When space probes like Voyager started sweeping past the outer planets like Jupiter and Saturn, they caused a sensation. What the first high-resolution images of the moons of the gas giants showed was that those satellites were a whole lot weirder than we’d ever imagined.
Io turned out to be one of the most volcanic places ever seen. Europa probably has a salty, liquid ocean under its covering of ice. Titan has a dense atmosphere and lakes and seas of liquid methane.
Saturn’s moon Enceladus is one of the most reflective bodies in the solar system. This is because it is covered in a layer of fresh, clean ice. As Cassini showed, this ice is the result of cryovolcanoes. That is, enormous geysers of water vapour which erupt from its south polar region, apparently from a liquid ocean deep under the ice. Some of this water vapour falls back on the surface as snow.
Water, of course, is of particular interest because it’s essential to life as we know it.
But Enceladus’ cryovolcanoes aren’t all water. There’s also methane — and that makes it all the more intriguing in the search for life beyond Earth.
Flying through the plumes and sampling their chemical makeup, the Cassini spacecraft detected a relatively high concentration of certain molecules associated with hydrothermal vents on the bottom of Earth’s oceans, specifically dihydrogen, methane and carbon dioxide. The amount of methane found in the plumes was particularly unexpected.
Methane is most famous as “swamp gas”, produced by microbes feasting on decaying organic matter. Seasonal methane blooms on Mars previously excited hopes that they might be the result of microbial activity. This is not widely believed anymore, but what about Enceladus?
“We wanted to know: Could Earthlike microbes that ‘eat’ the dihydrogen and produce methane explain the surprisingly large amount of methane detected by Cassini?” said Regis Ferriere, an associate professor in the University of Arizona Department of Ecology and Evolutionary Biology and one of the study’s two lead authors. “Searching for such microbes, known as methanogens, at Enceladus’ seafloor would require extremely challenging deep-dive missions that are not in sight for several decades.”
Ferriere and his team took a different, easier route: They constructed mathematical models to calculate the probability that different processes, including biological methanogenesis, might explain the Cassini data.
Hydrothermal vents – where volcanic activity spews out underwater geysers – are rich sources of organic compounds, and home to colonies of microbes. These microbes produce methane – but so does the volcanic activity itself.
The authors applied new mathematical models that combine geochemistry and microbial ecology to analyze Cassini plume data and model the possible processes that would best explain the observations. They conclude that Cassini’s data are consistent either with microbial hydrothermal vent activity, or with processes that don’t involve life forms but are different from the ones known to occur on Earth.
But the models suggest that abiotic methane production (methane not produced by living organisms) is insufficient to explain the amount of methane on Enceladus.
“Obviously, we are not concluding that life exists in Enceladus’ ocean,” Ferriere said. “Rather, we wanted to understand how likely it would be that Enceladus’ hydrothermal vents could be habitable to Earthlike microorganisms. Very likely, the Cassini data tell us, according to our models.
“And biological methanogenesis appears to be compatible with the data. In other words, we can’t discard the ‘life hypothesis’ as highly improbable. To reject the life hypothesis, we need more data from future missions,” he added.
Science Daily
Scientists and engineers are working on a next generation of robotic space probes that will be able to dive deep into the subsurface oceans of moons like Enceladus and Europa.
Who knows what they will find there?
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