HYDROTHERMAL SYSTEMS: DOORWAYS TO EARLY BIOSPHERE EVOLUTION

Hydrothermal systems provide important environments for the synthesis of simple precursor carbon compounds that may have been important in the origin of living systems. Such environments probably also played a primary role in the early evolution of the biosphere. The suggestion of a thermophilic last common ancestor that utilized hydrogen and/or sulfur is broadly consistent with independent geological evidence for the nature of Archean environments on Earth. Subsurface hydrothermal systems may have also been a refuge for thermophilic microorganisms during late, giant impacts that appear to have re-shaped the early biosphere at the end of heavy bombardment. Such events could account for the dominance of thermophilic groups observed in deep branches of the universal (RNA) phylogenetic tree.

Hydrothermal systems often exhibit both high rates of biological productivity and mineralization, providing optimal conditions for the preservation of certain classes of fossil microbial biosignatures. Thus, the deposits of these systems are often rich storehouses of paleobiological information. The terrestrial record of hydrothermal systems reaches back to the early Archean.

Today, planetary-scale biological productivity is primarily controlled by photosynthesis. An important question is: When did the switch-over from exclusively chemosynthetic production to photosynthetically-based ecosystems occur and what was the impact on the global biosphere? Modern, photosynthetically-dominated hydrothermal systems exhibit many diagnostic meso- to microscale structures and geochemical variations that provide useful biosignatures for photosynthetic processes. Comparative studies of subaerial hydrothermal deposits over a broad range of temporal and spatial scales may eventually provide answers to this question.

Hydrothermal processes appear to be inextricably linked to planetary formation and evolution. Such environments are likely to have existed on other bodies in our Solar System. Given the potential for preserving a record of prebiotic chemistry and/or fossil biosignatures, hydrothermal deposits are considered important targets in the search for a record of prebiotic chemistry and/or life on Mars, Europa and the dark asteroids.