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NASA’s Juno mission has successfully measured the subsurface temperature of Io, Jupiter’s volcanic moon, for the first time. This groundbreaking research reveals significant heating in the moon’s shallow subsurface, enhancing our understanding of the most volcanically active body in our solar system. The data collected during two flybys also indicate that a substantial portion of Io’s surface is unexpectedly smooth and composed of low-density materials.
These findings, published in the Journal of Geophysical Research: Planets, promise to expand our exploration of both fiery worlds and icy landscapes beyond Earth.
Io’s extreme volcanic activity stems from tidal heating. As the moon orbits in a slightly elliptical path, it experiences constant stretching and contraction due to Jupiter’s immense gravitational force, generating heat far exceeding that of Earth. Until now, most knowledge of Io’s heat was derived from infrared observations, which only gauge surface temperatures. The latest discoveries result from data analyzed from Juno’s Microwave Radiometer (MWR) instrument.
“The Juno microwave radiometer has directly observed Io’s heat output by looking beneath the surface,” stated study co-author Scott Bolton, principal investigator for Juno at the Southwest Research Institute in San Antonio. “Discovering that we could see beneath the rocky Moon’s surface has important implications for studying Earth’s volcanoes, as Juno shows similar signatures in subsurface temperature gradients observable with MWR-type instruments near terrestrial volcanoes.”
Bolton designed Juno’s microwave radiometer to investigate Jupiter’s deep atmosphere. MWR’s six microwave antennas work together to detect signals across a wide range of wavelengths, from approximately 0.5 inches to 20 inches (1.3 to 51 centimeters). During its extension phase, the MWR instrument offered the chance to study three of Jupiter’s Galilean moons: Ganymede, Europa, and Io.
“This innovative technique allows different wavelengths to probe varying depths, providing a unique perspective on the dynamics of giant planets’ deep atmospheres and the subsurface crusts of icy and rocky moons,” Bolton explained. “While we studied Ganymede and Europa’s ice shells, examining Io’s volcanic terrain was an unexpected and fascinating discovery.”
During close flybys on December 30, 2023, and February 3, 2024, the solar-powered Juno spacecraft came within approximately 930 miles (1,500 kilometers) of Io’s surface.
“The MWR instrument measured Io’s heat release from shallow depths to tens of feet below the surface. Everywhere we looked, temperatures rose by over 40 degrees Fahrenheit just beneath the surface, indicating a much steeper temperature gradient than solar heating could explain,” said lead author Shannon Brown from NASA’s Jet Propulsion Laboratory in Southern California.
The data presents two potential explanations for this phenomenon. First, heat may flow steadily through the conductive crust. While this background heat flow is modest (around 1 to 3 watts per square meter), Io releases up to 30 times more energy than the global average. Alternatively, the signal could be linked to cooling lava flows beneath a solidified crust that spans approximately 9 to 11 meters and covers about 10% of the moon’s surface at any given moment.
“Io serves as a unique window into the mechanics of tidal heating, a fundamental process that provides energy and heat to distant worlds,” Bolton remarked. “In Io’s case, this process not only creates the largest volcanic structures in the solar system but may also fuel the underground oceans of giant moons like Europa and Ganymede. Previously, we could only observe surface heat or eruptions; now we can study how heat ascends from the interior to the surface.”
Another notable insight from the flybys is Io’s remarkable smoothness. Historically recognized for its towering mountains, MWR shows that beyond this terrain, there are vast smooth expanses covering over 60 miles (100 kilometers). By flying over overlapping regions from various angles, researchers mapped how the surface reflects microwaves, akin to how passengers see sunlight glinting off ocean waves only from specific perspectives.
“Besides the mountains, the surface resembles the Great Plains of North America. Although Io is rocky, its surface material is very low density, comparable to pumice or light volcanic ash rather than solid rock,” Brown noted.
Managed by JPL, a division of Caltech in Pasadena, California, the Juno mission is a part of NASA’s New Frontiers program and is overseen by the Marshall Space Flight Center in Huntsville, Alabama. Lockheed Martin Space in Denver builds and operates the spacecraft. For more information about Juno, visit:
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Source: www.nasa.gov