How can you tell what’s in the atmosphere of a planet that’s over one billion miles from Earth?

Use a VERY long sampling straw?  Nope.
Use a spectrometer? Yup.  (Explore how this works in our “Is there life outside of Earth?” investigation.)
Scientists at NASA, the European Space Agency, and the Italian Space Agency collaborated to send the Cassini spacecraft to Saturn to get closer looks at the planet, its rings, and its many moons.  In November 2010, the exciting news came back that oxygen was discovered in the atmosphere of Rhea, one of the moons.
It took five years of measurements and data analysis to collect enough data to fully support the discovery.
So if there’s oxygen, there’s life, right?  Nope.  There are lots of organisms on Earth that don’t need oxygen.  And Rhea is too far from the Sun to be in a “habitable zone” anyways.
This discovery, relatively close to home, shows that we can detect molecules in the atmosphere of an orbiting body nearly one billion miles away.
And if we can make the measurement from that far away, it’s only a matter of time until we’ll be able to measure atmospheres of planets and moons outside of our solar system!

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