Martian Meteorite ALH84001 Research Articles

Novel nano-organisms from Australian sandstones

Other| December 01, 1998 Novel nano-organisms from Australian sandstones Philippa J. R. Uwins; Philippa J. R. Uwins University of Queensland, Centre for Microscopy and Microanalysis, St. Lucia, Queensl., Australia Search for other works by this author on: GSW Google Scholar Richard I. Webb; Richard I. Webb Search for other works by this author on: GSW Google Scholar Anthony P. Taylor Anthony P. Taylor Search for other works by this author on: GSW Google Scholar American Mineralogist (1998) 83 (11-12_Part_2): 1541–1550. https://doi.org/10.2138/am-1998-11-1242 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Philippa J. R. Uwins, Richard I. Webb, Anthony P. Taylor; Novel nano-organisms from Australian sandstones. American Mineralogist 1998;; 83 (11-12_Part_2): 1541–1550. doi: https://doi.org/10.2138/am-1998-11-1242 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract We report the detection of living colonies of nano-organisms (nanobes) on Triassic and Jurassic sandstones and other substrates. Nanobes have cellular structures that are strikingly similar in morphology to Actinomycetes and fungi (spores, filaments, and fruiting bodies) with the exception that they are up to 10 times smaller in diameter (20 nm to 1.0 mu m). Nanobes are noncrystalline structures that are composed of C, O, and N. Ultra thin sections of nanobes show the existence of an outer layer or membrane that may represent a cell wall. This outer layer surrounds an electron dense region interpreted to be the cytoplasm and a less electron dense central region that may represent a nuclear area. Nanobes show a positive reaction to three DNA stains, [4,6-diamidino-2 phenylindole (DAPI), Acridine Orange, and Feulgen], which strongly suggests that nanobes contain DNA. Nanobes are communicable and grow in aerobic conditions at atmospheric pressure and ambient temperatures. While morphologically distinct, nanobes are in the same size range as the controversial fossil nannobacteria described by others in various rock types and in the Martian meteorite ALH84001. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Phosphates and carbon on Mars: Exobiological implications and sample return considerations

Much of the surface of Mars may preserve chemical information contained in rocks from the Noachian era, with ages that overlap the correspondingly earliest Archean geological history of the Earth, or from before around 3800 Ma (Ma = 1×106years). Metabolically sophisticated life, which utilized phosphate and carbon and was capable of fractionating carbon isotopes, was apparently present already on Earth by ∼3800 Ma, or within 600 Ma after the formation of the planet. An early appearance of life on Earth opens the strong possibility for a similarly early and rapid emergence of life on planet Mars. This hypothesis remains within the realm of plausibility so long as it can be established that liquid water and energy sources were available there for inchoate life, and that the life that emerged reached a level of complexity which could be recognized by its chemical, and perhaps morphological remains. Hypotheses to be used in the search for an ancient Martian biosphere from future sample return missions are testable by examining the record of life in ancient terrestrial sedimentary rocks, including those that contain rare and recognizable “physical” microfossils (“morphofossils” identified on the basis of their shape alone) and stable, authigenic biominerals which include carbonaceous matter having characteristically fractionated carbon isotope signatures (here termed “chemofossils”). Prior to sample return, these tests can be applied to the mineral associations of the SNC meteorites, a group of meteorites believed to have originated on Mars. Recent claims of a biological origin for secondary minerals and their features as well as for trace organic compounds in the Martian meteorite ALH84001, are derived in part from the interpretation of putative “nanofossil” shapes and the nature of the associated mineral assemblage in small carbonate deposits of an igneous rock. Such igneous samples would not normally be the best candidate to search for evidence of past life, even on Earth. Investigations of these mineral occurrences in the Martian meteorites and of the oldest geological records on Earth provide a useful framework for (1) using mineral phase relationships, (2) analytical data of stable carbon isotopic distributions, and (3) the problematic task of morphofossil interpretations, in the search for life via future sample return missions from the ancient surface of Mars.

Correlated chemical and isotopic zoning in carbonates in the martian meteorite ALH84001

The meteorite ALH84001, a sample of the ancient martian crust, contains small quantities (∼1%) of strongly chemically zoned carbonate. High spatial resolution (10 μm) ion microprobe analyses show that the chemical zoning is strongly correlated with variations in oxygen isotope ratios. Early formed Ca,Fe-rich cores have δ 18O ∼ 7‰ increasing to 22‰ SMOW in the more Mg-rich outer cores and magnesite rims. Isolated areas of ankerite appear to be isotopically lighter with δ 18O ∼ 1‰. The large range in δ 18O requires a significant range in either fluid isotopic composition, or temperature, or both, in the course of the deposition sequence. Our data are inconsistent with formation of the zoned carbonates by closed system Rayleigh fractionation. There is no unique interpretation of the oxygen data, but the recent observation of existence of Δ 17O excesses in the carbonate appears to rule out models which involve high temperature isotopic exchange with silicate. Comparison with terrestrial analogues suggests that ALH84001 carbonates formed in a hydrothermal system with T<∼400°C, and which, at least in the early stages of formation, may have involved water with δ 18O < 0‰ SMOW. The later stages of deposition probably occurred at temperatures below 150°C, a conclusion which does not preclude the co-existence of thermophilic bacteria; temperatures during earlier stages of deposition are less likely to have been hospitable to bacteria.

Petrologic evidence for low-temperature, possibly flood evaporitic origin of carbonates in the ALH84001 meteorite.

High-temperature models for origin of the carbonates in Martian meteorite ALH84001 are implausible. The impact metasomatism model, invoking reaction between CO2 rich fluid and the host orthopyroxenite, requires conversion of olivine into orthopyroxene, yet olivine in ALH84001 shows no depletion in carbonate-rich areas; or else conversion of orthopyroxene into silica, which should have yielded a higher silica/carbonate ratio. The impact melt model implies that the fracture-linked carbonates, as products of melt injection, should appear as continuous planar veins, but in many areas they do not. Both vapor deposition and impact melting seem inconsistent with the zoned poikilotopic texture of many large carbonates. The popular hydrothermal model is inconsistent with the virtual absence of secondary hydrated silicates in ALH84001. Prior brecciation should have facilitated alteration. Hydrothermal fluids would be warm, and rate of hydration of mafic silicates obeys an Arrhenius law, at least up to approximately 100 degrees C. Most important, hydrothermal episodes tend to last for many years. Many areas of the ancient Martian crust show evidence for massive flooding. I propose that the carbonates formed as evaporite deposits from floodwaters that percolated through the fractures of ALH84001, but only briefly, as evaporation and groundwater flow caused the water table to quickly recede beneath the level of this rock during the later stages of the flood episode. The setting might have been a layer of megaregolith beneath a surface catchment of pooled floodwater, analogous to a playa lake. Carbonate precipitation would occur in response to evaporative concentration of the water. To explain the scarcity of sulfates in ALH84001, the water table must be assumed to recede quickly relative to the rate of evaporation. During the period when ALH84001 was above the water table, evaporation would have slowed, as the evaporation front passed beneath the surface of the debris layer, and possibly earlier, if the shrinking pool of surface water developed a porous sulfate crust. Alternatively, ALH84001 may have developed as a Martian form of calcrete, i.e., the evaporating flood(s) may have been entirely below ground as it (they) passed slowly through ALH84001. The greatest advantage of the flood evaporite model is that it exposes ALH84001 to carbonate precipitation without prolonged exposure to aqueous alteration. The model also seems consistent with the heavy and extremely heterogeneous oxygen isotopic compositions of the carbonates. However, this hypothesis seems no more than marginally consistent with the suggestion of McKay et al. [1996] that the carbonates are biogenic.

Microstructures and Microanalysis in ALH84001: Minerals or Martians?

Abstract The martian meteorite ALH84001 leapt into the spotlight in late 1996 when a NASA-led research team suggested that it contained evidence of biotic activity on ancient Mars. ALH84001 is unique among the martian meteorites- a cumulate rock made up predominantly of orthopyroxene, with interstitial maskelynite (feldspar turned into a diaplectic glass by shock), apatite, chromite and sulfides. Highly brecciated fracture zones cut through the rock, presumably a result of impacts, and within these zones is a variable amount of secondary carbonate. This carbonate, the most extensive secondary mineral found in any martian meteorite, formed during significant interaction between martian crust and volatiles, and represents a unique access point toward understanding the environment of ancient Mars. However, the conditions under which this carbonate formed are less clear. While claims for evidence of biotic activity are predicated on carbonate formation at low (&amp;lt;100°C) temperatures under water-rich conditions, other research suggests much less hospitable conditions, with little water present and formation temperatures approaching 700°C.

Exposure of Allan Hills 84001 and other achondrites on the Antarctic ice.

The enrichment of F on Antarctic meteorites is the result of their exposure to the atmosphere, and its measurement allows a subdivision of the terrestrial age into a duration of exposure on the ice and the time a meteorite was enclosed by the ice. In many cases, the periods of surface exposure are only small fractions of the terrestrial ages of meteorites collected in Antarctica. The enrichment of F on the surfaces of Antarctic achondrites was investigated by means of nuclear reaction analysis (NRA): scanning proton beams with an energy of 2.7 and 3.4 MeV were used to induce the reactions 19F(p, alpha gamma)16O and 19F(p,p gamma)19F, respectively. Gamma signals proportional to the F content were measured. The following Antarctic achondrites were investigated: Martian meteorite ALH 84001; diogenite ALHA77256; the eucrites ALHA81011 and ALHA78132; and in addition, the H5 chondrite ALHA79025. For ALH 84001, our data indicate a period of exposure on the ice of <500 years. Thus, this specimen was enclosed in the ice >95% of its terrestrial age of 13 000 years.

Atmosphere-surface interactions on Mars: delta 17O measurements of carbonate from ALH 84001.

Oxygen isotope measurements of carbonate from martian meteorite ALH 84001 (delta18O = 18.3 +/- 0.4 per mil, delta17O = 10.3 +/- 0.2 per mil, and Delta17O = 0.8 +/- 0.05 per mil) are fractionated with respect to those of silicate minerals. These measurements support the existence of two oxygen isotope reservoirs (the atmosphere and the silicate planet) on Mars at the time of carbonate growth. The cause of the atmospheric oxygen isotope anomaly may be exchange between CO2 and O(1D) produced by the photodecomposition of ozone. Atmospheric oxygen isotope compositions may be transferred to carbonate minerals by CO2-H2O exchange and mineral growth. A sink of 17O-depleted oxygen, as required by mass balance, may exist in the planetary regolith.

Formation and relative ages of maskelynite and carbonate in ALH84001

The morphology and stoichiometry of feldspathic glass in the Martian meteorite ALH84001 indicates it is maskelynite (a diaplectic glass) rather than a flowed glass, although this glass was heterogeneously affected by a tertiary set of processes. An impact event with shock pressures in excess of 31 GPa was needed to convert the original plagioclase (An 36Ab 60Or 4) to maskelynite. Carbonate is intimately associated with the maskelynite, and the carbonate’s radiating crystalline fabric and globular forms suggest it was produced after plagioclase was converted to maskelynite. Textures also suggest carbonate was produced at the expense of maskelynite in a dissolution-precipitation reaction that involved a carbonic fluid. This fluid system is tentatively estimated to have been active for at least a few years at temperatures <300°C, based on dissolution rates of plagioclase in mildly to strongly alkaline hydrothermal systems (which is the only analogue currently available). This reaction does not need to be mitigated by microbial life. Neither are bacteria needed to produce the radiating textures and globular forms of carbonate, which may instead reflect kinetic phenomena associated with crystal nucleation and growth. Because the carbonate was produced at the expense of maskelynite, it is younger than maskelynite, which has previously been shown to have last degassed 3.92 ± 0.04 Ga (Turner et al., 1997). However, the specific age of the carbonate and the carbonic fluid system remains unknown.

Reaction sequence of iron sulfide minerals in bacteria and their use as biomarkers.

Some bacteria form intracellular nanometer-scale crystals of greigite (Fe3S4) that cause the bacteria to be oriented in magnetic fields. Transmission electron microscope observations showed that ferrimagnetic greigite in these bacteria forms from nonmagnetic mackinawite (tetragonal FeS) and possibly from cubic FeS. These precursors apparently transform into greigite by rearrangement of iron atoms over a period of days to weeks. Neither pyrrhotite nor pyrite was found. These results have implications for the interpretation of the presence of pyrrhotite and greigite in the martian meteorite ALH84001.

A search for endogenous amino acids in martian meteorite ALH84001.

Trace amounts of glycine, serine, and alanine were detected in the carbonate component of the martian meteorite ALH84001 by high-performance liquid chromatography. The detected amino acids were not uniformly distributed in the carbonate component and ranged in concentration from 0.1 to 7 parts per million. Although the detected alanine consists primarily of the L enantiomer, low concentrations (<0.1 parts per million) of endogenous D-alanine may be present in the ALH84001 carbonates. The amino acids present in this sample of ALH84001 appear to be terrestrial in origin and similar to those in Allan Hills ice, although the possibility cannot be ruled out that minute amounts of some amino acids such as D-alanine are preserved in the meteorite.

Atomic force microscopy imaging of fragments from the Martian meteorite ALH84001.

A combination of scanning electron microscopy (SEM) and environmental scanning electron microscopy (ESEM) techniques, as well as atomic force microscopy (AFM) methods has been used to study fragments of the Martian meteorite ALH84001. Images of the same areas on the meteorite were obtained prior to and following gold/palladium coating by mapping the surface of the fragment using ESEM coupled with energy-dispersive X-ray analysis. Viewing of the fragments demonstrated the presence of structures, previously described as nanofossils by McKay et al. (Search for past life on Mars--possible relic biogenic activity in martian meteorite ALH84001. Science, 1996, pp. 924-930) of NASA who used SEM imaging of gold-coated meteorite samples. Careful imaging of the fragments revealed that the observed structures were not an artefact introduced by the coating procedure.

Oxygen Isotopic Constraints on the Genesis of Carbonates from Martian Meteorite ALH84001

Ion microprobe oxygen isotopic measurements of a chemically diverse suite of carbonates from Martian meteorite ALH84001 are reported. The δ 18O values are highly variable, ranging from +5.4 to +25.3‰, and are correlated with major element compositions of the carbonate. The earliest-forming (Ca-rich) carbonates have the lowest δ 18O values and the late-forming (Mg-rich) carbonates have the highest δ 18O values. Two models are presented which can explain the isotopic variations. The carbonates could have formed in a water rich environment at relatively low, but highly variable temperatures. In this open-system case the lower limit to the temperature variation is ∼125°C, with fluctuations of over 250°C possible within the constraints of the model. Alternatively, the data can be explained by a closed-system model in which the carbonates precipitated from a limited amount of CO 2-rich fluid. This scenario can reproduce the isotopic variations observed at a range of temperatures, including relatively high temperatures (> 500°C). Thus the oxygen isotopic compositions do not provide unequivocal evidence for formation of the carbonates at low temperature. Although more information is needed in order to distinguish between the models, neither of the implied environments is consistent with biological activity. Thus, we suggest that features associated with the carbonates which have been interpreted to be the result of biological activity were most probably formed by inorganic processes.

Evidence for the extraterrestrial origin of polycyclic aromatic hydrocarbons in the Martian meteorite ALH84001.

Possible sources of terrestrial contamination are considered for the observation of polycyclic aromatic hydrocarbons (PAHs) in the Martian meteorite ALH84001. Contamination is concluded to be negligible.

ArAr chronology of the Martian meteorite ALH84001: Evidence for the timing of the early bombardment of Mars

ALH84001, a cataclastic cumulate orthopyroxenite meteorite from Mars, has been dated by ArAr stepped heating and laser probe methods. Both methods give ages close to 3,900 Ma. The age calculated is dependent on assumptions made about 39Ar recoil effects and on whether significant quantities of 40Ar from the Martian atmosphere are trapped in the meteorite. If, as suggested by xenon and nitrogen isotope studies, Martian atmospheric argon is present, then it must reside predominantly in the K-rich phase maskelynite. Independently determined 129Xe abundances in the maskelynite can be used to place limits on the concentration of the atmospheric 40Ar. These indicate a reduction of around 80 Ma to ages calculated on the assumption that no Martian atmosphere is present. After this correction, the nominal ages obtained are: 3940 ± 50, 3870 ± 80, and 3970 ± 100 Ma. by stepped heating, and 3900 ± 90 Ma by laser probe (1σ statistical errors), giving a weighted mean value of 3,920 Ma. Ambiguities in the interpretation of 39Ar recoil effects and in the contribution of Martian atmospheric 40Ar lead to uncertainties in the ArAr age which are difficult to quantify, but we suggest that the true value lies somewhere between 4,050 and 3,800 Ma. This age probably dates a period of annealing of the meteorite subsequent to the shock event which gave it its cataclastic texture. The experiments provide the first evidence of an event occurring on Mars coincident with the time of the late heavy bombardment of the Moon and may reflect a similar period of bombardment in the Southern Highlands of Mars. Whether the age determined bears any relationship to the time of carbonate deposition in ALH84001 is not known. Such a link depends on whether the temperature associated with the metasomatic activity was sufficient to cause argon loss from the maskelynite and/or whether the metasomatism and metamorphism were linked in time through a common heat source.

No Such Correspondence

In a response to technical comments about a Research Article (1) on the presence of polycyclic aromatic hydrocarbons (PAHs) in the martian meteorite ALH84001, Simon J. Clemett and Richard N. Zare (20 Dec. 1996, p. 2122) stated (p. 2123), “Simoneit and Hites suggest that the PAHs originate from the ‘thermal degradation of (extraterrestrial) biopolymers.’ ” This statement and quotation were not supported by a citation. Later, a correction stated (4 Apr., p. 21) that the response by Clemett and Zare “should not have included (in the last paragraph, p. 2123) reference to unpublished correspondence by Simoneit and Hites… . ”

Petrography and bulk chemistry of Martian orthopyroxenite ALH84001: Implications for the origin of secondary carbonates

New petrologic and bulk geochemical data for the SNC-related (Martian) meteorite ALH84001 suggest a relatively simple igneous history overprinted by complex shock and hydrothermal processes. ALH84001 is an igneous orthopyroxene cumulate containing penetrative shock deformation textures and a few percent secondary extraterrestrial carbonates. Rare earth element (REE) patterns for several splits of the meteorite reveal substantial heterogeneity in REE abundances and significant fractionation of the REES between crushed and uncrushed domains within the meteorite. Complex zoning in carbonates indicates nonequilibrium processes were involved in their formation, suggesting that CO 2-rich fluids of variable composition infiltrated the rock while on Mars. We interpret petrographic textures to be consistent with an inorganic origin for the carbonate involving dissolution-replacement reactions between CO 2-charged fluids and feldspathic glass in the meteorite. Carbonate formation clearly postdated processes that last redistributed the REE in the meteorite.

Petrological evidence for shock melting of carbonates in the martian meteorite ALH84001.

The meteorite ALH84001--a shocked igneous rock of probable martian origin-contains chemically and isotopically heterogeneous carbonate globules, associated with which are organic and inorganic structures that have been interpreted as possible fossil remains of ancient martian biota. A critical assumption underlying this suggestion is that the carbonates formed from low-temperature fluids penetrating the cracks and voids of the host rock. Here we report petrological studies of ALH84001 which investigate the effects of shock on the various mineralogical components of the rock. We find that carbonate, plagioclase and silica were melted and partly redistributed by the same shock event responsible for the intense local crushing of pyroxene in the meteorite. Texture and compositional data show that, during the period of shock decompression, monomineralic melts were injected into pyroxene fractures that were subsequently cooled and resealed within seconds. Our results therefore suggest that the carbonates in ALH84001 could not have formed at low temperatures, but instead crystallized from shock-melted material; this conclusion weakens significantly the arguments that these carbonates could host the fossilized remnants of biogenic activity.

Low-temperature carbonate concretions in the Martian meteorite ALH84001: evidence from stable isotopes and mineralogy.

The martian meteorite ALH84001 contains small, disk-shaped concretions of carbonate with concentric chemical and mineralogical zonation. Oxygen isotope compositions of these concretions, measured by ion microprobe, range from delta18O = +9.5 to +20.5 per thousand. Most of the core of one concretion is homogeneous (16.7 +/- 1.2 per thousand) and over 5 per thousand higher in delta18O than a second concretion. Orthopyroxene that hosts the secondary carbonates is isotopically homogeneous (delta18O = 4.6 +/- 1.2 per thousand). Secondary SiO2 has delta18O = 20.4 per thousand. Carbon isotope ratios measured from the core of one concretion average delta13C = 46 +/- 8 per thousand, consistent with formation on Mars. The isotopic variations and mineral compositions offer no evidence for high temperature (>650 degrees C) carbonate precipitation and suggest non-equilibrium processes at low temperatures (< approximately 300 degrees C).

Stay the course

Reviewing NASA's current plans for Mars exploration, a panel of the National Research Council has advised the space agency to resist the urge to overhaul its program in the wake of the finding of possible life in the Martian meteorite ALH84001. In a report released on December 9, 1996, the Committee on Lunar and Planetary Exploration (COMPLEX) asserted that “it is inappropriate to predicate an important aspect of future Martian studies on the unconfirmed results in a single science paper.”

Cold water thrown on martian life hypothesis

A new report suggests that wormlike objects in the famous martian meteorite ALH84001 are not nanofossils of ancient life, but rather magnetite crystals grown from high-temperature vapor processes. Last August, a team headed by David S. McKay at the National Aeronautics & Space Administration's Johnson Space Center (JSC) in Houston and chemistry professor Richard N. Zare at Stanford University described several features in carbonate globules within an ancient meteorite from Mars that they say collectively point to the possibility that life once existed on the planet. The evidence includes striking scanning electron microscope images of wormlike shapes, which the group theorizes could be microfossils of martian nanobacteria [ Science , 273 , 924 (1996)]. A different group has now been able to characterize the chemical makeup and interior structure of these wormlike objects, or whiskers, using transmission electron microscopy. According to geology professor Harry Y. McSween Jr. at the University of Tenness...