Evidence that martian meteorite amino acids are contaminants

As the subunits that compose protein polymers in living systems, the detection of certain amino acids in a material is often interpreted as indicating a possible biological origin. The meteorite ALH84001, along with a number of other discovered meteorites, has a composition that suggests it was apparently ejected from the surface of Mars, and during the past year it has been proposed that microanalysis of this meteorite indicates the possible presence of bio-organics and biogenic fossils. This proposal, however, has met with considerable controversy, and the controversy is still in full force. ... ... Bada et al (4 authors at 3 installations, US) now report that the amino acids present in a sample of the ALH84001 meteorite appear to be terrestrial in origin and similar to those found in the ice where the meteorite was discovered, although the possibility remains that minute amounts of endogenous amino acids are preserved in the meteorite. The authors suggest that radiocarbon studies (cf. contiguous report: Jull et al, Science 279:366 1998), coupled with their own amino acid results, indicate that major and minor organic constituents in the Martian meteorites are contaminants.

QY:Jeffrey L. Bada jbada@ucsd.edu
Science 16 Jan 98

Related Background:
In 1984, a 1.9 kilogram meteorite the size of a potato (desig- nated ALH84001) was found in Antarctica, and because of its chemical composition the consensus is that this meteorite (and a dozen similar meteorites) originated from the planet Mars. The basis for the consensus is the detailed quantitative correspond- ence of the trapped gases in the meteorites to Martian atmosph- eric gases, and the specific distributions of oxygen isotopes. In 1996 a group of researchers, McKay et al (National Aeronautics and Space Administration Johnson Space Center, US; Stanford University, US) reported they had concluded that unusual charact- eristics of the meteorite ALH84001 can be most reasonably interpreted as vestiges of ancient Martian bacterial life. In particular, the authors noted the presence of tubules 20 to 40 nanometers in diameter (called by some "nannobacteria"), and they proposed these structures were fossilized bacteria or parts of microorganisms. The report provoked considerable controversy when it appeared, and the controversy continues, with many biologists objecting to the interpretation of the rock data, and partic- ularly to the idea of "bacteria" 20 to 40 nanometers in diameter. Now Gibson et al (National Aeronautics and Space Administration Houston, US; University of Georgia, US), this group including some of the authors of the 1996 report, in a review of the evidence for relic life on Mars, consider the ALH84001 meteorite not only the strongest evidence for Martian relic life, but also for the possibility of present Martian microbial life. The authors are hopeful that in 2005 a "sample return" mission will be launched to robotically collect Martian rocks and soil and return them to Earth.

QY: Everett K. Gibson
(Scientific American December 1997)

The term "nanofossils" (originally spelled "nannofossils" by the group that introduced the term) refers to elongated microscopic forms found in the Martian meteorite ALH84001. Several groups in the space and geology communities have proposed these forms are fossilized bacteria, but most biologists have rejected the idea on the basis that the forms are too small to be bacteria and should not be classified as such. Bradley et al (3 installations, US) now report that new analysis of material from the ALH84001 meteorite indicates the majority of the elongated microscopic forms can be resolved as either emergent substrate layers or magnetite whiskers, rather than biogenic nanofossils. Their report is followed by a response from McKay et al (3 install- ations, US CA), some of the original proponents of the nanofossil idea, and in their response McKay et al say the artifact possibilities mentioned by Bradley et al are already known to them, but are not related to their own observations. They add that living bacteria as small as 70 nanometers in diameter have been observed in mammalian blood, and that soil bacteria as small as 80 nanometers have also been observed. The references for these bacterial forms are one unpublished paper and two recently published papers in Proc. Soc. Photo-Opt. Instrum. Eng. 3111:420,429 (1997). It is evident that the nanofossil contro- versy has not yet been resolved.

QY: J. P. Bradley, Georgia Inst. Technol. 404-894-2000; David S. McKay
(Nature 4 Dec 97)

Apart from their heuristic significance, scientific controversies can be either amusing or irritating. In recent months, a controversy between some geologists and many biologists has developed, and it is apparently irritating the biologists. The issue concerns the minimum possible dimensions of bacteria. The geologists are led by Robert L. Folk (University of Texas, Austin TX US), and they have proposed that certain microscopic entities found in the Martian meteorite ALH84001 are fossils of what they term "nannobacteria" (their own unique spelling of the prefix nano-), which they say are similar to those found in Earth travertine and limestone rocks, and which have dimensions of 30 to 50 nanometers. This has caused a furor among biologists, whose understanding of bacteria and life forms in general is that the smallest dimensions possible for a life form with a bounding plasma membrane is about 200 nanometers (see Focus Report in this issue). In fact no membrane-bound bacterium with dimensions less than 340 nanometers has ever been identified, and one can make simple calculations that a 50 nanometer bacterium would not have enough internal volume to sustain its chemistry. Folk published papers on the subject in several geological journals in 1996, starting the debate, and in the Letters section of the 20 June 1997 issue of Science the debate continues, and this week it is being reported in the popular media as a "debate about life on earth". What evidently irritates biologists is the apparent misunderstanding by these geologists of experimental methods in biology. Characterizations of "living" vs. "non-living" by biologists are made on the basis of experimental laboratory replicability of an organism, and not on the basis of the visible structure of an entity. Which means the geologists involved need to attempt to culture their Earth-rock entities, and which means decisions that the Martian meteorite's so-called "nannobacteria fossils" are actually such will require demonstration of cultured entities with those dimensions. Biologists are not unwilling to admit the existence of new species of life forms, of which they have already recognized several million entities, but they argue that one does not classify pieces of rock as a life form on the basis of structure alone.

(New York Times 29 Jul 1997)

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