Amateur astronomers image extreme storms on Uranus

Giant storm on Uranus in the 2.2-micron IR band
Infrared images of Uranus (1.6 and 2.2 microns) obtained on 6th August 2014 with adaptive optics on the 10-metre Keck II telescope. The white spot is an extremely large storm that was brighter than any feature ever recorded on the planet in the 2.2 micron band. The cloud rotating into view at the lower-right limb grew into the large storm that was seen by amateur astronomers at visible wavelengths. Image credit: Imke de Pater (UC Berkeley) and W. M. Keck Observatory Images.

Discovered by Sir William Herschel in March 1781, gas giant Uranus is the penultimate planet of the Solar System and currently well placed for observation in the constellation of Pisces. Despite being four times the diameter of Earth, its immense distance from the Sun (19 Astronomical Units, or 2,870 million kilometres) means that most visual observers consider discerning its tiny 3.7-arcsecond, magnitude +6, blue-green disc in backyard telescopes is achievement enough. However, ambitious owners of large instruments equipped with CCD cameras may care to emulate observers in France and Australia who have recently succeeded in imaging enormous storms raging on the planet at infrared and visual wavelengths.

“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 detected eight large storms on Uranus’s northern hemisphere when observing the planet with adaptive optics on the W. M. Keck Observatory in Hawaii on 5th and 6th August 2014.

One event was the brightest storm ever seen on Uranus at 2.2 microns, a wavelength that reveals clouds just below the tropopause where the pressure ranges from about 300 to 500 mbar, equivalent to half the atmospheric pressure at the surface of the Earth. The storm accounted for 30 percent of all light reflected by the rest of the planet at this wavelength.

“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,” states co-investigator Heidi Hammel of the Association of Universities for Research in Astronomy. “But we predicted that such activity would have died down by now. Why we see these incredible storms now is beyond anybody’s guess.”

Australian astronomer Anthony Wesley images storm on Uranus.
Optical images of Uranus on 19th September and 2nd October, showing the dramatic appearance of a bright storm on a planet that normally displays only a diffuse bright polar region. Image credit: Anthony Wesley, Murrumbateman, Australia (16″ Newtonian, 650-850nm filter, PGR GS3-U3-23S6M camera).

Amateur astronomers soon learned of the bright Uranian storms. Australian amateur Anthony Wesley of Murrumbateman, NSW, succeeded in imaging storm features on 19th September and 2nd October 2014 with his 16-inch Newtonian.

Marc Delcroix in France processed amateur pictures and confirmed the discovery of a bright spot on an image by fellow French amateur Régis De-Bénedictis, then in others taken by amateurs in September and October 2014. Delcroix had his own chance to photograph it with the Pic du Midi one-metre telescope where, on 4th October, “I caught the feature when it was transiting, and I thought, ‘Yes, I got it!'” he said.

Delcroix, who works for an auto parts supplier in Toulouse, has observed with his backyard telescope since 2006 and has a particular interest in Jupiter. He has used the Pic du Midi telescope occasionally since 2012.

GIF animation showing the movement of the bright spot as Uranus rotated over a two hour period on 4th October, 2014. The images were taken at the Pic du Midi telescope in the French Pyrénées. Image credit: Marc Delcroix and F. Colas (S2P).
GIF animation showing the movement of the bright spot as Uranus rotated over a two hour period on 4th October, 2014. The images were taken at the Pic du Midi telescope in the French Pyrénées. Image credit: Marc Delcroix and F. Colas (S2P).

“I was thrilled to see such activity on Uranus. Getting details on Mars, Jupiter or Saturn is now routine, but seeing details on Uranus and Neptune are the new frontiers for us amateurs and I did not want to miss that,” said Delcroix. “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.”

Uranus’s atmosphere is composed mainly of hydrogen and helium with a blue tint caused by small amounts of methane, which condenses into highly reflective clouds of methane ice when it rises to the surface. Since the planet has no internal source of heat, its atmospheric activity is believed to be driven solely by sunlight which is now weak in the northern hemisphere, so astronomers were surprised when these observations showed such intense activity.

Interestingly, the extremely bright storm seen by the 10-metre Keck II telescope in the near infrared is not the one seen by the amateurs. De Pater’s colleague Larry Sromovsky, a planetary scientist at the University of Wisconsin, Madison, identified the amateur spot as one of the few features on the Keck Observatory images from 5th August that was only seen at 1.6 microns, and not at 2.2 microns. The 1.6-micron light is emitted from deeper within the Uranian atmosphere, which means that this feature is below the uppermost cloud layer of methane ice.

“These unexpected observations remind us keenly of how little we understand about atmospheric dynamics in outer planet atmospheres,” wrote De Pater, Sromovsky, Hammel and Pat Fry of the University of Wisconsin in their report delivered to a meeting of the American Astronomical Society’s Division for Planetary Sciences in Tucson, Arizona on 12th November.