As the years go by, the chances of Europa hosting life seem to keep going down. But it's not out of contention yet.
Europa is simply full of surprises. We're talking about the second moon of Jupiter, just a little smaller than our own moon. It sits just over three quarters of a billion kilometers from the sun. At that distance, the sun is less than 5% of the brightness that it is here on the Earth.
It's…cold. Desolate. Inhospitable. There's water on Europa, for sure—water is by far the most common molecule in the entire universe, so it's not exactly hard to come by—but it's all frozen. The entire world of Europa is covered in a sheet of ice tens of kilometers thick.
But Europa has a surprise. Tidal forces from Jupiter stretch and squeeze Europa on its elliptical orbit. This heats the interior, keeping it molten. And so you have a molten interior and a frozen exterior, which means in the middle you have an ocean. A globe-spanning liquid water ocean. An ocean with more water than the Earth has. An ocean that is completely and totally alien to anything we experience or encounter on the Earth: forever blocked from sunlight, with a depth reaching up to a hundred kilometers.
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