A fist-sized meteorite, one of only 18 rocks on Earth known to have come from Mars, has been found by Swiss scientists in the Oman desert — a prize discovery that could help determine if the planet ever sustained life. Scientists at the University of Bern announced the find Friday and said they are just beginning to examine the meteorite. Most of the other 17 Martian rocks have been snapped up by collectors, they said, so few are fully available for study.