluded to by a guest poster. Also it appears that several ices make up Sputnik Planum. I apologize for the oversight, and thank Dr. McKinnon and the New Horizons team for pointing out this inaccuracy.
BTW here are some links with some discussion: https://www.nasa.gov/feature/new-horizo ... s-on-pluto
Thanks for those links, RJN. I only checked out the first one. One thing that struck me there was this quote:
Additionally, new compositional data from New Horizons’ Ralph instrument indicate that the center of Sputnik Planum is rich in nitrogen, carbon monoxide, and methane ices. “At Pluto’s temperatures of minus-390 degrees Fahrenheit, these ices can flow like a glacier,” said Bill McKinnon, of Washington University in St. Louis, deputy leader of the New Horizons Geology, Geophysics and Imaging team.
If I got that correctly, the ices of Pluto can flow because
they are so cold. Fascinatingly, it looks to me as if they had melted and refrozen. Of course, different ices melt at different temperatures, so the melting and re-freezing scenario is certainly a possibility. Water ice melts at a much higher temperature than, say, nitrogen ice, so it makes sense if the mountains are made of water ice, which would make them as hard as rock at the temperatures of Pluto. The ice flows of Sputnik Planum would be made of other ices. Indeed, as the link says, Sputnik Planum is rich in nitrogen, carbon monoxide, and methane ices.