"This may be a window into how volcanic processes worked in the early history of the Earth, evidence of which is long gone."
Jupiter's moon Io can be called "EE-oh" or "EYE-oh." Just don't call it "ten," as once blurted out by a TV network newscaster.
Now, Io can be also called home to the hottest surfaces in the solar system, except for the sun.
Hundreds of millions of miles from the sun, volcanoes on Jupiter's moon Io sizzle at the highest recorded surface temperatures of any planetary body in the solar system. Planetary scientists from University of Arizona, Brown and five other institutions reported this finding earlier this month in a cover story in the weekly journal Science.
The report provides an important clue to understanding geophysical processes within Io, which may be similar to the early stages in the evolution of Earth, Venus and other planetary bodies.
"One of the really exciting things we're seeing is strong evidence of extremely high-temperature volcanism," says study co-author Jim Head, professor of geological sciences at Brown. "This may be a window into how volcanic processes worked in the early history of the Earth, evidence of which is long gone."
At least 12 different vents on Io spew lava at temperatures greater than 2,200 degrees Fahrenheit. One volcanic vent known as the Pillan Patera may be as hot as 3,100 degrees Fahrenheit - about three times hotter than the hottest sunlit surface of Mercury, the closest planet to the sun. Except for the volcanic hot spots, the prevailing surface temperatures on Io - 1,245 million miles from the sun - stay well below freezing at minus 240 degrees Fahrenheit.
Scientists have known for a while that Io is the solar system's most volcanically active planetary body. Io is "resurfaced" by fresh molten rock at a rate 25 times that of Earth, Head points out. Yet scientists were surprised by the extreme temperatures. The hottest lava on Earth and other terrestrial planets typically erupt at 1,800 to 2,100 degrees Fahrenheit.
Io's internal thermostat is turned up by its close neighbors, Head says. Neighboring moons Europa and Ganymede pull Io into an elliptical orbit, so that Io passes close and then swings farther away from Jupiter. During its orbit, Io actually changes shape slightly, molded by the massive gravitational forces of Jupiter at different distances. Just as metal heats up when it's bent back and forth, scientists believe Io heats up when it changes shape.
"It's almost as if Io is being kneaded by the tidal interactions between Jupiter and the other moons," Head says.
The data come from instruments on the Galileo spacecraft, which Head calls an orbiting volcano observatory. The latest temperature measurements are more than double the highest temperatures recorded by the Voyager spacecraft in 1979 and exceed more recent measurements made by Earth-based telescopes.
One instrument on the spacecraft, the solid state imaging camera, took high-resolution images during 11 eclipses in five orbits, when Io was eclipsed from the sun in Jupiter's shadow. From the spacecraft, Head says, hot lava literally glowed on the dark moon surface.
Io's hot spots were also studied by the spacecraft's near infrared mapping spectrometer during 11 orbits, mostly when Io was not in eclipse. The camera provides high spatial resolution to image the hottest features and map color variations, while the spectrometer can observe at many wavelengths and is sensitive to a wider temperature range. The combination of both instruments provides a powerful means to study Io's volcanism. The camera and spectrometer together have discovered a total of 41 hot spots on the moon, a dozen of them the highest surface temperatures in the solar system other than the sun itself.
"The most likely explanation for these very high temperatures is that the eruptions contain magnesium - rich silicates," says lead author Alfred McEwen, director of the Planetary Image Research Lab at the University of Arizona in Tucson. "We must now think of Io's volcanoes in terms of the type of very high-temperature silicate volcanism, which was found on Earth during its early days and which we suspect occurred also on Venus and Mars."
Head and his colleagues plan to gather more detailed information about Io with two planned close flybys in late 1999, as long as the Galileo spacecraft remains healthy. Galileo has been orbiting Jupiter and its four largest moons, including Io, for nearly three years. It is in the midst of an extended journey, known as the Galileo Europa Mission, with eight flybys of Europa and four of Callisto, in addition to the Io flybys.
Additional information and images taken by the Galileo spacecraft are available on the Internet. Other images are also available.