Life (?) in Martian Meteorite ALH 84001:
A Summary of Presentations at the
29th Lunar and Planetary Science Conference


The 29th Lunar and Planetary Conference (LPSC), March 16-20 1998, featured many talks, posters, and abstracts related to the martian meteorite ALH 84001 and its possible evidence for ancient martian life. The conference was held in Houston, Texas, at the NASA Johnson Space Center (JSC).

At least 41 presentations of research on (or related to) ALH 84001 were given at the 29th LPSC. Abstracts of these works are listed below, with short summaries of their contents. The abstracts are arranged with oral presentations first, poster presentations next, and print-only last. Oral presentations on ALH 84001 were scheduled for Friday morning (March 20). Most of the relevant posters were available for viewing Thursday evening (March 19).

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 from Adobe.

Papers cited within the summaries are listed after the abstracts. Prepared by Allan H. Treiman, L.P.I., 1998.

Regular Session Talks
Friday, March 20, 1998
ALH 84001: DOES IT BUG YOU?
8:30 a.m. Room B
Chairs: R. P. Harvey & I. P. Wright
1944

McKay G.* Mikouchi T. Schwandt C. Lofgren G.

Fracture Fillings in ALH84001 Feldspathic Glass: Carbonate and Silica

The carbonate globules in ALH 84001 contain putative evidence of ancient martian life, and are commonly associated with feldspathic glass. From the textures of these globules and glasses, the authors infer that the geological history of ALH 84001 is more complex that reported before. The authors discovered a fracture, partially filled with a zoned carbonate 'pancake', that extends across pyroxene and feldspar-composition glass, itself consisting of K-rich fragments in K-poor matrix. The fracture and carbonate pancake cut across the fragments and matrix, suggesting that the glass was brecciated (as by impact) before the fracture formed and the carbonate was deposited. Also, a shock event after carbonate deposition is required to leave the feldspar in its current glassy state. In other areas of ALH 84001, McKay et al. found that the feldspar-composition glass had been granulated or fractured, with silica (?) among the granules and filling the fractures. McKay et al. conclude that ALH 84001 must have once been very permeable, and was penetrated by a range of fluids which deposited the silica and the carbonate globules.

 

1786

Scott E.R.D.* Krot A.N.

Carbonates in Martian Meteorite ALH84001: Petrologic Evidence for an Impact Origin

Many petrographic observations tend to support the theory that the carbonates and most of the shock features in ALH 84001 formed at high temperature in a single impact event (Scott et al., 1997). New observations here include the recognition: that most carbonate 'disks' in fractures are thinner at their edges than their centers; that micro-disks of magnesite (10-30m diam.) are common near carbonate disks; that the proportion of carbonate in pyroxene is correlated with its degree of fracturing; and that carbonate grains can be dispersed in linear trains within the meteorite's crush zones. Scott and Krot explain all these carbonate textures, along with their previous observations, as having formed from carbonate melt ( plagioclase melt) formed during a single impact shock event.

 

1451

Brearley A.J.*

Magnetite in ALH 84001: Product of the Decomposition of Ferroan Carbonate

A biogenic origin has been suggested for sub-micron sized magnetite grains in the carbonate globules of ALH 84001. Based on SEM and TEM observations, Brearley suggests that these magnetites formed by the thermal decomposition of iron-bearing carbonate minerals. Fragments of carbonate globules in feldspathic glass are rich in defect-free 100 nm magnetite grains in cuboid, teardrop, and subrounded shapes. Many of these magnetites are associated with void spaces in the host carbonate. These magnetite grains were most abundant in ankeritic magnesite-siderite carbonates (Cc22Mg44Sd34). Bradley infers that the magnetites formed during the heating event, to ~900C, that melted the feldspathic glass surrounding the carbonate fragments. Magnetites are most common in the ferroan carbonate, as it has the lowest thermal stability of all the carbonate compositions. The void spaces represent the carbon dioxide evolved during decomposition of the carbonate minerals, and this decomposition could account for the differences in oxygen isotope compositions between the iron-rich and iron-poor carbonate minerals.

 

1347

Blake D.F.* Treiman A.H. Cady S. Nelson C. Krishnan K.

Characterization of Magnetite Within Carbonate in ALH84001

A biogenic origin has been suggested for sub-micron sized magnetite grains in the carbonate globules of ALH 84001. Blake et al. used TEM to examine magnetites in dark rims on a carbonate globule and nearby in the carbonates, and suggest that these magnetite grains formed by inorganic solution-precipitation at low temperature. First, the authors used Energy Electron Loss Spectroscopy (EELS) to confirm that this magnetite is truly magnetite and not the similar iron oxide maghemite. Magnetite grains are fairly common in the ankeritic carbonate near the dark rims. These grains are defect-free, and are commonly in epitaxial or near-epitaxial orientations with the host carbonate mineral. Many of the magnetites are associated with holes or void spaces in the carbonate, and some holes do not contain magnetite or other minerals. The authors interpret the magnetites as inorganic precipitates from the fluid that formed the carbonates, not as biogenic products.
1494

Thomas-Keprta K.L.* Bazylinski D.A. Golden D.C. Wentworth S.J. Gibson E.K.Jr. McKay D.S.

Magnetite from ALH84001 Carbonate Globules: Evidence of Biogenic Signatures?

McKay et al. (1996) suggested a biogenic origin for sub-micron sized magnetite grains in the carbonate globules of ALH 84001. The authors examined many of these magnetites by TEM, and found that a majority are similar in size, shape and structure to magnetite formed by magnetotactic bacteria. Thomas-Keprta et al. extracted thousands of magnetite grains from globule fragments by dissolving the host carbonate in acid. The magnetites range from ~20 to ~100 nm in size. Most are cuboidal or irregular, with a significant proportion being parallelepipeds. Elongate (whisker) magnetite grains were rare but present. The parallelepiped magnetite crystals are most similar in shape and size to magnetite grains produced by magnetotactic bacteria, and are not similar to known abiogenic magnetite grains. These parallelepiped magnetites may reflect a complex, partially biogenic, martian history.

 

1928

Foley C.N. Humayun M.* Davis A.M. Kagan O.

Chemical and SEM Studies of Mineral Assemblages Within ALH84001

This abstract, the first work from an extended study of ALH 84001, reports principally on sulfide minerals in carbonate-rich "pancakes" along fractures surfaces. The "pancakes" are abundant on some fracture surfaces and rare on others, do not appear to be associated with plagioclase glass, and commonly have sulfide-bearing rim zones. The authors have recognized a new type of carbonate: "pancakes" that are rich in Fe-sulfides (mineral species not reported), dark colored rather than orange (as with most "pancakes"), and contain up to 8% weight sulfur. Their Fe-sulfides occur as ~0.1m (100 nm) grains concentrated along cleavage traces in the carbonate grains of the "pancakes." These small sulfide grains are inferred to have been injected into the carbonate "pancakes" as sulfide-rich melts formed during the shock event that formed feldspar-composition glass in the meteorite.

 

1425

Jones J.H.* Schwandt C.S.

Experimental Investigations of the High-Temperature Stability of Siderite: Implications for the Origin of ALH 84001 Carbonates

The temperature of formation of the carbonate globules in a matter of dispute; here, the authors performed laboratory experiments at high pressure and temperatures to explore the stability of carbonate minerals like those in ALH 84001. Jones and Schwandt mixed magnesite, dolomite, and siderite carbonates to simulate the most common carbonate composition in ALH 84001, Cc20Ms40Sd40, and reacted the mix at 500-1200C and 10 kbar pressure at an oxygen fugacity of ~ NNO. At 600C, the siderite had begun to decompose, and above 800C it had completely reacted with the containment capsule to form (Fe,Ni)O and CO2. These results are comparable with those from the literature, and suggest that sideritic carbonate minerals, as in ALH 84001, are not likely to be produced above ~500C.

 

1630

Treiman A.H.* Ionov D.A. Amundsen H.E.F. Bunch T. Blake D.F.

A Terrestrial Analog for Carbonates in ALH 84001: Ankerite-Magnesite Carbonates in Mantle Xenoliths and Basalts from Spitsbergen (Svalbard), Norway

It has been difficult to understand the origin of the carbonate globules and pancakes in ALH 84001, in part because no similar carbonates have been reported from the Earth, where their history and geologic setting can be studied in detail. Here, the authors report a comparable (not identical) occurrence of zoned carbonate globules and pancakes in mantle xenoliths and basalts from Spitsbergen Island, Norway. The Spitsbergen globules are zoned from ankerite or siderite cores outward to magnesite rims, with many fine zones of Fe-richer or poorer carbonate. The globules are present in vesicles in the basalt and in glass within the xenoliths, and also as replacements of xenolith olivine. Pancakes are present along fractures in the xenoliths and host basalt. The carbonates are older than the silica and smectite clays with which they are found. Presence of carbonate globules in vesicles and fractures in the host basalt implies that they formed after the basalt solidified. The temperature of globule formation is poorly constrained, and it is not clear if the carbonate is derived from mantle carbonates (present in the xenoliths) or from Earth surface materials.

 

1787

Boctor N.Z.* Wang J. Alexander C.O.A. Hauri E. Bertka C.M. Fei Y. Humayun M.

Petrology and Hydrogen and Sulfur Isotope Studies of Mineral Phases in Martian Meteorite ALH84001

The authors measured relative abundances of the isotopes of hydrogen and of sulfur as clues the interactions of volatiles with ALH 84001. Hydrogen isotope values measured in carbonate globules and phosphate mineral grains (whitlockite) are nearly all "heavy" compared to Earth hydrogen, d D = +165 +287 . This range is much lower ("lighter" hydrogen) than the current martian atmosphere's d D ~ +5000 , suggesting that ALH 84001 and the current martian atmosphere have had little interaction. Two of the three sulfur isotope measurements on pyrite are consistent with previous results, d 34S ~ +7 ; the third analysis shows significantly "lighter" sulfur, d 34S ~ +2 , which is nearly identical to magmatic sulfur from other martian meteorites. The range of sulfur isotope abundances is not consistent with the actions of sulfate-reducing bacteria, but is consistent with pyrite formation at temperatures much below ~1000C.

 

1872

Farquhar J.* Thiemens M.H. Jackson T.

D 17O Measurements of Carbonate from ALH 84001: Implications for Oxygen Cycling Between the Atmosphere-Hydrosphere and Pedosphere of Mars

The authors' measurements of oxygen isotopes show that the carbonates in ALH 84001 had a different source oxygen than did the silicate minerals. The relative proportions of the three oxygen isotopes (16O, 17O, 18O) change during normal chemical processes, but the ratio 18O/16O must change twice as fast as the ratio 17O/16O ratio. However, the martian meteorites have relative ratios of 16O:17O:18O that are different from those on the Earth, which means that the meteorites cannot be from the Earth. This difference is given as the value D 17O with the Earth defined as zero; the martian meteorites have D 17O = +0.3. Water from the martian meteorites has D 17O = +0.8, distinctly different from the bulk meteorites and implying that water on Mars has not chemically equilibrated with the silicate minerals of its crust (Karlsson et al., 1992; Romanek et al., in press). Here, the authors measured oxygen isotope abundances of carbonates in ALH 84001, and found that they also had D 17O = +0.8. This D 17O value for the carbonates suggests that they formed at low temperature from water that had communicated with the martian atmosphere. The source of the high D 17O in the martian water may be photochemical reactions involving CO2 in the atmosphere.

 

1205

Becker L.* Popp B. Rust T. Bada J.L.

The Origin of Organic Matter in the Martian Meteorite ALH84001

No Abstract Available

 

1812

Clemett S.J.* Dulay M.T. Gillette S. Chillier X.D.F. Mahajan T.B. Zare R.N.

Are the Polycyclic Aromatic Hydrocarbons in ALH84001 of Extraterrestrial Origin?: A Reevaluation

Polycyclic aromatic hydrocarbons (PAHs) are among the evidence cited as possible traces of ancient martian life in ALH 84001(McKay et al., 1996). However, Becker et al. (1997) claimed that these PAHs were not martian, but were terrestrial contamination acquired by ALH 84001 in Antarctic. Here, Clemett et al. dispute Becker's claims, and conclude that the PAHs are indigenous to ALH 84001 and therefore martian. 1) Becker et al. claimed that carbonate minerals act as scavengers for PAHs in solution. The authors were unable to duplicate the scavenging experiments of Becker et al., and cite flaws in their experimental designs. 2) Non-martian meteorites from the same area of Antarctica contain considerably lower abundances of PAHs than does ALH 84001. 3) Clay-rich, carbonaceous micrometeorites, which are expected to be most efficient at scavenging PAHs, show variable amounts of PAHs, which vary from micrometeorite to micrometeorite and unlike those in ALH 84001 (esp. in being abundantly alkylated). 4) Meltwater from Antarctic ice contains a negligible PAH content, in contrast to analyses by Becker et al. 5) New data shows that PAHs in ALH 84001 are depleted near its fusion crust and absent in the fusion crust. This distribution is not consistent with Antarctic contamination, but with indigenous martian PAHs. The depletion near the fusion crust is ascribed to oxidation (burning) during the meteorite's flight through the Earth's atmosphere.

 

1263

Stephan T.* Rost D. Jessberger E.K. Greshake A.

Polycyclic Aromatic Hydrocarbons in ALH84001 Analyzed with Time-of-Flight Secondary Ion Mass Spectrometry

Polycyclic aromatic hydrocarbons (PAHs) are among the evidence cited as possible traces of ancient martian life in ALH 84001(McKay et al., 1996). Stephan et al. used time-of-flight secondary-ion-mass-spectrometry (TOF-SIMS) to confirm that ALH 84001 contains significant quantities of PAHs, and that these PAHs are principally unalkylated. The PAH mass spectra here is very different from that in McKay et al. (1996) because of the different analytical methods -- it is reasonable that both analyzed the same mix of PAHs. The PAHs detected here are not laboratory contaminants, as no PAHs were found in embedding material or standards. Unlike the results of McKay et al. (1996), the authors here found that the PAHs were evenly distributed throughout ALH 84001, not closely associated with the carbonate globules. So, the PAHs are not intimately related to the carbonates, their nanofossil forms, or other proposed signs of ancient martian life in ALH 84001.

 

1184

Jull A.J.T.* Beck J.W. Courtney C. Jeffrey D.A.

Carbon Isotopic Evidence for Terrestrial Organic Compounds Found in Some Martian Meteorites

The authors measured the isotopic composition of carbon in ALH 84001: 12C (stable), 13C (stable), and 14C (radioactive). Most of the organic (non-carbonate) carbon is terrestrial contamination, but a small portion may be pre-terrestrial. Carbon from martian meteorites tends to have much higher 13C/12C ratios (expressed as d 13C) than Earth carbon; young Earth carbon has detectable 14C, while old and extraterrestrial carbon contains almost no 14C. The authors extracted carbon from samples of ALH 84001 by acid dissolution, and by heating in oxygen at various temperatures. Carbon extracted at <400C is terrestrial contamination - it has significant 14C and low d 13C like terrestrial organics. Acid-soluble carbon (= most of the carbon released during heating from 400 and 700C), is from carbonate globules - that carbon is not terrestrial as it has no 14C and has high d 13C. A small part of the carbon is not acid-soluble and is released above 400C - it has no 14C and has moderate d 13C. This last carbon, about 20% of the non-carbonate (organic) carbon in ALH84001, may be pre-terrestrial. Much of this data is available in Jull et al. (1998).

Posters
Tuesday, March 17, 1998
POSTER SESSION I
7:30 P.M. Space Center Houston
1107

Gupta A. Freund F.

Fatty acids and glycolamide extracted from olivine single crystals

While this abstract does not mention ALH 84001 specifically, it suggests a way that that indigenous organic compounds in ALH 84001 could have formed without life. When a crystal grows from a magma, it incorporates small quantities of H2O, CO2, and nitrogen that were dissolved in the magma. When the crystal cools, these chemical could react with each other and form short- or long-chain aliphatic hydrocarbons, with or without functional groups containing oxygen (e.g., organic acids) or nitrogen (e.g., amines). The authors tested this hypothesis by extracting organic material from olivine crystals from an Earth basalt, after careful cleaning to avoid contamination. The olivine contained ppm levels of these organic compounds, including long-chain aliphatic hydrocarbons (mostly saturated), fatty acids, and glycolamide (C2H5NO2). These hydrocarbons could conceivably be non-biological precursors to the PAHs in ALH 84001.

Thursday, March 19, 1998
POSTER SESSION II
7:30 P.M. Space Center Houston

LIFE ON MARS AND EARTH

1505

Allen C. C. Graham C. R. Combie J. Albert F. G. Steele A. McKay D. S.

Biological Signatures in Carbonates: Yellowstone National Park

As part of a long-term study of possible terrestrial analogs to biogenic features in ALH 84001, the authors are studying mineral deposits and microorganisms in a hot spring from Yellowstone National Park, U.S.A. Aragonite (CaCO3) deposits, water (~70C, pH ~ 6.4), and bacterial filaments were collected. Most of the bacteria were Thermothrix sp. The aragonite needles were arranged in distinctive "streamer" fabrics, which are characteristic of bacterial precipitation. No intact cells were found in the aragonite samples, but incubated aragonite samples did yield a few microbes of unknown affinity. The aragonite is rich in biofilm, organic mucus, which contains acid-resistant spherules of 100-200 nm diameter. The spherules are similar to nanobacteria (?), but their actual nature is not yet known.

 

1509

Allen C. C. McKay D. S.

Biomarkers in Thermal Spring Carbonates: Implications for Mars

As part of a long-term study of potential signatures of biological activity (biomarkers) in planetary samples, especially from Mars, the authors focus here on thermal spring carbonates. Many types of microbes thrive in carbonate hot spring environments; these include filamentous, spherical (coccoidal), and rod-like (bacillar) shapes. Bacteria themselves decompose rapidly and are rarely preserved in carbonates. Bacterial organic matter is also rare in carbonate deposits that formed at >30C. Biofilms (polysaccharide slimes) are preserved through desiccation, and they are readily mineralized. "Nanofossil" spheres of 100-200 nm (0.1 - 0.2 m) are abundant in the deposits of some springs; larger spheres of 300-500 nm (0.3 - 0.5m) are common in others.

 

1506

Allen C. C. Taunton A. E. Taylor M. R. McKay D. S.

Microbes in Carbonate Thermal Springs: Hot Springs National Park

As part of a long-term study of possible terrestrial analogs to biogenic features in ALH 84001, the authors are studying mineral deposits and microorganisms in a hot spring from Hot Springs National Park, U.S.A. The Hot Springs, which are ~65C at a nearly neutral pH of ~ 7.3, precipitate aragonite and calcite at water surfaces. Bacteria are common in underground waters from the springs, including 1-2 m rods, and filaments of 0.1 m diameter and up to 6 m long. The waters also deposit orange films of amorphous Fe-Si-O material which is associated with a distinctive biota: spherical bacteria 5 - 15 m diameter, rod-shaped bacteria 0.5 - 1 m long, and spherical shapes < 1 m diameter. The carbonates are precipitated abiogenically, and experiments reported here also show that the orange films can form without biological action.

 

ALH 84001

1332

Cooney T.F. Scott E.R.D. Krot A.N. Sharma S.K. Yamaguchi A.

Confocal Raman Microprobe and IR Reflectance Study of Minerals in the Martian Meteorite ALH84001

As an aid to understanding the geological history of ALH 84001, the authors investigated some aspects of its mineralogy. Silica (SiO2) is an uncommon accessory phase. Raman spectroscopy showed that most of the silica is glassy (amorphous) with a little crystalline quartz mixed in. The Raman spectrum of the silica glass suggests shock pressures above 31 GPa. The authors found two distinct phosphate phases in ALH 84001: chlorapatite and merrillite. This is the first sure report of chlorapatite, based on infrared (IR) reflectance and Raman spectroscopy. There is no hint of water (hydroxyl) in the chlorapatite. ALH 84001 also contains the anhydrous phosphate merrillite, not whitlockite as has been commonly reported. Finally, the authors obtained Raman spectra of the cores of carbonate globules, average composition (Ca.19Mg.47Fe.34)CO3. The Raman spectrum was consistent with a single carbonate mineral, implying that these carbonates are metastable single phases and not ankerite and siderite interlayered (exsolved) at the submicron level.

 

1196

Treiman A H. Treado P.

Martian Maskelynite? Raman Spectra of Plagioclase Composition Glasses from ALH 84001, EETA79001, and ALHA77005

It has been unclear if plagioclase - composition glasses in ALH 84001 were originally maskelynite, a shock glass formed without melting, or are melt glasses from cooled liquids. The authors obtained Raman spectra of crystalline plagioclase and plagioclase-composition glasses to resolve this question. Raman spectra of plagioclase crystals and from maskelynite from the Manicouagan impact structure (Earth) all show sharp Raman emission lines consistent with crystalline material. Melt glasses from ALHA77005 show only broad Raman emissions. However, the martian meteorite EETA79001 contains classic maskelynite (shaped like plagioclase crystals, with chemical zoning and twins preserved as differences in refractive index) that has only broad Raman emissions, as is characteristic of melt glass. Plagioclase-composition glass in ALH 84001 also has only broad Raman emissions, and so is likely a melt glass. The concept of 'maskelynite' may not be useful in spectroscopic or micro-structural studies.

 

1830

Greenwood J.P. McSween H.Y.Jr.

Origin of Stoichiometric Feldspathic Glasses in ALH84001 by Mixing of Plagioclase and Orthoclase During Multiple Shock Events

In ALH 84001, all that remains of original feldspars and silica is feldspathic glass. The authors investigated reports that the glasses are non-stoichiometric (they lost or gained some elements during melting), using electron microprobe methods. Greenwood and McSween found that the glasses were nearly all stoichiometric, representing mixtures of plagioclase feldspar and orthoclase (actually, Or55Ab45 alkali feldspar: ed.) composition. There are four varieties of feldspathic glass. 1) Maskelynite (?) in the shapes and composition of igneous plagioclase. 2) Mobilized glass of plagioclase composition in veins and as crack fillings. 3) Glass with the composition of orthoclase feldspar (actually, Or55Ab45 alkali feldspar: ed.). 4) Mixed glasses, representing mixtures of plagioclase and orthoclase composition feldspars. This variety of glasses cannot be explained by a single shock event. At least two shocks are required - one to produce the mixed glasses, and another to make the mobilized glass veins that cross the mixed glasses.

 

1280

Shearer C.K. Adcock C.T.

The Relationship Between the Carbonate and Shock-produced Glass in ALH 84001

Shearer and Adcock examined mineral textures in thin sections of ALH 84001 to evaluate the hypothesis that its carbonate globules and feldspathic glass formed as immiscible melts. The authors find textural evidence that indicates otherwise -- that the carbonate globules were precipitated into fractures at low temperature, and were subsequently fragmented and disaggregated by feldspathic shock melts. Not only are the carbonate "pancakes" deposited in open fractures, the authors find evidence that the rounded carbonate globules were deposited in open cavities. They infer that feldspathic glass surrounding globules was emplaced after the globules formed. Shearer and Adcock also find that carbonate globules were detached from their substrates, shattered, and transported by mobile feldspathic glass.

 

1934

Sears D.W.G. Kral T.A.

SEM Imaging of Martian and Lunar Meteorites and Implications for Microfossils in Martian Meteorites

The authors examined five lunar meteorites, recovered from Antarctica, to see if the putative bacterial shapes in ALH 84001 might also occur without intervention of life. The lunar meteorites contain rare objects that are similar to the putative bacterial shapes. Sears and Kral examined original and fracture surfaces of anorthositic and basaltic lunar meteorites, using the same procedures as did McKay et al. (1996). Many kinds of sub-micron objects were found, including ovoid and elongate shapes similar in size and shape those in reported from ALH 84001. Sears and Kral did not determine the true nature of all these objects, but their presence in lunar meteorites suggests that they are not characteristic of biological activity on Mars.

 

1362

Westall F. Gobbi P. Gerneke D. Mazzotti G.

Microstructures in the Carbonate Globules of Martian Meteorite ALH84001: Preliminary Results of a High Resolution SEM Study

In order to understand the "fossil bacteria" structures found in ALH 84001, the authors are examining more fragments of the meteorite with a high-resolution field-emission gun SEM. Samples were uncoated and also coated with a very thin layer of C-Pt. No unambiguous bacteria or bacterial fossils were found. Surfaces of carbonate minerals are decorated with fine raised lamellae, as were described by Bradley et al. (1997). Many surfaces are also coated with 50-100 nm spherules of amorphous silica, such as commonly are found in hydrothermal (hot spring) deposits on Earth. The carbonate minerals contain rounded to sub-angular inclusions to 130 nm long which are unidentified at present. These inclusions could be mineralogical or biological in origin.

 

1496

Morris P.A. Allen C.C. Gibson E.K.Jr. McKay D.S. Thomas-Keprta K.

Reexamination of the Warrawoona Group Fossils (Towers Formation, Western Australia, 3.3 to 3.5 Ga): Analogs of Mars Meteorite Fossils?

As part of a long-term study of potential signatures of biological activity (biomarkers) in planetary samples, the authors are studying bacterial fossils in cherts of the Warrawoona Group (Australia). The Warrawoona carbonaceous cherts formed ~3.4 billion years ago, and contain some of the Earths oldest fossils. The cherts contain a variety of filament shapes of presumed biologic origin. Most striking are filaments ~2 m wide and over 100m long, which are associated with spherical objects (cocci?) ~2 m diameter. Also present are spheroidal objects in three groups: 26-33 m diameter with convoluted surfaces; 16-18 m diameter with smoother surfaces; and 5-12 m with depressions and protuberances. All these objects are mineralized (filaments by silica; spheres mostly by iron oxides and carbonates). These objects are similar to accepted fossil forms from the Warrawoona, and confirm the value of SEM analysis (as was done with ALH 84001) in recognizing mineralized fossils of bacteria.

 

1156

Flynn G.J. Keller L.P. Miller M. A. Jacobsen C. Wirick S.

Organic Compounds Associated with Carbonate Globules and Rims in the ALH 84001 Meteorite

Several varieties of organic materials are present in ALH 84001, including terrestrial contaminants and possible indigenous (Martian) material. The authors examined ultramicrotomed thin sections with Synchrotron X-ray Transmission Microscopy (SXTM) and infrared (IR) absorption spectroscopy at the National Synchrotron Light Source (Brookhaven NY). The analytical spot size for these methods is ~50 nm, 1000 times smaller than that available to McKay et al. (1996). SXTM data showed that the carbonate globules contain organic carbon (C-C, C=C, C-O and C-H bonds), that the carbon compounds vary somewhat within the globules, and that the organic carbon in the globule rims is different from that in their cores. IR spectra showed that the organics of the globule interiors is rich in -CH3 groups (characteristic of small organic molecules), while organics in the globule rims are dominated by -CH2- groups (characteristic of aliphatic chain hydrocarbons). These results seem inconsistent with the simplest model of terrestrial contamination -- infiltration and evaporation of a hydrocarbon-bearing water.

 

1195

Treiman A.H.

The History of ALH 84001 Revised: Multiple Shock Events

Understanding the origin of possible biological feature in ALH 84001 depends on understanding the meteorite's history, before and after formation of the possible traces of ancient martian life. Here, Treiman proposes that a complex history of 4 or 5 shock impact events is needed to explain the mineral textures and compositions of ALH 84001. The first impact granulated portions of the meteorite, and a second impact turned some of the feldspars in the meteorite to glass. Then followed deposition of the carbonate globules with their putative evidence of ancient martian life. After carbonate formation, one impact shock is required to produce the feldspathic glasses that cut across and disrupt the globules. A second impact is required by some paleomagnetic evidence, and a third could represent ejection from Mars. This complex history is consistent with martian geology -- land surfaces as old as ALH 84001 are heavily cratered. As for discussions about putative traces of ancient life in the meteorite, these traces must have been modified by at least two shock events. It is possible that elongate magnetites did in fact grow by vapor deposition at high temperature, but that growth could have been in an impact event after formation of the carbonate globules. This work will be published as Treiman (1998).

 

1825

Mathew K.J. Marti K.

Nitrogen and Noble Gas Isotopic Signatures in Bulk ALH84001 with Carbonates

Isotope abundance ratios of nitrogen and noble gasses are distinctive tracers for martian atmosphere in the martian meteorites. The authors measured isotope abundances in nitrogen and noble gases that were emitted from ALH 84001 as it was heated. Most of the nitrogen is relatively depleted in the heavy nitrogen isotope, 15N, compared to Earth standard -- the nitrogen has d 15N < 0. This "light" nitrogen may be from the interior of Mars. In contrast, nitrogen released while the carbonates decompose is "heavier," d 15N up to +7. This is much "lighter" than the current martian atmosphere, which has d 15N = +620, meaning that the carbonates have not reacted with the current martian atmosphere. Isotope abundances of xenon are like those reported earlier. Their sample of ALH 84001 was relatively rich in 38Ar, which is produced in interplanetary space as cosmic rays hit calcium atoms. The differences in 38Ar abundances in different samples suggest that the published 16,000 year exposure duration for ALH 84001 may be an oversimplification of a complex history.

 

1705

Barlow N.G.

Status Report on the Search for Source Craters of ALH84001

Last year, the author reported on a search for suitable source craters for ALH 84001, based on global imagery of Mars from the Viking orbiters. Now, the search has been extended to include circular craters larger than 20 km diameter and elliptical craters larger than 10 km diameter. The craters must be minimally degraded and be on ancient (Noachian) surfaces, near ancient surfaces, or on older crater ejecta that could likely contain ancient materials. Two suitable elliptical craters were reported last year. At this point, six more potential source craters on ejecta blankets or near the highlands-lowlands boundaries have been identified. It is estimated that approximately 600 craters in the ancient martian highlands may also be suitable as potential source craters for ALH 84001.

 

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1894

Bada J.L. Glavin D.P. McDonald G.D. Becker L.

Amino Acids in the ALH84001 Martian Meteorite

Short review of their recent Science paper (Bada et al., 1998), which showed that ALH 84001 contains amino acids that are terrestrial contamination.

 

1422

Bell M.S. Hörz F. Reid A.

Characterization of Experimental Shock Effects in Calcite and Dolomite by X-Ray Diffraction

Several published histories of ALH 84001 have its carbonate globules formed from carbonate shock-melts. The authors here experimentally shocked common carbonate minerals to pressures of 60 GPa. The minerals were deformed strongly, and showed broadened X-ray diffraction peaks and some peak position shifts. No evidence is given that these carbonate minerals either decarbonated or melted at 60 GPa.

 

1757

Bradley J. P. McSween H. Y. Harvey R. P.

Mechanisms of Formation of Magnetite in Martian Meteorite ALH84001

It has been argued that sub-micron magnetite grains in the carbonate globules of ALH 84001 are the products of biological activity. The authors here suggest that the magnetites formed via several non-biological mechanisms. Their earlier work showed that many elongate magnetite grains (whiskers) grew via screw dislocations -- a common mechanism of inorganic crystal growth -- and that some elongate magnetite crystals are oriented in specific directions with respect to their host carbonate, which implies that the magnetite crystals grew in place on the carbonate minerals (Bradley et al., 1996, 1997). Other magnetite whiskers lack screw dislocations, and magnetite is also abundant as parallelepipeds, plates, blades, and ribbons. The wide range of magnetite morphologies and structures means that they grew by many different mechanisms, which suggests that growth conditions (e.g., degree of supersaturation) and not biology controlled growth of these magnetite grains. The authors find it most reasonable that the magnetite crystals grew from a vapor phase at high temperature. A vapor phase seems required to form whisker-shaped magnetite grains, and the other magnetite morphologies are known to form during vapor phase growth.

 

1452

Brearley A. J.

Microstructures of Feldspathic Glass in ALH 84001 and Evidence for Post Carbonate Formation Shock Melting

To evaluate the hypothesis that ALH 84001s carbonate globules contain traces of ancient martian life, it is important to understand their formation and history. Using SEM and TEM methods, Brearley studied the carbonate globules in relation to the feldspathic glass that commonly surrounds them. In places, carbonate globules have been disrupted to irregular fragments embedded in feldspathic glass. Boundaries between carbonate and glass are 50-100 nm wide zones that contain elements characteristic of both carbonate and feldspathic glass; this suggest that glass has replaced or reacted with the carbonates. Away from the carbonates, the feldspathic glass consists of rounded domains of feldspar-like compositions separated by films of silica-rich compositions. This texture looks like partial replacement of feldspar by silica, as occurs in vapor-rich volcanic environments on Earth. Some of the silica is crystalline, possibly the high-pressure silica mineral stishovite. These and other observations suggest that: the carbonate globules were deposited into heterogeneous, non-stoichiometric material (possibly from fluid alteration); that the carbonates were deposited by a fluid (not as a melt); and that feldspathic glass was melted and mobilized after the carbonates formed.

 

1433

Gibson E.K.Jr., McKay D.S., Thomas-Keprta K.

Exobiological Features Within ALH84001: Current Observations

Authors of the original hypothesis that ALH 84001 contains traces of ancient martian life respond to recent criticisms and review available data. Bada et al. (1998) and Jull et al. (1998) both found that ALH 84001 contains a significant mass of terrestrial organic material, presumably contamination from its time in Antarctica. The authors point out that their hypothesis did not include amino acids as martian biomarkers, only PAH organic compounds. Further, they note that terrestrial contamination and alteration is nearly ubiquitous in Antarctic meteorites, and that these should contain terrestrial 14C. Gibson et al. again emphasize the need to examine many potential biomarkers, notably biofilms (which they have reported in ALH 84001).

 

1569

Protheroe W.J.Jr., Stirling J.

Cathodoluminescence Study of Fragments of the Martian Meteorite ALH 84001

Feldspathic glasses and pyroxene in ALH 84001 emit visible light when bombarded by electrons in an electron microscope (i.e., they cathodoluminesce). One grain of feldspathic glass appeared cross-hatched in emitted cathodoluminescent light. The cause of this pattern is not known.

 

1754

Shearer C. K. Adcock C. T.

The Composition and Distribution of Feldspathic Shock Glass in ALH 84001

Feldspathic glass in ALH 84001 is commonly associated with carbonate globules and may hold clues to the geologic history of the meteorite. In the glass compositions and structures, Shearer and Adcock see vestiges of a hydrothermal alteration event early in the history of the meteorite. Chemical compositions of the glass range from nearly stoichiometric plagioclase, to silica-enriched, to potassium-rich (K-rich), as reported in other studies. The K-rich glasses are also silica rich (i.e., are depleted in Al). The K-rich glasses are commonly found with broken carbonate globules and in fractures and cavities near olivine grains in the orthopyroxene. The plagioclase-composition glasses, however, tend to enclose the carbonate globules. The plagioclase glasses are, however, richer in sodium than typical martian igneous plagioclase, which suggests that their feldspar compositions have been altered. The authors suggest that the increased sodium content arose during low-temperature aqueous alteration before shock; silica was deposited in ALH 84001 at the same time, leading to the silica-rich glasses.

 

1281

Shearer C.K. & Adcock C.T.

The Origin of Olivine in Martian Meteorite ALH 84001. The Distribution of Olivine

Small quantities of the mineral olivine is present in ALH 84001, but its origin is not clear. It might be related to formation of the carbonate globules, or might not. To help resolve this uncertainty, the authors investigated the spatial distribution of olivine in ALH 84001. The olivine does not occur randomly or in pyroxene cores as might be expected if it were igneous. Rather, the olivine is present as clusters of grains in orthopyroxene adjacent to fractures containing disrupted carbonate globules or K-rich feldspathic glass. Shearer and Adcock infer that the olivine is not strictly magmatic, nor is it related to formation of the carbonate globules. They offer two hypotheses: 1) the olivine was originally magmatic, but was re-distributed during shock events, or 2) the olivine formed during shock metamorphism by reaction of orthopyroxene, CO2, and feldspathic glass.

 

1286

Shearer C.K. & Leshin L.A.

The Origin of Olivine in Martian Meteorite ALH 84001. The Oxygen Isotopic Systematics of the Olivine

Small quantities of the mineral olivine is present in ALH 84001, but its origin is not clear. It might be related to formation of the carbonate globules, or might not. To help resolve this uncertainty, the authors analyzed the oxygen isotope composition of the olivine and adjacent minerals. The oxygen isotope composition of the olivine ranges from d 18O = +4.3-5.3, identical within analytical uncertainty. The olivine d 18O is essentially identical to that of the host orthopyroxene, which implies that the oxygen in both minerals equilibrated with each other at high temperature. The carbonates in ALH 84001, however, have a huge range of d 18O, from about +5 to +25, which make it unlikely that the olivine and the carbonates formed in the same event.

 

1489

Thomas-Keprta K.L. McKay D.S. Wentworth S.J. Stevens T.O. Taunton A.E. Allen C.C. Gibson E.K.Jr. Romanek C.S.

Mineralization of Bacteria in Terrestrial Basaltic Environments: Comparison with Possible Life Forms in Martian Meteorite ALH84001

The possible martian bacteria shapes reported in ALH 84001 are apparently fossilized, i.e. mineralized or replaced by inorganic constituents. Little is known about mineralization of bacteria in igneous rocks, so the authors grew bacteria on basalt and examined the resultant samples with SEM and TEM methods. The samples inoculated with bacteria contained hollow bacteria-shaped objects 1-2.5 m long (slightly less in diameter) that consisted mostly of ferrihydrite (a ferric iron hydroxide) and contained no cellular structures or ultrastructures. These are likely to be mineralized bacteria, whether replaced by ferrihydrite or originally coated by ferrihydrite is not known. Other biogenic features included tubular forms (0.3 - 2.4 m long), commonly with an appendage and commonly embedded in biofilm. The authors also observed filaments of distinctive morphology, some attached to tubular bacteria, and some unattached and composed of ferrihydrite. These filaments are interpreted as bacterial appendages. These bacterial fossil forms are similar in size and shape to forms reported from ALH 84001, which are interpreted as biogenic.

 

1793

Wentworth S.J. Thomas-Keprta K.L. Taunton A.E. Velbel M.A. McKay D.S.

Possible Weathering Features in ALH84001

Using SEM and TEM methods, the authors investigated mineralogical and structural effects of low-temperature weathering in order to document Antarctic effects and search for martian features. Weathering minerals that definitely formed on Earth include Ca-sulfate, Mg-sulfate, amorphous(?) silica, and common salt. Weathering minerals of unknown origin (not on the fusion crust) include blade-like to rhombohedral crystals of a Mg-carbonate that may be hydrated (e.g., nesquehonite or hydromagnesite). Smectite clay, identified by TEM, is present as half-micron-sized patches. Wispy and fibrous coatings on some grains may also be clays. Surfaces of carbonate and pyroxene grains have pits or rounded surfaces that are unlike reported features from terrestrial weathering, and may represent the effects of weathering on Mars.

 

1594

Wright I. P. Grady M. M. Pillinger C. T.

On the 14C and Amino Acids in Martian Meteorites

Recent results on amino acid and 14C analyses of ALH 84001 (Bada et al., 1998; Jull et al., 1998) have shown that ALH 84001 is significantly contaminated by terrestrial organic matter. Here, the authors show that the data and their interpretation is somewhat ambiguous, and does not necessarily refute the hypothesis that ALH 84001 contains traces from ancient martian life. Bada et al. (1998) seem to demonstrate that all the amino acids in ALH 84001 are terrestrial contamination. Yet, the authors here point out that the abundances of D and L forms of the amino acid alanine seem to require more than one source of amino acids. Also, ALH 84001 contains so much amino acids that each half-gram sample that Bada analyzed would have had to scavenge all the amino acids from ~13 liters of Antarctic ice (~50,000 liters through the whole meteorite). This amount of water flowing through ALH 84001 should have produced some clay minerals and should have eliminated all vestiges of martian hydrogen — neither of which is observed. The author's comments on 14C mostly concern the martian meteorite EETA79001, and are not directly relevant to ALH 84001.

 

References:

Bada, J.L., Glavin D.P., McDonald G.D., and Becker L. (1998) A search for endogenous amino acids in martian meteorite ALH84001. Science 279, 362-365.

Becker L., Glavin D.P., and Bada J.L. (1997) Polycyclic aromatic hydrocarbons (PAHs) in Antarctic Martian meteorites, carbonaceous chondrites, and polar ice. Geochim. Cosmochim. Acta 61, 475-481.

Bradley J.P., Harvey R.P., and McSween H.Y.Jr. (1996) Magnetite whiskers and platelets in ALH 84001 Martian meteorite: Evidence of vapor phase growth. Geochim. Cosmochim. Acta 60, 5149-5155.

Bradley J.P., McSween H.Y.Jr., and Harvey R.P. (1997) Epitaxial growth of single-domain magnetite in martian meteorite ALH84001 (abstract). Meteor. Planet. Sci. 32, A20.

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.

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.

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.

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