SCIENCE -- May 12, 2011 at 5:02 PM ET
A Sea of Magma Feeds Hundreds of Volcanoes on Jupiter's Moon
New data confirms that an ocean of magma under the surface of Jupiter's moon, Io, feeds the moon's many active volcanoes.
Io is the most volcanically active body in our solar system, with about 100 volcanoes erupting at any one time. New measurements containing the first direct evidence of a sea of magma help to explain this phenomenon, along with the moon's high heat flow and unusual pattern of volcanic activity. The data was captured by the spacecraft Galileo during four flybys and published on Thursday in the journal Science.
"We used Jupiter's magnetic field as a sounding signal," said Krishan Khurana professor of geophysics and planetary physics at UCLA, and lead author of the study. "The principle we used is the same as what is used in metal detectors at the airport."
Metal detectors work by bouncing electromagnetic waves off metal objects, like metallic coins in your pocket, he explained. Similarly, while Jupiter's magnetic field penetrates most dry rock, it reflects off molten rock, which is an excellent conductor of electricity.
"We were looking for bounced-off signals from Io," Khurana said. We found them about 30 to 50 kilometers under the surface."
Galileo's magnetometer measured electrical current generated from the moon's interior. "Whenever you put an electrical conductor in a magnetic field, you generate currents," said Gerald Schubert, a geophysics professor and also an author on the study.
Similar techniques were used to measure subsurface saltwater oceans on Jupiter's other three moons: Europa, Ganymede and Callisto.
The finding is not surprising given the number of volcanoes and the amount of volcanic activity on Io, said Michael Manga, professor of earth and planetary science at the University of California, Berkeley. "It would be a great shock and surprise if there wasn't a magma ocean on Io," he said. "But confirming things that you suspect to be true is not a bad thing to do."
The latest data from Io reveals an ocean of magma that's 18 to 30 miles deep and about 1,400 degrees Celsius, hotter than lava on Earth. Scientists estimate that at least 20 percent of Io's rock is melted.
Io's active volcanoes produce roughly one hundred times more lava than all of Earth's volcanoes combined. But unlike Earth, where volcanoes generally erupt at the margins of tectonic plates, Io volcanoes are evenly distributed across the surface.
The existence of a magma ocean explains that distribution. The volcanoes have ready access to a subsurface aquifer at all latitudes and longitudes, Khurana said.
Like all of Jupiter's large moons, Io is tidally locked to Jupiter, meaning the same side of the moon faces the planet at all times. But the orbit is not perfectly circular; it gets tugged up and down by Jupiter's massive gravity, generating tides. Jupiter transfers huge amounts of energy, and heat, through these tides.
"The moon is pulling ocean water toward us," Manga said. "It's the same thing with Io. When you deform the material, that generates heat."
Many questions about Io remain. Has Io always been volcanically active? What is it that drives magma eruptions to the surface and what controls how and how long volcanoes erupt? But answering those questions would require another satellite mission to that part of the solar system, and that has been removed from the budget proposal.
Planetary scientists believe that studying Io may help us understand how the Earth transitioned from a planet with its own magma ocean 4.5 billion years ago to one driven by plate tectonics.
"Magma oceans are a really important part of planet formation," Manga said. "The earliest atmosphere of the Earth came out of that ocean, the earliest crust came out of that ocean, and the earliest oceans were formed from that ocean."
Correction: An earlier version of this article said the the Earth had a magma ocean 4.5 million years ago.