Clouds on Uranus Smell Like Rotten Eggs

An international team of scientists has found evidence of hydrogen sulfide—the gas that gives rotten eggs their unpleasant smell—throughout the upper atmosphere of Uranus, something that has been predicted by researchers but never proven.

Using the Gemini North telescope in Hawaii, the team discovered large quantities of the foul gas in the cloud tops of the planet. The findings, which are published in the journal Nature Astronomy, shine new light on the composition of Uranus's clouds—one of the Solar System's long-standing mysteries.

The gas was identified with the help of an extremely sensitive instrument, known as the Near-Infrared Integral Field Spectrometer (NIFS), which the scientists used to detect reflected sunlight from a region immediately above the main visible cloud layer on Uranus. Normally, the instrument is used to study objects such as black holes at the center of galaxies.

Scientists have long debated whether hydrogen sulfide or ammonia dominate the cloud tops of Uranus but have lacked the evidence to make a definitive conclusion. Detecting either of these substances has been tricky because when these cloud layers form by condensation, the cloud-forming gas is hidden away deep in an internal reservoir, with only tiny quantities remaining above the clouds.

But the capabilities of the NIFS—in addition to newly available data regarding the wavelengths at which different gases absorb light—enabled the researchers, for the first time, to identity even these faint traces, planetary physicist Patrick Irwin from the University of Oxford told Newsweek.

This image of a crescent Uranus, taken by Voyager 2 on January 24, 1986, reveals its icy blue atmosphere. Despite Voyager 2’s close flyby, the composition of the atmosphere remained a mystery until now. Scientists have found evidence of hydrogen sulfide throughout the upper atmosphere of Uranus. NASA/JPL

The new results demonstrate how the atmospheric compositions of the outer gas giants differ from those of the inner gas giants—Jupiter and Saturn—shining a light on the formation and evolution of planets in the Solar System.

"The implications of this are that during the formation of the solar system, Uranus (and probably Neptune) formed at a greater distance from the sun where it was cold enough that both ammonia and hydrogen sulfide were in condensed icy form and thus easily incorporated into the growing planetary embryos that would eventually become Uranus (and Neptune)," Irwin said.

"Closer to the sun, ammonia would be preferentially in gas form and thus less easily incorporated into the planetary embryos that became Jupiter and Saturn. When scientists model planetary formation they find that in some circumstances the planetary embryos can move towards/away from the sun, a process called migration, and, in some cases can even swap positions," he added. "What this study shows is that while we cannot rule out some migration, it would seem very unlikely that the embryos swapped position."