Radiolysis in the subsurface of rocky planets: An alternative to sunlight energy for life

Lisa M. PrattProvost’s Professor

Department of Geological Sciences, Indiana University

Astrobiology Short CourseLPSI

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. 2008 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 10-10 to 10-8 seconds of a decay event, the initial species

(H2O+, e-, H2O*) react further to produce:

Chemically reactive species:

Hydrated electron (eaq-),

Hydrogen (H•) radicals,

Hydroxyl (HO•) radicals,

Superoxide (O2•) radicals

Molecules:

Molecular hydrogen (H2)

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 Marsneutron spectrometer on Mars Odyssey spacecraft.

http://marsprogram.jpl.nasa.gov/odyssey/gallery/science/PIA04907.html

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.

http://www.nasa.gov/mission_pages/MRO/multimedia/20081204a.html

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 70o or more) warm intervals could allow for episodic surface blooms of microbes waiting in a subsurface refuge.