Mercury

Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days.

Description:The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits. The perihelion of Mercury's orbit precesses around the Sun at an excess of 43 arcseconds per century, a phenomenon that was explained in the 20th century by Albert Einstein's General Theory of Relativity.Mercury is bright when viewed from Earth, ranging from −2.3 to 5.7 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun is only 28.3°. Since Mercury is normally lost in the glare of the Sun, unless there is a solar eclipse it can be viewed from Earth's Northern Hemisphere only in morning or evening twilight, while its extreme elongations occur in declinations south of the celestial equator, such that it can be seen at favorable apparitions from moderate latitudes in the Southern Hemisphere in a fully dark sky.There are two geologically distinct plains regions on Mercury.Gently rolling, hilly plains in the regions between craters are Mercury's oldest visible surfaces,predating the heavily cratered terrain. These inter-crater plains appear to have obliterated many earlier craters, and show a general paucity of smaller craters below about 30 km in diameter.It is not clear whether they are of volcanic or impact origin.The inter-crater plains are distributed roughly uniformly over the entire surface of the planet. Smooth plains are widespread flat areas which fill depressions of various sizes and bear a strong resemblance to the lunar maria. Notably, they fill a wide ring surrounding the Caloris Basin. Unlike lunar maria, the smooth plains of Mercury have the same albedo as the older inter-crater plains. Despite a lack of unequivocally volcanic characteristics, the localisation and rounded, lobate shape of these plains strongly support volcanic origins.All the Mercurian smooth plains formed significantly later than the Caloris basin, as evidenced by appreciably smaller crater densities than on the Caloris ejecta blanket.The floor of the Caloris Basin is filled by a geologically distinct flat plain, broken up by ridges and fractures in a roughly polygonal pattern. It is not clear whether they are volcanic lavas induced by the impact, or a large sheet of impact melt.One unusual feature of the planet’s surface is the numerous compression folds, or rupes, which crisscross the plains. As the planet’s interior cooled, it may have contracted and its surface began to deform, creating these features. The folds can be seen on top of other features, such as craters and smoother plains, indicating that the folds are more recent.Mercury’s surface is flexed by significant tidal bulges raised by the Sun—the Sun’s tides on Mercury are about 17 times stronger than the Moon’s on Earth.The mean surface temperature of Mercury is 442.5 K, but it ranges from 100 K to 700 K due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles. The subsolar point reaches about 700 K during perihelion then drops to 550 K at aphelion.On the dark side of the planet, temperatures average 110 K.The intensity of sunlight on Mercury’s surface ranges between 4.59 and 10.61 times the solar constant (1,370 W·m−2). Although the daylight temperature at the surface of Mercury is generally extremely high, observations strongly suggest that ice exists on Mercury. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below 102 K; far lower than the global average.Water ice strongly reflects radar, and observations by the 70 m Goldstone telescope and the VLA in the early 1990s revealed that there are patches of very high radar reflection near the poles.While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely.The icy regions are believed to contain about 1014–1015 kg of ice, and may be covered by a layer of regolith that inhibits sublimation.By comparison, the Antarctic ice sheet on Earth has a mass of about 4×1018 kg, and Mars' south polar cap contains about 1016 kg of water.The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet’s interior or deposition by impacts of comets.Mercury is too small and hot for its gravity to retain any significant atmosphere over long periods of time; it does have a "tenuous surface-bounded exosphere" containing hydrogen, helium, oxygen, sodium, calcium, potassium and others. This exosphere 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.