- 1Universität Rostock, Institut für Physik, Germany
- 2LULI, CNRS, CEA, Sorbonne Universite, Ecole Polytechnique - Institut Polytechnique de Paris, 91128 Palaiseau, France
- 3SLAC, Stanford University, Menlo Park, CA 94025, USA
The atmospheres of Jupiter and Uranus are observed to be depleted in He/H, with a possible stronger depletion in Uranus than in Jupiter, depending on the assumed deep methane mixing ratio. Remote sensing data suggest an about protosolar He/H for Neptune while for Saturn a depletion, although its magnitude is uncertain [1].
The atmospheric He/H depletion of Jupiter together with the strong Ne/H depletion as observed by the Galileo entry probe are commonly taken evidence of helium rain at Mbar pressures [2]. This poses the question of the He/H and Ne/H abundances in the atmospheres of the ice giants if He/H phase separation takes place in their deep interiors.
How much of a light-element component (He-H) in the deep interiors of the ice giants is required to match the observed gravity data depends on model assumptions. If He/H is present at Mbar pressures and if particle exchange between atmosphere and deep interior occurs uninhibited, their atmospheres are predicted to be highly depleted in He/H, contrary to what is observed [1].
Here, we assume the presence of a barrier to convection between atmosphere and interior in models of the outer planets. This boundary layer (BL) inhibits heat and particle transport. The model unifies the thermal BL assumption for the ice giants [1] with the double-diffusive BL assumption for Jupiter [3].
We vary the diffusivity of He and Ne in H, the strength of partitioning of Ne in He-droplets, and the He/H phase diagram to compute possible atmospheric He/H and Ne/H ratios. They are benchmarked against Jupiter and serve as predictions for Uranus to be probed by a shallow (5 bars) entry probe. A measurement would provide unique constraints on the interior structure.
Acknowledgement: NN acknowledges support through DFG-grant NE 1734/3-1.
[1] Nettelmann N, Cano Amoros M, Tosi N, Helled R, Fortney JJ. Atmospheric Helium Abundances in the Giant Planets. SSRv 220:56 (2024)
[2] Wilson H, Militzer B. Sequestration of Noble Gases in Giant Planet Interiors. PRL 104:121101 (2010)
[3] Nettelmann N, Fortney JJ. Jupiter’s Interior with an Inverted Helium Gradient. PSJ (2025)
How to cite: Nettelmann, N., Bethkenhagen, M., and Bergermann, A.: Predicted He/H and Ne/H Abundances in the Atmospheres of the Ice Giants, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3223, https://doi.org/10.5194/egusphere-egu25-3223, 2025.
