Could Methane-Spewing Microbes Be Living in the Depths of a Subsurface Ocean on Saturn’s Moon Enceladus?

The hot, chemical plumes could be produced by something similar to Earthly microscopic life forms that consume hydrogen and carbon, then burp up methane

The plumes were first discovered in 2006 when the Cassini spacecraft spotted the geysers shooting water and other organic materials at high velocities hundreds of miles into space near the moon's south pole. (NASA/JPL/Space Science Institute under public domain)

By Elizabeth Gamillo

SMITHSONIANMAG.COM
JULY 8, 2021

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.

About Elizabeth Gamillo

Elizabeth Gamillo is a science journalist based in Milwaukee, Wisconsin. She has written for Science magazine as their 2018 AAAS Diverse Voices in Science Journalism Intern.

Read more from this author | Follow @elizgamillo

 

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POSTED ONJULY 8, 2021 BY NANCY ATKINSON

Cassini Saw Methane in Enceladus’ Plumes. Scientists Don’t Know How it Could be There Without Life

Even though the Cassini mission at Saturn ended nearly four years ago, data from the spacecraft still keeps scientists busy. And the latest research using Cassini’s wealth of data might be the most enticing yet.

Researchers say they’ve detected methane in the plumes of Saturn’s icy moon Enceladus. The process for how the methane is produced is not known at this time, but the study suggests that the surprisingly large amount of methane found are likely coming from activity at hydrothermal vents present on Enceladus’s interior seafloor. These vents could be very similar those found in Earth’s oceans, where microorganisms live, feed on the energy from the vents and produce methane in a process called methanogenesis.

“We are not concluding that life exists in Enceladus’ ocean,” said Régis Ferrière, an associate professor at the University of Arizona, and one of the study’s two lead authors.  “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.”

One of the biggest surprises of the 13-year Cassini mission came in Enceladus, a tiny moon with active geysers at its south pole. At only about 310 miles (500 km) in diameter, the bright and ice-covered Enceladus should be too small and too far from the Sun to be active. Instead, this little moon is one of the most geologically dynamic objects in the Solar System.

In 2005 Cassini discovered jets of water vapor and ice erupting form the surface of Enceladus. The water could be from an subsurface sea. Image Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

Stunning backlit images of the moon from Cassini’s camera show plumes erupting in Yellowstone-like geysers, emanating from tiger-stripe-shaped fractures in the moon’s surface. The discovery of the geysers took on more importance when Cassini later determined the plumes contained water ice and organics. Since life as we know it relies on water, this small but energetic moon has been added to the short list of possible places for life in our Solar System. 

For the new study, the research team analyzed one of those plume’s material ejected into space. They looked at Enceladus’ plume composition as the end result of several chemical and physical processes taking place in the moon’s interior, where dihydrogen, methane and carbon dioxide are being produced.

“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 Ferrière in a press release from the University of Arizona.

First, the researchers assessed what hydrothermal production of dihydrogen would best fit Cassini’s observations, and whether this production could provide enough energy to sustain a population of Earthlike hydrogenotrophic methanogens. To do that, they developed a model for the population dynamics of a hypothetical hydrogenotrophic methanogen, whose thermal and energetic niche was modeled after known strains from Earth.

Artist rendering showing an interior cross-section of the crust of Enceladus, which shows how hydrothermal activity may be causing the plumes of water at the moon’s surface. Credits: NASA-GSFC/SVS, NASA/JPL-Caltech/Southwest Research Institute

Then the researcher team ran the model to see whether a given set of chemical conditions, such as the dihydrogen concentration in the hydrothermal fluid, and temperature would provide a suitable environment for these microbes to grow. They also looked at what effect a hypothetical microbe population would have on its environment – for example, on the escape rates of dihydrogen and methane in the plume.

The team wrote in their paper, published in Nature:

“We find that the observed escape rates (1) cannot be explained solely by the abiotic alteration of the rocky core by serpentinization; (2) are compatible with the hypothesis of habitable conditions for methanogens; and (3) score the highest likelihood under the hypothesis of methanogenesis, assuming that the probability of life emerging is high enough. If the probability of life emerging on Enceladus is low, the Cassini measurements are consistent with habitable yet uninhabited hydrothermal vents and point to unknown sources of methane (for example, primordial methane) awaiting discovery by future missions.”

“And biological methanogenesis appears to be compatible with the data,” said Ferrière. “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.”

Further reading:

Nature paper
University of Arizona