In a study published this month in the journal Icarus, planetary researchers from the United States and Germany modeled chemical processes in the subsurface ocean of Enceladus, the sixth-largest of Saturn’s moons.
Enceladus’ tiger stripes are known to be spewing ice from the moon’s icy interior into space, creating a cloud of fine ice particles over the moon’s south pole and creating Saturn’s mysterious E-ring. Evidence for this has come from NASA’s Cassini spacecraft that orbited Saturn from 2004 to 2017. Pictured here, a high resolution image of Enceladus is shown from a close flyby. Tiger stripes are visible in false-color blue. Image credit: NASA / ESA / JPL / SSI / Cassini Imaging Team.
NASA’s Cassini mission to the Saturn system discovered a plume of ice grains and water vapor erupting from cracks on the surface of Enceladus.
This small moon has a global subsurface ocean in contact with a rocky core beneath its icy exterior, making it a promising location to search for evidence of extraterrestrial life in our Solar System.
The previous detection of molecular hydrogen in the plume indicates that there is free energy available for methanogenesis, the metabolic reaction of hydrogen with carbon dioxide to form methane and water.
Additional pathways could also provide sources of energy in Enceladus’ ocean, but they require the use of other compounds that have not been detected in the plume.
“The detection of molecular hydrogen in the plume indicated that there is free energy available in the ocean of Enceladus,” said Christine Ray, a researcher in the Space Science and Engineering Division at Southwest Research Institute and a Ph.D. student in the Department of Physics and Astronomy at the University of Texas at San Antonio.
“On Earth, aerobic, or oxygen-breathing, creatures consume energy in organic matter such as glucose and oxygen to create carbon dioxide and water.”
“Anaerobic microbes can metabolize hydrogen to create methane.”
“All life can be distilled to similar chemical reactions associated with disequilibrium between oxidant and reductant compounds.”
“This disequilibrium creates a potential energy gradient, where redox chemistry transfers electrons between chemical species, most often with one species undergoing oxidation while another species undergoes reduction.”
“These processes are vital to many basic functions of life, including photosynthesis and respiration.”
“We wondered if other types of metabolic pathways could also provide sources of energy in Enceladus’ ocean,” Ray explained.
This figure illustrates a cross-section of Enceladus, showing a summary of the processes Ray et al. modeled in the icy moon. Oxidants produced in the surface ice when water molecules are broken apart by radiation can combine with reductants produced by hydrothermal activity and other water-rock reactions, creating an energy source for potential life in the ocean. Image credit: Southwest Research Institute.
Because that would require a different set of oxidants that we have not yet detected in the plume of Enceladus, Ray and colleagues performed chemical modeling to determine if the conditions in the ocean and the rocky core could support these chemical processes.
For example, they looked at how ionizing radiation from space could create the oxidants oxygen and hydrogen peroxide, and how abiotic geochemistry in the ocean and rocky core could contribute to chemical disequilibria that might support metabolic processes.
They considered whether these oxidants could accumulate over time if reductants are not present in appreciable amounts.
They also considered how aqueous reductants or seafloor minerals could convert these oxidants into sulfates and iron oxides.
“We compared our free energy estimates to ecosystems on Earth and determined that, overall, our values for both aerobic and anaerobic metabolisms meet or exceed minimum requirements,” Ray said.
“These results indicate that oxidant production and oxidation chemistry could contribute to supporting possible life and a metabolically diverse microbial community on Enceladus.”
“Now that we’ve identified potential food sources for microbes, the next question to ask is ‘What is the nature of the complex organics that are coming out of the ocean?’” said Dr. Hunter Waite, also from the Space Science and Engineering Division at Southwest Research Institute and the Department of Physics and Astronomy at the University of Texas at San Antonio.
“This study is another step in understanding how a small moon can sustain life in ways that completely exceed our expectations!”
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Christine Ray et al. Oxidation processes diversify the metabolic menu on Enceladus. Icarus, published online December 5, 2020; doi: 10.1016/j.icarus.2020.114248
This article is an updated version of a press-release provided by Southwest Research Institute.
