How is Earth's mass related to its ability to have an atmosphere?

1 Answer
Dec 15, 2015

Surprisingly enough, very little.


Mass is proportional to gravity and gravity is necessary to hold an atmosphere, therefore mass would seem to be very important for the ability for a body to have an atmosphere. However there are quite a few examples that show that it doesn't require that much mass to get an atmosphere.

Venus is almost the same size as Earth (about 80% the mass). Somehow Venus has managed to amass an atmosphere more than 50 time denser than Earth. Given that Venus is closer to the sun and therefore the solar wind would be stronger (solar wind can strip atmospheres) it doesn't make much sense that Venus has that much more atmosphere.

The moon Titan is one of the largest moons in the solar system, but it still only has a small fraction of the mass of the Earth. Even with the much smaller mass it has an atmosphere that is almost 50% more dense than the Earth. Again, it doesn't seem to make sense, considering that Ganymede is a larger moon than Titan and has almost no atmosphere.

I think the answer is relative to each individual situation. Venus has a dense atmosphere because it primarily made of a heavier molecule (Carbon dioxide) than that of Earth's (primarily Nitrogen). Earth's atmosphere is protected by our strong magnetic field. Where Mars and Mercury have almost no protection from solar wind and subsequently have practically no atmosphere (Mercury) or a very thin atmosphere (Mars). Titan is a moon of Saturn and therefore very far from the sun, and therefore solar wind is much much less, and lets not forget that it is protected by Saturn's magnetic field which is much stronger than Earth's.