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Meteorites Suggest Galvanic Origins for Martian Organic Carbon

The nature of carbon on Mars has been the subject of intense research since NASA’s Viking-era missions in the 1970s, due to the link between organic (carbon-containing) molecules and the detection of extraterrestrial life. Analyses of Martian meteorites marked the first confirmation that macromolecular carbon (MMC)—large chains of carbon and hydrogen—are a common occurrence in Mars rocks. More recently, researchers have applied the lessons taken from studies of meteorites to the data being gathered by the Curiosity rover, finding similar MMC signatures on Mars itself. Now, the central question is “what is the synthesis mechanism of this abiotic organic carbon?”

To find clues, researchers used advanced microscopy and spectroscopy techniques to analyze the composition, morphology, and chemistry of three Martian meteorites at the nanoscale. They found that certain mineral grains showed signs of extensive corrosion, with MMC being layered between Fe-poor and Ti-rich grains that could act as natural “electrodes” in a nanosized galvanic cell. Scanning transmission x-ray microscopy (STXM) at ALS Beamline 5.3.2 revealed that the material was remarkably rich not only in aromatic (ring-shaped) hydrocarbons—known from Raman analysis—but that it also contained a high proportion of CO, COOH, and CN functional groups, hinting that the precursor of the material was probably CO2 in a brine that was electrochemically reduced to the organic material.

A similar process could occur anywhere that igneous rocks are surrounded by brines, including the subsurface oceans of Jupiter’s moon Europa, Saturn’s moon Enceladus, and even some environments on Earth, particularly early in this planets’ history.

A high-resolution transmission electron micrograph (scale bar = 50 nm) of a grain from a Martian meteorite. Reminiscent of a long dinner fork, organic carbon layers were found between the intact “tines.” This texture was created when the volcanic minerals of the Martian rock interacted with a salty brine and became the anode and cathode of a naturally occurring battery in a corrosion reaction. This reaction would then have enough energy—under certain conditions—to synthesize organic carbon. (Credit: Andrew Steele)

A. Steele, L.G. Benning, R. Wirth, S. Siljeström, M.D. Fries, E. Hauri, P.G. Conrad, K. Rogers, J. Eigenbrode, A. Schreiber, A. Needham, J.H. Wang, F.M. McCubbin, D. Kilcoyne, and J.D. Rodriguez Blanco, “Organic synthesis on Mars by electrochemical reduction of CO2,” Sci. Adv. 4, eaat5118 (2018), doi:10.1126/sciadv.aat5118.