NASA's Observatorium--Observation of the Week

April 14, 1999

Moon Formed by Impact of Mars-Sized Object on Young Earth

Earth as seen from the Apollo 8 Command Module some 70 miles above the Moon's heavily cratered surface (December 1968).

Artist's concept of NASA's Lunar Prospector, launched toward the Moon on January 6, 1998. The spacecraft, which now orbits the Moon between 15 and 23 miles above the surface, carries five science instruments for the study of the Moon's resources and origin.

Click on images for enlarged views.

Image Credit: National Aeronautics and Space Administration (NASA).

How did the Earth's Moon originate?

NASA's Lunar Prospector supports evidence gathered some 30 years ago during the Apollo program that the Moon was formed by the impact of a Mars-sized object on the young Earth. The impact ejected large quantities of matter, some of which entered into Earth orbit and collected by self-gravity to form the Moon.

The evidence is based on the following data. Prospector's gravity measurements show that the Moon possesses a very small, iron-rich core that adds up to, at most, about four percent of the Moon's total mass. In comparison, the Earth's core is much larger, amounting to approximately 30 percent of our planet's mass. Prospector also confirmed the Apollo missions' findings that the Moon and Earth have strikingly similar mineral compositions. Furthermore, the lunar surface is almost devoid of water and other volatiles (i.e., easily vaporized materials such as molecular nitrogen and carbon dioxide). Only in polar craters that are in permanent shadow did Prospector discover evidence of water, presumably deposited there by comet impacts after the Moon was formed.

The "giant impact" model for the origin of the Moon explains each of these findings. The impact occurred when the Earth was less than a hundred million years old (its present age is 4.6 billion years), but after most of its iron had sunk toward the center to form the terrestrial core. Hence, the material torn out of our planet's outer layers by the impact, and which subsequently collected to form the Moon, was iron-poor. Since it was iron-poor, the Moon was able to form only a very small core.

Secondly, since, according to this model, most of the lunar material came from the Earth (some came from the impacting body), the mineral compositions of the Moon and Earth are very similar. Thirdly, because the impact was extraordinarily violent, the ejected material was severely heated (up to nearly 20,000 °F according to numerical simulations of the impact event) and became spread out into a ring that orbited the Earth before collecting into the Moon. Thus, the volatiles were able to escape, which made them rare on the Moon.

Despite the compelling nature of this reasoning, Dr. Alan Binder of the Lunar Research Institute in Tucson, AZ, and principal investigator for the Lunar Prospector takes the conservative view: "Further analysis of Lunar Prospector data to refine the exact size of the lunar core and the amounts of elements like gold, platinum and iridium in lunar rocks--all of which are concentrated with metallic iron--is required. This will do much to pin down for good if the 'giant impact' model of the formation of the Moon is correct, or if the Moon formed in a different manner."

More Cool Stuff

The Lunar Prospector is a $63 million mission that is managed by NASA's Ames Research Center. It was developed under NASA's Discovery Program of lower-cost, highly focused, small scientific spacecraft. For more information on the spacecraft, its mission, and its science discoveries, go to:: http://lunar.arc.nasa.gov/
We have featured the Lunar Prospector mission before:: http://observe.ivv.nasa.gov/nasa/ootw/1998/ootw_980311/ob980311.html
Teachers, try the Observatorium's teacher's guide to our Prospector pages:: http://observe.ivv.nasa.gov/nasa/education/teach_guide/lunar.html
As part of its Learning Technologies Project (LTP), NASA supports a number of educational Web sites that have excellent material on the space sciences:: http://observe.ivv.nasa.gov/nasa/education/edu/edudocs/topic_space.html

Check out other observations in the Observation of the Week Archive.