The SNC meteorites, so named for the shergottite, nakhlite, and chassigny classes which comprise this group of petrologically similar specimens, consist of 12 meteorites that share a set of similar properties that are highly anomalous compared to other meteoritic samples. The investigation of these characteristic properties has led to speculation that the SNC meteorites may have originated from a planet-sized "parent body" in the inner solar system. Since it was first suggested in the mid-1970's that this parent body may have been the planet Mars, intensive study has not only upheld this radical theory, but also provided a convincing foundation of evidence to support it.

Careful study of the nakhlite group of meteorites conducted in the early- and mid-1970s by such workers as Papanastassiou and Wasserburg (1974) demonstrated that the nakhlite meteorites exhibited an anomalously young isochron crystallization age of 1.37 billion years, as determined by the rubidium-strontium [Rb-Sr] dating method. (At least one SNC meteorite, ALH84001, is much older, with an age of 4.5 billion years.) Such recent metamorphic activity could not be explained by origin on an asteroidal parent body, since small asteroids would have been unable to retain internal heat from the time of the formation of the solar system 4.6 billion years ago to the time of the solidification of the nakhlite meteorites a mere 1.3 billion years ago. The large difference in ages between other previously studied meteorites and the nakhlites indicated a highly singular origin. Further investigation of isotopic ratios by Papanastassiou and Wasserburg showed that the parent body of Nakhla had undergone physical and chemical differentiation more recently than 3.6 billion years ago. It also indicated that Nakhla's parent body had chemical properties such as Rb/Sr and K/U ratios as well as rare earth elemental abundances which were more similar to those found on Earth than to those measured in other meteorites.

The specific constraints imposed on the origin of SNC meteorites by age considerations led several workers in the late 1970s to tentatively suggest that the SNC meteorites were igneous rocks from Mars (see references in Wood and Ashwal 1981). The original proposals offered little concrete evidence that the SNC meteorites actually came from an ancient impact on Mars. Instead, they systematically eliminated all other possible origins, concluding by the process of elimination that the SNC meteorites could only have come from a planetary parent body. For example, Stolper et al. (1979) extended Papanastassiou and Wasserburg's discoveries of unusual chemical properties in SNC meteorites, finding the meteorites to have a high oxidation state, complicated distribution of rare earth abundances, unique chemical ratio of a number of such species as K/U, La/W, and Fe/Mn, high overall volatile content, high ratio of high-calcium pyroxene to low-calcium pyroxene, and enhanced alkali content in feldspar.

However, the difficulty of blasting material off a planetary surface an into an Earth-crossing orbit made Venus, Mercury, and Earth unlikely sources (in the case of Earth and Venus, it is nearly impossible to obtain sufficient energy to break away from the planets' gravity), and chemical comparisons to the accurately known properties of lunar samples returned by the Apollo missions eliminated the moon from the running. Mars remained the only possibility, and so was proposed as the SNC parent body despite the intrinsic difficulties of blasting large pieces of the planet into orbit and then transporting them the long way to Earth. Wood and Ashwal extended earlier suggestions in a groundbreaking paper which summarized earlier results and extended the conclusions drawn from them (Wood and Ashwal 1981). However, the researchers were at somewhat of a loss to propose a tenable scenario for how material could be ejected from Mars and subsequently reach the orbit of Earth. Shortly after their 1981 treatise, Ashwal and Wood published a follow-up paper (Ashwal et al. 1982) in which they demonstrated that "the petrologic, geochemical, and isotopic evidence was inconsistent with an asteroidal origin for SNC meteorites." In keeping with their prior arguments, they maintained that "a planet-sized parent body such as Mars still remained the most likely candidate for the SNC meteorites."

Bogard and Johnson provided the first direct experimental evidence that at least some of the SNC meteorites come from Mars. They measured the abundances of trapped argon, krypton, and xenon gases found in embedded glassy nodules in the Allan Hills (ALH77005) and Elephant Moraine (EET79001) shergottites, both of which had been discovered in the Antarctic ice sheet (Bogard and Johnson 1983). (The nodules formed from impact shock, which is a characteristic property of shergottite meteorites, while completely absent in nakhlites.) They found characteristic ⁴⁰Ar/³⁶Ar ( 2000) and ¹²⁹Xe/¹³²Xe ( 2.0) ratios which more closely resembled the Martian atmosphere, as reported by the Viking landers, than the noble gas content typically found in meteorites. The time of shock was determined by Rb-Sr dating to have been 180 million years, commensurate with that determined for the other two known shergottites. This age was assumed to correspond to the time at which the meteorites were ejected from their parent body, presumably by impact. After analyzing the observed abundances by considering the possibilities that they could have arisen from collision with or contamination by a chondrite, adsorption by gas from Earth's atmosphere, or from an achondritic parent body, the authors concluded that the only viable possibility was a Martian origin. They then went on to propose the very method whereby this conclusion could be corroborated.

Taking up the challenge presented by Bogard and Johnson, other researchers examined the glass nodules in EETA 79001 in search of the high isotopic content of heavy nitrogen discovered by Viking and which, according to Pepin, "distinguishes the atmosphere of Mars from virtually all other volatile reservoirs in the Solar System." The results were discussed by Pepin (1985) in a short paper summarizing evidence for a Martian origin of the SNC meteorites. When studies were performed on EETA 79001, a substantial enrichment of ¹⁵N was detected in the trapped gases. In addition, further analysis by other investigators extended the results to include carbon dioxide. The agreement between the laboratory-determined abundances and those sampled by Viking was found to be quite striking. The tight correlation provided the first real hard evidence for a Martian origin.

Despite the solid body of both circumstantial and hard evidence that had been accumulated, there was still no viable dynamical model which could explain how rocks could be ejected from Mars and transported all the way to Earth. Finally, several possible mechanisms were proposed and their consequences discussed by Vickery and Melosh (1987). Of the several scenarios they considered, they found a single ejection by a very large impact event 200 million years ago to be the most likely candidate. Vickery and Melosh attributed the differing cosmic ray exposure ages [which McSween (1985) had used as a basis for arguing that the SNC meteorites must have originated from multiple impact events-a possibility found "extremely unlikely" by Vickery and Melosh] to the shielding and subsequent fragmentation of the inner parts SNC meteorites. They speculated that the size of a crater large enough to have been formed in the impact event which ejected the SNC meteorites to be 175 km in diameter, and suggested that craters of this size should be possible to identify in young terrain somewhere on the surface of Mars.

An analysis of the Chassigny, Shergotty, and Zagami meteorites by Watson et al. (1994) found a high deuterium/hydrogen ratio relative to terrestrial values, as well as only a tenth as much water in the amphibole mineral phases as expected. Watson et al. interpreted these results as supporting the assertion that, in order for Mars to have lost the amount of water implied by the contrast between current martian conditions and the ancient flood features seen on the planet, the escape rate of hydrogen from Mars must have been higher in the past.

SNC meteorites continue to be found in Antarctica, and the current total is 12 (Sky & Telescope 1996). The meteorite Yamato 793605, although collected in 1981, was not identified as a SNC until 1995. Similarly, although ALH84001 was collected in 1984, it was not until 1993 that the stone was recognized as a member of the SNC family.

Interest in SNC meteorites was recently rekindled by the announcement from researchers at Stanford University and NASA's Johnson Space Flight Center that careful analysis of the ALH84001 meteorite had yielded evidence of ancient bacteria-like life forms on Mars. This work was set to appear in an issue of the journal Science but was leaked to the press on August 6, 1996. The resulting surge of interest led to a NASA press release on Aug. 7, 1996 followed by a press conference. The resulting tumult precipitated a flurry of media and NASA public relations activity which included a front-page story in The New York Times (Wilford 1996).

The ALH84001 meteorite was among the SNCs discovered in Antarctica. It is the oldest known SNC, with its crystallization age of 4.5 billion years indicating the rock is as old as Mars itself. The meteorite is thought to have been blasted off Mars 15 million years ago and remained in interplanetary orbit until entering the Earth's atmosphere and landing in Antarctica approximately 13,000 years ago.

Using scanning electron microscopy and laser mass spectroscopy, a team led by David S. McKay identified polycyclic aromatic hydrocarbons (PAHs) in the meteorite, as well as globules of carbonate and the minerals magnetite and iron sulfide. The carbonate globules are small elongated features resembling similar ones found on Earth which are believed to have formed in association with bacteria. McKay et al. (1996) determined their age to be approximately 3.6 billion years. All the unusual compounds found in the meteorite are associated with bacterial activity on Earth, PAHs with decay products of microorganisms and magnetite and iron sulfide with anaerobic bacteria. However, the presence of these compounds is not necessarily diagnostic for the presence of bacteria. McKay and his coworkers appear to have excluded the possibility that any of their discoveries represent terrestrial contamination, so the constituents they describe presumably must have formed while the meteorite was still on the surface (or shallow subsurface) of Mars.

While the reported discoveries are intriguing and consistent with formation through biological activity (especially when taken together), this conclusion requires additional confirmation. It should be recalled that initial experiments on soil samples from one of the Viking landers were heralded as clear evidence of organic material, whereas subsequent laboratory work showed that the observed results could be reproduced with entirely inorganic soil constituents. While the presence of past life on Mars would be a scientific discovery of epic proportions and warrants the closest possible scrutiny, it is premature at this juncture to state that the existence of life on Mars, past or present, has been conclusively demonstrated.

Lunar Meteorites, Mars, Meteorite