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Mercury

Mercury is the closest and smallest to the Sun planet with the most eccentric orbit and the smallest axial tilt, with its axis almost perpendicular to the plane of its orbit around our star. Its first recorded observations go back to the first millennium BC as Mercury is at times very bright with a magnitude ?2.6 down to only +5.7, which makes it almost invisible, given that it is always close to the Sun, as its maximum elongation is ~18^o.

Mercury is larger than the Moon with several similarities to our satellite. Mercury’s similarities with the Moon, with heavy cratering and very large areas covered by mare-like plains show that it is geologically inactive for billions of years. Mercury’s atmosphere is very tenuous because of the very low gravity and high temperature. Its tenuous surface-bounded exosphere is made mainly of hydrogen, helium, oxygen, sodium, calcium, potassium. The atoms of this exosphere are continuously lost but at the same time replenished from many sources. Hydrogen and helium are provided by the solar wind as they diffuse in the week but important magnetosphere. Although it is not expected for a small and slow rotating body to have a magnetic field, Mercury surprisingly hosts a dipolar magnetic field probably as a result of the rich in iron core of the small planet..

Mercury orbits the Sun every 87.97 days and rotates around its axis with respect to the stars every 58.7 days. The mercurian day is approximately 176 days and hence completes three rotations about its axis for every two orbits, i.e. Mercury is in a 3:2 spin–orbit resonance wit the Sun, as a result of its eccentric orbit and eccentric center of mass.

Due to the proximity to the Sun the temperature of Mercury is very high and due to the absence of an atmosphere it varies enormously from a freezing 100 K (-173 C) at the night side of the planet to a burning 700 K, with an equally large difference from the equator to the polar regions and an average of 440 K.

The first spacecraft to visit Mercury was NASA’s Mariner 10 (1974–75) that gave the first good images of Mercury, revealing the existence of extensive cratered terrains, giant scarps. MESSENGER, the new NASA mission to Mercury, was launched on August 3, 2004 and it is expected to lead to the understanding of Mercury's geological history, high density, origin of its dipolar magnetic field, the nature and structure of the core, search for possible existence of ice at its poles, and investigate nature and origin of the rarefied atmosphere.

BepiColombo,a joint mission of the European Space Agency (ESA) planned together with the Japanese Space Agency (JAXA),  is composed of two spacecrafts to map the planet and to probe the dynamic magnetosphere to be in operation around Mercury around 2019.

LINKS:

http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mercury

http://www.nasa.gov/mission_pages/messenger/main/index.html

http://www.esa.int/esaSC/SEMOHGO2UXE_index_0.html

PICTURES:

Courtesy NASA/JPL-Caltech

From http://pds.nasa.gov/planets/special/mercury.htm

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