Hubble Space Telescope data revealing Ganymede's ultraviolet aurorae overlays a visible-light image of the icy moon taken by NASA's Galileo orbiter. The ultraviolet aurorae are rocking back and forth in synchrony with Jupiter's magnetic field, suggesting the presence of a large ocean beneath Ganymede's surface. Credit: NASA/ESA/J. Saur/JPL/The Galileo Project

Scientists are finding liquid water, the cornerstone for life as we know it, in surprising nooks and crannies of the solar system.

Following Wednesday’s news that there seem to be hydrothermal vents churning away in the warm, alkaline seas inside Saturn’s moon Enceladus, researchers announced airtight evidence yesterday that Jupiter’s moon Ganymede also has a sizable ocean beneath its icy crust. Based on theoretical models and indirect evidence from the Voyager and Galileo missions, astronomers have assumed for decades that Ganymede harbors a subsurface ocean. But these latest measurements, obtained using the Hubble Space Telescope, make the case even more compelling. At the same time, our water-soaked solar system makes the failure to find extraterrestrial life all the more puzzling.

Ganymede is the largest moon in the solar system—only slightly smaller than Mars, and bigger than Mercury and Pluto. Thanks to its bulk, Ganymede also has a substantial magnetic field, which interacts with particles trapped in Jupiter’s larger magnetic field to create faintly glowing ultraviolet aurorae around the moon. As Jupiter rotates, its magnetic field causes Ganymede’s aurorae to slightly rock back and forth.

Researchers realized that, if Ganymede’s purported subsurface ocean were large and salty enough, it would be sufficiently electrically conductive to fight against Jupiter’s magnetic field and slightly dampen the auroral rocking. Looking with Hubble, that’s exactly what they saw. Instead of rocking back and forth by some six degrees, as would be expected if there were no subsurface ocean, Ganymede’s aurorae was rocking only by two degrees. That amount is broadly consistent with an ocean up to 100 kilometers deep, buried beneath perhaps 150 kilometers of ice, although the ocean’s actual depth and location could vary a bit based on just how much electrically conductive salt it contains.

You might think this new finding dramatically boosts hopes for finding life within Ganymede, but that’s not really the case. The latest Hubble data fit nicely with previous work that suggests Ganymede’s subsurface ocean is sandwiched between two layers of ice—the upper icy crust, and the lower icy mantle. That pattern would mean the ocean is isolated from Ganymede’s rocky core, and thus may be deprived of minerals and nutrients that can sustain metabolisms in the absence of sunlight. The subsurface oceans of Enceladus and Jupiter’s icy moon Europa, by comparison, both seem to sit on top of a rocky core, providing an abundance of usable energy and raw material for any aquatic life. Enceladus’s surface is also venting plumes of seawater into space, and Europa might be too—a sign that these oceans are connected to the surface, allowing nutrients to trickle down from above and making further investigations easier. No plumes have been observed on Ganymede, and unlike the geologically active and young surfaces of Enceladus and Europa that are being constantly replenished from below, its surface is crater-pocked, inert and ancient. Taken together, all this suggests Ganymede’s ocean is probably lifeless, and isolated almost inaccessibly deep beneath the surface.

Of course, we need to learn more, and the good news is that soon we will. The European Space Agency’s Jupiter Icy Moons Explorer (JUICE) is planned for launch in 2022 and would arrive at the giant planet in 2030, where it will spend most of its time studying Ganymede and its subsurface ocean.

The other good news is that Ganymede, Enceladus, and Europa are almost certainly only the tip of the iceberg for liquid water in the solar system. It increasingly seems that subsurface oceans are the norm for all icy worlds of a certain size—similar reservoirs are suspected within Saturn’s moon Titan, Neptune’s Triton, Jupiter’s Callisto, and even the dwarf planets Ceres, Eris, and Pluto. And if it can happen here, why not within other icy worlds, such as exomoons orbiting stars outside our solar system?

For decades, astrobiologists have been obsessively “following the water,” reasoning that, just as on Earth, wherever there is liquid water, there is also life. But as evidence mounts for an abundance of extraterrestrial oceans in our apparently lifeless solar system, some very smart people are getting nervous that the logic of our search is somehow flawed, or that we are ominously alone. On Twitter, SpaceX CEO and hopeful Mars colonizer Elon Musk had this to say about the Ganymede news:

The cosmic prevalence of liquid water and other conditions amenable to life would seem to suggest we should find cosmic company practically everywhere we look, whether microbes or little green men in flying saucers. And yet the universe so far seems sterile, silent, and dead beyond Earth. So where is everybody? That’s the “paradox,” as stated by the physicist Enrico Fermi in 1950. The current en vogue explanations are rather gloomy: They posit that the emergence of intelligence or even life itself faces long odds, or that technological civilizations are unstable and short-lived even if life and intelligence are common. Either way, we’d probably never find anyone else to talk to, and the latter scenario would suggest we’re probably sliding toward our own extinction.

To my mind, though, the prevalence of subsurface oceans—“roofed worlds,” as some call them—doesn’t exacerbate Fermi’s Paradox. Instead, it makes it easier to explain. To quote from a piece I wrote for Aeon Magazine last year:

Ice-roofed worlds might be the default abodes for biology in the Universe. Life within a roofed world could proceed swimmingly against any number of otherwise-fatal cosmic calamities, whether being slingshotted into the interstellar dark as a rogue planet, or being bathed in hard radiation from a nearby supernova or burping black hole. We could then guess why, like our solar system, the Universe at large looks so desolate to us. In this scenario, most life, even if it had eyes to see, would never glimpse sky, stars, light, or fire, and would have scant hope of ever reaching what lies above and beyond its icy shell.

So let’s keep a cool head. The universe is very big, and we’ve barely explored any of it. Life may be all around us, just hidden and locked away beneath dark, frozen-solid skies. It’s time to gear up for ice fishing in the outer solar system.