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Mercury Mariner10.jpg
Atmosphere of Mercury[1]
Molecular oxygen 42%
Sodium 29%
Hydrogen 22%
Helium 6%
Potassium 0.5%
Water 5-10%??
Possible trace

amounts of

Argon, neon, carbon dioxide

nitrogen, xenon, krypton

Mercury's primordial atmosphere dissipated shortly after the planet's formation because of both the low level of gravity on the planet, the high temperature, and the effects of the solar wind. However, at present there are traces of a very tenuous atmosphere containing hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor, with a combined pressure level poorly known: NASA estimates have varied from 10-15 bar (0.1 nPa)[1] (which can be considered negligible), to "about 0.00000000003 pound per square inch",[2] or approximately 200 nPa. This gives a rough mass estimate of 1.6*106 kg.



In 1974 Mariner 10 flew past Mercury and mapped half the surface. It also discovered that Mercury has a magnetic field. It is very weak, only about 1% of Earth's. The magnetosphere may be a result of the planet's large iron core which makes it heavy for its size. The core is three quarters of the way to the surface. Scientists now believe there is a thin layer of partly molten iron and sulfur above the inner core which helps to drive this magnetic field. This field also traps particles from the solar wind, replenishing Mercury's thin atmosphere which is being continually lost to the Sun.

Atmospheric composition

This atmosphere is not stable—atoms are continuously lost and replenished from a variety of sources. Hydrogen and helium atoms probably come from the solar wind, diffusing into Mercury’s magnetosphere before later escaping back into space. Radioactive decay of elements within Mercury’s crust is another source of helium, as well as sodium and potassium.

On 2008-07-03, it was revealed that the MESSENGER probe had discovered water vapor in the upper levels of Mercury's atmosphere, about "one ... for every three or four sodium ions".[3] This is believed to be formed on the spot via hydrogen from the solar wind and oxygen from the minerals in Mercury's crust.


Surface temperature variations on bodies without atmospheres, or with extremely weak atmospheres such as Mercury, are very pronounced. For example, during the day Mercury's surface reaches a temperature of 420 °C, while at night it dips to −180 °C. This is because Mercury's atmosphere cannot maintain a relatively constant temperature around the planet as, for example, Earth's atmosphere does, because there is not enough atmosphere to even out the distribution of heat energy. Due to these huge changes in the temperature, thermal stresses may cause significant effects on Mercury's surface.

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