sign in sign up
<<
Home , Block Island meteorite, Heat Shield Rock, Kamacite, Meridiani Planum, Schreibersite, Taenite

CosmoELEMENTS

THE STUDY OF EXOGENIC ROCKS ON —AN EVOLVING SUBDISCIPLINE IN

James W. Ashley*

History is replete with entertaining anecdotes predicts that -type proportions on the stony- candidates. They are brecciated of found and lost on . That Mars would approximate those found on and contain varying amounts of (an chronicle has recently been expanded to Earth, where some 94 percent are stony (chon- Fe–Ni ) and troilite (FeS), but they do not include the exotic saga of meteorite search, drite and ) varieties. resemble known meteorite varieties (Schröder discovery, and assessment on another planet et al. 2010). They may represent either a type using roving spacecraft. The inventory of OBSERVATIONS of meteorite not found in Earth-based collec- confi rmed and candidate meteorites on Mars tions or some species of impact that As with most discoveries in the planetary sci- currently stands at a minimum of 21 fi nds preserves materials from the impacting . ences, some observations confi rm theory while from three widely separated rover landing If meteoritic, the rocks are probably members others raise baffl ing questions. For example, to sites (TABLE 1). Using a combination of remote of a single-fall strewn fi eld. Further studies date not a single chondritic meteorite has been sensing and direct-measurement instruments are underway. The tendency for most to identifi ed on Mars, and the suite is dominated on and (Mars Exploration be found in groups is also almost certainly instead by large (30–200 cm) iron– mete- Rovers, MERs), and most recently on an indication of specimens being members orites (see TABLE 1). A selection bias probably (Mars Science Laboratory, MSL), a morphologic of common falls (“pairing” in meteoritics results from the nature of meteorite survival, and chemical database has been compiled for vernacular). preservation, and the built-in mission require- this suite of rocks. Finding nonindigenous ments that tend to overlook small rocks, but materials on Mars forces us to rethink mete- Overall, the observed effects of weathering in identifying this lost population would solve oritical defi nitions and language (for example, fi nds are complex and are likely com- an important mystery (Ashley et al. 2015). to avoid confusion about whether the term binations of relic/fossil components and prod- Mini-TES was indeed useful for identifying Martian meteorites refers to meteorites found ucts of contemporary processes. Our knowledge the fi rst meteorites found on another planet, on Mars or to the SNC meteorite association). of each rock depends on how complete rover but not in the manner anticipated. Steve Ruff, Here we will refer to these rocks as Martian reconnaissance has been on a case-by-case of Arizona State University’s Mars Space Flight fi nds. basis, which is largely a function of mission Facility, recognized that the spectrum mea- priorities at each time of discovery. Unknown sured for at residence times for the meteorites complicate THEORY was similar to the spectrum of the Martian sky. the diffi culty of sorting out their Martian his- Martian fi nds provide new ways to address a Only a metallic object has the refl ectivity to tories. The irons tend to present rounded and variety of science topics (e.g. Chappelow and produce these spectra, and so a meteorite was pitted morphologies with enlarged hollows and Sharpton 2006; Yen et al. 2006; Chappelow suspected and later confi rmed (Schröder et al. sculpted surfaces that are likely the result of and Golombek 2010; Ashley et al. 2011a). For 2008). Zhong Shan and Allan Hills, located in eolian abrasion (FIG. 1). However the intensities example, the reactive metallic iron in most Crater on the opposite side of the planet, of these features vary among specimens and meteorites provides clues to aqueous alteration were identifi ed in a similar way. Subsequent even within the same specimen, from virtu- that indigenous rocks do not. This is signifi - iron discoveries were identified based on ally unmodifi ed to cavernously excavated. This cant for the purpose of assessing weathering morphology, , texture, luster, and is consistent with some terrestrial examples, processes near the Martian equator (where chemistry. Other anomalies are found among where similar morphologies led Buchwald to the Mars rovers are conveniently located). On LIST OF METEORITE CANDIDATES FOUND ON MARS. The Opportunity discoveries were on Earth, weathering is a serious nuisance to the TABLE 1 Meridiani Planum while the Spirit and Curiosity fi nds were in Gusev and craters, respectively. study of a meteorite’s preterrestrial history (e.g. Type Velbel 2014). On Mars the effects of – First Instrumentation Meteorite Rover (suspected or interactions permit the probing of encountered employed confi rmed) aqueous geologic scenarios and related paleo- climate/habitability questions. Moreover the Barberton Opportunity 121 stony-iron PancamNavcam occurrence of meteorite falls throughout Mars’ Heat Shield Rock* Opportunity 339 IAB complex iron MTES/Pancam/APXS/MB/MI history means that some falls (if their residence Allan Hills Spirit 858 iron MTES/Pancam/ times can be determined) may assist under- Zhong Shan Spirit 858 iron MTES/Pancam/Navcam standing of subtle reactions related to the low Santa Catarina Opportunity 1034 stony-iron MTES/Pancam/APXS/MB/MI water–rock ratios of more recent ( Joacaba Opportunity 1046 stony-iron MTES/Navcam age) climate situations—a valuable tool for Mafra Opportunity 1151 stony-iron MTES/Navcam Mars science (e.g. Kraft et al. 2014). Paloma Opportunity 1190 stony-iron MTES/Navcam A reference library of thermal emission spectra Santorini Opportunity 1713 stony-iron Pancam/APXS/MB/MI for fresh and weathered meteorites was pre- Kasos Opportunity 1889 stony-iron Pancam/APXS/MB/MI pared to assist with the detection and evalua- Block Island Opportunity 1961 IAB complex iron Pancam/APXS/MB/MI tion of weathering effects using the miniature Shelter Island Opportunity 2022 IAB complex iron Pancam/APXS/MB/MI thermal emission spectrometers (Mini-TES) on the MER rovers (Ashley and 2004). Mackinac Island Opportunity 2034 iron PancamNavcam The approach was based on the weathering Oileán Ruaidh Opportunity 2368 iron PancamNavcam behavior of ordinary in Antarctica, Ireland Opportunity 2374 iron PancamNavcam a Mars-analog environment. Current under- Bingag Cave Opportunity 2642 iron PancamNavcam standing of meteorite delivery mechanisms Dia Island Opportunity 2642 iron PancamNavcam and inner Solar System dynamical models Canegrass Opportunity 3346 unknown PancamNavcam Lebanon Curiosity 634 iron Mastcam/ChemCam RMI

* Jet Propulsion Laboratory Lebanon B Curiosity 634 iron Mastcam/ChemCam RMI California Institute of Technology Littleton Curiosity 634 iron Mastcam/ChemCam RMI Pasadena, CA 91109-8099, USA E-mail: james.w.ashley@jpl..gov * Offi cial name: Meridiani Planum (Connolly et al. 2006)

ELEMENTS 10 FEBRUARY 2015 CosmoELEMENTS

A B A B

C D

FIGURE 1 (A) Released images of Lebanon, Lebanon B, and Littleton taken by Curiosity’s Mastcam on MSL sol 640 in Gale Crater. (B) A ChemCam Remote Micro-Imager view of the circled area in (A). Note the deep incision, the scalloped and polished surface, and the enlarged hollows. IMAGES COURTESY OF NASA/ JPL/LANL speculate on the role of sulfuric acid produced when troilite nodules are exposed to water on Earth (Buchwald 1975); indeed, this mecha- nism is the preferred explanation for many (but not all) of the hollows observed on the Martian examples. Thus, separating eolian scouring effects from the effects of acidic corrosion is not always straightforward. Comparing fi nds among sites can be a qualitative but thought- provoking pastime. Four of the Meridiani irons exhibit Widmanstätten patterns and iron oxide/oxyhydroxide coatings, while six show signs of cav- The found by Opportunity on Meridiani Planum FIGURE 2 ernous weathering (see Schröder et al. 2008; Johnson et al. 2010; Ashley on sol 1961. (A) The outline shows the Microscopic Imager (MI) et al. 2011b; Fleischer et al. 2011). None of these features are obvious mosaic area shown in D; north is toward the lower left. (B) A full-scale, resin 3-D in the images of Gale Crater irons, where meteorite surfaces appear print of Block Island prepared by Kris Capraro of JPL from a geometric model using Pancam images collected at six circumferential rover standoff positions around the more polished (FIG. 1). Many reasons are possible for this, of course, meteorite. (C) A hammerhead-shaped metal protrusion together with its shadow including differences in residence time and incomplete reconnaissance along the rim of Block Island’s conspicuous pit. (D) Map of the MI mosaic area in by the rovers, as well as actual differences in site-specifi c weathering (A). Blue indicates Widmanstätten pattern (may be or schreibersite lamellae), processes. Other features are unique to individual samples. Lebanon red is iron oxide / oxyhydroxide coating, is bare metal, and black is shadow. IMAGES COURTESY OF NASA/JPL/MIPL/PANCAM/MI (see TABLE 1 for the locations of the meteorites) shows deep incision, presumably along internal weaknesses. Shelter Island shows large-scale differential mass removal, and Mackinac Island appears to have been simple exposure to the small amount of oxygen in the Martian atmo- hollowed to its core. The upper surface of Block Island presents a gaping sphere is insuffi cient to produce the coating and that water (probably pit decorated along its rim with delicate and highly angular protrusions ice) was therefore involved. Moreover, it means that this exposure was (FIG. 2). Clearly a large mass of unknown mineralogy has been removed recent, because otherwise the relatively soft coatings would have been from its metal groundmass. removed by wind abrasion. Thus, we have recent rust formation by water A beautiful sequence of events is recorded on Block Island, Shelter at the Martian equator alternating with periods of eolian scouring (see Island, and Oileán Ruaidh. The presence of acid-etched (or more likely Ashley et al. 2011b for further details). The most off-the-shelf explana- sandblasted) Widmanstätten patterns (FIG. 2D) confi rms postfall surface tion is one involving obliquity cycling where water ice is brought to modifi cation. The presence of iron oxide / oxyhydroxide coatings in equatorial latitudes every few hundred thousand years. An alternate crosscutting relationships with these features (FIG. 2D) shows that they hypothesis involves ripple migration (now dormant; Golombek et al. too occurred after fall. They must therefore be a weathering product 2010) over the meteorites on comparable timescales, where frost forms rather than fusion crusts produced during atmospheric fl ight. Finally, on meteorite surfaces while buried (e.g. Yen et al. 2005). Future studies clear indications of rust destruction in the current epoch show that will seek to differentiate between these competing theories.

REFERENCES mentation as a possible contributor Fleischer I and 6 coauthors (2011) Schröder C and 14 coauthors (2010) to anomalous stony meteorite New insights into the miner- Properties and distribution of Ashley JW, Wright SP (2004) Iron scarcity on Mars — Insights from alogy and weathering of the paired candidate stony meteorites oxidation products in Martian Antarctica and MER. 46th Lunar Meridiani Planum meteorite, Mars. at Meridiani Planum. Journal of ordinary fi nds as pos- and Planetary Science Conference, Meteoritics and Planetary Science Geophysical Research 115: E00F09, sible indicators of water exposure abstract 2881 46: 21-34 doi: 10.1029/2010JE003616 at landing sites. 35th Lunar and Planetary Buchwald VF (1975) Handbook of Johnson JR and 6 coauthors (2010) Velbel MA (2014) Terrestrial weath- Science Conference, abstract 1750 Iron Meteorites. University of Pancam visible/near-infrared ering of ordinary chondrites in California Press, Oakland, 1418 pp spectra of large Fe-Ni meteorites at nature and continuing during Ashley JW and 16 coauthors (2011a) Meridiani Planum, Mars. 41st Lunar laboratory storage and pro- Chappelow JE, Golombek MP (2010) The scientifi c rationale for studying and Planetary Science Conference, cessing: review and implications Entry and landing conditions that meteorites found on other worlds. abstract 1974 for Hayabusa sample integrity. produced Block Island. Journal of In: Visions and Voyages for Meteoritics and Planetary Science Geophysical Research, Planets 115: Kraft MD, RJ, Christensen PR Planetary Science in the Decade 49: 154-171 2013-2022. National Academies E00F07, doi: 10.1029/2010JE003666 (2014) Perspectives on Amazonian weathering on Mars. Geological Yen AS and 7 coauthors (2005) Press, 400 pp Chappelow JE, Sharpton VL (2006) Society of America Meeting, Subsurface weathering of rocks and Atmospheric variations and mete- Ashley JW and 9 coauthors (2011b) abstract 111-10 soils at Gusev Crater. 46th Lunar orite production on Mars. Icarus Evidence for mechanical and chem- and Planetary Science Conference, 184: 424-435 Schröder C and 22 coauthors (2008) ical alteration of iron-nickel mete- abstract 1571 Meteorites on Mars observed orites on Mars: Process insights Connolly HC and 11 coauthors with the Mars Exploration Yen AS and 18 coauthors (2006) for Meridiani Planum. Journal of (2006) The Meteoritical Bulletin Rovers. Journal of Geophysical Nickel on Mars: Constraints on Geophysical Research 116: E00F20, No. 90, 2006 September. Research 113: E06S22, doi: meteoritic material at the sur- doi: 10.1029/2010JE003672 Meteoritics and Planetary Science 10.1029/2007JE002990 face. Journal of Geophysical 41: 1383-1418 Ashley JW, Velbel MA, Golombek MP Research 111: E12S11, doi: (2015) Weathering-induced frag- 10.1029/2006JE002797

ELEMENTS 11 FEBRUARY 2015