Saturn Atmospheric Science in the Next Decade

Introduction

The Cassini mission to Saturn, presently in its first extended mission (ending in July 2010), has provided a wealth of new information, established new ideas and posed new questions about the Solar System’s second largest planet. But even though the ringed world has been regularly observed from Earth throughout the last few decades, and despite the Pioneer 11 and Voyagers 1 and 2 flybys a full Saturnian year (nearly 30 earth years) earlier, there are many fundamental atmospheric properties and processes which remain poorly characterized. Cassini’s second extended mission is expected to dominate Saturn atmospheric science in the 2013-2023 timeframe, but many questions will remain outstanding, either because (a) Cassini lacks the instrumentation to probe the necessary atmospheric levels; (b) studies of bulk and altitudevarying composition and related chemistry cannot be adequately achieved via remote sensing; or (c) Cassini’s temporal or spatial coverage will be insufficient to probe the seasonal timescales or full range of latitudes necessary to understand Saturn’s weather layer in full. This white paper supports the goals of Cassini’s extended mission (see white paper by Spilker et al.), and advocates continued studies of Saturn post-Cassini by the next flagship mission (TSSM or otherwise), dedicated temporal-monitoring from ground-based and space-based platforms, in addition to multiple entry probes and deep-atmosphere remote sensing.

As the second largest gas giant planet in our Solar System, Saturn’s atmospheric composition, structure and dynamics are most closely compared to those of Jupiter. Indeed, comparisons between the two gas giants, in terms of their bulk composition and their responses to differing degrees of seasonal insolation provide a wealth of information about the evolutionary processes at work on gas giants in general. Saturn’s atmosphere is distinguished from Jupiter’s in several important respects. The bulk rotation of the planet is poorly understood (e.g., Anderson and Schubert, 2007), a result of the close alignment of the magnetic field and planetary rotation axes, which minimizes the variable periodicity of the kilometric radiation used to assess the rotation rate. Atmospheric stability arguments, in tandem with gravitational and radio occultation data, are beginning to suggest a faster rotational period than previously thought, altering our understanding of the relationship of the cloud-top jet streams and the bulk rotational state of the interior. Saturn’s temperature field, molecular composition (para-H2, PH3, hydrocarbons, etc.) and aerosol content exhibit strong hemispheric asymmetries driven by seasonal variations that are unique in our Solar System. The zonal organization of Saturn’s weather layer apparently persists to much higher latitudes than observed on Jupiter thus far, resulting in two cyclonic hot polar vortices (irrespective of season), encircled by strong prograde jets and consistent with strong subsidence creating a polar ‘eyewall’ and a polar atmosphere depleted of volatiles (e.g. Fletcher et al., 2008; Dyudina et al., 2009; Baines et al., 2009).