Martian meteorites are rocks which originated on Mars but have fallen on Earth. The 99 Martian meteorites known at the time of writing represent less than 0.2% of all known meteorites.
It is possible to pinpoint a meteorite as being from Mars by its chemical composition, which matches the chemical composition of Mars itself. No other body in the solar system matches the unique chemical characteristics of the Martian atmosphere and surface. Freshly fallen Martian meteorites may also have a black fusion crust from their passage through Earth’s atmosphere, which will not leave a streak on a ceramic tile. They are always stony and never contain iron in metallic form.
Martian meteorites include shergottites, nakhlites and chassignites, which together compose the SNC group of meteorites. Their names come from the location where the first known meteorite of that type was found. Most Martian meteorites are shergottites. There are 13 identified nakhlites, while only 2 meteorites have been identified as chassignites.
The SNC meteorites are all basaltic meteorites. They also include trapped gases in their structure which are similar to the modern Martian atmosphere, as measured by spectroscopy and by the various Martian landers.
These Martian meteorites are generally much younger than other meteors, although sometimes that means they haven’t been in space for a long time by comparison with non-Martian meteorites. Shergottites may have crystallized as recently as 180 million years ago, although they could also be much older.
At the other end of the scale, nakhlites are thought to have crystallized 1.3 billion years ago. However, they were ejected from Mars just 10.75 million years ago, during an asteroid strike on the surface of Mars. Some nakhlites also contain olivine crystals. Because of meteoric evidence, it is now thought that as much as half of the upper mantle of rocky planets, such as the Earth, may be made of olivine.
The nakhlites also show signs of having been suffused with liquid water around 620 million years ago. Their existence was one of the earliest indications that Mars once had liquid water on or near its surface. A few Martian meteorites were originally believed to have contained fossils of early microscopic life on Mars. This has since been found to have other, non-biological explanations, but the presence of grains of unusually pure magnetite is still best explained through biological means.
The exact number of known Martian meteorites constantly changes with reclassification of previous meteorites as well as new finds. Many recent new finds end up in private collections without being classified or studied.