Cassini Enceladus
An illustration of NASA's Cassini spacecraft flying through a plume above the surface of Saturn's moon Enceladus. Credit: NASA/JPL-Caltech

WASHINGTON — Discoveries involving two “ocean world” moons in the outer solar system announced April 13 are likely to bolster the case for planned and proposed spacecraft missions to those worlds.

At a press conference, NASA announced that its Cassini spacecraft, orbiting Saturn, had detected hydrogen gas in previously-discovered plumes emanating from the surface of the icy moon Enceladus. Scientists suspect that the moon has an ocean of liquid water beneath the surface that provides the source material for the plumes.

The hydrogen, scientists said at a briefing, is likely produced by hydrothermal activity, and could serve as an energy source for any life there. “Although we can’t detect life, we’ve found that there’s a food source there for it. It would be like a candy store for microbes,” said Hunter Waite, team lead for Cassini’s ion and neutral mass spectrometer, in a statement about the discovery.

In a separate finding presented at the same briefing, scientists using the Hubble Space Telescope detected new evidence of plumes emanating from Europa, the icy moon of Jupiter also thought to have a subsurface ocean. Such plumes had been seen in past Hubble observations of the moon, but limited data, in part because the plumes appear to be intermittent, had left some scientists skeptical about whether they exist.

The new observations give scientists greater, but not complete, confidence Europa has plumes. William Sparks, an astronomer with the Space Telescope Science Institute, said at the briefing the detection was made at the “four sigma” level of confidence, which made it unlikely, but not impossible, that the detections were simply random noise or instrumental effects.

“That’s not quite as strong evidence as you’d really like,” he acknowledged. “I wouldn’t say it’s completely unequivocal the way it is with Enceladus. We’re still at the limits of what Hubble can do. But we’re growing in confidence.”

Follow-up observations will require new missions. NASA is already developing a mission to Europa, called Europa Clipper, that will go into orbit around Jupiter and make dozens of close flybys of the moon to better determine how habitable it is.

Among the suite of instruments on Europa Clipper is an ultraviolet spectrometer. “This imager is going to be the plume finder,” said Jim Green, director of NASA’s planetary science division.

He added that, hopefully, the spacecraft will be able to fly through a plume, much as Cassini did at Enceladus. “Now we’ll have the right set of observations to make,” he said. That includes an advanced mass spectrometer that will be able to measure the composition of the plume.

Europa Clipper is being developed for launch as soon as 2022, and could arrive at Jupiter in late 2024, depending on the choice of launch vehicle. However, scientists acknowledge that it’s unlikely that the spacecraft will alone be able to determine if there is life in Europa’s oceans. A follow-on lander mission is in the early stages of development, although the Trump administration’s fiscal year 2018 budget blueprint, released March 16, included no funding for a lander despite supporting Europa Clipper and other planetary programs in that budget proposal.

The prospects for missions to Enceladus are less clear. Cassini is nearing the end of its mission, as NASA plans a final series of close-in investigations of Saturn and its rings prior to flying the spacecraft into Saturn itself in mid-September. With Cassini running low on fuel for its thrusters, that maneuver is intended to prevent the spacecraft from possibly crashing into, and contaminating, the potentially habitable moons of Enceladus and Titan.

Cassini also lacks the instrumentation to better understand if Enceladus is inhabited. “Cassini can look for habitability, but we don’t have the instruments to look for life,” said Linda Spilker, Cassini project scientist, at the briefing. “We’ve come as far as we can go, so it remains for a future mission to detect life at Enceladus.”

NASA currently has no mission to Enceladus, or Saturn in general, on the books. However, so-called “ocean worlds,” which include Enceladus and Titan, are one of the proposed destinations for the next mission in the New Frontiers program of medium-class planetary missions. NASA issued an announcement of opportunity for that next mission late last year, with proposals due to NASA April 28. The agency expects to select several proposals in November for additional study, with a final selection in mid-2019.

Congress, in its fiscal year 2016 appropriations bill, directed NASA to develop an “Ocean Worlds Exploration Program” to search for life on such worlds using a mix of small and large missions. That language was included by Rep. John Culberson (R-Texas), chairman of the appropriations subcommittee that funds NASA and a strong advocate for missions to Europa in particular.

A formal program is still being established within the agency, officials said at the briefing. Mary Voytek, astrobiology senior scientist at NASA Headquarters, said a “roadmap” document outlining studies of ocean worlds is in the final stages of development by an advisory group. “They’re about to deliver that to us any day now,” she said. That document, she said, will set science priorities and technology requirements for any future missions.

Some scientists have questioned whether Enceladus, with its constant plumes containing chemical energy that could support life, may be a better initial target than Europa. Voytek noted that the presence of hydrogen in the plume indicates that it is not being consumed by any life that might exist in the oceans in Enceladus. “It means that there might not be life there at all, and if there is life, it’s not very active,” she said.

She speculated that could be linked to the age of Enceladus, which may be much younger Saturn itself. Europa, by contrast, was formed at the same time as Jupiter, more than four billion years ago. “That’s a lot more time for life to have emerged and start taking advantage of these energy sources,” she said. “So my money, for the moment, is still on Europa.”

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...