The Martian surface is dominated by volcanic rocks, so the search for life on Mars should consider potential habitats in these rocks. Volcanic terrains present numerous possible habitats, including porosity in fractures and gas cavities, sites admitting light beneath translucent phenocrysts and a range of surface detritus. Photosynthetic activity may be more limited than on Earth due to sediment opacity, but volcanic systems provide sources of energy for chemolithotrophs, through high-temperature hydrothermal reactions and low-temperature weathering of ferromagnesian minerals. Both are likely processes on Mars.

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Conference Proceedings of the III European Workshop on Exo-Astrobiology
Citation
Parnell, J. (John), Cockell, C. (Charles), Edwards, H. (Howell), & Ellery, A. (2004). The range of life habitats in volcanic terrains on mars. In European Space Agency, (Special Publication) ESA SP (pp. 81–84).