“The weather on Uranus is incredibly active,” said team leader Prof Imke de Pater of the University of California, Berkeley.
“This type of activity would have been expected in 2007, when Uranus’s once-every-42-year equinox occurred and the Sun shined directly on the equator. But we predicted that such activity would have died down by now.
Why we see these incredible storms now is beyond anybody’s guess,” said team member Heidi Hammel of the Association of Universities for Research in Astronomy.Uranus is an ice giant, about 4 times the diameter of Earth. It has an atmosphere of hydrogen and helium, with just a bit of methane to give it a blue tint.
Because it is so distant – 19 times farther from the Sun than Earth – the astronomers were able to see little detail on its surface until adaptive optics on both Keck Observatory telescopes revealed features much like those on Jupiter.
Prof De Pater and her colleagues have been following the planet for more than a decade, charting the weather, including bands of circulating clouds, massive swirling storms and convective features at its north pole.
Bright clouds are probably caused by gases such as methane rising in the atmosphere and condensing into highly reflective clouds of methane ice.
Because Uranus has no internal source of heat, its atmospheric activity was thought to be driven solely by sunlight, which is now weak in the northern hemisphere.
Hence the astronomers were surprised when the observations showed such intense activity.
They detected eight large storms on Uranus when observing the planet with the Keck Observatory on August 5 and 6, 2014.
One was the brightest storm ever seen on the planet at 2.2 microns, a wavelength that senses clouds just below the tropopause, the lower boundary of the stratosphere – where the pressure ranges from about 300 to 500 mbar, or half the pressure at Earth’s surface.
The storm accounted for 30 % of all light reflected by the rest of the planet at this wavelength.
Another storm is below the uppermost cloud layer of methane ice in Uranus’s atmosphere and may be tied to a vortex in the deeper atmosphere. Such vortices could be anchored much deeper in the atmosphere and extend over large vertical distances, as inferred from similar vortices on Jupiter, including its Great Red Spot.
Prof De Pater and her colleagues reported the results on November 12 at the46th Annual Meeting of the American Astronomical Society’s Division of Planetary Sciences in Tucson, Arizona.