Exchange of ejecta between Telesto and Calypso: Tadpoles, horseshoes, and passing orbits

Abstract

We have numerically integrated the orbits of ejecta from Telesto and Calypso, the two small Trojan companions of Saturn's major satellite Tethys. Ejecta were launched with speeds comparable to or exceeding their parent's escape velocity, consistent with impacts into regolith surfaces. We find that the fates of ejecta fall into several distinct categories, depending on both the speed and direction of launch.The slowest ejecta follow sub-orbital trajectories and re-impact their source moon in less than one day. Slightly faster debris barely escape their parent's Hill sphere and are confined to tadpole orbits, librating about Tethys' triangular Lagrange points L 4 (leading, near Telesto) or L 5 (trailing, near Calypso) with nearly the same orbital semi-major axis as Tethys, Telesto, and Calypso. These ejecta too eventually re-impact their source moon, but with a median lifetime of a few dozen years. Those which re-impact within the first ten years or so have lifetimes near integer multiples of 348.6 days (half the tadpole period).Still faster debris with azimuthal velocity components ∼ > 10 m/s enter horseshoe orbits which enclose both L 4 and L 5 as well as L 3 , but which avoid Tethys and its Hill sphere. These ejecta impact either Telesto or Calypso at comparable rates, with median lifetimes of several thousand years. However, they cannot reach Tethys itself; only the fastest ejecta, with azimuthal velocites ∼ > 40 m/s, achieve "passing orbits" which are able to encounter Tethys. Tethys accretes most of these ejecta within several years, but some 1% of them are scattered either inward to hit Enceladus or outward to strike Dione, over timescales on the order of a few hundred years.

Small Impact Crater Populations on Saturn's Moon Tethys and Implications for Source Impactors in the System

“…Cratering in the inner solar system is primarily caused by the impact of asteroids onto the surface (Marchi et al, 2009;Neukum et al, 2001;Strom et al, 2005) whereas craters on the bodies in the outer solar system have a wider range of sources. Specifically, at Saturn, potential sources for these craters include heliocentric material, planetocentric material, or debris formed from impacts onto the co-orbital moons of Tethys and Dione (Dobrovolskis et al, 2010;Dones et al, 2009;Nayak & Asphaug, 2016;Smith et al, 1981Smith et al, , 1982Zahnle et al, 2003).…”

“…This difference in shape of the size frequency diagrams suggests that the small crater population has been preferentially enhanced in Region 1 compared to the other areas on Tethys. Potential sources of the enhancement are secondary craters or sesquinary craters, or where the debris that created the smaller craters came from one of Tethys' co-orbital satellites, Telesto or Calypso (Alvarellos et al, 2017;Bierhaus et al, 2012Bierhaus et al, , 2018Nayak & Asphaug, 2016; see Dobrovolskis et al, 2010, for the models of ejecta exchange between the moons). Overall, we are seeing size distributions that are suggestive of a Saturn-specific planetocentric population along with an enhancement of the small crater population near the anti-Saturn point (Region 1).…”

Sesquinary Catastrophe for Close-in Moons with Dynamically Excited Orbits

“…However, Trojan moons that were imaged in good resolution (Telesto, Calypso, and Helene) do not appear to be ellipsoidal like Methone but have more irregular shapes, even if they appear blanketed by loose material (Thomas et al 2013). Behavior of their ejecta may be rather complex, with slowly ejected particles staying in tadpole orbits, while some faster fragments may be accreted by their parent moons (Dobrovolskis et al 2010;Nayak et al 2016;Ferguson et al 2020Ferguson et al , 2022aFerguson et al , 2022b. Conversely, ejecta from the larger moons may impact the Trojans, and finally the Trojans may also exchange ejecta between themselves.…”

Orbital pathways for a Lunar-Ejecta Origin of the Near-Earth Asteroid Kamo`oalewa

“…We explore the dynamical fates of lunar ejecta in a similar vein as [22] for satellite ejecta in the Saturnian system. Our dynamical model includes the eight major planets from Mercury to Neptune and the Moon, and we use the IAS15 integrator within REBOUND [23].…”