We’ve had a few good blows this fall but none compare to what’s underway on the solar system’s seventh planet. Storm clouds have billowed up on Uranus over the past several months, so big and bright that even amateur astronomers have photographed them.
“The weather on Uranus is incredibly active,” said Imke de Pater, professor and chair of astronomy at the University of California, Berkeley, and leader of the team that first noticed the activity when observing the planet with the 10-meter (393.7-inch) Keck telescope in Hawaii.
How big is this bad boy? Observing at a variety of wavelengths, the Hubble Space Telescope tracked multiple storm fronts extending over a distance of more than 5,760 miles (9,000 km) and clouds at a variety of altitudes. That’s nearly a fifth of the planet’s 31,518 mile-diameter!
Uranus, four times the size of Earth and nearly twice as far from the Sun as Saturn, is a bitter cold planet rich with water, methane and ammonia ice in its interior swaddled in an atmosphere of hydrogen and helium with just enough methane to give it a blue tint.
Through a telescope, Uranus is normally a featureless dot that few amateurs astronomers bother to photograph. But once news of the storm got out, French amateur Marc Delcroix used the 39-inch (1-m) Pic du Midi telescope to give it a try. He nailed it on the second night out:
“I was so happy to confirm myself these first amateur images on this bright storm on Uranus, feeling I was living a very special moment for planetary amateur astronomy.” said Delcroix, who works for an auto-parts supplier in Toulouse, France. Anthony Wesley of Australia also succeeded in photographing the storm on September 19th and October 2nd with his 16-inch reflecting telescope. I’ve not heard whether anyone has actually seen it. Perhaps it might be possible with a large amateur scope, the right filter and a darn good night.
Bright clouds seen by amateurs and pros alike are probably caused by gases such as methane rising in the atmosphere and condensing into highly reflective clouds of methane ice.
De Pater and team detected eight large storms in all in Uranus’s northern hemisphere when observing the planet with the Keck Observatory on Aug. 5 and 6. Interestingly, the extremely bright storm in the Keck photos is not the one seen by the amateurs, which is much deeper in the atmosphere, below the uppermost cloud layer of methane ice crystals. Clearly, this Uranian hurricane rages at multiple levels in the planet’s atmosphere.
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. Had it occurred during during Uranus’ every-42-year-equinox in 2007, when the Sun shined directly over the equator, no one would have been surprised, but what could cook up a meteorological megastorm with little input from the Sun is anyone’s guess.
“The colors and morphology of this cloud complex suggests that the storm may be tied to a vortex in the deeper atmosphere similar to two large cloud complexes seen during the equinox,” said Larry Sromovsky, de Pater’s colleague and planetary scientist at the University of Wisconsin-Madison. So the storm may have deeper roots. Yet Uranus is the coldest planet in the solar system, even colder than Neptune, which radiates 2.6 times the energy it receives from the Sun into space. Uranus hardly leaks at all.
Maybe the storm is a dust devil writ large, where warmer and denser air from the lower atmosphere suddenly rises through cooler, low-pressure air higher up and begins to rotate when conditions are just right.
“These unexpected observations remind us keenly of how little we understand about atmospheric dynamics in outer planet atmospheres,” wrote De Pater and team in their report. Still, it’s nice to know the team’s willing to put up with years of bland to wring a new discovery now and then.