New data analysis from NASA’s Galileo spacecraft reveals a subsurface ocean of molten or partially molten magma beneath the surface of Jupiter’s volcanic moon Io.


This graphic shows the internal structure of Jupiter’s moon Io as revealed by data from NASA’s Galileo spacecraft. The low-density crust about 30 to 50 kilometers (20 to 30 miles) thick is shown in gray in the cross-section. The newly discovered subsurface molten or partially molten magma “ocean” – also known as the asthenosphere – is shown in red-brown beneath the crust. This global magma layer is believed to be more than 50 kilometers (30 miles) thick, with the outer part making up at least 10 percent of the entire mantle by volume. The mantle has been colored gold in this graphic.

Io is bathed in magnetic field lines (shown in blue) that connect the north polar region of Jupiter to the planet’s south polar region. As Jupiter rotates, the magnetic field lines draping around Io strengthen and weaken. Because Io’s magma ocean has a high electrical conductivity, it deflects the varying magnetic field, shielding the inside of the moon from magnetic disturbances. The magnetic field inside of Io maintains a vertical orientation, even as the magnetic field outside of Io dances around. These variations in the external magnetic field signatures enabled scientists to understand the moon’s internal structure. In the animation, the magnetic field lines move with Jupiter’s rotation period of about 13 hours in Io’s rest frame.

Io’s mantle is made of “ultramafic” rocks, a class of rocks rich in iron and magnesium silicates, which become capable of carrying substantial electrical current when melted. Ultramafic rocks are igneous in origin – that is, formed through the cooling of magma. On Earth, they are believed to derive from the mantle.

Io’s ultramafic mantle must have a temperature exceeding 1,200 degrees Celsius (2,200 degrees Fahrenheit) to support rock melting in the asthenosphere. Io’s core, about 600 to 900 kilometers (400 to 600 miles) in radius is composed of iron and iron sulfide and shown in a metallic silver hue.

The finding heralds the first direct confirmation of this kind of magma layer at Io and explains why the moon is the most volcanic object known in the solar system. The research was conducted by scientists at the University of California, Los Angeles, the University of California, Santa Cruz, and the University of Michigan. The study is published this week in the journal Science.

“Scientists are excited we finally understand where Io’s magma is coming from and have an explanation for some of the mysterious signatures we saw in some of the Galileo’s magnetic field data,” said Krishan Khurana, lead author of the study and former co-investigator on Galileo’s magnetometer team at UCLA. “It turns out Io was continually giving off a ‘sounding signal’ in Jupiter’s rotating magnetic field that matched what would be expected from molten or partially molten rocks deep beneath the surface.”

Io produces about 100 times more lava each year than all the volcanoes on Earth. While Earth’s volcanoes occur in localized hotspots like the “Ring of Fire” around the Pacific Ocean, Io’s volcanoes are distributed all over its surface. A global magma ocean about 20 to 30 miles (30 to 50 kilometers) beneath Io’s crust helps explain the moon’s activity.

“It has been suggested that both the Earth and its moon may have had similar magma oceans billions of years ago at the time of their formation, but they have long since cooled,” said Torrence Johnson, a former Galileo project scientist based at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif. He was not directly involved in the study. “Io’s volcanism informs us how volcanoes work and provides a window in time to styles of volcanic activity that may have occurred on the Earth and moon during their earliest history.”

NASA’s Voyager spacecraft discovered Io’s volcanoes in 1979, making that moon the only body in the solar system other than Earth known to have active magma volcanoes. The energy for the volcanic activity comes from the squeezing and stretching of the moon by Jupiter’s gravity as Io orbits the largest planet in the solar system.

Galileo was launched in 1989 and began orbiting Jupiter in 1995. Unexplained signatures appeared in magnetic field data from Galileo flybys of Io in October 1999 and February 2000. After a successful mission, the spacecraft intentionally was sent into Jupiter’s atmosphere in 2003.

“During the final phase of the Galileo mission, models of the interaction between Io and Jupiter’s immense magnetic field, which bathes the moon in charged particles, were not yet sophisticated enough for us to understand what was going on in Io’s interior,” said Xianzhe Jia, a co-author of the study at the University of Michigan.

Recent work in mineral physics showed that a group of rocks known as “ultramafic” rocks become capable of carrying substantial electrical current when melted. Ultramafic rocks are igneous in origin, or form through the cooling of magma. On Earth, they are believed to originate from the mantle. The finding led Khurana and colleagues to test the hypothesis that the strange signature was produced by current flowing in a molten or partially molten layer of this kind of rock.

Tests showed that the signatures detected by Galileo were consistent with a rock such as lherzolite, an igneous rock rich in silicates of magnesium and iron found in Spitzbergen, Sweden. The magma ocean layer on Io appears to be more than 30 miles (50 kilometers) thick, making up at least 10 percent of the moon’s mantle by volume. The blistering temperature of the magma ocean probably exceeds 2,200 degrees Fahrenheit (1,200 degrees Celsius).

The Galileo mission was managed by JPL for NASA’s Science Mission Directorate in Washington. For more information about the Galileo mission and its discoveries, visit: http://solarsystem.nasa.gov/galileo


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