The final images are in, and the resulting portrait of Jupiter's moon Io, after a challenging series of observations by NASA's Galileo spacecraft, is a peppery world of even more plentiful and diverse volcanoes than scientists imagined before Galileo began orbiting Jupiter in 1995.

Light from the setting Sun falls across the Loki volcanic region on Io in this image taken by NASA's Galileo spacecraft on Oct. 16, 2001. The image was taken to examine the relative depths and heights of features in the region. The Sun illuminates the surface from the right. Galileo's camera caught the large volcanic crater, or "patera" of Loki near the boundary between night and day. The image also shows several smaller craters plus shadows cast by the high peaks of several mountains. Credit: NASA/JPL/University of Arizona

Now that Galileo's observations of Io have ended, scientists are focusing on trying to understand the big picture of how Io works by examining details.

Thirteen previously unknown active volcanoes dot infrared images from Galileo's final successful flyby of Io, volcanologist Dr. Rosaly Lopes of NASA's Jet Propulsion Laboratory reported at the spring meeting of the American Geophysical Union in Washington, D.C.

That brings the total number of known Ionian hot spots to 120. Galileo images revealed 74 of them.

"We expected maybe a dozen or two," said Dr. Torrence Johnson, Galileo project scientist at JPL in Pasadena, Calif. That expectation was based on discoveries by NASA's Voyager spacecraft in 1979 and 1980, and subsequent ground-based observations.

"The volcanoes on Io have displayed an assortment of eruption styles, but recent observations have surprised us with the frequency of both giant plumes and crusted-over lakes of molten lava," said planetary scientist Dr. Alfred McEwen of the University of Arizona, Tucson.

These images of an area called Telegonus Mensa on Io reveal a complex interplay of geologic processes. A fracture runs northwest from the lower right corner of the mosaic into the amphitheater in the center of the frame. A high-resolution image along this fracture reveals that lava has erupted from it. The amphitheater itself is the site of extensive erosion, as the cliff has slumped southeastward under the influence of Io's gravity. Flat tops of massive slump blocks -- up to 6 kilometers long and 0.5 kilometers across -- are illuminated by the Sun and cast shadows down the face of the cliff. Based on these shadows, Galileo scientists estimate that the cliff is 1 to 2 kilometers high. Just to the left of center a series of landslides can be seen, the longest of which extends 4 kilometers. Credit: NASA/JPL/University of Arizona

Some high-resolution views taken as Galileo skimmed past Io on Oct. 16, 2001, are aiding analysis of the connection between volcanism and the rise and fall of mountains on Io. Few of Io's volcanoes resemble the crater-topped volcanic peaks seen on Earth and Mars, said planetary scientist Dr. Elizabeth Turtle of the University of Arizona. Most of Io's volcanic craters are in relatively flat regions, not near mountains, but nearly half of the mountains Io does have sit right beside volcanic craters.

"It appears that the process that drives mountain-building -- perhaps the tilting of blocks of crust -- also makes it easier for magma to get to the surface," Turtle said. She showed a new image revealing that material slumping off a mountain named Tohil Mons has not piled up in a crater below, suggesting that the crater floor has been molten more recently than any landslides have occurred. Galileo's infrared-mapping instrument has detected heat from the crater, indicating an active or very recent eruption.

From the analysis of Galileo's observations, scientists are developing an understanding of how that distant world resurfaces itself differently than our world does.

Nine previously unknown volcanoes have been discovered from this infrared image of Io. The infrared image, on the right, serves as a thermal map to a section of Io's surface from pole to pole. An image from Galileo's camera showing the same face of Io (left) is included for correlating the heat-sensing infrared data with geological features apparent in visible wavelengths. Some of the hot spots visible in this image were not seen in a similar infrared image taken just 10 weeks earlier of an overlapping section of Io. Three sites of major activity in the images are Prometheus, which is a bright spot at center left; Amirani, which is an elongated feature in the upper right; and the site where a giant plume was erupting in August, which is the bright spot near the top of the image. Credit: NASA/JPL/University of Arizona

"On Earth, we have large-scale lateral transport of the crust by plate tectonics," McEwen said. "Io appears to have a very different tectonic style dominated by vertical motions. Lava rises from the deep interior and spreads out over the surface. Older lavas are continuously buried and compressed until they must break, with thrust faults raising the tall mountains. These faults also open new pathways to the surface for lava to follow, so we see complex relations between mountains and volcanoes, like at Tohil."

"Io is a weird place," Johnson said. "We've known that since even before Voyager, and each time Galileo has given us a close look, we get more surprises. Galileo has vastly increased our understanding of Io even though the mission was not originally slated to study Io."

Extensions to Galileo's original two-year orbital mission included six swings close to Io, where exposure to Jupiter's intense radiation belts stresses electronic equipment on board the spacecraft. Researchers presented some results today from two Io encounters in the second half of 2001. Observations were not made successfully during Galileo's final Io flyby, in January 2002, because effects of the radiation belts put the spacecraft into a precautionary standby mode during the crucial hours of the encounter.

This image shows the contrast in volcanism styles found on Io. The central feature is a large patera, or volcanic depression, almost 100 kilometers (60 miles) long. It may have formed after eruptions of lava emptied a subsurface magma chamber and left an empty space into which the crust collapsed. Evidence of lava flows associated with this patera, however, is difficult to find. Either the flows have been buried, or perhaps they never erupted and simply drained back deep into the crust. Credit: NASA/JPL/University of Arizona

Galileo will make its last flyby of a moon when it passes close to Amalthea, a small inner satellite of Jupiter, on November 5. No imaging is planned for that flyby. With fuel for altering its course and pointing its antenna nearly depleted, the long-lived spacecraft will then loop one last time away from Jupiter and perish in a final plunge into Jupiter's atmosphere in September 2003.