: About 100 known meteorites have come to Earth from Mars. Most of them formed between 150 million and 600 million years ago. One of the exceptions is a meteorite found in the Sahara desert. A previous analysis of one of its five fragments put its age at 2 billion years. But a new examination of one of the other fragments has yielded an age of 4.4 billion years, making it the oldest known Martian meteorite. Munir Humayan of Florida State University says that based on its composition, the meteorite formed when Mars itself was just 100 million years old and likely originated from highlands in the planet's southern hemisphere. Because that period of time was highly volcanic and liquid water was believed to be present, the rock could hold some evidence of past life on Mars. However, the meteorite has been on Earth for so long that it may have been contaminated by native life forms. The part of the rock that led to the earlier dating likely was added by a later event, possibly even the impact by an asteroid or comet that ejected the rock from Mars.