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 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 .

Phases and eclipses

Observations of the Moon from Earth, reveal different phases of the Moon with respect to the percentage of lunar surface reflecting sunlight back to Earth. As the Moon orbits Earth, it comes between Earth and the Sun once a month at the time of a new Moon, and orbits Earth in very nearly the same plane that the Earth orbits the Sun. If the Moon's orbital plane had zero tilt off the Earth's orbital plane, a total lunar eclipse would be visible every month.

There is actually about a 20° tilt, so total eclipses over selected area occur only about seven times a year, when the Earth is directly between the Sun and the Moon. The Earth's shadow falls on the full Moon, and it slowly becomes dark and then bright again as it moves out of the Earth's shadow.

The lunar surface

Although surface features were observed as early as 1600 by Galileo, most information regarding the surface is from spacecraft exploration.

In 1959 the Soviet Union sent the first spacecraft to orbit the Moon, sending back tantalizing photographs. American astronauts orbited the Moon in 1968 and landed manned expeditions from 1969 to 1972. Six landed during those years, providing copious data from the lunar surface. Apollo experimentation included soil-testing devices, cameras, seismometers, solar wind collectors, and rock collection. The Soviet Union also landed an unmanned craft during this time (1970), collecting additional data for analysis.

Moon rocks

Spacecraft experimentation found mostly igneous rock on the Moon (rock formed by cooling lava) but some sedimentary. The sedimentary rock was probably formed by falling debris after meteoritic impacts. The igneous rock in the marina is mostly basalt, but the highlands are mostly anorthosites. Both types form from cooling lava but under different conditions and at different cooling rates.

Radioactive dating of rocks brought back by the Apollo astronauts yields an absolute age (the time since the rocks solidified) of the highlands as 3.9 to 3.8 billion years, with the final lava flow around 3 billion years ago.

The Moon has no overall magnetic field. According to the currently favored model of planetary magnetic fields (the dynamo model), this means either that the Moon probably has no molten core or that only a very small part of the core is molten. There is a weak magnetic field frozen into the rocks, however, or the rocks have a north pole and a south pole, so it's possible that the Moon once had a magnetic field surrounding it.

Lunar ice

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.

Lunar origin

The fact that the oldest rocks on the Moon are about the same age as the oldest rocks found on Earth tells indicates that the two were formed around the same time. Rather than forming by gravitational clumping of matter in orbit around the Earth, however, the Moon is probably a captured asteroid. Studies of the differences in the compositions of the Earth and Moon indicate that, during the early stages of the Earth's formation, a large asteroid struck the Earth a glancing blow, and pieces of it, as well as pieces of Earth, flew back up to orbit the Earth in a ring-like system around the planet. Eventually, the force of gravity caused the fragments to coalesce and form our Moon. This is called the "ring ejection theory" of lunar origin.

Dynamic Moon

In 2003, astronomers announced that data and observations indicated one particular crater might well be classified as the youngest crater thus far discovered. The crater, formed in 1953 by an asteroid impact, is the only known lunar crater to have been formed during recorded human history. New and more powerful telescopes, along with orbiting satellite photos now allow examination of the impact site and astronomers observed indications of a fresh crater in the impact zone (i.e., the area corresponding to the impact flash observed in 1953).

Although impacts continue, and thus new crater formation is a continuous event, the crater at the 1953 impact site is the first new crater to be observed.

The object that struck the Moon is estimated to have been about 328 yd (300 m) across and the estimated energy released from impact would have been the equivalent of approximately that observed from the detonation of one-half megaton of TNT (more than 30 times more energetically destructive than the atomic bomb dropped on Hiroshima).

See also Planetary geology; Spacecraft, manned.

Resources

books

Bakich, Michael A. The Cambridge Planetary Handbook. New York: Cambridge University Press, 2000.

Light, Michael, and Andrew Chaikin. Full Moon. New York: Knopf, 2000.

Wlasuk, Peter. Observing the Moon (Practical Astronomy). New York: Springer Verlag, 2002.

other

Williams, David R., NASA, National Space Science Data Center. "The Moon" [cited March 10, 2003]. <http://nssdc.gsfc.nasa.gov/planetary/planets/moonpage.html>.