Radiolysis in the subsurface of rocky planets: An alternative to sunlight energy for life Lisa M. Pratt Provost’s Professor Department of Geological Sciences, Indiana University Astrobiology Short Course LPSI May 1, 2010

Head frame for the Evander gold mine near Johannesburg, South Africa.

Gold ore hosted by quartz-pebble conglomerate deposited 2.5 billion years ago

High-retention ceramic filters for sampling of cells, membranes, and DNA.

Anaerobic chamber for transfer of deep-groundwater samples into nutrient media to assess metabolic pathways.

A single-species ecosystem in the deep subsurface Groundwater sampled at a depth of 2.8 km below the surface in the Witwatersrand Basin of South Africa has yielded a single, complete genome of a bacterial microorganism. Chivian et al Science Magazine

Vegetative cells and resting spores in groundwater sample Rod-like shape shown by scanning electron micrograph

Radiolytic Splitting of Water as Energy for Microbes

Some Crazy Radiolytic Chemistry Within to seconds of a decay event, the initial species (H 2 O +, e-, H 2 O*) react further to produce:  Chemically reactive species:  Hydrated electron (e aq - ),  Hydrogen (H) radicals,  Hydroxyl (HO) radicals,  Superoxide (O 2 ) radicals  Molecules: Molecular hydrogen (H 2 )

About to seconds following decay event OXIDANTS  hydrogen peroxide (H 2 O 2 )  hydroxyl radicals (HO) REDUCTANTS  H atoms  molecular hydrogen (H 2 ) With continuous irradiation, steady-state concentrations are reached for:  molecular hydrogen (H 2 )  hydrogen peroxide (H 2 O 2 )  small amounts of molecular oxygen (O 2 )

Sealed silica tubes showing products of reaction between pyrite and hydrogen peroxide Red and yellow minerals include iron oxide, elemental sulfur, and numerous iron sulfates similar to minerals identified on Mars

Estimated water content near surface of Mars neutron spectrometer on Mars Odyssey spacecraft.

This view covers an area about 1.15 kilometers (0.7 mile) wide. Individual layers in the scene average 3.6 meters (12 feet) thick. l Rhythmic bedding in Martian sedimentary rocks (Becquerel crater) indicates climate cycles.

 Water is not a limiting molecular resource although liquid water may be a limiting physical state for life on Mars.  Energy sources (redox gradients) do not appear to be limiting near the surface and radioactive minerals could drive radiolysis in the deep subsurface.  High-obliquity (tilt 70 o or more) warm intervals could allow for episodic surface blooms of microbes waiting in a subsurface refuge.