Thin surface bound exosphere of Mercury, sometimes referred to as its atmosphere. Its main constituents are Hydrogen and Helium (believed to originate from solar winds) and atomic Oxygen (believed to originate from the surface). Sodium and to a lesser degree Potassium are found near the poles. Calcium is centered around the equator. Magnesium was first detected by Cassini during a flyby. Originally speculated about when it was thought to be tidally locked, found to be mostly absent by Mariner 9. Molocules are constantly escaping to space, forming a tail away from the Sun due to heat, so must be replenished somehow.