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Fragments of rocks that hit the moon

This artist's impression illustrates how the early Moon (and Earth) were reshaped by an intense period of impact bombardment. Asteroids appear to be the dominant impactors.
[Image Dan Durda/FIAAA]

While looking at rocks collected on the Moon during the Apollo mission, scientists have found tiny fragments of meteorites that hit the Moon long ago.

Early in our solar system's history, the Earth and Moon were bombarded by many more objects than they are today, and researchers have wondered whether these objects were primarily asteroids, comets, or an even mixture of both.

The new discovery suggests that asteroids were probably the more common type of impactor.

Astronauts collected this 3.77 billion-year-old ancient regolith breccia at the Apollo 16 landing site. The sample contains fragments of the asteroids that bombarded the Earth-Moon system.
[Image courtesy of Dr. David A. Kring]

In the study, Katherine Joy of the Lunar and Planetary Institute in Houston and colleagues describe small meteorite fragments preserved in ancient lunar rocks called "regolith breccias," which came from the landing site of the Apollo 16 spacecraft. Regolith breccias are hardened clumps of lunar soil containing small rock fragments and other debris.

These tiny fragments are samples of the small objects that crossed inner Solar System 3.4 to 3.8 billion years ago. By analyzing the chemistry of these fragments, Dr. Joy and her coauthors conclude that these objects belonged to a certain class of asteroids, instead of comets.

The research appears online in the 17 May 2012 edition of ScienceExpress.