New Clues About Martian Meteorites Baffle Scientists
By Greg Clark

http://www.space.com/scienceastronomy/solarsystem/martian_meteorite_000830.html

"The age of the Martian surface has been calculated by examining the number of craters on Mars, Mittlefehldt explained. The oldest surfaces would have been exposed to meteorite impacts from space for the longest time, and thus would have the most craters on them. Young surfaces would be relatively free of craters."

"Using the current understanding of crater density, a maximum of 15 percent of the Martian surface could be as young as 175 million years old, Mittlefehldt said. Even getting that high of a percentage is really stretching the model, he said. It simply doesn't make sense that half the meteorites from Mars and half the big impact events that sent them to Earth just happened to hit the 15 percent of the surface that is young, he said."

"What that means is that Mars may be much younger than most scientists have thought, he said. The accepted models for how fast craters build up on Mars could be entirely wrong, which would throw into question a whole host of conclusions about Martian surface history."