Io’s Volcanoes are Windows into its Hot Interior

NASA’s Juno spacecraft was sent to Jupiter to study the gas giant. But its mission was extended, giving it an opportunity to study the unique moon Io. Io is the most volcanically active body in the Solar System, with over 400 active volcanoes.

Researchers have taken advantage of Juno’s flybys of Io to study how tidal heating affects the moon.

In recent months, Juno performed several flybys of Io, culminating in one that brought the spacecraft to within 1500 km of the surface. This gave Juno unprecedented close-up views of the volcanic moon. One of its instruments, the Jovian Infrared Auroral Mapper (JIRAM), is an infrared spectrometer, and its data is at the heart of new research into Io’s volcanic activity and how tidal heating drives it.

The new research letter, “JIRAM Observations of Volcanic Flux on Io: Distribution and Comparison to Tidal Heat Flow Models,” was published in the journal Geophysical Research Letters. Madeline Pettine, a doctoral student in astronomy at Cornell University, is the lead author.

Though Io is dead, the tidal heating that keeps it warm could contribute to habitability elsewhere.

“Studying the inhospitable landscape of Io’s volcanoes actually inspires science to look for life,” said lead author Pettine.

“It’s easier to study tidal heating on a volcanic world rather than peering through a kilometers-thick ice shell that’s keeping the heat covered up.”

Madeline Pettine, Cornell University

Io is one of the four Galilean moons. The other three, Callisto, Ganymede, and Europa, are all suspected of having liquid oceans under frozen layers of surface ice. If these oceans truly exist, they could potentially support life. Jupiter’s tidal heating provides the heat to keep those oceans warm. Io is valuable scientifically because we can witness the effects of tidal heating on its surface.

Juno isn’t the only spacecraft to have visited Jupiter’s moon Io. This global view of Io was obtained during the tenth orbit of Jupiter by NASA’s Galileo spacecraft. It’s a false colour image that highlights differences on Io’s surface. Image Credit: NASA

“Tidal heating plays an important role in the heating and orbital evolution of celestial bodies,” said co-author Alex Hayes, the Jennifer and Albert Sohn Professor of Astronomy in the College of Arts and Sciences at Cornell. “It provides the warmth necessary to form and sustain subsurface oceans in the moons around giant planets like Jupiter and Saturn.”

Io’s volcanoes aren’t distributed evenly on its surface. The majority of them are in the equatorial region. However, in this work, the researchers found that the volcanoes on Io’s poles may act to regulate the moon’s interior temperature.

“I’m trying to match the pattern of volcanoes on Io and the heat flow that they’re producing with the heat flow we expected from theoretical models,” said Pettine.

Jupiter is the most massive planet in the Solar System and its gravitational pull is second only to the Sun’s. Jupiter’s powerful gravity does more than dictate Io’s orbit. It warps the moon and forces it to deform, generating heat.

This simple schematic shows how a planet can create tidal heating on an orbiting moon. The stretching and heating are most extreme when the moon is at its pericenter, the closest distance to the planet. Image Credit: Caltech.

“The gravity from Jupiter is incredibly strong,” Pettine said. “Considering the gravitational interactions with the large planet’s other moons, Io ends up getting bullied, constantly stretched and scrunched up. With that tidal deformation, it creates a lot of internal heat within the moon.”

Io has no ocean, so the heat melts rock, creating a likely magma ocean inside the moon. That magma works its way up through the surface, erupting as volcanoes and lava flows. The gases from the magma colour the surface of the moon in reds, yellows, and browns.  

To understand what’s happening inside Io, Pettine and her colleagues worked with a mathematical equation called spherical harmonic decomposition. This equation allows scientists to analyze data from a spherical surface and break it down, revealing patterns and important features.

Previous research shows that most of Io’s volcanic activity is in its equatorial region, although some volcanoes have been detected on its poles. In this work, it revealed systems of bright volcanoes at high latitudes.

“Our observations confirm previously detected systems of bright volcanoes at high latitudes,” the authors write. “While our map agrees with previous studies that suggest that low?to mid?latitude areas see the highest areas of volcanic activity, our map suggests that the poles of Io are comparably active to the equator.”

This figure’s perspective shows the sub-Jovian, north-polar view of Io in the left column and the anti-Jovian, south-polar view of Io in the right column. The topmost row shows the coverage map achieved for JIRAM during this study. The second row is a global map of volcanic flux. The hot spot in the north polar region is clear. Image Credit: Pettine et al. 2024.

Pettine and her co-researchers compared their global heat flux maps with three different models that attempt to explain what’s going under Io’s surface: the Deep Mantle model, the Asthenospheric model, and the Global Magma model.

The Deep Mantle Model says that tidal heating keeps a large portion of the mantle in a molten state. The Asthenospheric Model says that less of the mantle is molten and that only the asthenosphere is in a molten state due to tidal heating. This is more similar to Earth. The Global Magma Ocean model is a more extreme interpretation of the data and says that a greater portion of Io’s interior is molten, perhaps extending from just below the surface to greater depths.

This figure shows what Io’s surface heat flux should look like for three different interior models. Image Credit: Pettine et al. 2024.

The researchers also created a complete global map of heat flux produced by volcanic hot spots. “Viewing this flux on both a linear and a logarithmic scale better illustrates individual volcanic behaviour and global heat flow variations, particularly the lowest-flux regions,” the authors write.

“Our study finds that both poles are comparably active and that the observed flux distribution is inconsistent with an asthenospheric heating model, although the south pole is viewed too infrequently to establish reliable trends,” the authors explain.

These global volcanic flux maps show the average flux in milliwatts per square meter. The top is on a linear scale, while the bottom is on a logarithmic colour scale. The coloured bars and the line plots beside each map show the average flux projected horizontally (to the right of each map) and the average flux projected vertically (below each map) to show trends in flux by latitude and longitude. Image Credit: Pettine et al. 2024.

The researchers say that their heat flux maps don’t favour any of the models. “Using spherical decomposition, we find that the distribution of flux is much more uniform than in-line with any of the models,” they write.

For now, a more complete understanding of Io’s tidal heating and volcanic activity is elusive. Juno’s JIRAM observations are just a snapshot of the moon. Over longer time periods, the heat maps will look different and may support different models and conclusions.

“I’m not solving tidal heating with this one paper,” said Pettine. “However, if you think about icy moons in the outer solar system, other moons like Jupiter’s Europa, or Saturn’s Titan and Enceladus, they’re the places that if we’re going to find life in the solar system, it will be one of those places.”

A better understanding of tidal heating will do more than explain aspects of our own Solar System. It may help us understand habitable zones in other solar systems and how exomoons might be heated by giant exoplanets.

Artist’s illustration of a large exomoon orbiting a large exoplanet. While we have no way of observing exomoons, that day will come soon enough. A better understanding of tidal heating will help us understand what we will see. Image Credit: NASA/ESA/L. Hustak

That’s why, although Jupiter’s icy moons are prime targets for exploration, with two missions heading to study Europa, Ganymede, and Callisto, we need to keep a scientific eye on Io.

“We need to know how the heat is being generated,” Pettine said. “It’s easier to study tidal heating on a volcanic world rather than peering through a kilometers-thick ice shell that’s keeping the heat covered up.”

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