Saturn's moon Enceladus features a warm subterranean ocean covered in ice. In an extraordinary new finding, scientists have confirmed the existence of a chemical energy source within this moon's water that's capable of sustaining living organisms here on Earth. Enceladus is now officially the best place beyond Earth to look for life.
Molecular hydrogen is being produced in the ocean of Enceladus, according to a new study published today in Science. The most plausible source of this hydrogen is hydrothermal reactions between hot rocks and water in the ocean beneath the moon's icy surface. So in addition to warm water, organic molecules, and certain minerals, this moon is also producing an accessible source of energy that could conceivably support alien microbes.
Indeed, hydrothermal processes near volcanic vents are known to sustain complex ecosystems here on Earth. The new study marks an important development in our ability to assess the habitability of distant celestial objects, while setting the stage for future missions.
Underrated compared with Jupiter's icy moon Europa, Enceladus is one of the most fascinating objects in the Solar System. A mid-sized Saturnian moon that measures about 313 miles (504 km) in diameter, it features a geologically young, dynamic surface. Because it's in an eccentric (i.e. non-circular) orbit around its gas giant host, scientists think gravitational forces are causing Enceladus to twist and contort — and that those contortions are generating heat in the moon's rocky core. The warmth generated by these tidal forces is probably what allows the moon to sustain water in a liquid state — lots of it. Enceladus may be covered in an icy shell, but beneath the surface lies a globe-spanning liquid ocean about 37 miles (60 km) deep.
This hidden ocean — which may be as warm as 90 degrees Celsius (194 degrees Fahrenheit) at the bottom — is one of many reasons that Enceladus is considered a prime candidate for extraterrestrial life.
Geysers erupting thorough Enceladus' icy surface. (Image: NASA)
In 2005, NASA's Cassini probe spotted plumes erupting from the Enceladus' south polar terrain, sending water vapour and solid particles from that subterranean ocean off into space. Back in 2015, NASA directed Cassini to perform a deep dive through this vapour, collecting valuable information with its instruments, most notably the Ion and Neutral Mass Spectrometer (INMS). Chemical analysis of the plume indicated the presence of organic and nitrogen-bearing molecules, as well as salts and silicates, which strongly suggest ocean water is in contact with a rocky core.
In a subsequent trip through the plume, Cassini's INMS was put into a mode that minimized analytical artifacts that had compromised the measurements of the energy source molecular hydrogen, or H2, during previous flybys. An analysis of this improved data by scientists J. Hunter Waite, Christopher Glein, Jonathan Lunine, and others, confirmed that the molecular hydrogen being detected by Cassini is in fact produced within Enceladus. As scientific discoveries go, that's huge.
Molecular hydrogen is light and chemically reactive, so it's not the kind of thing that would just normally stick around on this moon without a source to replenish it. The confirmation essentially means that some kind of chemical process is actively making the molecule within the moon itself.
"In our paper we looked at several ways Enceladus might make molecular hydrogen," Lunine, an astronomer at the Cornell Center for Astrophysics and Planetary Science, told Gizmodo. "The one that seems to explain the large amount of molecular hydrogen observed is the reaction at the seafloor of certain kinds of minerals with hot water, which makes molecular hydrogen." In other words, a hydrothermal reaction.
"Cassini found a lot of hydrogen — so much that it must be actively produced," Lunine continued. "If the hydrothermal activity that makes the molecular hydrogen were to shut down, the molecular hydrogen would be consumed by reactions until there was very little left — much less than what is observed by INMS."
If hydrothermal chemical reactions are indeed responsible for the molecular hydrogen, that means the methane previously detected by Cassini might be generated from carbon dioxide (also detected by Cassini) through a reaction with hydrogen. When Cassini flew through the plume in 2015, it measured upwards of 1.4 per cent hydrogen per volume of sample, and up to 0.8 per cent per volume carbon dioxide. Together, these are signatures of a process known as methanogenesis — a metabolic reaction that sustains microbes in deep, dark undersea environments on Earth.
"These mineral-water reactions are the restaurant at the bottom of the ocean of Enceladus, making goodies [i.e. molecular hydrogen (H2) and methane (CH4)] that primitive microbes could eat," said Lunine. "This doesn't tell us whether life is there or not — it just makes the case for the ocean being able to support life that much stronger."
A hydrothermal event on Earth. (Image: NOAA)
Deep sea hydrothermal vent environments are also speculated to exist on Jupiter's moon, Europa. These vents are of critical importance to astrobiologists, as they're known to sustain entire marine ecosystems on Earth. Moreover, the recent discovery of what may be the world's oldest fossil in Quebec suggests that life on Earth may have originated around hydrothermal vents.
"The H2 discovery completes the case for going back to Enceladus to look for life," said Lunine. "The discovery of native molecular hydrogen (H2) completes the set of what I would call the 'basic' requirements for life as we know it: Liquid water, organic molecules, minerals, and an accessible source of "free" energy. The H2 gives us the last of these."
Excitingly, it may be easier to detect traces of life on this moon than we realise. We could potentially do so by flying a spacecraft through a plume equipped with more modern instruments than those aboard Cassini (remember, Cassini was launched 20 years ago). Scientists could look at the plume's chemistry in more detail, searching for the molecular signatures of life in the subsurface ocean; in other words, we would let the ocean come to us. This basically describes the Enceladus Life Finder (ELF) mission, which would involve no landing, drilling or melting — just ten or so deep dives into that tantalising south polar plume.
"So Enceladus is, in my view, the best place beyond Earth to go look for life — a demonstrably habitable ocean that is being spewed into space for us to sample," Lunine told Gizmodo. "What are we waiting for?"