Atmospheric Density Required for Mercury´s Polar Regions to be Warm?

[snorkack]

TL;DR Summary

At which atmospheric density would Mercury´s poles be neither hot nor cold?

Venus has a dense atmosphere (about 90 bar).
Although Venus has a low axial inclination and the poles of Venus are therefore always in twilight, the day side atmosphere heats to about +460 C and the heat transport at the 90 bar atmosphere is so efficient that the poles are no more than 10 C cooler than day side.
Mercury has a thin atmosphere.
Day side of Mercury heats to about +430 C; but the poles are only around -170 C because of thin atmosphere and weak atmospheric heat transport.
At which atmospheric density would Mercury have just enough heat transport to provide for +20 C at poles?

 

[Ken G]

Interesting point, that with the right atmosphere the poles might be livable, but we know the atmosphere must be thinner than Earth because Earth is about the place where you get good (but not complete) sharing of heat. So if it was like Earth, and way closer to the Sun, it would be too hot. So if it's going to be thinner than Earth, it probably wouldn't be all that livable after all, but at least it could be temperature.

Note also that if you use a weak atmosphere to share the heat around, it might not be enough to raise the temperature of the ground. Let's say an atmosphere like Mars could warm it up to +20 C, but that would be the air, the ground would still be radiating away so could be much colder. In other words, there is a density where the T of the air is right, but if that density is too low to get the ground to come to that T, then the ground T could still be much lower. You probably wouldn't care what the T of the thin air was if you are walking around on -100 C ground.

But your question also points out something else that's interesting-- since the T of the actual Mercury goes from -170 to +430, there is a latitude where it already is +20 C. Since Mercury's axis isn't tilted, that place is always at +20 C, except for the darn eccentricity of the orbit. If it were a circular tidally locked orbit, there would be a curve on the surface of Mercury that was always +20 C, all the time, with a nice big Sun close to the horizon all the time. If not for the lack of air, that might be an interesting spot to hang out. Or, given how long Mercury's day is, maybe it would be possible to keep moving on its surface such that you always stay somewhere that is +20 C. It's not something you think to do on that supposedly hot planet!

 

[snorkack]

But your question also points out something else that's interesting-- since the T of the actual Mercury goes from -170 to +430, there is a latitude where it already is +20 C. Since Mercury's axis isn't tilted, that place is always at +20 C, except for the darn eccentricity of the orbit.

No, there isn´t. Because of the 88 day nights. In the equatorial regions, the surface hears to +430C in the afternoon - and manages to cool to -160 Celsius by morning.
At high latitudes where the Sun is low, daytime may only warm to +20, but night is still 88 days, so necessarily even colder than at equator.