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.
Mercury/Tail - A tail of atoms escaping from Mercury's gravity due to irradiation by the Sun. Its composition and intensity varies by season. Comprised mostly of Sodium, but Calcium has also been detected. Its length was much diminished during Messenger's third flyby as Mercury was traveling at a different speed along its orbit than it was during the first two flybys. Earthbased observations show that the tail extends more than a thousand planetary radii.
Mercury/Calcium - Element found in Mercury's atmosphere in equatorial regions, and in higher concentrations near sunrise than sunset, which oddly persisted during all three Messenger flybys while most of the rest of the environment was ever-changing. It is a mystery why Sodium and Magnesium are distributed differently. Also detected in Mercury's tail. Emissions from ionized Calcium were observed near the equatorial plane in the exosphere by Messenger during its third flyby.
Mercury/Sodium - Element found in Mercury's atmosphere in polar regions. Its distribution is different than that for Calcium and Magnesium. It's also main constituent in Mercury's tail, which was much diminished in length during Messenger's third flyby due its its different speed than during the first two.
Mercury/Magnesium - Element thought to exist on the surface of Mercury. It was detected in much higher abundancies by Messenger during a flyby than expected (similar to that of Calcium). It is evenly distributed over the globe, for some reason, unlike Calcium (near the equator), which is a puzzle. Also differently distributed than Sodium.
Mercury/Iron - One of the main constituents of Mercury's molten core. Likely bonded with a lighter element, such as Sulfur, in order to stay molten. Not completely molten in the core. Suspected to exist in trace amounts in the atmosphere, but has not been detected yet.
Mercury/Aluminum - Surface element on Mercury, less abundant compared to Silicon than the Earth, in favor of more Potassium. It is suspected of existing in Mercury's atmosphere.
Mercury/Silicon - Surface element on Mercury. It is suspected of existing in Mercury's atmosphere.