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Keck Telescope's adaptive optics let astronomers study volcanic activity on Io from armchair on Earth
03 June 2002

By Robert Sanders, Media Relations
and Laura K. Kraft, Keck Observatory

Berkeley - New adaptive optics on Hawaii's Keck Telescope have produced the sharpest infrared images yet of the entire surface of Io, one of Jupiter's moons, allowing astronomers to study the moon and its volcanoes regularly from Earth rather than rely on space-based observations from the Galileo satellite orbiting the planet.

  Io's volcanoes
Infrared images of Jupiter's moon Io captured by the Keck Telescope with (left) and without adaptive optics. Volcanoes are clearly visible.
Courtesy W. M. Keck Observatory

The images were assembled by University of California, Berkeley, postdoctoral fellow Franck Marchis and converted into an animated movie by a team at the W.M. Keck Observatory. The movie shows Io during one full rotation, a period of about one and three-quarters days.

"These new observations give important clues about the temperature and composition of Io's volcanic structures such as lava lakes, fire fountains, calderas and lava tubes," Marchis said. "With the Galileo mission ending, adaptive optics systems will be the only tools available to study this exotic extra-terrestrial volcanism."

"The volcanoes on the surface are always changing, so this is an ideal way to continue to monitor Io's activity," added colleague Imke de Pater, professor of astronomy at UC Berkeley. She and Marchis regularly collect data on Io at several different wavelengths to determine the temperature distribution in Io's volcanoes and gather information about how the volcanoes evolve and vary over time.

Constant monitoring can turn up surprises, such as the enormous eruption in February 2001 of a relatively quiet volcano, Surt, that de Pater and Marchis say was the most energetic volcanic eruption ever seen in our solar system. It was so bright that it outshone most of the moon's surface, she said. Surt's last previous eruption was in 1996.

The success of Keck's adaptive optics system, consisting of a wavefront sensor to detect the distortions caused by atmospheric turbulence - stellar twinkling - and a deformable mirror that adjusts several hundred times a second to remove the distortion, proves its value for studying solar system objects from Earth that, until now, have been hidden to all but satellites and probes.

"Adaptive optics technology is now able to produce eight-fold improvements in image quality beyond what has previously been possible," said Frederic H. Chaffee, PhD, director of the Observatory. "The Ionian images released today show features on its surface as small as 60 to 120 miles across. This is equivalent to being able to distinguish the two headlights of a single car in St. Louis while looking at them from Los Angeles, 1,800 miles away."

The movie of Io's full surface was produced from 14 pictures taken with Keck's Near InfraRed Camera Two (NIRC2) placed behind the adaptive optics system. The telescope is able to get rid of the atmospheric distortion, producing high-quality images as if the 10-meter Keck telescope were in space.

Marchis was able to boost detail in the pictures by using an image enhancement process developed by the French Office National d'Etudes et de Recherche Aerospatiales to enhance planetary observations. Called MISTRAL (Myopic Iterative Step Preserving Restoration Algorithm), the method improved the contrast and resolution. The images were then converted by a Keck team to an animation and a Java applet, which gives interactive control with simultaneous comparisons to close-up pictures taken at visible wavelengths by the Galileo orbiter.

Though the movie images of Io were taken at a wavelength of 3.5 microns (L-band), NIRC2 also obtained images at two other wavelengths. Observations at the 2.2 micron infrared wavelength (K -band) give astronomers information about reflected sunlight modulated by various surface features. The 3.5 micron infrared band (L-band) gives astronomers information about thermal volcanic activity with the highest level of contrast between hot magma and reflected sunlight.

The movie of Io's surface previewed this week is only the starting point for UC Berkeley studies, de Pater emphasized. She and Marchis will combine all the NIRC2 data to determine the temperatures of Io's many volcanoes and infer the type of activity at each.

Io's surface is pockmarked with volcanoes of varying activity, interspersed with terrain covered by sulfur dioxide "frost" at a temperature of about 120 Kelvin (-153C or -218F).

The project to image the surface of Io and demonstrate the imaging performance of the Keck adaptive optics with NIRC2 is being led jointly by Marchis and David Le Mignant, PhD, of the observatory. Other Keck team members include P. Amico, R. Campbell, A. Conrad, A. Contos, B. Goodrich, G. Hill, S. Kwok, D. Sprayberry, P. Stomski and P. Wizinowich. This observational study of Io was partially supported by the France-Berkeley Fund and by UC Santa Cruz's Center for Adaptive Optics, a National Science Foundation-funded Science and Technology Center.

The observatory, located at the summit of Mauna Kea, provides astronomers access to two 10-meter optical telescopes, the world's largest. Each telescope features a revolutionary primary mirror composed of 36 hexagonal segments that work in concert as a single piece of reflective glass to provide unprecedented power and precision. Both Keck telescopes are equipped with adaptive optics. Funding for the telescopes and the Keck II adaptive optics system was provided by the W.M. Keck Foundation.

The observatory is operated by the California Association for Research in Astronomy, a partnership of the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration (NASA).