In the past I've run into papers describing N2 eutectics with methane, carbon monoxide, and neon - the former two whose spectrums have been seen with the nitrogen in Sputnik (AFAIK neon hasn't been detected on Pluto). Neon significantly lowers the triple point temperature (as its triple point is lower), while one of the former two (I forget which one) allows one to have liquids at a mildly colder than pure N2 when a small amount is mixed in (although the triple point is still rather warm by Pluto standards). I'm sure if you google you'll find the papers yourself.

Really, though, it wouldn't take too much internal heat at all to boost the temperature 10-20 degrees; you've got an insulating ice cap, and radiative heat loss is proportional to T^4, so it drops to extremely low rates at cryogenic temperatures. If the heat is there then there should be liquids dozens of meters under the surface at Sputnik - you have your heat, you have your pressure... it's what should happen. But whether said heat is actually there and whether there actually are liquids, I'm not going to speculate.

No, not a good idea. 'Mare' means sea. It is used on the Moon for purely historical reasons*, and on Titan where there are real seas. Regio just means region, it's a more general name but far more appropriate.

Phil

* Leonardo da Vinci thought the bright areas were seas, bright because each wave reflected a flash of sunlight. Galileo took issue with this, after seeing the surface through a telescope. He said, if any parts of the Moon are seas they would have to be the dark areas, because the bright areas are mountainous. He didn't say they WERE seas, but the name took hold after that.

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Now that is just weird. I don't recall ever seeing a mountain coming up out of the center of a canyon, rift, chasm, or whatever.

That source of energy would be traced back to energy introduced by solar insolation, but rather than directly heating the surface it would be a kind of second order effect to the system. Still, any heat generated through that effect would have to be equal or less than just pure insolation?

It's a bit surprising to me how shallow the big crater/basin seems to be. Also it now seems very unlikely that Ceres has anything resembling Tethys' Ithaca Chasma (there were some speculations on that here earlier on). It probably was simply an albedo feature plus shadows within crates in the earlier, lower resolution images.

The bright spots are interesting. To me they simply look like fresh ice that has been exposed by impacts but apparently they might be something else.