The original version of this story appeared in Quanta Magazine.
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
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