Out of 20,000 found worldwide, the 4.8-pound rock is only the 13th meteorite proven to be from the red planet.
Meteorite experts hope this latest discovery, called Lucky 13, will tell scientists more about environmental conditions on Mars and aid in the search for evidence of life on the planet.
"This is another piece in the jigsaw puzzle," said Colin Pillinger, a space scientist at the Planetary Sciences Research Institute at Open University in Milton Keynes, 50 miles northwest of London. "And this particular meteorite is exciting because it seems to be from a different formation and possibly a different age than the others."
The meteorite's age is not yet known. But Pillinger said it left Mars at least a million years ago, when a comet or asteroid smashed into the planet's surface. After drifting through space, the meteorite eventually crashed onto Earth, where it probably lay undiscovered in the desert for about 40,000 years, Pillinger said.
Other Martian meteorites have been found to be 4.5 billion years old.
He does not know who found the rock, but Pillinger said it is owned by a private citizen. That makes it the first of the 13 meteorites in private hands. Six of the others are owned by the U.S. government, and museums own the other six. The last Mars meteorite was found in 1994 in Antarctica.
Scientists at the planetary institute learned of the grapefruit-sized meteorite last week when they were given a small sample to test for authenticity.
Pillinger described the process, using laser technology to isolate oxygen isotopes, as a "genetic test for rocks."' The results confirmed the rock did originate on Mars. Now the sample held by the British scientists will be subjected to a series of experiments.
"Clearly, we won't ever be looking for any advanced forms of life," said Ian Wright, a meteorite researcher in Britain. "But in the microscopic record, there could be a rich variety of things."
In 1996, NASA scientists stunned the world when another Mars meteorite was hailed as containing evidence of life. NASA said they had discovered the fossilized remains of tiny, bacteria-like animals that may have once thrived on Mars. But since the announcement, their theory has remained unproven.