Why is it that Venus has been able to retain an atmosphere that exerts a
pressure about 90x that of the atmosphere on Earth? The mass of Venus is less
than that of Earth, so its gravitational acceleration is only 8.9 m/s2. Its
atmosphere is hotter (about 737K vs. 288K). Even though its atmosphere is mostly
CO2, with a molar mass of 44 compared to an average of about 29 for Earth's
atmosphere, it seems counterintuitive that it could retain such a dense
atmosphere compared to Earth. Even when you factor in that at higher temperatures
gases exert greater pressure with less molecules/volume, this is only a factor of
about 2.5.
I cannot find any explanation of this, although I imagine there are many
available--just can't find them. Many Websites even discuss the factors that
allow a planet to retain an atmosphere and then give many examples--Mercury, the
moon, Mars, etc. that go along with the smaller mass, less gravity, less
atmosphere argument and "higher temperature-less atmosphere" argument, but then they
never explain Venus.