Experts have determined the origin of these rocks as a result of gas measurements made in the thin Martian atmosphere by two NASA Viking spacecraft in 1976. All Martian meteorites contain iron-rich oxide minerals with no iron in metallic form and an iron sulfide mineral called pyrrhotite, instead of the troilite typically found in iron metal-bearing meteorites. In addition, the pyroxene and olivine minerals within them have distinctive ratios of iron to manganese, but most significantly, these meteorites have oxygen isotopic compositions different from those of any other similar meteorites.
The above factors, taken together, show that all of the 56 or so proposed Martian meteorites found throughout the world come from the same mass, and the atmospheric gas evidence has proven conclusively that the mass is from Mars. Paradoxically, we know that these specimens come from Mars even though humans have not yet directly obtained any rock samples from there. Even more exciting is the fact that none of the Martian meteorites (with one possible exception) seem notably similar to rock outcrops at landing sites explored by robotic craft.
Over half (54%) of all Martian meteorites found since 1975 are from Algeria, Morocco and nearby regions. Although most meteorites crash into the sea, we know they have also fallen onto solid earth. Every rocky desert and forest is a potential gold mine, once a seeker knows what s/he is looking for.
How to recognize a Martian meteorite
Most people think Martian meteorites are red and, to a lesser extent, green. But none are a true red. Some are dark green and a few have light greenish parts, but most are grayish in color, and some are brown or black, as a result of shock darkening. Most will have a black fusion crust, although over time the crust may have worn away.