The 31st Lunar and Planetary Conference (LPSC), March 13-17, 2000, will feature several talks, posters, and abstracts related to the martian meteorite ALH 84001, its possible evidence for ancient martian life, and the larger questions of the origin of life and its distribution through space. The conference will be in Houston, Texas at the NASA Johnson Space Center (JSC).
Accepted abstracts closely related to ALH84001 (in my opinion) are listed below, with short summaries of their contents. The abstracts are arranged by topic, by guess, and by gosh. My opinions are given in italic face.
There will be sixteen presentations of research directly related to ALH 84001 at the 31st LPSC, much lower than last year’s 33 presentation. One scientist explained "The community is moving on."
To me, the most important abstract is from D.C. Golden and colleagues. In lab experiments (sterile conditions), the made carbonate globules like those in ALH84001, complete with submicron magnetites in the proper shapes and sizes. The globules were grown from water solution at 150°C, and the magnetites were made from the most iron-rich carbonates in the globules by heating the globules to 470°C. This inorganic hypothesis for the globules and magnetites is consistent with abstracts at this meeting by A. Koziol (stability of magnetite + siderite) and A. Brearley (post-carbonate heating event), and a just-published paper (Baker et al., 2000). On the other hand, K. Thomas-Keprta and colleagues have greatly refined the case that the submicron magnetites look like those made by a terrestrial bacterium.
More broadly significant is the work of A. Steele and colleagues, who continue to document the widespread infestation of terrestrial bacteria and fungi into meteorites.
Surprisingly to me, D. S. McKay is not first author of any abstract this year. Also surprisingly, his group submitted nothing on the bacteria-shaped objects in Nahkla and Shergotty that they described last year. Previous contributors notable this year for their absences include: E.R.D. Scott, S. Clemett, L. Becker and J. Bada, G. Flynn, J. Bradley, and H. McSween.
As usual, I’ve provided a topic and author index, all keyed to the abstract numbers.
Topic Index Author Index
To read a full abstract, click on the abstract number, which will connect you to the online LPSC abstract in .PDF format. To view the abstracts, you need the Adobe Acrobat Reader, which can be obtained free of charge.
Prepared by Allan H. Treiman, Lunar and Planetary Institute, 2000.
1799 Golden D.C., Ming D.W., Schwandt C.S., Lauer H.V., Socki R.A., Morris R.V., Lofgren G.E., and McKay G.A.
Inorganic Formation of Zoned Mg-Fe-Ca Carbonate Globules with Magnetite and Sulfide Rims Similar to those in Martian Meteorite ALH84001
Carbonate globules like those in ALH84001 were produced by precipitation from water solution followed by heating. Following Golden et al. (1999, 2000), carbonate globules enclosing rare magnetite and pyrite grains were grown at 150°C from solutions of Ca-Mg-Fe bicarbonates with added sulfate. Chemical zoning very similar to that in ALH84001 was mimicked by varying the composition of the solution. Heating to 470°C decomposed the most iron-rich carbonate to submicron grains of magnetite in irregular and parallelepiped shapes. These submicron magnetite grains, formed inorganically, are reported to have the same sizes, shapes, and compositions as those in the carbonate globules of ALH84001.
1424 Koziol A.M.
Carbonate and magnetite parageneses as monitors of carbon dioxide and oxygen fugacity.
Siderite (FeCO3) and magnetite (Fe3O4), if formed in thermochemical equilibrium, can be used to estimate formation temperature and pressure (= fugacity) of oxygen gas. Magnetites in the ALH84001 carbonate globules could have formed by thermal decomposition of siderite (Brearley, 1998), but could also have formed if oxygen gas pressure increased. Siderite and magnetite are stable together in a restricted range of temperature and oxygen pressure -- the pure minerals may be unstable above about 200°C (although this is based on old thermochemical data).
1203 Brearley A.J.
Hydrous phases in ALH84001: Further evidence for preterrestrial alteration.
Brearley has found several more occurrences of water-bearing minerals, micas to be precise, in carbonate globules of ALH84001. The micas occur only as pockets and veinlets that crosscut the structure of the carbonates; one occurrence shows signs of carbonate having dissolved before the micas were deposited. The micas have layer spacings of 1.0 nanometers and are rich in alumina and magnesium -- this combination of layer spacing and composition suggests that they are the mica phlogopite. A deficiency in K suggests that the mica may be interlayered with illite mica or with clays. These micas are not likely to be Antarctic, as they are only in the carbonates and are decomposed when they abut the feldspathic glass that surrounds the carbonate fragments. Micas described earlier (Brearley, 1998) probably formed with the carbonates. The micas described here formed in a high-temperature reaction between from clay minerals originally in the carbonate globules.
1690 Thomas-Keprta K.L., Wentworth S.J., McKay D.S. and Gibson E.K.
Field emission gun scanning electron (FEGSEM) and transmission electron (TEM) microscopy of phyllosilicates in martian meteorites ALH84001, Nakhla, and Shergotty.
Water-bearing minerals, here the layer silicates (=phyllosilicates), bear witness to the history of water-rock interactions on Mars. In ALH84001, altered ~ 3.9 billion years ago, smectite clay minerals are rare on pyroxene grain surfaces and at pyroxene-carbonate boundaries. These smectites have layer spacings of ~1.0 nanometers, are relatively rich in Mg and poor in Al. They are interpreted to have formed on Mars. The Nakhla meteorite, altered ~ 700 million years ago on Mars, contains veinlets rich in smectite clay, similar in composition to that in ALH84001. With the smectite are nanometer-sized grains of iron oxides (hematite and goethite), crystalline silicate minerals (quartz, crystobalite, and tridymite), and amorphous silica. The Shergotty meteorite was altered within the last 180 million years, and contains some smectite, NaCl, Ca sulfate, and possibly Mg chloride. The different alteration minerals in these meteorites bespeak different water compositions. Alteration of the Nakhla meteorite was more oxidizing that that of ALH84001 and Shergotty. The presence of water-bearing alteration minerals in martian meteorites of such a wide age range suggests that liquid water has been present in Mars throughout its geologic history.
1683 Thomas-Keprta K.L., Clemett S.J., Bazylinski D.A., Kirschvink J.L, McKay D.S., Wentworth S.J., Vali H., and Gibson E.K.
Statistical analyses comparing prismatic magnetite crystals in the ALH84001 carbonate globules with those from the terrestrial magnetotactic bacteria strain MV-1.
The shapes of "prismatic" submicron magnetite grains from the carbonate globules of ALH84001 have been compared to the intracellular magnetite grains in the Earth bacterium MV-1. Here, the authors expand on that similarity. Previous work on the shapes was done from TEM images, which show the grains as shadows rather than their full 3D shapes. A single 3D shape can cast all sorts of shadows depending on how it is oriented, and the authors produced a statistical model to go from the observed "shadow" outlines to the real range of 3D grain shapes. Of those, 63% are the sizes of magnetic single domain grains, 17% were too small (superparamagnetic), and 20% were in between. The modeled range of grain shapes is almost an exact match for those in MV-1, as are their composition, crystallographic perfection, and crystal morphology. These similarities may suggest that the ALH84001 magnetites are biogenic.
2006 VanCleve K.A., Robbins L.L. and Bell M.S.
Microbial alteration of maskelynite: Implications for ALH84001.
Some terrestrial bacterial precipitate carbonate globules as they dissolve and alter silicate glass, and the authors propose that the carbonate globules in ALH84001 may have formed this way. In experiments they found that the Earth bacterium Desulfovibrio desulfuricans does dissolve and alter (i.e. lives on) plagioclase composition glass (maskelynite) like that in ALH84001.
1670 Steele A., Toporski J.K.W., Westall F.W., Thomas-Keprta K., Gibson E.K., Avci R., Whitby C., Griffin C., and McKay D.S.
The microbiological contamination of meteorites: A null hypothesis.
Meteorite researchers have, until now, assumed that the organic material and micron-scale structures they observed were extraterrestrial -- that (except in extreme cases) terrestrial biological contamination was not significant. The authors have shown, however, that a wide range of meteorite samples contain living (and dead) Earth bacteria and fungi, including some specific to human skin. These meteorites also contain organic compounds, biomarkers, produced by Earth microbes. Earth bacteria and fungi can inhabit meteorites very rapidly, as shown by the contamination of the Nakhla, which was collected shortly after it fell. So, meteorite researchers should take the starting position that organic material in meteorites represents terrestrial contamination; an extraterrestrial origin would need to be proved.
2078 Weiss B.P., Kirschvink J.L, Baudenbacher F.J., Vali H., Peters N.T., Macdonald F.A., and Wikswo J.P.
Reconciliation of magnetic and petrographic constraints on ALH84001? Panspermia lives on!
Weiss and colleagues have mapped the locations and orientations of magnetism trapped in minerals of ALH84001, using a new instrument (scanning SQUID microscopy). Magnetism is trapped in magnetite and in the sulfide mineral pyrrhotite, which are both associated with fractures and probably with carbonate globules. To retain its magnetic signature, the pyrrhotite could not have been warmer than 40°C since it was magnetized. As these fractures cut across the "granular bands" in the rock, they and their magnetic minerals must post-date formation of the granular bands, event D1 (Treiman, 1998). However, the magnetism could not have survived the significant heat proposed for the post-carbonate-deposition deformation event D3 (Treiman, 1998). D3 is recognized by formation and flow of feldspathic glass; the authors suggest that the glass is really a late igneous product, and that its flow features occurred at low temperatures in the ~ 4 billion years since the carbonates formed.
1225 Treiman A.H.
Heterogeneity of Remnant Magnetism in ALH84001: Petrologic Constraints
Small grains of the sulfide mineral pyrrhotite in ALH84001 are magnetized, and the magnetism of each pyrrhotite grain points in a different direction. The pyrrhotite grains could not have been hotter than 40°C since they became magnetic. Earlier descriptions of this magnetism inferred that all of the pyrrhotite magnets were once aligned, and were then disarranged in the major deformation that ALH84001 experienced, named D1 (Weiss et al., 1999; Treiman, 1998). For the most part, however, D1 shuffled fragments in ALH84001 without rotating them much. Also, D1 was followed by a significant thermal event, possibly to ~875°C. Thus, the directions of the pyrrhotite magnets could not have been dispersed during D1; it is still not clear how this happened.
1326 Stephan T. and Jessberger E.K.
Polycyclic aromatic hydrocarbons in ALH84001—Implications from time-of-flight secondary mass spectrometry analyses.
In this and earlier works, the authors have used time-of-flight secondary ion mass spectrometry (TOFSIMS) to show that polycyclic aromatic hydrocarbons (PAHs) were irregularly distributed and not associated with carbonate globules (vis. McKay et al., 1996). Earlier studies analyzed PAHs on the surfaces of polished thin sections, and were criticized because cutting and polishing could redistribute the PAHs. Here, Stephan and Jessberger report TOFSIMS analyses for PAHs on fracture surfaces of ALH84001. Their results for fracture surface are similar to those on polished thin sections: the PAHs in ALH84001 are not concentrated in the carbonate globules.
1909 Bell M.S., McHone J., Kudryavtsev A., and McKay D.S.
Raman mapping of carbonates in ALH84001 martian meteorite
The authors imaged the surface of a chip of ALH84001 using Raman spectroscopy, a technique that can be sensitive to specific minerals or mineral compositions. The goal of the study was to locate grains of MgO and CaO that had formed by decarbonation of MgCO3 and CaCO3 in the carbonate globules, and to locate areas of pyroxene-composition glass (which the authors investigated earlier). No grains of these oxides or glass were located, but the Raman technique allowed mapping of the locations of the various globule minerals (e.g., magnetite, magnesite) and of variations in the compositions of the carbonate minerals.
1218 Jagoutz E. and Kubny A.
Vibrational spectroscopic study of feldspathic glasses in SNC meteorites.
The authors acquired infrared and Raman spectra of felspar-composition glass (maskelynite) in ALH84001. The glasses are amorphous to these methods, and their broad spectral bands are typical of thermal (melt) glasses.
1762 Corrigan C.M., Harvey R.P. and Bradley J.
Sodium-bearing pyroxene in ALH 84001.
While investigating secondary (alteration) minerals in ALH84001, the authors found grains of a sodium-rich, iron-rich silicate mineral in micron-sized cavities [in pyroxene] which look like they were once filled by chromite. The mineral is tentatively identified as a pyroxene rich in acmite component NaFe3+Si2O6. This mineral could have formed in "...low- to moderate-grade [metamorphic] conditions...," or could have formed as a low-temperature precipitate (which scenario has not been described in terrestrial settings). Some similar cavities contain carbonate + magnetite rather than acmite. The authors continue to explore the origin of the acmite.
1981 Delaney J.S. and Dyar M.D.
Correction of the calibration of ferric/ferrous determinations in pyroxene from Martian samples and achondritic meteorites by synchrotron microXANES spectroscopy.
Earlier, the authors had reported that pyroxene in ALH84001 contained about 15% of its iron in the ferric, oxidized form. The new revised calibration shows the pyroxene to have essentially no ferric, oxidized, iron. This result is consistent with other data on ALH84001 and the martian meteorites.
1866 Langenhorst F., Shaw C.S.J., and Metzler K.
Mineral chemistry and microstructures in ALH84001.
ALH84001 orthopyroxene contains thin lamellae (<50 nm) of clinopyroxene, which are a result of intense shock. Material of feldspar and silica compositions is completely amorphous, again from shock, and commonly has protrusions into surrounding pyroxenes. Carbonate consist of unzoned grains, ~ 10 micrometers in diameter, but adjacent grains have somewhat different compositions. The carbonate grains are free of defects at the TEM scale, but contain rounded areas of different compositions, which may have formed by shock-degassing.
2021 Protheroe W.J.Jr. and Stirling J.A.R.
Preliminary results of cathodoluminescence spectral analysis of b -Ca-phosphates ("whitlockite") in the Mars meteorite ALH84001.
The Ca-phosphate mineral "whitlockite" [actually, merrillite] in ALH84001 luminesces white when bomabarded by electrons, as in an electron microscope or a cathodoluminescence microscope. Optical spectra of the luminescence show distinct emissions from the "whitlockite" attributable to the rare earth elements cerium, dysprosium, samarium, and neodymium.
Baker L.L., Agenbroad D.J., and Wood S.A. (2000) Experimental hydrothermal alteration of a Martian analog basalt: Implications for Martian meteorites. Meteorit. Planet. Sci. 35, 31-38.
Brearley A.J. (1998) Magnetite in ALH 84001: Product of the decomposition of ferroan carbonate (abstract). Lunar Planet. Sci. XXIX, Abstract #1757, Lunar and Planetary Institute, Houston (CD-ROM).
Golden D.C., Ming D.W., Schwandt C.S., Morris R.V., Yang S.V., and Lofgren G.E. (1999) An experimental study of kinetically-driven precipitation of Ca-Mg-Fe carbonates from solution: Implications for the low-temperature formation of carbonates in martian meteorite ALH84001. Lunar Planet. Sci. XXX, Abstract #1973, Lunar and Planetary Institute, Houston (CD-ROM).
Golden D.C., Ming D.W, Schwandt C.S, Morris R.V, Yang S.V, and Lofgren G.E. (2000) An experimental study on kinetically-driven precipitation of Ca-Mg-Fe carbonates from solution: Implications for the low temperature formation of carbonates in martian meteorite Allan Hills 84001. Meteorit. Planet. Sci. 35, in press.
Treiman A.H. (1998) The history of ALH 84001 revised: Multiple shock events. Meteorit. Planet. Sci. 33, 753-764.
Weiss B.P., Kirschvink J.L., Baudenbacher F.J., Peters N.T., MacDonald F.A., Wikswo J.P., and Vali H. (1999) Evidence for a low-temperature transfer of ALH84001 from Mars to Earth: Support for the Pansperimia Hypothesis. EOS (Trans. A.G.U.) 80, F94.