We've known of Europa’s existence for more than four centuries, but for most of that time, Jupiter’s fourth-largest moon was just a pinprick of light in our telescopes— a bright and curious companion to the solar system’s resident giant. Over the last few decades, however, as astronomers have scrutinized it through telescopes and six spacecraft have flown nearby, a new picture has come into focus. Europa is nothing like our moon.

Observations suggest that its heart is a ball of metal and rock, surrounded by a vast saltwater ocean that contains more than twice as much water as is found on Earth.

In the depths of its ocean, or perhaps crowded in subsurface lakes or below icy surface vents, Jupiter’s big, bright moon could host life.

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