A great roar of acoustic waves near the north and south poles of Jupiter's moon Io shouts about the power of the volcanic moon.

The wave data, new pictures and other information collected recently by NASA's Galileo spacecraft provide insight into what happens above Io's surface, at its colorful volcanoes and inside its hot belly. Scientists presented the findings Monday at a meeting of the American Geophysical Union in San Francisco.

Galileo has returned infrared imagery of a new hot spot on Io that was the source of a towering plume in August 2001, indicating a sulfur-dioxide concentration that may have been fallout from the plume. Galileo's near-infrared mapping spectrometer captured the image on the left during an Oct. 16, 2001 flyby of Io. Coloring indicates the intensity of glowing at a wavelength of 4.1 microns. Yellow, red, and white represent high temperatures. Black is where the near-infrared glow was so intense the image was saturated. Greens and blues are cold. The visible-light image on the right was obtained by Galileo's camera in 1999, before any volcanic activity was seen at this site. The first sign of activity came in August 2001, when Galileo detected an infrared hot spot and the tallest volcanic plume ever seen at Io. The dark blue band north of the hot spot in the new infrared image represents a concentration of sulfur-dioxide, which has a strong signature in the infrared. The sulfur-dioxide is thought to be from the fallout of the plume. The image shows high temperatures corresponding to yellow flows in the center of the visible-light image, and from a small caldera at the 8 o'clock position. Credit: NASA/JPL

Galileo, managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif., has been orbiting Jupiter for six years. As it flew near Io's poles in August and October, the density of charged particles it was passing through suddenly increased about tenfold when the spacecraft crossed the path of a magnetic-field connection between Io and Jupiter, reported Dr. Donald Gurnett of the University of Iowa, Iowa City. The waves, indicating the density, travel in a plasma of charged particles, and would be silent to the ear, but Iowa researchers converted them to sound waves to make the patterns audible.

"You hear a whistling sound from Jupiter's radio emissions, then, just when you go over the pole, you hear a tremendous roar that starts abruptly, then stops abruptly," Gurnett said. "It's like the noise from a huge electrical power generator." Io actually generates as much wattage as about 1,000 nuclear power plants.

The region of increased density is where electrons and ions come up from Io's tenuous atmosphere and follow a "flux tube" where field lines from Jupiter's strong magnetic field intersect Io. In a 1999 flyby of Io, Galileo had provided some indication of the higher density over the moon's poles. This year's two Io flybys were the first to show that those denser areas coincide with the magnetic-field flux tube, Gurnett said.

Dramatic shadows across a mountainous landscape on Io reveal details of the topography around a peak named Tohil Mons in this mosaic created from images taken by Galileo in October 2001. Tohil Mons rises 5.4 kilometers (18,000 feet) above Io's surface, according to analysis of stereo imaging from earlier Galileo flybys of Io. The new images, with a resolution of 327 meters (1,070 feet) per picture element, were taken when the Sun was low in the sky, producing informative shadows. North is to the top and the Sun illuminates the surface from the upper right. The topographic features revealed include a very straight ridge extending southwest from the peak, 500- to 850-meter-high (1,640- to 2,790-foot-high) cliffs to the northwest and a curious pit immediately east of the peak. Major questions remain about how Io's mountains form and how they are related to Io's ubiquitous volcanoes. Although Io is extremely active volcanically, few of its mountains appear to be volcanoes. However, two volcanic craters do lie directly to the northeast of Tohil's peak, a smaller dark-floored one and a larger one at the very edge of the mosaic. Furthermore, the shape of the pit directly east of the peak suggests a volcanic origin. Galileo scientists will use these images to investigate the geologic history of Tohil Mons and its relationship to the neighboring volcanic features. Credit: NASA/JPL/University of Arizona

Recent magnetic-field measurements tell us something new about the plumes erupting from Io's volcanoes and about the moon's molten core, said Dr. Margaret Kivelson of the University of California, Los Angeles.

Galileo detected electrical currents flowing along magnetic field lines above two areas of volcanic activity on Io, Kivelson said. Material shot high from eruptions is apparently affecting conductivity more than 100 kilometers (about 60 miles) above the surface.

"If this is the mechanism that's producing the currents, it may help us in the search for active plumes," she said.

Galileo's routes near Io's north pole in August and near its south pole in October were chosen for gaining measurements to determine whether Io generates an intrinsic magnetic field of its own within the greater magnetic field generated by Jupiter.

"There's no intrinsic field," Kivelson said. "We can put that question to rest." That means Io's molten iron core does not have the same type of convective overturning by which Earth's molten core generates Earth's magnetic field. Lack of that overturning fits a model of Io's core being heated from the outside, by tidal flexing of the layers around it, rather than being heated from the center.

The heat generated inside Io by the tidal tug of Jupiter makes this moon the most volcanically active world in the solar system. A new color picture of one large volcanic crater, Tupan Patera, shows various red, green, yellow and black surface materials laid down by volcanic interactions of molten rock and sulfur compounds, said Dr. Elizabeth Turtle of the University of Arizona, Tucson. Tupan, named for a Brazilian god of thunder, is one of Io's most persistent volcanoes. Another new image reveals roofed-over portions of a long lava channel, indicating that insulation provided by the cover helped lengthen a large lava flow.

Tupan Caldera, a volcanic crater on Io, has a relatively cool area, possibly an island, in its center, as indicated by infrared imagery from Galileo. A thermal portrait of Tupan collected by the near-infrared mapping instrument on Galileo during an Oct. 16, 2001 flyby is presented on the right, beside a visible-light image from Galileo's camera for geographical context. The infrared image uses false color to indicate intensity of glowing at a wavelength of 4.7 microns. Reds and yellows indicate hotter regions; blues are cold. The hottest areas correspond to the dark portions in the visible-light image and are probably hot lavas. The central region in the crater may be an island or a topographically high region. Parts of it are cold enough for sulfur-dioxide to condense. Credit: NASA/JPL

New infrared imagery from Galileo shows that darker areas at Tupan correspond to hotter surface materials, said Dr. Rosaly Lopes of JPL. The infrared data also confirm sulfur- dioxide deposits near the source of a tall plume seen in August above a previously inactive volcano.

NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages Galileo for NASA's Office of Space Science, Washington, D.C.

Nighttime temperatures in the southern hemisphere of Io, mapped here with data from Galileo, give hints about the textures of surface materials, as well as the locations of volcanic hot spots. The photopolarimeter/radiometer instrument on Galileo collected the information during an Aug. 6, 2001 flyby of Io, yielding the most detailed temperature map yet for this region. Features are visible as small as 150 kilometers (93 miles) across. The temperature map is superimposed here on images from Galileo's camera, covering volcanoes Pele (Pe), Pillan (Pi), Babbar (Ba) and others. It extends southward almost to Iošs south pole. The contour interval is 2.5 degrees Kelvin (4.5 degrees Fahrenheit), but contours above 110 Kelvin (minus 262 Farenheit) have been omitted to reveal the volcanic hot spots that produce these relatively high temperatures. Some of the most interesting features on this map occur in the regions between the hot spots. NASA/JPL/Lowell Observatory