NASA's Juno spacecraft has measured subsurface temperatures on Io, Jupiter's most volcanically active moon, for the first time. The data come from two close flybys and reveal intense heat just beneath Io's surface, alongside evidence that most of the moon's terrain is unusually smooth and composed of low-density material.
Juno carries an infrared radiometer that detects thermal emissions from celestial bodies. During the flybys, the instrument penetrated beneath Io's surface to gauge heat trapped in shallow subsurface layers. The results confirm that Io maintains enormous heat in its upper crust, consistent with the moon's status as the solar system's most volcanically active body.
Io orbits Jupiter within the intense gravitational environment of the Jovian system. The constant tidal stress from Jupiter's gravity flexes Io's interior repeatedly, generating frictional heat that powers the moon's extensive volcanic activity. This heating mechanism, called tidal heating, makes Io fundamentally different from Earth and other rocky bodies where internal heat derives primarily from radioactive decay.
The temperature measurements provide quantitative confirmation of this process. The subsurface heat detected by Juno exceeds what scientists would expect from a geologically dead world, validating decades of observations from telescopes and previous spacecraft showing Io's active volcanism.
The data also reveal that Io's surface composition differs markedly from initial expectations. The smooth terrain and low-density material suggest widespread deposits of sulfur and sulfur dioxide frost, which cover much of the moon's landscape. This composition explains Io's distinctive coloration and contributes to the moon's harsh, alien appearance.
Understanding Io's thermal structure advances knowledge of how tidal heating operates in icy and rocky bodies throughout the solar system. Similar processes may generate internal heat on other moons like Europa and Enceladus, where subs