NASA’s Juno Mission Detects Subsurface Heating Beneath Io’s Crust

For the first time, scientists have peered beneath the crust of the most volcanically active world in the solar system. Using the Microwave Radiometer (MWR) instrument, the Juno spacecraft detected significant heating within the shallow subsurface of Io, moving beyond previous infrared observations that could only sense the top surface temperature. The findings were published Wednesday in the Journal of Geophysical Research: Planets.

MWR Data and the Subsurface Temperature Gradient

The MWR instrument, which was built by JPL, uses six microwave antennas to detect wavelengths ranging from about half an inch to 20 inches (1.3 to 51 centimeters). Because different wavelengths explore different depths, the tool allowed the team to probe thermal emissions from a few inches down to tens of feet. Specifically, the lowest frequency microwave channels (0.6 and 1.25 gigahertz) on the MWR can penetrate between about 6 and 20 feet (2 and 6 meters) into the crust.

Everywhere we looked, we found the temperature rising by more than 40 degrees Fahrenheit just several feet into the surface — a gradient far steeper than solar heating alone can explain. — Shannon Brown, lead author at NASA’s Jet Propulsion Laboratory

This thermal data reveals a distinct gradient across the moon. The coolest subsurface temperatures are found near the northern pole, dropping to around -298°F (-183°C, or 90 Kelvin). Intermediate yellow regions warm up to near -190°F (-123°C, or 150 Kelvin) as they approach the equator.

Zal Montes Patera and Regional Heat Anomalies

The mission identified specific “hot spots” where internal heating is most intense. The most prominent red anomaly, located between 60 and 120 degrees west longitude, shows subsurface temperatures 18 to 36 degrees Fahrenheit (10 to 20 degrees Celsius, or 10 to 20 Kelvin) warmer than the surrounding areas. This heat source coincides with the Zal Montes Patera complex, where Juno’s Stellar Reference Unit observed an active lava flow.

A second major heat source was detected near the equator, stretching from 0 to 50 degrees west longitude. Together, these distinct microwave anomalies indicate that significant internal heating is occurring within the upper tens of meters of the crust.

Tidal Heating and Two Possible Explanations

Io’s extreme volcanism is driven by tidal heating, as Jupiter’s gravity constantly stretches and squeezes the moon during its elliptical orbit. This process generates internal heat output many times greater than that of Earth.

Conductive Crust: Heat may rise steadily through the crust. This background flow is measured at 1 to 3 watts per square meter. While locally gentle—comparable to a small nightlight under every square yard—it represents a total energy release up to 30 times Earth’s average.

Cooling Lava: The signal could originate from cooling lava flows capped by 30 to 35 feet of solidified crust, which cover about 10% of the moon’s surface at any given time.

Implications for Earth’s Volcanoes and Icy Moons

The ability to probe volcanic rock was an unexpected discovery. During the mission’s extended phase, the MWR provided the opportunity to observe three of the planet’s Galilean moons: Ganymede, Europa, and Io. Previously, the MWR explored tens of miles beneath the surfaces of Ganymede and Europa, assuming those ice shells were mostly pure water.

“The surprising discovery that we could see below a rocky moon’s surface has important implications for studying Earth’s volcanoes. Juno has taught us that if we look with an MWR-type instrument near a volcano on Earth, we might see a similar signature in the subsurface temperature gradient, providing new information on how terrestrial volcanoes work.” — Scott Bolton, Juno’s principal investigator at Southwest Research Institute in San Antonio and study coauthor

Beyond the thermal data, the flybys revealed that Io is remarkably smooth. While the moon is known for tall mountains, the MWR indicates expansive smooth patches of low-density material that stretch for 60 miles or more.

Understanding this heat movement is critical for broader planetary science. According to Bolton, tidal heating not only creates the most volcanic body in the solar system but also fuels the subsurface oceans on the moons of giant planets, such as Europa and Ganymede.

The solar-powered Juno spacecraft came within about 930 miles (1,500 kilometers) of the moon’s surface during its flybys. The Dec. 30, 2023, pass marked Juno’s 57th close pass of Jupiter and captured the north polar region of the moon. The Juno mission is managed by NASA’s Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, for principal investigator Scott Bolton of the Southwest Research Institute. Juno is part of NASA’s New Frontiers Program, managed at NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the agency’s Science Mission Directorate in Washington. The spacecraft was built and is operated by Lockheed Martin Space in Denver.