Impact of Jupiter’s heating and self-shadowing on its Circumplanetary disk structure

An understanding of the structure of the circumplanetary disk (CPD) that surrounded Jupiter after the gas runaway accretion phase is essential for comprehending the formation of the Galilean system.  Insights from three-dimensional hydrodynamic simulations indicate that the CPD could have been optically thick¹ and heated by the hot young Jupiter^2,3. Therefore, an analysis of the impact of Jupiter’s radiative heating and subsequent self-shadowing of the CPD on its structure and composition, may provide insights into the formation conditions of the Galilean satellites.

To assess the impact of Jupiter radiative heating and disk self-shadowing, we have developed a two-dimensional quasi-static CPD model and used a grey-atmosphere radiative transfer to determine its thermal structure. In the model, the CPD actively accretes material from the protoplanetary disk, which accretion rate is derived from a Jupiter formation model^4,5, that predicts a depletion over a timescale of 100 kyr. The CPD evolution is simulated over a period of 400 kyr.

The model demonstrates that the CPD undergoes a transition from a fully optically thick and hot state, characterized by a maximum midplane temperature above 2000 K, to a fully optically thin and cold state, with a temperature below 400 K, over a timescale of less than 200 kyr. This transition is linked to the rapid depletion of the accretion rate. During this period, shadows are projected on the disk at distances greater than 10 Jupiter radii. Shadows only influence the CPD temperature when heating by accreting material and viscous stress become negligible compared to Jupiter’s radiative heating, after 150 kyr of evolution. Between 150 and 200 kyr of evolution, the shadowed area, centered around 10 Jupiter radii, experiences a temperature drop of approximately 100 K compared to its surroundings. The extent and duration of shadows are significantly influenced by the CPD metallicity. A higher material metallicity results in longer shadow durations and larger shadowed areas.

The shadowed area can act as a cold trap for volatile species, such as NH_3, CO₂, and H₂S, where their ices are trapped closer to the planet compared to their respective iceline position. Depending on the CPD metallicity, cold traps can last between 35 and 120 kyr at distances centered around 10 Jupiter radii. The existence of the shadows may have influenced the composition of the Galilean moons building blocks, potentially shaping their characteristics, providing an observational test to this model.

A significant distinction between our methodology and that of previous studies is the utilisation of an accretion rate derived from a numerical simulation that exhibits a much faster decay than the more conventional 1 to 3 Myr accretion rate depletion timescale.⁶. To assess the variability of our results with the accretion rate prescription, we performed a simulation with an accretion rate depletion timescale of 1 Myr over 3 Myr. By doing so, we constat that the CPD remained hotter after 3 Myr than the case with rapidly decreasing accretion rate after 200 kyr. Since accretion and viscous heating dominate over the planet's heating, the shadowed areas do not exhibit significant temperature drops. Therefore, the consequences of CPD self-shadowing only exist for a rapidly decreasing accretion rate after Jupiter's formation.

 

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