Scientists have long sought the reasons for the relatively young age of the
majority of Martian meteorites discovered on Earth in comparison with the age
of Mars, an enigma exarcebated by the fact that the Martian rocks were ejected
Line by only six or seven separate impact events. Previous tests had predicted that
(5) driving meteorites to Earth would require a collision with an asteroid immense
enough to make a crater 12-kilometers across, but because such huge impacts
are extremely infrequent, it was unlikely that enough of them could have
occurred to explain our planet’s Martian meteorite collection.
Now astronomer James Head’s higher-resolution models demonstrate that
(10) collisions making craters only three kilometers across can jettison 10 million
fragments, each about 10 centimeters across, into space, a distribution
sufficient to cause some of them to be found on Earth. Sections of the planet
covered by debris (thus likely to be made up of older terrain) would require
larger and hence rarer impacts, and thus meteorites which reach Earth are
(15) predictably biased toward younger ages.

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