The best chances of finding extraterrestrial life might lie within the water-rich moons in our own solar system.
On its deepest dive through the spray that shoots up from Saturn's icy moon Enceladus, NASA's Cassini spacecraft picked up signs of molecular hydrogen — an ingredient that, coupled with carbon dioxide, feeds many microbes living around hydrothermal vents on the bottom of Earth's oceans, NASA officials said Thursday.
The findings, described in the journal Science, break ground in showing that an icy world beyond our own might truly have the right environment for Earth-like life.
"It is life-friendly," lead author J. Hunter Waite, a planetary scientist at the Southwest Research Institute, said in an interview.
He ticked off several of the ingredients necessary for life that have been found on Enceladus, including water and key elements such as carbon, hydrogen, nitrogen and oxygen. "This drives Enceladus to the top of the heap on conditions that demonstrate habitability."
Ever since Enceladus was spotted shooting what turned out to be plumes of water vapor and ice from cracks in its frozen crust more than a decade ago, scientists have wondered what lies beneath the surface of the Saturnian moon. Data gathered by Cassini have shown that this tiny world may host a salty global ocean; the spacecraft's Cosmic Dust Analyzer also recently discovered the presence of silica in the plume, a hint that geothermal activity may be happening beneath the surface.
Enceladus is too far from the sun for life to employ photosynthesis, but the chemical energy released by geothermal activity could help fuel any hypothetical microbes in the moon's ocean. The presence of silica is a hopeful sign that water indeed interacts with rock, though it doesn't prove that the right chemical ingredients are present.
However, when Waite and his colleagues used Cassini's Ion and Neutral Mass Spectrometer to study the plumes, they discovered molecular hydrogen — which happens to be a key food source for microbes living around hydrothermal vents on Earth.
"Silicon is not very appetizing, apparently," but molecular hydrogen is, Waite said.
On Earth, certain microbes living in the dark, high-pressure depths of the ocean where oxygen is scarce consume hydrogen and carbon dioxide, producing methane in the process. Cassini's discovery that the plume could contain as much as 1.4 volume percent molecular hydrogen and 0.8 volume percent carbon dioxide shows that the same thing could theoretically happen at Enceladus.
"If correct, this observation has fundamental implications for the possibility of life on Enceladus; chemical disequilibrium that is known to support microbial life in Earth's deep oceans is also available to support life in the Enceladus ocean," Jeffrey Seewald of the Woods Hole Oceanographic Institution, who was not involved in the study, wrote in a related commentary.
The paper can't say whether any microbes do, or ever did, exist beneath Enceladus' icy crust, Waite was quick to add — only that the right ingredients are there. Researchers are not even sure whether all the hydrogen they found is coming from a hydrothermal process exactly like the one seen on Earth, or whether it's more similar to a different process that has been produced in the lab.
Keep in mind that different forces are heating the water on each world: Earth's deep oceans are warmed by the planet's internal heat, while the ocean within Enceladus is likely heated by tidal forces squeezing and stretching the little moon. Whether or not Enceladus hosts an underwater ecosystem like the Lost City, the famous hydrothermal vents in the Atlantic Ocean, remains to be seen.
In a related discovery, another team of scientists announced Thursday that the Hubble Space Telescope had picked up signs of a fresh plume of water spewing out of Jupiter's moon Europa. That finding, described in Astrophysical Journal Letters, lends support to efforts to send a spacecraft to the icy world.
Many scientists believe Europa offers the best chance for finding a life-friendly environment elsewhere in the solar system. Enceladus is a relatively tiny moon. Europa is much larger, and may have about twice as much water beneath its surface as all of Earth's oceans combined. Enceladus might be relatively young, which means life might not have had much time to emerge. Europa, by comparison, is billions of years old.
"My money for the moment is still on Europa, but it could be on any of those moons," said Mary Voytek, astrobiology senior scientist at NASA headquarters who wasn't involved with either study.
The water plume detected by Hubble appears to be rising from the same relatively "warm" spot that released a plume a few years back. The 2014 geyser rose about 30 miles above the surface; the new one, snapped in 2016, reached about 62 miles high.
NASA has been eyeing Europa and other water worlds for years. Plans to launch a Europa Clipper mission in the 2020s are already in the works.
But Christopher McKay, a planetary scientist at NASA Ames Research Center who was not involved in the work, appeared to lean more toward Enceladus. While Enceladus is only about 310 or so miles across, compared to the roughly 1,940-mile diameter of Europa, it seems to have the strongest plumes — which makes its ocean easier to study, he said.
Regardless, he added, either would make a good target.
"Europa and Enceladus are sisters here," McKay said.
The findings could also encourage astronomers to look for different kinds of life-friendly worlds, and to develop new methods to find them. Ocean-bearing moons, for example, could potentially be identified by their extreme optical brightness, McKay suggested, thanks in large part to their frozen exteriors.
All the new findings reinforce the idea that scientists should continue looking to water worlds — both in our solar system and beyond — as some of the best possible places to search for life, Waite and others said.
"We still have a long way to go," Seewald wrote.
"Future missions to explore oceans beyond Earth will answer many of these questions and further constrain the possibility of life elsewhere in our solar system," he added.
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8:15 p.m.: The story was updated with additional details from NASA and planetary scientist Christopher McKay.