Io’s subsurface temperatures have been measured directly for the first time by NASA’s Juno spacecraft, revealing that the most volcanic body in the Solar System releases heat up to 30 times greater than Earth on average, according to findings published by Earth.com based on mission data.
How NASA’s Juno Spacecraft Measured Io’s Subsurface
For decades, planetary scientists relied solely on infrared instruments to measure thermal radiation escaping from the top of Io’s crust. That changed when NASA’s Juno spacecraft made close flybys of the Jovian moon, passing approximately 1,500 kilometers above the surface on December 30, 2023, and February 3, 2024.
During these close approaches, Juno’s Microwave Radiometer used six distinct antennas to detect microwave wavelengths across multiple depths. This instrument successfully mapped thermal emissions from a few centimeters down to dozens of meters beneath the rocky crust. Shannon Brown, the study’s lead author from NASA’s Jet Propulsion Laboratory, confirmed that the instrument captured subsurface thermal activity spanning that entire depth range.
Internal Heat Drivers and Underground Lava Fields
Data from the microwave radiometer show that temperatures spike by more than 22 degrees Celsius over just a few meters of depth. This sharp thermal gradient far exceeds anything solar heating alone can produce, proving that heat pours upward from deep inside the moon.
Researchers identified two primary sources driving this subterranean heat. The first is a steady conductive heat flow through solid rock, measured between 1 and 3 watts per square meter. While modest at any single spot, that rate scales up across the entire moon to release energy up to 30 times greater than Earth’s average. The second mechanism involves buried lava fields cooling slowly beneath roughly 9 to 10 meters of hardened crust. According to the research data, these subterranean thermal reservoirs may account for approximately 10 percent of Io’s surface area at any given time.
Tidal Heating Mechanics and Planetary Resonances
Did you know? Io’s intense volcanism is driven by tidal heating caused by its slightly elliptical orbit around Jupiter. The giant planet’s immense gravitational pull constantly flexes and compresses the moon, generating massive internal friction and heat.
Scott Bolton, a Juno co-investigator at the Southwest Research Institute, noted that Io provides a crucial natural laboratory for studying tidal heating processes across the cosmos. This same gravitational flexing mechanism likely maintains underground oceans on other moons in the outer Solar System, including Europa and Ganymede, as well as on exoplanets orbiting distant host stars.
Surprising Surface Density and Planetary Comparisons
Despite its reputation for towering mountains and explosive eruptions, Juno’s data revealed that vast stretches of Io’s landscape are surprisingly smooth. Shannon Brown reported that away from its rugged mountain chains, the terrain bears a strong resemblance to the Great Plains of North America. Furthermore, the moon’s surface material exhibits an exceptionally low density, likening its consistency more to volcanic ash or pumice than to solid rock.
This porous, low-density layer plays a direct role in how thermal energy migrates upward through the moon’s uppermost strata. Bolton suggested that microwave radiometry techniques developed during the Juno mission could eventually be adapted to study terrestrial volcanoes on Earth, capturing subsurface temperature gradients that traditional surface-level monitoring cannot reach.
Frequently Asked Questions
What makes Io the most volcanic body in the Solar System?
Io’s extreme volcanism is powered by tidal heating. Jupiter’s gravitational pull constantly flexes the moon’s interior as it travels along an elliptical orbit, generating immense internal heat that melts rock and fuels constant eruptions.
How deep did Juno measure beneath Io’s surface?
Using its Microwave Radiometer, Juno measured thermal emissions from a few centimeters down to dozens of meters beneath the crust during close flybys in late 2023 and early 2024.
What is tidal heating and why does it matter?
Tidal heating is the generation of internal heat through gravitational flexing. According to researchers, this process not only melts Io’s interior but also powers subterranean oceans on other icy moons like Europa and Ganymede.
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