HET618 Essay – Chris Blaxall
Evidence of past life on Mars in ALH84001
“Extraordinary claims require extraordinary proof!” - Carl Sagan
Introduction
A 1.9kg potato shaped rock was found in Alan Hills,
Antarctica in 1984 1. Despite sitting on a shelf untouched for
10 years, this 4.5 billion year old meteorite 2, now referred to
as ALH84001, has created an ongoing debate over more than
a decade about whether it provides evidence that life may
have once existed on Mars. This paper examines some of the
findings of the research that followed, providing support and
counter argument about whether or not the composition of
ALH84001 provides conclusive evidence that life did exist
on Mars in the past.
Diagram 1 - Meteorite ALH84001
Source: http://exobio.ucsd.edu/Space_Sciences/lifefrom.htm
The key evidence
The origin of the meteorite is the starting point. It appears that there is general agreement,
although not unanimous, that ALH84001 did originate from Mars, through a comparison of
the gases trapped within the meteorite with data captured about the Mars atmosphere by the
Viking Landers in the 1970s 3.
Assuming this Mars origin, the original research in 1996 by McKay et al 4 identified four key
measures that provided strong circumstantial evidence that life once existed on Mars:

the presence of carbonate materials 4;

the mineral magnetite being present in ALH84001 4;

complex organic molecules, Polycyclic Aromatic Hydrocarbons (PAHs) in the
meteorite 4; and

rod-like fossils located in the meteorite 4.
These four measures are investigated and discussed in detail below.
Carbonate materials
Carbonate materials were found to be present in fractures in ALH84001 and in particular iron,
calcium, sulphur, magnesium and phosphorous carbonates were identified 4. McKay et al
argued that these carbonates had been formed when liquid water flowed through the rock4
some 4 billion years ago on Mars 2. Whilst not confirming the actual presence of life, the
presence of water satisfies one of the key ingredients that are required for life to form and
survive.
It is argued, however, that such carbonate deposits could also have been formed deep inside
Mars at very high temperatures, without the need for water to be present, just as such
carbonates have been found on the Earth 2. Much research has been carried out to determine
at what temperatures these carbonates formed, with both high temperatures (>600°C) and low
temperatures (<100°C) being proposed 5. Warren discounts the high temperature theory 6,
putting forward a flood water evaporation theory whereby the carbonates would have formed
in a warm, wet environment as the oceans on Mars evaporated. This explanation does not
prove that the carbonate materials were biogenic, but it does support the theory that Mars was
once covered in seas of water in a warm environment that could have been supportive of life.
A further theory about the carbonates was that they had been formed by shock vaporisation of
material, without water being present, but this seems to have been discounted by more recent
research of the pressures that the carbonate could withstand 5.
On balance it appears that the evidence is likely to support the fact that water was present on
the surface of Mars some 4 billion years ago and thus may have provided an appropriate
environment for life to develop there, but not direct evidence that life has existed on Mars.
Magnetite
McKay et al identified the presence of magnetite (a form of iron oxide) in ALH84001 and
noted that grains around the carbonate globules bore a strong resemblance to the magnetite
that is produced by magnetotactic bacteria 4. These bacteria are found on the Earth and
through the production of tiny amounts of iron magnetic particles use these as a form of
navigational aid through interaction with the Earth’s magnetic field 7. Further research by
Thomas-Keprta et al has indicated three separate forms of magnetite in the meteorite namely:

about 25% likely to have been produced by the terrestrial magnetotactic bacteria as
described above, as their shape is indistinguishable from magnetites produced by the
MV-1 bacterial strain 8, 5, 9 ;

<10% being ‘needle shaped whiskers’ that are more likely to have been formed by high
temperature impact shock 5; and

The remainder being irregular shaped magnetites that could have been formed by
either iron-reducing bacteria or through abiotic processes 5.
Further support for biogenic processes comes from the work done by Kirschvink and Vali
whereby they indicate that the samples from ALH84001 meet five out of the six Darwinian
characteristics being crystal size and shape, crystal structural perfection, elongation of
magnetite crystal structures, anisotropic growth of certain crystal faces and chemical purity
with only the presence of magnetite chains missing 10. Friedmann et al go further in
identifying magnetite chains and conclude, therefore, that the magnetites are likely to have
been of biological origin and comment that, whilst it is unlikely that the bacteria were ever
alive in ALH84001, they may have been deposited into fissures in the rock while on Mars11.
These positive findings are challenged by Buseck et al who conclude that none of the crystals
found in ALH84001 are ‘identical enough’ to those found on Earth for there to be conclusive
proof that they were formed by the MV-1 bacteria or indeed any other bacteria 12. In addition,
work by John Bradley identifies cracks and defects in the magnetite crystal structures that are
likely to have been caused by extremely high temperatures of hundreds of degrees at which
life could not survive 12.
Whilst there appears to be strong support for some of the magnetite having been produced by
bacteria on Mars, the proof remains inconclusive and thus does not provide absolute support
for past life on Mars.
Polycyclic Aromatic Hydrocarbons (PAHs)
PAHs are molecules that form through the breakdown of biological material such as the
decomposition or burning of plant life and are found in plentiful supply on the Earth 2. They
occur in coal and oil and can be released into the atmosphere by, for example, car exhausts 2.
It should be recognised, however, that PAHs can also form by other than biological means
and have been found in interstellar material 2.
The initial claim by McKay et al was that the PAHs found in the meteorite were the result of
the decay and fossilisation of bacteria that occurred on Mars 4. This conclusion was reached
after firstly identifying PAHs as being present and showing that contamination from the local
environment on the Earth could be ruled out 4. The PAHs identified were a few specific
strains rather than a much larger number of different strains that would be expected if they
had been formed on the Earth 4. A further argument against contamination, led by Clemett et
al, was that micrometeorites also found in the Antarctic were each found to have different
carbon abundances. If contamination from the Earth was occurring then the expected result
would be for all these objects to have similar concentrations and types of PAHs 5. Clemett et
al also identified the concentrations of PAHs within ALH84001 and showed that there was a
greater density of PAHs in the interior rather than at the edges 5 indicating less likelihood of
Earth contamination being the reason for the PAHs being present, as it would be more likely
that the concentrations would be reversed if that were the case.
Luann Becker et al challenged this argument by showing that PAHs found in a similar but
much younger meteorite from Mars were similar to those found in ALH84001 2, 13. The age
of the younger meteorite would have meant that there would have been little chance of
conditions existing where water was present on the surface and hence would lead to a
conclusion that the PAHs were formed other than by biological means or that there had been
contamination of both meteorites in the Antarctic before testing began. Contamination of the
meteorite is also supported by measurements of 14C in the meteorite which appears to confirm
that most of the organic carbon in the meteorite is terrestrial 14.
Even assuming no contamination having occurred whilst the meteorite was on the Earth, there
is still the question as to whether the PAHs were formed by life on Mars as originally
postulated. The PAHs found in ALH84001 are similar to those found in micrometeorites
that have been formed by abiogenic processes and it would seem more likely, therefore, that
the PAHs in ALH84001 were similarly produced other than by life forms on Mars 14.
It appears, therefore, that whilst the existence of PAHs tends to suggest the potential existence
of life on Mars at the time the meteorite was forming, there remains significant doubt about
whether the PAHs were formed by organic decomposition on Mars or whether there was
some form of contamination as the meteorite sat in the Antarctic ice for 13,000 years before
being found.
Nanofossils
In the original research on ALH84001 by McKay et al,
they identified features that appeared to be fossils of a
size of 20 to 100 nm (often referred to as nanofossils
due to the nanometre scale of their size) 4. These
fossils were considered to be very similar to fossils of
terrestrial nanobacteria that had been found on the
Earth in Italy 4.
Diagram 2 - Comparison of nanofossils from
ALH84001 and Columbia River Basalt 5
Further support to this hypothesis was provided by
Thomas Keprta et al 15 when strong similarities in both
size and shape were found between fossilised bacteria
found in Columbia River Basalt and those in
ALH84001. Diagram 2 illustrates this point with the
ALH84001 nanofossils in the left column and the
Columbia River Basalt in the right column.
This
research helped to counter an argument that the sizes
of the fossils identified initially were too small to be
formed from a life form. This area is still not fully
understood and the nanofossils could be inorganic and
not formed from bacteria 5.
Further arguments that the structures identified are not biological fossils include an idea that
the structures have been created by gold coatings or that they are actually mineral spherical
structures that look like bacteria fossils 5. Debate also still continues about whether
contamination of the meteorite, either during the 13,000 years it was in the ice in Antarctica
or during the laboratory investigations, has provided some of the features and structures that
are now being considered as emanating from Mars 3. Research is continuing in this area and
there appears to be no conclusive evidence at this stage that the identified structures are
biogenic fossils.
Conclusion
After more than a decade of research and hundreds of published papers on the subject, there
appears to be little argument that ALH84001 did originate from Mars but debate still
continues as to whether it provides evidence that life did once exist on Mars.
The original paper by McKay et al in 1996, provided circumstantial evidence that conditions
may have been appropriate for life to form, but did not provide conclusive evidence that life
would necessarily have needed to be present for all the circumstantial evidence to remain true.
Subsequent research has provided both support and challenge to each of the four key strands
of the original argument as outlined above and provides little further clarity on confirming
that life did exist on Mars in the past.
The presence of carbonate materials in ALH84001 appears to indicate the likelihood that
water could have been present on Mars some 4 billion years ago, but this would provide only
evidence that conditions may have been appropriate for life possibly to form. The presence of
magnetite and PAHs also provides some support that life could have been present but the
proof is not conclusive. The presence of nanofossils continues to be an area of debate and
once again there is no absolute proof that what has been identified is conclusive evidence of
life having existed on Mars in the past.
It would appear that this debate is likely to continue for many decades to come until we are in
a position to explore and investigate Mars locally rather than relying on a small sample of
random rocks that have found their way to the Earth.
References
1.
What is ALH 84001?
www.lpi.usra.edu/lpi/meteorites/The_Meteorite.html
2.
A Martian Chronicle, Andrew H Knoll, the Sciences, July/August 1998, 20 – 26
3.
It’s dead Jim. But was it ever alive?
www.astrobiology.com/adastra/its.dead.jim.html
4.
Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001
McKay, David S; Gibson, Everett K Jr; Thomas-Keprta, Kathie L; Vali, Hojatoll et al, Science, Aug 16,
1996, 273, 5277
5.
Life on Mars: evaluation of the evidence within Martian meteorites ALH84001, Nakhla and Shergotty
Gibson, E K Jr; McKay, D S; Thomas-Keprta, K L; Wentworth, S J et al; Precambrian Research 106 (2001)
15-34
http://ares.jsc.nasa.gov/astrobiology/biomarkers/pubs/PrecambrianResearch_106_15-34_2001.pdf
6.
Petrological evidence for low-temperature possibly flood evaporitic origin of carbonates in the ALH84001
meteorite
http://www.lpi.usra.edu/meetings/metsoc98/pdf/5204.pdf
7.
Magnetic microbes
http://commtechlab.msu.edu/sites/dlc-me/curious/caOc96SC.html
8.
Truncated hexa-octahedral magnetite crystals in ALH84001: Presumptive biosignatures
Thomas-Keprta, Kathie L; Clemett, Simon J; Bazylinkski, Dennis A; Kirschvink, Joseph L et al; PNAS,
February 27, 2001, 2164-2169
http://www.pnas.org/cgi/reprint/98/5/2164
9.
Magnetite from ALH84001 carbonate globules: evidence of biogenic signatures? Thomas-Keprta, K.L.,
Bazylinski, D.A., Golden, D.C., Wentworth, S.J., Gibson, E.K., Jr., McKay, D.S., 1998., Lunar Planet. Sci.,
XXIX, 1433-1434
http://www.gps.caltech.edu/users/jkirschvink/pdfs/magnetofossils.pdf
10. Criteria for the identification of bacterial magnetofossils on Earth or Mars
http://www.lpi.usra.edu/meetings/LPSC99/pdf/1681.pdf
11. Chains of magnetite crystals in the meteorite ALH84001: Evidence of biological origin
http://www.pnas.org/cgi/reprint/98/5/2176.pdf
12. Magnetite morphology and life on Mars
Buseck, Peter R; Dunin-Borkowski, Rafal E; Devouard, Bertrand; Frankel, Richard B et al, PNAS 2001; 98;
13490 – 13495
13. The origin of organic matter in the Martian meteorite ALH84001
Becker, Luann; Popp, Brian; Rust, Terri; Bada, Jeffrey L; Earth and Planetary Science Letters 167 (1999)
71-79
14. Biomarkers in ALH84001
http://mars.jpl.nasa.gov/mgs/sci/fifthconf99/6019.pdf
15. Thomas-Keprta K.L., McKay D.S., Wentworth S.J., Stevens T.O., Taunton A.E., Allen C.A., Coleman A.,
Gibson E.K.Jr., and Romanek C.S. (1998) Bacterial mineralization patterns in basaltic aquifers: Implications
for possible life in martian meteorite ALH84001. Geology 26, 1031-1035