These Moons Are Dark and Frozen. So How Can They Have Oceans?

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,”...


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