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 Moon's rotation is locked in phase with its orbit so that the same side is always facing toward the Earth. 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 is receding away from the Earth at about 3.5 centimeters per year. The gravitational coupling between the Moon and the Earth in addition to causing the tides also transfers rotational energy from the Earth to the Moon. This slows down the Earth's rotation (by about 1.48 milliseconds/century) and accelerates the Moon into a higher orbit (the opposite effect happens to Phobos and Triton).

• 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 almost certainly not hot enough to be 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 concensus 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 landing site 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 mosiac 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. Moon from Galileo (unusual perspectives) 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. Apollo images ftp directory
20. Clementine images ftp directory; image browser; a few processed images from usgs
21. ... 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 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
• Apollo info and images html
• Apollo 11 at Twenty-Five (ExInEd Hypercard stack)
• Dr. Cain's class notes
• Lunar Nomenclature Table
• Chinese Moon Legends

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