Stephen Berg
HET 618
April 4, 2007
Word Count 2012
Can Abiotic Processes Account for Martian Meteorite ALH84001?
Ever since the announcement that meteorite ALH84001 contained possible evidence of
biologic activity on Mars, this potato-sized rock has become a source of endless
controversy. Found on an Antarctic ice field in 1986, this meteorite was identified as
being of Martian origin by analyzing oxygen gases trapped inside it, whose isotope ratios
differ from Earth’s atmosphere, but closely match those measured by the Viking Mars
landers in 1976 (Gibson, 1997). In 1996, McKay et al, after extensive microanalysis of
the meteorite, published evidence that ALH84001 contained numerous features that they
argued were best explained as formed by some sort of primitive life, perhaps similar to
terrestrial bacteria, and when taken together, seemed difficult to explain in any other way.
Subsequent analysis by other scientists has since found a variety of alternative, nonbiological (“abiotic”) mechanisms to explain the formation of each of McKay’s proposed
bio-markers. However, the processes that might have produced one structure may not
account for others. In this essay, I explore several mechanisms to see if any one scenario
can account for all of McKay’s supposed evidence for Martian Life.
Electron microscope image from ALH84001, showing possible nanofossils (McKay).
Background
ALH84001 is formed primarily of igneous orthopyroxenite (opx), (Mg,Fe)SiO3, mixed
with diverse mineral impurities, with those containing Mg, Fe and Ca being the most
significant. Analysis of various radioactive isotopes has shown that the rock first
crystallized about 4.5 billion years ago (Ga), followed by a shock event (presumably an
impact) at 4.0 Ga. Carbonate globule formation was estimated at 3.6 Ga (McKay, 1996),
although more recent studies place this near to or contemporaneous with the shock event
(Gibson, 2001, Barber, 2003). About 16 million years ago, the rock was ejected into
space, probably by another impact, and then landed in Antarctica about 13,000 years ago
(Gibson, 1997). These events are generally agreed upon, although additional shocks have
been hypothesized in some scenarios,.
With mounting evidence that Mars early history included extensive water with a more
substantial atmosphere and warmer climate, the question naturally arises whether life
formed under these conditions, and if so, might ALH84001 harbor evidence of this life?
McKay (1996) hoped to find such indicators, and found four distinct lines of evidence
that in combination they felt strongly suggested microbe-like life. Briefly they are as
follows:
1. Water appears to have been present in the rock, contributing to the formations
found inside, and being available for possible biological activity.
2. Tiny carbonate globule formations contain a number of features resembling
evidence of terrestrial biological activities. Among them are ovoid shapes that
appear to resemble certain terrestrial microfossils.
3. Crystals of magnetite were observed to resemble chains of magnetite formed by
certain Earth bacteria, ostensibly for the purpose of aiding navigation within the
Earth’s magnetic field. These are also found in close proximity to iron sulfide
particles, a not uncommon biological association, but difficult to explain
abiotically.
4. Complex hydrocarbons called PAH’s (polycyclic aromatic hydrocarbons) are
found in profusion deep within the meteorite. While these are also found in nonMars meteorites, the types found are suggestive of the decay of biological
material.
Critical response was immediate, with questions raised at almost every point. Early
criticism centered on possible contamination as the source of the PAH’s, while the size of
the ovoids appeared to be too small for viable living organisms. The contamination issue
seems to have been put to rest by studies showing that the indigenous Antarctic PAHs not
only have far lower concentrations than those in the meteorite (McKay), but are not even
of the same chemical type (Gibson, 2001). Other researchers have considered the PAH
origin too uncertain to be meaningful, since they could have been produced either with or
without biological activity (Treiman, 2000). The ovoids, with dimensions ranging from
20 to 100 nm in size, bear resemblance to certain terrestrial features thought to be
fossilized nanobacteria (McKay). However, they are considerably smaller than terrestrial
bacteria, and may be too small to contain sufficient chemistry to make life possible
(Gibson, 2001). Although this does not preclude a biological origin, neither does it
present a compelling case in its favor (Treiman, 2000). At present, the ovoids seem also
to have lapsed into ambiguity.
Magnetites
More recent debate has focused on the carbonate globules and the associated magnetite
and sulfide crystals. The globules are profuse, and are found mainly along cracks and
fractures in the rock, where they could have formed either by precipitating from water, or
from biogenic activity. Some are spherical, while most are flattened, probably due to the
restricted space within the fractures. The largest are 10 or more microns in diameter, and
several have been examined carefully with electron microscopes. McKay (1996) found
significance in the structure of the globules they studied, especially the presence of thin,
concentric bands having contrasting mineral contents, whose presence near each other is
difficult to explain. The cores contain alternating Fe and Mg-rich bands, while the outer
edges show three concentric bands, with the first and third rich in Fe and S, while the
intermediate band lacks Fe, containing Mg instead. Magnetite crystals (Fe3O4) were
found throughout the globules, with high concentrations near the rim (Thomas-Keprta,
2005). McKay (1996) suggested that the carbonates, magnetites and sulfur compounds
could all have precipitated inorganically under conditions of moderate temperature and
high pH. However, porous structuring in the carbonates show evidence of later acidic
conditions, which are not mirrored in either the magnetites or pyrrhites. Since these can
both be produced by Earth bacteria, McKay found their juxtaposition to be more
plausibly explained by biotic precipitation. The magnetites vary considerably in size and
shape, with about 25% displaying a crystalline structure remarkably similar to those
produced by terrestrial bacteria. These crystals are very pure, and are not known to occur
abiotically in nature (Thomas-Keprta, 2001). Some of these also form chains similar to
those produced by bacteria (Gibson, 1997), while magnetite clustering suggests formation
by colonies of microbial life (Gibson, 2001).
An extensive study by Barber (2003) found the magnetites to be closely associated with
the numerous voids near the carbonate rim. They argue that biologically produced chains
would not have been maintained once the organisms died, and that chains form easily and
more permanently along ledges on fracture surfaces. They found atoms from the
carbonates had diffused over short distances, indicating that heating to at least 400º C
must have occurred, and conclude that cool formation of the magnetites was not possible
(Barber). Their proposed scenario has the magnetites and pyrrhites forming inside the
carbonates by exsolution from the existing solid mixture as a result of the shock at 4.0
Ga, with neither melting nor dehydration being required. Because the magnetites formed
inside the carbonates, no life could have been involved (Barber). Golden et al in 2003,
and again in 2004 were able to produce magnetites synthetically that they describe as
“indistinguishable” from the ALH84001 magnetites. Both processes required heating
(550º C and 350º C) in a CO2 environment, simulating early Mars. Although significant,
the processes required both gradual heating and cooling, neither of which resembles the
shock approach proposed by Barber, or other realistic scenarios. Thomas-Keprta (2005)
concludes that the way the magnetites are embedded in the opx precludes formation by a
simple precipitation method, but also finds that thermal decomposition alone cannot
explain their origin. While the Barber study casts considerable doubt on the value of the
magnetites as biological markers, it is unclear if either Barber or Golden produce a viable
alternative. The ambiguity is perhaps best stated by Trieman (2000), who questioned
why bacteria living inside a rock would even need magnetites!
Sulfides
The Fe sulfide crystals McKay found in close proximity to magnetites require that the Fe
have been both reduced and oxidized in nearly the same location. The McKay team
found this difficult to understand unless bacteria-like organisms were present. However,
this argument ignores that the sulfides were found in two sizes, only the smaller of which
was proposed by McKay as being biogenic (Gibson, 2001). Conversely, Barber (2003)
does not see a problem with the two crystals forming side by side if the sulfides formed
as a result of shock heating. Golden (2004) was able to precipitate both sulfides and
magnetite, as well as carbonate globules, from solution after heating gradually to 550º C
in a CO2 atmosphere, followed by rapid cooling. Other experiments also produced
carbonate globules as well, although fracturing patterns indicate that the carbonates had
to have formed prior to the magnetites (Trieman, 2002). I join Gibson (2001) in finding
the sulfide evidence inconclusive.
Carbonate Globules
Carbonate globules from ALH84001. Globules are crushed inside feldspathic glass and
opx (Treiman, 2005). Sections of the concentric rings are clearly visible.
The carbonate globules, tiny disks ranging from 20 to 250 microns in diameter, are found
covering the walls of fractures inside the meteorite. Gibson (1997) found that they
appeared to have precipitated out of a fluid saturated with CO2. Thomas-Keprta (2005)
noted their well-formed internal structure of chemically differentiated rings, radiating
concentrically outward from the center, similar to the structure reported by McKay.
Controversy was already reported by McKay in their original paper with two quite
different temperatures of formation indicated. Electron microprobe suggested the
carbonates formed at about 700º C, while oxygen isotope data indicated a temperature
between 0 and 80º C. Biogenic activity would presumably require the much cooler
temperature range, and McKay notes similarly structured formations are found
precipitated by Earth bacteria in fresh water. Arguments have also been made for other
temperatures. Kirschvink (1997) used magnetic orientation to derive a temperature of
about 150º C. Others have argued for temperatures near 200º C, or above 650º C
(Barber). Barber concludes the globules formed as a result of sudden and transient heat
generated by an impact event, while Treiman (2005) believes them to be better explained
as deposits from an aqueous solution into cavities within the rock. Barber believes
subsequent shocks were of much lower intensity, producing temperatures of less than
400º C, but assumes the magnetites formed with the carbonates. Gibson (2001) argues
against high temperature shock formation, stating that CaO and MgO should also have
formed, and they are absent. Clearly no conclusions can be reached until the formation
temperature is more precisely known.
Conclusion
With the obvious confusion concerning ALH84001, it is difficult to know if a single
abiotic scenario can account for all of this meteorites supposed biological markers.
Golden has duplicated the most difficult features of magnetites, sulfides and carbonates,
but at the expense of conditions that do not match the sudden heating of an impact.
Barber offers the most comprehensive analysis, but requires the magnetites to form with
the carbonates in a single shock, despite fracturing evidence that the magnetites were
formed later (Treiman, 2002). Treiman (2005) offers a quite different scenario based on
similar terrestrial rocks found in Norway. Early fracturing opened these rocks to water,
which dissolved exposed olivine crystals, and replaced them with carbonates. Small
olivines were less likely to be exposed by the fractures and have survived, while later
shocks filled in the remaining voids. This low temperature version requires no biological
assistance, although it is not clear if it can account for the magnetites and sulfides.
Extensive studies of ALH84001 by numerous scientists have cast strong doubt on the
origin of McKay’s proposed biogenic markers. While it now seems likely that most or all
of these features are not biological in origin, we are still awaiting a satisfactory scenario
capable of explaining them all. We must also be more certain of the temperatures
involved, and whether or not water played a part. Until these issues are resolved,
ALH84001 will continue to tease us with the possibility that it gives testimony to early
Martian life. Despite unprecedented analysis, this small meteorite has kept its secrets
well. Like Mona Lisa’s smile, we wonder what da Vinci is hiding from us. Is there a tale
of Martian life? Or is there no secret at all? We do not yet know the answer.
References:
Barber, D. & Scott, R., 2003, Transmission electron microscopy of minerals in the
martian meteorite Allan Hills 84001, Meteoritics & Planetary Science 38, pp. 831, 841-3,
845, 841-3, 845, 832-3, 846, 844.
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pp. 2-3, 4, 6, 4.
Gibson, E., et al, 2001, Life on Mars: evaluation of the evidence within Martian
meteorites ALH84001, Nakhla, and Shergotty, Precambrian Research 106, pp. 17, 19, 22,
28, 28 & 19, 19, 18.
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(2005).
Acknowledgements
The NASA ADS retrieval system was used to locate and retrieve publications for use in
this paper.