And but, defiantly, these alien seas stay liquid.

A Mirror-Wrapped Ocean

Scientists suspect {that a} handful of moons orbiting Jupiter and Saturn—and possibly even some spinning round Uranus and Neptune—harbor oceans. Hefty Ganymede and crater-scarred Callisto produce weak, Europa-like magnetic alerts. Saturn’s haze-covered Titan, too, very most likely has a liquid-water subsurface ocean. These “are the five that most scientists in the community feel pretty confidently about,” mentioned Mike Sori, a planetary scientist at Purdue University.

So far, the one absolute oceanic certainty is Enceladus. “That’s a no-brainer,” mentioned Carly Howett, a planetary scientist on the University of Oxford.

In the Eighties, some scientists suspected Enceladus had plumes; Saturn’s E ring was so clear and glossy that one thing—maybe from considered one of its moons—have to be leaking into area and continuously refreshing it. After Cassini lastly witnessed that planet-garnishing magic in motion, scientists briefly questioned whether or not the moon’s south-polar plumes could be the work of daylight vaporizing ice within the moon’s shell—a bit like dry ice boiling away when heated, maybe by daylight.

“For a while, there was this argument about whether there needed to be an ocean at all,” Nimmo mentioned. “What really nailed that was when [Cassini] flew through the plume and they found salt—sodium chloride. That’s an ocean.” There was nonetheless an opportunity that these plumes may very well be erupting from a smaller, extra remoted sea. But additional Cassini observations revealed that Enceladus’ shell is rocking forwards and backwards so acutely that it have to be separated from the moon’s deeper inside by a worldwide ocean.

The plumes additionally pump out hydrogen and quartz, indicators of deep-sea hydrothermal vent exercise, mentioned Frank Postberg, a planetary scientist on the Free University of Berlin. On Earth, such vents produce the warmth and chemistry wanted to energy ecosystems that exist past the attain of daylight—communities of organisms that scientists as soon as thought couldn’t exist in our photosynthetically dependent world.

But what may very well be powering a vent system sturdy sufficient to warmth a complete ocean? Another moon—this one of many fiery selection—would supply these clues.

The Eternal, Infernal Tides

In June 1979, a month earlier than Voyager 2’s shut flyby of Europa, scientists announced that Voyager 1 had glimpsed titanic, umbrella-shaped plumes billowing into area above Io—the eruptive fingerprints of a number of volcanoes.

This remark ought to have been baffling: Volcanism requires an inside warmth supply, and Io, like the opposite icy moons, ought to have been nothing greater than embers. But a couple of months earlier, an unbiased workforce of scientists had appropriately predicted that Io could be a hyperactive volcanic world.