"The 61st Meeting of the Meteoritical Society"

A Preview of Upcoming Presentations Related to Martian

Meteorite ALH 84001.

The martian meteorite ALH 84001, and its possible evidence of ancient martian life, are among the featured topics at the upcoming 61st Meeting of The Meteoritical Society. The meeting, hosted by Trinity University, will be July 20-25, 1998, at the university in Dublin, Ireland.

Talks related to ALH 84001 will occupy a full session, entitled "Martian Meteorites I," a talk in the later session "Martian Meteorites II," and a number of poster presentations.

Abstracts of talks for the meeting of the Meteoritical Society are online; below, I've summarized the abstracts about ALH 84001 and the possible traces of life in it. To read a full abstract from the conference, double-click on the highlighted title, which will connect you to the online abstract in pdf format. To view the abstracts, you need a pdf reader, which can be obtained free of charge from Adobe. Abstracts are summarized in presentation order. References are listed after the summaries. Summaries and brief comments (in small italics)are by Allan Treiman Lunar and Planetary Institute.

Stephan T., Rost D., Jessberger E.K., and Greshake A. Polycyclic aromatic hydrocarbons are everywhere in Allan Hills 84001.

Among the lines of evidence cited by McKay et al. (1996) for martian biologic activity in ALH 84001 was the presence of organic molecules, polycyclic aromatic hydrocarbons (PAHs), associated with the meteorite's carbonate globules. Here, the authors continue their analyses for PAHs in ALH 84001, and find (in contrast to McKay et al.) that the PAHs are nearly everywhere in ALH 84001, except that they are less abundant in the carbonate globules! The authors have analyzed three polished pieces of ALH 84001 and found the same results on all, and found no detectable PAHs in a polished surface of another martian meteorite, Chassigny. [It is puzzling that the reported PAHs are present even in the cores of igneous mineral grains, and are neither more or less abundant along grain boundaries. These results are in stark contrast to results from other techniques (Clemett et al., 1998; the abstract here by Flynn et al.), which found that organic matter is strongly concentrated in the carbonate globules.]

Sephton M.A. and Gilmour I. A "unique" distribution of polycyclic aromatic hydrocarbons in Allan Hills 84001, or a selectiveattack in meteorites from Mars?

Among the lines of evidence cited by McKay et al. (1996) for martian biologic activity in ALH 84001 was the presence of organic molecules, polycyclic aromatic hydrocarbons (PAHs), associated with the meteorite's carbonate globules. An unusual feature of the PAHs from ALH 84001 is that they were unalkylated, that is that the PAH molecules ("poly-unsaturated") did not have long-chain "saturated" (i.e., aliphatic) hydrocarbon groups on them; PAHs from recently living organisms usually have aliphatic groups attached. To see if exposure and weathering in the Antarctic could affect the PAHs in meteorites, the authors compared PAHs from five carbonaceous chondrite falls (observed to fall and collected soon after) with those in five Antarctic carbonaceous chondrites. They found that PAHs in the meteorite falls had significantly more aliphatic hydrocarbon groups than the Antarctic meteorites. The authors attribute the difference to weathering in the Antarctic: oxidation and decarbonation of the aliphatic chains to leave only the PAH cores. "...the unalkylated PAH distribution in ALH84001 appears more of a characteristic of the Antarctic environment than fossil Martian life." [If martian, the PAHs in ALH 84001 have also experienced at least one intense shock event (Treiman, 1998), and so may retain limited information about Mars!]

Steele A., Goddard D.T., Toporski J.K.W., Stapleton D., Wynn-Williams D.D., and McKay D. Terrestrial contamination of an Antarctic chondrite.

McKay et al. (1996) reported finding "microfossil" structures in ALH 84001, but without direct evidence that the structures were martian rather than earthly. Here, the authors examined four non-martian meteorites from the Allan Hills (ALH) ice field of Antarctica to see if they showed evidence for terrestrial micro-organisms. One of the meteorites contains biogenic tendrils or fibers, which appear to be the hyphae of a fungus species that is known to inhabit rocks in Antarctica (an "endolithic" organism). This is apparently the first reported instance of terrestrial life colonizing a meteorite. The fibers here are 10-100 times wider than those reported in ALH 84001, yet this work does show that terrestrial microbiota can take up residence in meteorites and could conceivably be interpreted as extraterrestrial. [In conjunction with Sears and Kraal (1998) and Burckle and Delaney (1998, below) this paper suggests that all sorts of terrestrial objects, biogenic or biomimetic, can enter meteorites in Antarctica.]

Baker L., Franchi I.A., Wright I.P., and Pillinger C.T. Oxygen isotopes in water from martian meteorites.

Abundance ratios of the isotopes of oxygen (O) are important tracers of the origin and history of water in meteorites (and Earth rocks); here, the authors extracted water from ALH 84001 and Nakhla (another martian meteorite) and analyzed their O isotope ratios. Their results for Nakhla are similar to earlier reports (Karlsson et al., 1992), and their results for ALH 84001 are a first. For both meteorites, the water extrtacted at low temperatures (below about 300°C) has oxygen isotopes characteristic of Earth water. Water extracted at higher temperatures, above 400°C, has oxygen isotope ratios (i.e., D17O) characteristic of martian water and of carbonates in ALH 84001 (Karlsson et al., 1992; Farquhar et al., 1998; Romanek et al., 1998). Thus, at least some of the water in ALH 84001 is of martian origin, consistent with earlier results on its hydrogen isotopes (Leshin et al., 1996). [For ALH 84001, the sharp change in oxygen isotope ratios of the released water at 600°C may suggest that much of the martian water is held in or among its carbonate minerals (Boctor et al., abstract here; Farquhar et al., 1998). On the other hand, could the high-temperature water releases represent chemical reactions between carbonates and organics and not really water?]

McSween H.Y.Jr and Harvey R.P. Brine evaporation: An alternative model for the formation of carbonates in ALH84001.

As an alternative to high-temperature and hydrothermal models for the formation of carbonates in ALH 84001, the authors suggest that the carbonates could have formed by "room" temperature evaporation of briny waters. On Earth, carbonate minerals including magnesite (as in the ALH globules) can form as seawater or other brines evaporate away. Some geomorphic features on Mars have been interpreted as playa lakes or evaporite deposits; brine evaporation can also explain (at least qualitatively) the lack of hydrous silicate minerals associated with the carbonates, the range of oxygen isotope ratios in the carbonates, and the presence of sulfide and silica minerals. On the other hand, the absence of sulfate minerals (like gypsum) associated with the ALH 84001 carbonates is difficult to explain, given the abundance of sulfur in the martian soil. "A role for microbial organisms in the precipitation of the carbonates seems possible in this environment but unnecessary..." [The authors have been among the strongest proponents of high-temperature, impact-induced formation of the carbonates (Harvey and McSween, 1996). Are they abandoning that model? It is quite interesting that this abstract and Warren’s abstract should appear at the same time. ]

Warren P.H. Petrologic evidence for low-temperature, possibly flood-evaporatic origin of carbonates in the ALH84001 meteorite.

The author notes significant flaws in the impact and hydrothermal theories for formation of the ALH 84001 carbonates, and suggests rather that the carbonates formed during the evaporation of flood waters on Mars. The greatest advantage of this model is in allowing carbonate formation without requiring times or temperatures that would allow production of hydrous silicate minerals. Mars shows the scars of many large floods (like the Mars Pathfinder landing site), and carbonate minerals could have formed from the remnant flood waters as they evaporated and soaked into the martian soil (regolith). To explain the absence of sulfates in ALH 84001 (martian soil is rich in sulfate), the flood waters must recede or flow away rapidly relative to the evaporation rate (before sulfate saturation is reached). On the other hand, the ALH 84001 carbonates could have formed in a calcrete or hardpan soil. The flood-evaporite hypothesis "...does not seem consistent with the suggestion that the ALH84001 carbonates are biogenic." [It is quite a coincidence that this and Harvey and McSween's abstract should appear at the same time. Treiman et al. (1995) and Treiman (1997) have suggested that near-surface hardpan soils are widespread on Mars.]

Schwandt C.S., Hörz F., Haynes G., and Lofgren G. Shock experiments using Homestake formation as an analog for the carbonate in meteorite ALH 84001.

Using high-pressure shock experiments, the authors evaluate the theory that carbonate globules and patches in ALH 84001 formed from shock-induced carbonate melts (Scott et al., 1997). The authors shocked a natural rock (Homestake Formation) consisting mostly of siderite, FeCO3 and ankerite, CaFe(CO3)2. At shock pressures below 34.4 GPa, effects in the carbonates are limited to cracks and fractures; above 34.4 GPa to 51.2 GPa, the carbonates partially decompose to oxides and gas. In these experiments, there is no evidence that either ankerite or siderite melted. Thus, it seems unlikely that the ALH 84001 carbonates formed from shock melts. [These results may be difficult to reconcile with the theory of Scott et al., unless shock pressures were well above 51 GPa. However, intense deformation of ALH84001 suggests shock up to 75 GPa (Treiman, 1998). If this deformation were contemporaneous with carbonate formation, as Scott et al. suggest, then experiments at higher pressures would be needed.]

Scott E.R.D. and Krot A. Formation of pre-impact interstitial carbonates in the ALH84001 martian meteorite.

The authors consider how carbonate material could have originally entered ALH 84001, to be shock melted to its current mineralogy and textures (Scott et al., 1997). Recent oxygen isotope measurements prove that the carbonates in ALH 84001 did not chemically equilibrate with its silicate minerals (Farquhar et al. 1998), which means that the carbonates could not have formed originally from carbonate magmas, from late magmatic "metasomatic" fluids, or by replacement of silicate minerals by high-temperature water-rich fluid. Rather, the carbonates must originally have been deposited from low-temperature waters that had been in contact with Mars' atmosphere. The presence of martian atmosphere gas in ALH 84001 further suggests that it was near Mars' surface when it was shocked so as to melt its original carbonates into their present shapes and compositions. [The oxygen isotope data of Farquhar et al. (1998) seem to be a serious challenge to Scott’s idea that the carbonates and feldspathic glasses in ALH 84001 formed as contemporaneous impact melts. If the carbonates and glass were molten together and in contact, why didn't their oxygen isotopes equilibrate? It would be very helpful to have more oxygen isotope data on adjacent carbonates and glass that Scott et al. believe to have been molten together. ]

Eiler J.M., Valley J.W., Graham C.M., and Fournelle J. Geochemistry of carbonates and glass in ALH 84001.

To understand the origin of carbonate minerals in ALH 84001, the authors continued their earlier studies of oxygen isotope ratio d 18O (a measure of 18O/16O), and augmented it with bulk and trace-element chemical analyses. Ankeritic carbonates [Ca(Fe,Mg)(CO3)2] occur principally in irregularly shaped patches and have essentially constant d 18O = +5.8 per mil. In the carbonate disks and globules, d 18O varies regularly from magnesite (MgCO3, 0 per mil) through siderite (FeCO3, +26 per mil). These results are consistent with most previous work. Abundances of Mn are closely correlated with abundances of Ca in the carbonates, but Ca/Mn in ankerite is different from that in magnesite and siderite. Similarly, ankerites have distinct trace-element abundances compared to magnesite-siderite: richer in Sr and Y, higher La/Nd and Sr/Ba. Rare-earth-element abundances are depleted in light rare earth elements (LREE) compared to nearby feldspathic glass. These differences between carbonates and feldspathic glass seem inconsistent with simultaneous formation of both at high temperature (Scott et al., 1997). The bulk chemical and trace-element variations seem inconsistent with fractional precipitation from a carbonate-rich fluid (Leshin et al., 1998). If the carbonates formed from a single fluid composition as temperature changed, the distribution coefficients between carbonate minerals and fluid must change in weird ways. On the other hand, mixing of groundwaters with different compositions could give rise to such chemical and isotopic variations. [Mixing between different compositions of groundwater can commonly cause carbonate minerals to precipitate and grow (e.g., Harrison, 1990).]

Boctor N.Z., Wang J., Alexander C.M.O'D., Hauri E., Bertka C.M., and Fei Y. Hydrogen isotope studies of carbonate and phosphate in martian meteorite ALH84001.

The authors have extended their earlier analyses of hydrogen isotopes in minerals of ALH 84001, using a different ion-microprobe method and correcting for possible contamination from epoxy. Carbonate mineral areas (unspecified composition or texture) had ~500 ppm of water, which was somewhat enriched in heavy hydrogen (deuterium or D) relative to normal hydrogen; d D = +175 - +210 per mil (parts per thousand enriched relative to standard ocean water). Hydrogen in whitlockite, a phosphate mineral, ranged from d D = +180 - +500 per mil; samples with more hydrogen had higher d D. These d D values are lower than found in the martian atmosphere and in phosphate minerals of other martian meteorites, d D = +4000 per mil, but still are so high that all their hydrogen (i.e. water) cannot be from the Earth. Hydrogen in the whitlockite may be from the magma that originally formed ALH 84001, or could be from reaction with ancient groundwater that did not have the extreme enrichment in D in Mars' current atmosphere.

McKay G.A., Schwandt C., and Mikouchi T. Feldspathic glass and silica in Allan Hills 84001.

The authors are studying the relationships between carbonate formation and the shock events that have affected ALH 84001. Following earlier studies, they have studied the compositions and textures of feldspathic glass that are commonly associated with the carbonate globules. The chemical compositions of the feldspathic glasses are commonly those of mineral feldspar, (Na,Ca)(Si,Al)Si2O8, plus silica, SiO2. High-contrast back-scattered electron (BSE) imagery shows that these areas are composed of angular fragments of feldspar-composition glass separated by thin films and layers of silica-composition glass. In this texture, the authors see three consecutive events: 1) granulation of crystalline feldspar (probably in an impact event), 2) filling the spaces among the granules with silica, and 3) shock melting of the feldspar and silica. The latter impact shock event may be the one that disrupted and broke the carbonate globules. The silica among the feldspar fragments is probably not magmatic, because very little such silica is present in ALH 84001, and because its thin layers suggest deposition from a thin fluid, not a viscous magma . This silica probably didn't form at high temperature because it contains little aluminum, and high-temperature silica in the other martian meteorites typically contains noticeable aluminum. [Deposition of the silica could have been contemporaneous with deposition of the carbonate globules; the authors' earlier work showed that the crystalline feldspar was granulated before deposition of the carbonates. If so, silica deposition may be intimately related to carbonate formation (e.g., Harvey and McSween, 1996). Oxygen isotope abundances in one silica grain suggest, if it formed with the carbonates, a temperature less than approximately 300°C (Valley et al., 1997). ]

Burckle L.H. and Delaney J.S. Microfossils in chondritic meteorites from Antarctica? Stay tuned.

To assess the idea that ALH 84001 contains fossil traces of martian life (McKay et al., 1996), it is important to understand possible sources of biological contamination on Earth. Winds off the oceans can carry biological material deep into Antarctica. First, storms from the southern ocean can occasionally penetrate far into Antarctica, and carry particulates (including biologic particulates) that were swept from the ocean. Second, the general atmospheric circulation can transport micron-sized particles from moderate latitudes high into the atmosphere, then to Antarctica. Once in Antarctica, biologic particulates can be distributed widely by Antarctica';s own inversion and katabatic winds. Modern ocean microfossils, diatom tests and phytoliths, have been found high in the Transantarctic mountains; wind transport is the only plausible mechanism for emplacing them. The authors are now searching for these microfossils in chondrite meteorites from Allan Hills and Queen Alexandra Range. Dust from cracks in the meteorites is removed by ultrasonication, and examined microscopically. Results of this search will be presented. [Just as life is nearly ubiquitous on Earth, so is "contamination" with living and defunct terrestrial life. Steele et al. (1998, above) have shown a living organism in an Antarctic meteorite, and Sears and Kraal (1998) show "biogenic-like" structures in Antarctic meteorites that originated on the Moon. ]

Flynn G.J., Keller L.P., Jacobsen C., and Wirick S. Carbon in Allan Hills 84001 carbonate and rim.

To understand the nature and abundance of organic carbon in ALH 84001, the authors performed X-ray absorption near-edge spectroscopy (XANES) on a fragment of carbonate globule. XANES is capable of distinguishing among many varieties of organic material by probing their carbon bonding patterns. The authors mounted their fragment using a carbon-free technique, and found that the fragment showed both fine-grained rim (magnesite, magnetite, sulfides) and coarse-grained globule interior (mostly carbonate) separated by a thin porous rind. Rim material showed three XANES peaks related to p bonds (unsaturated or aromatic bonds) between carbon and carbon (organics) and one related to carbon-oxygen bonds (carbonate). The organic and carbonate carbon are intimately mixed on scales of 100 nm (0.1 micrometer). The rim and interior of the globule show the same organic carbon peaks, suggesting that both contain the same kind of organic matter. Organic matter is locally present at percent abundances. A sample described in an earlier paper, one rich in feldspathic glass and chromite, showed a different set of p bonds implying different organic compounds. [This work continues to affirm the presence and abundance of "organic" carbon associated with the carbonate globules, despite the results above of Stephan et al. These p bonded organics could be the PAHs described by McKay et al. (1996) and elaborated on in Clemett et al. (1998). It remains unclear whether the PAHs are terrestrial or extraterrestrial (martian); a convincing measurement of carbon isotope abundances might resolve this issue for good.]

Wright I.P., Grady M.M., and Pillinger C.T. Further carbon isotopic measurements on carbonates in ALH 84001.

An earlier experiment to determine carbon isotopic ratios in ALH 84001 carbonates (Wright et al., 1997) gave bizarre results, so the authors modified and repeated the experiment. Earlier, they had repeatedly etched a sample of ALH 84001 with acid, collected the carbon dioxide gas that evolved at each etch step, and removed the residual materials after each step. Most of the sample's carbon was lost in this procedure. Here, the authors performed a similar experiment on a new sample, but without removing residual material after each step. In the new experiment, the total carbon produced equaled their estimate of the carbon originally in the rock, and the overall carbon isotope ratio is comparable to ratios analyzed on bulk samples, d 13C = + 40 per mil. These data confirm that most of the carbonate minerals in ALH 84001 are martian, despite having fairly abundant 14C (Jull et al., 1998). However, the present results do not explain why the earlier experiments lost most of the carbon and why the carbon analyzed there had d 13C < 0 per mil. [The authors are to be commended for repeating a difficult experiment. The results of their earlier experiment remain a puzzle, but may be important for understanding the distribution of organic and carbonate carbon in the rock; see the Flynn et al. abstract above.]

References: See the ALH papers website for more on most of these references.

Clemett S.J., Dulay M.T., Gilette J.S., Chillier X.D.F., Mahajan T.B., and Zare R.N. (1998) Evidence for the extraterrestrial origin of polycyclic aromatic hydrocarbons (PAHs) in the martian meteorite ALH 84001. Faraday Discussions (Royal Soc. Chem.) 109, in press.

Farquhar, J., Thiemans M.H., and Jackson T. (1998) Atmosphere-surface interactions on Mars: D 17O measurements of carbonate in ALH 84001. Science 280, 1580-1582.

Harvey R.P. and McSween H.Y. Jr. (1996) A possible high-temperature origin for the carbonates in the martian meteorite ALH84001. Nature 382, 49-51.

Harrison W.J. (1990) Modeling fluid/rock interactions in sedimentary basins. pp. 195-231 in Cross T.A. ed., Quantitative Dynamic Stratigraphy, Prentice Hall, NJ.

Jull A.J.T., Courtney C., Jeffrey D.A., and Beck J.W. (1998) Isotopic evidence for a terrestrial source of organic compounds found in Martian meteorites Allan Hills 84001 and Elephant Moraine 79001. Science 279, 366- 369.

Karlsson H.R., Clayton R.N., Gibson E.K.Jr., and Mayeda T.K. (1992) Water in SNC meteorites: Evidence for a martian hydrosphere. Science 255, 1409-1411.

Leshin L.A., Epstein S., and Stolper E.M. (1996) Hydrogen isotope geochemistry of SNC meteorites. Geochim. Cosmochim. Acta 60, 2635-2650.

Leshin L.A., McKeegan K.D., Carpenter P.K., and Harvey R.P. (1998) Oxygen isotopic constraints on the genesis of carbonates from Martian meteorite ALH 84001. Geochim. Cosmochim. Acta 62, 3-13.

McKay D.S. Gibson E.K.Jr., Thomas-Keprta K.L., Vali H. , Romanek C.S., Clemett S.J., Chillier X.D.F., Maechling C.R., and Zare R.N. (1996) Search for past life on Mars: Possible relic biogenic activity in martian meteorite ALH 84001. Science 273, 924-930.

Romanek C.S., Perry E.C., Treiman A.H., Socki R.A., Jones J.H., and Gibson E.K.Jr. (1998) Oxygen isotopic record of mineral alteration in the SNC meteorite Lafayette. Meteor. Planet. Sci. 33, in press.

Scott E.R.D., Yamaguchi A., and Krot A.N. (1997) Petrological evidence for shock melting of carbonates in the martian meteorite ALH 84001. Nature 387, 377-379.

Sears D.W.G. and Kraal T.A. (1998) SEM imaging of martian and lunar meteorites and implications for microfossils in martian meteorites (abstract). In Lunar Planet. Sci. XXIX, Abstract #1934, Lunar and Planetary Institute, Houston (CD-ROM).

Treiman A.H. (1997) Near-surface geologic units in Ares Vallis and adjacent areas: Potential sources of sediment at the Mars Pathfinder landing site. J. Geophys. Res. 102, 4219-4229.

Treiman A.H. (1998) The history of ALH 84001 revised: Multiple shock events. Meteor. Planet. Sci. 33, in press.

Treiman A.H., Fuks K.H, and Murchie S. (1995) Diagenetic layering in the upper walls of the Valles Marineris, Mars: Evidence for drastic climate change since mid-Hesperian. J. Geophys. Res. 100, 26,339-26,344.

Valley J.W., Eiler J.M., Graham C.M., Gibson E.K.Jr., Romanek C.S., and Stolper E.M. (1997) Low-temperature carbonate concretions in the martian meteorite ALH 84001: Evidence from stable isotopes and mineralogy. Science 275, 1633-1638.