NEEP602 Course Notes (Fall 1997)
Resources from Space
Lecture #17: You Old Martian Fossil You!
Title: The Martian Meteorite ALH8001 Saga
Notes:
General Information:
The work of McKay, et al., (1996) on Martian Meteorite ALH8001 from Antarctica suggested the presence of evidence of past life on Mars. This investigation has excited researchers through out the world to add to data and comment on this particular meteorite as well as others of the SNC suite
The principal arguments (see McSween, 1997) that, in aggregate, the evidence supports the conclusion that these are fossils are as follows:
Presence of a carbonate host inbreccia zones that cut older minerals
Figure: ALH8001 in
thin section (NASA Life on Mars #21)Presence (~1 ppm) of fused hydrocarbon rings (C14-C22) or polycyclic aromatic hydrocarbons (PAHs) which on Earth are present as product of organic decay (SNC EETA79001 also has PAHs at this level)
Figure: PAH structures
(NASA Life on Mars #25)Presence of magnetite grains morphologically similar to those produced by terrestrial magnetotactic bacteria
Coexistence of magnetite grains with the sulfides pyrrhotite and greigite that might also be biogenic
Presence of small "ovoid and elongated forms," 50-200 nanometers, that resemble terrestrial nanobacteria
Figure: E. coli form
(NASA Life on Mars #27)Lacy networks structures in acid etched cargbonates and pyroxenes that resemble biogenic polymers or biofilms (McKay, et al., 1997)
Isotopic evidence that at least some of the carbonates formed at temperatures below 300oC (Valley, et al., 1997, and Kirschvink, et al., 1997)
Arguments against this conclusion are as follows:
Inorganic explanations are equally plausible (Anders, 1996)
PAHs are not definitive also form by abiotic processes as in combustion of fossil fuels and are common in chondritic meteorites
PAHs observed are similar to those found in Antarctic ice and could be contamination by melt water (Becker, et al., 1997)
Carbon isotope ratios in the PAHs are indistinguishable from terrestrial ratios
Highly diverse magnetite morphologies argue against a biogenic origin which is usually species specific (Bradley, et al 1996), but such diversity can occur if biogenic magnetite forms outside the cell (Kirchvink, 1997, reported in Science, 276, p30)
Sulfur isotope ratios do not support a biogenic origin (Shearer et al., 1996)
Proposed fossil nanobacteria, roughly 1/500 to 1/000 the size of a terrestrial baterium and their fossils, are too small to carry on required metabolic and genetic functions (Morowitz, 1996) but might have been struck prior to encapsulation.
Proposed nanofossils are whiskers of magnetite similar in size and shape (Bradley, et al., 1996)
Carbonate chemistry and phase relations suggest formation above 650oC (Mittlefehdt, 1994, and Harvey and McSween, 1996) but McKay and Lofgren (1997, and Valley, et al., 1997 argue that zonal deposition in the carbonates suggests disequilibrium precipitation.
It has been suggested that oblique impacts with vapor entraining might be required for ejection of meteorites from Mars.
Summary statement on Resources from Mars:
Although self-sufficiency is assured, and some exports to deep space customers are likely, there are no known resources, standing alone, that future Martians could export to the Earth at a net profit.
It should be noted, however, that this is what we said about the Moon until 1985 and the realization that 3He was in sufficient concentrations in the lunar regolith to be of interest as a fusion fuel on Earth.
It should also be noted that potential, high value resources from Mars, if found in significant concentrations, could be of value to Earth if produced as a consequence of other activities financed for other reasons.
References:
Carr, M. 1996, Water on Mars, Oxford University Press, New York, 249p.
Horowitz, N.H., 1998, The Biological Question of Mars, in D.B. Reiber, editor, The NASA Mars Conference, AAS Science and Technology Series, v 71, 177185.
Levin, G.V., 1988, A Reappraisal of Life on Mars, in D.B. Reiber, editor, The NASA Mars Conference, AAS Science and Technology Series, v 71, 187-208.
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