Chris Peterson wrote:
ceelias wrote:I know that a lot of Mars atmosphere drifted off into space, and some chemically combined on the surface. How big (how much gravity) does a planet have to be (have) to retain an reasonably (capable of sustaining life) mixed atmosphere? Earth's size works well for us, but is there a lower limit? Obviously gas molecules would need to stay below escape velocity, and different chemical molecules would have different masses. I suspect the calculations have been done, but couldn't find it in a quick Google search.
It's not just a question of mass, but of having a strong enough magnetic field to deflect solar wind that would otherwise carry away the atmosphere.
This is certainly true, and one of the great realizations of recent times. Indeed, there are a number of factors that could come into play. My quick Google search found a very interesting discussion of it here:
http://worldbuilding.stackexchange.com/ ... -time-scal
While said discussion mentions a number of possible confounding factors, it does focus in on the question you've asked. I can't actually vouch for any of its validity, I don't have time to read it right now, but it does look fun.
One might do well enough by empirical observation ... Pluto has a thin atmosphere, but you may not like that example, because you may view it like a comet ... which only has an atmosphere when it comes in close, and loses that atmosphere rapidly as it goes. On the other hand, Earth is obviously losing its atmosphere (https://en.wikipedia.org/wiki/Atmospheric_escape
), if you want to get picky.