In its quest to find extant life in the Solar System, NASA has focused its gaze on the Jovian moon Europa, home to what is likely the largest ocean known to humans. Over the next decade, the space agency is slated to launch not one, but two multi-billion dollar missions to the ice-encrusted world in hopes of finding signs of life.

Europa certainly has its champions in the scientific community, which conducts surveys every decade to establish top priorities. The exploration of this moon ranks atop the list of most desirable missions alongside returning some rocky material from Mars for study on Earth. But there is another world even deeper out in the Solar System that some scientists think may provide an even juicer target, Saturn’s moon Enceladus. This is a tiny world, measuring barely 500km across, with a surface gravity just one percent of that on Earth. But Enceladus also has a subsurface ocean.

“I have a bias, and I don’t deny that,” says Carolyn Porco, one of the foremost explorers of the Solar System and someone who played a key imaging role on the Voyagers, Cassini, and other iconic NASA spacecraft. “But it’s not so much an emotional attachment with objects that we study, it’s a point of view based on the evidence. We simply know more about Enceladus.”

Tiny moon

This is true. Whereas NASA’s Galileo probe explored the Jupiter system during the 1990s, the space agency sent a more capable probe to Saturn in the 2000s. That spacecraft spent 13 years in orbit around Saturn, and it found conclusive evidence of not only an ocean on Enceladus, but one that is accessible through its large geysers. These jets of icy particles soar as much as 500km above the surface because there is so little surface gravity to restrain them.

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NASA/JPL/Space Science Institute

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NASA/JPL/GSFC/Space Science Institute

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NASA/JPL-Caltech

Moreover, Cassini was able to fly through these sprays on several occasions. Scientists studying data collected during multiple passes have confirmed that the ocean below is akin to deep oceans on Earth, and they have found the presence of large and complex organic molecules. All of this points to the possibility of life.

Scientists know less about Europa, said Porco, who recently spoke with Ars when she accepted the Eliza Scidmore Award at the 2018 National Geographic Explorers Festival. In contrast to Enceladus, for the plumes on Europa, scientists aren't sure whether these plumes originate from the ocean below Europa’s ice, or whether they contain any organic material at all.

"We just don't know that much about Europa with certainty," Porco said. “There is a lot of excitement, but it’s speculation at this point. Of course I’d choose Enceladus. We know it the best, and it stands the greatest chance of making that next big step.”

Truthfully, this is a rather happy debate for scientists to be having. The planetary science community had pretty much lost all hope for finding any signs of life (beyond Earth) in the Solar System during the 1970s. The Viking landers had touched down on Mars, finding nothing but cold, dry, dead soil. And scientists at the time didn’t have much hope for the gas giants beyond Mars, as they expected the moons of the outer Solar System to be a lot like Earth’s moon, with ancient, static features.

Water everywhere

Since the 1970s, however, a steady stream of robotic missions to Mars has found increasingly strong evidence of water beneath the surface today, and as surface lakes and rivers in the past. Further out in the Solar System, the Voyagers and subsequent Galileo and Cassini probes found myriad interesting and active worlds around Jupiter, Saturn, and beyond with strong evidence for subsurface oceans because they possessed warm interiors.

With ice on the outside, a core on the inside, and an ocean in touch with this core, some of these worlds started to look a lot like the bottom of Earth’s ocean, where life thrives near hydrothermal vents. Scientists began to get really excited as they built exploration programs around following the water.

It’s not just Europa or Enceladus that beg for more exploration. Another Jovian moon, Ganymede, probably has a subsurface ocean. Titan, of Saturn, has an interior water ocean too, as well as liquid hydrocarbons on its surface. New Horizons might even have found a subsurface ocean on Pluto. “It’s de rigueur,” Porco said. “Everybody wants to have a subsurface ocean these days.”

But NASA’s resources are limited, and exploring the outer Solar System costs a lot of money. One great advantage of Enceladus is that it brings the ocean into space, so NASA’s next mission wouldn’t have to land to look for life, Porco said. It could make multiple flybys (because of Enceladus’ tiny size, it is virtually impossible to get into orbit around the Moon). With a mass spectrometer, such a spacecraft could better characterize the composition of the icy particles, and a very capable microscope might be able to detect organisms.

The debate over Enceladus, Europa, and other intriguing worlds of the outer Solar System seems unlikely to be resolved soon, and probably will only be done when we finally fly the missions and gather in situ data. For now, it's probably best to content ourselves with the knowledge that there are are indeed some wonderful horizons yet to explore around our Sun, rather than just a few cold, dead, craters.