Presentation Information
[PPS01-P19]Global N-body simulation of propeller structures in Saturn's rings
*Ryo Tamon1, Keiji Ohtsuki1 (1.Graduate School of Science, Kobe University)
Keywords:
Saturn,Ring
All of the giant planets in our solar system have rings. In addition, four asteroids which have rings have been discovered. Examining ring structures may provide clues to their origin and evolution. Thus, we investigate these structures to better understand their possible formation processes.
Moonlets in Saturn's rings generate gravitational perturbation on the ring depending on their mass. The perturbation creates global or local structures on the ring. A sufficiently massive satellite forms a gap across the entire circumference of the ring, such as the Encke gap created by Pan, or the Keeler gap created by Daphnis. On the other hand, a smaller moonlet creates a local structure known as a “propeller” (see [1] for a review). This kind of a local structure had been theoretically predicted before the arrival of the Cassini spacecraft at Saturn, and was discovered by Cassini [2, 3] (Figure 1). The propeller structure is formed by a small satellite significantly larger than ring particles [4, 5]. The size and orbital distributions of these embedded moonlets inferred from observations of propeller structures can provide constrains on the origin and evolution of the moonlets and the planetary rings.
Recently, to investigate structures of complete gaps within Saturn's rings such as the Keeler gap, global simulations accounting for collisions and mutual gravity between particles were performed [6, 7]. On the other hand, in previous N-body simulations of propeller structures, local simulations have been used (e.g., [3-5]). While local simulations allow us to examine fine structures in detail at high resolution, it is difficult to investigate how such structures can be compared to complete gaps created by more massive satellites.
In the present work, we performed global simulations to investigate the continuous structural transition from a complete gap to a propeller as a function of moonlet mass. The code used in this work is modified from “nbody-with-center” created by J. Makino, and is similar to the code used in [6, 7]. In our simulation, the maximum number of particles is N≈3×106, and we performed simulations by varying ms/Mp from 10-5 to 10-8, where ms and Mp are moonlet mass and planet mass, respectively.
We found that the transition from the complete gap structure into the local propeller structure occurs by decreasing the mass ratio ms/Mp. Moreover, the result of the relation between the semi-major axis of the ring particles and the dynamical optical depth averaged in the azimuthal direction shows that the width of the gap structure can be well scaled by the Hill radius of the embedded moonlet.
References:
[1] Spahn, F. et al. (2018) In Planetary Ring Systems (Tiscareno, M. S. & Murray, C. D., eds.), pp. 157-197,Cambridge Univ. Press
[2] Tiscareno, M. S. et al. (2006) Nature, 440, 648
[3] Sremčević, M. et al. (2007) Nature, 449, 1019
[4] Lewis, M. C. & Stewart, G. R. (2009) Icarus, 199, 387
[5] Michikoshi, S. & Kokubo, E. (2011) ApJ, 732, L23
[6] Torii, N. et al. (2024) Icarus, 415, 116029
[7] Torii, N. et al. (2025) Icarus, 439, 116608
Moonlets in Saturn's rings generate gravitational perturbation on the ring depending on their mass. The perturbation creates global or local structures on the ring. A sufficiently massive satellite forms a gap across the entire circumference of the ring, such as the Encke gap created by Pan, or the Keeler gap created by Daphnis. On the other hand, a smaller moonlet creates a local structure known as a “propeller” (see [1] for a review). This kind of a local structure had been theoretically predicted before the arrival of the Cassini spacecraft at Saturn, and was discovered by Cassini [2, 3] (Figure 1). The propeller structure is formed by a small satellite significantly larger than ring particles [4, 5]. The size and orbital distributions of these embedded moonlets inferred from observations of propeller structures can provide constrains on the origin and evolution of the moonlets and the planetary rings.
Recently, to investigate structures of complete gaps within Saturn's rings such as the Keeler gap, global simulations accounting for collisions and mutual gravity between particles were performed [6, 7]. On the other hand, in previous N-body simulations of propeller structures, local simulations have been used (e.g., [3-5]). While local simulations allow us to examine fine structures in detail at high resolution, it is difficult to investigate how such structures can be compared to complete gaps created by more massive satellites.
In the present work, we performed global simulations to investigate the continuous structural transition from a complete gap to a propeller as a function of moonlet mass. The code used in this work is modified from “nbody-with-center” created by J. Makino, and is similar to the code used in [6, 7]. In our simulation, the maximum number of particles is N≈3×106, and we performed simulations by varying ms/Mp from 10-5 to 10-8, where ms and Mp are moonlet mass and planet mass, respectively.
We found that the transition from the complete gap structure into the local propeller structure occurs by decreasing the mass ratio ms/Mp. Moreover, the result of the relation between the semi-major axis of the ring particles and the dynamical optical depth averaged in the azimuthal direction shows that the width of the gap structure can be well scaled by the Hill radius of the embedded moonlet.
References:
[1] Spahn, F. et al. (2018) In Planetary Ring Systems (Tiscareno, M. S. & Murray, C. D., eds.), pp. 157-197,Cambridge Univ. Press
[2] Tiscareno, M. S. et al. (2006) Nature, 440, 648
[3] Sremčević, M. et al. (2007) Nature, 449, 1019
[4] Lewis, M. C. & Stewart, G. R. (2009) Icarus, 199, 387
[5] Michikoshi, S. & Kokubo, E. (2011) ApJ, 732, L23
[6] Torii, N. et al. (2024) Icarus, 415, 116029
[7] Torii, N. et al. (2025) Icarus, 439, 116608