The Scientific Rationale for Studying found on Other Worlds Submitted to: 2013-2022 Planetary Science Decadal Survey Committee

Primary author: James W. Ashley Phone number: 602.705.5492 Institution: Space Flight Facility School of and Space Exploration Arizona State University E-mail address: [email protected]

Co-authors and respective institutions:

M. D. Fries1, G. R. Huss2, J. E. Chappelow3, M. P. Golombek1, M. A. Velbel4, S. W. Ruff5, C. Schröder6, W. H. Farrand7, D. D. Durda8, P. A. Bland9, I. Fleischer6, A. C. McAdam10, S. P. Wright11, A. T. Knudson12, L. A. Leshin10, and A. Steele13

1Jet Propulsion Laboratory, Pasadena, California 2 Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, Hawaii 3Arctic Region Supercomputing Center, University of Alaska Fairbanks, Fairbanks, Alaska 99775-6020 4 Department of Geological Sciences, Michigan State University, East Lansing, Michigan 5Mars Space Flight Facility, School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 6 Institut für Anorganische und Analytische Chemie, Johannes Gutenberg-Universität, Maintz, Germany 7 Space Science Institute, Boulder, Colorado 8 Southwest Research Institute, Boulder, Colorado 9Impacts and Astromaterials Research Centre (IARC), Department of Earth Science and Engineering, Imperial College London, Kensington Campus, London SW7 2AZ, UK 10 NASA Goddard Space Flight Center, Greenbelt, Maryland 11 Institute of , University of New Mexico, Albuquerque, New Mexico 12Planetary Science Institute, Seattle, Washington 13 Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC

PSDS; Meteorites on Mars

Introduction:

The scientific value of meteorites is traditionally viewed in terms of their relevance to the formation and evolutionary history of their parent bodies, the as a whole, or to environments within the presolar nebula. Meteorites that fall on Earth are not generally considered to provide information about the Earth. However, meteorites on the surfaces of other solar system bodies have the potential to provide information about local climate history, atmospheric chemistry, the age of the surface on which they are found, and a variety of other topics that will be outlined below. Conceptually, meteorites on other worlds are the equivalent of artificially inserting highly sensitive, unweathered rocks of known character into the surface environment, and allowing them to alter over their entire exposure lifetimes, a time period potentially spanning billions of years, depending on the environment. The Mars Exploration Rovers (MERs) surprised researchers by permitting the identification and investigation of several meteorites on the surface. Such meteorites are a resource that should be utilized in future Mars missions, both for in situ studies and for sample return. The current number of martian find1 candidates stands at a minimum of 11 for the MER mission [Ashley et al., 2009; Schröder et al., 2006; Schröder et al., 2008; Schröder et al., 2009] (see Figure 1 for the most recent example). It is thus probable that current and future roving spacecraft will continue to encounter meteoritic specimens. Providing that the ability to recognize meteorites in situ by these machines is comparable to that of the MERs, opportunities for further study can be expected and planned for. With a 2012 landing date currently anticipated, the two-year nominal mission for Mars Science Laboratory (MSL) will kick off the 2013-2022 decade. Relevant instruments on the spacecraft include the Mast Camera (MastCam), Mars Hand Lens Imager (MAHLI), Alpha Particle X-ray Spectrometer (APXS), Chemistry & Camera (ChemCam) instrument, Chemistry & Mineralogy X-ray Diffraction (CheMin) instrument, and Sample Analysis at Mars (SAM) instrument suite. The SAM and CheMin XRD instruments, for example, will be useful for mineralogical determinations within candidates; and SAM derivatization and pyrolysis experiments could assist in assessing differences in organic compound oxidation from between the interior and exterior of target samples. We recommend consideration of the meteorite problem when designing the detailed mission objectives for MSL, as well as when selecting instruments for future missions. This topic falls under the Planetary Science Decadal Survey category “Recommendations for supporting research required to maximize the science return from the flight investigations.” In the remainder of this document, we will describe several different studies that are enabled by the presence of meteorites on other planets. We will focus this discussion on Mars, but many of the concepts are equally valid for other planetary bodies. For example, some researchers are exploring the possibility that Earth rocks may be resting on our [Armstrong et al., 2002; Crawford et al., 2008]. Such rocks could yield clues to the origins of life on an early Earth. The topics for Mars will be separated into two categories: In situ Studies and Sample Return (including any precursor missions optimized for sample return). However, topics 1, 2, and 3 below span both categories.

1 Point of nomenclature: The term ‘martian find’ will be used in place of ‘’ to distinguish meteorite candidates found on Mars from the SNC association of meteorites found on Earth, commonly also referred to as ‘martian meteorites’.

1 PSDS; Meteorites on Mars

In-situ Studies: 1. Witness samples for weathering and climate history assessment; Among the most valuable scientific applications of martian finds is as control or ‘witness’ samples for probing both physical and chemical weathering environments [Ashley and , 2004]. The hypersensitivity of most meteorites to mineral-volatile interactions makes them particularly well suited to the latter purpose. The chemical weathering of primary minerals constitute a suite of processes that can provide clues to climate behavior in the environment of their occurrence. On Earth, aqueous alteration in meteorites is of interest because of its potential to obscure such samples’ records of pre-terrestrial () processes. However, on Mars is less relevant to the meteorites themselves than to the assessment of past and present climatic processes and questions relating to habitability. From an astrobiological standpoint, it is liquid exposure, of course, that represents one of the strongest markers for habitability potential. Liquid water interactions with surface materials are therefore the to these questions. The well-known starting chemistries and normative mineralogies of ordinary have been used to evaluate terrestrial climatic behavior as a function of oxidation intensity in hot

Figure 1. Timely for the case in point, the MER-B rover has recently completed an investigation of the large iron- meteorite informally named Block Island at . Preliminary indications appear to present evidence for both physical and corrosive (aqueous) alteration. Such findings confirm the utility of meteorites as in situ witness samples for a variety of martian processes.

Image credit: NASA/JPL/Pancam. desert environments [Bland et al., 1998]. Iron oxidation might be similarly used as a kind of litmus test for the past availability of . Reduced iron metal is present in some 88% of all terrestrial falls [Dodd, 1981] and will oxidize readily in the presence of even trace amounts of water, though results may vary depending on the phase of water (vapor, liquid or ice). The metal content ranges from <1% for LL ordinary chondrites [Dodd, 1981]; to more than 20% for H ordinary chondrites [Pun et al., 1990]; as subequal volumes with silicate phases in the case of many stony- and CB chondrites [McCoy et al., 1990; Weisberg et al., 2002]; to close to 100% for irons (minus sulfides, occasional silicates, graphite nodules, and other non-metal fractions). In the case of reduced metallic iron (Fe°), an even stronger thermodynamic drive toward oxidation is present than is the case for ferrous iron (Fe2+) in minerals such as olivine, pyroxene or magnetite. Meteoritic iron-nickel metal is therefore likely to be the most sensitive material to this type of alteration on the surface of Mars. A large portion of the iron grains within meteorite matrices is found within the cryptocrystalline size fraction, magnifying the alteration susceptibility through increased granular surface area [Burns and Martinez, 1991]. Where weathering intensities might be subtle or undetectable for indigenous rocks, they may be conspicuous in this responsive material. In addition to iron oxidation reactions, stony and stony- iron meteorites are typically sensitive to silicate alteration because of their high-temperature mineral content. Moreover, many are unequilibrated mixtures of primordial materials in both coarse-grained and matrix constituents, which adds to their overall tendency toward alteration

2 PSDS; Meteorites on Mars when water is present to facilitate reactions. Finally, the unaltered, ‘starting’ conditions of these rocks are well known in terms of their mineralogies, bulk chemistries, and textures because we are likely to have identical copies of these samples available on Earth, making departures from the pristine case more easily recognizable than with indigenous rocks. Calculations for thermodynamically spontaneous aqueous alteration reactions on Mars show that water vapor interaction with the meteoritic sulfide mineral should produce a hydrated iron sulfate [Gooding et al., 1992]. Such a result might be anticipated in the presence of a high water vapor or acid fog situation [e.g. Schiffman et al., 2006]. The formation of oxyhydroxide minerals is favored in the event of a liquid water interaction [Gooding et al., 1992]. Well- 3+ crystallized products may include goethite (α-FeOOH), akaganéite (Fe 7.6Ni0.4O6.4(OH)9.7Cl1.3) (β-FeOOH), and lepidocrocite (γ-FeOOH), where akaganéite sequesters chlorine from the environment [Buchwald and Clarke, 1988; 1989]. The presence of the hydroxyl radical is accepted as evidence of water exposure. Relevant to the context of Mars, these oxyhydroxides can revert to hematite given appropriate reaction temperatures [Glotch and Kraft, 2008]. Even in a situation where is lacking as a separate volatile, the oxygen atom in the water molecule alone is sufficient to oxidize ferrous iron in olivine on Mars [Zolotov, 2007]. Meteorites on Mars have been estimated to decompose completely from chemical weathering on a one-Gyr timescale [Bland and , 2000]. If we thus assume that many or most meteorites resting on the surface are likely to be more recent ( to ) additions to the planet, then their aqueous alteration would address the effects of more recent (and therefore more subtle) water availability. It has been demonstrated that Mars continues to sustain frequent impacts [Malin et al., 2006]. Any fragments of this material that are not vaporized upon hypervelocity impact (increasingly likely for smaller diameters) will automatically begin altering upon arrival if conditions favor such alteration. Study of this fresh material in-situ could also contribute significantly to the understanding of impact processes (refer to Point #6 below). In summary, the arguments for meteorites as witness samples for weathering are: 1) they are more sensitive to chemical alteration than most other rock types on geochemically evolved planetary surfaces; and 2) their unaltered, ‘starting’ conditions are already well-known. 2. Assisting with martian habitability assessments; Martian finds may be useful as a unique test of the hypothesis of past or present life. On Earth, it is known that meteorites accumulate environmental microbes even in Antarctic conditions [Burckle and Delaney, 1999; Steele et al., 1998; Steele et al., 1999; Steele et al., 2000]. These microbes can be detected directly [same references apply] and microbial alteration can be discerned through isotopic alteration signatures [Gronstal et al., 2009]. Microbial colonization of meteorites occurs rapidly because meteorites represent a local concentration of carbonaceous compounds, metastable mineral phases suitable for use in metabolic processes, and possibly even slight relative warmth due to solar heating. This is especially true of organic compound-rich carbonaceous chondrites, and is true to a lesser degree in low petrographic grade ordinary chondrites. It is reasonable to expect that if these meteorites came into contact with martian microorganisms then the meteorites would be colonized and altered in a similar fashion to that seen on Earth. Analyzing martian falls, either in situ or as part of a sample return suite will provide unique and important information pertaining to martian life and habitability. 3. Understanding better the contributions of extraterrestrial materials to martian soils and sedimentary rocks; Our best estimates of the meteoritic contribution to surface sediments on Mars are currently based on APXS data from both MER-A and MER-B rock and soil targets, and

3 PSDS; Meteorites on Mars place the amount at 1 to 3% chondritic [Yen et al., 2006]. However, the authors of this study indicate that other factors could be affecting this estimate, which is based primarily on nickel abundance. Yen et al. [2006] further point out that understanding the influx of organic material is necessary for “constraining carbon oxidation rates in support of Martian habitability assessments.” Other important considerations for the study of meteoritic components to martian regolith materials are: 1) addressing mysteries relating to a ferromagnetic component of the martian dust [e.g. Bertelsen et al., 2004], 2) assessment of past and present meteoritic flux rates, and 3) assigning Mars-arrival ages to any sizable meteoritic materials for possible determination of sedimentation rates (an idea that should be explored when making sample selections). 4. Constraints on the martian atmosphere and meteorite populations; passing through an atmosphere are ablated, decelerated, and have their trajectories altered by aerodynamic forces. Thus meteoritic bombardment of a planetary surface is mediated by any overlying atmosphere, and modulated by its temporal variations. Assuming that a meteoroid’s entry conditions (entry mass, velocity, and angle) and planet’s atmospheric density are such that the impactor survives passage and impact, a meteorite is produced. Therefore meteorite populations, and individual meteorites found on Mars preserve records of Mars’ impact history and atmospheric conditions [Chappelow and Sharpton, 2006a]. We have only just begun to learn how to ‘read’ these records. Given the masses and compositions of meteorites found on Mars by the MER rovers, estimates of the entry conditions and atmospheric density that held at the time they dropped on Mars can be backed out using a computer model of meteoritic passage through the atmosphere. When ‘scanned’ over appropriate ranges of initial conditions, sub-ranges of these conditions that result in impacts of the observed mass and low enough velocity to survive impact can be identified, and a minimum atmospheric density value obtained. This analysis has already been carried out on ‘’ [Chappelow and Sharpton, 2006b], and is now being conducted on ‘Block Island’. The sizes of both of these objects strongly suggest they landed at times of significantly denser martian atmosphere. Application of this procedure to greater numbers of meteorites would lead directly to increasingly better estimates of the mass distribution of small (cm- to m-sized) Mars-crossing meteoroids, which is currently only poorly constrained. In addition, good estimates of meteorite ages would yield information on martian atmospheric density vs. time, and establish martian meteorite production rates. Conversely, meteorite populations may be used in a manner somewhat analogous to the way crater populations are currently used — to estimate martian surface ages. And since meteorite production and impact cratering are complementary processes, it should be possible to establish a relationship between meteorite and crater populations on Mars, thus enabling estimation of meteorite populations from crater counts, which can be obtained for large areas, from orbit. 5. Compositional variations among main-belt ; Aspects of meteoritical knowledge gleaned from the surface of Mars might extrapolate to the asteroidal disciplines. One would expect martian finds to compare directly to those found on Earth, and indeed this is the case for Heat Shield Rock, a IAB complex meteorite which may be genetically related to other IAB meteorites on Earth. However, ‘Santa Catarina’ and related meteorite candidates identified by the Opportunity rover [Fleischer et al., 2009; Schröder et al., 2009] appear to have no direct terrestrial counterparts, and may sample a new parent body source. The possibility that temporal variations in meteoroid type population could be represented better by the martian situation than by meteorite collections on Earth because it preserves dramatically older suites of rocks can be

4 PSDS; Meteorites on Mars explored and contrasted in situ with roving missions. Evaluation of the increasing number of martian finds permits the assessment of such population differences if they exist. 6. Improving our understanding of impact processes on Mars; Finding meteoritic material associated with fresh impact craters or crater clusters offers the potential benefit of using the craters to help date the impact events, which can improve knowledge of the timescale for meteorite alteration. As an example, the Opportunity rover recently visited a fresh crater cluster. The craters are superposed on the ubiquitous granule ripples of Meridiani Planum, suggesting a very young age. Small, dark pebbles are found on top of the craters, suggesting that these pebbles are fragments of the impactor. Crater counts of similarly fresh impact craters can be used to estimate an age for the impact and thus the fragments. Combining this information with morphometric information of the craters could help improve the nature of the impact (primary versus secondary; [i.e. McEwen et al., 2005]) and the dispersion of the craters in the cluster can be used to infer the density of the impactor [Ivanov et al., 2008; 2009]. Similar studies could be envisioned for future rovers on Mars or other worlds. 7.Unanticipated applications; The epic MER experience has demonstrated that many unanticipated uses for meteorites present themselves to the researcher following discovery. Dust- thickness determinations using iron surfaces [Ashley et al., 2008], impact studies, atmospheric fragmentation studies, and the study of erosive forces in the presence of iron materials are current and proposed projects inspired by the martian finds identified thus far. It has even been conjectured that future settlers on Mars might find meteoritic iron a valuable resource material [Landis, 2009]. We can thus anticipate with confidence that new ideas will be forthcoming with additional finds. This is an exciting posture for any scientific program.

Mars Sample Return and Precursor Missions: A report prepared by the Mars Exploration Program Analysis Group (MEPAG) Next Decade Science Analysis Group (ND-SAG) for a future Mars Sample Return (MSR) mission included the following reference to the topic of meteorite consideration: “Obviously, allocating precious return mass to a meteorite would require a strong justification for the hypothesis being tested,” [MEPAG, 2008]. We hope here to present satisfactory justification that recovered meteoritic materials would constitute a valuable resource, and not necessarily a source of ‘contamination’ to be avoided. As discussed, the presence of meteoritic material on the surface of Mars (or indeed, upon any world with an atmosphere) provides for a kind of ‘litmus test’ approach for aiding the assessment of climate history on that world through a study of the type and intensity of weathering processes. With a successful sample return, we would have the ability to study the surface- volatile interaction histories of materials with well-known starting mineralogies, chemistries (isotopic and elemental), and textures. Laboratory comparison and contrasting of relative rates of different alteration mechanisms within the suite of weathering effects detectable, as measured for meteoritic materials from Mars and those found in terrestrial desert environments would be useful. Such a study is possible only with returned samples, which would allow not only the direct measurement of trace element chemistry at an unprecedented level of accuracy, but also the probing of individual grains on the microscale. Determining the post-fall, martian residence time will be important for providing constraints on martian weathering behavior. Both cosmogenic stable nuclides and radionuclides have been discussed for the martian situation [Nishiizumi and Reedy, 2000]. These authors suggest that the 10Be-26Al-21Ne isotope system should be useful for determining exposure ages for rocks at Mars’

5 PSDS; Meteorites on Mars surface up to 106-107 years b.p. If sufficient spallation/ablation and/or fragmentation had occurred during (a reasonable assumption), such an age determination should be obtainable up to this maximum for a meteorite sample returned to the laboratory. Determining ages for older exposure histories may be more problematic, but could be possible within the context of contributing stratigraphic, geomorphologic, and/or geochemical factors. While discussions for possibly performing in situ geochronology of Mars rock crystallization ages using the SAM and CheMin instruments on MSL remain ongoing, any isotopic determination of a meteorite’s surface exposure age would require sample return for laboratory analysis. Looking ahead to a time when the NASA vision of human missions to Mars is realized, we can anticipate that meteorites will comprise a non-trivial fraction of the rocks recovered. The planetary sciences will benefit from having thoughtfully considered, field-tested models for meteorite-based problem solving. What we learn from the careful study of these targets in the 2013-2022 decade will help guide this effort.

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Fleischer, I., et al. (2009), Cobbles at Meridiani Planum, paper presented at European Planetary Science Congress. Glotch, T., and M. Kraft (2008), Thermal transformations of akaganeite and lepidocrocite to hematite: assessment of possible precursors to Martian crystalline hematite, Physics and Chemistry of Minerals, 35(10), 569-581. Gooding, J. L., et al. (1992), Physical and chemical weathering, in Mars, edited by H. H. Keiffer, et al., pp. 626-651, University of Arizona Press, Tucson, Arizona. Gronstal, A., et al. (2009), Laboratory experiments on the weathering of iron meteorites and carbonaceous chondrites by iron-oxidizing bacteria, Meteoritics and Planetary Science, 44(2), 233-247. Ivanov, A., et al. (2008), Small impact crater clusters in high resolution HiRISE images, paper presented at Lunar and Planetary Science Conference XXXIX. Ivanov, A., et al. (2009), Small impact crater clusters in high resolution HiRISE images - II, paper presented at Lunar and Planetary Science Conference XL. Landis, G. A. (2009), Meteoritic steel as a construction resource on Mars, Acta Astronautica, 64(2-3), 183-187. Malin, M., et al. (2006), Present-day impact cratering rate and contemporary gully activity on Mars, Science, 314, 1573-1577. McCoy, T. J., et al. (1990), Classification of four ordinary chondrites from Spain, Meteoritics and Planetary Science, 25, 77-79. McEwen, A. S., et al. (2005), The rayed crater and interpretations of small impact craters on Mars, Icarus, 176(2), 351-381. MEPAG, N.-S. (2008), Science priorites for Mars sample return, unpublished white paper, edited, p. 73. Nishiizumi, K., and R. C. Reedy (2000), In-situ measurements of cosmogenic radionuclides on the surface of Mars, paper presented at Concepts and Approaches for Mars Exploration. Pun, A., et al. (1990), Classification of five new chondrite finds from Roosevelt County, New Mexico, Meteoritics and Planetary Science, 25, 233-234. Schiffman, P., et al. (2006), Acid-fog deposition at Kilauea volcano: A possible mechanism for the formation of siliceous-sulfate rock coatings on Mars, Geology, 34(11), 921-924. Schröder, C., et al. (2006), A stony meteorite discovered by the Mars Exploration Rover Opportunity on Meridiani Planum, paper presented at 69th Annual Meeting of the Meteoritical Society, Zürich, Switzerland. Schröder, C., et al. (2008), Meteorites on Mars observed with the Mars Exploration Rovers, Journal of Geophysical Research, 113(E06S22). Schröder, C., et al. (2009), Santorini, another meteorite on Mars and third of a kind, paper presented at Lunar and Planetary Science Conference XL, Lunar and Planetary Institute, The Woodlands, Texas. Steele, A., et al. (1998), Terrestrial contamination of an Antarctic chondrite, Meteoritics & Planetary Science, 33(4), A149-A149. Steele, A., et al. (1999), Imaging of the biological contamination of meteorites: A practical assessment, paper presented at Lunar and Planetary Science Conference XXX. Steele, A., et al. (2000), Investigations into an unknown organism on the martian meteorite , Meteoritics & Planetary Science, 35(2), 237-241. Weisberg, M. K., et al. (2002), Gujba and origin of the Bencubbin-like (CB) chondrites, paper presented at Lunar and Planetary Science Conference XXXIII, League City, Texas. Yen, A. S., et al. (2006), Nickel on Mars: Constraints on meteoritic material at the surface, Journal of Geophysical Research, 111(E12S11). Zolotov, M. Y. (2007), Serpentinization of olivine with limited oxygen in a Martian environment, edited by J. W. Ashley, p. Personal communication.

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