It may seem surprising to people, but you can look at
something like Mars, which has a very
thin atmosphere, and you can look at
something like Venus which we tend to think of as sort of having this rather heavy, clouded atmosphere, which [is] hellishly warm because of runaway greenhouse effect, and on both of those planets you are seeing this phenomenon of the atmosphere leaking away, is actually what directly has led to those very
different outcomes
for those planets; the specifics of what happened as the atmosphere started to go in each case [made] all the difference.