Galileo's near infrared mapping spectrometer instrument has found extremely high temperatures inside the volcanoes, which are more abundant than previously believed and contain surprising substances. The spectrometer detects heat from lava and shows the location of different materials on Io's surface.

The volcano Pele, named for the mythological Polynesian fire goddess, showed much higher temperatures inside its volcano than any found now on Earth -- about 1,500 Celsius (more than 2,700 Fahrenheit). "One of the most interesting questions about Io is: do all Io's volcanoes erupt such hot lavas, or are most volcanoes similar to basaltic volcanoes on Earth that erupt lavas with lower temperatures, about 1,200 Celsius (2,192 F)?" said Dr. Rosaly Lopes-Gautier, the instrument's science coordinator for Io at NASA's Jet Propulsion Laboratory (JPL), Pasadena, California. Billions of years ago on Earth, such hot volcanoes may have been common.

Before the three Io flybys by Galileo in late 1999 and early 2000, scientists knew Io had two volcanoes with very high temperatures. As the Galileo spacecraft moved close to Io during its flybys, it revealed more high-temperature areas than could be detected by distant observations. This means that Io could contain many smaller volcanoes with very hot lava.

A small, inactive volcano in the Chaac region was found to have a bright white floor covered in sulfur dioxide. The fact that this deposit is confined within the caldera walls indicates that it could have originally been a liquid, rising from lower layers. Because Io's atmosphere is so thin that it is almost a vacuum, the liquid would normally boil off.

"Our calculations indicate that given sufficiently large quantities, some of the liquid could freeze to form a layer of sulfur dioxide ice inside the caldera," said Dr. Bill Smythe, a JPL research scientist.

The infrared instrument can measure different substances by the light that they absorb or reflect. When it showed an unusual light pattern in a part of Io, scientists knew they had found a mysterious and unexpected substance.

"It was possible that it could be a mineral containing iron, such as pyrite, present in silicate lavas. If this was true, we would expect the band to be stronger in areas of the surface where lava deposits are new," said JPL's Dr. Robert Carlson, principal investigator for the spectrometer team.

However, the higher spatial resolution observations taken during the flybys showed that the opposite is true: the band is weaker in the dark volcanic areas.

"This means that, whatever the compound is, it probably doesn't come to the surface in the lava, but instead could be ejected in the volcanic plumes," Carlson said. Identifying this mysterious compound will probably require experiments in the laboratory as well as using the spacecraft observations.

The spectrometer team from JPL is joined in their studies by Dr. Sylvain Doute of the University of California, Los Angeles (UCLA).

Io may be giving off so much total heat, the best explanation would be that virtually the whole sphere is covered with lava spewed so recently it is still cooling, new calculations suggest.

Earlier estimates of Io's heat output totaled the amounts from active volcanoes and other localized areas warm enough to be measured. That approach sets a lower limit to the total output, but excludes about nine-tenths of Io's surface, said Dr. Dennis Matson, a JPL planetary scientist. He and four JPL colleagues calculated an upper limit to the estimate of Io's total heat output.

Their result, about 13.5 watts of energy per square meter (about 1.3 watt per square foot), is about five times as much as the heat output from the ground in the Yellowstone hot springs area of Wyoming, Matson said. Heat output does not include energy absorbed from the Sun, which is much less on Io than on Earth.

Recent measurements by Galileo of nighttime temperatures on Io's surface, averaging 90 to 95 degrees Kelvin (-297 to -288 degrees Fahrenheit), correspond closely with the team's new upper-limit estimate of the moon's heat flow.

The measurements do not vary much by latitude or time of night, implying that most of the heat comes from Io itself, rather than absorbed sunshine. That suggests Io's actual total heat output is close to the new upper-limit calculation, Matson said.

The JPL researchers say that for Io to be putting out that much heat, most of its surface would have to be covered with lava in various stages of cooling. Knowing the moon's heat output helps scientists check theories about the interiors of both Io and Jupiter, Matson said.

Not all of Io is hot. It has a solid metal core, surrounded by a rocky mantle, much like Earth. But Earth is only distorted slightly when the moon's gravity pulls the surface water into high tides. Jupiter pulls Io's crust into a permanent oval shape, due to its rotation and the tidal influence of Jupiter.

Io has no long-term strength to resist these forces, behaving as though it were a fluid. Galileo measured Io's polar gravity when it flew by this large moon in May 1999. From the gravitational field, says Dr. Jerry Schubert of the UCLA, it's possible to determine Io's internal structure.

The relationship between the polar and equatorial gravity shows that Io has a large metallic core, which is mostly iron. Measuring Io's gravity at the poles confirms an earlier idea, derived from measurements of Io's gravity at its equator, that Io's core is made of iron. On Earth, the metallic core generates Earth's magnetic field. It is not yet known if Io's metallic core also generates a magnetic field.

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