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R.N. Clark, G.A. Swayze, T.V.V. King, K.E. Livo, R.F. Kokaly, J.B. Dalton (USGS)
Hydrothermal alteration systems have been considered a likely environment in which to find evidence for life on Mars, as such systems would provide a source of heat as well as water and minerals for growth. Detection and characterization of hydrothermal systems from both orbit and in situ are critical to determine whether a hydrothermal system might be suitable for such a search. The mineralogy exposed at the surface is key to such detection and evaluation. Hydrothermal systems on the Earth have been mapped and characterized with visible-near-infrared imaging spectrometers, showing that a wealth of information can be derived from the surface expressions of minerals. The minerals exposed are indicative of the temperature of formation of the hydrothermal system. Depending on the level of erosion, and the depth of the heat source, different minerals may be exposed, ranging from high temperature minerals such as pyrophyllite and dickite, to intermediate temperature minerals such as alunites, muscovites, and kaolinites (which also depend on pH), to the lowest temperature expressions of OH-bearing silica. Hydroxylated silica is commonly found at the top of hydrothermal systems, such as those in the geysers of Yellowstone National Park. Some of the silica is biologically precipitated. Other hydrothermal systems, such as Cuprite, Nevada, are more deeply eroded and show the higher temperature minerals that were formed deep in the hydrothermal system. Identification of sulfate mineralogy, such as jarosites and alunites, can also be used to determine the paelo-pH of the water in the system, and thus the potential for hosting different forms of life.
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