July 7, 1998 PROVIDENCE, R.I. -- 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 University and five other institutions report this finding in the cover story of the July 3 issue of the weekly journal Science.
Editors: The Io image on the cover of Science shows about 20 dark volcanic vents, some also shaded red by sulfur. It is available at http://pirlwww.lpl.arizona.edu/hiips/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.
"The very hot lavas erupting on Io are hotter than anything that has erupted on Earth for billions of years," says lead author Alfred McEwen, director of the Planetary Image Research Lab at the University of Arizona. "They are the highest surface temperatures in the solar system other than the sun itself."
At least 12 different vents on Io spew lava at temperatures greater than 2,200 degrees Fahrenheit. One volcanic vent 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. The surface temparatures on Io, which is 1,245 million miles from the sun, stay well below freezing (minus 243 degrees Fahrenheit) except for the volcanic hot spots.
The latest temperature measurements are more than double the highest temperatures recorded by the Voyager spacecraft in 1979 and also exceed more recent measurements made by telescopes.
McEwen and his colleagues calculated the temperatures of Io's volcanoes using two instruments on the Galileo spacecraft. The instruments read the infrared "signatures" of the volcanic vents, which emit light beyond the color red, which is the longest wavelength visible to the human eye. Scientists calculated the lava temperatures needed to fit the infrared signatures. These lava temperatures and the visible color properties of the dark flows are consistent with lava compositions rich in heavy elements like magnesium.
Io's neighbors turn up its internal thermostat. 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," says co-author James Head, professor of geological sciences at Brown University.
In its chilly corner of the universe, Io needs to release its inner heat, just as a cup of hot coffee cools by releasing steam. Scientists have known for a while that Io is the solar system's most volcanically active planetary body. Yet scientists were surprised by the extreme temperatures.
The findings raise new questions about the composition and evolution of Io. For example, the hot temperatures suggest that the lava is composed of dense material that tends to sink, not rise, within a planet. Typically, lighter material in a volcanically active planetary body tends to melt first and rise to the surface where it cools and forms a crust. The process is called differentiation.
"Given Io's intense vulcanism, we expect extreme differentiation," McEwen says. "The evidence suggests we're seeing heavy magma erupt to the surface. How do we explain that? It's harder for dense material to rise through a low-density crust, although this has occurred on Earth's moon. Perhaps some process mixes the crust back into Io's interior, so the crust has a higher density."
On Earth, the tectonic plates move slowly around the surface, forming new crust at mid-ocean ridges, for example, and recycling oceanic crust into the hot mantle where two plates collide, one diving under the other. Scientists don't know yet how to explain what's happening on Io.
"We have a lot of the same questions about early Earth," McEwen says. "Early Earth is hard to understand because the evidence has been so degraded by an active environment and plate tectonics. I like to think of Io as a grand experiment in planetary vulcanism and differentiation. This experiment may ultimately help us to understand the evolution of Earth and other planets, such as Venus and Mars."
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