It never rains but it storms on Uranus. Astronomers have spotted a slew of squalls on this usually quiet, distant planet — two of them giant storms that took professional and amateur astronomers by surprise.
The findings, presented this week at the American Astronomical Society's Division of Planetary Sciences meeting in Tucson, show that the mysterious, blandly blue planet might be far more complicated than scientists had thought.
Team lead Imke de Pater, a planetary astronomer at UC Berkeley, said she and colleagues first spotted the strange activity in August while using the Keck Observatory in Hawaii to study weather patterns on Uranus — which they had been doing regularly for more than a decade.
To their surprise, they saw eight large storms, including the brightest tempest ever seen on the planet at 2.2 microns — the wavelength of light that picks up clouds hanging just below the tropopause (the boundary between the troposphere and the stratosphere).
"It was just by chance I was observing and noticed these incredible storms that we really had never seen before," De Pater said in an interview. "You always have to be looking, I guess, to see things like that — so this was one of those lucky times."
The squalls were highly unusual, De Pater said. The scientists might have expected this kind of activity in 2007 — that's when Uranus had its equinox (a once-in-42-years event) and the sun would have been shining directly over the equator.
But for such storms to brew up some seven years later?
"People had predicted that the maximum in cloud activity would be around 2007 … and indeed we did see cloud activity," De Pater said. "But it was nothing compared to what we saw in August."
The scientists wanted to study this strange phenomenon further, but they didn't have a lot of telescope time. So they alerted the amateur astronomy community to the storms to see if its members could help.
Amateur astronomers picked up another bright feature, but this one was visible only at a wavelength of 1.6 microns — which meant that this storm was brewing deeper in the atmosphere. The scientists also later used the Hubble Space Telescope to find storm systems at a variety of depths and across a distance of 5,760 miles.
These storms on an usually quiet Uranus have astronomers scratching their heads, De Pater said. The three other gassy giant planets — Jupiter, Saturn and Neptune — all seem to have strong internal heat sources, and that energy could help stir up storms in the atmosphere. But Uranus doesn't appear to have one — which means that the sun must be mostly responsible for generating such disturbances in the planet's atmosphere, astronomers had thought.
But seven years after the equinox, there is not enough sun to explain these massive storms, De Pater pointed out. So it means that the inner workings of Uranus, which remain hidden from view, are more complex than scientists expected.
"Well, at least it tells us that the theories have to be adjusted," De Pater said. "They are not representative of reality."