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Mercury is the innermost and smallest planet in the solar system, orbiting
the Sun once every 88 days. Mercury is bright when viewed from
Earth, ranging from −2.0 to 5.5 in apparent magnitude, but is
not easily seen as its greatest angular separation from the Sun
is only 28.3°. It can only be seen in morning or evening
twilight. Comparatively little is known about it; the first of
two spacecraft to visit Mercury was Mariner 10, which mapped
only about 45% of the planet’s surface from 1974 to 1975. The
second is the MESSENGER spacecraft, which mapped another 30%
during its flyby of January 14, 2008. MESSENGER will make one
more pass by Mercury in 2009, followed by orbital insertion in
2011, and will then survey and map the entire planet.
Mercury is similar in appearance to the Moon: it is heavily
cratered, 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.
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 2439.7 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. 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 600 km mantle consisting of silicates.
Astronomers have postulated that, early in Mercury’s history, a
giant impact with a body several hundred kilometers across
stripped the planet of much of its original mantle material,
resulting in the relatively thin mantle compared to the sizable
core.
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, some extending over 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. 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 has been proposed
to explain the formation of Earth’s Moon (giant impact theory).
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 of
these hypotheses predicts a different surface composition, and
two upcoming space missions, MESSENGER and BepiColombo, both aim
to make observations to test them. |
