Daniel Carrera wrote:
To trap hydrogen and helium you'd need a gas giant. Pretty much any terrestrial planet bigger than Mercury will trap everything from Oxygen onwards and fail to trap Hydrogen and Helium unless it is cryogenic cold. I think we have quite a lot more leeway on gravity compared to other factors.
To expand on gravity: Anything planet from about 0.1 Earth masses up to maybe 10 earth masses (we've found terrestrial planets with 5 earth masses) will trap a reasonable atmosphere. Note also that there's nothing wrong with trapping Helium, it's an inert gas. What we are really concerned about is trapping too much gas (any gas) and becoming a gas giant. Or else, not trapping anything at all and having no atmosphere.
The point is, we have a factor of about 100 between the smallest and the biggest planet that will support life as we know it. So, while we can't ignore gravity altogether, it is not something we need to fine-tune very carefully.
Some readers may know that Mars has 0.1 Earth masses and it has minimal atmosphere. First I note that Mars does have some atmosphere, it's just very thin. So that proves that it can hold on to one. Low gravity is not the only probable reason why Mars lost its atmosphere. It is likely that if Mars had had a stronger magnetic field, like Earth does, it would have retained its atmosphere. Keep in mind that the main thing that removes the atmosphere from the inner planets is not heat but the solar wind. You can compensate for lower gravity by having a stronger magnetic field.
Cheers, Daniel. --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
