If the martian crust has acted through time as a significant sink for surface water, then we might expect to find abundant hydrated minerals formed by weathering processes at the martian surface. It is not apparent that such concentrations are sufficient to produce the amount of degassed water required to generate martian fluvial features. This drop of water extracted from a martian meteorite has the unique stable isotope composition of martian crust. Those weathering products in martian meteorites that contain water include silicate clays, hydoxides, and various salts, all clearly formed by interaction at low temperature of liquid martian water with igneous minerals that comprise the meteorites. Binocular view shows carbonate grains 100 microns across in ALH84001.
Colored rings are due to variations in carbonate composition.
One interpretation is that they formed in a warmer, more reducing aqueous environment than the Nakhlites-perhaps hydrothermal. Another interpretation is that they formed in a cool reducing aqueous environment. Preliminary analyses indicate that the carbonates may have been depositied in ALH84001 as early as 3.6 billion years ago. 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 mineral diversity and higher concentrations of water-bearing phases, which is consistent with Viking chemical analyses of martian soils.