What lives in the seas of Enceladus? Despite 10 years orbiting Saturn鈥檚 icy moon and sampling the material gushing from its plumes, NASA鈥檚 is far from having an answer. Now two proposed missions hope to change that by searching for life more directly.
Cassini has made a series of fly-bys through water-rich plumes erupting from deep inside the icy moon out of cracks in the surface. These have yielded tantalising clues that the ingredients for life might be buried there.
The subsurface ocean is probably an alkaline solution with a pH of 11 or 12, which could have been produced by water reacting with certain iron-rich types of rock 鈥� a process called serpentinisation. This creates hydrogen, a source of energy that is favourable for life and probably powered ancient life on Earth.
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But despite focusing on these details during its most recent fly-by on 28 October, Cassini researchers haven鈥檛 been able to determine precise hydrogen levels because of complications in how the probe鈥檚 instruments take measurements.
The final fly-by on 19 December will offer more data, but it won鈥檛 be able to confirm whether microbes live in the salty sea: the instruments on board, designed more than 20 years ago, simply don鈥檛 have the capability.
That鈥檚 where the Enceladus Life Finder (ELF), proposed by at Cornell University in Ithaca, New York, aims to step in. Equipped with better instruments, the solar-powered probe would follow in Cassini鈥檚 footsteps by flying through the plumes and building on what we already know.
鈥淭he plume of Enceladus is waiting for us. It could be the place where we find out if life had a second genesis in our own solar system,鈥� Lunine said in a talk at the in San Francisco, California, on 14 December. 鈥淲e must go back.鈥�
The ELF would use mass spectrometers with much higher sensitivity, range and resolution than those of Cassini. One, called MASPEX, would analyse the gas streaming from the vents. Another, the Enceladus Icy Jet Analyzer, would focus on solid particles.
This combination would allow space scientists to detect and identify amino acids, the building blocks of life. It would also shed light on whether the ocean has the basic requirements of habitability 鈥� the right pH, temperature, available chemical energy and types of chemical reactions taking place in the thermal vents.
The ELF was initially proposed to NASA鈥檚 Discovery Program, which offers up to $500 million for small space missions. It was, however, rejected earlier this year in favour of less risky mission concepts focused on asteroids and Venus.
Lunine鈥檚 team plans to resubmit the ELF proposal at 鈥渢he first opportunity available鈥�.
Collecting samples
A separate mission dubbed LIFE (Life Investigation For Enceladus) offers an alternative approach: to collect samples from the plumes and then take them back to Earth in a capsule for investigation. LIFE would run on a plutonium battery, which means it can鈥檛 be funded through NASA鈥檚 Discovery Program because it prohibits the use of nuclear power sources.
of Sample Exploration Systems in La Ca帽ada, California, heads up LIFE. He was previously deputy principal investigator on NASA鈥檚 2006 Stardust Mission to capture and return comet particles, and argues that his plan will be more likely than the ELF to deliver definitive results.
鈥淲e don鈥檛 even have a set of instruments on Earth to detect life. We aren鈥檛 even sure what life is. So if we go out there with any in situ instruments, they will always be extremely limited and the information you get won鈥檛 be very definitive,鈥� he says. 鈥淎 more productive means to learn of the possibility of life would be to bring samples to terrestrial laboratories for the scientists of the world to reach a consensus.鈥�
Tsou argues that his proposals are being hampered not only by NASA鈥檚 desire to conserve its dwindling plutonium supply, but also by concern for planetary protection. 鈥淭hey want to make sure we do not bring any live germs to kill everybody on Earth,鈥� he said.
Lunine points out that such a sample-return mission is scientifically risky as well, because it won鈥檛 make as many fly-bys. The team would also have to wait three or four years to get the sample back to Earth, leaving plenty of room for something else going wrong.
鈥淲e have a much lower-risk approach to understanding whether there鈥檚 life on Enceladus,鈥� he says.
Image credit: JPL-Caltech/NASA
