NASA’s Juno mission scientists have reported the first successful temperature measurements below the surface of Jupiter’s moon Io, including indications of significant heating within the moon’s shallow subsurface.
Collected during a pair of close flybys of the Solar System’s most volcanically active world, the Juno mission measurements were described as a complete “surprise” since the instrument was designed for a different purpose.
“[The readings] break new observational ground for both fiery and icy worlds beyond our planet,” the team said.
MWR Instrument Designed to Probe Jupiter’s Atmosphere Provides Double Duty
Due to the extreme gravitational pull of its host planet, Jupiter, Io is repeatedly stretched and pulled as its slightly elliptical orbit carries it around the massive gas giant. This repeated motion generates a tremendous amount of heat, which the scientists note is “many times greater” than the same type of heat generated on Earth.
However, a detailed understanding of this process has remained elusive, as the only data on the moon’s inner heat came from infrared telescope observations. The team notes this data is limited to the temperature of Io’s upper rocky surface layer.
Conversely, Juno is equipped with an instrument called the Microwave Radiometer (MWR). According to the team’s statement announcing the successful subsurface temperature detection, Juno’s Microwave Radiometer was custom designed by Scott Bolton, the mission’s principal investigator at Southwest Research Institute in San Antonio and a co-author of the study detailing the team’s findings, to “peer beneath Jupiter’s top clouds” for a closer look at the gas giant’s complex atmosphere.
“The MWR’s six microwave antennas serve as a single instrument, simultaneously detecting microwaves at a wide range of wavelengths, from about half an inch to 20 inches (1.3 to 51 centimeters),” the team explains.
“Everywhere We Looked, We Found the Temperature Rising”
Bolton said the team’s novel approach allowed the instrument to observe wavelengths from different depths, “providing a new way to characterize the deep atmosphere of giant planets and the subsurface crusts of icy and rocky moons.” This included observations of Jupiter’s other ‘Galilean’ moons, Ganymede and Europa.
“At Ganymede and Europa, we explored tens of miles below the surface, assuming their ice shells were mostly pure water,” the mission scientists explained.
However, Bolton added, using the instrument to successfully probe into Io’s volcanic rock “was an unexpected discovery.”
The feat was accomplished during a pair of flybys on December 30, 2023, and February 3, 2024, where Juno came within roughly 930 miles (1,500 km) of Io’s surface. Shannon Brown, the paper’s lead author at NASA’s Jet Propulsion Laboratory in Southern California, said that during these flybys the MWR instrument measured the moon’s thermal emission “at depths ranging from a few inches down to tens of feet.”
“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,” Brown said.
Although more information is likely needed to make a conclusive determination, the team said that the data suggests two possible explanations for the readings. The first option is heat rising steadily through what the team termed a “conductive crust.”
“While this background heat flow — measured at 1 to 3 watts per square meter — is relatively gentle on a local scale (roughly equivalent to a small nightlight glowing under every square yard), across the entire moon it represents a release of energy up to 30 times Earth’s average,” they explained.
Another possibility proposed by the joint research team suggested that the heat signal detected by Juno’s MWR could be coming from cooling lava flows covered by 30 to 35 feet (9 to 11 meters) of solidified crust that cover approximately 10% of Io’s surface “at any given time.”
A Similar Instrument Could be Used to Scan Earth
When discussing the implications of the team’s findings, Bolton said the “surprising” ability to see below Io’s surface “has important implications for studying Earth’s volcanoes.” He also noted that if scientists used a similar instrument to measure areas on Earth near volcanoes, they might see a similar heat signature in the planet’s subsurface temperature gradient, “providing new information on how terrestrial volcanoes work.”
“Io provides a unique window into learning how tidal heating works throughout the cosmos, a fundamental process that provides energy and heat to worlds that are far from their parent star,” Bolton explained. “This process can not only create the most volcanic body in the solar system, in the case of Io, but also fuels the subsurface oceans on the moons of giant planets, such as Europa and Ganymede.”
“Up until this point, we could only observe the heat escaping at the surface or through eruptions,” the mission scientists added. “Now we can characterize how the heat is moving from the interior toward the surface.”
NASA has additional information about Juno available at the mission’s official webpage.
Christopher Plain is a Science Fiction and Fantasy novelist and has spent the last six years as Associate News Editor and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him at christopher@thedebrief.org.