Mapping Io’s Hidden Heat With NASA’s Juno

Mapping Io’s Hidden Heat With NASA’s Juno

Jupiter’s moon Io is the most volcanically active body in the solar system, with hundreds of erupting volcanoes that paint its surface in vivid reds, yellows, and whites. But much of its thermal activity lies hidden beneath the crust, radiating heat that can only be detected by instruments sensitive to microwave wavelengths. NASA’s Juno spacecraft, equipped with the Microwave Radiometer (MWR), has now delivered the most detailed maps yet of that subsurface heat, revealing where Io’s internal fires burn hottest.

How Juno’s MWR Peers Beneath Io’s Surface

Unlike visible-light cameras that capture surface eruptions, the MWR measures thermal microwave emissions that penetrate through Io’s thin, patchy atmosphere and even through the uppermost layers of the moon’s crust. This allows scientists to see heat sources that are not directly exposed to space. The instrument works by detecting the natural microwave radiation emitted by warm material; the intensity and wavelength reveal the temperature and depth of the heat source.

During two close flybys—Perijove 57 on December 30, 2023, and Perijove 58 on February 3, 2024—Juno swooped within about 930 miles (1,500 kilometers) of Io’s surface. The spacecraft spins at two revolutions per minute, creating sweeping, overlapping patterns of MWR footprints across the moon’s disk.

Perijove 57: Northern Hemisphere Focus

The first flyby (black overlapping lines in the graphic) primarily mapped Io’s northern hemisphere. The MWR captured thermal data from high latitudes down to the mid-latitudes, revealing several hot spots that correspond to known volcanic centers such as Loki Patera and Pele. The data also indicated areas of diffuse heat that may be linked to large-scale magma reservoirs.

Perijove 58: Mid-Latitudes and Equator

The second pass (blue lines) concentrated on the mid-latitudes and equatorial regions. This flyby filled in gaps left by the first, providing a more complete picture of the hemisphere that always faces Jupiter—the sub-Jovian hemisphere. The equatorial scans showed intense thermal anomalies near the equator, likely associated with active lava flows and possibly shallow magma chambers.

Why Mapping Io’s Heat Matters

Io’s volcanic activity is driven by tidal heating: Jupiter’s immense gravity, combined with the gravitational tugs of Europa and Ganymede, flexes Io’s interior, generating enormous friction that melts rock. Understanding where and how this heat escapes to the surface helps scientists model the moon’s internal structure, composition, and thermal evolution.

• Volcanic hazard assessment: Future missions to the Jupiter system may need to avoid or study active volcanic plumes.
• Planetary geology: Io serves as a natural laboratory for extreme volcanism, offering insights into early Earth and other rocky worlds.
• Tidal heating models: Precise heat maps constrain how tidal forces deform Io’s interior.

The Juno Mission and MWR Instrument

Juno, launched in 2011 and orbiting Jupiter since 2016, was originally designed to study the gas giant’s atmosphere, magnetosphere, and interior. Its extended mission includes flybys of the Galilean moons. The MWR, built by NASA’s Jet Propulsion Laboratory (JPL), operates at six wavelengths ranging from 1.3 cm to 50 cm, allowing it to probe different depths.

Scott Bolton of the Southwest Research Institute in San Antonio is the principal investigator. The mission is managed by JPL, a division of Caltech in Pasadena, California. Lockheed Martin Space in Denver built and operates the spacecraft. Juno is part of NASA’s New Frontiers Program, managed at NASA’s Marshall Space Flight Center in Huntsville, Alabama.

What’s Next for Io Studies

The MWR data are still being analyzed, but early results confirm that Io’s heat flow is far from uniform. Some regions radiate more heat than can be explained by surface volcanism alone, suggesting extensive subsurface magma networks. Future flybys—if approved—could target the anti-Jovian hemisphere or observe temporal changes in volcanic activity.

Combined with visible-light imaging from Juno’s JunoCam and infrared data from other instruments, the MWR maps will help build a three-dimensional picture of Io’s thermal structure. This could answer long-standing questions about whether Io has a global magma ocean or if its volcanism is driven by isolated mantle plumes.

Explore More

For more details on Juno’s Io flybys and MWR data, visit:

• NASA’s Juno mission page
• Southwest Research Institute Juno site

Keep checking the Photojournal for updated images and scientific findings as the Juno team continues to unlock the secrets of Io’s hidden heat.