NASA’s Juno measures subsurface heat on Jupiter’s moon Io for first time

NASA’s Juno spacecraft has made the first direct measurements of heat beneath the surface of Jupiter’s moon Io, revealing a steep temperature gradient in the shallow subsurface of the most volcanically active world in the solar system. The findings, published Wednesday in the Journal of Geophysical Research: Planets, were collected during two close flybys on Dec. 30, 2023, and Feb. 3, 2024, when Juno came within about 930 miles (1,500 kilometers) of Io’s surface.

The data, gathered by Juno’s Microwave Radiometer (MWR) instrument, shows that temperatures rise by more than 40 degrees Fahrenheit just several feet below the surface—a gradient far steeper than solar heating alone can explain. The MWR was originally designed to probe Jupiter’s deep atmosphere but has also been used to study three of the planet’s Galilean moons: Ganymede, Europa, and Io. The ability to probe into volcanic rock at Io was an unexpected discovery.

These measurements matter because Io’s extreme volcanism is powered by tidal heating, a fundamental process that also fuels subsurface oceans on icy moons like Europa and Ganymede. Until now, scientists could only observe heat escaping at the surface or through eruptions. The new data allows them to characterize how heat moves from the interior toward the surface, providing insights into tidal heating throughout the cosmos.

The MWR data suggests two possible explanations for the observed heat. First, heat could be rising steadily through a conductive crust, with a background heat flow of 1 to 3 watts per square meter—gentle locally but across the entire moon representing up to 30 times Earth’s average heat output. Alternatively, the signal could come from cooling lava flows capped by about 30 to 35 feet (9 to 11 meters) of solidified crust, covering roughly 10% of Io’s surface at any time.

Additionally, the flybys revealed that Io’s surface is remarkably smooth, with expansive plains stretching for 60 miles (100 kilometers) or more. The surface material has very low density, resembling pumice or fluffy volcanic ash rather than solid rock. These findings break new observational ground for both fiery and icy worlds, and the technique could be applied to study volcanoes on Earth using similar microwave instruments.