Moon

Earth's only known natural satellite (searches for Earth-orbiting moonlets having so far drawn a blank) and the only extraterrestrial body to have been visited by humans—12 in all, aboard six Apollo missions between July 1969 and December 1972. The Moon has been the target of numerous robotic probes including, most recently, Clementine (1994) and Lunar Prospector (1999). A total of 382 kg of rock samples were returned to Earth by Apollo and the Russian Luna programs; in addition, a number of lunar meteorites have been found. Most of these samples have been dated between 3 and 4.6 billion years (the one exception is a lunar meteorite dated at 2.8 billion years), and provide information about the early history of the solar system which is missing on Earth due to a lack of rocks more than about 3.8 billion years old. The lunar rocks have also provided strong support for the impact theory of the Moon's origin, namely, that the Moon formed from material splashed out of Earth's core and mantle by a colliding body as big or bigger than Mars shortly after Earth formed. Proof of this theory could come from the European Space Agency's SMART-1 probe, scheduled for launch in the second half of 2003, which will map the Moon in X rays. These X-ray measurements will show conclusively if the Moon contains less iron than Earth, compared to lighter elements such as magnesium and aluminum—as it should if, as predicted, the Moon came mainly from the light mantle rocks of the young Earth and its impactor.

The Moon has two main types of terrain: bright, densely cratered, ancient highlands, dating back over 4.5 billion years, and dark, relatively smooth, younger maria (Latin for “seas”), aged about 3.8 billion years. The maria, which make up about 17% of the Moon's surface, are huge impact basins that were later flooded by molten lava. Most of the surface is covered with a mixture of fine dust and rocky debris, known as regolith, produced by meteor impacts that vary in depth from 3 to 5 m in the maria to 10 to 20 m in the highlands.

The Moon's crust ranges in depth from 60 km on the nearside to 100 km on the farside. This uneveness may account for the fact that the Moon's center of mass is displaced from its geometric center by about 2 km in the direction toward Earth. Crustal uneveness may also explain differences in lunar terrain, such as the striking dominance of maria on the Earth-facing hemisphere. Below the crust is a mantle and probably a small core, some 340 km radius and containing about 2% of the lunar mass. Curiously, the Moon's center of mass is offset from its geometric center by about 2 km in the direction toward the Earth.

The Moon has no atmosphere. But evidence from Clementine suggests that there may be water-ice in some deep craters near the Moon's south pole that are permanently shaded. This was confirmed by Lunar Prospector, which also indicated there may be ice at the north pole. The Moon has no global magnetic field, though some of its surface rocks display remnant magnetism suggesting that there may have been a global magnetic field early in the Moon's history. With no atmosphere and no magnetic field, the Moon's surface is exposed directly to the solar wind. Lunar temperatures range from −184°C at night to 214°C during the day, except at the poles where it's a constant −96°C.

Mean diameter:	3,476 km .
Mass (Earth = 1):	0.0123
Mean density:	3.34 g/cm3
Surface gravity (Earth = 1):	0.165
Escape velocity:	2.38 km/s
Mean albedo:	0.07
Axial tilt:	6° 41′
Orbit
Mean distance from Earth:	384,404 km (1.28 light-seconds)
Eccentricity:	0.0549
Inclination:	5° 9′
Period:	27d 7h 43m 11s

The gravitational interaction between Earth and the Moon leads to some important effects, the most obvious of which is the tides. Although the solid rock of Earth and the Moon is distorted a bit by the mutual tugging, the effect is most noticeable on Earth's oceans. These are pulled into an elliptical shape. As Earth rotates under the ocean bulge, it causes high tides to propagate onto beaches. Because there are two bulges, there are two high tides and two low tides every day. In the case of Earth's rocky body, which isn't completely fluid, the bulges raised by the Moon's gravity are carried by Earth's rotation slightly ahead of the point directly beneath the Moon. The Moon's gravity, acting on these out-of-line bulges, then produces a twisting force, or torque, which slows Earth's rate of spin, by about 1.5 milliseconds/century. A similar but much more powerful braking effect by Earth on the Moon, long ago, slowed the Moon's spin rate to match its orbital period. To conserve angular momentum as Earth's spin slows, the size of the Moon's orbit gradually increases, by 3.8 cm/year. At this rate the Moon will look about 15 percent smaller from Earth in about one billion years. See also lunar eclipse, lunar phases, and transient lunar phenomenon.

Some Notable Features on the Moon

Feature	Description
Aitken Basin	An impact basin in the south polar region. With a diameter of about 2,500 km, a maximum depth of over 12 km, and an average depth of about 10 km, it is the biggest, deepest impact basin in the solar system.
Apennines	A mountain range that rises to 4,572 m at the southeastern edge of Mare Imbrium. The highest escarpment on the Moon, it is higher above the adjacent flatlands than the Himalayan front is above the plains of India and Nepal. The landing site of Apollo 15 was chosen to allow the astronauts to drive from the Lunar Module to the base of the Apennines during two excursions.
Bailly	The largest crater on the nearside of the Moon with a diameter of 295 km and maximum depth of 3.96 km. A highly eroded structure, it is named after the French astronomer Jean Bailly.
Copernicus	A 93-km-wide crater that is one of the most prominent features on the lunar nearside. Made less than 1 billion years ago, and thus one of the Moon's youngest major markings, it has a system of bright rays seen most clearly at full Moon.
Imbrium Basin	The largest and youngest of the giant impact basins on the nearside of the Moon. The asteroid collision that formed it, about 3.9 billion years ago, went close to breaking the Moon apart; in the event, it threw ejecta over much of the lunar surface and created deep fissures. Through these cracks, lava poured out, filling much of the basin and leaving the 1,300-km-wide dark feature known as Mare Imbrium.
Orientale Basin	The youngest and best preserved impact basin on the Moon, visible from Earth only at the extreme western limb as a libration feature. Formed some 3.8 to 3.9 billion years ago, it shows three concentric rings of mountains. Strong radial lineations made by the scouring flow of ejecta are also evident.
Tycho	A spectacular, 85-km-wide crater associated with the brightest and most extensive ray system on the Moon. In some cases, the rays extend for over 1,500 km; their prominence suggests that Tycho was formed relatively recently, perhaps within the past 3 billion years.

Moon An oblique view of the crater Copernicus on the lunar nearside, photographed from Apollo 17 in lunar orbit. NASA



Moon The sharpest image ever taken of the Moon by a ground-based telescope. The Very Large Telescope photographed this small patch of the Moon's surface, some 6° north of the lunar equator and near the intersection of Mare Tranquillitatis and Mare Foecunditatis, on April 30, 2002, revealing detail as small as 130 meters across. The large crater at the top is named Cameron and has a diameter of about 10 km. The size of the field is about 60 km × 45 km. European Southern Observatory