Experimental Study of Radiolytic Oxidation of Pyrite: Implications for Mars- Relevant Crustal Processes

Introduction: Radiolytic dissociation of water produces a highly reactive combination of oxidizing (e.g., H 2 O 2 , OH radicals, and O 2) and reducing (e.g., H atoms and H 2) species [1]. In subsurface environments , radiolysis can produce gradients of both electron acceptors and electron donors that are possible sources of metabolic energy [2]. Radiation-induced chemical reactions have particular significance in geo-logic environments where molecular oxygen derived from the atmosphere is a negligible input. Results from geochemical studies of unconformity-related uranium deposits indicate that radiolysis is considerably under recognized as a naturally occurring source of chemical energy for biotic and abiotic reactions. In particular, radiolysis of water coupled to oxidation of sulfide minerals [3] or elemental sulfur [4] can produce gradients of partially to fully oxidized sulfur species that might be suitable for microbial metabolism. Recent data from Mars Exploration Rovers provide multiple lines of evidence indicating the extensive presence of sulfates on Mar's surface. Measurements by NASA's Mars Pathfinder and Viking landers showed that sulfur is a substantial component of soil dust and surface rocks [5]. Evidence of hydrated sul-fate salt deposits in the Martian tropics comes from near-infrared spectral data on Mars Express [6]. In addition, sulfates have been identified in SNC meteorites , which contain salt minerals including sulfates, up to 1% by volume. These evidences taken together strongly suggest that sulfate minerals are on the Mars surface and within the upper lithosphere. Sulfate minerals are a potential archive of information on both the sulfur geochemical cycle and history of water on Mars. On Earth, H 2 O 2 has been detected as a stable product of radiolysis in anoxic subsurface environments associated with uranium deposits. Although H 2 O 2 is generally regarded as of little or no geochemical significance on Earth, it is increasingly clear that the H 2 O 2 molecule plays a pivotal role in Martian atmospheric and soil chemistry. Trace-level concentrations of H 2 O 2 in Mars atmosphere have been measured recently using mid-infrared [6] and sub-millimeter spec-troscopy [7] employing ground-based telescopes. Traditional models for the oxidation of sulfide minerals in aerobic environments involve the presence of molecular oxygen and water, as the key oxidants for sulfides. In recent years, however, geochemists have increasingly recognized that radiolysis could be an