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The closest planet to the Sun, Mercury
is a small, rocky world. It's elliptical orbit takes
it as close as 47 million km and as far as 70 million
km from the Sun. Mercury completes a trip around the
Sun every 88 days, speeding through space at nearly
50 km per second, faster than any other planet. Because
it is so close to the Sun, temperatures on its surface
can reach a scorching 467 degrees Celsius. But because
the planet has hardly any atmosphere to keep it warm,
nighttime temperatures can drop to a frigid -183 degrees
Celsius.
Because Mercury is so close to the Sun,
it is hard to see from Earth except during twilight.
Until 1965, scientists thought that the same side of
Mercury always faced the Sun. Then, astronomers discovered
that Mercury completes three rotations for every two
orbits around the Sun. If you wanted to stay up for
a Mercury day, you'd have to stay up for 176 Earth days!
Like our Moon, Mercury has almost no atmosphere.
What little atmosphere exists is made up of atoms blasted
off its surface by the solar wind and has less than
a million-billionths the pressure of Earth's atmosphere
at sea level. It is composed chiefly of oxygen, sodium,
and helium. Because of Mercury's extreme surface temperature,
these atoms quickly escape into space and are constantly
replenished. With no atmosphere to protect the surface,
there has been no erosion from wind or water, and meteorites
do not burn up due to friction as they do in other planetary
atmospheres. Mercury's surface very much resembles Earth's
Moon, scarred by thousands of impact craters resulting
from collisions with meteors. While there are areas
of smooth terrain, there are also cliffs, some soaring
up to a mile high, formed by ancient impacts.
The Caloris Basin, one of the largest
features on Mercury, is about 1,300 km in diameter.
It was the result of an asteroid
impact on the planet's surface early in the solar system's
history, the probable cause of the strange surfaces
on the opposite side of the planet. Over the next half-billion
years, Mercury actually shrank in radius from 2 to 4
km as the planet cooled from its formation. The outer
crust, called the lithosphere, was compressed and grew
strong enough to prevent the planet's magma from reaching
the surface, effectively ending the planet's period
of geologic activity. Evidence of Mercury's active past
is seen in the smooth plains in the Caloris basin.
Mercury is the second smallest planet
in the solar system, larger only than Pluto,
the most distant planet in our solar system. If Earth
were the size of a baseball, Mercury would be the size
of a golf ball. Viewed from Mercury, the Sun would look
almost three times as large as it does from Earth. Mercury
is the second densest body in the solar system after
Earth, with an interior made of a large iron core with
a radius of 1,800 to 1,900 km, nearly 75 percent of
the planet's diameter and nearly the size of Earth's
Moon. Mercury's outer shell, comparable to Earth's outer
shell (called the mantle) is only 500 to 600 km thick.
Only one spacecraft has ever visited Mercury:
Mariner 10 in 1974-75. Mariner 10's discovery that Mercury
has a very weak magnetic field, similar to but weaker
than Earth's, was a major surprise. In 1991, astronomers
using radar observations showed that Mercury may have
water ice at its north and south poles. The ice exists
inside deep craters. The floors of these craters remain
in perpetual shadow, so the Sun cannot melt the ice.
NASA is planning a new mission to Mercury
called Mercury Surface, Space Environment, Geochemistry,
and Ranging (MESSENGER), which will orbit Mercury toward
the end of this decade. MESSENGER will investigate key
science questions using a set of miniaturized instruments:
Why is Mercury so dense? What is the geologic history
of Mercury? What is the structure of Mercury's core?
What is the nature of Mercury's magnetic field? What
are the unusual materials at Mercury's poles? What volatiles
are important on Mercury?
Related Links:
Source: NASA
Last Updated : 08.20.2003
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Distance from the Sun:
(Semimajor axis of orbit)
57,909,175 km
0.38709893 A.U.
Radius:
2,439.7 km
(0.3825 of Earth's radius)
Volume:
0.054 (Earth = 1)
Mass:
0.3302 x 1027 g
Density:
5.43 gm/cm3
Surface Gravity:
370 cm/s2
Escape Velocity:
4.25 km/s (at equator)
Sidereal Rotation Period:
58.646225 days
Sidereal Orbit Period:
0.2408467 sidereal years
Mean Orbit Velocity:
47.8725 km/s
Orbit Eccentricity:
0.20563069
Orbit Inclination:
7.00487 degrees
Equatorial Inclination:
0.0 degrees
Mean Surface Temperature:
440 K
Natural Satellites:
None
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