MERCURY

MASS: 5.5% of earth's
DIAMETER: 3,031 miles
ROTATION: 58.6 earth days
ORBIT AROUND SUN: 88 earth days

Mercury is the closest planet to the sun and the second smallest planet next to Pluto. Since it is so close to the sun it is hard to see except during twilight. Its average distance from the sun is 36 million miles. It follows an elliptical orbit (non-circular), which can bring it as close as 30 million miles and as far as 43 million miles away from the sun. Mercury has very little atmosphere, which means that the temperature changes drastically getting as hot as 750 degrees F during the day and as cold as -320 degrees below zero during the night. Although Mercury spins very quickly around the Sun, it spins on its axis very slowly, taking about 58.6 Earth days to complete a circle.Like our Moon, Mercury has very little atmosphere, mostly burned off millions of years ago by the planet's close proximity to the Sun. It consists of argon, neon, and helium. Because of Mercury's extreme surface temperature, these atoms quickly escape into space and are constantly replenished. Since there isn't much of an atmosphere there isn't any erosion from wind or water or anything to stop meteors from crashing into the planet. The surface of Mercury is covered with thousands of craters. While there are areas of smooth terrain, there are also cliffs, some soaring up to two miles high. A network of faults helped to create this rough and rocky terrain. The long ridges where the elevation rises or drops steeply appear to be faults where the outer crust (lithosphere) wrinkled when the planet shrunk. Because of the abundance and length of faults, scientists believe the diameter of the planet shrunk 1 to 2 miles, probably as Mercury's metallic core cooled. The Caloris Basin, one of the largest features on Mercury, is about 800 miles in diameter. It was a result of an asteroid impact. There is evidence of Mercury's active past in the smooth plains of the basin.The high density and weak magnetic field of the planet indicate that more than 60% of Mercury consists of metal, iron to be more specific. The diameter of the core is 1120 - 1180 miles. It makes up more than ? of the planet. The outer shell (mantle) is only 310 to 370 miles thick.