NASA's Galileo spacecraft soars above Jupiter's volcanic moon Io in this artist's impression. Photo: NASA/JPL

Lava flows, cliffs and depressions from collapsed volcanic eruptions are seen in new images of Jupiter's moon Io, taken by NASA's Galileo spacecraft during its most recent flyby on February 22. The new pictures give scientists more information to help them in their ongoing quest to understand the rampant, fiery volcanic activity on Io and its similarities to past and present volcanism on Earth.

The new batch of images also includes a high-resolution view of the side of Europa that faces Jupiter. That hemisphere shows variations in surface materials that may indicate the presence of sulfuric acid (common battery acid) and salty minerals, possibly from a subsurface ocean.

Galileo has been orbiting Jupiter and its moons since December 1995. After its primary mission ended in December 1997, Galileo successfully completed a two-year extended mission, and it is currently embarking on another extension, called the Galileo Millennium Mission.

Europa: Sea salts or battery acid?

Europa. Photo: NASA/JPL

This composite image of the Jupiter-facing hemisphere of Europa was obtained on November 25, 1999 by two instruments onboard NASA's Galileo spacecraft. The global black-and-white view, by the spacecraft's camera, provides the highest resolution view ever obtained of this side of Europa. The superimposed false-color image, obtained by Galileo's near-infrared mapping spectrometer instrument, reveals the presence of materials with differing compositions on Europa's surface.

In this image, blue areas represent the cleanest, brightest icy surfaces, while the reddest areas have the highest concentrations of darker, non-ice materials. The mixture of colors seen here is most likely the result of both variations in the ages and composition of surface materials. The dark materials are believed to fade with the passage of time.

This area is highly unusual compared to many other areas on Europa because of its high concentration of fresh-appearing bright ridges and fractures. On other parts of Europa, the darker areas appear to be the most recently formed, but here the ridges and fractures appear to "overprint" the underlying darker mottled terrain.

Scientists disagree about the chemical makeup of the dark materials; both sulfuric acid (common battery acid) and salty minerals, perhaps from a subsurface ocean, have been suggested. Analysis of images like this one may help to resolve this controversy. Surprisingly, either material could help to produce conditions below the surface that could be favorable to the formation of living organisms.

The colored area is centered near the intersection of the equator and the Europan "prime meridian," where the longitude is assigned the value of 0 degrees. This is the sub-Jupiter point, where Jupiter always appears to be almost directly overhead. This phenomenon occurs because Europa takes the same period of time to rotate as it does to orbit around Jupiter (3.55 days). The area imaged in color is about 400 by 400 kilometers (250 by 250 miles), an area of about 160,000 square kilometers (about 62,000 square miles).

Highest resolution of lava flows on Io

Io lava flows. Photo: NASA/JPL

Lava flows similar to those found in Hawaii are seen in the black and white image at top, taken by NASA's Galileo spacecraft. It is one of the highest resolution images (7 meters or 23 feet per picture element) ever obtained of Jupiter's volcanic moon Io. The two horizontal black stripes are places where data were lost during transmission to Earth.

The image shows the textures of lava flows on the floor of the caldera Chaac, which is shown in false color at lower resolution (185 meters or 607 feet per pixel element) in the bottom image. Calderas are depressions caused by collapse during volcanic eruptions. The one shown here is approximately 100 kilometers (63 miles) long and 30 kilometers (19 miles) across. Using shadow lengths from the new high-resolution observations, the northeastern (upper right) scarp, or line of cliffs, has been estimated to be 2.8 kilometers (9200 feet) high.

The lava flows are similar in texture to lava flows within the caldera at Hawaii's Kilauea volcano. This suggests that the floor of Chaac has been covered by a combination of lava flows and lava lakes.

The light-colored material surrounding the caldera may be composed of sulfur-dioxide frost or some other sulfur-rich material on the surface of Io. Galileo scientists believe that the greenish color on the caldera floor is a form of contaminated sulfur created when sulfur-rich material escaping from volcanic vents reacts chemically with warm lava flows. The high-resolution view shows numerous lava flows. The darkest flows are thought to be the most recent because they have not been covered by the sulfurous materials which coat most of Io's surface.

The top image was acquired by Galileo on February 22, 2000. It was taken at a distance of 600 kilometers (370 miles) and is centered at 11.9 degrees north latitude and 157.6 degrees west longitude. North is to the top, and the Sun illuminates the surface from the right.

The color image was created by combining a black and white image taken on February 22, 2000 at a distance of 18,800 kilometers (11,700 miles) from Io with lower-resolution (1.3 kilometers or 0.81 miles per picture element) color images taken on July 3, 1999 at a distance of 130,000 kilometers (81,000 miles). The image is centered at 11.6 degrees north latitude and 157.7 degrees west longitude. North is to the top and the Sun illuminates the surface from the left.

Terrain near Io's south pole

Io south pole. Photo: NASA/JPL

Volcanic calderas, lava flows and cliffs are seen in this false color image of a region near the south pole of Jupiter's volcanic moon Io. It was created by combining a black and white image taken by NASA's Galileo spacecraft on February 22, 2000 with lower resolution color images taken by Galileo on July 3, 1999. The three black spots (top center and middle left) are small volcanic calderas about 10-20 kilometers (6-12 miles) in size, which are dark because their floors are covered by recent lava flows. Two of these three calderas are surrounded by diffuse dark material, which may have been thrown out of the calderas by explosive eruptions.

The bright, white material is thought to be sulfur-dioxide frost and is concentrated near the cliffs in this image. It may be formed when liquid sulfur dioxide seeps out at the base of mountain scarps, vaporizes into a plume of gas, liquid and solid, and then condenses again on the surface. Part of this process, called sapping, occurs in arid environments on Earth when ground water seeps out at the bases of cliffs. The vaporization and production of plumes is much more dramatic on Io due to the lower gravitational acceleration and especially the very low atmospheric pressure. It may be one of the dominant erosion processes on Io.

The mountain at the center left, named Telegonus Mensae, exhibits a number of ridges parallel to its margins. These ridges have been observed on a number of other Ionian mountains and they suggest that as the mountain ages, it is collapsing outward under the influence of gravity.

The yellow lava flow at the southern end of the image appears to be fed by a dark channel that connects to a dark caldera. This is a likely candidate for a lava flow composed of sulfur (rather than silicate material).

The image is centered at 53.8 degrees south latitude and 117.1 degrees west longitude and north is to the top. The higher resolution image has a resolution of 350 meters (or yards) per picture element and is illuminated from the upper left. It was taken at a range of 34,000 kilometers (21,000 miles). The color images have resolutions of 1.3 kilometers (0.81 miles) per picture element and are illuminated from almost directly behind the spacecraft. They were taken at a distance of about 130,000 kilometers (81,000 miles) by Galileo's onboard camera.