Scientists Find Solar System's Hottest Surfaces On Jupiter's Moon Io

Editors: The Io image on the cover of Science shows about 20dark volcanic vents, some also shaded red by sulfur. It is available athttp://pirlwww.lpl.arizona.edu/hiips/science/.

The report provides an important clue to understanding geophysical processeswithin 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 eruptedon Earth for billions of years," says lead author Alfred McEwen, director ofthe Planetary Image Research Lab at the University of Arizona. "They are thehighest surface temperatures in the solar system other than the sun itself."

At least 12 different vents on Io spew lava at temperatures greater than2,200 degrees Fahrenheit. One volcanic vent may be as hot as 3,100 degreesFahrenheit - about three times hotter than the hottest sunlit surface ofMercury, the closest planet to the sun. The surface temparatures on Io, whichis 1,245 million miles from the sun, stay well below freezing (minus 243degrees Fahrenheit) except for the volcanic hot spots.

The latest temperature measurements are more than double the highesttemperatures recorded by the Voyager spacecraft in 1979 and also exceed morerecent measurements made by telescopes.

McEwen and his colleagues calculated the temperatures of Io's volcanoesusing two instruments on the Galileo spacecraft. The instruments read theinfrared "signatures" of the volcanic vents, which emit light beyond the colorred, which is the longest wavelength visible to the human eye. Scientistscalculated the lava temperatures needed to fit the infrared signatures. Theselava temperatures and the visible color properties of the dark flows areconsistent with lava compositions rich in heavy elements like magnesium.

Io's neighbors turn up its internal thermostat. Neighboring moons Europa andGanymede pull Io into an elliptical orbit, so that Io passes close and thenswings farther away from Jupiter. During its orbit, Io actually changes shapeslightly, molded by the massive gravitational forces of Jupiter at differentdistances. Just as metal heats up when it's bent back and forth, scientistsbelieve Io heats up when it changes shape.

"It's almost as if Io is being kneaded by the tidal interactions betweenJupiter and the other moons," says co-author James Head, professor ofgeological 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 fora 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 densematerial that tends to sink, not rise, within a planet. Typically, lightermaterial in a volcanically active planetary body tends to melt first and riseto the surface where it cools and forms a crust. The process is calleddifferentiation.

"Given Io's intense vulcanism, we expect extreme differentiation," McEwensays. "The evidence suggests we're seeing heavy magma erupt to the surface. Howdo we explain that? It's harder for dense material to rise through alow-density crust, although this has occurred on Earth's moon. Perhaps someprocess mixes the crust back into Io's interior, so the crust has a higherdensity."

On Earth, the tectonic plates move slowly around the surface, forming newcrust at mid-ocean ridges, for example, and recycling oceanic crust into thehot mantle where two plates collide, one diving under the other. Scientistsdon't know yet how to explain what's happening on Io.

"We have a lot of the same questions about early Earth," McEwen says. "EarlyEarth is hard to understand because the evidence has been so degraded by anactive environment and plate tectonics. I like to think of Io as a grandexperiment in planetary vulcanism and differentiation. This experiment mayultimately help us to understand the evolution of Earth and other planets, suchas Venus and Mars."

More information

http://www.jpl.nasa.gov/galileo/http://www.lpl.arizona.edu/faculty/mcewen.htmlhttp://porter.geo.brown.edu/planetary/