Juno’s Microwave Radiometer Reveals Subsurface Heat Flow on Io

During close flybys on December 30 2023 and February 3 2024, NASA’s Juno spacecraft used its Microwave Radiometer (MWR) to probe several inches to tens of feet below Io. The instrument recorded temperatures rising by more than 40 °F just a few feet beneath the surface, a gradient far steeper than solar heating alone can explain. These observations constitute the first direct subsurface temperature measurements on a rocky moon.

NASA’s Juno spacecraft finds temperatures surging just below Io’s volcanic surface Earth.com

Io is the most volcanically active body in the Solar System, powered by tidal heating as Jupiter’s gravity repeatedly stretches and squeezes the moon. Prior to Juno, scientists could only infer internal heat from infrared observations of surface emissions or eruptive events. Microwave radiometry, previously applied to the icy shells of Ganymede and Europa, offered a way to look deeper into Io’s crust.

Earth.comNASA’s Juno spacecraft finds temperatures surging just below Io’s volcanic surface

• Temperature rise: > 40 °F within a few feet of the surface (Brown).

NASA’s Juno spacecraft finds temperatures surging just below Io’s volcanic surface

• Heat flow: Estimated conductive flux of 1–3 W m?², roughly 30 × Earth’s average surface heat flux.

NASA’s Juno spacecraft finds temperatures surging just below Io’s volcanic surface

• Surface smoothness: Broad, smooth regions extending over 60 mi.

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• Bulk density of upper layer: 0.7–1.1 g cm?³, consistent with porous volcanic ash or pumice.

NASA’s Juno spacecraft finds temperatures surging just below Io’s volcanic surface

• Porosity estimates: ? 75 % for sulfur-rich material or ? 85 % for basalt under model assumptions.

NASA’s Juno spacecraft finds temperatures surging just below Io’s volcanic surface

• Heat-transport scenarios: Two end-member models were evaluated—steady conductive flow through solid rock and cooling of buried lava flows covering roughly 10 % of Io’s surface.

NASA’s Juno spacecraft finds temperatures surging just below Io’s volcanic surface

Bolton highlighted Io’s role as a natural laboratory for tidal heating, a process that can power both surface volcanism and subsurface oceans on other moons. He noted that the observed gradient cannot be explained by solar heating alone, indicating internal processes dominate heat transport.

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• “The Juno Microwave Radiometer directly observed Io’s heat output by looking below the surface,” — Scott Bolton, Juno principal investigator.

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• “The instrument measured Io’s thermal emission at depths ranging from a few inches down to tens of feet,” — Shannon Brown, lead author.

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The team evaluated two primary mechanisms—steady conduction and cooling of buried lava fields. Neither model fully reproduces the measured temperature profile across all microwave frequencies. Uncertainties remain regarding the thermal conductivity of Io’s porous surface layer and the depth-dependent absorption of microwave radiation. Further observations with higher-resolution instruments are needed to discriminate between the competing explanations.

NASA’s Juno spacecraft finds temperatures surging just below Io’s volcanic surface Earth.com

Understanding how tidal heating distributes energy beneath Io’s crust informs broader planetary science. The same tidal processes are thought to sustain subsurface oceans on icy moons such as Europa and Enceladus, potentially creating habitable environments far from the Sun. Moreover, the ability to probe subsurface temperatures on a rocky body opens new avenues for comparative volcanology and could improve monitoring of volcanic hazards on Earth.

NASA’s Juno spacecraft finds temperatures surging just below Io’s volcanic surface