Now astronomers from the University of California at Berkeley have imaged thermal emission from Jupiter’s slowly reemerging South Equatorial Belt (SEB) in unprecedented detail, using the 10-m Keck II telescope atop Mauna Kea, Hawaii. The key to their success was the creative way in which they used Keck’s adaptive-optics system to take advantage of the proximity of Jupiter’s moon Europa.
Jupiter’s SEB, which is usually broad and dark, disappears for about a year at irregular intervals, and astronomers are still working out what prompts the fading. In visible light, most of Jupiter’s darker areas are places where one can see deeper into the planet’s swirling atmosphere. The lighter areas are wispy, high-altitude clouds of ammonia concealing what's below. It seems likely that Jupiter’s SEB disappears because of an alteration in the planet’s normal wind patterns, allowing bright ammonia clouds to cover the SEB.
The 5-micron mid-infrared wavelength, shown in bright red and yellow in this Keck image, allows astronomers to peer deeper into Jupiter’s clouds and, perhaps, to help find out why the SEB fades. “At shorter wavelengths we see reflected sunlight,” says team member Michael Wong, but wavelengths of 5 microns or longer show thermal radiation — heat glow — from Jupiter’s interior, indicating breaks in the planet's multilayered upper cloud decks.
A good deal of Jupiter’s thermal radiation is absorbed in the planet’s lower atmosphere. However, "at 5 microns Jupiter has an atmospheric window where there is very little absorption from methane, hydrogen, or other gases,” Wong explains. “So at this wavelength, we can see quite deeply into spots where there are no clouds.”
The challenge was getting a 5-micron image with enough resolution, since Jupiter poses an interesting challenge for adaptive-optics users. Normally, Keck astronomers use a powerful laser to create an artificial guide star in Earth's upper atmosphere, allowing the telescope to correct for atmospheric seeing at up to 2,000 times per second. For this method to work the guide star must be very near to the object observed, so that the same atmospheric distortions affect both. However, Jupiter is so bright that it drowns out the laser guide star. The Berkeley team needed another solution, and they found one in one of Jupiter’s moons.
“Europa is bright enough to be seen against the glare of Jupiter,” says Franck Marchis, and sometimes it's near enough for adaptive optics, though “this configuration doesn’t happen often and doesn’t last more than one hour.”
One such hour came on November 30, 2010, and the resulting high-resolution infrared image revealed much about Jupiter’s cloud cover. One might assume that dark regions in the image are cloud-free, Wong explains, but some deeper clouds must be there because they’re not all bright at 5 microns. So “the revival of the whole belt seems to be progressing more rapidly in the upper clouds than in the deeper clouds.”
The more astronomers can improve their understanding of the fade and revival process, Wong concludes, the closer they come to figuring out the cause of the entire cycle.