Scientists have estimated that Mars is bombarded by more than 200 small asteroids or bits of comets per year forming craters at least 3.9 meters across, U.S. space agency NASA said Wednesday.

It said these asteroids or comet fragments typically are no more than 1 to 2 meters in diameter, too small to reach the ground on Earth. These space rocks cause craters on Mars because the Red Planet has a much thinner atmosphere.

"It's exciting to find these new craters right after they form, " said Ingrid Daubar of the University of Arizona, Tucson, lead author of the paper published online in the journal Icarus . "It reminds you Mars is an active planet, and we can study processes that are happening today."

According to NASA, researchers have identified 248 new impact sites on parts of the Martian surface over the past decade, using images from the Mars Reconnaissance Orbiter to determine when the craters appeared.

The 200-per-year planetwide estimate is a calculation based on the number found in a systematic survey of a portion of the planet, it said, adding that earlier estimates pegged the cratering rate at three to 10 times more craters per year. Endit

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