Io's lava lakes are ringed by narrow bands of incandescent magma — the hot peripheral rings where fresh lava surfaces. Previous thermal models focused on these bright rings as the primary heat source. They were wrong by up to a factor of 10.
The dominant thermal output comes from the cold central crusts (arXiv:2603.22062). Thirty lava lakes analyzed with Juno's JIRAM instrument show that the low-temperature regions covering most of the lake surface radiate more total energy than the dramatic hot rings at the edges. The cooler crust is larger in area; the area advantage overwhelms the temperature disadvantage.
Each lake resurfaces stochastically on timescales of roughly a decade. The crust thickens, cools, and eventually overturns — replaced by fresh magma that begins cooling immediately. The age of the crust at any given moment determines its temperature, and the temperature determines its contribution to the heat budget. Old, cold crusts radiate less per unit area but cover more area than young, hot ones.
The measurement error was systematic: instruments preferentially detected the bright rings because they're easier to see. The cold crust, spread across the lake's center, emitted at longer wavelengths with lower intensity per pixel. Previous estimates captured the conspicuous minority and missed the dominant majority.
This matters for understanding Io's global heat budget — the rate at which tidal dissipation converts Jupiter's orbital energy into Io's internal heat. If the lakes emit 10 times more than previously calculated, the planetary energy balance needs revision.
The pattern: the loudest signal isn't the largest contribution. The hot ring is visible; the cold crust is where the energy actually goes. Measuring only what you can see underestimates everything you can't.