Mercury's elliptical orbit
takes the small planet as close as 47 million kilometers (29 million miles)
and as far as 70 million kilometers (43 million miles) from the Sun. If one
could stand on the scorching surface of Mercury when it is at its closest
point to the Sun, the Sun would appear almost three times as large as it does
when viewed from Earth. Temperatures on Mercury's surface can reach 430
degrees Celsius (800 degrees Fahrenheit). Because the planet has no atmosphere
to retain that heat, nighttime temperatures on the surface can drop to -170
degrees Celsius (-280 degrees Fahrenheit).
Because Mercury is so close to the Sun, it is hard to directly observe from
Earth except during twilight. Mercury makes an appearance indirectly, however
13 times each century, Earth observers can watch Mercury pass across the face
of the Sun, an event called a transit. These rare transits fall within several
days of May 8 and November 10. The first two transits of Mercury in the 21st
century occur May 7, 2003, and November 8, 2006.
Scientists used to think that the same side of Mercury always faces the Sun,
but in 1965 astronomers discovered that the plan-et rotates three times during
every two orbits. Mercury speeds around the Sun every 88 days, traveling
through space at nearly 50 kilometers (31 miles) per second faster than any
other planet. The length of one Mercury day (sidereal rotation) is equal to
58.646 Earth days.
Rather than an atmosphere, Mercury possesses a thin exo-sphere made up of
atoms blasted off its surface by the solar wind and striking micrometeoroids.
Because of the planet's ex-treme surface temperature, the atoms quickly escape
into space. With the thin exosphere, there has been no wind erosion of the
surface and meteorites do not burn up due to friction as they do in other
planetary atmospheres.
Mercury's surface resembles that of Earth's Moon, scarred by many impact
craters resulting from collisions with meteoroids and comets. While there are
areas of smooth terrain, there are also lobe-shaped scarps or cliffs, some
hundreds of miles long and soaring up to a mile high, formed by early
contraction of the crust. The Caloris Basin, one of the largest features on
Mercury, is about 1,300 kilometers (800 miles) in diameter. It was the result
of an asteroid impact on the planet's surface early in the Solar System's
history. Over the next half-billion years, Mercury shrank in radius about 1 to
2 kilometers (0.6 to 1.2 miles) as the planet cooled after its formation. The
outer crust contracted and grew strong enough to prevent magma from reaching
the surface, ending the period of geologic activity.
Mercury is the smallest planet in the Solar System
(see What Defines a Planet), larger only than previously measured dwarf
planets, such as Pluto. Mercury is the second densest planet after Earth,
with a large iron core having a radius of 1,800 to 1,900 kilometers (1,100
to 1,200 miles), about 75 percent of the planet's radius. Mercury's outer
shell, comparable to Earth's outer shell (called the mantle), is only 500 to
600 kilometers (300 to 400 miles) thick. Mercury's magnetic field is thought
to be a miniature version of Earth's, but scientists are uncertain of the
strength of the field.
Only one spacecraft has ever visited Mercury: Mariner 10, which imaged about
45 percent of the surface. In 1991, astronomers using radar observations
showed that Mercury may have water ice at its north and south poles inside
deep craters that are per-petually cold (below -212 degrees Celsius or -350
degrees Fahrenheit). Falling comets or meteorites might have brought ice to
these regions of Mercury, or water vapor might have outgassed from the
interior and frozen out at the poles.
A new NASA mission to Mercury called MErcury Surface, Space ENvironment,
Geochemistry, and Ranging (MESSENGER) will begin orbiting Mercury in March
2011 to investigate key scien-tific
areas such as the planet's composition, the structure of the core, the
magnetic field, and the materials at the poles.
Source:Nasa-Wikipedia
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Mariner 10's first image of Mercury.
