Venus and Mars show how a planet's air can go badly wrong
Venus should be chilly, with a calculated equilibrium temperature near −40 °C. Instead its surface sits close to 460 °C beneath air about 80 times denser than ours. Mars went the opposite way, keeping barely 0.6% of Earth's surface pressure. Gravity, distance from the Sun, water and magnetic fields decided each fate.
An atmosphere is gas held to a world by its gravity, gathered early on from the surrounding nebula or breathed out of the young planet's hot interior. Keeping it is a contest. Strong gravity and cold temperatures help; the stellar wind strips gas away, and a magnetosphere slows the loss. That is why the chilly giants Jupiter, Saturn, Uranus and Neptune still hold vast envelopes of light hydrogen and helium with no true surface beneath, while the small rocky planets near the Sun kept only heavier gases built from carbon, nitrogen and oxygen.
Venus and Mars both have air made mostly of carbon dioxide, nitrogen and argon, yet they ended up worlds apart. Venus has no oceans or rain to pull carbon dioxide out of the sky, so the greenhouse gas piled up. Lacking a magnetic field and orbiting close to the Sun, it lost its water within about two billion years. Mars is small, cold and also unshielded, and has probably lost at least 80 to 85% of its original water to space. If all its frozen carbon dioxide turned to gas, surface pressure might reach about what climbers feel atop Mount Everest.
Earth's air, by contrast, is shaped by the life it supports, and its magnetosphere deflects the solar wind about ten Earth radii out. Elsewhere, Saturn's moon Titan and Neptune's moon Triton wrap themselves in nitrogen, and Pluto's nitrogen and methane freeze out as it swings away from the Sun.
Astronomers now read the air of planets around other stars. In 2002, watching a gas giant called HD 209458b pass in front of its star, they detected sodium, the first such find; its atmosphere, above 1,000 K, is leaking into space. Super-Earths hugging their stars may carry vapours boiled off magma oceans.
Source: Atmosphere
