Cassini’s mission might be winding down, but there’s no end in sight to the revelations its data will bring.
Scientists from the University of Cambridge, the University of East Anglia, and the University of Eastern Finland joined forces with American scientists from Johns Hopkins, Cornell, and the Southwest Research Institute to analyze information gathered by Cassini in 2015 when it dove into a vapory plume on the south pole of Saturn’s moon Enceladus.
While the spacecraft wasn’t looking for life itself, it was looking for signs that life could exist on Enceladus. In addition to water, Cassini found organic molecules such as ammonia and carbon dioxide. That was a major finding and it set the stage for another one: that the vapors erupting from hydrothermal vents in Enceladus’ ocean contain molecular hydrogen.
Enceladus South Pole plumes. Credit: NASA/JPL
NASA puts it this way: “Life needs three things, right? It needs water. It needs chemistry and it needs energy.” Scientists knew about the water on Enceladus, but they didn’t have confirmation about the chemistry and energy until now.
In a publication for Science, researchers explain the significance of the discovery of molecular hydrogen—namely, that the only thing that could generate it is “hydrothermal reactions between hot rocks and water in the ocean beneath the moon’s icy surface,” or a process called chemosynthesis (think photosynthesis, but with chemicals instead of sunlight). Chemistry and energy—check and check.
Enceladus. Credit: NASA
On Earth, hydrothermal vents exist in oceans (as well as volcanoes, hot springs, and geysers), where they gush geothermally heated water that contains hydrogen sulfide, iron sulfides, calcium, silicon, and other minerals, which combine with the oxygen in the water to create sustenance for organisms. It’s even possible that life originated around such vents, which serve as the nexus of entire ecosystems and support rare organisms such as extremophiles, life forms such as bacteria and worms that live in areas of extreme temperature or pressure. It’s possible that such organisms—or wildly different ones—thrive in the chemical soup created by the vents on Enceladus. And while we don’t have direct confirmation of the existence of life there, we now know that all of the necessary ingredients for life (as we know it, at least) do indeed exist there.
Hydrothermal vents. Credit: NOAA
The hydrothermal vents on Enceladus spew both molecular hydrogen and carbon dioxide, key ingredients for methanogenesis, a process akin to respiration by which microbes form methane. Some readers might remember that the Curiosity Rover has (thus far unsuccessfully) searched for methane on Mars, as methane is a sign of biological life. Methanogenesis allows organisms—including some of Earth’s oldest life forms—to survive in extreme areas.
And while we don’t know for sure whether that process is happening on Saturn’s moon, we know that all the necessary components are there. These findings provide the strongest circumstantial evidence yet for the likelihood of microbes on Enceladus, and will hopefully spur future missions designed specifically to detect such life.