The SNC meteorites are from Mars

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Abstract

The 14 SNC meteorites are all igneous rocks, either basalts or basaltic cumulates. They are inferred to be from Mars, based on direct comparison with Martian materials and on consistency with inferences about Mars. Most telling is that the SNC meteorites contain traces of gas which is very similar in elemental and isotopic compositions to the modern Martian atmosphere as measured by Viking landers on Mars and spectroscopy from Earth. The Martian atmosphere appears to have a unique composition in the solar system, so its presence in the SNCs is accepted as strong direct evidence that they formed on Mars. Independent of this link, the SNC meteorites must be from a planet with a significant atmosphere because they contain several abundant gas components, one of which carries large isotopic fractionations characteristic of atmospheric processing. Further, the elemental compositions and oxidation state of the SNC meteorites are consistent with data from in situ analyses of Martian soils and rocks, and are quite distinct from compositions of other meteorites, rocks from the Earth, and rocks from the Moon. The range of formation ages for the SNC meteorites, 4.5–0.18Ga, is consistent with the varied ages of the Martian surface (based on its cratering record) and inconsistent with surface ages on any other solar system body. The extreme chemical fractionations in the SNC meteorites suggest complex internal processes on a large planet, which is inconsistent with an asteroidal origin. Some SNCs were altered by aqueous solutions at <0.7Ga, consistent with the recent presence of liquid groundwater in Mars inferred independently from the geology of its surface. There seems little likelihood that the SNCs are not from Mars. If they were from another planetary body, it would have to be substantially identical to Mars as it now is understood.

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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