![]() The temperature of Mercury's exosphere depends on species as well as geographical location. ![]() This corresponds to a surface pressure of less than 10 −14 bar (1 nPa). ![]() ![]() Mariner 10 's ultraviolet observations have established an upper bound on the exospheric surface density at about 10 5 particles per cubic centimeter. The near-surface abundance of this newly detected constituent is roughly comparable to that of sodium. During its 2009 flyby, the Ultraviolet and Visible Spectrometer (UVVS) channel of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on board the MESSENGER spacecraft first revealed the presence of magnesium in the Mercurian exosphere. Other ions such as H 3O + ( hydronium), OH ( hydroxyl), O 2 + and Si + are present as well. Their abundances relative to sodium are about 0.2 and 0.7, respectively. In 2008 the MESSENGER probe's Fast Imaging Plasma Spectrometer (FIPS) discovered several molecular and different ions in the vicinity of Mercury, including H 2O + (ionized water vapor) and H 2S + (ionized hydrogen sulfide). Further observations by Messenger reported in 2014 note the atmosphere is supplemented by materials vaporized off the surface by meteors both sporadic and in a meteor shower associated with Comet Encke. Observations by the MESSENGER probe in 2009 showed that calcium is concentrated mainly near the equator-opposite to what is observed for sodium and potassium. In 1998 another element, calcium (Ca), was detected with column density three orders of magnitude below that of sodium. The properties and spatial distribution of these two elements are otherwise very similar. A year after the sodium discovery, Potter and Morgan reported that potassium (K) is also present in the exosphere of Mercury, though with a column density two orders of magnitude lower than that of sodium. Some research has claimed a correlation of the sodium abundance with certain surface features such as Caloris or radio bright spots however these results remain controversial. Its abundance is also enhanced near the dawn terminator as compared to the dusk terminator. Sodium is observed to concentrate near the poles, forming bright spots. The average column density of this element is about 1 × 10 11 cm −2. It was discovered in 1985 by Drew Potter and Tom Morgan, who observed its Fraunhofer emission lines at 589 and 589.6 nm. The fourth species detected in Mercury's exosphere was sodium (Na). Mercury's exospheric hydrogen and helium are believed to come from the Solar wind, while the oxygen is likely to be of crustal origin. In 2008 the MESSENGER probe confirmed the presence of atomic hydrogen, although its concentration appeared higher than the 1974 estimate. The near-surface concentrations of these elements were estimated to vary from 230 cm −3 for hydrogen to 44,000 cm −3 for oxygen, with an intermediate concentration of helium. The first constituents discovered were atomic hydrogen (H), helium (He) and atomic oxygen (O), which were observed by the ultraviolet radiation photometer of the Mariner 10 spaceprobe in 1974. The Mercurian exosphere consists of a variety of species originating either from the Solar wind or from the planetary crust. Later, in 2008, improved measurements were obtained by the MESSENGER spacecraft, which discovered magnesium in the Mercurian exosphere. This conclusion was confirmed in 1974 when the unmanned Mariner 10 spaceprobe discovered only a tenuous exosphere. The existence of a Mercurian atmosphere was contentious until 1974, although by that time a consensus had formed that Mercury, like the Moon, lacked any substantial atmosphere. Solar light pushes the atmospheric gases away from the Sun, creating a comet-like tail behind the planet. ![]() The exospheric species originate either from the Solar wind or from the planetary crust. Mercury, being the closest to the Sun, with a weak magnetic field and the smallest mass of the recognized terrestrial planets, has a very tenuous and highly variable atmosphere (surface-bound exosphere) containing hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor, with a combined pressure level of about 10 −14 bar (1 nPa). Mercury's surface, with the atmosphere too thin to be visible.Ĭolumn density cm −2 Surface density cm −3 ![]()
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