Lunar Exploration - Wavelength Virtual Astronomical Tours

Planet Profile - The Moon

orbit: 384,400 km from Earth
diameter: 3476 km
mass: 7.35e22 kg

Would you like to know your weight on the Moon?

Due to its size and composition, the Moon is sometimes classified as a terrestrial "planet" along with Mercury, Venus, Earth and Mars. A total of 382 kg of rock samples were returned to the Earth by the Apollo and Luna programs. These provide most of our detailed knowledge of the Moon. They are particularly valuable in that they can be dated.

Most rocks on the surface of the Moon seem to be between 4.6 and 3 billion years old. This is a match with the oldest terrestrial rocks which are rarely more than 3 billion years old. Thus the Moon provides evidence about the early history of the Solar System not available on the Earth.

The gravitational forces between the Earth and the Moon cause some interesting effects. The most obvious is the tides. The Moon's gravitational attraction is stronger on the side of the Earth nearest to the Moon and weaker on the opposite side. Since the Earth, and particularly the oceans, is not perfectly rigid it is stretched out along the line toward the Moon. From our perspective on the Earth's surface we see two small bulges, one in the direction of the Moon and one directly opposite. The effect is much stronger in the ocean water than in the solid crust so the water bulges are higher. And because the Earth rotates much faster than the Moon moves in its orbit, the bulges move around the Earth about once a day giving two high tides per day.

But the Earth is not completely fluid, either. The Earth's rotation carries the Earth's bulges slightly ahead of the point directly beneath the Moon. This means that the force between the Earth and the Moon is not exactly along the line between their centers producing a torque on the Earth and an accelerating force on the Moon. This causes a net transfer of rotational energy from the Earth to the Moon, slowing down the Earth's rotation by about 1.5 milliseconds each century and raising the Moon into a higher orbit by about 3.8 centimeters per year. The opposite effect happens to satellites with unusual orbits such as Phobos and Triton.

The asymmetric nature of this gravitational interaction is also responsible the Moon rotating synchronously, it is locked in phase with its orbit so that the same side is always facing toward the Earth. As the Earth's rotation is being slowed by the Moon's influence so in the distant past the Moon's rotation was slowed by the action of the Earth, but in that case the effect was much stronger. When the Moon's rotation rate slowed to match its orbital period the bulge always faced toward the Earth there was no longer an off-center torque on the Moon and a stablity was achieved. The same thing has happened to most of the other satellites in the solar system. Eventually, the Earth's rotation will be slowed to match the Moon's period, too, as is the case with Pluto and Charon.

The Moon appears to wobble a bit due to its slightly non-circular orbit so that a few degrees of the far side can be seen from time to time, but the majority of the far side was completely unknown until the Soviet spacecraft Luna 3 photographed it in 1959.

Highland Anorthosite
When the moon first formed it probably had a surface composed mostly of feldspar-rich igneous rocks. This rock type still exists today and makes up the lunar highlands, which is the lighter-colored part of the moon visible from Earth. This 4.4-billion-year-old rock sample is an anorthosite collected from the lunar highlands of the moon by Apollo 16 astronauts.

Mare Basalt
This igneous rock was collected from one of the darker areas on the moon known as mare. The mare formed billions of years ago. Large meteorites impacted the surface of the moon and broke up the crust. Later lavas formed by melting of rock within the moon due to the decay of radioactive elements. The broken crust under the big impact craters allowed the lava to come to the surface. Over time the craters came to be filled with lava flows. This basalt sample, estimated to be 3.7 billion years old, was collected by Apollo 17 astronauts.

Highland Breccia
On Earth, the surface is eroded by the action of water and wind. The most important process for altering the surface of the moon, however, is that of meteorites impacting upon and breaking up the surface. This lunar sample is a rock type named breccia. On the moon a breccia is made when meteorites break up the surface and the pieces are welded together by the heat and pressure of impact processes. This breccia was collected by Apollo 16 astronauts and is 3.9 billion years old.

Our Closest Neighbor

The Moon is the only natural satellite of the Earth. Called Luna by the Romans, Selene and Artemis by the Greeks, and many other names in other mythologies. The Moon is the second brightest object in the sky after the Sun.

As the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes; we see this as the cycle of the Moon's phases. The time between successive new moons is 29.5 days (709 hours), slightly different from the Moon's orbital period (measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time.

The Moon was first visited by the Soviet Luna 2 spacecraft in 1959. The first manned landing was by the United States on July 20, 1969. The last Apollo mission was in December of 1972. In the summer of 1994, the Moon was mapped extensively by the Clementine spacecraft and again in 1999 by the Lunar Prospector.

The Moon has no atmosphere. But evidence from Clementine suggested that there may be water ice in some deep craters near the Moon's south pole which are permanently shaded. This has now been confirmed by Lunar Prospector. There is apparently ice at the north pole as well.

The Moon's crust averages 68 km thick and varies from essentially 0 under Mare Crisium to 107 km north of the crater Korolev on the lunar far side. Below the crust is a mantle and probably a small core (roughly 340 km radius and 2% of the Moon's mass). Unlike the Earth's mantle, however, the Moon's is only partially molten. Curiously, the Moon's center of mass is offset from its geometric center by about 2 km in the direction toward the Earth.

There are two primary types of terrain on the Moon: the heavily cratered and very old highlands and the relatively smooth and younger maria. The maria which comprise about 16% of the Moon's surface are huge impact craters that were later flooded by molten lava. Most of the surface is covered with regolith, a mixture of fine dust and rocky debris produced by meteor impacts. For some unknown reason, the maria are concentrated on the near side.

Most of the craters on the near side are named for famous figures in the history of science such as Tycho, Copernicus, and Ptolemaeus. Features on the far side have more modern references such as Apollo, Gagarin and Korolev with a distinctly Russian bias since the first images were obtained by Luna 3.

In addition to the familiar features on the near side, the Moon also has the huge craters South Pole-Aitken on the far side which is 2250 km in diameter and 12 km deep making it the the largest impact basin in the solar system and Orientale on the western limb which is a splendid example of a multi-ring crater.

The Moon has no global magnetic field. But some of its surface rocks exhibit remanent magnetism indicating 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. Over its 4 billion year lifetime many hydrogen ions from the solar wind have become embedded in the Moon's regolith. Thus samples of regolith returned by the Apollo missions proved valuable in studies of the solar wind.

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