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A new research paper compares the carbonates and clays of Nili Fossae to the archean rocks of Warrawoona (Western Australia) associated with stromatolites fossiles.
Research lead by Adrian J. Brown (SETI Institute, 515 N. Whisman Rd, Mountain View, CA 94043, USA), published in June 2010 (copyright Elsevier). Please cite this article as: Brown, A.J., et al., Hydrothermal formation of Clay-Carbonate alteration assemblages in the Nili Fossae region of Mars, Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.06.018 Synthesis and comment by Pierre Brisson. NB: This synthesis is an interpretation of the quoted scientific paper (hereafter “the paper”). Please read it directly in case of need or specific interest. Data collected by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard NASA Mars Reconnaissance Orbiter (MRO) led to the identification of magnesium carbonate (i.e. “magnesite”) in the Nili Fossae area (fracture system; 22°N, within Syrtis Major, on the rim of Isidis Planitia.
The ancient grounds of Nili Fossae (Noachian / Phyllosian) display a remarkably large variety of exposed rocks (clays, rich in iron and magnesium, olivines, carbonates).
HiRISE Image PSP_ 010206_1976 Credit NASA. The carbonate outcrops are circled in red.
There are four potential formation scenarios for the carbonate-bearing unit – 1.) groundwater percolating through fractures altering olivine to Mg-carbonate at slightly elevated temperatures, 2.) olivine-rich material, heated by impact or volcanic processes, was deposited on top of a water-bearing phyllosilicate rich unit and initiated hydrothermal alteration along the contact, 3.) olivine-rich rocks were weathered to carbonate at surface (cold) temperatures in a manner similar to olivine weathering of meteorites in Antarctica, and 4.) the carbonate precipitated from shallow ephemeral lakes. The authors of the paper favour the above hypothesis (2). They propose that the phyllosilicate (see below “spectral interpretation”) and overlying carbonate bearing unit, were formed at the same time by a single hydrothermal event. In this scenario, the phyllosilicate (argillic) and carbonate (propylitic i.e. resulting of the hydrolysis of mafic materials) zones reflect different temperature zones achieved during the hydrothermal alteration. The Nili Fossae region contains large amounts of olivine (rocks coming from the mantle of the planet) apparently included in volcanic basalt. Olivine is precisely a mafic material (mafic rocks are igneous rocks rich in heavy elements, “ma”gnesium + “f”erum). In order for the carbonate bearing units to form the observed Mgcarbonate unit and stratigraphically lower Mg-phyllosilicate (talc)-bearing unit, the authors propose that the carbonate-bearing units have undergone alteration in a circum-neutral (6-8) pH environment, similar to those of the mafic/ultramafic Warrawoona Group in Western Australia. The area is well known for its archean fossils (-3.5 billion years). The so-called “North Pole Dome” zone in the East Pilbara district where the mafic and ultramafic rocky outcrops of the Warrawoona group have been found, were scrutinized from an airplane with a spectrometer. In this “group”, there are large areas where weathering rinds do not obscure inherent mineralogy from the air, making it an excellent analogue to Martian rocks observed from a Martian orbit. The rocks of the Warrawoona Group provide an excellent example of low-grade metamorphism. Their age is spanning a period going from -3.515 till -3.426 billion years. They come from komatiitic lavas (lavas coming from the mantle of the planet and containing high levels of magnesium). As they get younger in age, they get progressively less mafic. They display layers of talc and carbonate resulting from a hydrothermal alteration at that early period. The North Pole Dome also displays some of the oldest evidence of life on Earth in the form of stromatolites. Microfossils were likely to have formed in a volcanic plateau setting enjoying an abundant hydrothermal activity. Carbonates (X-CO3 minerals) are expected to form from basalt in an aqueous alteration environment under a CO2 rich atmosphere. Because of this (evidence of past liquid surface water and of a rich CO2 atmosphere), they have been searched for extensively on Mars using remote sensing methods in order to know whether Mars had known conditions similar to Earth in its early age. Trace amounts of carbonate had been detected in the Martian dust (thanks to ESA’s OMEGA spectrometer) and in Martian meteorites. It is only recently that the rover Spirit found rocks with strong evidence of the presence of this mineral. We now have the CRISM instrument aboard MRO which identified sizable patches of the same. Remote sensing is done by spectrometers which analyze the light reflected from the ground. The presence of such or such rock is inferred from detecting their specific spectral absorption bands. A margin of uncertainty may remain on account of the proximity of some rocks and, of course, of possible impurities between the instrument and the rocks. CRISM is a visible and infrared imaging hyperspectral spectrometer covering the 0.36-3.92 μm region with a 6.55 nm/channel resolution. In high resolution targeted mode (relevant to all observations discussed in this paper) CRISM has a ground sampling distance of 15-19 m/pixel, and a swath width of approximately 10.8 km on the ground. HyMap is a visible to near infrared imaging spectrometer used in Australia. Spectra were also taken by the PIMA spectrometer on the ground which confirmed the HyMap analysis. In the Nili Fossae area, the carbonate-bearing layer is stratigraphically beneath the flat, smooth Syrtis Major volcanic flow layer (which subsequently was punctually eroded or perforated by the meteoritic bombardment, or ripped open by a weak tectonic process which caused the fractures in this place). As in the Pilbara district, the hydrothermal talc-carbonate alteration heat source was likely a later volcanic flow but this could only be confirmed with landed instruments (for instance with the MSL rover Curiosity in case it is sent to Nili Fossae). Spectral Interpretation: In addition to the Mg-carbonate layer, a stratigraphically lower Mg-phyllosilicate bearing layer has been discovered in many locations throughout Nili Fossae. The Mg-phyllosilicate has been associated with saponite as well as talc. The absorption bands of these two minerals are very close and the discrimination power of CRISM does not allow choosing with certainty between both. But, in Western Australia, HyMap has shown that in an olivine rich environment of mafic origine, the same spectral bands are most often associated with talc. Geologic implication: The presence of olivine and its alteration products within the same layers, would suggest that not enough heat (<150°C) and water were available for the alteration reaction to run to completion. This contrasts with the situation in the Warrawoona group mafic/ultramafic rocks, were olivine signatures are no longer visisble from airborne observations. Biologic implication: By analogy with what happened on Earth, conditions would have been ideal for the creation and preservation of microfossils like those found in the Warrawoona rocks. Therefore the Nili Fossae clay-carbonate sequence may be a good location to search for biomarkers in future landed astrobiology missions to Mars. Implications for the atmosphere: The patches of carbonate identified by CRISM are not enough to justify a CO2 trap to an atmosphere in which this gas is abundant. It still maybe that we have not yet discovered the whole picture of carbonated rocks on this planet (a large part of which might be preserved below the lava mantle covering the Northern hemisphere). Methane outgases seasonally from the Nili Fossae region. A geologic process, serpentinization could explain the phenomena (see below) as well as biologic emanations. For this to occur, it would be necessary to have: 1.) pressure of 0.5 bar (i.e. subsurface Mars); 2.) temperature However this process being a source of methane on Mars today, would imply that clay carbonate alteration is occurring kms below the Martian surface. It would then not be possible to directly link to the Nili Fossae, the carbonate observed by CRISM. Here again we would need further investigation from a landed spacecraft. Further investigation of this area by landed missions with mineralogical characterization capability (Visible Near Infra Red, Raman or X Ray Diffraction spectrometers) would enable a more detailed analysis of the clay and carbonate components of these rock units and should be a high priority of the Mars Exploration Program. Definitions: Serpentinization (Serpentine: 2Mg3Si2O5 (OH) 4): Mg2SiO4 + Fe2SiO4 +CO2 +26H2O => 2Mg3Si2O5 (OH) 4 +12Fe3O4 +CH4 Forsterite+ Fayalite + Carbonic gas + Water => Serpentine + Magnesite + Methane Forsterite, Mg2SiO4 (« Fo ») and fayalite: Fe2SiO4 (« Fa ») are olivines. Olivine is the main mineral of the peridotes (rocks constitutive of the planetary mantle). Magnesite is the common name of magnesium carbonate. Comments : With time and as research progress, the geologic picture of Mars becomes clearer and its similarities or differences with Earth become more visible. Today, it seems that Mars was very similar to Earth at the beginning of its geologic history. However it seems that liquid water never was as abundant as it was on Earth and the hydrating process stopped very early. Likewise the tectonic movements hampered by a crust much thicker than Earth’s and therefore a much more difficult access to magmatic mantle rocks, were only barely sketched (rifts faults, fractures, volcanism). Fortunately, this incipient history allowed Mars to so proceed forward as to create an environment similar to Earth when life emerged. Data collected until now lead us to this point and not further. To be more precise, it is not because stromatolites are associated on Mars with rocks very similar to those found on Earth, that they will also be found on Mars but it is possible. Likewise it remains possible that the persistent production of methane observed today, results from a biological and not a geochemical process. We reached a critical point to know whether life did also emerge outside of Earth. We know where to look and what to look for. More than ever, we must go and see whether a same environment allowed the blossoming of the same phenomena. This is not trivial and it fully warrants our, intellectual, financial and even physical, accrued investment. Pierre Brisson
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