NASA's Juno mission has, for the first time, measured temperatures beneath the surface of Jupiter's moon Io, offering new insights into how heat moves through the most volcanically active body in the solar system.
The findings, published in the Journal of Geophysical Research: Planets, are based on data collected during two close flybys of Io in December 2023 and February 2024.
The study also found that much of the moon's surface is smooth and composed of low-density material.
Io's extreme volcanic activity is driven by tidal heating, caused by Jupiter's powerful gravity repeatedly stretching and compressing the moon as it travels in a slightly elliptical orbit.
Unlike previous infrared observations, which measured only surface temperatures, Juno's Microwave Radiometer (MWR) was able to probe beneath the surface.
Scott Bolton, Juno's principal investigator at the Southwest Research Institute and co-author of the study, said the instrument directly observed Io's heat output below the surface. He said the ability to study beneath the surface of a rocky moon could also help scientists better understand volcanic processes on Earth.
Originally designed to study Jupiter's deep atmosphere, the MWR has also been used during Juno's extended mission to observe the planet's moons Ganymede, Europa and Io.
During the flybys, Juno passed within about 1,500 kilometres of Io's surface.
Shannon Brown, the study's lead author at NASA's Jet Propulsion Laboratory, said the instrument measured thermal emissions from depths ranging from a few inches to several feet. The team found temperatures increasing by more than 40 degrees Fahrenheit just a few feet below the surface, a rise that could not be explained by sunlight alone.
Researchers said the heat may be moving steadily through Io's crust or originate from cooling lava beneath a hardened crust.
Bolton said the findings provide a new way to study how heat travels from Io's interior to its surface, improving scientists' understanding of tidal heating in the solar system.