NASA’s Juno spacecraft has produced the first measurements of subsurface heat flow on Jupiter’s moon Io, the most volcanically active body in the solar system. The findings, published in the Journal of Geophysical Research: Planets, give researchers a new, more direct way to quantify how much internal heat Io is releasing through its crust rather than through visible eruptions alone.
Using Juno’s microwave radiometer, which can sense radiation emitted from beneath a surface rather than just off it, the mission team measured a background heat flow of roughly 1 to 3 watts per square meter across Io’s surface between eruptions and lava flows. According to NASA’s Jet Propulsion Laboratory (JPL), that works out to a release of energy up to about 30 times Earth’s average heat flow — a striking figure given that Io is a rocky body roughly the size of Earth’s Moon, not a gas giant. The result offers scientists a subsurface complement to the visible hot spots and lava lakes that ground- and space-based telescopes have tracked for decades, and a way to constrain models of how tidal heating from Jupiter’s gravity actually moves heat through Io’s interior.
Why a subsurface measurement matters
Most of what’s known about Io’s volcanism comes from surface observations: bright eruptions, lava flows, and plumes captured by cameras and infrared instruments during Juno’s close passes. Those observations are dramatic but incomplete — they capture heat that has already reached the surface and radiated or erupted away. A background, between-eruptions heat-flow measurement is a different kind of data point: it approximates the steady conductive heat moving up through Io’s crust, independent of any single eruption. That distinction matters for testing competing models of how tidal flexing heats Io’s interior, including whether the heating is concentrated in a global magma ocean beneath the crust or distributed more unevenly.
A competitively selected, PI-led mission
Juno is a good illustration of a funding and governance model that’s easy to conflate with NASA’s more familiar center-directed missions but is structurally different. Juno was selected through NASA’s New Frontiers Program, a competed line of medium-class planetary science missions in which proposal teams — led by a named Principal Investigator (PI) and their home institution — compete for selection, rather than a mission being assigned top-down to a NASA center. Juno’s PI is Scott Bolton of the Southwest Research Institute (SwRI) in San Antonio, Texas; the spacecraft itself was designed and built by Lockheed Martin Space in Denver, while JPL, a division of Caltech, manages the mission for NASA as part of the New Frontiers Program portfolio.
For CASRAI’s audience, this distinction is a useful real-world case of two funding mechanisms that coexist within the same funder: PI-led, competitively awarded mission lines (New Frontiers, and NASA’s smaller Discovery Program) sit alongside directorate- or center-directed flagship missions, where NASA itself defines the mission concept and assigns implementation to a center such as JPL. The two models carry different accountability structures — a competed PI-led mission is answerable to the proposing institution’s named lead investigator and to the peer-review panel that selected it, while a directed mission’s accountability runs primarily through NASA’s own program management. Research administrators tracking funder mechanisms across science agencies will recognize the pattern: it closely parallels the difference between an investigator-initiated grant and an agency-directed contract or cooperative agreement.
Open image processing
As with most Juno results, the imagery supporting this work draws on data from JunoCam, the mission’s visible-light camera, whose raw frames NASA releases publicly for anyone to process. The instrument’s images are routinely turned into finished views by members of the public as well as by the mission team, a practice NASA has maintained since Juno’s arrival at Jupiter in 2016 as a deliberate open-participation element of the mission.
Source
NASA/JPL: NASA’s Juno Takes the Temperature of Jupiter’s Fiery Moon Io (published July 22, 2026).
