Of all the objects in the solar system that might be hospitable for life as we know it, no place is shouting louder for attention than Saturn’s moon Enceladus.
That could become a roar now that NASA’s Cassini orbiter has completed its latest and closest flyby. The spacecraft dipped deep into the plumes of water ice erupting from fissures in the moon’s icy surface to help answer the question: Could life truly thrive in the liquid-water ocean beneath the moon’s ice?
Up to now, evidence has favored yes. If Wednesday’s encounter reinforces the case, it could make Enceladus one of the most compelling objects in the solar system for further study.
Indeed, from Jupiter’s moon Europa, which is also thought to have an under-ice ocean, to Saturn’s moon Titan, where liquid methane falls as rain, runs as rivers, and collects in vast seas, some of the most intriguing places for exploration could be among the hardest to explore.
How, after all, do you explore an ocean that could be under miles of ice – or made of frigid methane? Scientists have already begun to consider how to answer these challenges, from a nuclear-powered lander equipped with a driller-probe to a mission that could return a sample of Enceladus ice to Earth.
Whether any of these visions become a reality could hinge in no small part on Cassini’s quick passage through Enceladus’s tenuous veil Wednesday.
Depending on the results, “I hope we are able to use this flyby as a motivator for moving forward to the next steps,” says Jonathan Lunine, a planetary scientist at Cornell University in Ithaca, N.Y.
In the search for life beyond Earth, Enceladus is particularly enticing because of its plumes. When researchers sample the plume, they are in effect sampling the under-ice ocean. No landing or drilling required.
Europa’s ocean, meanwhile, is also hidden beneath miles of ice, and only one, brief plume has ever been observed there. For its part, Titan’s hydrocarbon lakes and seas represent an environment so alien that some researchers say they aren’t sure how they would look for evidence of life.
Yet finding even microbial life on one of these outer-solar-system moons could be even more profound than finding similar life on Mars, some researchers suggest.
Life on Enceladus would represent strong evidence that life emerged in the inner and outer solar systems independently. Mars and Earth have exchanged enough material early in their histories that simple microbes, or at least some of the more complex chemical building blocks for life, could have originated on either planet and colonized the other, some argue.
As for Wednesday’s flyby, Cassini’s instruments aren’t set up to detect evidence for life, mission officials say. But they can spot key pieces of geochemical evidence that the ocean is livable.
Cassini has the capability “to tell us about the characteristics of that ocean,” says Curt Niebur, NASA’s Cassini program scientist.
Measurements of molecular hydrogen serve as direct evidence of hydrothermal activity on the ocean floor, which is thought to be driving the plume’s eruptions. Confirmation of the presence of hydrogen, and better measurements of its amounts, could also provide clues about hydrothermal activity, notes Linda Spilker, the mission’s project scientist. The more hydrogen Cassini finds, the higher the level of hydrothermal activity beneath the plume region.
Past flybys have revealed carbon dioxide, methane, and organic molecules in Enceladus’s plume.
“With our much deeper dive through the plume, we’ll have a chance to sample potentially larger particles," which could be new organics that researchers haven’t seen before, Dr. Spilker adds.
The team also is trying to tease out the nature of the plume sources. Some send material into space in narrow jets, suggesting conduits rising through the ice. Others spew material in curtain-like eruptions, suggesting that cracks in the ice may run all the way down to where ice meets ocean.
Each tells a story of how much heat is driving the eruptions, Spilker says.
The Cassini team hopes to have images from the flyby on the ground over the next day or two. Researchers anticipate a quick first-look at the data on composition next week.
What would a mission look like?
What comes next for Enceladus, and when, remains an open question.
Late last year, NASA bypassed a mission proposal to send an orbiter to sample the plume with two instruments capable of detecting evidence for life, says Dr. Lunine, who leads the team working on the mission, dubbed the Enceladus Life Finder. The team plans to refine and resubmit the proposal when another call comes for proposals for Discovery-class missions, which cost about $450 million each.
A variation on the orbiter theme involves a sample-return mission dubbed LIFE, for Life Investigation For Enceladus. An orbiter would gather samples from Enceladus’s plumes, as well as from Titan’s atmosphere and Saturn’s E-ring. Enceladus’s plumes provide the material for the ring. This, too, is envisioned at a Discovery-class mission.
An example of a more ambitious idea for perhaps the 2030s involves an orbiter with a lander and an ice drill to melt its way into Enceladus’s crust. Dubbed the Enceladus Explorer, the mission’s obiter/lander combination would orbit Enceladus while instruments on the orbiter identified potential landing sites near where the plumes emerge from the crust – a daunting task given the jumbled nature of the crust in the region.
After the lander touches down, it would release a driller called the IceMole. Researchers have tested an IceMole prototype in alpine glaciers in Europe and Antarctica, says Konstantinos Konstantinidis, a graduate student at the Bundeswehr University in Munich, Germany, and one of the architects of the Enceladus Explorer (EnEx) concept.
Late last year, a team led by the German Aerospace Center used the probe to burrow through more than 60 feet of ice to return uncontaminated samples of water from a lake beneath Blood Falls glacier in Antarctica.
On Enceladus, the IceMole would look for cells or cell-like structures in the water, for biochemical evidence of cell reproduction, and for byproducts of cell activity. Methane is one byproduct.
Such a mission presents enormous challenges, from the size of the rocket needed to get it into space to the size of the budget needed to get the project off the ground. Mr. Konstantinidis and colleagues estimate that such a mission would cost about 3 billion euros ($3.3 billion US), meaning several nations would need to pitch in to bear the cost, he says.
Similar hopes for a mission to Europa rose in 1996 when it became increasingly clear that the moon had a subsurface ocean. NASA responded with a plan for a Europa Orbiter, with a proposed launch in 2003. But NASA canceled the program.
NASA and the European Space Agency continued to weigh concepts, with the ESA now planning a Jupiter Icy Moons Explorer mission in 2022 and NASA moving forward on its own version to launch later in the decade. Earlier this year, NASA chose the nine science instruments that will be on its mission.
“Here we are, 16 years later, and we’re just getting going on another Europa mission,” Lunine says. “I really hope that’s not the story we’re going to see with the Saturn system.”