In search of the RNA world on Mars

Abstract

Advances in origins of life research and prebiotic chemistry suggest that life as we know it may have emerged from an earlier RNA World. However, it has been difficult to reconcile the conditions used in laboratory experiments with real-world geochemical environments that may have existed on the early Earth and hosted the origin(s) of life. This challenge is in part due to geologic resurfacing and recycling that have erased the overwhelming majority of the Earth’s prebiotic history. We therefore propose that Mars, a planet frozen in time, comprised of many surfaces that have remained relatively unchanged since their formation >4 Gya, is the best alternative to search for environments consistent with geochemical requirements imposed by the RNA world. In this study we synthesize in situ and orbital observations of Mars and modeling of its early atmosphere into solutions containing a range of pHs and concentrations of prebiotically relevant metals (Fe²⁺, Mg²⁺, and Mn²⁺), spanning various candidate aqueous environments. We then experimentally determine RNA degradation kinetics due to metal-catalyzed hydrolysis and evaluate whether early Mars could have been permissive towards the accumulation of long-lived RNA polymers. Our results indicate that a Mg²⁺-rich basalt sourcing metals to a slightly acidic (pH 5.4) aqueous environment mediates the slowest rates of metal-catalyzed RNA hydrolysis, though geologic evidence and modeling of basalt weathering suggest that aquifers on Mars would be near neutral (pH ∼7). Moreover, oxidizing conditions on Mars have major consequences regarding the availability oxygen-sensitive prebiotic metals (i.e., Fe²⁺ and Mn²⁺) very early in its history due to increased RNA degradation rates and precipitation. Overall, 1) low pH better preserves RNA than basic conditions at high concentrations; 2) acidic to neutral pH environments with Fe²⁺ or Mn²⁺ will hydrolyze more RNA; and 3) alkaline environments with Mg²⁺ dramatically hydrolyze more RNA.