The Moon

Luna

Moon Facts

• The Moon is the only natural satellite of Earth:
  • distance from Earth: 384,400 km
  • diameter: 3476 km
  • mass: 7.35e22 kg

• Called Luna by the Romans, Selene and Artemis by the Greeks.

• The Moon, of course, has been known since prehistoric times. It is the second brightest object in the sky after the Sun.

• Due to its size and composition, the Moon is sometimes classified as a terrestrial "planet" along with Mercury, Venus, Earth and Mars.

• The Moon was first visited by the Soviet spacecraft Luna 2 in 1959. It is the only extraterrestrial body to have been visited by humans (picture 4, 3). The first landing was on July 20, 1969 (do you remember where you were?); the last was in December 1972. The Moon is also the only body from which samples have been returned to Earth. In the summer of 1994, the Moon was very extensively mapped by the little spacecraft Clementine.

• 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 get slighly 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 1.48 milliseconds/century and raising the Moon into a higher orbit by about 3.8 centimeters per year. (The opposite effect happens to satellites with retrograde orbits such as Phobos and Triton).

  The asymmetric nature of the gravitational force is also responsible for the fact that the Moon rotates synchronously, i.e. it is locked in phase with its orbit so that the same side is always facing toward the Earth. Just as the Earth's rotation is now 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 was slowed to match its orbital period (such that the bulge always faced toward the Earth) there was no longer an off-center torque on the Moon and a stable situation 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.

• Actually, 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 (picture 2) was completely unknown until the Soviet spacecraft Luna 3 photographed it in 1959.

• The Moon has no atmosphere. Recent evidence from Clementine that suggested that there might be water ice in some craters near the Moon's poles has turned out to be inconclusive. But the possibility still exists that ice may exist mixed with lunar soil.

• 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 farside. Below the crust is a mantle and possibly a small core. 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. Also, the crust is thinner on the near side.

• 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.

• In addition to the familiar features on the near side, the Moon also has 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.

• 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. Even today, 20 years after the last Moon landing, scientist still study these precious samples. (picture 18)

• Most rocks on the surface of the Moon seem to be between 4.6 and 3 billion years old. This is a fortuitous 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.

• Prior to the study of the Apollo samples, there was no consensus about the origin of the Moon. There were three principal theories: co-accretion which asserted that the Moon and the Earth formed at the same time from the Solar Nebula; fission which asserted that the Moon split off of the Earth; and capture which held that the Moon formed elsewhere and was subsequently captured by the Earth. None of these work very well. But the new and detailed information from the Moon rocks led to the impact theory: that the Earth collided with a very large object and that the Moon formed from the ejected material. There are still details to be worked out, but the impact theory is now widely accepted.

• 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. This lunar hydrogen may also be of use someday as rocket fuel.

Pictures

1. (above) Nice image of full moon 106k gif

2. the farside 342k gif

3. Apollo 17 94k jpg

4. Buzz Aldrin, second man on the Moon 98k jpg

5. The moon 106k gif
6. Apollo 17 landing site 291k gif
7. GOES satellite image of Earth showing Moon in same frame 92k gif
8. Classic image of the Earth and moon, taken from Voyager, low resolution. 35k gif

9. Earth and Moon taken from Galileo 54k gif; 36k gif

10. False-Color mosaic of the moon, showing part of the dark side 94k gif
11. Black and white image of the moon showing the North polar regions 153k gif
12. Close up view of the north pole of the moon 136k gif

13. Earth rise over the moon 150k gif

14. The east side of the moon (hidden part) 105k gif
15. Moon in 2nd Galileo flyby with grid overlaid P-41475 154K gif

16. from Galileo 38k gif; 13k jpg

17. Color image of the Moon from Galileo 62k jpg

18. Highland Anorthosite, 4.4 billion years old 197k gif

19. A nice lunar eclipse image html
20. Apollo images ftp directory
21. Clementine images image browser; a few processed images from usgs
22. ... more Moon images

Movies

1. Earth-Moon animation 382K fli; 373K pics; 140K quicktime
2. Astronauts walking near US flag and lunar lander 118k mpg
3. A dramatic view the moon with Venus in the distance 82k mpeg
4. Galileo Moon Encounter 8000k AVI
5. Time-lapse sequence of Earth moon system from Galileo 382k FLI
6. Clementine images showing the Apollo 16 lunar landing site 212k MPEG

More about the Moon

• from LANL
• from RPIF
• from RGO
• from NSSDC
• Lunar rocks and regolith
• from NASA Spacelink
• Io and Luna, a comparison from LANL
• Exploring the Moon from LPI
• Apollo info and images html
• Apollo 11 at Twenty-Five (ExInEd Hypercard stack)
• Lunar Nomenclature Table
• Chinese Moon Legends
• Scientific Discoveries Made During Apollo

Open Issues

• Is there water on the Moon? Even small quantities would be enormously valuable to future astronauts.
• Why are the maria concentrated on the near side?
• Why is the Moon's center of mass off center? Because of the tidal lock with the Earth?
• Only twelve men have ever on walked the surface of the Moon. Who will be the 13th? Who will be the first woman?

Express to Mars