The SNC meteorites are from Mars
Introduction
Planning for Mars Sample Return has been, and will continue to be, strongly influenced by claims that Martian life has been detected in meteorites from Mars (McKay 1996, McKay 1999). Until 15–20 years ago, it was difficult to imagine that some meteorites might actually be rocks from Mars. But data from these meteorites, the SNC meteorites, are now the foundations for much of our present understanding of Mars (e.g., McSween, 1994).
The importance of the SNCs in Mars exploration has increased dramatically since they were reviewed last (McSween, 1994). Since then, four new SNC meteorites have been discovered (Yamato 793065, QUE94201, Dar al Gani 476, and Dar al Gani 489), and McKay 1996, McKay 1999 reported putative signs of Martian life in two SNCs. These new developments have initiated an avalanche of new data on the SNC meteorites and their implications for the geology, atmosphere, and history of Mars. But, as shown by debates at the SOCFI conference in Paris, France (February 1–5, 1999), there is still some uncertainty in the international scientific community about the link between SNC meteorites and Mars (e.g., Jagoutz, 1999; Ott, 1999).
We feel it is useful to explore and restate the evidence linking the SNC meteorites with Mars, because so much mission planning for Mars Sample Return ultimately rests on inferences from these meteorites. First, we will give a short overview of the SNC meteorites themselves and why they are thought to have formed on a single parent body. The compendium of Meyer (1998) gives a full review of the “Martian meteorites” and their properties. The remainder of the paper will summarize the evidence and arguments linking the SNCs to Mars: trapped atmospheric gases, bulk chemistry, geochronology, and hydrology.
Section snippets
Why S+N+C?
The SNC meteorites are 14 samples of basaltic rock and cumulate rocks derived from basaltic magma, that share a common geochemical and isotopic heritage (Table 1). The acronym SNC comes from the three classes of meteorite involved: S is for shergottites, which are basalts and peridotites, named for the type meteorite Shergotty; N is for nakhlites, which are clinopyroxenites, named for the type meteorite Nahkla; and C is for Chassigny, which is a unique dunite. ALH84001, an orthopyroxenite, is
Volatile components
Initially, the idea that the SNC meteorites might be from Mars was based on indirect evidence — comparing characteristics of the meteorites in hand with reasoned hypotheses about Mars’ rocks (e.g., Wood and Ashwal, 1981). These indirect arguments were augmented in 1983 by the recognition of Martian atmosphere gas trapped in the SNC meteorites. These gases constitute the strongest direct evidence that the SNC meteorites are from Mars. In comparing SNC volatiles to the Martian atmosphere and
Chemical composition
The Viking and Mars Pathfinder landers produced chemical analyses of soils and rocks on the Martian surface, and these analyses provide additional links between the SNC meteorites and Mars. From the beginning, the Viking analyses of Martian fines were compared to the Shergotty meteorite (Clark et al., 1982), and that similarity is still cited (e.g., Rieder et al., 1997).
Many geochemical parameters are useful in distinguishing basaltic materials that come from the various solar system bodies,
Geochronology and history
One important part of the earlier, indirect arguments linking the SNC meteorites to Mars (Wood and Ashwal, 1981) was their chronological histories. Although a significant amount of additional data have been obtained, these arguments remain (for the most part) as valid as they were in the early 1980s. The young isotopic ages of the SNCs and their extreme chemical fractionations, the mix of young and old ages, and evidence for recent aqueous alteration, are all consistent with Mars as the source
Aqueous alteration
All of the SNC meteorites were altered or modified by water-borne solutions before they arrived at Earth (e.g., Bunch and Reid, 1975; Gooding and Muenow, 1986; Gooding et al., 1988; Gooding, 1992; Romanek et al., 1994). While many asteroidal meteorites show similar signs of aqueous alteration (e.g., Zolensky and McSween Jr, 1988), alteration of the SNC meteorites was relatively recent (Gooding, 1992; Swindle et al., 1997; Shih et al., 1998), except for ALH84001 (Borg et al., 1999). Of the
From Mars to The Earth
Although once very controversial, it is now generally accepted that rocks can be ejected from Mars, and can traverse the solar system to land on Earth. Before the mid-1980s it was not clear how unmelted, unvaporized material (i.e., rock) could be ejected from a planetary surface. That problem was solved by the first recognition of a meteorite from the Moon (Mason, 1982; Treiman and Drake, 1983) and by recognition of the physical ejection mechanism of spallation (Melosh, 1989). There also was
Conclusion
At this point, everything known about the SNC meteorites is consistent with the constraints we know or can reasonably surmise about Mars. The presence of Martian atmosphere gas (or an identical facsimile) in the SNC meteorites is the most telling link between the SNCs and Mars. The presence of distinct atmospheric and crust/mantle gas components in the SNCs are consistent with their source on a differentiated planet with an atmosphere, and seems to exclude small bodies and those without
Acknowledgements
We appreciate careful, constructive reviews by S. Clifford, J. Jones, G. Ryder, and an anonymous reviewer. Supported in part by NASA grant NAG5-8270 to AHT and NASA RTOP 344-31 to DDB. Lunar and Planetary Institute contribution #995.
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