|Source region of outflow channel, Viking Orbiter.|
Martian meteorites are known to contain martian water (Figure 3), some in igneous minerals (e.g., amphibole and mica) that crystallized from water-bearing magmas, and some in weathering products. However, water identified in igneous minerals appears to exist in relatively low abundance (~0.1-0.5%), and it is not apparent that such concentrations are sufficient to produce the amount of degassed water required to generate martian fluvial features. Further, an analysis of the deuterium/hydrogen (D/H) isotopic ratio of water in meteorite apatite and other minerals shows large fractionated values similar to atmospheric hydrogen (see next topic), suggesting that even the water in this igneous phase may have been derived from the martian surface.
|Figure 3. SEM images of hydrous minerals in Nakhla: A & B (top left and center): calcium carbonate, C (bottom left): magnesium sulfate, D (bottom center): calcium sulfate, (Bottom right): TEM image of smectite clay and ferrihydrite in Lafayette.|
Those weathering products in martian meteorites that contain water include silicate clays, hydoxides, and various salts, all clearly formed by interaction of liquid martian water with igneous minerals that comprise the meteorites. Several martian meteorites contain traces of precipitated minerals such as Ca-carbonate and Ca-sulfate (Figure 3). The nakhlite meteorites contain the most diverse suite of hydrated phases, including carbonates, sulfates, and layer-structured silicate clays (iron-rich smectites), often in association with ferric iron oxides and hydroxides. These mineral assemblages, as well as oxygen isotopic analyses of water extracted from the nakhlites (see photo), suggest that the altering water was cool by geologic standards (<100°C), possibly cold (~0°C), and strongly oxidizing.
Unlike the other martian meteorites, ALH84001 contains abundant inclusions of distinctively layered sequences of Ca-Mg-Fe-carbonates (see photo), and their formation might have involved martian water that was chemically reducing and hotter. Current interpretations of water-borne mineral assemblages in the nakhlites suggest ground water infusion. Are carbonates in ALH84001, by contrast, consistent with hydrothermal fluids associated with an active geologic heat source, perhaps similar to subaqueous volcanic vents on Earth? Although the formation times of the carbonate formation episodes are unknown, their slightly different nature and the much older age of the host meteorite compared to other martian meteorites suggest that alteration may have occurred relatively early. Thus, martian meteorites suggest aqueous activity may have persisted throughout martian history. Because martian meteorites never spent substantial time at the very surface, more highly weathered surface materials are likely to contain greater species diversity and higher concentrations of water-bearing phases, which is consistent with Viking chemical analyses of martian soils.
|Carbonates in ancient highlands (M. Grady, BMNH). Binocular view shows carbonate grains 100 microns across in ALH84001. Colored rings are due to variations in carbonate composition.||Water from Mars (H. Karlsson, E. Gibson, JSC). This drop of water extracted from a martian meteorite has the unique stable isotope composition of martian crust.|
Meteorites from Mars: In-depth
Volcanism and Impact