51st Lunar and Planetary Science Conference (2020) 1477.pdf

SOME OF SATURN’S SMALL SATELLITES HAVE SURPRISINGLY STYGIAN OR STRANGELY SNOWY SURACES. M.M. Hedman1, P. Helfenstein2, R.O. Chancia1, P. Thomas2, E. Roussos3, C. Paranicas4, A.J. Verbiscer5, 1Department of Physics, University of Idaho, Moscow ID 83844, 2Cornell Center for Astrophysics and Planetary Science, Cornell Universit, Ithaca NY 14853, 3Max Planck Institute or Solar System Research, Goettingen, Germany 37077, 4APL, Johns Hopkins University, Laurel MD 20723, 6Department of Astronomy, University of Virginia, Charlottesville VA 22904.

The Saturn system contains a large number of any other nearby moon, despite orbiting within the inner moons with a range of surface features and photometric flank of the E ring. properties that, in part, reflect various aspects of their The comparatively dark surfaces of Methone, Pal- environments. In particular, the whole-disk albedos of lene and Aegaeon imply that something besides the E the mid-sized icy moons Mi- mas, Enceladus, Tethys, ring is affecting their surface properties, and we pro- Dione and Rhea are strongly correlated with the pose that the most likely explanation are Saturn’s expected fluxes of the E-ring particles that surround all radiation belts. Cassini measured high fluxes of high- of these moons [1, 2]. Since the E-ring originates from energy (> 25 MeV) protons between the orbits of the Enceladus’ plume, this correlation means that larger moons Janus/Epimetheus, Mimas and Enceladus Enceladus’ geological activity strongly affects the [5]. These four moons are all large enough that they surface properties of Saturn’s mid-sized moons [1]. create persistent regions of low energetic proton flux However, these mid-sized moons are not the only along their orbits known as macrosignatures, and so objects in this region. Much smaller moons can be found their surface weathering is limited by the slow ra- dial at several locations between the main rings and Mimas’ diffusion that works to fill in the macrosignature. By orbit (Janus, Epimetheus and Aegaeon), be- tween the contrast, Aegaeon, Methone and Pallene are too small orbits of Mimas and Enceladus (Methone, Anthe and to produce such depletions and so orbit in regions with Pallene), and along the orbits of Tethys and Dione much higher fluxes of energetic protons, and so are (Telesto, Calypso, Helene and Polydeuces). These weathered at a much higher rate. The ex- act mechanism moons lack much of the internal processes operating by which this radiation darkens the sur- faces of these inside the mid-sized moons and might be expected to small moons is still not completely clear. However, provide a fiducial for testing models of how much further investigations of both the spectral and various internal and external processes affect satellite photometric properties of these and other satellites surface properties For example, hese moons are also should help clarify how radiation affects the surfaces of embedded in the E ring and so one might expect their icy moons in the outer Solar System. surface brightnesses to follow the same trends as their On the other hand, we also find that Calypso and larger neighbors. However, quantitatively testing this Helene have substantially brighter surfaces than their hypothesis with albedos obtained from available Cassini co-orbital companions Tethys and Dione. This suggests imaging data is a challenge, firstly because these smaller that Calypso and Helene might have experienced higher moons have significantly non-spherical shapes [3], and particle fluxes, but there are no obvious structures in the secondly because most of the observational geometry E ring that would preferentially brighten these particular coverage needed to accurately constrain models of their moons, so there must either be something subtle in the photometric behavior are low spatial resolution images E-ring particles’ orbital properties that leads to that are suitable only for disk-integrated measurements substantial asymmetries in the particle flux, or [4]. something happened recently to temporarily increase We have developed a new disk-integrated both these moons’ brightnesses. photometric model that explicitly accounts for the smoothly ellipsoidal shapes of the small moons and thus [1] Verbiscer, A., et al. (2007) Science, 315, 815. [2] allows us to derive estimates of whole-disk albedos that Hendrix, A., et al. (2018) Icarus, 300, 103-114. [3] can be compared between moons of different shapes [5]. Thomas, P., et al. (2018) in Enceladus and the Icy We find that Aegaeon, Methone and Pallene are Moons of Saturn, P. Schenk, R. Clark, C. Howett, A. surprisingly dark at visible wavelengths. More Verbiscer, H. Waite, Eds. [4] Verbiscer, A., et al. specifically, Methone and Pallene are darker than (2018) in Enceladus and the Icy Moons of Saturn, P. Mimas. This result was unexpected because both of Schenk, R. Clark, C. Howett, A. Verbiscer, H. Waite, these moons orbit between Mimas and Enceladus, and Eds. [5] Hedman, M.M. et al. (2019) arXiv: so the E-ring flux is higher onto these moons than it is 1912:09192, accepted for publication in AJ. [6] onto Mimas, which would naturally tend to make them Kollmann,P. et al. (2013), Icarus, 222, 323-341. brighter. Aegaeon is even more extreme, being about as dark as Iapetus’ leading side and thus much darker than