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Uranian satellite formation by evolution of a water vapour disk generated by a giant impact



The ice-giant planet Uranus probably underwent a giant impact, given that its spin axis is tilted by 98 degrees1,2,3. That its satellite system is equally inclined and prograde suggests that it was formed as a consequence of the impact. However, the disks predicted by the impact simulations1,3,4 generally have sizes one order smaller and masses two orders larger than those of the observed system at present. Here we show, by means of a theoretical model, that the Uranian satellite formation is regulated by the evolution of the impact-generated disk. Because the vaporization temperature of water ice is low and both Uranus and the impactor are assumed to be ice-dominated, we can conclude that the impact-generated disk has mostly vaporized. We predict that the disk lost a substantial amount of water vapour mass and spread to the levels of the current system until the disk cooled down enough for ice condensation and accretion of icy particles to begin. From the predicted distribution of condensed ices, our N-body simulation is able to reproduce the observed mass–orbit configuration of Uranian satellites. This scenario contrasts with the giant-impact model for the Earth’s Moon5, in which about half of the compact, impact-generated, solid or liquid disk is immediately incorporated into the Moon on impact6.

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Fig. 1: The mass (M) and orbital radius (r) distribution of the current Uranian satellite systems and that predicted by N-body simulation.
Fig. 2: The evolution of the disk of a mixture of H/He gas and water vapour and the associated ice condensation.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Code availability

The codes used in this study are available from the corresponding author upon reasonable request.


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This study was supported by MEXT ‘Exploratory Challenge on Post-K computer’ (hp180183 and hp190143), by ‘Priority Issue on post-K computer’ (hp190156), by JSPS KAKENHI 15H02065 and 19K03950, and by MEXT KAKENHI 18H05438. The N-body simulation in this work was carried out at the Yukawa Institute Computer Facility.

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S.I, S.U. and T.S. initiated the project. S.I. and Y.I. performed the theoretical calculations. S.I. wrote the manuscript. All authors contributed to the discussion and the interpretation of the results.

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Correspondence to Shigeru Ida.

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Ida, S., Ueta, S., Sasaki, T. et al. Uranian satellite formation by evolution of a water vapour disk generated by a giant impact. Nat Astron 4, 880–885 (2020).

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