Could icy moons like Saturn’s Enceladus in the outer solar system be home to microbes or other forms of alien life?
Intriguing new findings from data collected by NASA’s Cassini spacecraft suggest the possibility.
Plumes of gas erupting out of Enceladus — a small moon with an ocean of liquid water beneath its icy crust — contain hydrogen. Scientists infer a lot from that: that there are hydrothermal chemical reactions similar to those that occur at hot fissures at the ocean bottoms on Earth.
On Earth at least, hydrothermal vents thrive with microbial life, offering up the potential that icy moons far away from Earth — dubbed “OceanWorlds” by NASA — could be habitable.
“That’s just going to be a tremendous opportunity to test our theories and see if there’s life there,” said James L. Green, director of planetary science at NASA.
This is the latest discovery by Cassini, a spacecraft that is heading into its final months after 13 years of exploring Saturn, its moons and rings. On April 22, Cassini begins a journey that will take it between the planet and its rings for 22 orbits before its mission finally ends with a crash into Saturn’s atmosphere in September.
Cassini’s findings also show that levels of carbon dioxide, hydrogen and methane measured in the Enceladus plume were out of equilibrium, an imbalance that could provide an energy source that organisms could tap into for food, according to a paper published Thursday in the journal Science.
“It indicates there is chemical potential to support microbial systems,” said J. Hunter Waite Jr., program director for the space science and engineering division at the Southwest Research Institute in San Antonio and lead author of the Science paper.
In a separate paper published Thursday in The Astrophysical Journal Letters, another team of researchers using the Hubble Space Telescope once again spotted what appears to be a similar plume rising from Europa, one of Jupiter’s big moons that also possesses an ocean beneath an icy exterior.
Cassini had earlier found that there are seas of methane on Saturn’s largest moon, Titan, a finding that has inspired some scientists to suggest sending a boat there.
At a mere 310 miles wide, Enceladus was considered too small to be geologically interesting; scientists suspected that its interior had froze solid long ago. But a decade ago, Cassini spotted plumes rising from the south pole region, one of the biggest, most surprising discoveries of the mission, which has been in orbit around Saturn for nearly 13 years.
The tidal forces of Saturn pulling and squeezing Enceladus appear to generate enough heat to melt the ice. From additional Cassini observations, scientists concluded that not only is there a pool of water near the south pole of Enceladus to generate the plumes, but a global ocean that lies beneath the moon’s ice.
In October 2015, Cassini swooped to within 30 miles of the surface of Enceladus, and one of the instruments collected and identified particles in the plume spray. It was mostly water molecules, but Dr. Waite and his colleagues also found hydrogen molecules, up to 1.4 percent by volume.
While hydrogen is the most common element in the universe, it wasn’t expected to be found in any quantity on a moon as small as Enceladus, where the gravity is too slight to hold on to the gas for long.
“Just finding hydrogen was a surprise,” said Christopher R. Glein, a geochemist at the Southwest Research Institute and another author of the Science paper.
After considering a variety of ways that could continually generate hydrogen, the scientists concluded that hydrothermal reactions offered the most likely explanation for producing that much of the gas. Each water molecule consists of two hydrogen atoms and one oxygen atom. Geophysical models indicated that as hot water flows past the rocks, minerals in the rocks were grabbing the oxygen atoms and releasing hydrogen, the scientists reported.
There appeared to be enough energy to support microbes. “This is the first time we’ve been able to make a calorie count of an alien ocean,” Dr. Glein said.
“This is a great result for the habitability of Enceladus,” said Christopher P. McKay, a planetary scientist at the NASA Ames Research Center in Mountain View, Calif., who was not involved with the research.
Dr. McKay said the hydrogen levels were far above what microbes need.
Still,the presence of hydrogen doesn’t prove that life exists on Enceladus. In fact, it might suggest the opposite.
At hydrothermal vents on Earth, the hydrogen is quickly gobbled up by microbes. That so much hydrogen is rising through Enceladus’s ocean and reaching space could mean there is no life on the little moon to take advantage of it.
Or life could exist, but is limited by other factors. “If there is biology there, it isn’t very active,” said Mary A. Voytek, head of NASA’s astrobiology program.
Scientists will not get any more data for a long time.
Cassini will make no more close flybys of Enceladus. The spacecraft is low on fuel, and the mission will come to a close in September. For the last few months, Cassini has shifted to a different orbit that will allow it to probe the interior properties of Saturn and take a close look at the inner part of its rings.
NASA has at present no plans to return to Saturn or Enceladus. But it is currently soliciting proposals for a mission with a price tag of up to $850 million, and one of the areas that NASA specified an interest is a mission to explore Enceladus and Titan, Saturn’s largest moon.
The Enceladus findings also aid the design of Europa Clipper, NASA’s next big planetary mission, which is to launch in the 2020s to study Europa.
In the new Astrophysical Journal Letters paper, researchers led by William B. Sparks of the Space Telescope Science Institute in Baltimore report on a Hubble Space Telescope observation in 2016 that revealed a likely plume of water vapor rising from Europa from the same spot that the researchers saw a similar plume two years earlier.
In addition, NASA’s Galileo spacecraft in the 1990s found that this location on Europa was unusually warm, and scientists would not be surprised to find hydrothermal vents there, too. “The geophysics is similar everywhere,” Dr. Voytek said.