Why does Uranus emit far less internal heat than Neptune? · revision history

Revision 2

Accepted · core change · record

Changes against revision 1:

Title
Why does Uranus emit far less internal heat than Neptune?
Plain
Neptune radiates more than twice the energy it receives from the Sun, while its near twin Uranus radiates only slightly more than it absorbs. Either something inside Uranus traps heat or Uranus lost its heat early.
Precise · changed · core
Revision 1: Neptune's internal heat flux is about $4e2\,\mathrm{erg}/(\mathrm{s}\,\mathrm{cm}^2)$ (Voyager-era estimate), whereas a 2025 full-orbit energy-budget analysis gives $78\pm 18\,\mathrm{erg}/(\mathrm{s}\,\mathrm{cm}^2)$ for Uranus (Wang and coauthors, GRL 2025, https://doi.org/10.1029/2025GL115660). Determine the cause: a stably stratified interior with composition gradients that suppresses convection, early heat loss after a giant impact, or a different bulk composition. An answer is an interior and evolution model reproducing both planets' present fluxes, radii and gravity harmonics.
Revision 2: Neptune's internal heat flux is about $4 \times 10^{2}\,\mathrm{erg}/(\mathrm{s}\,\mathrm{cm}^2)$ (Voyager-era estimate), whereas a 2025 full-orbit energy-budget analysis gives $78\pm 18\,\mathrm{erg}/(\mathrm{s}\,\mathrm{cm}^2)$ for Uranus (Wang and coauthors, GRL 2025, https://doi.org/10.1029/2025GL115660). Determine the cause: a stably stratified interior with composition gradients that suppresses convection, early heat loss after a giant impact, or a different bulk composition. An answer is an interior and evolution model reproducing both planets' present fluxes, radii and gravity harmonics.
Settled by
A Uranus orbiter measuring gravity harmonics, tidal response and thermal emission, with evolution models fitting both ice giants simultaneously.
Kind
phenomenon
Answer type
mechanism
References
(empty)
Literature status
open
Status note
The 2025 analysis found that Uranus does emit internal heat, at $78 \pm 18\,\mathrm{erg}/(\mathrm{s}\,\mathrm{cm}^2)$, though far less than Neptune.
Posed since
(empty)

Decisions