For most of humankind’s existence, Earth was the only known ocean-draped world, seemingly unlike any other cosmic isle.

But in 1979, NASA’s two Voyager spacecraft flew by Jupiter. Its moon Europa, a frozen realm, was decorated with grooves and fractures — hints that there might be something dynamic beneath its surface.

“After Voyager, people suspected that Europa was weird and might have an ocean,” said Francis Nimmo, a planetary scientist at the University of California, Santa Cruz.

Then, in 1996, NASA’s Galileo spacecraft passed by Europa and detected a strange magnetic field coming from within. “We didn’t understand what it was,” said Margaret Kivelson, a space physicist at the University of California, Los Angeles who was in charge of the spacecraft’s magnetometer. Eventually, she and her team realized that an electrically conductive fluid — something inside the moon — was convulsing in response to Jupiter’s immense magnetic field. “The only thing that made any sense,” Kivelson said, “was if there was a shell of liquid melt beneath the surface of the ice.”

In 2004, NASA’s Cassini spacecraft arrived at Saturn. When it observed Saturn’s small moon Enceladus, it found coruscating icy plumes erupting from vast chasms at the moon’s south pole. And when Cassini flew through these spouts, the evidence was unmistakable — this was a salty ocean vigorously bleeding into space.

Now Earth’s oceans are no longer unique. They’re just strange. They exist on our planet’s sunlit surface, while the seas of the outer solar system are tucked beneath ice and bathed in darkness. And these subterranean liquid oceans seem to be the rule for our solar system, not the exception. In addition to Europa and Enceladus, other moons with ice-covered oceans almost certainly exist as well. A fleet of spacecraft will explore them in detail over the next decade.

All of this raises an apparent paradox. These moons have existed in the frosty reaches of our solar system for billions of years — long enough for residual heat from their creation to have escaped into space eons ago. Any subsurface seas should be solid ice by now. So how can these moons, orbiting so far beyond the sun’s warmth, still have oceans today?

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