An icy moon of Jupiter is looking more and more like it could hold alien life deep in its subsurface sea.
On Monday, NASA announced that scientists had officially measured water vapor on the moon, called Europa, for the first time.
The discovery is yet another sign that Europa has all the right ingredients for aliens — given the right chemicals and a little deep-sea volcanic activity, it's possible that life could spring up (or already has) deep in the saltwater ocean below Europa's surface.
Here's why scientists are increasingly looking to Europa in their hunt for alien life.
On Monday, NASA announced that scientists had measured water vapor on Europa for the first time.
The discovery is yet another clue that Europa could host alien life.
Scientists have long suspected that Europa conceals an ocean below its icy surface — possibly with twice the volume of Earth's oceans.
Liquid water is promising, but it's not enough. For life to arise, Europa needs two other ingredients: a few essential chemical elements, and a source of energy.
The necessary chemical compounds seem to be abundant on the icy moon: Scientists think Europa first formed with carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur.
In June, scientists spotted sodium chloride (also known as table salt) in Europa's icy surface, indicating that the ocean below is more like Earth's oceans than they previously thought.
But there's one major problem: The sunlight that fuels life on Earth is 25 times fainter on Europa.
But Europa's oceans may be much warmer than its surface, thanks to its oval-shaped orbit around Jupiter.
These tides stretch and relax Europa, cracking its surface ice and building friction that heats the moon from the inside.
These tides could crack Europa's mantle and give rise to deep-sea hydrothermal vents. On Earth, such vents produce intense heat that rips apart molecules and sparks chemical reactions.
Life on Earth crops up around these vents. These ecosystems don't need sunlight to survive.
Nobody knows yet if Europa has deep-sea vents, much less alien life. NASA plans to investigate these questions with its Europa Clipper mission.
The Clipper spacecraft is expected to fly through Europa's water vapor plumes to analyze what might be in the ocean.
That investigation could help scientists prepare to land a future spacecraft on Europa's surface and punch through the ice.