A new analysis of data from the Galileo spacecraft has found that Jupiter’s violently volcanic moon Io has a moon-wide ocean of magma just below its surface.
The study was conducted by scientists from UCLA, UC Santa Cruz and the University of Michigan, Ann Arbor. The researchers used data from Galileo to measure Io’s ability to carry an electrical current, called its conductivity.
“We were able to infer there was a magma ocean by the conductivity measurement, which tells us the melt fraction [of the rocks on Io],” said the study’s lead author Krishan Khurana, a research geophysicist with UCLA’s Institute of Geophysics and Planetary Physics and a former co-investigator on Galileo’s magnetometer team.
Melt fractions is the percentage of rock that melts when heated.
Khurana compared the magnetometer technique to metal detectors here on Earth. Metal detectors create electromagnetic waves that pass through a person but reflect off of metal objects such as coins. A sensor detects those reflected waves.
The Galileo spacecraft acted as a big metal detector orbiting Jupiter during the 1990s and early 2000s. A sensor was mounted on a 10-meter extension that was connected to an instrument inside the spacecraft. The electromagnetic (EM) waves came from Jupiter itself due to its magnetic field, similar to the one we experience on Earth. Here on Earth, the magnetic field is what directs compasses to point to the magnetic north pole.
The EM waves from Jupiter would normally pass through most material without change.
“However, if there is a conducting material like a magma ocean, the EM waves from Jupiter can bounce off,” Khurana said. “It reflected a signal around Io which our sensors detected.”
All rocks have at least a little conductivity. The study’s authors inferred that the rocks on Io are partially or completely molten by recent work with ultramafic rocks – the same kind of rocks that make up the molten magma layer beneath Earth’s crust.
“Ultramafic rocks become highly capable of carrying electric current when melted,” said study co-author Xianzhe Jia, an assistant research scientist at the department of atmospheric, oceanic and space sciences at the University of Michigan, Ann Arbor.
As rocks are heated, they are better able to conduct electricity so their conductivity goes up. When these ultramafic rocks are heated, the conductivity increases by about 100,000 times, and increases even more when the rocks start to melt.
“The required conductivity is so high that a hot solid interior is unable to explain it,” Jia said. “That led us to test the hypothesis that the source of the strange signature in the magnetic field data was a molten or partially molten version of this kind of rock.”
The recent analysis that indicated Io has a molten ocean was possible with advances in computer models not available ten years ago.
The research helps explain the heavy volcanic activity of Io. Whereas Earth’s volcanism is mostly restricted to a few hot-spots such as the Pacific Ring of Fire, Io has 4,000 active volcanoes distributed evenly around the moon and 100 of them are erupting at any one time. Such violent activity requires an immense amount of heat caused by tidal action – similar to the ocean tides we get on Earth – but requires a specific kind of material in order to erupt.
“If a material is too rigid, it can’t move and heating can’t take place,” Khurana said. “However, if the substance is too low viscosity, like water, it moves around easily without very much heat.”
A molten magma layer is the perfect explanation for this activity, Khurana added.
A magma ocean also helps explain why Io doesn’t have a magnetic field like Earth and Jupiter; the hot magma layer inhibits the very specific movement required to generate a consistent magnetic field.
“Our insight into current conditions on Io may help us understand the structure of the ancient, partially-molten and tidally-heated Moon and perhaps even of tidally-heated super-Earths,” Jia said.
AMY STEWART can be reached at email@example.com.