Nov 5, 1999

A Hawaiian-Style Volcano on Io

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New images from Galileo reveal unexpected details of the Prometheus volcano on Io including a caldera and lava flowing through fields of sulfur dioxide snow.




Galileo image of an Io lava field
November 5, 1999: A volcanic crater several times larger than one found at Hawaii's Kilauea volcano has been photographed on Jupiter's moon Io during a close flyby performed by NASA's Galileo spacecraft.

"It appears that the Prometheus volcano on Io has characteristics remarkably similar to those of the Kilauea volcano in Hawaii, although Prometheus is much larger," said Dr. Laszlo Keszthelyi (KEST-ay), a Galileo research associate at the University of Arizona, Tucson, AZ. "Both volcanoes are long- lived eruptions, with flows that apparently travel through lava tubes and produce plumes when they interact with cooler materials."

Right: This is a high-resolution image of part of Prometheus, an active volcano on Jupiter's volcanic moon Io. In earlier, lower resolution images, it appeared that all the dark material at Prometheus comprised a single, long lava flow. The new image shows for the first time that the northeastern end of this dark feature is actually a lava-filled caldera 28 kilometers (17 miles) long and 14 kilometers (9 mile s) wide. The underground source of the Prometheus lava is probably beneath this newly discovered caldera. Galileo scientists are intrigued also by the snowfield containing hummocks, seen to the east of the Prometheus caldera. [more information].


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The sharp images of Prometheus released today come from two of Galileo's onboard instruments -- the camera, and the near- infrared mapping spectrometer which observes in wavelengths not visible to the naked eye. The images were taken during the close flyby of Io by Galileo on October 10, 1999, and are part of a large batch of data currently being transmitted to Earth.
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"We've been having a feast looking at the material from Io," said Dr. Rosaly Lopes-Gautier NASA's Jet Propulsion Laboratory, Pasadena, CA. "We have been waiting for such high-resolution images of Io for more than 10 years." Scientists will present an assortment of new images and describe their latest discoveries at a press briefing scheduled for November 19 at NASA Headquarters, Washington, DC.

Prometheus is the "Old Faithful" of Io's many volcanoes. It has been active during every observation over the past 20 years by NASA's Voyager and Galileo spacecraft and the Hubble Space Telescope. The new spectrometer images show two distinct hot spots at Prometheus -- a large one to the west and a fainter, cooler one to the east. The images reveal numerous lava flows near the western hot spot and enable scientists to identify a crater, or caldera, 28 kilometers (17 miles) long and 14 kilometers (9 miles) wide near the hot spot to the east.
Left: The active volcano Prometheus on Jupiter's moon Io was imaged by the near-infrared mapping spectrometer instrument onboard NASA's Galileo spacecraft during the close flyby of Io on October 10, 1999. The spectrometer can detect active volcanoes on Io by measuring their heat in the near-infrared wavelengths (just beyond the red end of human vision). It can also obtain information on the composition of materials on Io¹s surface using the same wavelengths. The image on the left, taken at an infrared wavelength, shows the different compositions of materials on the volcano. The dark material is thought to be silicate lava, and the white material is sulfur dioxide frost. The image on the right was taken at a longer infrared wavelength that shows heat coming out of the volcano. The hottest areas appear white and the coolest appear black. [more information]

Previously, it was thought that the 50 to 100 kilometer- (30 to 60 mile-) tall plume observed at Prometheus formed where the lava erupts onto the surface. Now, however, it now appears that the plume forms at the far end of the lava flows. The caldera and eastern hot spot are thought to be associated with the vent where the molten rock rises to the surface. It appears that after the lava reaches the surface, it is transported westward through lava tubes for about 100 kilometers (60 miles) before breaking out onto the surface again. Here, numerous lava flows wander across a plain covered with sulfur dioxide-rich snow. The plume is created by the interaction of the hot lava with the snow.
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This plume feature is just one of several similarities between Prometheus and Hawaii's Kilauea. Volcanologists say that Prometheus has been erupting for more than 20 years and Kilauea has been erupting for more than 16 years. The current vent at Kilauea consists of a small lava lake about 100 meters (330 feet) across that produces a relatively small thermal hot spot. From this vent, lava is transported 10 kilometers (6 miles) in lava tubes to the Pacific Ocean where large steam plumes are generated by the interaction between the hot lava and the ocean. Galileo scientists believe the plume seen on the western end of Prometheus is similar to this Hawaiian steam plume, except the Ionian plume is composed largely of sulfur dioxide and rises much higher because of Io's low atmospheric density and gravity.

Another Io flyby, this time at an altitude of 300 kilometers (186 miles), is planned for November 25 at 8:40 p.m. Pacific Time (11:40 p.m. Eastern Time). (Times given are in Earth-received time -- or the time when the signal of the event is received on Earth.) The Io flybys are challenging and risky, because Io lies in an area of intense radiation from Jupiter's radiation belts, and that radiation can harm spacecraft components. Because of the risk, the flybys were scheduled for the final portion of Galileo's extended mission.

Additional information about the Galileo mission is available on the Galileo home page at a new web address of

JPL manages the Galileo mission for NASA's Office of Space Science, Washington, D.C. JPL is operated for NASA by the California Institute of Technology, Pasadena, CA.

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The Frosty Plains of Europa -- Dec. 3, 1998. As Galileo returns new images of Europa, NASA scientists prepare to study samples from a potentially similar environment here on Earth.
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Related Sites:

Ice, Water and Fire the Galileo Europa Mission
Galileo home page at JPL, with the latest on Europa, Callisto and Io
Jet Propulsion Laboratory home page
Io from the SEDS Nine Planets web site
Callisto from the SEDS Nine Planets web site
Jupiter from the SEDS Nine Planets web site
Io: The Prometheus Plume Aug. 18, 1997 Astronomy Picture of the Day
Close-up of an Io volcano Aug. 4, 1995 Astronomy Picture of the Day
Sizzling Io July 6, 1998 Astronomy Picture of the Day


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