HYDROTHERMAL ENVIRONMENTS AND PLANETARY HABITABILITY: IS THERMOPHILY THE MOST COMMON PATHWAY OF BIOSPHERE EMERGENCE IN THE COSMOS?

Evidence from molecular biology suggests that hydrothermal systems may have been a “cradle” for early biosphere evolution on Earth. The deepest and shortest branches of the Domains Bacteria and Archaea are all populated by hyperthermophiles that only grow at temperatures >80 °C (Woese 1987; Stetter, 1996). The most deeply rooted organisms are chemolithotrophs that utilize hydrogen and sulfur in their metabolism, features that are widely regarded as likely properties of the last common ancestor of life on Earth. The view from molecular biology is consistent with a wide variety of independent geological evidence for widespread hydrothermal conditions on the early Earth, including model-based predictions of declining crustal temperatures (e.g. Turcotte 1980), due to such things as decreasing impact rate and size following the Hadean interval of heavy bombardment, the gradual decay of radioactive energy sources and progressive loss of internal heat through magmatic and geothermal processes. In addition, the presence of komatiitic lavas during the Archean (representing surface eruptions of high temperature peridotite magmas) indicates that average crustal temperatures were much higher at that time, while oxygen isotope fractionations observed in well-preserved siliceous sediments (cherts) suggest the early Earth was an open hydrothermal system, characterized by a steady decline in average surface (climatic) temperature from early Archean highs of 50-70 °C, to present values (Knauth and Lowe 2003). In so far as the Earth's history represents a general pattern of planetary evolution, hydrothermally-dominated crustal environments may be a common feature of the early evolutionary history of terrestrial planets and thus, a fundamental aspect early planetary habitability. It follows that thermophily could be a common pathway for biosphere emergence elsewhere in the Cosmos.