Mars Exploration Program & Sample Return Missions
On February 2-5, 1999, the Centre National dEtudes Spatiales (CNES) hosted a conference on Mars Sample Return missions and cooperation between the U.S., France, and other international partners. The conference was held at the Carré des Sciences, Paris, France.
A few talks and posters emphasized the Allan Hills 84001 meteorite and its putative traces of ancient martian life. Other general topics included current architectures of the NASA and ESA Mars programs, instruments in spacecraft and on Earth that would be useful for understanding life on Mars, a review of recent results from current spacecraft missions and Earth-based studies, and other planetary missions under consideration.
Below are summaries of the abstracts most relevant to ALH84001; the abstracts are available only in the program volume. By Allan H. Treiman, Lunar and Planetary Institute.
Gillet Ph., Barrat J.A., Blichert-Toft J., Fiquet G., Guyot F., Jambon A., Lecuer V.C., Sheppart D., Malavergne Martinez I. From chemical and mineralogical models to geophysical tests of the martian interior.
The authors continue to investigate carbonate-rich weathering products in the Tatahouine meteorite. After 63 years of Earth weathering in the Sahara, fragments of that meteorite contained calcite-rich discs along fractures, and the discs were decorated with elongated objects, 0.1 micrometer long, which the authors think may be bacteria (Barrat et al., 1998, 1999). The authors have now grown bacteria from the soil around the Tatahouine meteorite find site, and some of them deposit crystals of calcite as they grow. The live bacteria are ~10 times as long as the possible bacteria found in the meteorite (i.e., 1000 times the volume), and the authors suggest that the smaller objects are stressed or starved versions of the bacteria they cultured.
Maurette M., Hofmann B., Farmer J., Matrajt G., and Mustin C. New types of biogenic structures in mineral and rocks from non-sedimentary environments: future application to the search for ancient life forms in martian rocks.
The authors have found that biological organisms commonly cause their mineral hosts to be partially dissolved, i.e. etched. This biological etching yields characteristic patterns on mineral surfaces, quite distinct from the patterns produced by non-biological etching. Biological etch patterns can remain long after the organisms (and their organic materials) have been removed. These biogenic etch patterns are potential biomarkers for extraterrestrial life. Etch patterns characteristic of biological organisms have not been detected in ALH84001.
McKay D.S., Gibson E.K.Jr., Thomas-Keprta K.L., Romanek C., Allen C.C., Westall F., and Steele A. The Search for Life in Martian Meteorite ALH84001: A Status Report.
The controversial idea that ALH84001 contains remnants and traces of ancient martian life has invigorated studies of Mars, and inspired significant new studies of martian meteorites and of Earth life. ALH84001 experienced a complex geologic history. Its carbonate globules, hosts to the reported traces of martian life, formed at about 3.9Ga. Most available data suggest that they formed from water-rich fluids at temperatures amenable to life as we know it. Possible microfossils reported from ALH84001 are an order of magnitude smaller than known viable bacteria, but new results may suggest they could be appendages of bacteria or could be stressed (dehydrated) bacteria. A significant fraction of the magnetite grains in the carbonate globules are like known biogenic magnetites in their sizes, shapes, structures, and compositions. Most of the organic matter in ALH84001is terrestrial, but a significant fraction is martian. New images of the Nakhla meteorite, another martian rock, showed rounded and globular structures associated with the meteorites clay-rich veinlets. These rounded structures are interpreted as mineralized bacteria; some are clearly older than the clay, which formed on Mars.
Stephan T., Jessberger E.K., Rost D., and Heiss C.H. TOF-SIMS analyses of martian rocks.
The authors are continuing to analyse organic material in ALH84001 using "time-of-flight secondary ion mass spectrometry," TOF-SIMS. As reported earlier, their analyses of ALH84001 showed relatively uniform distributions of polycyclic aromatic hydrocarbon (PAH) molecules. PAHs are present even in the cores of pyroxene grains, and they may have entered the pyroxenes along microfractures. The carbonate minerals in ALH84001 contain considerably less PAH material that does anything else. These results suggest that PAHs in ALH84001 are not concentrated in (and near) the carbonate globules, contradicting the work of McKay et al. (1996) and Clemett et al. (1998).
Klossa B., Lorin J.-C., and McKay C.P. SIMS imaging and analyses of carbonates in the SNC meteorite ALH 84001.
The authors have begun research. Stay tuned. [I did not see their poster for results of their work.]
Westall F., McKay D.S., Gibson E.K.Jr., DeWit M.J., Dann J., Gernke D., DeRonde C.E.J. Terrestrial microfossils as analogues for possible fossil martian life.
Recognition of microfossils in martian rocks will require a clear understanding of microfossils in terrestrial rocks. The authors have studied morphological fossils (preserved shapes) in laminated cherts from the ~3.4 Ga Barberton Greenstone belt, South Africa. These fossils represent bacterial mats that were silicified soon after they grew. Macroscopically, finely laminar and stromatolitic (humpy) textures in the cherts suggest biological structures. Microscopic examination shows smooth films coating bedding surfaces (biofilms?) and bacteriomorphic structures (i.e. mineralized bacteria). Bedding plane surfaces are coated discontinuous layers of wavy, ropy, twisted, and/or interwoven material. These are interpreted as representing extracellular polysaccaride slime from bacteria, i.e. biofilm. Trace element composition may also suggest the presence of biofilms, as they commonly permit concentration of heavy metals. Spherical and rod-shaped structures in the chert, 0.6 4 micro-meters long, are interpreted as mineralized bacteria based on their: size and shape, cellular complexity, colonial and consortial structures, association with biofilm, and carbon isotope compositions.
Barrat J.A., Gillet Ph., Lecuyer C., Sheppart S.M.F., and Lesourd M. (1998) Formation of carbonates in the Tatahouine meteorite. Science 280, 412-414.
Barratt J.A., Gillet Ph., Lesourd M., Blichert-Toft J., and Poupeau G.R. (1999) The Tatahouine diogenite: Mineralogical and chemical effects of sixty-three years of terrestrial residence. Meteorite. & Planet. Sci. 34, 91-97.
Clemett S.J., Dulay M.T., Gilette J.S., Chillier X.D.F., Mahajan T.B., and Zare R.N. (1998) Evidence for the extraterrestrial origin of polycyclic aromatic hydrocarbons (PAHs) in the martian meteorite ALH 84001. Faraday Discussions (Royal Soc. Chem.) 109, 417-436.
McKay D.S., Gibson E.K.Jr., Thomas-Keprta K.L., Vali H., Romanek C.S., Clemett S.J., Chillier X.D.F., Maechling C.R., and Zare R.N. (1996) Search for past life on Mars: Possible relic biogenic activity in martian meteorite ALH 84001. Science 273, 924-930.