Mars meteorites include three rare groups of achondritic (stony) meteorites (16 objects total) with isotope ratios that are said to be consistent with each other and inconsistent with the earth. It should be pointed out, however, that the isotope ratios do not actually match Mars ratios especially well, to the extent that Mars ratios are known, although they do differ substantially from Earth isotope ratios and from what is known of Lunar ratios.
All the meteorites are igneous rocks. Lherzolitic shergottites (one from Antarctica, 2 from California) are identified by their Deuterium/Hydrogen ratios. The crystals appear to be 154-187 million years old and they appear, from cosmic ray analysis, to have spent 2.5 to 3.6 million years in space. There are also basaltic shergottites, some of which appear (from the presence of hydrated carbonates and sulfates) to have been exposed to liquid water prior to injection into space. One of the shergottites, known as ALH84001, is much older than the others - about 4.5 billion years. In this respect, it resembles a typical meteorite rather than the other shergottites. The other two types of Martian meteorites are Chassignites and Nakhlites. All appear to have been on earth no more than 35,000 years. Together, these objects are called SNC meteorites.
Although common wisdom is that the SNC meteorites are from Mars, the Mars origin theory does have some problems. The isotope ratios are not an especially good match. A widely published graph showing a near-perfect match is alleged to be constructed from examples selected to "prove" the hypothesis and to be a poor representation of the real data. For example, the Ar40/Ar36 ratio for one meteorite (1650) is almost exactly half-way between Earth (300) and Mars (3000). Carbon Dioxide, the predominant gas in the current Mars atmosphere, is unaccountably rare in the trapped gases in the SNC meteorites. The SNC meteorites do not show shock artifacts that would be expected in small objects ejected with enough velocity to escape Mars. The majority of SNC meteorites are quite young by geologic standards and seem to imply that volcanic activity was present on Mars only a few hundred million years ago. Cosmic ray traces in the meteorites indicate relatively short stays (3 to 3.5 million years) in space. It is asserted that there are no large young craters on Mars that are candidates as sources for the SNC meteorites (How does one identify a young crater on Mars?).
Possible evidence of life has been hypothesized in three meteorites.
- A 1300-million-year-old meteorite from near Nakhla, Egypt. Small structures that look vaguely like Earth Bacteria. More like bacteria than those in the better-known Allen Hills meteorite.
- A 165-million-year-old meteorite from Shergotty, India. Still to be analyzed
- A 4500-million-year-old meteorite found in the Allen Hills of Antarctica (ALH84001). Ejection from Mars seems to have taken place about 16 million years ago. Arrival on Earth was about 13000 years ago. Cracks in the rock appear to have filled with carbonate materials between 4000 and 3600 million years ago. Evidence of Polycyclic Aromatic Hydrocarbons (PAHs) have been identified with the levels increasing away from the surface. Other antarctic meteorites do not contain PAHs. Earthly contamination should presumably be highest at the surface. Several minerals in the crack fill are deposited in phases, specifically, iron deposited as magnetite, that are claimed to be typical of biodepositation on Earth. There are also small ovoid and tubular structures that might possibly be nanobacteria fossils in carbonate material in crack fills (investigators McKay, Gibson, Thomas-Keprta, Zare). Micropaleontologist Schopf, who described several important terrestrial bacterial assemblages examined ALH84001 and opined that the structures are too small to be Earthly bacteria and don't look especially like lifeforms to him.) The size of the objects is consistent with Earthly "nanobacteria", but the existence of nanobacteria itself is controversial.