Mystery of bright spot on Uranus

Amateur astronomers with large telescopes and CCD cameras are being urged to turn them on the distant planet Uranus following reports of the appearance of a brilliant new feature.

The bright spot on Uranus observed with Gemini
The bright spot on Uranus observed with Gemini

Professional images taken using the 8.1-metre Gemini Telescope North on Hawaii have recorded a region said to be ten times brighter than the planetary background.

Leading planetary scientist Dr Heidi B Hammel, a key figure with the James Webb Space Telescope whose special interest is in the ice planets Uranus and Neptune, used her Facebook page yesterday to appeal for more observations. She said that if the feature was confirmed independently by enough amateur astronomers, it would be seen as a “target of opportunity” that would allow NASA’s Hubble space telescope to be switched from its scheduled observing programme to watch it.

Our image here was taken using Gemini by planetary scientisy Larry Sromovsky, of the University of Wisconsin-Madison, using Gemini. A line scan through the spot gives the brightness curve below the disk. Superimposed lines of latitude and longitude show how Uranus is tilted right over, believed due to an impact many aeons ago, so that it now rolls around the solar system on its side. Its north pole is at about 4 o’clock in the image.

The bright spot is thought to be some sort of eruption of methane ice high in atmosphere of Uranus. But Dr Sromovsky warned that it is unlikely to be so prominent for amateurs unless they are observing with specialised CCD equipment at longer wavelengths.

He told Skymania News today: “This is an H-band image, centered at 1.6 microns, close to the wavelength of maximum contrast for such features. Its contrast will decrease with decreasing wavelength, and will likely not be detectable by amateur astronomers, except possibly at the longer CCD wavelengths where the Rayleigh scattering background can be suppressed. Looking with a methane band filters at 890 nm might be productive, especially if the feature continues to brighten.

“The feature is not very large; instead its prominence is due to its high altitude, placing it above the intense absorption of methane in the deeper atmosphere. This is much higher than the 1.2-bar methane condensation level and thus it is expected to be predominantly composed of methane ice particles.”

Dr Sromovsky added: “The latitude of the feature is approximately 22.5° north planetocentric, which is a latitude nearly at rest with respect to the interior. So it should rotate around Uranus’ axis with nearly a 17.24-hour period. At the time of the image, the feature’s longitude was 351° West. That could change slowly in either direction.

“The low latitude is unusual. Previous exceptionally bright cloud features on Uranus were at close to 30° North, both in 1998 (Sromovsky et al. 2000, Icarus 146, 307-311) and in 2005 (Sromovsky et al. 2007, Icarus 192, 558-575). The 2005 feature oscillated ±1° about its mean latitude. The new feature might also oscillate in latitude, in which case its longitudinal drift rate might also vary with time.”

Uranus is a puzzle to astronomers because it is a cold planet that emits very little energy of its own, unlike Jupiter and Saturn which pump out heat as they contract. There also appears to be no large-scale convection in its atmosphere and the Voyager probes detected little weeather as they flew by in the early 1980s.

Dr Chris Arridge, of University College London’s Mullard Space Science Laboratory told us that the mega impact in its distant past may also have caused a massive loss of primordial heat left over from the formation of the solar system.

He said: “The consequence of that is that because there is so little heat coming up from inside Uranus, its atmosphere is completely driven by force of sunlight essentially. And so because it has got this large tilt in its axis one pole gets continually illuminated by sunlight for 42 years during half of its orbit while the other is in darkness and then the situation is reversed for 42 years. The atmosphere is very strongly forced and that is a situation we don’t get anywhere else in the Solar System.”

Dr Arridge, who is pushing for a joint NASA-ESA mission to Uranus, said that more cloud-like features and convection patterns were seen around the planet’s equinox in 2007-8, suggesting this was a seasonal effect.

Last month we published a stunning infrared image of Uranus taken by Mike Brown using the Keck telescope.

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By Paul Sutherland

Paul Sutherland has been a professional journalist for nearly 40 years. He writes regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy, plus he has authored three books on astronomy and contributed to others.

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