Uranus Clouds Reveal a Rotten-Egg Clue About Its Formation

Astronomers using Gemini North spectroscopy detected trace hydrogen sulfide above Uranus’s cloud tops (about 0.4–0.8 parts per million), confirming a sulfur-bearing layer and showing Uranus is chemically distinct from the inner gas giants. The planet’s blue color comes from methane, not sulfur, and while the “rotten egg” smell is a real footnote, any visitor would be overwhelmed by the extreme cold and unbreathable atmosphere long before odor could be noticed. The finding helps constrain how Uranus formed and where its clouds originate.