In the not-too-distant future, a spacecraft could deliver samples from an alien ocean to Earth, where scientists would scrutinize the material for signs of life.
Scientists are developing a mission concept that would send a probe flying through the plume created by the 100-odd geysers erupting from the south polar region of Saturn's ice-covered moon Enceladus.
These geysers blast water, salts and organic compounds from the satellite's subsurface ocean far out into space. The mission, known as Life Investigation for Enceladus (LIFE), would collect samples of this stuff, then send it winging back to Earth in a return capsule. [Inside Enceladus, Icy Moon of Saturn (Infographic)]
"Getting a sample from Enceladus would be phenomenal," said LIFE leader Peter Tsou, of Sample Exploration Systems in La Canada, California. "This 'are we alone' question — potentially we can shed tremendous light on it in a single mission."
LIFE is not on NASA's books; it remains a concept at the moment. Tsou estimates the sample-return effort could be mounted for $700 million or so — about 30 percent the cost of NASA's Mars rover Curiosity mission.
A potentially habitable moon
Many astrobiologists regard the 310-mile-wide (500 kilometers) Enceladus and the much larger Jupiter moon Europa as the solar system's best bets to host life beyond Earth.
Enceladus and Europa appear to possess subsurface oceans of liquid water that are in contact with their rocky mantles, making possible many complex chemical reactions. And recent studies suggest that, while both moons' oceans are beyond the reach of sunlight, they may still harbor energy sources sufficient to sustain microbial life.
NASA is already working on a flyby mission to Europa, which the agency hopes to launch in the early to mid-2020s. But many scientists are also pushing for a dedicated Enceladus effort, in large part because of the satellite's dramatic geysers, which NASA's Saturn-orbiting Cassini spacecraft discovered in 2005.
These powerful jets, which emanate from fractures near Enceladus' south pole, merge to form a plume — a frigid cloud of ocean particles that extends many miles into space, just waiting to be snagged and studied. [See Enceladus' Geysers in Action (Video)]
"It's free samples," Jonathan Lunine, of Cornell University, told Space.com. "We don't need to land, drill, melt or do anything like that."
Cassini has flown through the plume on multiple occasions, finding evidence of carbon-containing organic compounds with its mass spectrometer instrument. But Cassini is not equipped to look for signs of life.
The argument for sample return
Lunine is principal investigator of another mission concept called Enceladus Life Finder (ELF), which aims to search for signs of life in plume particles. But the ELF probe would do all this work onboard in the Saturn system, rather than send the samples back to Earth for analysis.
Tsou thinks sample-return is a better way to go, saying that it may be tough for a robotic spacecraft millions of miles from its handlers to make a definitive detection of alien life.
"Right now, no biologist or astrobiologist has a generally agreed-upon definition of life," Tsou said. "So, in order for us to determine that there's life on Enceladus, it's not going to be a simple, binary, 1-or-0 answer," Tsou said. "You'll have to do many, many studies."
As an example, Tsou and his team cite the protracted analysis of pieces of Comet Wild 2, which were delivered to Earth by NASA's Stardust mission in 2006. (Tsou served as Stardust's deputy principal investigator.)
"Final confirmation of the cometary origin of the amino acid glycine from Comet Wild 2 was obtained over 3 years after the samples were returned to Earth," the LIFE team wrote in a paper presented at the 45th Lunar and Planetary Science Conference, which was held last year in The Woodlands, Texas.
"Significant advancement in assessing the biological potential of Enceladus can be made on returned samples in terrestrial laboratories, where the full power of state-of-the-art laboratory instrumentation and procedures can be utilized, without serious limits on power, mass or cost," they added. "Terrestrial laboratories provide the ultimate in analytical capability, adaptability, reproducibility, reliability and synergy amongst scientists." [5 Bold Claims of Alien Life]
How it would work
The LIFE probe would launch to Saturn orbit, which it could reach after 5 years if orbital dynamics allowed a speed-boosting flyby of Jupiter, Tsou said. The journey would take 7 or 8 years without such a gravity-assist maneuver, he added. (The trip would be much shorter, however, if NASA's in-development Space Launch System megarocket were used.)
Once in orbit, LIFE would perform multiple plume-sampling flybys of Enceladus, collecting material in a cushioning aerogel similar to that employed by the Stardust mission. LIFE would also carry a return capsule, a camera, a mass spectrometer (which would allow some in situ analysis) and a dust counter, which would let mission scientists know that the probe had indeed flown through the plume.
After collection was complete, LIFE would send the samples speeding back toward Earth in the return capsule. The plume material would have to be handled extremely carefully once it got here, because of the possibility that it could harbor alien lifeforms (which could theoretically harm or alter Earth life and ecosystems).
Enceladus samples would therefore likely be lodged and studied in a facility capable of "biosafety level 4" (BSL-4) containment, the most secure classification, Tsou said. Researchers studying extremely contagious and dangerous infectious agents, such as the Ebola virus, do their work at BSL-4.
The United States does not have a BSL-4 facility outfitted to handle material from space, and previous studies have estimated the cost of constructing a custom-made one at $500 million or more, Tsou said. But LIFE would not necessarily have to incur that cost.
The Japan Agency for Marine-Earth Science and Technology (JAMESTEC) is planning to build a BSL-4 facility onboard its oceangoing research vessel Chikyu, Tsou said. And JAMESTEC officials have responded favorably to the possibility of storing and studying Enceladus samples on Chikyu, he added.
"They will have the staff; they will have the experience," Tsou said. "To recover [samples] from another [world's] ocean — they were very excited."
LIFE's collaboration with Japan could be extensive, if the mission ends up going ahead: Tsou said there's a good chance that Japan would supply LIFE's sample-return capsule. (The nation has some expertise in this area; the Japan Aerospace Exploration Agency, or JAXA, successfully returned pieces of the asteroid Itokawa to Earth in 2010, and launched another asteroid sample-return mission last year.)
There are no firm commitments at this point, but Tsou said he has had productive meetings with officials from JAXA and the nation's Institute of Space and Aeronautical Science.
Indeed, Tsou expressed optimism that Japan might be able to pick up about $200 million of the potential mission's price tag, leaving NASA to pay the remaining $500 million or so. LIFE therefore might someday be able to fly as part of NASA's Discovery Program, which launches focused, relatively low-cost missions.
Future in doubt
The space agency is currently considering about two dozen proposals for a Discovery mission that will launch by the end of 2021, with a cost cap of $450 million (not including post-launch operations). NASA is expected to select a handful of finalists this month, then make its choice in September 2016.
ELF is in the running for that mission, but Tsou and his colleagues did not submit LIFE as a possibility. The current Discovery call prohibited the use of nuclear power sources — such as radioisotope thermoelectric generators, which convert the heat created by the radioactive decay of plutonium-238 into electricity — apparently in an effort to conserve NASA's dwindling stockpile of plutonium.
Tsou thinks nuclear fuel is crucial for probes flying all the way to Saturn, which lies 9.5 times farther from the sun than Earth does and thus receives much less solar energy. (Lunine, on the other hand, is confident that ELF can succeed using solar power.)
So the future of LIFE is up in the air. Tsou said he would like to propose the concept as a Discovery mission down the road, though he could foresee submitting LIFE via NASA's medium-class exploration program, known as New Frontiers, if the next Discovery call is nuclear-free as well. (The $720 million New Horizons mission, which performed the first-ever flyby of Pluto in July, is a New Frontiers project.)
"We're still pushing ahead without any funding, doing the best we can," Tsou said.
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Michael Wall is a Senior Space Writer with Space.com and joined the team in 2010. He primarily covers exoplanets, spaceflight and military space, but has been known to dabble in the space art beat. His book about the search for alien life, "Out There," was published on Nov. 13, 2018. Before becoming a science writer, Michael worked as a herpetologist and wildlife biologist. He has a Ph.D. in evolutionary biology from the University of Sydney, Australia, a bachelor's degree from the University of Arizona, and a graduate certificate in science writing from the University of California, Santa Cruz. To find out what his latest project is, you can follow Michael on Twitter.