A NASA probe peered beneath the surface of the most volcanic world in the solar system, revealing scorching heat rising from below its crust.
During two recent close flybys of Jupiter’s moon Io, NASA’s Juno mission measured the temperature below its surface for the first time. The data showed significant heating within the shallow subsurface of the moon that may be rising steadily through a conductive crust.
The findings were published in the Journal of Geophysical Research: Planets, and could help provide new insight for both fiery and icy worlds of the solar system.
Jupiter’s tortured moon
Io is covered in hundreds of volcanoes, which spew fountains of lava that constantly refill impact craters on the surface with scorching molten lakes.
Io’s volcanic activity is the result of a gravitational tug-of-war between Jupiter’s strong pull on the moon and precisely timed gravitational pulls from neighboring moons: Ganymede and Europa. The tortured moon is the subject of extreme tidal forces, causing its surface to bulge up and down by as much as 330 feet (100 meters) at a time, according to NASA.
The moon’s torment generates an internal heat output that’s much greater than that of Earth’s. Until now, scientists have only known about Io’s heat through infrared observations that sense the temperatures of the top surface.
Using Juno’s Microwave Radiometer (MWR) instrument, scientists were able to collect data from beneath the surface. The instrument is designed to investigate the dynamics and composition of Jupiter’s deep atmosphere using its ability to peer through the gas giant’s cloud tops.
“The Juno Microwave Radiometer directly observed Io’s heat output by looking below the surface,” Scott Bolton, Juno’s principal investigator at Southwest Research Institute in San Antonio and co-author of the study, said in a statement. “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.”
Heat from below
On December 30, 2023, and February 3, 2024, the Juno spacecraft came within about 930 miles (1,500 kilometers) of Io’s surface.
“The instrument measured Io’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,” Shannon Brown, the paper’s lead author at NASA’s Jet Propulsion Laboratory in Southern California, said in a statement.
The scientists behind the study interpret Io’s internal heat through two possible explanations. Heat could be rising steadily through a conductive crust. This type of background heat flow may be relatively gentle on a local scale, similar to a small nightlight glowing under every square yard, according to NASA. When it occurs across the entire moon, it represents a release of energy up to 30 times Earth’s average.
The heat signal could also be coming from cooling lava flows, which are capped by roughly 30 to 35 feet (9 to 11 meters) of solidified crust that cover about 10% of the moon’s surface at any given time.
“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 said. “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. Now we can characterize how the heat is moving from the interior toward the surface,” Bolton added.
