The Game of the Seven Moons Astrophys

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research highlights RINGS The game of the seven moons Astrophys. J. Suppl. Ser. 232, 28 (2017) We were almost deprived of the magnificence of Saturn’s ring system. In fact, a planetary ring tends to disperse radially: numerical models have predicted that Saturn’s A ring, the outermost of the system (except for the faint F ring), would have dispersed in ~108 years. This phenomenon was averted due to Saturn’s small moons. If a satellite is in orbital resonance with the inner or outer edge of a ring, it applies a torque on it, limiting its dispersion. It is usually believed that the A ring was contained by its 7:6 resonance with the moon Janus (with some help from the smaller Epimetheus). Radwan Tajeddine and colleagues instead show that seven small moons contribute to confinement. The small moons can act on the rings with two main mechanisms: individual resonances, like the one described above, or 'envelopes' of overlapping resonances generated by different moons — a shepherding effect. Tajeddine et al. compute the variations of the angular momentum flux across the A ring due to resonances induced by all the small moons around it and show that the second mechanism is in play: Pan, Atlas, Prometheus, Pandora, Janus, Epimetheus and Mimas all have a significant effect, confining the ring and defining its internal structure. Also, as the rings exert a corresponding torque on the satellites, they affect the moons’ orbital evolution. Tajeddine et al. could infer an upper limit of 1 Gyr for the age of these satellites. Interestingly, the B ring seems instead to be controlled only by the 2:1 resonance between Mimas and its internal edge. Luca Maltagliati Published online: 9 November 2017 https://doi.org/10.1038/s41550-017-0330-y NATURE AstrONOMY | VOL 1 | DECEMBER 2017 | 817 | www.nature.com/natureastronomy 817 © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved..

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Arxiv:1912.09192V2 [Astro-Ph.EP] 24 Feb 2020
Draft version February 25, 2020 Typeset using LATEX preprint style in AASTeX62 Photometric analyses of Saturn's small moons: Aegaeon, Methone and Pallene are dark; Helene and Calypso are bright. M. M. Hedman,1 P. Helfenstein,2 R. O. Chancia,1, 3 P. Thomas,2 E. Roussos,4 C. Paranicas,5 and A. J. Verbiscer6 1Department of Physics, University of Idaho, Moscow, ID 83844 2Cornell Center for Astrophysics and Planetary Science, Cornell University, Ithaca NY 14853 3Center for Imaging Science, Rochester Institute of Technology, Rochester NY 14623 4Max Planck Institute for Solar System Research, Göttingen,Germany 37077 5APL, John Hopkins University, Laurel MD 20723 6Department of Astronomy, University of Virginia, Charlottesville, VA 22904 ABSTRACT We examine the surface brightnesses of Saturn's smaller satellites using a photometric model that explicitly accounts for their elongated shapes and thus facilitates compar- isons among different moons. Analyses of Cassini imaging data with this model reveals that the moons Aegaeon, Methone and Pallene are darker than one would expect given trends previously observed among the nearby mid-sized satellites. On the other hand, the trojan moons Calypso and Helene have substantially brighter surfaces than their co-orbital companions Tethys and Dione. These observations are inconsistent with the moons' surface brightnesses being entirely controlled by the local flux of E-ring par- ticles, and therefore strongly imply that other phenomena are affecting their surface properties. The darkness of Aegaeon, Methone and Pallene is correlated with the fluxes of high-energy protons, implying that high-energy radiation is responsible for darkening these small moons. Meanwhile, Prometheus and Pandora appear to be brightened by their interactions with nearby dusty F ring, implying that enhanced dust fluxes are most likely responsible for Calypso's and Helene's excess brightness.
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