Exploring the hypothesis of an inverted Z gradient inside Jupiter

Context. Reconciling models of Jupiter's interior with measurements of the atmospheric composition still poses a significant challenge. Interior models favour a subsolar or solar abundance of heavy elements, Z, whereas atmospheric measurements suggest a supersolar abundance. One potential solution may be to account for the presence of an inverted Z gradient, namely, an inward decrease of Z, which implies a higher heavy-element abundance in the atmosphere than in the outer envelope. Aims. We investigate two scenarios in which the inverted Z gradient is either located at levels where helium rain occurs (∼Mbar) or at higher levels (∼kbar) where a radiative region could exist. Here, we aim to assess the plausibility of these scenarios. Methods. We calculated interior and evolution models of Jupiter with such an inverted Z gradient and we set constraints on its stability and formation. Results. We find that an inverted Z gradient at the location of helium rain is not feasible, as it would require a late accretion and would involve too much material. We find interior models with an inverted Z gradient at upper levels due to a radiative zone preventing downward mixing, could satisfy the current gravitational field of the planet. However, our evolution models suggest that this second scenario cannot be validated. Conclusions. We find that an inverted Z gradient in Jupiter could indeed be stable, however, its presence either at the Mbar or kbar levels is rather unlikely.