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Mercury is the closest planet to the sun. Itís average distance from the
sun is approximately fifty-eight million km and itís diameter is 4875
km, making it the second smallest planet in our solar system. Itís
volume and mass are about 1/18 that of the earth and itís average
density is approximately equal to that of the earth. Mercuryís magnetic
field is one-hundred times weaker than that of Earthís. Mercury has the
shortest revolution of all the planets in our solar system and revolves
around the sun in about eighty-eight days. Radar observations of the
planet show that its period of rotation is 58.7 days, or two-thirds of
its period of revolution. That means that Mercury has one and one-half
days in itís year.
Mercury doesnít have an atmosphere, but
it does have a thin layer of helium. The helium is actually solar wind
that is trapped by Mercuryís weak gravity. Scientists think that
collisions with protoplanets early in the history of the solar system
may have stripped away lighter materials, making Mercury a very dense
planet with an iron core extending outwards 4/5 of the way to the
Mercury bares a very similar resemblance
to our moon because it has a lot of craters. The craters, which cover
seventy-five percent of Mercuryís surface, were formed by huge rocks
that smashed into the planetís surface. The largest crater is called the
Caloris Basin and it is 1400 km in diameter and is flooded with molten
lava. Mercury also has many cliffs that are usually over 300 miles long
and two miles high. The rest of the planetís surface is smooth and may
have been formed by lava flowing out of cracks in the surface.
Temperatures on Mercury vary greatly because of itís closeness to the
sun. The surface temperature on the sunlit side is about 430 degrees
Celsius, while the dark side may reach temperatures of -170 degrees
Mercury was a difficult planet to study
before the invention of the telescope. Even then, you could only see
Mercury in the morning and evening. Then the Mariner 10 was built in the
1970ís to go observe Mercury. The Mariner 10 spacecraft passed Mercury
twice in 1974 and once in 1975 and it took hundreds of pictures of the
planet. After this, the Mariner 10 came too close to the sun and is now
orbiting the sun.
Mercury has no known moons and it also
has a double sunrise at perihelion (the point closest to the sun).
Mercury also has the widest temperature range (500 degrees between
coldest and hottest) of all the planets. But even with all this
information, scientists still donít know that much about Mercury.
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