Though it looks like a smooth, solid, pale blue orb, there’s more going on beneath the clouds of Uranus than meets the eye.
A polar cyclone has been spotted at the planet’s north pole researchers report in the May 28 Geophysical Research Letters. Observed with radio telescopes, the find is the first direct evidence of a cyclone on Uranus. A previous spacecraft flyby hinted at a similar storm at the planet’s south pole.
“It’s really exciting to see this polar structure come into view,” says Michael Roman, a planetary scientist at the University of Leicester in England who was not involved with the research. The observations “show a rather unique structure that we simply have never been able to study before.
In 1986, NASA’s Voyager 2 spacecraft revealed winds at the center of Uranus’ south pole were moving faster than those in neighboring areas and were rotating. This evidence pointed to something dynamic like a cyclone occurring at the pole. But the spacecraft’s instruments weren’t sensitive enough to confirm the storm.
In recent years, as the north pole of Uranus turned more toward Earth, scientists were able to probe the other side of the ice giant, where they spotted similar hints of a swirling storm. Using the Very Large Array radio observatory in New Mexico, planetary scientist Alex Akins and colleagues probed the temperature under the clouds for more atmospheric clues. “What we saw with the VLA was kind of the last piece of [evidence],” says Akins, of the Jet Propulsion Laboratory in Pasadena, Calif.
New thermal emission observations from 2021 and 2022 show a spot on the north pole where the gas beneath the clouds is warmer and drier than its surroundings, suggesting the presence of a low-pressure region in the midst of those spinning winds. “These contrasts look similar to what we see in hurricanes on Earth,” Akins says.
Excluding Mercury, all the planets in our solar system have now been observed to host some kind of swirling air mass at their poles (SN: 10/14/08).
Akins plans to continue to observe the cyclone to see how it changes. Previous observations from 2015 suggest it is growing stronger. Researchers wouldn’t expect Uranus’ atmospheric circulation to change on such a relatively short timescale, Akins says, so a continued strengthening of the storm would suggest there’s more to learn about how the planet’s atmosphere works.
In 2022, experts brought together by the National Academies of Sciences, Engineering, and Medicine recommended NASA send a probe to Uranus (SN: 4/20/22). “The more we can learn about Uranus as we begin to plan a mission,” Roman says, “the better we can focus our planning for that mission.””“That’s a perfectly reasonable explanation,” says atmospheric physicist Richard Sonnenfeld of the New Mexico Institute of Mining and Technology in Socorro, who wasn’t involved in the study. Alternatively, he says, the signals could be produced as pulses of electrical current propagate back and forth along tendrils of lightning that have already formed, rather than from the stop-and-go advancements of a new bolt. On Earth, such currents cause some bolts to appear to flicker.
But stop and go seems like a sound interpretation, says atmospheric physicist Yoav Yair of Reichman University in Herzliya, Israel. Kolmašová and her colleagues “show that if you’re discharging a cloud … the physics remains basically the same [on Jupiter as on Earth], and the current will behave the same.”
If that universality is real, it could have implications for the search for life elsewhere. Experiments have shown that lightning strikes on Earth could have smelted some of the chemical ingredients needed to form the building blocks of life (SN: 3/16/21). If lightning is discharging in a similar way on alien worlds, Yair says, then it could be producing similar ingredients in those places too.
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