The NASA spacecraft Juno has detected increased emissions on Io, the most volcanically active body in the Solar System, using its microwave radiometer (MWR). Unlike infrared instruments, which mainly sense the surface, the longer wavelengths of the MWR channels penetrate the subsurface and reveal temperature distributions below the surface.
How the deep view works
Microwaves interact with rock differently than visible light or infrared radiation. Penetration depth depends on wavelength, composition, porosity and moisture of the material. Juno's lowest frequency channels, around 0.6 and 1.25 gigahertz, can—depending on the medium—reach depths of several to a few dozen meters. That allows the detection of thermal signatures that would remain hidden to pure infrared observations.
What the patterns suggest
The data show locally elevated emissions originating from shallow depths. This points to heat transport in near-surface layers—not only hot surface crusts from recent lava flows. Possible causes include shallow magma chambers or very hot subsurface structures that transfer heat to overlying layers, as well as recently solidified lava plates that are still cooling. In this way the MWR complements known infrared hotspots such as lava lakes, flow channels and vents by providing clues about the immediate sources beneath them.
Contextualizing the results
Io's volcanism is driven by intense tidal forces from Jupiter and the other Galilean moons. The new measurements refine our understanding of these processes by making the distribution of heat in near-surface regions visible. This affects interpretations of eruption frequency, eruption intensity and lava flow paths. Such data also help test models of tidal heating—insights that can be applied to other moons and to exoplanets experiencing strong tidal forces.
Technical note
The MWR was originally designed to probe deep atmospheric layers of Jupiter. Its successful use on a largely airless, solid body demonstrates the instrument's flexibility and opens additional possibilities for future observations of other moons or asteroids.
What comes next
Microwave observations provide starting points for coordinated follow-up campaigns with infrared and visible cameras as well as radio telescopes to track the temporal evolution of hotspots. Laboratory measurements of the dielectric properties of Io-analog materials can refine interpretations of penetration depths and temperature profiles. For future missions, targeted flybys or landing sites with active, shallow heat reservoirs are particularly promising—for example for sampling, heat-flow measurements and seismic investigations.
NASA Earth Observatory / Earth Science
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