Ocean Worlds: Enceladus Looks Increasingly Habitable, and Europa’s Ocean Under the Ice More Accessible to Sample

NASA’s Cassini spacecraft completed its deepest-ever dive through the icy plume of Enceladus on Oct. 28, 2015. (NASA/JPL-Caltech)

It wasn’t that long ago that Enceladus, one of 53 moons of Saturn, was viewed as a kind of ho-hum object of no great importance.  It was clearly frozen and situated in a magnetic field maelstrom caused by the giant planet nearby and those saturnine rings.

That view was significantly modified in 2005 when scientists first detected signs of the icy plumes coming out of the bottom of the planet.  What followed was the discovery of warm fractures (the tiger stripes) near the moon’s south pole, numerous flybys and fly-throughs with the spacecraft Cassini, and by 2015 the announcement that the moon had a global ocean under its ice.

Now the Enceladus story has taken another decisive turn with the announcement that measurements taken during Cassini’s final fly-through captured the presence of molecular hydrogen.

To planetary and Earth scientists, that particular hydrogen presence quite clearly means that the water shooting out from Enceladus is coming from an interaction between water and warmed rock minerals at the bottom of the moon’s ocean– and possibly from within hydrothermal vents.

These chimney-like hydrothermal vents at the bottom of our oceans — coupled with a chemical mixture of elements and compounds similar to what has been detected in the plumes — are known on Earth as prime breeding grounds for life.  One important reason why is that the hydrogen and hydrogen compounds produced in these settings are a source of energy, or food, for microbes.

A logical conclusion of these findings:  the odds that Enceladus harbors forms of simple life have increased significantly.

To be clear, this is no discovery of extraterrestrial life. But it is an important step in the astrobiological quest to find life beyond Earth.

“The key here is that Enceladus can produce fuel that could be used by biology,” said Mary Voytek, NASA’s senior scientist for astrobiology, referring to the detection of hydrogen.


This graphic illustrates how scientists on NASA’s Cassini mission think water interacts with rock at the bottom of the ocean of Saturn’s icy moon Enceladus, producing hydrogen gas (H2). It remains unclear whether the interactions are taking place in hydrothermal vents or more diffusely across the ocean. (NASA)

“So now on this moon we have many of the components associated with life — water, a source of energy and many of the important chemical building blocks.  Nothing coming from Cassini will tell is if there is biology there, but we definitely have found another important piece of evidence of possible habitability.”

The finding of molecular hydrogen (H2 rather a single hydrogen atom) in the Enceladus plumes was described in a Science paper lead by authors Hunter Waite and Christopher Glein of the Southwest Research Institute, headquartered in San Antonio.

They went through a number of possible sources of the hydrogen and then concluded that the clearly most likely one was that chemical interaction of cool water and hot rocks — both heated by tidal forces in the complex Saturn system — at the bottom of the global ocean.

“We previously thought that the water was heated but now we have evidence that the rocks are as well,” Waite told me.  “And the evidence suggests that the rock is quite porous, which means that water is seeping through on a large scale and producing these chemical interactions that have a byproduct of hydrogen.”

The moon Enceladus is the sixth largest in the Saturn system. This image was taken by Cassini in 2008. (NASA/JPL-Caltech, Space Science Institute.)

He said that the process could be taking place in and around those chimney-like hydrothermal vents,  or it could be more diffuse across the ocean floor.  The vent scenario, he said, was “easier to envision.”

What’s more, he said, the conditions during this water-rock interaction are favorable for the production of the gas methane, which has been detected in the Enceladus plume.

This is another tantalizing part of the Enceladus plume story because the earliest lifeforms on Earth are thought to have both consumed and expelled that gas.  At this point, however, Waite said there is no way to determine how the methane was formed, which would be a key finding if and when it is made.

“Our results leave us agnostic on the presence of life,” he said. “We don’t have enough information for that.”

“But we now can make a strong case that we have a very habitable environment on this moon.” It’s such a strong case, he said, that it would be almost as scientifically interesting to not find life there than to detect it.

One of the more interesting remaining puzzles is why the hydrogen is present in the plume in such unexpectedly substantial (though initially difficult to detect) amounts.  If there was a large microbial community under the ice, then it could plausibly be argued that there wouldn’t be so much hydrogen left if they were consuming it.

The possibilities:  Waite said that it could mean there is just a lot of “food” being produced for potential microbes to survive on in the ocean, or that other factors limit the microbe population size.  Or, of course, it could mean that there are no microbes at all to consume the hydrogen food.

Astronomers have twice found evidence of a plume of water vapor coming from the same location on Europa. Both plumes, photographed in UV light by Hubble, were seen in silhouette as the moon passed in front of Jupiter. (NASA/ESA/STScI/USGS)

News of the Enceladus discovery came on the same day that other researchers announced that strong evidence of detecting a similar plume on Jupiter’s moon Europa using the Hubble Space Telescope.

This was not the first plume seen on that larger moon of Jupiter, but is perhaps the most important because it appeared to be was spitting out water vapor in the same location as an earlier plume.  In other words, it may well be the site of a consistently or frequently appearing geyser.

“The plumes on Enceladus are associated with hotter regions,” said William Sparks of the Space Telescope Science Institute. “So after Hubble imaged this new plume-like feature on Europa, we looked at that location on the Galileo thermal map. We discovered that Europa’s plume candidate is sitting right on the thermal anomaly,”

Sparks led the Hubble plume studies in both 2014 and 2016, and their paper was published in The Astrophysical Journal.  He said he was quite confident, though not completely confident of the result because of the limits of the Hubble resolution.  A 100 percent confirmation, he said, will take more observations.

Since Europa has long been seen as a strong candidate for harboring extraterrestrial life, this is extraordinarily good news for those hoping to test that hypothesis.  Now, rather than devising a way to blast through miles of ice to get to Europa’s large, salty and billions-of-years-old ocean, scientists can potentially learn about the composition of water by studying the plume — as has happened at Enceladus.

As their paper concluded, “If borne out with future observations, these indications of an active Europan surface, with potential access to liquid water at depth, bolster the case for Europa’s potential habitability and for future sampling of erupted material by spacecraft.”

This is particularly exciting since NASA is actively developing a mission to Europa that would orbit the moon and could target the plume area for study.

NASA teams have also proposed a Europa lander — a mission that was rejected by the Trump administration in its budget proposals.  But discovery of  what might be a regularly-spurting plume just might change the equation.

The plumes of Enceladus originate in the long tiger stripe fractures of the south polar region pictured here. (Cassini Imaging Team, SSI, JPL, ESA, NASA)

The news about both Enceladus and Europa illustrates well the process by which the search for life beyond Earth — astrobiology — moves forward.

Like few other disciplines, astrobiology needs expertise coming from a broad range of fields, from astrophysicists, geochemists, biochemists, geologists, and more.

Hunter Waite, for instance, trained as an atmospheric  scientists and now builds mass spectrometers for spacecraft such as Cassini,  operates them in flight, and analyzes and reports the data.  He is something of a “plume” expert as well, and will follow up his team leading work on Enceladus as principal investigator of the Europa mass spectrometer that surely will investigate that other moon’s new-found plumes. (The Europa mission, called the Europa Clipper, is loosely scheduled to launch in 2022.)

His colleague, Christopher Glein, is a geochemist.  And the leader of the Europa plume-spotting team, William Sparks, is an astronomer.

Mary Voytek, NASA senior scientist for astrobiology.  (NASA)

Each discipline focuses on a part of the larger system that might, or might not, be habitable.  No single scientists or discipline of scientists is capable of detecting extraterrestrial life.

This has long been the view of NASA’s Voytek, who views astrobiology as a kind of very long-term scientific full-court press.

She is wary of overselling discoveries that involve the search for life beyond Earth and the origin of life here, saying that they sometimes are well-meaning “science fiction” more than science.

However, the Enceladus findings in particular have her excited.  A lot of questions remain, such as whether the water with molecular hydrogen is coming from a hydrothermal vent or across the ocean floor, and whether the amount of methane detected in the plume increases or decreases the likelihood of life on the ocean floor.

But her conclusion: “I think this puts Enceladus into a different category and definitely higher up on the index of habitability.”  Any potential life, she said, would almost surely be microbial, though it might be larger “if we get lucky.”