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Mercury, the closest planet to the Sun, looks much like Earth's Moon at first glance.
Unlike the Moon, it is crisscrossed by large thrust faults that formed as the planet contracted while it cooled. Mercury is
the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days.
Comparatively little
is known about Mercury; ground-based telescopes reveal only an illuminated crescent with limited detail. The first of two
spacecraft to visit the planet was Mariner 10, which mapped about 45% of the planet’s surface from 1974 to 1975. The
second is the MESSENGER spacecraft, which mapped a further 30% during its flyby of January 14, 2008. MESSENGER's last flyby
took place in September 2009 and it is scheduled to attain orbit around Mercury in 2011, where it will begin mapping the rest
of the planet. Mercury is similar in appearance to the Moon: it is heavily cratered with regions of smooth plains, has no
natural satellites and no substantial atmosphere. However, unlike the Moon, it has a large iron core, which generates a magnetic
field about 1% as strong as that of the Earth. It is an exceptionally dense planet due to the large relative size of its core.
Surface temperatures range from about 90 to 700 K (-183 °C to 427 °C, -297 °F to 801 °F), with the subsolar
point being the hottest and the bottoms of craters near the poles being the coldest.
Recorded observations of Mercury
date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two
separate objects: one visible only at sunrise, which they called Apollo; the other visible only at sunset, which they called
Hermes. The English name for the planet comes from the Romans, who named it after the Roman god Mercury, which they equated
with the Greek Hermes. The astronomical symbol for Mercury is a stylized version of Hermes' caduceus.
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.[9]
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, Mercury can only be viewed in morning or evening twilight.
Mercury is one of four terrestrial
planets in the Solar System, and is a rocky body like the Earth. It is the smallest planet in the Solar System, with an equatorial
radius of 2,439.7 km. Mercury is even smaller—albeit more massive—than the largest natural satellites in the Solar
System, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material. Mercury's density is
the second highest in the Solar System at 5.427 g/cm³, only slightly less than Earth’s density of 5.515 g/cm³.
If the effect of gravitational compression were to be factored out, the materials of which Mercury is made would be denser,
with an uncompressed density of 5.3 g/cm³ versus Earth’s 4.4 g/cm³.
Mercury’s density can be used to infer details of its inner structure. While the
Earth’s high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller
and its inner regions are not nearly as strongly compressed. Therefore, for it to have such a high density, its core must
be large and rich in iron.
1. Crust—100–300 km thick
2. Mantle—600 km thick
3. Core—1,800
km radius
Geologists estimate that Mercury’s core occupies about 42% of its volume; for Earth this proportion
is 17%. Recent research strongly suggests Mercury has a molten core. Surrounding the core is a 500–700 km mantle consisting
of silicates. Based on data from the Mariner 10 mission and Earth-based observation, Mercury’s crust is believed to
be 100–300 km thick. One distinctive feature of Mercury’s surface is the presence of numerous narrow ridges, and
these can extend up to several hundred kilometers. It is believed that these were formed as Mercury’s core and mantle
cooled and contracted at a time when the crust had already solidified.
Mercury's core has a higher iron content than
that of any other major planet in the Solar System, and several theories have been proposed to explain this. The most widely
accepted theory is that Mercury originally had a metal-silicate ratio similar to common chondrite meteors, thought to be typical
of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass. However, early in the Solar System’s
history, Mercury may have been struck by a planetesimal of approximately 1/6 that mass and several hundred kilometers across.
The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component.
A similar process, known as the giant impact theory, has been proposed to explain the formation of Earth’s Moon.
Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized. The planet would
initially have had twice its present mass, but as the protosun contracted, temperatures near Mercury could have been between
2,500 and 3,500 K (Celsius equivalents about 273 degrees less), and possibly even as high as 10,000 K. Much of Mercury’s
surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" which could have
been carried away by the solar wind. A third hypothesis proposes that the solar nebula caused drag on the particles from which
Mercury was accreting, which meant that lighter particles were lost from the accreting material. Each hypothesis predicts
a different surface composition, and two upcoming space missions, MESSENGER and BepiColombo, both aim to make observations
to test them.
Image from MESSENGER's second Mercury flyby. Kuiper crater is just below center. An extensive ray
system emanates from the crater near the top.Main article: Geology of Mercury. Mercury’s surface is overall very similar
in appearance to that of the Moon, showing extensive mare-like plains and heavy cratering, indicating that it has been geologically
inactive for billions of years. Since our knowledge of Mercury's geology has been based on the 1975 Mariner flyby and terrestrial
observations, it is the least understood of the terrestrial planets. As data from the recent MESSENGER flyby is processed
this knowledge will increase. For example, an unusual crater with radiating troughs has been discovered which scientists are
calling "the spider."
Mercury was heavily bombarded by comets and asteroids during and shortly following
its formation 4.6 billion years ago, as well as during a possibly separate subsequent episode called the late heavy bombardment
that came to an end 3.8 billion years ago. During this period of intense crater formation, the planet received impacts over
its entire surface, facilitated by the lack of any atmosphere to slow impactors down. During this time the planet was volcanically
active; basins such as the Caloris Basin were filled by magma from within the planet, which produced smooth plains similar
to the maria found on the Moon.
Mercury’s Caloris Basin is one of the largest impact features in the Solar System.Craters
on Mercury range in diameter from small bowl-shaped cavities to multi-ringed impact basins hundreds of kilometers across.
They appear in all states of degradation, from relatively fresh rayed craters to highly degraded crater remnants. Mercurian
craters differ subtly from lunar craters in that the area blanketed by their ejecta is much smaller, a consequence of Mercury's
stronger surface gravity.
The largest known crater is Caloris Basin, with a diameter of 1,550 km,. The impact that
created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric ring over 2 km tall surrounding
the impact crater. At the antipode of the Caloris Basin is a large region of unusual, hilly terrain known as the "Weird
Terrain". One hypothesis for its origin is that shock waves generated during the Caloris impact traveled around the planet,
converging at the basin’s antipode (180 degrees away). The resulting high stresses fractured the surface. Alternatively,
it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin’s antipode.
Overall, about 15 impact basins have been identified on the imaged part of Mercury. A notable basin is the 400 km
wide, multi-ring Tolstoj Basin which has an ejecta blanket extending up to 500 km from its rim and a floor that has been filled
by smooth plains materials. Beethoven Basin has a similar-sized ejecta blanket and a 625 km diameter rim. Like the Moon, the
surface of Mercury has likely incurred the effects of space weathering processes, including Solar wind and micrometeorite
impacts.
The so-called “Weird Terrain” was formed by the Caloris Basin impact at its antipodal point.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.
Despite the generally extremely high temperature
of its surface, 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.[48] 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 for its gravity to retain any significant atmosphere over long periods
of time; however, 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. MESSENGER found high proportions of calcium, helium, hydroxide, magnesium, oxygen, potassium,
silicon and sodium. Water vapor is present, released by a combination of processes such as: comets striking its surface, sputtering
creating water out of hydrogen from the solar wind and oxygen from rock, and sublimation from reservoirs of water ice in the
permanently shadowed polar craters. The detection of high amounts of water-related ions like O+, OH-, and H2O+ was a surprise.
Because of the quantities of these ions that were detected in Mercury's space environment, scientists surmise that these molecules
were blasted from the surface or exosphere by the solar wind.
Sodium, potassium and calcium were discovered in
the atmosphere during the 1980–1990s, and are believed to result primarily from the vaporization of surface rock struck
by micrometeorite impacts. In 2008 magnesium was discovered by MESSENGER probe. Studies indicate that, at times, sodium emissions
are localized at points that correspond to the planet's magnetic poles. This would indicate an interaction between the magnetosphere
and the planet's surface.
Despite the generally extremely high temperature of its surface, 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.
Mercury is too small for its gravity to retain any significant atmosphere over long periods of time; however, 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. MESSENGER
found high proportions of calcium, helium, hydroxide, magnesium, oxygen, potassium, silicon and sodium. Water vapor is present,
released by a combination of processes such as: comets striking its surface, sputtering creating water out of hydrogen from
the solar wind and oxygen from rock, and sublimation from reservoirs of water ice in the permanently shadowed polar craters.
The detection of high amounts of water-related ions like O+, OH-, and H2O+ was a surprise. Because of the quantities of these
ions that were detected in Mercury's space environment, scientists surmise that these molecules were blasted from the surface
or exosphere by the solar wind.
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