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. 2015 Nov 1;15(11):998–1029. doi: 10.1089/ast.2015.1374

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© The Author(s) 2015; Published by Mary Ann Liebert, Inc.

This Open Access article is distributed under the terms of the Creative Commons Attribution Noncommercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.

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FIG. 1. — Heterogeneous or “punctuated” habitability on Mars. (a) Stage 1, prebiotic period ∼4.4 Ga. Much volcanic and impact activity; heavy rain of IDPs, meteorites, and micrometeorites importing volatiles, including prebiotic molecules. (b) Stage 2, origin of life, ∼4.2 Ga. Bodies of stable water with associated hydrothermal activity; possible origin of life (red star) around hydrothermal vents; continued rain of IDPs, meteorites, and micrometeorites (prebiotic molecules). (c) Stage 3, established life, ∼3.8 Ga. Water bodies are coated with ice; established life in habitable niches, e.g., water-lain sediments (black dots), cracks/fractures in the upper crust, or subsurface aquifers; continued rain of IDPs, meteorites, and micrometeorites (prebiotic molecules); possible emergence of life in another, geographically isolated habitable area (red star). (d) Stage 4, extinct/surviving life (red dots) <3.0 Ga. Surface water has disappeared; former ice-coated water bodies are covered with sediment/lava flows; surviving life in cracks/fractures in the upper crust, subsurface aquifers; continued rain of IDPs, meteorites, and micrometeorites (prebiotic molecules) but oxidation of carbon at the surface. (e) Stage 5, possiblility of ephemeral life as cells in the subsurface (red dots) are brought up to the surface by impact-related hydrothermal activity and survive for as long as there is liquid water.