Atmosphere

Mercury has a very thin almost-airless atmosphere. At one time it was believed that the planet didn’t have an atmosphere at all, but it does contain small concentrations of the gases helium, hydrogen and oxygen as well as calcium, potassium and sodium. Because of Mercury’s size, it doesn’t have a strong enough gravitational pull to keep a stable atmosphere. It is constantly being lost and replenished via the solar wind, impacts and radioactive decay of elements in the crust.