A new study published in the Astrophysical Journal Letters examines whether radioactive decay could support life on ocean worlds like Jupiter‚Äôs moon Europa.
In the icy planetary bodies around the Solar System, radiation emitted by long-lived radionuclides contained in rocky cores could break up water molecules and support hydrogen-eating microorganisms.
To address this possibility, the study‚Äôs authors modeled a natural water-cracking process called radiolysis, and applied the model to several known or suspected ocean worlds: Enceladus, Ceres, Europa, Titania, Oberon, Pluto, and Charon.
‚ÄúThe physical and chemical processes that follow radiolysis release molecular hydrogen (H2), which is a molecule of astrobiological interest,‚ÄĚ said lead author Alexis Bouquet, a PhD student at the University of Texas at San Antonio and a researcher at the Southwest Research Institute.
Radioactive isotopes of elements such as uranium (235U and 238U), potassium (40K), and thorium (232Th) are found in a class of rocky meteorites known as chondrites.
The cores of the icy worlds studied by Bouquet and co-authors are thought to have chondrite-like compositions.
Ocean water permeating the porous rock of the core could be exposed to ionizing radiation and undergo radiolysis, producing molecular hydrogen and reactive oxygen compounds.
‚ÄúMicrobial communities sustained by H2 have been found in extreme environments on Earth,‚ÄĚ Bouquet said.
‚ÄúThese include a groundwater sample found nearly 2 miles (3.2 km) deep in a South African gold mine and at hydrothermal vents on the ocean floor.‚ÄĚ
‚ÄúThat raises interesting possibilities for the potential existence of analogous microorganisms at the water-rock interfaces of ocean worlds such as Enceladus or Europa.‚ÄĚ
‚ÄúWe know that these radioactive elements exist within icy bodies, but this is the first systematic look across the solar system to estimate radiolysis,‚ÄĚ said co-author Dr. Danielle Wyrick, a principal scientist in the Space Science and Engineering Division at the Southwest Research Institute.
‚ÄúThe results suggest that there are many potential targets for exploration out there, and that‚Äôs exciting.‚ÄĚ
One frequently suggested source of molecular hydrogen on ocean worlds is serpentinization.
This chemical reaction between rock and water occurs, for example, in hydrothermal vents on the ocean floor.
The key finding of the study is that radiolysis represents a potentially important additional source of molecular hydrogen.
While hydrothermal activity can produce considerable quantities of hydrogen, in porous rocks often found under seafloors, radiolysis could produce copious amounts as well.
Radiolysis may also contribute to the potential habitability of ocean worlds in another way.
In addition to molecular hydrogen, it produces oxygen compounds that can react with certain minerals in the core to create sulfates, a food source for some kinds of microorganisms.
‚ÄúRadiolysis in an ocean world‚Äôs outer core could be fundamental in supporting life,‚ÄĚ Bouquet said.
‚ÄúBecause mixtures of water and rock are everywhere in the outer solar system, this insight increases the odds of abundant habitable real estate out there.‚ÄĚ
Alexis Bouquet et al. 2017. Alternative Energy: Production of H2 by Radiolysis of Water in the Rocky Cores of Icy Bodies. ApJL 840, L8; doi: 10.3847/2041-8213/aa6d56