In contrast, although the volcano Pele spouts Io's largest plume, which produces the large orange ring seen in the right-hand image, Pele emits very little total heat. This is because Pele's activity, though vigorous, is confined to a small volcanic crater, where there is no room for large, warm lava flows to radiate heat.
The temperature map comes from observations made during Galileo's 27th orbit of Jupiter, in February 2000. Blue indicates the coldest temperatures, near 90 degrees Kelvin (minus 297 degrees Fahrenheit), while oranges and yellows indicate the highest temperatures, in excess of 170 K (minus 153 F).
Small areas of the volcanoes are far hotter than this, exceeding 1,500 K (2,240 F). However, in this relatively low-resolution view, which shows no features smaller than about 340 kilometers (210 miles) across, radiation from these small, hot regions is mixed with radiation from surrounding colder regions, so the high temperatures are not detected directly.
The cooler regions, shown in blues and purples, are dominated by radiation from the surface between the volcanoes, which was warmed by sunlight the previous day and retains some of that heat through the night. Because high-latitude regions receive less sunlight during the day, we would expect them to be cooler at night, but the image shows that temperature varies little with latitude, which is puzzling.
It is possible that the moon's volcanic heat is shunted off at its poles or the poles are made of stuff that cools off less easily at night. Or it could just be a trick of the light.
Some 250 scientists are meeting this week in Boulder, Colo., to discuss Io, Europa and other members of the Jupiter system. John Spencer, of the Lowell Observatory, Flagstaff, Ariz., will describe some possible explanations for Ios odd heat balance. For example, the poles may have more volcanic heating than the lower latitudes, or they may be surfaced with materials that cool off slower at night.
Static Europa
Europa appears as a thick crescent in a newly released enhanced-color image from NASA's Galileo spacecraft. The view combines images taken in violet, green and near-infrared filters in 1998 and 1995.
The colors have been stretched to show the subtle differences in materials that cover the icy surface of Europa. Reddish linear features are some of the cracks and ridges, thousands of kilometers long, which are caused by the tides raised by the gravitational pull of Jupiter. Mottled, reddish "chaotic terrain" exists where the surface has been disrupted and ice blocks have moved around. The red material at the ridges and chaotic terrain is a non-ice contaminant and could be salts brought up from a possible ocean beneath Europa's frozen surface.
Also visible are a few circular features, which are small impact craters. Europa's surface has very few craters, indicating that recent or current geologic activity has removed the traces of older impacts.
The paucity of craters, coupled with other evidence, has led scientists to surmise that there could be an ocean of liquid water beneath Europa's surface. Where there is water, there could be life. This is why Europa is a target of current interest for study of the possibility of non-Earth life. A follow-up spacecraft to Galileo will be Europa Orbiter, which should determine whether or not Europa has an ocean.
While at the University of Arizona, Tucson, Cynthia Phillips used this image in a detailed search for current geologic activity on Europa. Phillips is currently with the SETI Institute, Mountain View, Calif. She compared this image with similar images of Europa taken in 1979 by the Voyager spacecraft and found no signs of change due to geologic activity.
That suggests a minimum surface age for Europa of about 30 million years, though the result does not rule out current geologic activity altogether, as the study was limited by the resolution of 2 kilometers (1.6 miles) or more per pixel in this image and the Voyager images. Future images by Europa Orbiter may allow a search for smaller-scale changes on Europa's surface.
A revealing eclipse
During an eclipse of Jupiter's moon Io on January 1, 2001, NASA's Cassini spacecraft recorded glows from auroras and volcanoes on Io.
The camera on Cassini captured images of eclipsed Io in several colors ranging from the near-ultraviolet to the near-infrared. In the adjacent picture, two colors have been added to show the type of evidence used by imaging scientists in determining the source of Io's auroral glows. White dots near the equator are volcanoes, some of which are much brighter than the faint atmospheric glows.
A thin oxygen atmosphere at Io probably gives the moon its red glow. Sulfur dioxide at Io may generate the bright blue glows. The blue glows are restricted to areas deep down in the atmosphere near the surface of Io, while the red glows are much more extensive, reaching heights of up to 900 kilometers (560 miles).
That layering is what you'd expect if the blue glows are from sulfur dioxide as it is a heavier molecule than oxygen so it should stick closer to the surface due to gravity.
Blue and red regions at Io's equator dance back and forth as the orientation of Jupiter's magnetic field shifts.
A faint blue emission is visible near the north pole of Io, possibly due to a volcanic plume erupting from the volcano Tvashtar at high northern latitude on the side of Io opposite Cassini. This eruption, observed by both Galileo and Cassini, left an enormous red ring around Tvashtar.
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