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."
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/.
At 3 km/pixel, this is the highest resolution color picture of Io yet taken by Galileo. The satellite is seen against a backdrop of Jupiter's cloud tops, which appear blue in this false-color composite constructed from 1-micron, green and violet filter images. Among the surprises that can be seen on the surface of Io are several small, distinctly greenish patches, and subtle violet hues at the cores and margins of bright SO2-rich regions like the one in the lower right. Dark spots, many flagged by bright red pyroclastic deposits, mark the sites of current volcanic activity. Most of Io's riotous color can be explained by sulfur compounds, but the dark materials that make up the flows and calderas are probably silicates. (Image processed by Paul Geissler, University of Arizona)
New images of Io taken by the Galileo spacecraft are available on the Galileo home page
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
July 2, 1998
"The most likely explanation for these very high temperatures is that the eruptions contain magnesium-rich silicates," said Dr. Alfred McEwen of the University of Arizona, Tucson, AZ, a member of Galileo's solid state imaging camera team. "We've tentatively identified magnesium-rich orthopyroxene in lava flows around these hot spots. This leads us to conclude that silicate volcanism is taking place with lava compositions expected to melt at a very high temperature. 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."
The new findings by the Galileo camera and the spacecraft's near infrared mapping spectrometer have updated scientists' information on Io's volcanic processes. Previously, Io observations made by the Voyager spacecraft in 1979 put the highest temperature estimates at about 650 Kelvin (710 degrees Fahrenheit). This led many scientists to believe that Io's volcanic activity was caused by low-temperature sulfur volcanism. In 1986, ground-based telescope observations increased the temperature estimates to above 900 Kelvin (1,160 degrees Fahrenheit), which suggested that silicate volcanism was occurring at least occasionally, just as it does on Earth today. In 1996 and 1997, Galileo identified 30 locations with temperatures higher than 700 Kelvin (800 degrees Fahrenheit).
"This new data indicate that high-temperature eruptions on Io are a basic and common part of its active volcanic processes," said Dr. Torrence Johnson of JPL, Galileo project scientist. Johnson led the group that found the high temperature eruption in 1986. He is also a member of the near infrared mapping spectrometer team. "Io's current volcanic activity may have a lot in common with ancient volcanic processes on Earth and other planets. Since the geologic record from those times is very sparse, it's quite exciting to be able to study this type of volcanism going on today."
"This discovery of high-temperature silicate volcanism provides us with an extremely important clue to understanding the geophysical processes within Io," McEwen explained. Io is heated by periodic tides as it orbits Jupiter, along with the other Galilean satellites (Europa, Ganymede and Callisto).
Armed with this new information, scientists also hope to learn more about the composition of Io's crust. "Io's extreme volcanic activity is expected to result in a low-density crust rich in silica, sodium and potassium," said McEwen. "However, the high-temperature volcanism suggests that the crust may be composed of heavier lavas."
Galileo's solid state imaging camera observed Io during 11 eclipses in 5 orbits, when Io was in Jupiter's shadow, and sunlight was blocked so the camera could better see the glowing volcanic vents. 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. Thus, 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 Io.
Scientists hope 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 2-1/2 years. It is currently 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.
Galileo Europa Mission is managed by JPL, a division of California Institute of Technology, Pasadena, CA.