Enceladus ocean jets spew hydrogen: Hint of life?
Step aside freezing Martian deserts. The pitch black seas of at least one of the solar system鈥檚 far-flung water worlds has nearly everything needed for life, NASA says.
The Cassini spacecraft has detected molecular hydrogen on Saturn鈥檚 moon Enceladus, a potential energy source for rare forms of life much like those found near Earth鈥檚 deep sea vents, according to a in the journal Science and a NASA press conference. This detection adds to the growing list of ingredients for life found on the icy moon.
But Enceladus isn鈥檛 the only one of our solar system鈥檚 icy moons sparking excitement at NASA.
NASA scientists also announced that what appear to be water plumes spewing out of Jupiter鈥檚 moon Europa have been spotted using the Hubble Space Telescope, again.
Together, the two announcements paint a picture of two water worlds protected by ice and fueled by warm cores. And, as these moons have surprised scientists, they could redefine how scientists think about the habitability of the solar system and hunt for alien life.
鈥淲ith this research, we鈥檙e making a big step forward towards answering the question: 鈥業s there life out there?鈥� 鈥� Thomas Zurbuchen, associate administrator of NASA鈥檚 Science Mission Directorate, said during the press conference.
The search for life in the solar system starts at home, which means water. 鈥淭he one kind of life that we know about, which is life on Earth, requires liquid water,鈥� explains Jonathan Lunine, an astrophysicist at Cornell University in New York and a co-author on the Science paper. 鈥淭he one common denominator of every form of life on Earth is that it requires liquid water to live.鈥�
Besides the wet stuff, life also needs an energy source (aka food), and the right chemical ingredients to make an organism, namely carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Firm answers for Jupiter's moon will have to wait for NASA鈥檚 Europa Clipper mission, slated for the 2020s. But Cassini has provided the strongest evidence yet that Enceladus has what it takes in what Cassini project scientist Linda Spilker called 鈥渁 capstone finding for the mission.鈥� The final holdouts are phosphorus and sulfur, which scientists suspect exist in the moon's core.聽
A new Goldilocks zone
The search for extraterrestrial life has long focused on so-called Goldilocks zones, the area around a star with just enough energy to melt surface ice without boiling surface water. Jupiter and Saturn sit outside this zone, where it鈥檚 too dark and cold for the sun to make liquid water stable on the surface of any planet or moon.
But sunlight isn鈥檛 the only source of heat. Gravity is an option too.
As the moon creates tides on Earth by gravitationally tugging on the nearside more than the far side, resulting in the two bodies stretching each other as they orbit, Saturn and Jupiter also pull at their moons.
Saturn similarly churns Enceladus鈥檚 core by 鈥渁s much as 90 feet every two to three days,鈥� according to Jim Green, NASA鈥檚 director of Planetary Science Division. This motion generates the energy needed to power hot water vents where ocean meets rock, playing the role drifting continents do on Earth and giving rise to a new Goldilocks zone sandwiched between fire and ice.
And yet, nothing can live on heat and water alone. You also need food, and almost all Earth-life ultimately runs on sunlight. Plants form the foundation of land based food chains while photosynthesis-capable phytoplankton support those of the ocean. Even beings lurking in dark depths benefit from scavenging what sinks from the surface.
It wasn鈥檛 until the late '70s that science discovered life had found another energy source: chemicals. Holes in the sea floor spewing molecule-rich jets of hot water known as hydrothermal vents turned out to house whole ecosystems thriving in perfect darkness.
So-called chemosynthetic microbes underpin these webs of life, underscoring biology鈥檚 broad resilience and opening the door to the possibility of life on harsh moons of the outer solar system, where dozens of miles of ice keep oceans even darker than their terrestrial counterparts.
鈥淭he Enceladus case is a perfect example of how these amazing discoveries that Cassini is making really require an understanding of, in this case, what's going on at the bottom of Earth's oceans to be able to interpret,鈥� Dr. Lunine says. And without that data, 鈥淲e really wouldn't be able to interpret what we're seeing for Enceladus.鈥�
Cassini has been gradually building the case for habitability during its nearly 13 years in the Saturn system, revealing tantalizing hints of vent activity via silica it sniffed with its instruments during repeated dives through Enceladus鈥檚 icy plumes. Now the discovery of molecular hydrogen supplies the last piece of the puzzle, confirming that the moon鈥檚 floor has all the same elements as certain microbe friendly vents on Earth, such as the
鈥淭he big question was [hydrogen], so the confirmation of [hydrogen] in the plume is very important and confirms that the ocean of Enceladus would be a nice place for life, methanogens in particular,鈥� writes Chris McKay, a NASA Ames Research Center astrobiologist who was not involved with the paper, in an email.
Methanogens are microbes that process hydrogen and carbon dioxide for energy, giving off methane. Cassini has also detected methane in the ice plumes, and while non-biological sources of methane are still possible, this discovery of hydrogen supplies another intriguing piece of the puzzle.
The team considered other sources besides the vents, but concluded that the moon is too small to have attracted hydrogen with its gravity and held onto it for hundreds of millions of years, leaving vents as the only remaining option.
"I think that Cassini has pretty much put the nail in the ocean on this one and the Enceladus ocean is habitable," says Lunine.
Questions remain
But don鈥檛 start planning your Enceladus fishing trip just yet. While the presence of hydrogen is exciting, its abundance is almost concerning. If there鈥檚 something swimming around down there, it isn鈥檛 very hungry. 鈥淲e have this buildup of food that鈥檚 not being used,鈥� said NASA astrobiologist Mary Voytek during the press conference. 鈥淢y money for the moment is still on Europa, but it still could be on any of these moons.鈥�
Furthermore, not everyone agrees that the discovery of molecular hydrogen on Enceladus means the moon is certainly habitable.
"I would say that it is definitely a possibility, but to say straight out that it is a habitable environment is maybe going a little bit too far," as habitability requires more than just energy and water, Alexandra Pontefract, a geomicrobiologist at MIT who was not involved in the research, says in a phone interview.
"It would also be incredibly interesting to understand how life could have arisen in such an environment," she adds. "On Earth, though vent systems have long been the focus of origins of life research, we have yet to adequately explain how RNA and DNA (the basis for heredity) could be synthesized" in such a wet environment.聽As far as scientists know, the chemical soup needs to dry out for the chemistry to be catalyzed to form these key molecules.
But, Dr. Pontefract says, the detection of molecular hydrogen does raise the possibility that there could be a food source for microbial life deep in Enceladus鈥� oceans. "Enceladus represents a really fantastic target for life detection within our solar system," she says.
This finding of molecular hydrogen in Enceladus鈥檚 plumes comes just as the Cassini mission is coming to a close. The mission will conclude as the spacecraft, which was launched in October 1997, will dip into Saturn鈥檚 atmosphere in September where extreme pressure and heat will permanently retire the craft.
Cassini鈥檚 instruments aren鈥檛 up to the task of detecting life directly, so more answers about the water worlds will have to wait for upgraded toys on future missions.
As for Europa, it won鈥檛 be long until scientists are able to take a closer look at what NASA is now almost completely certain are water plumes, after observing repeated bursts in the same location. The agency plans to launch a spacecraft to that icy moon of Jupiter in the early 2020s. The Europa Clipper, as the mission has been named, will point cameras and spectrometers at the moon to study its icy crust and search for subsurface lakes and oceans that are suspected to be fueling icy plumes.
By that time Cassini will be long gone, but the Clipper will continue its mission of probing these alien water worlds, probing our fundamental questions about life of both the terrestrial and extraterrestrial varieties.
鈥淸These missions] are an expression of our human desire to explore in a peaceful way, our ability to create marvelous machines that can be our proxies all the way across the solar system,鈥� says Lunine.
[Editor's note: This article has been updated to clarify Alexandra Pontefract's comments.]
