NASA released on Thursday close-up images of Prometheus, a volcano on the jovian moon Io that has erupted continuously for at least 20 years.
The new image shows that Prometheus -- named for the Titan who in Greek mythology gave fire to humankind -- appears to be remarkably similar to another long-lived volcano here on Earth, Hawaii's Kilauea.
The new, black-and-white image (inserted in a mosaic that includes an earlier color picture of the massive volcano) shows a 17-mile (28-kilometer) wide, bean-shaped crater, or caldera, that has collapsed.
Scientists said the image shows that volcanic flows erupting near the caldera apparently breach the surface before traveling 60 miles (100 kilometers) to the west through tubes of hardened lava before surfacing again.
"That's exactly what happens with the volcanic eruptions at Kilauea," said Laszlo Keszthelyi, a research associate at the University of Arizona in Tucson. "As far as we can tell, it's an identical type of eruption, but it's 10 times bigger."
Prometheus has erupted every time it has been imaged, beginning with Voyager 1 in 1979, through observations by the Hubble Space Telescope and Galileo.
The new images released Thursday also include a look at the volcano in the near-infrared. They show the composition of surface materials on Prometheus, including silicate lava and sulfur dioxide frost, as well as gauge relative temperatures.
The high-resolution look at Prometheus' hot spots apparently confirms the lava tube theory, since the highest temperatures crop up not at the vent, where any lava would be expected to be hottest, but miles away. That implies that once the lava erupts, it travels in well-insulated tubes before again breaching the surface, Keszthelyi said.
Similar lava tubes are common in volcanically active regions like Hawaii and the Pacific Northwest, and also at Lava Beds National Monument in northeastern California.
On Prometheus, the hottest hot spots coincide with the area where the 60-mile (100-kilometer) high plumes associated with the volcano also appear. Previously, based on the theory the gases were being released from the magma itself, scientists thought the plumes originated where the lava first erupted onto the surface.
Now it appears the plume forms miles from the vent, and only when and where the lava comes into contact with the sulfur dioxide-rich snow that is abundant in the region.
"It flips the old idea right around," Keszthelyi said.
Galileo acquired the images with its camera and near-infrared mapping spectrometer during its October 10 flyby of Io -- its first of this jovian moon with all its instruments functioning.
Scientists will present more images and discoveries from the Io flyby during a November 19 briefing at NASA headquarters.
"We've been having a feast looking at the material from Io," said Rosaly Lopes-Gautier at NASA's Jet Propulsion Laboratory in Pasadena, California, which manages the mission.
Galileo is scheduled to fly by Io again on November 25, passing within 186 miles (300 kilometers) of its surface.
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