New evidence for a liquid sea beneath the icy crust of Saturn's moon Enceladus has elevated its already considerable status as a potential habitat for organic life to a level that, for some researchers, makes a sample-return mission to the moon irresistible.
US and Italian scientists announced the evidence – based on detailed measurements of the moon's gravity field – Thursday and formally published their analysis in Friday's issue of the journal Science.
The sea is sandwiched between the moon's rocky core and a 31-mile-thick crust of ice. Centered on the moon's south pole, the sea is up to 6 miles deep and holds more water than Lake Superior. Moreover, it appears to contain several chemical compounds that serve as the foundation for organic life, as well as sustain it. The telltale signs of these chemicals appear in the plumes of water-ice crystals that erupt from fissures at Enceladus's south pole – plumes that are widely thought to originate at the sea.
The new evidence, gathered by NASA's Cassini spacecraft, "is quite convincing," says Christopher McKay, an astrogeophysicist at NASA's Ames Research Center at Moffett Field in California.
Enceladus is not the only object in the solar system casting a "come hither" look to astrobiologists. Two years ago, scientists reported evidence based on gravity measurements that Saturn's moon Titan may host a subsurface sea. Titan already was of keen interest for the lakes of liquid hydrocarbons on its surface.
Enceladus' larger cousin, Jupiter's moon Europa, also is widely considered to host a subsurface ocean – one that appears to lurk below a crust only about half as thick as that of Enceladus. Last December, researchers reported detecting ice plumes erupting from Europa – a sign of current geological activity long sought but not seen on the moon during NASA's Galileo mission, which toured the Jovian system between 1995 and 2003.
In addition, Jupiter's moons Ganymede and Callisto also are thought to host subsurface seas or oceans.
But advocates for a mission to Enceladus argue than none of these other bodies shows the range of prerequisites for potential habitability that Enceladus does: liquid water, inorganic salts, hydrocarbons, and energy sources for biologically important chemical reactions.
"No other world has such well-studied indications of habitable conditions," wrote Dr. McKay, Carolyn Porco, who leads the Cassini imaging team, and two other colleagues in an opinion piece that appears in this month's issue of the journal Astrobiology.
Perhaps most important, beside Europa, no other world hosts an interior ocean as easy to analyze: Just swing through the plumes and snatch samples of the ice and dust.
That's an approach a working group of US and Japanese scientists have been exploring for the past year and a half, McKay says.
The US and Japan are the only two countries with a proven ability to capture and return samples from solar system objects beyond the moon. The group has been drawing up the outlines of a financially modest mission to sample Enceladus's plumes.
Like the past US comet-sample-return mission, Stardust and its solar-wind sibling Genesis, the total price tag for an Enceladus sample-return mission couldn't exceed $425 million, including launch costs, putting it in NASA's Discovery class of space-science missions.
The team has been considering a Cassini-like approach to cost-sharing, where the US would pick up the tab for the main spacecraft, while the Japanese would be responsible for the sample collection and storage portion of the craft. The craft also would carry a mass spectrometer for analyzing the composition of the material the craft encounters, as well as an imaging camera, and perhaps a dust analyzer, similar to the one Cassini has.
One sticking point with a direct bearing on mission cost involves planetary protection. The chemistry evident in Enceladus's plumes and the regional sea's contact with the moon's rocky core suggest that, even now, the sea could be swimming with microbes. These could become involuntary astronauts if they find their way through the plumbing that connects the sea to the surface, where the water flash freezes as it's ejected to form the plumes.
Bacteria from another spot in the solar system would be a scientific bonanza, but if any escaped on Earth, the consequences could be serious, McKay says.
"It's not likely that we're going to bring back Enceladus measles," he notes. "But infection is not the only concern."
Citing the importation of rabbits to Australia in 1788, he says: "They're not infectious; they're kind of cute. But they're an ecological disaster."
By international agreement, samples returned from a habitable planet would need a high level of isolation and control – something NASA currently isn't equipped to provide.
Indeed, in the run-up to the planetary science community's Decadal Survey, a priority-setting document published in 2010, NASA's Jet Propulsion Laboratory looked at the prospects for an Enceladus orbiter and put the price tag at $1.5 billion to $2 billion. The orbiter would have been larger and would have had a more ambitious science agenda than just collecting and returning samples. And much of the risk associated with trying to pull off such a mission in the 2010 to 2023 time frame was the need to develop the tools to meet planetary protection requirements.
McKay argues that planetary protection is critical and that the burden of developing it shouldn't be attached to a specific mission. Rather, it should be a part of NASA's ongoing technology development program, since any mission that returns samples from a potentially habitable body would need to use such facilities.
Planetary protection aside, the two biggest challenges facing the more-modest sample-return mission the US-Japanese team is exploring involved reentry and the hardware needed to contain the samples. Such containers would likely be an evolved version of self-sealing containers the Japanese have developed for their Hayabusa-2 mission to an asteroid. The mission currently is slated for launch in July.
Reentry would involve a heat shield and a container that would fall freely – no parachutes – and slam into the ground for retrieval. Parachute-based reentry systems add too much avoidable risk when considering samples that must be isolated from the environment, McKay says.
With respect to reentry, the biggest hurdle may be a mental one, he suggests.
"To me, the hardest part is getting to the realization that crashing is a feature, not a problem," he says.
As currently envisioned, the craft would launch in the early 2020s, spend two years orbiting Saturn and making multiple flybys of Enceladus, then bring back the samples. Total time from launch to sample-container impact: 15 years.