NASA Juno Spacecraft Detects Subsurface Heat on Io

Juno Captures First Subsurface Temperature Measurements on Io

Previously, scientists relied almost entirely on infrared observations that could only sense the temperature of Io’s top surface. During close flybys on December 30, 2023, and February 3, 2024, the solar-powered Juno spacecraft came within about 930 miles (1,500 kilometers) of the moon’s surface and utilized its Microwave Radiometer (MWR) instrument to probe roughly six to 20 feet (two to six meters) underground.

Understanding the Mechanics of Tidal Heating

Io’s extreme volcanism is powered by tidal heating resulting from a gravitational tug-of-war between Jupiter and neighboring moons Ganymede and Europa. As Io travels along its slightly elliptical orbit, its distance from Jupiter varies by about 3,500 kilometers, causing extreme tidal deformation that forces the moon’s surface to bulge up and down by as much as 330 feet (100 meters) at a time, according to Space. This torment generates an internal heat output far greater than that of Earth.

Data collected by the MWR instrument—which was originally designed by IFLScience to peer through Jupiter’s thick cloud tops—revealed that everywhere researchers looked, temperatures rose by more than 40 degrees Fahrenheit just several feet into the surface. According to Shannon Brown, the paper’s lead author at NASA’s Jet Propulsion Laboratory in Southern California, this temperature gradient is far steeper than solar heating alone can explain.

Two Leading Explanations for Subsurface Heat

Scientists analyzing the data have interpreted the internal heat through two possible explanations:

* Conductive Crust: Heat could be rising steadily through a conductive crust. Measured at 1 to 3 watts per square meter, this gentle background heat flow represents a release of energy up to 30 times Earth’s average across the entire moon. * Cooling Lava Flows: The signal could alternatively originate from cooling lava flows capped by roughly 30 to 35 feet (9 to 11 meters) of solidified crust, which cover about 10% of the moon’s surface at any given time.

Additionally, Juno’s subsurface heat map revealed localized regions of elevated heat measuring between 18 and 36 degrees Fahrenheit warmer than surrounding terrain, while data also showed that most of Io’s surface is remarkably smooth and composed of material of very low density.

Implications for Earth and Icy Worlds

Researchers stated that the ability to probe into Io’s volcanic rock was an unexpected discovery, contrasting with previous missions where instruments explored Ganymede and Europa assuming their ice shells were mostly pure water.

Scientists believe that understanding how heat moves through these planetary crusts provides a unique window into tidal heating, a fundamental process that fuels subsurface oceans on icy moons and drives ancient volcanism on worlds like Mars, Venus, and Earth’s moon. Furthermore, researchers noted that utilizing an MWR-type instrument near a volcano on Earth could reveal a similar signature in subsurface temperature gradients, potentially helping scientists better monitor terrestrial volcanoes and improve eruption forecasting.