Spacecraft’s data reveal magma ocean under Jupiter moon
The discovery explains why Io is the most volcanic object known in the solar system.
May 13, 2011
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 20 to 30 miles (30 to 50 kilometers) thick is shown in gray in the cross-section. NASA/JPL/University of Michigan/UCLA
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. Scientists at the University of California, Los Angeles, the University of California, Santa Cruz, and the University of Michigan conducted the research.
"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 from the University of California, Los Angeles. "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 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 from NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "Io's volcanism informs us how volcanoes work and provides a window in time to styles of volcanic activity that may have occurred on Earth and our 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 from 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 km) thick, making up at least 10 percent of the moon's mantle by volume. The blistering temperature of the magma ocean probably exceeds 2200° Fahrenheit