Question: If Earth’s early atmosphere is similar to Venus’ and Mars’, is there any possibility that their atmospheres will change the same way Earth’s did?

Susan Cartwright answered on 25 Jun 2020:

Earth’s atmosphere changed for two reasons: Earth had liquid water on its surface, which dissolved much of the CO2 that used to be a dominant compnent of our atmosphere, and second, life evolved the process of photosynthesis, which converts CO2 and water to sugars and oxygen. The O2 in our atmosphere is, as they say in the ads, a plant-based produc.

Venus lost all its water early on: because it was too hot for liquid oceans, the water was in the atmosphere as vapour, and was then destroyed by sunlight (the UV rays broke up the water into H2 and O2, the H2 escaped into space, and the O2 combined with surface rocks – which it would do here on Earth if plants didn’t keep pumping more out). Venus is now very dry, so there is no way for it to get rid of its CO2 as Earth did.

Meanwhile, Mars is just too small. Because of this, it has a lower gravity and no magnetic field, so atmospheric gases find it very easy to escape. Mard has lost nearly all its original atmosphere, and there is no way for it to get that back.

So, by themselves, Mars and Venus cannot evolve to an Earthlike atmosphere. Venus is probably past saving, but there have been a number of suggestions fr giving Mars an atmosphere, such as craching a xomet into it to deliver lots of water, which would vaporise to give Mars an atmosphere of water vapour.