Scientists show Jovian moon Io's mantle is similar to Earth

Washington University in St. Louis
September 2000

Researchers at Washington University in St. Louis studying chemical data from the Hubble Space Telescope have determined the eruption conditions of Jupiter's volcanically active satellite, Io, and concluded that the moon has a differentiated mantle similar to that of Earth. Differentiation is the process where various rock types are produced from a common magma.

The researchers, Mikhail Zolotov, Ph. D., senior research scientist in earth and planetary sciences, and Bruce Fegley, Jr., Ph.D., professor of earth and planetary sciences, were able to take the Hubble measurements and infer the temperature of the magma and plume pressure of Pele, one of Io's most active volcanoes.

This is the first time that scientists have used chemical data obtained from a telescope to study present-day interior processes of a solar system body.

Surprisingly, the magma temperature inferred from the Hubble chemical data corresponds to the temperatures deduced from infrared measurements taken by the Galileo spacecraft orbiting Jupiter. The study shows that Earth orbit observations can compete successfully with expensive planetary missions.

Zolotov and Fegley published their results in the Sept. 1, 2000 issue of "Geophysical Research Letters." Their study was funded by NASA.

Io, one of the four major satellites of Jupiter, is approximately the same size as our moon, but has tremendous volcanic activity occurring on its surface.

From Earth's orbit, the Hubble Space Telescope took pictures and spectra of the massive eruptions of Pele, one of the most active volcanoes of Io, with plumes reaching over 1,300 feet high. Through the analysis of these spectra, collaborators Melissa McGrath, Ph.D., of the Hubble Space Telescope Institute, and John Spencer, Ph.D., of the Lowell Observatory, detected gaseous sulfur species and sulfur oxides inside the plumes.

Zolotov and Fegley then used the relative abundances of those gases to calculate the temperature and pressure inside the vent of Pele.

"The composition of the plume reflects the conditions in the vent through relatively simple calculations," explains Zolotov. "By observing the composition of the plume of quenched volcanic gas in the volcanic atmosphere, we can determine the temperature, pressure and concentrations of undetected species in the vent."

In addition, Zolotov and Fegley evaluated the oxidation state of the magma and exsolved gases. They found that the abundances of sulfur oxides and sulfur vapors suggest a lack of iron metal from Io's mantle.

"If there is no free iron metal in the mantle, intense oxidation and differentiation in the internal structure of Io could have occurred in the first half billion years, or billion years, early in the history of Io," says Zolotov.

For this differentiation process to occur, an oxidizing agent must be present to remove all of the iron metal from the mantle. Water acted as an oxidizing agent when the Earth differentiated its iron billions of years ago.

"Io's mantle is about as oxidized as the Earth's mantle," says Zolotov. "And water, which is gone now, also could have caused the high oxidation state of Io."

When this reaction occurs, water reacts with iron, hydrogen is released and the iron then becomes oxidized.

"In this process, the iron metal is removed from Io's mantle and the interior of the moon becomes oxidized," says Zolotov. "Another satellite of Jupiter, water-rich Europa, may have differentiated in similar fashion to Io."