The ocean interior of Saturn's moon Enceladus may have the "temperatures and chemical energy sources necessary for habitable conditions," according to new research published in Science on Thursday.
This major finding is the result of an extremely tight flyby of Enceladus conducted by NASA's Cassini spacecraft, which has been in orbit around the ringed gas giant since 2004. The data sourced from this close encounter adds to Enceladus' growing reputation as one of the solar system's leading candidates in the search for alien life, and hospitable worlds beyond Earth.
"We feel pretty fortunate that we got this information about habitability," said J. Hunter Waite, the principal investigator of the mass spectrometer (INMS) onboard Cassini and the lead author of the paper, in a phone interview with Motherboard. "It will continue to build an interesting case for going back [to Enceladus]."
On October 28, 2015, Cassini zoomed by Enceladus at an altitude of 30 miles above the surface of this tiny moon, which is only 314 miles in diameter, small enough to fit within the length of the United Kingdom. The orbiter passed through one of the billowing plumes of vapor that frequently erupt from this world's icy surface, and sampled its chemical contents.
Waite and his team now reveal that this taste-test indicated that the plume's constituent elements were water vapor (96 to 99 percent), carbon dioxide (0.3 to 0.8 percent), methane (0.1 to 0.3 percent), ammonia (0.4 to 1.3 percent), and perhaps most importantly, molecular hydrogen (0.4 to 1.4 percent).
According to the researchers, the most plausible explanation for this relatively high hydrogen reading is that there are hydrothermal reactions between rocks and water deep inside Enceladus's subsurface ocean.
Similar hydrothermal reactions on Earth support anaerobic organisms called methanogens. Unlike humans and all other aerobic (oxygen-reliant) life on Earth, these exotic microbes metabolize carbon dioxide and molecular hydrogen, and "breathe out" methane as a byproduct in a process called methanogenesis.
All the right ingredients for methanogenesis were detected in the plume Cassini flew through, but it's impossible to conclusively tell whether they were produced by alien beasties.
"We know the hydrogen's there, we have a range, but it's not really tight enough to be able to say anything about methanogenesis," Jonathan Lunine, an Enceladus expert based at Cornell and a co-author on the paper, told me over the phone.
"It could be geological activity, the reaction of these molecules and water with rock at the base of the ocean," he said. "It's intriguing but I would be very skeptical if somebody were to say we could take the current Cassini data and be able to definitively say whether the methane is coming from biological processes."
This ambiguity can be partly chalked up to the fact that Cassini was not designed to look for signs of life. The orbiter was launched nearly 20 years ago, when it was not definitely known that Enceladus regularly blasts out its ocean guts for curious spacecraft to enjoy.
"There was a suspicion that stuff was coming out [of Enceladus] but nobody had any clue what the density was, the gas-to-grain ratio, and so on," Lunine said, referring to the years leading up to Cassini's launch. "All of this was really theoretical, based on the brightness of Enceladus, the smooth surface, and the fact that it was orbiting within an E-ring that everybody presumed was supplied by the moon itself."
The orbiter was outfitted with a mass spectrometer and dust analyzer that made the 2015 findings possible, but it would take a more refined suite of instruments to root out clear signs of biological processes.
"I think we can make significant leaps by just going back and flying through the plume," said Waite. "Not with very different instrumentation, but improved instrumentation, and a focused mission to substantiate this habitability, and start looking for life."
To that end, Lunine is the principal investigator of the Enceladus Life Finder (ELF), a proposed mission that would include two spectrometers designed to analyze gas and solids within the plumes for signs of biological activity, with much more sensitivity than Cassini.
"We'll be able to see metabolic products, and get a much tighter number for [molecular hydrogen]," Lunine said. "We'll really be able to test the methanogenesis hypothesis very precisely." He said NASA's new focus on ocean worlds, outlined in detail in 2016, suggests that the agency increasingly regards Enceladus as a "scientifically powerful target in the search for life."
If NASA greenlights ELF, the spacecraft would likely be launched around 2024, arriving at Saturn a decade later. Sure, that's a painfully long time to wait for such tantalizing results. But great discoveries—like say, methane-exhaling creatures living in an ocean inside a tiny walnut-shaped moon—are rarely rushed.