Juno Data Suggests Io Is Far Hotter Than Believed — Heat Concentrated in Few Volcanoes

Io may be emitting hundreds of times more heat than earlier estimates

New analyses of data from NASA's Juno spacecraft reveal that Jupiter's moon Io — the most volcanically active body in the solar system — may be releasing heat at rates hundreds of times higher than previous calculations suggested. The difference stems not from a lack of observations but from how Juno's infrared measurements were interpreted.

Why earlier estimates were low

Much of the earlier work used observations from a narrow infrared range known as the M-band, collected by Juno's Jovian InfraRed Auroral Mapper (JIRAM). While M-band data are excellent at pinpointing the very hottest, most incandescent spots on Io, they are relatively insensitive to broader, cooler surfaces that nevertheless radiate significant thermal energy.

"The M-band is sensitive only to the highest temperatures, and therefore tends to favor the most incandescent areas of volcanoes, neglecting the colder but much more extensive ones," said team leader Federico Tosi of the National Institute for Astrophysics (INAF). "It's like estimating the brightness of a bonfire by observing only the flames and not the surrounding embers."

What the reanalysis found

By reprocessing JIRAM data and combining it with additional Juno observations and more detailed thermal models, the team found that roughly half of Io's total heat output is concentrated in just 17 of the moon's 266 catalogued volcanic sources. Many lava lakes on Io appear to have bright, hot peripheral rings surrounding a cooler, solidified central crust. That cooler crust is faint in the M-band but can cover large areas and contribute substantially to the total heat budget.

"When this 'hidden' component is also considered, the actual heat flux is up to hundreds of times higher than that calculated by analyzing the M-band alone," Tosi said. "This changes the scale of the satellite's energy balance."

Implications for a global magma ocean

The finding that heat is highly concentrated weakens one line of evidence previously used to argue for a global magma ocean or continuous subsurface lava layer beneath Io's crust. However, the authors are careful to note that their results do not definitively rule such an ocean in or out — only that M-band JIRAM measurements alone cannot be used to confirm it.

"We're not saying that such an ocean doesn't exist, but that it can't be deduced from these observations," Tosi said. "It's important to recognize the limitations of available data before drawing strong conclusions on such a complex issue."

What comes next

Juno obtained its closest, highest-resolution views of Io in 2023 and 2024, but its changing orbit means similar close passes are unlikely in the near term. Upcoming missions like ESA's JUICE and NASA's Europa Clipper will focus mainly on Ganymede and Europa and will not observe Io with comparable spatial detail. The researchers say their revised interpretation provides a framework to extract more accurate heat estimates from remote or lower-resolution data and could inform the design of future missions dedicated to Io.

Publication: The research was published Nov. 5 in Frontiers in Astronomy and Space Sciences.