Following two flybys of Jupiter’s innermost Galilean moon Io, groups utilizing knowledge from NASA’s Juno spacecraft have revealed the primary measurements of the temperature under Io’s floor. The flybys revealed substantial subsurface heating on the moon, in addition to a floor that’s extraordinarily clean and composed of low-density supplies.
Io is essentially the most volcanically lively celestial physique within the photo voltaic system, a attribute brought on by the immense tidal forces from its mother or father planet. As Io orbits Jupiter, the planet’s excessive gravity stretches and squeezes Io’s floor, producing an inner warmth output considerably larger than that of Earth’s. Nevertheless, till these new measurements, all the pieces about Io’s thermal nature was realized by means of infrared observations of its floor.
To make the subsurface temperature measurements, Juno used its Microwave Radiometer (MWR) instrument. Designed to make observations deep into Jupiter’s thick ambiance, the MWR is multi-wavelength, supporting radiation observations from 1.3 cm to 51 cm wavelengths (600 MHz to 22 GHz frequencies). MWR achieves these observations through six otherwise sized antennas mounted to the edges of Juno. The MWR has additionally performed observations of Ganymede and Europa.
MWR knowledge map exhibiting the place warmth is rising from Io’s subsurface atmosphere. The colours signify a temperature gradient throughout Io. (Credit score: NASA/JPL-Caltech/SwRI/USGS)
“The Juno Microwave Radiometer instantly noticed Io’s warmth output by wanting under the floor. The shocking discovery that we may see under a rocky moon’s floor has necessary implications for finding out Earth’s volcanoes. Juno has taught us that if we glance with an MWR-type instrument close to a volcano on Earth, we’d see an identical signature within the subsurface temperature gradient, offering new info on how terrestrial volcanoes work,” stated co-author and Juno principal investigator Scott Bolton of the Southwest Analysis Institute.
The staff, led by Shannon Brown of NASA’s Jet Propulsion Laboratory, used Juno knowledge from two flybys of Io: the primary occurred on Dec. 30, 2023, and the second on Feb. 3, 2024. Throughout each of those flybys, Juno got here inside roughly 1,500 km of Io’s floor. MWR knowledge revealed that temperatures dramatically improve simply toes into the moon’s floor.
“The instrument measured Io’s thermal emission at depths starting from a number of inches all the way down to tens of toes. In every single place we regarded, we discovered the temperature rising by greater than 40 levels Fahrenheit simply a number of toes into the floor — a gradient far steeper than photo voltaic heating alone can clarify,” Brown stated.
Brown et al. have two theories for the sharp improve in temperatures slightly below the floor. The primary principle means that Io has a conductive crust, permitting warmth to steadily rise to the floor. The measured background warmth movement throughout the crust — roughly one to a few watts per sq. meter — equates to an vitality launch nearly 30 occasions Earth’s common throughout all the moon, although localized measurements are far much less energetic.
Io imaged by Juno throughout its Dec. 30, 2023, flyby. (Credit score: NASA/JPL-Caltech/SwRI/MSSS/Gerald Eichstädt)
The second principle means that cooling lava flows might be heating Io’s subsurface atmosphere. These lava flows, trapped below roughly 9 to 11 meters of stable crust, are estimated to cowl round 10% of Io’s floor at any time.
“Io gives a novel window into studying how tidal heating works all through the cosmos, a elementary course of that gives vitality and warmth to worlds which might be removed from their mother or father star. This course of cannot solely create essentially the most volcanic physique within the photo voltaic system, within the case of Io, but additionally fuels the subsurface oceans on the moons of big planets, comparable to Europa and Ganymede. Up till this level, we may solely observe the warmth escaping on the floor or by means of eruptions. Now we are able to characterize how the warmth is shifting from the inside towards the floor,” Bolton stated.
Io’s subsurface temperatures weren’t the one factor revealed from Juno’s flybys in 2023 and 2024.
MWR knowledge additionally confirmed that the moon’s floor is remarkably clean. Whereas Io options many tall mountains, beneath them lay patches of clean floor that stretch for 100 km or additional. Brown et al. decided the smoothness of Io’s floor by mapping the way it displays microwaves, a measurement made potential by Juno flying over overlapping areas at totally different angles.
Map highlighting the place Juno’s MWR instrument measured Io’s floor. (Credit score: NASA/JPL-Caltech/SwRI/USGS)
“Away from its mountains, the floor is extra just like the Nice Plains of North America, and despite the fact that Io is a rocky physique, the floor materials has a really low density — extra like pumice or a fluffy volcanic ash than stable rock,” Brown stated.
Juno, launched in August 2011, just lately celebrated a decade at Jupiter, inserting itself into orbit across the planet on July 5, 2016. Launched as a part of NASA’s New Frontiers program, Juno accomplished its most up-to-date cross of Jupiter, referred to as “perijoves,” on July 5, 2026 — the eighty fifth of its mission.
(Lead picture: Io imaged by Juno in October 2023. Credit score: NASA/JPL-Caltech/SwRI/MSSS/Ted Stryk)
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