Io’s magma ocean provides a view into Earth’s past

On January 1610, after observing the night sky with a simple telescope, Galileo Galilei wrote in a letter to the Tuscan court

In the same spirit of discovery some 400 years later, his namesake, NASA’s Galileo spacecraft, has collected magnetic field data that provides evidence of a sea of magma beneath the surface of one of these “four new planets.” As Galileo drew analogies between their behavior and that of other celestial bodies, so are scientists today making connections between magmatic heating processes Galileo observed inside Jupiter’s moon Io and those inside Earth.

Galileo departed Earth for Jupiter in 1989, entered orbit around the planet in 1995, and stayed until 2003. On board was a broad range of instrumentation to study the atmosphere of the gas giant and its moons. One of those, Io, had been shown by visual imaging to have active volcanoes and a thermal output of over 2 W/m², roughly 30 times that of Earth.

Galileo‘s magnetometer measured Jupiter’s magnetic field but during flybys of Io detected a perturbation in all three field components. Scientists initially thought this signal was caused by the interaction of Io’s atmosphere with Jupiter’s magnetospheric plasma. But after models of this proposed interaction failed to describe the signal, the problem lay dormant until new mineral physics experiments showed that melted igneous rocks are good conductors of electricity and could provide the necessary current to induce a magnetic response.

An induced magnetic field

A study led by geophysicist Krishan Khurana of the Institute of Geophysics and Planetary Physics at UCLA published recently in Science last month models the measured magnetic field perturbation. The paper supports theories that proposed a magma ocean as an explanation for another of Io’s geophysical phenonomena: its source of internal heating. An electrical current in response to Jupiter’s rotating magnetic field generates an induced magnetic field, which is detected by the probe’s magnetometer. But an electrically conductive surface is critical for that induction response to occur, and Khurana’s models verify that a layer of molten rock beneath Io’s surface provides the necessary conductivity.

“We are guided by experiments done in the last five to ten years on lherzolite,” says Khurana. This ultramafic rock—low silica, high magnesium, and iron bearing—rock is higher in magnesium and iron than is basalt, the high volatile-content-material typically extruded during volcanic activity on Earth. Derived from the upper mantle and extruded during large magmatic activity at higher temperatures, lherzolite as a solid lacks the high electrical conductivity needed to induce a magnetic signal. “But if you melt the same rocks,” explains Khurana, “conductivity increases several orders of magnitude and approaches the value associated with Earth’s water ocean conductivity,’ around 3 siemens per meter. That level of conductivity is more than enough to induce a magnetic signal. “It is the ultramafic rocks that we believe are present inside Io, because any volatile component [basalt, for example] would have come out and formed the crust,” he adds.

A similar magnetic induction response can be generated on Earth from magma at tectonically active mid-ocean ridges. Experimenters impose a radio wave at frequencies around 1 to 20 Hz that penetrates through ocean water and the upper crust. “In some ways, we’re using knowledge already used on Earth to say something about Io” Khurana says, noting that we don’t impose a wave on Io but use the wave created by Jupiter’s field. The Galileo probe collects signals from an electromagnetic wave that has a 13-hour period and can penetrate tens of thousands of kilometers of solid rock without attenuation. But Khurana warns that “even the 13-hour signal reaches saturation level when the conductivity is very high.” We cannot get an induced response stronger than the strength of the signal itself, which has a maximum strength of 800 nT.

It’s quite likely that the magma ocean is deeper than 50 km, the thickness believed to produce the maximum induced response. Laszlo Kestay, research geologist on the US Geological Survey’s astrogeology team , says that “by carefully timing [flyby measurements] . . . we can get Io in different parts of Jupiter’s magnetic field and collect some additional useful magnetic data.” Since Io circles Jupiter in a slightly eccentric orbit, the induced field varies. Multiple flyby measurements should separate different wave periods and identify their perturbations.

Magnetometers carried by other large space missions have collected data that is “instrumental in the discovery of water oceans beneath the icy crusts of the other large moons of Jupiter,” adds Kestay. The electrolyte-rich waters induce a magnetic field. In contrast, Earth’s moon sits inside a constantly changing interplanetary magnetic field created by the Sun, and induction measurements have shown that it, too, has a core and mantle. But a changing field is required to create an electromagnetic response.

“A constant field can penetrate through everything and does not create a response,” says Khurana. Saturn, for example, has an axially symmetric field which is constant all the time. However, even in Saturn’s symmetric magnetosphere, plasma shows rotating signals of 2 to 4 nT, very weak compared to Io’s 800 nT. Thus Khurana and colleagues propose a large number of flyby measurements as a technique to look for systematic signatures near Saturn’s icy moons.

Io’s tidal heat source and volcanoes

With a magma ocean in place as the mechanism behind Io’s induced magnetic field, the Jovian moon’s heat balance can also be explained in more detail.

Whereas Earth generates heat from radioactivity and remnants of early planet-forming events, Io’s heat source is tidal. The slight eccentricity of Io’s orbit around Jupiter creates variable solid-body tides of 100-m amplitude on top of the permanent several-kilometer tides. “Imagine a point on the surface that moves up and down by 100 m in the course of an Ionian day,” Khurana explains. That kinetic energy is converted to heat and must be dissipated. The heat is dissipated inside the magma ocean and volcanoes bring it to the surface.

But the magma ocean remains molten because the heat hasn’t escaped yet. “This confirms how powerful the tidal heating of Io’s mantle is!” exclaims Kestay. The big puzzle, then, is that this hot body does not generate a magnetic field in its core. A magnetic field requires energy derived from convection inside the body. However, if heat is not escaping the core, convection stops and a magnetic field cannot be generated.

“Heating occurs in a location where the material has high viscosity because it’s rubbing between adjacent layers that creates frictional heating,” explains Khurana. That is, the melt in Io’s interior requires a high viscosity and some mobility: An entirely rigid body can’t create heat. Strong heating can occur in a magma ocean with 20% melt and 80% crystals. That’s not too different from Earth’s magma.

“Io lets us see what ancient Earth might have been like,” Kestay says. It is believed that Earth, like most planetary bodies, had a magma ocean when it was formed. The magmatic interior is cooled through volcanic action. But once the magma ocean solidifies, heat from the interior will be released through plate tectonics and other mechanisms.

“By learning how heat is dissipated within Io, we can get a good handle on when [Earth’s] magma ocean solidified and plate tectonics took over,” Khurana says. Calculating heat dissipation rates through Io’s crust is next on his agenda for using Galileo‘s observations to explain a bit more not only about the Galilean moons, but also about our own corner of the solar system.

In his 17th century letter to the Tuscan court Galileo “concluded, and decided unhesitatingly, that there were [four] stars in the heavens moving about Jupiter, as Venus and Mercury about the sun; which at length was established as clear as daylight by numerous other observations.” Now, centuries later, conclusions from his namesake mission are being used to establish the history and dynamics of these ‘stars’ and our planet.

Rachel Berkowitz