Things are always heating up on Jupiter’s moon Io, a volcano-covered world that’s constantly pushed and pulled by its host planet’s gravity. NASA’s ongoing Juno mission, while investigating the rocky moon, saw sweltering temperatures in its depths. With this unprecedented view, astronomers are now wondering what’s causing so much heat to rise from the moon's subsurface.

Previously, heat signatures from Io could only be seen escaping at its surface or through volcanic eruptions. But thanks to Juno’s observations, researchers have been able to finally catch underground heat coming up to the surface, as explained in a new study published in the Journal of Geophysical Research: Planets. The process that generates this heat, called tidal heating, could provide insight into other moons across the outer Solar System.

“Io provides a unique window into learning how tidal heating works throughout the cosmos, a fundamental process that provides energy and heat to worlds that are far from their parent star,” said study co-author Scott Bolton, Juno’s principal investigator at the Southwest Research Institute, in a statement.

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A Heated Tug-of-War with Jupiter

Io is one of the Galilean satellites — Jupiter’s four largest moons — but it stands apart for how volcanically active it is, with over 400 volcanoes rising from its surface.

So where did all these volcanoes come from? The answer lies in tidal heating, which the University of California Museum of Paleontology defines as the repeated deformation of a body (such as a moon) due to tides from another body (such as a planet), causing the heating of the former’s interior.

Tidal forces from our own moon heat Earth’s interior, but the effect is very minor. Looking at it the other way around, tides on the moon created by Earth’s gravity cause the lunar crust to rise and fall, according to Smithsonian’s National Air and Space Museum.

In the case of Io, though, tidal heating has a much more drastic effect. The moon is often described as being in a “tug-of-war” with Jupiter, meaning it is always stretched and squashed by the gas giant’s gravity as it proceeds on its slightly elliptical orbit. This tug-of-war powers Io’s volcanoes, yet astronomers have never seen how the heat arises from below the surface.

Surveys Below Io's Surface

NASA’s Juno spacecraft, launched in 2011 to study Jupiter and its moons, has finally provided a view of Io’s subsurface heat. The spacecraft’s Microwave Radiometer (MWR), with six microwave antennas that are able to detect microwaves at a wide range of wavelengths, was able to look deeper into Io, along with two of Jupiter’s other Galilean moons, the ocean-bearing Europa and Ganymede.

During flybys of Io in December 2023 and February 2024, the instrument measured the moon’s thermal emission at depths ranging from a few inches down to tens of feet, recording temperatures rising by 40 degrees F only a few feet into the surface.

The heat may either be rising steadily through a conductive crust, or it could be coming from cooling lava flows that are capped by 30 to 35 feet of solidified crust. This heat output, scaled to the entire moon, represents a release of energy up to 30 times Earth’s average.

The details learned from studying Io’s tidal heating, according to the researchers, could even be applicable to Earth's volcanoes.

“The surprising discovery that we could see below a rocky moon’s surface has important implications for studying Earth’s volcanoes. Juno has taught us that if we look with an MWR-type instrument near a volcano on Earth, we might see a similar signature in the subsurface temperature gradient, providing new information on how terrestrial volcanoes work,” said Bolton.

Smoother Than It Seems

In a separate finding using the MWR instrument, Juno observed a surprising smoothness to Io’s surface. While the moon does have plenty of tall mountains, Juno’s data has shown that the surface is also rife with smooth patches that stretch for 60 miles or more.

“Away from its mountains, the surface is more like the Great Plains of North America, and even though Io is a rocky body, the surface material has a very low density — more like pumice or a fluffy volcanic ash than solid rock,” said lead author Shannon Brown, a scientist at NASA’s Jet Propulsion Laboratory.

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Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• This article references information from a study published in the Journal of Geophysical Research:Planets: Io Sub-Surface Temperature Profile Observed by the Juno Microwave Radiometer
• This article references information from the University of California Museum of Paleontology: tidal heating
• This article references information from the Smithsonian’s National Air and Space Museum: Earth’s Tidal Forces Linked to Shrinking Moon