The sluggish speed of making abiotic methane

Abstract

In searching for life beyond Earth, scientists have focused considerable attention on terrestrial bodies containing liquid water. These bodies are considered to be among the most promising locations for hosting the conditions and chemistry required for life (1). To understand how to detect the presence of habitable or inhabited environments, the study of analog systems on Earth has been especially useful. Our knowledge of the chemistry of terrestrial materials and systems facilitates predictions about the theoretically spontaneous chemical reactions that might be common. This knowledge also enables experiments aimed at understanding how fast these reactions might occur. With this knowledge in hand, we can begin to better understand the chemical gradients of which emerging life might take advantage. In PNAS, McCollom (2) provides critical experimental constraints on the rates at which methane can be produced under hydrothermal conditions. Terrestrial planets are likely to be replete with a rock known as peridotite, which reacts with water in a process called serpentinization (3). This reaction transforms peridotite into serpentine and other minerals and produces alkaline fluids. Arguably, the most important consequence of this reaction is that it produces abundant molecular hydrogen (H2). Hydrogen is a feedstock for a wide range of chemical reactions, many of which are capable of supporting microbial metabolism. For example, methanogenic microbes make their living by capturing the energy released when hydrogen is used to chemically reduce carbon dioxide to methane. In terrestrial settings with temperatures amenable to hosting life, the high concentrations of hydrogen produced by serpentinization promote methanogenesis, rendering this metabolism thermodynamically feasible wherever there is peridotite and water. Given this understanding, reports have pointed to the transient detection of methane in the Martian atmosphere as a possible indication of biological activity (4). Complicating this interpretation, however, is the fact that hydrothermal environments with active … [↵][1]1Email: abradley{at}eps.wustl.edu. [1]: #xref-corresp-1-1