The Interior of Enceladus Looks Really Great for Supporting Life
When NASA’s Voyager spacecraft visited Saturn’s moon Enceladus, they found a body with young, reflective, icy surface features. Some parts of the surface were older and marked with craters, but the rest had clearly been resurfaced. It was clear evidence that Enceladus was geologically active. The moon is also close to Saturn’s E-ring, and scientists think Enceladus might be the source of the material in that ring, further indicating geological activity.
Since then, we’ve learned a lot more about the frigid moon. It almost certainly has a warm and salty subsurface ocean below its icy exterior, making it a prime target in the search for life. The Cassini spacecraft detected molecular hydrogen—a potential food source for microbes—in plumes coming from Enceladus’ subsurface ocean, and that energized the conversation around the moon’s potential to host life.
Now a new paper uses modelling to understand Enceladus’ chemistry better. The team of researchers behind it says that the subsurface ocean may contain a variety of chemicals that could support a diverse community of microbes.
As far as the search for life elsewhere in the Solar System goes, Enceladus checks many boxes. Saturn’s sixth-largest moon is about 500 km (310 mi) in diameter and seems to have a global ocean buried under a sheet of ice. And that ocean is likely warm and salty and contains some interesting chemicals. According to the new research, there are several pathways among these chemicals that could support life.
The title of the paper is “Oxidation processes diversify the metabolic menu on Enceladus.” The lead author is Christine Ray, a Ph.D. student at the Department of Physics and Astronomy, University of Texas, San Antonio. Ray is also with the Space Science and Engineering Division at Southwest Research Institute (SwRI.) The paper is published in the journal Science Direct.
This work was spurred on by Cassini’s discovery of molecular hydrogen in plumes of vapor coming from Enceladus.
“The detection of molecular hydrogen (H2) in the plume indicated that there is free energy available in the ocean of Enceladus,” said lead author Ray in a press release. “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 a disequilibrium between oxidant and reductant compounds.”
The disequilibrium that Ray references creates an energy gradient critical to life. The energy gradient allows energy exchange between an organism and its environment. Processes around these gradients are critical to many aspects of biology, like photosynthesis and respiration. If a system is in equilibrium, there’s no energy gradient, creating a barrier to life.
When molecular hydrogen was discovered in the plumes coming from Enceladus, it drew a parallel with deep ocean hydrothermal vents here on Earth. At those vents, hydrogen provides an energy source for an entire ecosystem. People immediately began to wonder if the same might be true for Enceladus.
But this study takes it further. The authors wanted to know if there could be other energy pathways in Enceladus’ ocean that were conducive to life.
“We wondered if other types of metabolic pathways could also provide sources of energy in Enceladus’ ocean,” Ray said. “Because that would require a different set of oxidants that we have not yet detected in the plume of Enceladus, we performed chemical modeling to determine if the conditions in the ocean and the rocky core could support these chemical processes.”