What Does Uranus Smell Like?
QUICK ANSWER
Uranus's upper atmosphere contains hydrogen sulfide, the same chemical that gives rotten eggs their notorious smell. A 2018 study confirmed the presence of hydrogen sulfide in Uranus's clouds. So technically, Uranus does smell like rotten eggs, at least where its atmosphere can be sampled.
Yes, the joke writes itself. Uranus's upper atmosphere contains hydrogen sulfide, the chemical that makes rotten eggs smell like rotten eggs. The discovery came in 2018 from observations of Uranus's clouds, and it confirmed something astronomers had long suspected: Uranus and Neptune have fundamentally different atmospheric chemistry than Jupiter and Saturn.
Does Uranus actually smell like rotten eggs?
Chemically, yes. A 2018 study published in Nature Astronomy, led by Patrick Irwin at Oxford University, detected hydrogen sulfide (H2S) in Uranus's upper atmosphere using observations from the Gemini North Telescope in Hawaii. Hydrogen sulfide is the same compound that gives rotten eggs their distinctive smell. If a human could somehow breathe Uranus's upper atmosphere (which would be impossible for many other reasons), they would smell rotten eggs. The discovery had been long suspected but not directly confirmed until then.
Why does Uranus have hydrogen sulfide?
Because of how it formed. Jupiter and Saturn have atmospheres rich in ammonia, not hydrogen sulfide. The difference comes from where each planet formed in the early solar system: Jupiter and Saturn formed in regions where ammonia was abundant, while Uranus and Neptune likely formed farther out, where temperatures were colder and hydrogen sulfide was more chemically stable. The atmospheric composition is essentially a chemical fingerprint of where in the early solar system the planet was built.
Can spacecraft directly smell Uranus?
Not really. Smelling requires breathing the atmosphere through biological sensors, which spacecraft don't have. What spacecraft can do is analyze the chemical composition of an atmosphere using spectrographic instruments, which is how the hydrogen sulfide was detected. Future Uranus missions could include atmospheric probes that directly sample the gas composition, providing even more detailed chemical readings. The 2023 planetary decadal survey recommended a Uranus orbiter and probe mission, which could include this kind of instrument.
Does Neptune smell the same?
Probably similar. Neptune has very similar atmospheric chemistry to Uranus, including hydrogen sulfide. The 2018 study that detected hydrogen sulfide on Uranus did so by studying clouds that are thought to be similar to clouds on Neptune. Direct confirmation for Neptune is still pending more detailed observations, but the chemical similarity between the two ice giants makes the rotten-egg signature likely on both planets. The smell would persist on any future mission that ventured close enough to actually sample the gas.
Uranus's upper atmosphere genuinely contains hydrogen sulfide, the chemical responsible for the smell of rotten eggs. A 2018 study confirmed the presence of the gas in Uranus's clouds, finally settling a question that had been speculated about for decades. The same chemistry probably applies to Neptune. The rotten-egg punchline turns out to be scientifically accurate.
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