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Based on the make-up of the planet and atmosphere, researchers have concluded that Mars smells like rotten eggs.
But since no human has directly visited any of these planets, how do we know how they smell? Researchers use a range of methods, including studying their chemical composition and using techniques such as spectroscopy, which detects individual wavelengths of light emitted by particular molecules. Hydrogen, oxygen, methane, nitrogen etc. all have their own unique light-emitting fingerprints. So when researchers look at sunlight bouncing off planetary atmospheres, they can match these fingerprints to their corresponding gas, learning not just which gasses are in the atmosphere, but also their abundance.