SAN ANTONIO — July 29, 2025 — NASA’s Juno mission has made the first direct measurements of the temperature below the surface of Jupiter’s moon Io, which reveal significant heating in the upper few feet of the most volcanically active world in the solar system.

These findings, derived from data collected by Juno’s Microwave Radiometer (MWR) during two close flybys, were published July 22 in the Journal of Geophysical Research: Planets. Southwest Research Institute’s Dr. Scott Bolton, co-author of the paper and Juno’s principal investigator, invented and designed the MWR to investigate Jupiter’s atmosphere below the cloud tops. The novel instrument has also proven invaluable for studying Jupiter’s ice-covered and volcanic moons as well.

New data from MWR also show that the majority of Io’s surface is remarkably smooth at large scales and more similar in density to volcanic ash or pumice than to solid rock. These results break new observational ground for both fiery and icy worlds beyond our planet.

“The Juno microwave radiometer directly observed Io’s heat output by looking below the surface,” Bolton said. “The discovery that we could see below the surface of a rocky moon was a surprise with potentially important implications for the study of Earth’s volcanoes.”

Io’s extreme volcanism is powered by tidal heating. As the moon travels around the gas giant in a highly elliptical orbit, it is constantly stretched and squeezed by Jupiter’s immense gravity, which generates internal heat many times greater than Earth’s. Until now, virtually everything known about Io’s heat came from infrared observations, which sense only the surface temperature.

The mission’s extended phase provided the opportunity to observe three of the gas giant’s Galilean moons, Ganymede, Europa and Io. The Juno radiometer uses six microwave antennas acting as a single instrument capable of simultaneously detecting microwaves at a wide range of wavelengths. The instrument is collectively referred to as a Bolton Radiometer.

“The novel aspect of the instrument is that each wavelength explores a different depth to characterize the deep atmospheres of giant planets or the subsurface crusts of icy and rocky bodies,” Bolton said. “At Ganymede and Europa, we probed tens of miles into their ice shells. At Io, the MWR measured thermal emissions at depths ranging from a few inches down to tens of feet.”

Dr. Shannon Brown, senior research scientist at NASA’s Jet Propulsion Laboratory in Southern California and the paper’s lead author, said that everywhere MWR searched on Io they found the temperature rising by more than 40 degrees Fahrenheit (>20 degrees Celsius) just several feet into the surface. “That is a gradient far steeper than solar heating alone can explain,” Brown said.

Scientists offer two possible explanations. First, heat could be rising steadily through a conductive crust at a rate of 1 to 3 watts per square meter — up to 30 times Earth’s average. Or, more likely, the signal comes from cooling lava flows, capped by many feet of solidified crust, covering about 10% of the moon’s surface at any given time.

“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 Bolton. “This process can create both the most volcanic body in the solar system, in the case of Io, and the liquid subsurface oceans within the icy moons of giant planets, such as Europa and Enceladus.”

For more information, visit https://www.swri.org/markets/earth-space/space-research-technology/space-science/planetary-science.