Life (?) in Martian Meteorite ALH 84001:
A Summary of Presentations at the
28th Lunar and Planetary Science Conference


The 28th Lunar and Planetary Science Conference, March 17-21, 1997, was the first major planetary science conference since Dr. D. McKay and co-workers announced the possible presence of traces of ancient martian life in the meteorite ALH 84001. The conference was held in Houston, Texas at the NASA Johnson Space Center (JSC) and the Lunar and Planetary Institute (LPI).

At least 37 presentations of new research on the martian meteorite ALH 84001 were given at the 28th LPSC. Abstracts of these works are listed below, in the order of presentation, with short descriptions of their contents. The abstracts are arranged in the order they were presented.

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 version 3.0 of the pdf reader, which can be obtained free of charge.

Papers cited within the summaries are listed after the abstracts.


Regular Session Talks
Monday, 8:30 a.m.
These talks were given as part of the regular conference sessions.
1799  McKay G. A. and Lofgren G. E.
Carbonates in ALH 84001:  Evidence for kinetically controlled growth.

The authors investigated chemical zoning in the carbonates of ALH 84001, and conclude that the carbonate grew rapidly into open spaces. Rapid growth permitted the carbonates to have their unusual compositions, without the need to call on unusual chemical or physical conditions. Besides the well-known carbonate occurrences as zoned pancakes and ellipsoids, the authors also describe carbonates as veins filling cracks and as pockets in orthopyroxene crystals.
 
1192  Treiman A. H.
Chemical disequilibrium in carbonate minerals of martian meteorite ALH 84001:  Evidence against high formation temperature.

The author evaluated Harvey and McSween’s (1996) arguments that the carbonates of ALH 84001 formed at high temperature, and found them all flawed. Their arguments for high temperature all assumed chemical equilibria among carbonate and silicate minerals in ALH 84001; mineral compositions and chemical theory suggest no chemical equilibria, so the inferences of high temperature are not valid.
1789  Scott E. R. D., Yamaguchi A., and Krot A. N.
Shock melting of carbonate, plagioclase, and silica in the martian meteorite ALH 84001.

From the textures and compositions of the carbonates and the surrounding feldspar-composition glass, the authors conclude that both formed at high temperature as shock melts. Micron-wide veinlets of feldspar-composition glass in pyroxene grains, and veinlets of this glass and carbonate minerals, suggest that all the shock effects in ALH 84001 all came from a single shock event, which must have melted both the feldspar and the carbonate and injected them into the pyroxene.
1842  Kirschvink J. L., Maine A., and Vali H.
Paleomagnetic evidence supports a low-temperature origin of the carbonate in martian meteorite ALH 84001.

The natural remnant magnetism is aligned differently in the carbonates and orthopyroxene of ALH 84001. This means that the carbonates and orthopyroxene could not have been hot at the same time; if they were both hot, their remnant magnetic fields would be aligned the same. From their sizes, shapes, and magnetic properties, the magnetite crystals in ALH 84001 must have formed at a temperature below 150°C. [In addition this work suggests that Mars had a strong magnetic field when the orthopyroxenes and carbonates formed; Mars now has very little, if any, magnetic field.]
1671  Greenwood J. P., Riciputi L. R., and McSween H. Y. Jr.
Sulfur isotopic variations in sulfides from shergottites and ALH 84001 determined by ion microprobe:  No evidence for life on Mars.

The authors measured the relative abundances of sulfur isotopes in pyrite and carbonate rims in ALH 84001, and did not find the isotope ratios characteristic of sulfur-eating bacteria on Earth. These bacteria tend to use the lighter isotope of sulfur, so the bacteria’s products are isotopically “light,” with d34S much less than 0‰. Sulfur in ALH 84001 pyrite is somewhat heavy, with d34S between +2.0 and +7.3‰ (as was found by Shearer et al., 1996). Sulfide in carbonate dark rims is not light either, with d34S = +6 6.7‰. “The significant enrichments in 32S expected if the sulfides were products of sulfate-respiring bacteria . . . are not found.”
1445  Valley J. W., Eiler J. M., Graham C. M., Gibson E. K. Jr., and Romanek C. S.
Ion microprobe analysis of oxygen and carbon isotope ratios in the ALH 84001 meteorite.

The authors analyzed oxygen and carbon isotope ratios in the carbonates (using ion microprobe). The oxygen had d18O from 9.5 to 20.6‰, with two carbonate ellipsoids having different average d18Os. “The high values of d18O in Carb. #1 cannot represent isotopic equilibrium with the host orthopyroxene at temperatures above 100-200°C .... On Earth, such high and variable d18O is proof of low-temperature exchange because isotopic fractionations are small at high temperature.” Carbon isotope ratios were mostly constant, but occasionally varied widely, suggesting the presence of an unidentified compound with very light carbon (d13C < -50‰).
1657  Gilmour J. D., Lyon I. C., Saxton J. M., Turner G., and Whitby J. A.
Oxygen and noble gas isotope constraints on the origin of ALH 84001 carbonate.

Xenon isotope measurements again confirm the martian origin of ALH 84001; 129Xe/132Xe ratios range up to 2.4, which is current martian air. Oxygen isotope ratios for the carbonate globules are inconclusive for their formation temperature, although their oxygen isotopes must have been through some chemical processes at a low temperature. A relatively low value of the abundance ratio Cl/36Ar suggests that the carbonates formed at high temperature.
1544  Gibson E. K. Jr., McKay D. S., Thomas-Keprta K., Romanek C. S., Clemett S. J., and Zare R. N.
Possible relic biogenic activity in martian meteorite ALH 84001:  A current assessment.

A review of potential biogenic features in ALH 84001, a review and guide to current work and abstracts on these features (especially biofilms), and an affirmation of the inferences of McKay et al. (1996).
1615  Westall F., de Wit M. J., and Dann J.
What do fossil bacteria look like?  Examples of 3.5-billion-year old mineral bacteria and the search for evidence of life in extraterrestrial rocks.

For comparison with possible fossil bacteria in ALH 84001, the authors report on confirmed fossil bacteria from 3.5-billion-year-old cherts from Barberton, South Africa. These fossil bacteria are short rods, 0.65-1.0 µm long; they appear individually, in clusters of identically sized cells, and as fossilized bacterial mats. The authors emphasize that these fossils have no remaining organic matter, and are preserved only as shaped mineral grains. So, potential fossils in ALH 84001 might be recognized only by size and shape, not organic material.
1345  Thomas-Keprta K. L., Wentworth S. J., McKay D. S., Stevens T. O., Golden D. C., Allen C. C., and Gibson E. K. Jr.
The search for terrestrial nanobacteria as possible analogs for purported martian microfossils in the martian meteorite ALH 84001.

Very small bacteria, nanobacteria, have been proposed as terrestrial analogs for the bacteria-shaped objects in ALH 84001. The authors investigate whether terrestrial nanobacteria occur in environments like those proposed for ALH 84001:  lightless, with only the rock and water for sustenance. Basalt lava rocks beneath the Columbia River plateau (eastern Washington) have been affected by bacterial action, and the authors attempted to culture nanobacteria from these rocks in fresh basalt rock. After culturing, the basalt rocks contained small filaments and rounded shapes, on the order of 0.35 µm long and 0.02 µm wide. These sizes and shapes are comparable to those of potential microfossils in ALH 84001.
1681  Steele A., Goddard D. T., Stapleton D., Smith J., Tapper R., Grady M., McKay D. S., Gibson E. K., Thomas-Keprta K. L., and Beech I. B.
Atomic force microscopy imaging of ALH 84001 fragments.

Published images of possible bacteria shapes from ALH 84001 have been criticized as being artifacts of sample preparation. To study this possibility, the authors examined untreated, uncoated surfaces of carbonate globules using environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM). ESEM was used to locate regions of the globules rich in magnetite and iron sulfides in these regions, AFM imaging showed that the globule surfaces were covered with rounded protrusions of 0.1 to 0.2 µm diameter, and showed a single segmented structure 0.5µm long. The authors conclude that the bacteria shapes in earlier published images were real, and not products of sample preparation.
1413  Wright I. P., Grady M. M., and Pillinger C. T.
An investigation into the association of organic compounds with carbonates in ALH 84001.

By selectively dissolving carbonate globules in acid, the authors attempted to separate organic material and carbonate minerals for carbon isotope analyses. The experiment was a partial failure, as most of the carbon was lost during processing. The authors hypothesize that the lost carbonate material might have been mostly magnesite (which is intrinsically resistant to acid), or that its mineral grains might have been coated with acid-resistant biofilms.
1548  Flynn G. J., Keller L. P., Kirz J., Wirick S., Bajt S., and Chapman H. N.
Carbon mapping and carbon-XANES measurements on carbonate globules in ALH 84001.

Using an X-ray microscope and X-ray Absorption Near-Edge Structure (XANES) spectra, the authors have searched for organic carbon in the carbonate globules. Mapping of a small sample (analysis spot size only 0.05 µm) showed many areas with 1% or more organic carbon. Work continues.

Mars Remote Sensing
Tuesday, 8:30 a.m.
This talk was given as part of the regular conference sessions.
1455  Bishop J. L., Pieters C. M., and Hiroi T.
Spectroscopic properties of martian meteorite ALH 84001 and identification of minerals and organic species.

As an aid to mineral identification and application to Mars, the authors obtained visible and infrared reflection spectra of chips and powders of ALH 84001. All major mineral species (recognized by microscope) were detected in reflection spectroscopy. Some light absorption features in infrared light (3.3-3.5 µm wavelength) are from organics, and are not always associated with the carbonates.

Posters
Tuesday Evening
1810  Griffith L. L. and Shock E. L.
Orthopyroxene hydrothermal alteration pathways:  Low vs. high temperature.

Using chemical equilibrium modeling, the authors investigate whether it would be possible for the carbonate globules in ALH 84001 to form at low temperature without also forming clays and other water-bearing silicates. In fact, the authors find that orthopyroxene (as in ALH 84001) can react with carbonated water at ~75°C to yield magnesite and quartz without clays, talc, or other water-bearing minerals. So, the absence of water-bearing minerals with the ALH 84001 carbonates is not proof that they formed at high temperature.
1687  Steele A., Goddard D. T., Grimes G. W., Stapleton D., Smith J., Tapper R., Grady M., McKay D. S., Gibson E. K., Thomas-Keprta K. L., and Beech I. B.
Scanning proton microprobe imaging of ALH 84001 fragments.

The authors applied proton microprobe imaging to examine the three-dimensional distribution of carbon and other elements in ALH 84001 carbonate globules. Preliminary results are consistent with element distribution maps from other methods.
1675  Golden D. C., Thomas-Keprta K. L., McKay D. S., Wentworth S. J., Vali H., and Ming D. W.
Size distribution of magnetite in carbonate globules of ALH 84001 martian meteorite.

Using transmission electron microscopy, the authors measured the sizes of many magnetite grains from the carbonate globules. The magnetite crystals, from both cores and rims of globules, were mostly cubes and octahedrons; no ribbons or whiskers were observed. These ALH 84001 magnetites are similar in sizes and shapes to those deposited by a common strain of bacteria on Earth, and so are consistent with a biogenic origin.
1224  Flynn G. J., Sutton S., and Keller L. P.
Element abundance patterns in carbonate globules and rims from ALH 84001.

To understand the formation temperature of the carbonate globules, the authors analyzed carbonate chips for many trace elements using analytical X-ray microscopy. Many elements are distributed irregularly; abundances of S, Cl, and Br vary by approximately an order of magnitude. Average element/iron abundance ratios are nearly constant, suggesting that the globule rims formed in place by alteration of material like core carbonate. The chlorine/bromine ratio is always much higher than in Antarctic ice, suggesting little Antarctic contamination. The low abundances of volatile elements may suggest that the carbonate globules formed at high temperature.

Mars:  Water, Climate, Life
Wednesday, 8:30 a.m.
1259  Allen C. C., Thomas-Keprta K. L., McKay D. S., and Chafetz H. S.
Nanobacteria in carbonates.

Using scanning and transmission electron microscopy, the authors looked for nanobacteria (like those hypothesized for ALH 84001) in carbonate mineral deposits from a hot spring. Mineral grains from the hot spring were found to be coated with thin layers of “mucus,” a biofilm. In the film are spheroids of 0.05 to 0.5 µm diameter, similar in size to the possible fossil bacteria in ALH 84001. It is not yet clear if the spheroids are fossil bacteria or abiogenic mineral deposits.
1661  Barlow N. G.
The search for possible source craters for martian meteorite ALH 84001.

ALH 84001 was ejected from Mars ~16 million years ago by an asteroid impact. The impact crater source of ALH 84001 is not known; two young craters in ancient highlands, eroded by water, are suggested as the most likely candidates.

Plenary Session
Wednesday, 1:30 p.m.
These talks were presented at an all-conference session,
held in the large auditorium in Bldg. 2 of Johnson Space Center.
1817  McKay D. S., Gibson E. K., Thomas-Keprta K., Romanek C. S., and Allen C. C.
Possible biofilms in ALH 84001.

Bacteria on Earth commonly produce thin films of organic polymers, so-called biofilms. The authors searched fragments of ALH 84001 for these biofilms. On lightly etched surfaces of carbonate and silicates from ALH 84001, the authors observed films of material that appear similar to terrestrial biofilms. The films do not appear to be clays, but are otherwise uncharacterized. The physical association of the films with martian carbonate materials suggests to the authors that the films are also martian.
1545  Bradley J. P., Harvey R. P., and McSween H. Y. Jr.
Magnetite whiskers and platelets in the ALH84001 martian meteorite:  Evidence of vapor phase growth.

Using transmission electron microscopy, the authors found tiny magnetite crystals, whiskers and ribbons ~0.05 µm long, in the carbonate globules. These magnetite crystals are similar to crystals that grow at high temperature from a vapor, and are unlike magnetite crystals formed by bacteria. The authors also found a group of aligned magnetite whiskers that looks like one published image of potential fossil bacteria in ALH 84001; could these potential fossil bacteria be aligned magnetite crystals?
1461  Thomas-Keprta K. L., Romanek C. S., Wentworth S. J., McKay D. S., Fisler D., Golden D. C., and Gibson E. K.
TEM analysis of fine-grained minerals in the carbonate globules of martian meteorite ALH 84001.

Using transmission electron microscopy, the authors examined minerals and structures in the carbonate globules, especially in the dark rim zones. Their work confirms earlier findings of magnetite (iron oxide) and pyrrhotite (iron sulfide) in a porous carbonate matrix. The authors also found a few patches of clay minerals in the orthopyroxene; this is the first report of water-bearing minerals in ALH 84001. Finally, the authors dispute Bradley et al.’s (1996, 1997) claims that that magnetite crystals shaped like whiskers or ribbons must have formed at high temperature by citing references to similar magnetite crystals formed by Earth bacteria.
1235  Shearer C. and Papike J. J.
The petrogenetic relationship between carbonates and pyrite in martian meteorite ALH 84001.

The authors are concerned with whether the carbonates and pyrite in ALH 84001 formed together, because their earlier sulfur isotope studies of the pyrite showed no sign of biologic processes. Their textural and chemical analyses suggest that the carbonate globules grew from a water-rich solution at low temperature, that the earliest carbonates could have formed from the orthopyroxene, and that the pyrite may have formed at the same time as the early carbonates.
1399  Leshin L. A., McKeegan K. D., and Harvey R. P.
Oxygen isotopic constraints on the genesis of carbonates from martian meteorite ALH 84001.

To test Romanek et al.’s (1994) inference that the carbonates formed at low temperature, the authors analyzed the oxygen isotopic composition of the carbonates by ion microprobe. They found that the carbonates had a wide range of oxygen isotope compositions, measured as d18O from +5.6‰ to +21.6‰, a much wider range than Romanek et al. found. Many of the analyses, those with d18O = +5.6 to +8.5‰ are consistent with high-temperature equilibration of oxygen isotopes with the surrounding silicates, so the authors favor a high-temperature origin.
1820  Vali H., Zhang C., Sears S. K., Lin S., Phelps T. J., Cole D., Onstott T. C., Kirschvink J. L., Williams-Jones A. E., and McKay D. S.
Formation of magnetite and Fe-rich carbonates by thermophilic bacteria from deep terrestrial subsurface:  A possible mechanism for biomineralization in ALH 84001.

The authors searched for, and found, a bacterial system on Earth that would produce the same minerals (magnetite and Fe-rich carbonates) as in the carbonate globules of ALH 84001. They incubated bacteria from deep rock strata with amorphous iron and various “foods” (like glucose or acetate). The bacteria grew and deposited small magnetite crystals of sizes and shapes like those found in ALH 84001. When conditions were alkaline and rich in carbon dioxide, the bacteria also deposited crystals of iron carbonate (siderite), as is found in ALH 84001.

Posters
Thursday Evening
1264  Hua X. and Buseck P. R.
Magnetite in carbonaceous chondrites.

For comparison with magnetites in ALH 84001, the authors describe the shapes and compositions of magnetites in the carbonaceous chondrite meteorites, which contain carbon compounds that were produced without life. Carbonaceous chondrites contain many varieties of magnetite, and work is continuing on magnetites that are the same sizes as those in ALH 84001.

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1530  Browning L. B. and Bourcier W. L.
Did the porous carbonate regions in ALH 84001 form by low temperature inorganic processes?

McKay et al. (1996) identified porous areas of ALH 84001 carbonates with magnetite and greigite(?) crystals as probably biogenic in origin. The authors here suggest some inorganic mechanisms for formation of the porous areas, and note again that the absence of clay minerals seems inconsistent with a low-temperature origin for the carbonate globules.
1554  Eiler J., Valley J. W., and Graham C. M.
Standardization of SIMS analyses of O and C isotope ratios in carbonates from ALH 84001.

Companion paper to Valley et al., describing their analytical methods. Ion microprobe analyses for O and C isotope ratios in carbonate minerals are very sensitive to the compositions of the minerals. To get real isotope ratios, they had to calibrate and correct for this sensitivity.

1411  Kurat G., Hoppe P., Brandstätter F., and Koeberl C.
Fluid precipitation of chromite and feldspar-rich glass in martian orthopyroxenite ALH 84001.

Paired with the following abstract. Based on analyses of mineral compositions, they suggest that most of the rarer minerals in ALH 84001 (chromite, feldspar glass, apatite) were deposited by “fluids,” not magma. Neither the composition nor temperature of the fluids are specified.
1415  Kurat G., Nazarov M. A., Brandstätter F., Ntaflos T., and Koeberl C.
Precipitation and reaction products of fluids in martian orthopyroxenite ALH 84001.

Paired with the preceding abstract. They infer a complex history, including deposition of chromite and feldspar-rich glass from a CO2-rich aqueous fluid.
1859  Rice J. W. Jr.
Searching for the ALH 84001 “smoking gun” (parent crater).

The author lists the 19 youngest craters on ancient (Noachian) areas of Mars; among these, he suggests that the source crater of ALH 84001 is probably in Memnonia at 5°S, 146°W.
1222  Shearer C. K.
Sulfur isotopic systematics in ALH 84001. Open- and closed-system behavior of sulfur in a martian hydrothermal system.

An attempt to model earlier analyses of sulfur isotopes in ALH 84001 pyrite if it were formed by bacterial activity (with metabolism like Earth bacteria), at low temperature (McKay et al., 1996), and from starting materials with a sulfur isotope composition like common basalts (and the other martian meteorites; Greenwood et al.,). The observed isotope composition cannot be modeled by bacterial growth with any water flow rate or starting material, so he concludes that either “the pyrite did not the precipitate during biogenic activity,” or “the solutions precipitating carbonate and pyrite were highly enriched in the heavy sulfur isotope.”
1433  Treiman A. H.
Thinking about life on Mars:  Dangers and visions.

Many of the commonly accepted norms of Earth life (including cell division, standard biochemical pathways, and homochirality) are not followed in all Earth organisms. With so much intrinsic variability in Earth life, it is dangerous to extrapolate from the Earth norm to martian life.

1414  Wright I. P., Grady M. M., and Pillinger C. T.
Isotopically light carbon in ALH 84001:  Martian metabolism or Teflon contamination?

In October, these authors reported finding organic matter in ALH 84001 that was strongly enriched in the light isotope of carbon, d13C » -60‰, which was widely reported as “proof” of biogenic activity. The interpretation of this isotopically light carbon is not clear. The light carbon was released from the samples at temperatures consistent with it being from carbonate mineral, not organic material. However, no analyses of carbonate carbon have give such low d13C. Teflon does behave like this isotopically light carbon in ALH 84001, and has a comparably low d13C. For the light carbon to be from Teflon, the whole analyzed sample would have to have been 45% Teflon, which the authors doubt.
1601  Wright I. P., Grady M. M., and Pillinger C. T.
Evidence relevant to the life on Mars debate. (1) 14C results.

The martian meteorite EETA 79001 (NOT the subject of the recent “life on Mars” articles) contains carbonate minerals that formed at low temperatures, but analyses of 14C (carbon 14) in the carbonates had suggested that they formed recently on Earth. The authors dispute this interpretation, arguing that the observed 14C is a minor component added to original martian carbonates. Thus the organic material associated with the EETA 79001 carbonates is also martian.
1602  Wright I. P., Grady M. M., and Pillinger C.T.
Evidence relevant to the life on Mars debate.  (2) Amino acid results.

The martian meteorite EETA 79001 (NOT the subject of the recent “life on Mars” articles) contains amino acids similar to those of terrestrial life, and was interpreted earlier as having entered the meteorite while it was in Antarctica. The authors dispute this view, and claim that the amino acids in EETA 79001 are too abundant to have come from Antarctic ice or meltwater. In addition, the “left-handed-ness” of the EETA79001 amino acids could represent not only terrestrial biological contamination, but also amino acids produced inorganically by a number of processes, or even martian biological “contamination.”

References:

Bradley J. P., Harvey R. P., and McSween H. Y. Jr. (1997) Magnetite whiskers and platelets in ALH 84001 Martian meteorite: Evidence of vapor phase growth. Geochim. Cosmochim. Acta, 60, 5149-5155.

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.

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., Grady M. M., Wright I. P., Mittlefehldt D. W., Socki R. A., Pillinger C. T., and Gibson E. K. Jr. (1994) Record of fluid-rock interactions on Mars from the meteorite ALH 84001. Nature, 372, 655-657.

Shearer C. K., Layne G. D., Papike J. J. and Spilde M. N. (1996) Sulfur isotope systematics in alteration assemblages in martian meteorite ALH 84001. Geochim. Cosmochim. Acta, 60, 2921-2926.

Allan Treiman
Lunar and Planetary Institute
February 1997