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Vol. 7, No. 7 July 1997 INSIDE • Call for Editors, p. 15 GSA TODAY • Employment Service, p. 21 • 1997 GSA Annual Meeting, p. 28 A Publication of the Geological Society of America Evidence for Life in a Martian Meteorite? Harry Y. McSween, Jr. Department of Geological Sciences, University of Tennessee, Knoxville, TN 37996 ABSTRACT The controversial hypothesis that the ALH84001 mete- orite contains relics of ancient martian life has spurred new findings, but the question has not yet been resolved. Organic matter probably results, at least in part, from terrestrial contamination by Antarctic ice meltwater. The origin of nanophase magnetites and sulfides, suggested, on the basis of their sizes and morphologies, to be biogenic remains con- tested, as does the formation temperature of the carbonates that contain all of the cited evidence for life. The reported nanofossils may be magnetite whiskers and platelets, proba- bly grown from a vapor. New observations, such as the possi- ble presence of biofilms and shock metamorphic effects in the carbonates, have not yet been evaluated. Regardless of the ultimate conclusion, this controversy continues to help define strategies and sharpen tools that will be required for a Mars exploration program focused on the search for life. INTRODUCTION Since the intriguing proposal last summer that martian mete- orite Allan Hills (ALH) 84001 contains biochemical markers, bio- genic minerals, and microfossils (McKay et al., 1996), scientists and the public alike have been treated to a variety of claims sup- porting or refuting this hypothesis. Occasionally, the high visibil- ity of the controversy has overshadowed the research effort (e.g., Begley and Rogers, 1997), but I believe that science will benefit significantly from this experience. If the hypothesis is confirmed, Figure 1. Mars is thought to be the parent body of a dozen igneous it will rank among the major discoveries of all time. If not, McKay meteorites (achondrites). The ALH84001 achondrite was recovered in and his colleagues have still demonstrated that microparticles, sol- Antarctica in 1984, but it was not recognized as a martian sample until uble minerals, and possibly organic matter can survive on the red a decade later. This orthopyroxene cumulate rock (cube is 1 cm) is planet for billions of years, which provides a hopeful outlook for a crosscut by fracture zones (see thin section view; long axis is ~0.5 cm), Mars exploration program focused on the search for life. within which tiny carbonate globules were deposited. The carbonate grains are associated with organic matter and contain microparticles This article reviews recently published research bearing on suggested to be martian nanofossils and biogenic minerals. this important topic. Most of this literature exists only as confer- Photographs courtesy of NASA. ence abstracts and technical comments in scientific journals. There seems to be virtually no argument, at least within the plane- tary science community, that ALH84001 is a sample of the ancient (~4.5 Ga) martian crust (e.g., Mittlefehldt, 1994; McSween, 1994; a sample containing ~1 ppm PAHs ranks as a triumph of analyti- Clayton and Mayeda, 1996; Goswami et al., 1997). This ultramafic cal ingenuity. Although PAHs do not play a significant role in the igneous rock (Fig. 1) is crosscut by breccia zones containing small biochemistry of terrestrial organisms, they can form by geochem- carbonate grains, which are the hosts for organic matter as well as ical transformation of certain hydrocarbons present in decayed reported biogenic materials and microfossils. This article addresses organisms. PAHs also form by abiotic processes such as the and expands upon the four lines of evidence for possible biologic combustion of fossil fuels, and they are common constituents activity cited by McKay et al. (1996). of chondritic meteorite kerogens (Zenobi et al., 1989). Anders (1996) noted that PAHs are produced by pyrolosis (thermal ORGANIC MATTER decomposition) whenever graphite formation is kinetically inhib- McKay et al. (1996) described fused hydrocarbon rings (poly- ited. He further pointed out that the “few specific PAHs” from C14 cyclic aromatic hydrocarbons, or PAHs) associated with the car- bonates in ALH84001. The identification of specific molecules in Meteorite continued on p. 2 IN THIS ISSUE GSA TODAY July Vol. 7, No. 7 1997 Evidence for Life in a Research and Fellowship Opportunities ..... 19 Martian Meteorite ................... 1 Officers and Past Chairs of GSA TODAY (ISSN 1052-5173) is published In Memoriam ............................ 2 Sections and Divisions .................. 20 monthly by The Geological Society of America, Inc., Correction ............................... 2 GSA Employment Service ................. 21 with offices at 3300 Penrose Place, Boulder, Colorado. GSAF Update ............................. 6 AIPG Becomes Associated Society ......... 21 Mailing address: P.O. Box 9140, Boulder, CO 80301- 9140, U.S.A. Periodicals postage paid at Boulder, Col- Rock Stars—Florence Bascom ............. 8 July Bulletin and Geology Contents ......... 24 orado, and at additional mailing offices. 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Claims are honored for one year; please allow sufficient delivery time for overseas copies, S-wave (A) and P-wave (B) velocity variations along a section through the southern up to six months. United States.” The published description reverses the S-wave and P-wave images. STAFF: Prepared from contributions from the GSA staff and membership. Executive Director: Donald M. Davidson, Jr. Meteorite continued from p. 1 technique similar to that used by McKay Science Editors: Suzanne M. Kay, Department of Geo- et al. (1996) for ALH84001. EETA79001