When NASA's Cassini spacecraft circled Saturn and its icy moons from 2004 to 2017, scientists learned one moon may not be a frozen, lifeless celestial object after all. Enceladus, Saturn’s sixth largest moon, is an active moon with an ocean laying underneath its crust and hydrothermal vents deep beneath its icy shell that spew water ice, hydrogen and methane—all the ingredients microscopic life forms love here on Earth.
Now, new research shows those plumes shooting from the Enceladus' surface contain high amounts of methane and may be a sign that the moon can potentially harbor life, according to a study published last month in Nature Astronomy. Researchers speculate the methane could be produced by something similar to Earthly methanogenic microbes that consume hydrogen and carbon and burp up methane near deep-sea vents on the ocean floor, reports Charlie Wood for Popular Science.
The plumes were first discovered in 2006 when the Cassini spacecraft spotted the geysers shooting water ice and other organic materials at high velocities hundreds of miles into space near the moon's south pole, reports Passant Rabie for Inverse. The geysers are thought to feed Saturn's E ring, the planet's second outermost ring.
Ten years later, when Cassini cruised around Enceladus, the spacecraft dove directly into the plumes vapor 30 miles from the moon's surface, reports Paul Scott Anderson for EarthSky. During the dive, Cassini took samples of the spray and used mass-spectroscopy to reveal that the plumes contained high concentrations of methane, carbon monoxide, carbon dioxide, hydrogen, and various other materials, Inverse reports. The hydrogen may be produced by the deep-sea hydrothermal vents on the moon's seafloor, in a similar way that may have started life on Earth as well, reports Mike Wall Space.com.
On Earth, microorganisms that live within deep-sea vents use hydrogen and carbon dioxide to produce methane in a method called methanogenesis, reports Popular Science. Researchers suspect Saturn's moon may have microbes producing the plumes because of the amount of methane Cassini detected. However, methane can be made without the help of microbes.
Methane can be produced non-biologically through a chemical reaction called serpentinization when hot water interacts with minerals in rocks and creates hydrogen. But the amount of methane detected by the Cassini spacecraft was too much to be made by serpentinization alone, reports EarthSky.
To see how Enceladus may produce the abundance of methane and hydrogen, scientists at the University of Arizona and Paris Sciences & Lettres University used mathematical models that combined plausible serpentinization rates that Enceladus may use to make hydrogen and methane on its own. They also used another model that looked at how the rates would change if the moon had microbes creating methane through methanogens, Popular Science reports.
The researchers found that the amount of methane detected is too high to be produced on its own without something else also releasing methane. However, the amount of methane detected by Cassini may match the amount produced if it were occurring on Enceladus through both serpentinization and microbes, the researchers explain in a statement.
"Obviously, we are not concluding that life exists in Enceladus' ocean," says study author Régis Ferrière, an astrobiologist at the University of Arizona, in a statement. "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."
The abundance of methane could also be rising from the moon's core, if it formed from colliding comets or other unknown reasons yet to be discovered, per Popular Science.
More missions and research are needed to determine whether methane is genuinely being produced by microbes or some other process entirely. Researchers are hoping for another mission focused on astrobiology that would probe and measure the chemical compounds on Enceladus and its ocean.
"The ultimate dream for people like me would be to drill through the cracks on Enceladus, and having some kind of submarine hovering around in Enceladus' ocean and taking all kinds of cool measurements," says Marc Rovira-Navarro, a planetary scientist not involved with the study, to Inverse.