Combining meteorites and missions to explore Mars SPECIAL FEATURE Timothy J. McCoy1, Catherine M. Corrigan, and Christopher D. K. Herd2 Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10th and Constitution Avenues NW, Washington, DC 20560-0119 Edited by Mark H. Thiemens, University of California San Diego, La Jolla, CA, and approved August 24, 2011 (received for review March 30, 2011) Laboratory studies of meteorites and robotic exploration of Mars can and must contribute to our understanding of all of these re- reveal scant atmosphere, no evidence of plate tectonics, past servoirs if we are to understand Mars as a system. evidence for abundant water, and a protracted igneous evolution. Among the meteorites now recognized as martian, the first Despite indirect hints, direct evidence of a martian origin came to be recovered fell October 3, 1815 in Chassigny, France, only with the discovery of trapped atmospheric gases in one meteorite. 20 y after meteorites were recognized to have an extraterrestrial Since then, the study of martian meteorites and findings from mis- origin. In hindsight, hints of a martian origin were available from sions have been linked. Although the meteorite source locations 1950, but final recognition awaited a remarkable finding in 1983. are unknown, impact ejection modeling and spectral mapping By that time, it was recognized that a duo of basaltic meteorites, of Mars suggest derivation from small craters in terrains of Ama- Shergotty and Zagami, differed from the abundant basaltic zonian to Hesperian age. Whereas most martian meteorites are eucrite meteorites. This duo (later joined by other meteorites to young (<1.3 Ga), the spread of whole rock isotopic compositions be called shergottites after the archetypal meteorite) was linked results from crystallization of a magma ocean >4.5 Ga and forma- to the pyroxenitic nakhlites and the olivine-rich dunite Chassigny tion of enriched and depleted reservoirs. However, the history through a common oxygen isotopic signature (1). The common inferred from martian meteorites conflicts with results from recent parent body to these disparate meteorites clearly produced a Mars missions, calling into doubt whether the igneous histor y in- diverse range of igneous lithologies. Further, all of these meteor- ferred from the meteorites is applicable to Mars as a whole. Allan ites shared apparently young ages of 1.3 Ga or less (2), either Hills 84001 dates to 4.09 Ga and contains fluid-deposited carbo- requiring resetting during heating in a later impact event or young nates. Accompanying debate about the mechanism and tempera- crystallization ages and a planet-sized body to explain recent, GEOLOGY ture of origin of the carbonates came several features suggestive near-surface igneous activity. Among the earliest suggestions of past microbial life in the carbonates. Although highly disputed, of a martian origin was that of McSween and Stolper (3) based the suggestion spurred interest in habitable extreme environments on similarities between the bulk compositions of these meteorites on Earth and throughout the Solar System. A flotilla of subsequent and soil compositions measured by the Viking lander. Direct evi- spacecraft has redefined Mars from a volcanic planet to a hydro- dence for a martian origin came with the discovery of high ratios 40 36 129 132 logically active planet that may have harbored life. Understanding of Ar∕ Ar and Xe∕ Xe in trapped gases in shock-altered the history and habitability of Mars depends on understanding the phases of the Antarctic meteorite Elephant Moraine A79001 that coupling of the atmosphere, surface, and subsurface. Sample re- closely resembled martian atmosphere measured by the Viking turn that brings back direct evidence from these diverse reservoirs lander (4). This remarkable discovery set the stage for three dec- is essential. ades of investigations about Mars that, like the initial findings, coupled laboratory analyses of these meteorites with observations aboratory studies of over 50 meteorites derived from Mars, from spacecraft. This approach recently came full circle with the Lplus data collected by numerous orbital and landed spacecraft, discovery on Meridiani Planum by the Mars Exploration Rover paint a picture of a rocky planet that evolved in a fundamentally Opportunity of an isolated rock whose textural, mineralogical different way than Earth. It has scant atmosphere, no evidence of and chemical properties are very similar to that of certain basaltic crustal recycling (plate tectonics), evidence for copious amounts shergottites, including Elephant Moraine (EET) A79001 (5) of water that—if still present—is frozen and largely underground, (Fig. 1). and crustal ages ranging from approximately 4.0 Ga to no more Among the earliest questions was how these meteorites than 180 Ma. reached Earth and where they originated on Mars. Initial skepti- Since its inception, the study of martian meteorites has been cism about a martian origin centered on the shock pressures intimately linked to the exploration of Mars by orbital and landed required for impact ejection, which should have produced com- spacecraft. The very recognition of martian meteorites was plete melting of the ejecta. Proof that meteorites could be ejected enabled by our exploration of Mars. From that initial stage, both unmelted from large bodies came with the discovery of the Allan the number of martian meteorites and missions to Mars has Hills (ALH) A81005 Antarctic meteorite and the recognition increased dramatically, unveiling fundamental insights into the of its lunar origin (6). Early analytical solutions to ejecting material as spallation blocks indicated that craters of the order of history of Mars and posing additional questions, many driven – by ground- and space-based technological advances. In most re- 10 15 km were required. Later workers called for a single, either spects, the studies of martian meteorites and martian rocks and very large [e.g., 200 km; (7)] or oblique (8) impact event on a terrains from missions have been highly complementary, with geologically diverse terrain with ejection of multimeter sized each asking questions and framing answers for the other. Among blocks and subsequent collisions in space to explain the diverse the most interesting areas of exploration—driven by a controver- cosmic-ray exposure ages. As the number of recognized martian sial finding from a martian meteorite—is the search for ancient meteorites increased, these latter models became increasingly or extant life. Achieving this goal requires a better understanding of linkages between the interior, crust, and atmosphere that have Author contributions: T.J.M. designed research; T.J.M., C.M.C., and C.D.K.H. performed driven, and may continue to drive, the evolution of the planet research; and T.J.M., C.M.C., and C.D.K.H. wrote the paper. and, ultimately, an increasingly necessary (if elusive) Mars sample The authors declare no conflict of interest. return mission. In this paper, we discuss how martian meteorites, This article is a PNAS Direct Submission. when coupled with Mars missions, have elucidated these impor- 1To whom correspondence should be addressed. E-mail: [email protected]. — tant reservoirs. We argue that Mars sample return which is 2Permanent address: Department of Earth and Atmospheric Sciences,University of Alberta, widely accepted as the highest priority for Mars exploration— 1–26 Earth Sciences Building, Edmonton, AB, Canada T6G 2E3. www.pnas.org/cgi/doi/10.1073/pnas.1013478108 PNAS Early Edition ∣ 1of6 Downloaded by guest on October 2, 2021 geologic context coupled with the apparent bias toward ejection of martian meteorites from younger geologic units is one of the most significant limitations in coupling martian meteorite and spacecraft-based studies to understand the history of Mars. With the availability of visible to near-infrared data from the OMEGA spectrometer on Mars Express, revisiting this exercise to identify possible source regions for the shergottites might prove pro- ductive. In spite of the lack of “field context,” detailed studies of these samples have yielded insights into the geologic evolution of Mars. Much of the first geochemical modeling of the martian interior derived from the analysis of radiogenic isotopes for geochronol- ogy. Results for the shergottites showed a significant and large variation in initial isotopic ratio, especially for Sr (12), and it was apparent that the initial 87Rb∕86Sr and 87Sr∕86Sr values for meteorite whole rock samples plot on a line with a slope corre- sponding to 4.5 Ga (13). This led to the postulation that 4.5 Ga was the true crystallization age of the shergottites, and that ap- proximately 180 Ma ages obtained by mineral separate Rb–Sr and Sm–Nd reflected resetting by impact metamorphism—a genuine concern because each meteorite experienced at least one impact event, associated with its liberation from Mars, with peak shock pressures in the range of 5–55 GPa and postshock heating up to 1,000 °C (14). This view was further supported by 39Ar–40Ar (giving ages of >250 Ma) and U–Th–Pb results (2). Following arguments that shock metamorphism is unable to fully reset isochrons (2, 12), an alternative view was proposed to explain the 4.5 Ga apparent age in which the spread of whole rock iso- topic compositions is attributed to mantle source characteristics or mixing (13, 15). In this scenario, isotopically enriched and depleted reservoirs are formed by 4.51 Ga (15), and remain un- Fig. 1. Images of martian meteorite Elephant Moraine (EET) A79001 (above) mixed and undisturbed until the melting event that makes a sher- and martian rock Bounce Rock (fasle color; below). Dark patches in EET gottite magma. The geochemical characteristics of a shergottite A79001 are shock-melt pockets containing trapped martian atmosphere. are dictated by which reservoir, or combination of reservoirs, is Scale cube above is 1 cm and length of Bounce Rock is approximately involved in melting.