No one ever said that Uranus was anything but a strange world. Nineteen times farther from the Sun than the Earth, the planet’s equator is tilted 98 degrees from its orbital plane. The tilt is so profound that if you work out the averages, the Uranian poles get more sunlight than the equator. That could lead to interesting weather patterns on a world with an 84-year orbit where seasons last twenty-one years. Such seasonal subjects have been the subject of recent study using imagery from the Keck II instrument in Hawaii, the results presented at the Division for Planetary Sciences meeting this week in Ithaca, NY.

Uranus reached equinox in 2007 when the Sun attained a position directly over the planet’s equator. Having equal amounts of sunlight over northern and southern hemispheres is obviously not a routine occurrence for this planet, but it’s a good chance to look at what’s happening on the meteorological front. Lawrence Sromovsky (University of Wisconsin) notes that seasonal weather changes are driven by the variance in solar energy caused by the tilt of a planet on its axis (the term for this is ‘seasonal forcing’). But the data gathered by Keck shows that Uranus features a unique lag in responding to that input. Says Sromovsky:

“Although both hemispheres were symmetrically heated by sunlight at equinox, the atmosphere itself was not symmetric, implying that it was responding to past sunlight instead of current sunlight, a result of Uranus’s cold atmosphere and long response time.”

Cold indeed. We’re talking about atmospheric temperatures at the cloud tops of minus 215 degrees Celsius. We’re also talking about winds that can reach speeds of 900 kilometers per hour. The image below displays recent atmospheric features and their changes:

Image: Near-infrared images from the Keck II telescope show the planet Uranus in 2005 (left), with the rings at an angle of 8 degrees, and at equinox in 2007 (right pair), with the planet’s ring system edge-on. In all images, the south pole is at the left and the equator is directly below the rings. Credit: Imke de Pater, University of California, Berkeley; Heidi Hammel, Space Science Institute; Lawrence Sromovsky and Patrick Fry, University of Wisconsin-Madison. Obtained at the Keck Observatory, Kamuela, Hawaii.

Notice the cloud structure in the planet’s southern hemisphere (at left of the image and near the bright band at bottom), which may be dissipating as it drifts north, a motion that is probably the result of seasonal change. The vortex has evidently been in existence for years, if not decades, at between 32 degrees and 36 degrees south latitude. At the same time, the bright feature in the northern hemisphere in the 2005 image seems to correspond to the smaller bright area in the rightmost 2007 image. Note that the southern band is not as bright as it was in 2005, while a new northern band is now brightening. The expectation is that the bands will completely reverse by the time of the next equinox.