Origins of the Moon

Students: Please read the information below. Please note that this page is copied from Nick Strobel's Astronomy Notes web site.  A more complete version of this information is available at his web site.

Nick Strobel's Astronomy Course, Bakersfield College
http://www.astronomynotes.com

The Earth's Moon

The Moon is about one-quarter the diameter of the Earth---if placed on the United States, it would extend from Los Angeles to almost Washington D.C. The Moon has held a special place in history. This is because it moves quickly among the stars and it changes---it goes through a cycle of phases, like a cycle of birth, death, and rebirth (see the Moon motions document). The Moon is also primarily responsible for the tides you experience if you spend any time near the coast (see the tides section of the gravity chapter). When Galileo looked through his telescope, he discovered a wondrous place. The Moon became a place to explore. Galileo discovered impact craters, mountains and valleys. The Moon is rough like the Earth.

The Moon also has large, dark smooth areas covering about 17% of the Moon's surface that people originally thought were seas of liquid water so they are called mare (Latin for ``seas''---they are what make out the face on the Moon). Now it known that the mare are vast lava flows that spread out over many hundreds of square miles, covering up many craters that were originally there. The mare material is basaltic like the dark material on the Earth's ocean crust and that coming out many of our volcanoes (e.g., the Hawaiian islands). Mercury also has maria but they are lighter in color because of the different chemical composition and they do not stand from its heavily cratered areas.

Liquid water cannot exist on the Moon because of the lack of an atmosphere---the Moon has only 1/81 the Earth's mass and about 1/6th the Earth's surface gravity. If there is any water to be found on the Moon, it will be in a frozen state in a place of constant shade such as deep craters near the poles. Recent missions have discovered some of those ice blocks near the poles. The ice blocks will be the source of water for any humans that decide to set up bases on the Moon.

Craters

The Moon's surface is almost as old as the Earth. The rough highland regions are 3.8 to 4.2 billion years old and the younger maria are between 3.1 and 3.8 billion years old. All of the planets and moons experienced a period of heavy bombardment about 3.8 billion years ago that lasted for about 500 million years as most of the remaining chunks of rock left over from the formation of the solar system pelted the planet and moon surfaces. The Moon, some of the moons of the giant planets, and Mercury preserve a record of this bombardment. The record of this heavy cratering was erased on the Earth long ago because of erosion and geologic activity that continues to this day. The Moon has no erosion because of the lack of liquid water and an atmosphere and it is small enough that its interior cooled off long ago so geologic activity has essentially ceased (an occasional very small moonquake can still occur). The small size of the Moon meant that not much heat could be stored from its formation and its small size also means that any remaining heat can easily escape to space (the ratio of its surface area to its volume is larger than that for the Earth).

Volcano craters are above the surrounding area on mountaintops while the craters from impacts are below the surrounding area with raised rims. The craters on all of the moons except Io, Mercury, and most of the ones on Mars are from impacts. The kinetic energy of the impacting meteorite or asteroid is converted into heat, sound, and mechanical energy---the projectile explodes on impact. The explosion is what carves out the crater so all craters are round (otherwise they would be oblong in shape). The rock on the surface of the planet or moon is bent backward, upward, and outward so the amount of material ejected is much larger than the projectile. Large craters will have a central peak formed by the rock beneath the impact point rebounding upward and they may also have terracing of the inner walls of the crater from the collapsing of the crater rim inward. The size of the craters having central peaks depends on the size (gravity) of the planet or moon: on the Moon craters larger than about 60 kilometers in diameter have central peaks while the crater diameter on the Earth need be larger than just 1 to 3 kilometers.

The number of craters per unit area on a surface can be used to determine an approximate age for the planet or moon surface if there is no erosion. The longer the surface has been exposed to space, the more craters it will have. If you know how frequently craters of a given size are created on a planet or moon, you can just count up the number of craters per unit area. This assumes, of course, that the cratering rate has been fairly constant for the last few billion years. The heavy bombardment of about 3.8 billion years must be taken into account when using the crater age dating technique. For example, the highland regions on the Moon have ten times the number of craters as the maria, but radioactive dating (explained in the next chapter) shows that the highlands are approximately 500 million years older than the maria, not ten times older. Careful studies of how the craters overlap other craters and other features can be used to develop a history or sequence of the bombardment on the moons and planets.

Interior and Composition

The strong tides from the Earth pulled the early Moon's liquid interior toward the Earth, so the far side's crust is now about 130 kilometers thick while the near side's crust is about 65 kilometers thick. The thinness of the near side's crust is also why there are more mare on the near side than the far side. The near side was thin enough to be cracked apart when large asteroids hit the surface and formed the mare but the far side crust was too thick.

Our knowledge about the Moon took a huge leap forward during the Apollo missions. One main science reason for going to the Moon was to return rock samples to find about their ages and composition. Using their knowledge of geology gained from the study of Earth rocks, scientists were able to put together a history for the Moon. The Apollo astronauts also left seismometers on the Moon to detect moonquakes that can be used to probe the interior using seismology.

The Moon's density is fairly uniform throughout and is only about 3.3 times the density of water. If it has an iron core, it is less than 800 kilometers in diameter. This is a sharp contrast from planets like Mercury and the Earth that have large iron-nickel cores and overall densities more than 5 times the density of water. The Moon's mantle is made of silicate materials, like the Earth's mantle, and makes up about 90% of the Moon's volume. The temperatures do increase closer to the center and may be high enough to partially liquify the material close to the center. Its lack of a liquid iron-nickel core and slow rotation is why the Moon has no magnetic field.

Lunar samples brought back by the Apollo astronauts show that compared to the Earth, the Moon is deficient in iron and nickel and volatiles (elements and compounds that turn into gas at relatively low temperatures) such as water and lead. The Moon is richer in elements and compounds that vaporize at very high temperatures. The Moon's material is like the Earth's mantle material but was heated to very high temperatures so that the volatiles escaped to space.

Formation

There has been a variety of scenarios proposed to explain the differences between the Moon and Earth. The one that has gained acceptance after much study is the giant impact theory. Earlier theories came in a variety of flavors.

  1. The capture (pick up) theory proposed that the Moon formed elsewhere in the solar system and was later captured in a close encounter with the Earth. The theory cannot explain why the ratios of the oxygen isotopes is the same as that on the Earth but every other solar system object has different oxygen isotope ratios. The theory also requires the presence of a third large body in just the right place and time to carry away the extra orbital motion energy.
  2. The double planet (sister) theory said that the Moon formed in the same place as the Earth but it could not explain the composition differences between the Earth and Moon.
  3. The spin (daughter or fission) theory said that the Earth rotated so rapidly that some of its mantle flew off to the form the Moon. However, it could not explain the composition differences. Also, the spun-off mantle material would more likely make a ring, not a moon and it is very unlikely that the Earth spun that fast.

The giant impact theory proposes that a large Mars-sized object hit the Earth and blew mantle material outward which later recoalesced to form the Moon. The Earth had already differentiated by the time of the giant impact so its mantle was already iron-poor. The impact and exposure to space got rid of the volatiles in the ejecta mantle material. Such an impact was rare so is was not likely to have also occurred on the other terrestrial planets. The one ``drawback'' of the theory is that it has a lot of parameters (impactor size, speed, angle, composition, etc.) that can be tweaked to get the right result. A complex model can usually be adjusted to fit the data even if it is not the correct one (recall Ptolemy's numerous epicycles). But the giant impact theory is the only one proposed that can explain the compositional and structural characteristics of the Moon.

Useful Links:

Lunar Prospector
http://lunar.arc.nasa.gov/

Lunar Propsector Impact Page
http://www.ae.utexas.edu/~cfpl/lunar/

The Lunar Prospector Simulation
http://www.exploringspace.arc.nasa.gov/lp.htm

Apollo Lunar Surface Journal, transcripts, summaries of the missions, everything!
http://www.hq.nasa.gov/office/pao/History/alsj/

Apollo 15 Flight Journal
http://www.hq.nasa.gov/office/pao/History/ap15fj/index.htm

Lunar and Planetary Institute
http://cass.jsc.nasa.gov/moon.html

International Lunar Exploration Working Group
http://ilewg.jsc.nasa.gov/

Information on Blue Moons
http://nssdc.gsfc.nasa.gov/planetary/lunar/blue_moon.html

Astromaterials Curation (where they keep the moon rocks)
http://www-curator.jsc.nasa.gov/curator/curator.htm

The Richard Nixon Library and Birthplace (moon rock from Apollo 15 and 30th Anniversary Exhibit)
Put link here.

Top Ten Discoveries Made during Apollo Missions to the Moon
http://www-curator.jsc.nasa.gov/curator/lunar/lunar10.htm

Pictures of the Moon
http://seds.lpl.arizona.edu/nineplanets/nineplanets/pxmoon.html

Bad Astronomy: Tides, the Earth, the Moon, and why our days are getting longer
http://www.badastronomy.com/bad/misc/tides.html

Orbits: A few good lessons on planetary orbits and tides
http://www.bc.cc.ca.us/programs/sea/astronomy/gravappl/gravapplb.htm