"We just don't know that much about Europa with certainty."
Ars Technica said: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.
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’s 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. Tellingly, the scientific team responsible for imaging the worlds the Voyagers passed by did not even include a geologist.
...