Aug. 3, 2006 Astronomers from the University of California, Berkeley, and the W. M. Keck Observatory in Hawaii last month snapped high-resolution near-infrared images of the Great Red Spot, a persistent, high-pressure storm on Jupiter, as an upstart storm, Red Spot Jr., breezed by it on its race around the planet.
The image, which also shows Jupiter's moon Io, was taken on July 20 Hawaii time (July 21 Universal Time) by the Keck II telescope on Mauna Kea using adaptive optics to sharpen the image.
The spots are of interest to astronomers because Red Spot Jr. formed from the merger of three white spots only recently, between 1998 and 2000, and in December 2005 turned red like the much older Great Red Spot. While the new red spot is about the size of Earth, the Great Red Spot is nearly twice that diameter and has been circling the planet for at least 342 years.
The images captured by the second-generation Near Infrared Camera (NIRC2) on Keck II show that, though the two red spots are about the same color when seen in visible wavelengths, they differ markedly at infrared wavelengths. When the astronomers viewed the planet through a narrow-band filter centered on the 1.58 micron, near-infrared wavelength, Red Spot Jr., which was called Oval BA before it changed from white to red, was a lot darker, indicating that the tops of the storm clouds may be lower than those of the Great Red Spot. With more atmosphere above its cloud tops, more infrared light is absorbed by molecules like methane in the atmosphere.
"Red Spot Jr. is either not as high as the Great Red Spot, or it's just not as reflective, that is, as dense," said lead astronomer Imke de Pater, professor of astronomy at UC Berkeley. "These images will put some constraints on the altitude of Red Spot Jr."
The Great Red Spot is thought to tower about 8 kilometers (5 miles) above the surrounding cloud deck. The fact that Red Spot Jr. turned red may indicate its swirling storm clouds are rising higher also, though apparently they are not as high as those of its larger companion, or the clouds are thinner.
Why the spots are red is a subject of great debate. Some people think the hurricane-like winds in the Great Red Spot, which can reach 400 miles per hour, dredge up material from deeper in the planet's atmosphere that, when exposed to ultraviolet sunlight, turns red. One candidate is phosphine gas, PH3, which has been detected on Jupiter. Ultraviolet light might catalyze its conversion to red phosphorus, P4, according to one of the leading theories. Other, more complicated theories have phosphine interacting in the atmosphere with chemicals such as methane or ammonia to form complex compounds such as methylphosphane or phosphaethyne.
Recent studies, however, suggest that the red color also may be attributed to sulfur allotropes, that is, different molecular configurations, including chains and rings, of pure sulfur (S3-S20). The new work hypothesizes that ammonium hydrosulfide particles are carried upwards in the Great Red Spot and are broken up by ultraviolet light. Subsequent chemical reactions ultimately lead to long-chained sulfur allotropes , which can vary in color from red to yellow.
"The jury is still out on the exact processes that lead to the red coloration of the Great Red Spot - and Oval BA," de Pater is quoted as saying in the August 2006 issue of Sky & Telescope magazine.
Christopher Go, an amateur astronomer who first noticed the color change of Red Spot Jr., joined de Pater's team earlier this year. He noted that during the close encounter between the two spots, Red Spot Jr. was squashed slightly, stretching in its direction of motion. The same thing happened in 2002 and 2004 when the Great Red Spot and Red Spot Jr. passed one another, though then Junior was white.
The Great Red Spot rotates westward, opposite to the eastward rotation of the planet. Because alternating bands on the Jovian surface move in opposite directions, the adjacent Red Spot Jr. moves eastward. The planet rotates about once every 10 hours.
Another of de Pater's colleagues, UC Berkeley mechanical engineering professor Philip Marcus, predicted several years ago that Jupiter's climate was changing, based on the disappearance of the cyclonic storms or spots within the bands. The formation of Red Spot Jr. from three smaller storms is an example of this. The mixing of the atmosphere by these cyclones keeps the temperature about the same over the entire planet, he argued, so loss of this mixing will cause the equator to heat up and the poles to cool.
Earlier this year, on April 16, de Pater and her team captured near-infrared, ultraviolet and visible light photos of the planet using the Hubble Space Telescope to look more closely at the two red spots. The observations with the Keck Telescope were a follow-up study to try to measure the speeds of the swirling winds in the spots. Jupiter's brightness and size, however, confused the adaptive optics (AO) system, forcing the astronomers to miss some good shots of the planet as the guide star was being positioned optimally relative to Jupiter.
"This was probably the most challenging observation ever tried with the AO system at Keck," said de Pater, referring to use of the laser guide star system next to an object as big and bright as Jupiter. Adaptive optics can take the twinkle out of an object caused by thermal motion in the atmosphere, but to do this well, the target must be near another bright object that can serve as a reference. For some of the images, Jupiter's moon Io was used as the reference "star." But until Io got close enough for this, a laser guide star was created near Jupiter to serve this purpose.
"This was our first attempt using the laser to obtain AO-corrected images of Jupiter's surface," said Dr. Al Conrad, a support astronomer at the Keck Observatory. "The technique shows promise and, if we perfect it, will provide us with many more opportunities to observe this fascinating, ever-changing object."
The team, which included Keck observing support members Terry Stickel, David le Mignant and Marcos van Dam and UC Berkeley post-doc Michael Wong, also obtained a close-up of the two spots through a narrow-band filter centered on 5 microns, which samples thermal radiation from deep in the cloud layer. Both spots appear dark because the clouds completely block heat emanating from lower elevations, though narrow regions around the spots that are devoid of clouds show leakage of this heat out into space.
"These 5 micron images reveal details in the cloud opacity not seen at the other wavelengths and will help unravel the vertical structure of the spots," Wong added. "The smooth, narrow arcs visible to the south of each spot probably result from the interaction between the spots and high-speed winds that are deflected around them."
The resolution using both the narrow and wide views on the camera was about 0.1 arcseconds, or only half as good as can be obtained on a clear night with optimal seeing.
The W. M. Keck Observatory operates twin 10-meter telescopes located on the summit of Mauna Kea on the island of Hawaii and is managed by the California Association for Research in Astronomy, a non-profit corporation whose board of directors includes representatives from Caltech, the University of California and NASA. For more information, please visit http://www.keckobservatory.org.
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