Juno’s Microwave Measurements Map Hidden Heat Sources on Io

A volcanic moon in focus

Io, the innermost of the four Galilean moons, is considered the most volcanically active body in the solar system. Vast lava flows and plumes reaching up to a hundred kilometers shape its surface. This persistent activity is driven mainly by tidal forces: Jupiter’s strong gravity and orbital resonances with Europa and Ganymede periodically flex Io, generating internal friction and thus heat.

Why microwaves provide new insights

Previous observations in the visible and infrared mainly detect fresh lava and surface temperatures. They are, however, susceptible to interference from dust, ash or plumes and respond strongly to the properties of the very topmost surface layers. The Microwave Radiometer (MWR) aboard Juno complements that view: microwaves penetrate into the material and deliver information about heat flows beneath the surface—even where the line of sight is obscured.

How the measurement campaign was conducted

During several close flybys (perijoves), Juno pointed the MWR at Io. At Perijove 57 on 30 December 2023 the focus was on the moon’s northern hemisphere; a subsequent pass shortly after extended the coverage. The instrument’s measurement tracks overlap densely, allowing a more precise spatial attribution of the signals.

How the instrument works

The MWR operates at multiple frequencies. As wavelength increases, penetration depth into the subsurface also increases. This makes it possible not only to infer surface temperatures but also to estimate temperature gradients in deeper layers. On Io this approach helps distinguish thermal anomalies of active lava fields from heat released from greater depths.

First results

The processed data show a mosaic of concentrated hotspots alongside extensive, more weakly warmed regions. Repeated flybys separate short-lived eruptions from longer-term anomalies. Notably, areas that appear inconspicuous in optical images show elevated signals in the microwave maps—an indication of subsurface heat sources or recently solidified, still-warm lava fields. These findings refine the overall picture of Io’s heat output and provide clues about the distribution of geothermal sources within the crust.

Context for planetary science and Earth observation

Io is an example of a body whose internal activity is powered by external gravitational interactions. A better understanding of the spatial and temporal distribution of heat refines models for other moons such as Europa or Enceladus. Methodologically, the study demonstrates transferability across remote sensing domains: microwave measurements are established in Earth system science—for example, to determine soil moisture, sea ice thickness or near-surface temperature profiles. The principle of revealing hidden heat by using differing penetration depths links planetary research and climate observation.

Outlook

The current MWR data are a first building block. Further flybys and the combination with infrared, visible and particle measurements are expected to produce a more complete thermal inventory. A multi-parameter approach will not only map active volcanoes but also better estimate Io’s total heat budget—central to understanding its internal dynamics.