For more than 30 years, planetary scientists have pointed infrared instruments at Jupiter’s moon Io and tallied its volcanic heat output. The numbers were impressive on their own: Io is the most volcanically active body in the solar system, with hundreds of erupting vents and vast lava lakes dotting a sulfur-stained surface. But a peer-reviewed study published in May 2025 in Frontiers in Astronomy and Space Sciences now argues that the standard measurement technique captured only a sliver of the real picture. Depending on surface temperature, the total thermal energy radiating from Io’s volcanic terrain could be hundreds of times greater than what that single infrared band detected.
If the finding holds up under further scrutiny, it would resolve a puzzle that has nagged researchers since the Galileo mission era: why theoretical models of tidal heating consistently predicted Io should be producing far more energy than instruments could account for. The heat, it turns out, may never have been missing. The instruments were just looking through too narrow a window.
The measurement gap hiding in plain sight
The problem centers on a workhorse tool aboard NASA’s Juno spacecraft. The Jupiter InfraRed Auroral Mapper, or JIRAM, images Io in what scientists call the M-band, a narrow slice of the mid-infrared spectrum. That band is excellent at picking up the glow of fresh, searingly hot lava. But Io’s volcanic landscape is not all molten rock. Most of the surface area in a typical lava lake consists of cooled crust that still radiates significant thermal energy, just at longer wavelengths the M-band cannot see.
The Frontiers study put numbers to this blind spot. For cooler volcanic surfaces, the ratio between total thermal power (across all wavelengths) and the power detected in the M-band alone can reach factors of hundreds, up to roughly 800. In practical terms, the M-band captures the bright, narrow ring of exposed lava at a lake’s edge while missing the vast majority of energy pouring off the cooler crust that covers most of the lake’s area.
JIRAM images of multiple volcanic depressions on Io, called paterae, show exactly this structure. NASA’s Jet Propulsion Laboratory has released infrared images of one such feature, Chors Patera, revealing a cold central crust surrounded by a bright hot ring at the margins. A separate preprint analyzing lava-lake shapes across Io concluded that most of the thermal power originates from the cooler crust component, not the narrow hot margins. Because earlier global heat estimates leaned heavily on M-band brightness, they systematically overweighted the hottest material and discounted the cooler but far more extensive surfaces.
A decades-old puzzle starts to crack
This is not the first time scientists have suspected something was off. Io’s global heat flow has been estimated using datasets stretching back to the Galileo spacecraft and ground-based infrared radiometry from 1983 to 1993. Those studies placed Io’s total volcanic power output on the order of 1014 watts, with large warm volcanic regions accounting for most of the budget, according to research published in Icarus.
But tidal-heating models, which calculate how much energy Jupiter’s gravitational pull should squeeze into Io’s interior, consistently predicted the moon should be producing more. Researchers called it the “missing heat” problem. The new M-band analysis offers a direct, physical explanation: the heat was always there, radiating from cooler surfaces at wavelengths the primary measurement band could not detect.
Juno’s polar orbits have also sharpened the picture of where Io’s heat emerges. A 2023 study in Nature Astronomy compared the observed distribution of volcanic heat flow with predictions from competing models of Io’s interior. The authors found that the pattern of hotspots favored scenarios in which tidal energy is dissipated relatively close to the surface, or within a global magma ocean, rather than deep in the mantle. Even then, the researchers noted that JIRAM detects only a substantial fraction of the estimated global volcanic power, not the full budget. The instrument was already known to be seeing only part of Io’s thermal story before the latest analysis quantified just how large the gap might be.
Dramatic eruptions hint at the scale
One event from late 2024 illustrates both the potential and the uncertainty. JIRAM observed a synchronized eruption on Io on December 27, 2024, with an estimated total power output between roughly 140 and 260 terawatts, described in a preprint as more than 1,000 times brighter than earlier estimates for that region.
That figure is not a revised global average. Extrapolating from a single eruption to a steady-state heat budget would be a mistake. But it signals that Io’s volcanic system can concentrate energy at levels far beyond what prior catalogs anticipated, and it underscores how sensitive inferred power is to assumptions about temperature and emitting area.
What scientists still need to pin down
No single study has yet produced a revised global heat flow number for Io that integrates the new M-band correction factors with full spatial coverage. The Frontiers paper quantifies the measurement bias but stops short of recalculating Io’s total output. The lava-lake morphology analysis demonstrates the mechanism by which cooler crusts dominate power output, but it remains a preprint that has not completed formal peer review.
The implications for Io’s interior are also in flux. If the true global heat flow turns out to be several times higher than previous estimates, and if the distribution of that extra energy differs from the previously mapped hotspots, the relative support for different interior models could shift. The Nature Astronomy team’s conclusion favoring shallow heating was based on then-available power estimates. No study has yet repeated that model comparison using a corrected heat budget.
There is also the question of timing. Historical heat-flow estimates relied on observations taken decades ago, while Juno provides snapshots during specific orbital passes. If Io’s volcanic output varies significantly over years or decades, part of the discrepancy between models and measurements could reflect real changes rather than instrument bias alone. Separating temporal variability from spectral undercounting will require longer monitoring and careful cross-calibration across instruments and missions.
Why Io’s heat budget matters beyond Io
Io is not just a curiosity. It is the most extreme example of tidal heating in the solar system, and the physics governing its interior apply to other worlds scientists are desperate to understand. Europa, Io’s neighboring Jovian moon, harbors a subsurface ocean kept liquid by the same tidal forces. Enceladus, orbiting Saturn, vents water vapor from tidally heated fractures. If scientists have been systematically underestimating tidal heating at Io, the models used to predict conditions inside Europa and Enceladus, both prime targets in the search for habitable environments, may need recalibrating too.
ESA’s JUICE spacecraft, launched in 2023 and expected to reach the Jupiter system in 2031, will carry instruments capable of observing across a broader infrared range. Combined with continued Juno observations and potential future dedicated Io missions, those data could finally close the gap between what a single infrared band reveals and what Io actually radiates.
As of mid-2026, the scientific community has not converged on a new global number for Io’s heat output. The peer-reviewed evidence establishes that decades of thermal estimates built on M-band data alone are incomplete, especially for cooler volcanic surfaces that radiate most of their energy outside that window. Newer analyses suggest the undercount may be large enough to resolve the long-standing mismatch between tidal-heating models and measurements. But the process of nailing down a revised figure is still unfolding in the literature, and any single heat-flow number should be read with attention to which wavelength bands it draws on and whether the underlying work has passed peer review.
Io’s true power output may be far greater than a generation of scientists believed. Proving it will take more than one infrared band and more than one spacecraft.
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*This article was researched with the help of AI, with human editors creating the final content.
