Moon

Earth's moon is a roughly spherical, rocky body orbiting Earth at an average distance of 238,000 mi (382,942 km). Its diameter is about one-fourth Earth's diameter. Compared to moons of other planets, this is a The first footprint on the moon's surface, made by astronaut Neil A. Armstrong on July 20, 1969, shows the fineness of the lunar soil. The soil is produced when moon rock is weathered by impacting meteors, solar winds and extreme temperature changes. In many areas the soil appears to be a few meters thick. U.S. National Aeronautics and Space Administration (NASA). A photo of the full moon taken from Apollo 17. The mare regions, which are relatively flat, appear as dark areas because they reflect less light. The highlands are lighter in color and have a more rugged surface than the mare. About 1/3 of the surface shown in this picture cannot be seen from the Earth. U.S. National Aeronautics and Space Administration (NASA). JLM Visuals. Reproduced by permission. large relative size. The diameters of other moons are a much smaller fraction of their planets' diameter.

The mass of the Moon is only about 1/80 the mass of the Earth, so the force of gravity is smaller. On the Moon the acceleration due to gravity is only one-sixth Earth's. Accordingly, masses "weigh" one-sixth as much on the Moon. The astronauts who landed there could leap high and long with very little effort because of the reduced gravitational force. This is also why the Moon has no atmosphere. The escape velocity of the Moon, which is related to the mass of the planet, is very low—about the same as the velocity due to collisional motion in an atmospheric gas mixture—so any atmosphere that might have once formed would have easily escaped the pull of the Moon's gravity.

The view of the Moon remains the same (viewing the same face) because it rotates on its axis at the same rate that it travels around the Earth—once every 29.5 days. This is no mere coincidence. The side of the Moon facing us is attracted more strongly by Earth's gravitational force than the opposite side. The force of gravity depends on the mass of the two interacting objects (here the Earth and the Moon) and the Moon has a mass asymmetry. There is more mass concentrated in the half of the Moon we see than in the other half. The effect is called "gravitational locking" and is a common occurrence in our solar system.

In 1998 ice was found in the Moon's craters by the orbiting American spacecraft Lunar Prospector. The frozen water is in the form of ice crystals mixed with dirt, and was found covering each of the Moon's poles in relatively large amounts. It is possible that such water could someday be retrieved out of the Moon's dirt and used by human colonies living permanently on the Moon's harsh surface.