Journal Article

Early geochemical environment of Mars as determined from thermodynamics of phyllosilicates

Chevrier, Vincent, Poulet, Francois, Bibring, Jean-Pierre

2007

Overview

Alternative pasts for Mars There is abundant evidence that liquid water once existed on the surface of Mars. Today's martian atmosphere is too thin to sustain the sort of greenhouse effect that could create a climate warm enough to support liquid water but, one theory goes, there may have been a carbon dioxide-rich atmosphere in the past. The apparent absence of carbonates and the low escape rates of carbon dioxide, however, have argued against that explanation. Now a thermodynamic study of clay minerals recently discovered by the OMEGA spectro-imaging instrument on-board Mars Express, has provided a clearer picture of the primitive geochemical conditions on Mars. What emerges is an atmosphere with very low carbon dioxide levels. Other greenhouse gases — methane seems the most likely — may therefore have been involved in sustaining a warm and wet climate on Mars. The apparent absence of carbonates and the low escape rates of carbon dioxide are indicative of an early Martian atmosphere with low levels of carbon dioxide. Calculations of aqueous equilibria of phyllosilicates that were recently observed on Mars now suggest low partial pressure of carbon dioxide at the time. This implies that other greenhouse gases may have played a key role in sustaining a warm and wet climate on early Mars. Images of geomorphological features that seem to have been produced by the action of liquid water have been considered evidence for wet surface conditions on early Mars 1 . Moreover, the recent identification of large deposits of phyllosilicates, associated with the ancient Noachian terrains 2 , 3 suggests long-timescale weathering 4 of the primary basaltic crust by liquid water 2 , 5 . It has been proposed that a greenhouse effect resulting from a carbon-dioxide-rich atmosphere sustained the temperate climate required to maintain liquid water on the martian surface during the Noachian 6 , 7 . The apparent absence of carbonates and the low escape rates of carbon dioxide 8 , however, are indicative of an early martian atmosphere with low levels of carbon dioxide. Here we investigate the geochemical conditions prevailing on the surface of Mars during the Noachian period using calculations of the aqueous equilibria of phyllosilicates. Our results show that Fe 3+ -rich phyllosilicates probably precipitated under weakly acidic to alkaline pH, an environment different from that of the following period, which was dominated by strongly acid weathering 9 that led to the sulphate deposits identified on Mars 10 , 11 , 12 . Thermodynamic calculations demonstrate that the oxidation state of the martian surface was already high, supporting early escape of hydrogen. Finally, equilibrium with carbonates implies that phyllosilicate precipitation occurs preferentially at a very low partial pressure of carbon dioxide. We suggest that the possible absence of Noachian carbonates more probably resulted from low levels of atmospheric carbon dioxide, rather than primary acidic conditions 13 . Other greenhouse gases may therefore have played a part in sustaining a warm and wet climate on the early Mars.

• Publisher: Nature Publishing Group UK,Nature Publishing,Nature Publishing Group
• Subject: Carbon Dioxide / Carbonates / Climate / Cosmochemistry. Extraterrestrial geology / Deposition / Earth sciences / Earth, ocean, space / Environmental aspects / Exact sciences and technology / Extraterrestrial Environment / Extraterrestrial geology / Geochemistry / Greenhouse Effect / Greenhouse gases / Humanities and Social Sciences / Iron / letter / Mars / Mars (Planet) / Mars surface / multidisciplinary / Observations / Oxidation / Phyllosilicates / Science / Science (multidisciplinary) / Silicates / Sulfates / Thermodynamics / Water / Weathering / Wet climates