Interestingly, they don't look like mountains formed by collisional or extensional tectonic processes to my eye (a geologist who studies mountain formation). They look sort of like diapirs (i.e. lower density material pushing up through a higher-density medium; frozen methane and nitrogen are both less dense than water ice[1]) or vaguely like eroded volcanoes, but without obvious vents. Very different erosional processes could explain some of it, but it certainly doesn't look like typical plate-tectonic or ice-sheet topography. Super cool!
Actually, that's really interesting. I had no idea there was such a thing as a diapir. Your comment on eroded volcanoes made me wonder if water on Pluto acts a lot like rock on Earth and if there are water volcanoes that result from sub-surface heat transforming water into liquid.
Probably a naive question, but why is it the case? we have the basic formula of thermodynamic: PV = nRT, with n and R being constant, so if I keep the volume the constant and decrease the temperature, it would decrease the pressure P, making the material less hard. What am I missing?
phase transitions are complicated. when something is a gas that rule applies because the forces between the constituent particles are neglible except for the electromagnetic interaction causing scattering (i.e. particles bouncing off each other, or the surroundings).
this is the pressure from that formula. increasing temperature increases the energy in the system, resulting in more collisions, and more pressure. decreasing it results in less as you correctly point out.
in a solid the electromagnetic interaction is constantly present and locks the constiuent particles nearly in place, so that when excited they 'vibrate' around their location, rather than moving freely like in a gas. this has different consequences, so that decreasing the temperature will increase the rigidity of the material since the particles will become ever less likely to achieve the energy required to escape their electromagnetic 'traps'.
Postulating here: Is it possible the mountains are a result of gravitational pull of Charon on Pluto's water-ice mantle during pluto's formative years? Or the gravitational affects of Neptune and Pluto's orbital resonance?
Keeping in mind that I don't know anything about Charon's orbit or gravitational effects, I think it's possible. It also doesn't have to be early in Pluto's formation, either; it can be a slow, continuous, ongoing process. Salt diapirs on Earth move up through the brittle crust and 'erupt' on the surface.
I believe they're saying that this is not the case since they've seen fewer craters on Pluto than they would have expected, which makes them think these mountains are relatively 'recent' formations (that would have hidden any older craters that were present).
orbital trajectories are in free fall... tidal effects are all that are left and they are huge, smooth gradients, not tiny highly variable things of the sort that could create surface features like this.
Tidal forces do serve as an energy source to drive other geological mechanisms, however they stated in the press conference that Charon is to small to do this, so we need to find another mechanism for how Pluto is still active.
[1]: https://extras.springer.com/2006/978-1-4020-4351-2/Jenam/Ses...