SATURN RING SKIMMER MISSION CONCEPT M. S. Tiscareno1, M. M. Hedman2, M

51st Lunar and Planetary Science Conference (2020) 2834.pdf

SATURN RING SKIMMER MISSION CONCEPT M. S. Tiscareno1, M. M. Hedman2, M. Vaquero3, H. Cao4, P. R. Estrada5, J. Fuller6, A. P. Ingersoll6, K. E. Miller7, M. Parisi3, C. S. Paty8, J. N. Cuzzi5, A. R. Hendrix9, R. E. Johnson10, T. Koskinen11, W. S. Kurth12, J. I. Lunine13, P. D. Nicholson13, R. Schindhelm14, M. R. Showalter1, L. J. Spilker3, W. Tseng15. 1SETI Institute, 2Univ of Idaho, 3Jet Propulsion Laboratory, 4Harvard Univ, 5NASA Ames, 6Caltech, 7Southwest Research Institute, 8Univ of Oregon, 9Planetary Science Institute, 10Univ of Virginia, 11Univ of Arizona, 12Univ of Iowa, 13Cornell Univ, 14Ball Aerospace, 15National Taiwan Normal Univ.

Abstract: The innovative Saturn Ring Skimmer mission concept will observe individual ring particles for the first time, will directly measure the magnetosphere in the region where it is shaped by the rings, and will directly measure the atmosphere of a disk. • By taking a broad look at how the rings, the magnetosphere, the upper atmosphere, and the planetary interior compose a coherent interconnected system, the Saturn Ring Skimmer will address questions about this system that could only be posed after Cassini’s grand finale. Figure 1. Using an innovative trajectory design [1], • By studying disk dynamics at the individual parti- the Saturn Ring Skimmer would repeatedly pass cle level, the Saturn Ring Skimmer will use this nat- across the face of the rings at altitudes of a few hun- ural laboratory to help us understand exo-disks and dred km. planetary formation. • By determining the role played by Saturn’s rings in pretation of these fluxes may imply that Saturn’s rings driving the Saturn system to be very different from are young and/or short-lived, but we lack a clear picture Jupiter, the Saturn Ring Skimmer will help us un- of the total mass and composition of the material being derstand a whole class of exoplanets. transported across the rings and into the planet. We advocate for the New Frontiers list to include an What is happening inside Saturn? Cassini’s entry that addresses these science objectives. Grand Finale also revealed that Saturn’s gravitational field is far more dynamic than anyone had expected. Background: The Cassini mission revolutionized Variations in the gravitational accelerations felt by Cas- our understanding of the Saturn system. Its close prox- sini during different passages between the planet and the imity to the planet, especially during the mission’s final rings imply that Saturn has time-variable and/or azi- year, combined with its 13-year time domain, enabled it muthal structures in its deep interior that are unlike an- to substantially address dozens of scientific questions ything seen at Jupiter. Further study of these variations and mysteries. As always happens with ground-break- would not only help us to better understand the interior ing science, the answers provided by Cassini revealed a structure and dynamics of Saturn in particular, but give whole new suite of pressing scientific questions. us new insights into the history and workings of giant The new set of fundamental questions about the in- planets in general. ner Saturn system includes the following top-level sci- How does the presence of the rings alter Saturn ence questions: and its system? Saturn’s rings are the only broad dense How do rings and disks work at the particle disk in our solar system, but many exoplanets may have level? Saturn’s rings span 300,000 km, but are com- similar ring systems [3]. Many of the unique aspects of posed primarily of particles millimeters to meters Saturn’s magnetosphere, atmosphere, and interior are across. Cassini never had the resolution to directly de- likely connected to the ring system’s influence [4,5], tect typical individual ring particles, and doing so likely and the best way to confirm and understand that influ- holds the key to understanding many of Cassini’s most ence is to directly explore the interface region between enigmatic discoveries [2]. Furthermore, since analogous the rings and the rest of the system. processes probably occurred in protoplanetary disks like The Saturn Ring Skimmer mission concept would the one that gave rise to our solar system, more detailed address these important science questions. Using an in- information about these particle-level dynamics would novative trajectory design [1], the Ring Skimmer ex- allow us to better understand our own origins. plores the inner Saturn system with a proximity and How long will the rings last? Measurements made frequency that were previously thought to be impossi- during Cassini’s Grand Finale revealed surprisingly ble with current technology and feasible costs. high fluxes of material into Saturn. A steady-state inter-

51st Lunar and Planetary Science Conference (2020) 2834.pdf

The Ring Skimmer approaches ~100´ closer to Sat- Ring Atmosphere and Neutral Gas urn’s rings than Cassini’s distance when the best ring • How do meteoroid bombardment and solar irradia- images to date were taken, but that’s not all. The Ring tion affect production of gas from ice-organic sur- Skimmer also enables in situ observations immediately faces? above and below the ring plane, as well as close-range • What physical processes govern gas-solid interac- observations of Saturn’s atmosphere, magnetosphere tions in a disk configuration, including protoplane- and gravitational fields, and does so with a frequency tary disks? and longevity that enable both spatial mapping and time-domain science. Saturn’s Atmosphere • What are the basic cloud types at Saturn, and how The science topics addressed by the Ring Skimmer do they vary with location and time? are broadly based within the inner Saturn system. The • How do Saturn's atmospheric processes compare to role of Saturn’s rings in these investigations can be un- those at other planets? derstood via three main themes: • What are the patterns of cloud motion at Saturn? Theme 1: Study disk processes, using the rings as How do they vary regionally and with time? a natural laboratory. Saturn’s rings are an exemplary • What is the vertical structure of clouds at Saturn? astrophysical disk in a location amenable to close obser- vation. Understanding the detailed workings of disk Saturn’s Interior processes in Saturn’s rings informs the study of less ac- • How does Saturn's interior structure vary with azi- cessible disks, including protoplanetary disks. muth and/or time? Theme 2: Study Saturn system origin and his- • How is Saturn's deep interior changing over time- tory, using the rings as a tracer. The structural and scales of years to decades? chemical makeup of Saturn’s rings constitute clues that Relationship to other Saturn system missions: A constrain the possible processes that may have produced Saturn Ring Skimmer could also explore Titan, Encela- them. dus, or other moons of Saturn, or carry a Saturn atmos- Theme 3: Use the rings as a detector to under- phere descent probe. Indeed, flybys of Titan are a re- stand surrounding processes. Saturn’s rings are large quired part of ring skimming, and close proximity to En- and delicate, and in many cases serve to amplify subtle celadus, other moons, and Saturn occurs naturally. Alt- processes, such as interior gravity perturbations or mi- hough polar moon flybys and ring skimming cannot be crometeoroid impacts [6]. done at the same time, a mission could operate multiple We are in early stages of developing the science case phases focusing on one and then the other. for this mission concept. We have identified the follow- The New Frontiers list: A robust multi-discipli- ing key science questions, grouped in terms of five sci- nary Ring Skimmer mission is best suited for a New ence disciplines: Frontiers budget, but current New Frontiers rules do not Saturn’s Rings allow Saturn system mission proposals that are not fo- • What do individual ring particles look like? cused on either an Ocean World or an Atmospheric En- • What happens when ring particles collide, frag- try Probe. We petition the Planetary Science Decadal ment, and/or form aggregates? Survey committee to consider adding Inner Saturn Sys- • What are the properties of micrometeoroid impacts tem Science to the New Frontiers list, so that a Saturn and how do they sculpt the rings? Ring Skimmer can receive full consideration in the fu- • How polluted are the rings, and what are the impli- ture. cations for the rings' origin and evolution? References: [1] Vaquero M, Senent J, and Tis- Plasma and Magnetosphere careno MS (2019). American Astronautical Society • How does the material transport between the rings, Meeting Abstracts, 19–265. [2] Tiscareno MS et al. the magnetosphere, and the atmosphere constrain (2019). Science 364, aau1017. [3] Schlichting HE and the origin and evolution of the rings? Chang P (2011). Astrophys. J. 734, 117. [4] Khurana K • What is the angular momentum exchange rate be- et al. (2018). Geophys. Res. Lett. 45, 10068. tween Saturn and its rings and how does it contrib- [5] Waite JH et al. (2018). Science 362, aat2382. ute to the long term evolution of Saturn rings? [6] Mankovich C et al. (2019). Astrophys. J. 871, 1.

• How do the periodicities in Saturn's magnetosphere affect the rings?