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Scars from a violent past

A new study settles an ongoing debate about the origin of planetary features.
November 11, 2005
Asteroids, not comets, bombarded the inner solar system 3.9 billion years ago, according to a recent scientific study. Robert Strom of the Lunar and Planetary Laboratory (LPL) at the University of Arizona at Tucson and his team concluded that this bombardment of asteroids lasted between 20 and 150 million years. The study resolves two ongoing debates: What formed the craters found on inner solar system bodies? And, when did they form?

The inner solar system — Mercury, Venus, Earth, and Mars — presents two types of cratered surfaces. Earth's Moon provides examples of both: maria (dark plains), and rugged highlands. The highlands show the effects of the initial bombardment. The maria record a later, longer epoch that extends to the present. Mercury and Mars also show both surface types. Tectonic processes on Venus and Earth have obliterated features from the initial heavy bombardment, but Venus retains a record of the later epoch that's consistent with Mars, Mercury, and the Moon.
Lunar Crater Walter
The Crater Walter sports a large central peak and several internal, secondary craters. This image comes from The Consolidated Lunar Atlas.
LPL / UA
The team studied cratered surfaces throughout the inner solar system. "There is a direct translation from the size of craters to the size of their impactors," explains team member Renu Malhotra. From this correlation, the team assembled an accurate picture of the size distribution of the impacting debris.

"The connection we made has to do with the size distribution of asteroids," Malhotra says. "The population of main-belt asteroids matches the complex, densely cratered surfaces almost perfectly." The cratering record for the first bombardment fits an asteroid source far better than a cometary one.

Additionally, says Malhotra, "the distribution of near-Earth asteroids [NEAs] matches the younger surfaces beautifully." So NEAs are the culprits in the later impact wave. While NEAs also come from the main asteroid belt, their impact rate amounts to a gradual rain of rock compared to the initial heavy bombardment.

The researchers have proposed two options for a time line. In the first option, the initial bombardment occurs at the end of the solar system's accretion. The second suggests a quiescent period after solar system accretion, followed by a spike in impacts.

"To match the results we see, we conclude that there must have been a spike. The event must have happened fairly quickly, due to a major dynamic instability in the asteroid belt," Malhotra explains.

What triggered that instability remains a mystery, but the LPL researchers believe they have found answers to two fundamental questions of our solar system's history.

Michael Carroll is a science writer and astronomical artist, and author of nearly 20 children's science books. He lives with his family in Colorado.
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