Submitted by: Audra Martin, La Puente
Answered by: Bill Bottke, Postdoctoral Fellow, Division of Geological and Planetary Sciences
The short answer is that we do not know where the moon came from. It's difficult to know, because we have too few examples. Earth is the only terrestrial planet (that is, the only planet within the inner solar system, and made of rock as opposed to gas) that has a large satellite.How do we know that meteorites come from Mars?
Meteorites are rocks which fall to Earth from space. Most are thought to be fragments of asteroids which have survived fiery entry through Earth's atmosphere. Twelve of the thousand or so meteorites held in worldwide collections, however, are thought to come from Mars. Eleven of these meteorites, formed roughly 1.3 billion years ago, were named the SNC meteorites after the sites where they were found: "S"hergotty (India), "N"akhla (Egypt), and "C"hassigny (France).
The twelveth Martian meteorite is much older and different from the rest. It is called ALH 84001, named for the year it was discovered, 1984, and the location it was found, Allen Hills, Antarctica. It was formed 4.5 billion years ago, such that it was present when Mars had a much thicker atmosphere and liquid water on its surface. Recently, scientists have suggested that ALH 84001 might even contain fossil evidence for ancient Martian life.
How do we know that these meteorites are from Mars when people have never been there and no rocks have been collected on its surface? In 1976, two NASA spacecraft named Viking I and II landed on Mars and analyzed its atmosphere and surface. These spacecraft examined soil and air samples using on-board instruments, making careful measurements and radioing their data back to scientists on Earth. After careful study, it was determined that Mars' atmosphere was very different from Earth's atmosphere or any other combination of gases found in the solar system. Then, by analyzing small traces of gas trapped in the interior of these twelve meteorites, scientists were able to identify the characteristic "fingerprint" of the Martian atmosphere, proof that these rocks were blasted off Mars' surface at some time in the past.
Even before gas was discovered in these meteorites, scientists were suspicious that they might have originated on Mars. The SNC meteorites have young formation ages, and all twelve Martian meteorites have complex chemical compositions which set them apart from other known meteorites classes. Moreover, the abundance of oxygen isotopes (different kinds of oxygen) in the meteorites are inconsistent with oxygen isotopes found in Earth rocks.
However, the idea that makes most sense is that the moon arose from a giant impact during the formation of Earth about 4.5 billion years ago. While Earth was being formed, it was hit by very large objects, including bodies as large as Mars (about one-tenth the mass of Earth). If you had hit the growing Earth with such a body, material would have splashed out from the impact site, and a sizable amount of that would have gone into Earth orbit. This material may have come partly from the impacting body and partly from Earth itself. We think that this material could have then aggregated rather quickly into the moon as we know it.
According to this view of the origin of the moon, you would therefore have created a body that started out close to Earth (much, much closer than the present distance between Earth and the moon). This body would have been very hot, probably molten, because of the intense energy created in the impact, and it would have been a body with a composition similar to the outer parts of Earth, not Earth as a whole. The central part of Earth is an iron core; Earth as a whole has a lot of iron but the outer parts of Earth do not.
All of these characteristics are in agreement with the moon as we see it. The moon is moving away from Earth steadily and was therefore once much closer to Earth; it moves away because the tides that are raised by the moon in the oceans of Earth cause angular momentum to be transferred from the spin of Earth to the orbital motion of the moon.
In addition, we know from looking at lunar rock that the moon was once very hot, perhaps completely molten. When you look at the moon in the night sky or through a telescope, you see dark-colored regions and bright-colored regions; the bright-colored regions, called lunar highlands, are composed of very ancient rocks that arose through crystallization from molten rock, and this fits in with the idea of a very traumatic beginning.
The moon also has very little if any metallic iron; it does not have a significant core. This fits in with the idea that the moon is derived >from material that was in the outermost parts of the early Earth or of the projectile, most likely both. The absence of a core in the moon is otherwise very difficult to explain-- it's one of the main reasons we do not think that the moon formed somewhere else in the solar system and then got captured, since if it had formed somewhere else it would be very difficult to understand why it has this composition.
Perhaps the main reason why this idea of a giant impact is attractive is that the angular momentum of Earth and moon together is about what you would get from such an impact. It turns out that this amount of angular momentum, which was once in the spin of Earth and is now mostly in the orbital motion of the moon, is about what you would get if an object approximately the mass of Mars hit Earth (obliquely, not head on). So all of these things fit in together.