Results from the Cassini Grand Finale at Saturn using Cassini INMS, UVIS, CIRS, and RSS
Dr Hunter Waite, Southwest Research Institute, USA
Waite, Perryman, Miller, Bell, Koskinen, Guerlet, Hubbard, Glein, and Stevenson
The Grand Finale phase of the Cassini-Huygens mission was completed in September of 2017. Both the Saturn bulk atmosphere (H2 and He) and infalling material from the rings were measured. The most surprising result was the amount of methane, carbon dioxide, carbon monoxide, molecular nitrogen, water, ammonia and organic compounds falling from the rings into the atmosphere at a rate of over 10,000 kg s-1. This rate of infall over the lifetime of the rings can have significant observable effects on the observed composition of the atmosphere and ionosphere. This will be discussed in the presentation.
The He/H2 ratio in the well mixed atmospheres of Jupiter and Saturn serves as an important parameter for assessing formation and evolution models of the giant planets. The smaller size of Saturn relative to Jupiter provides different predictions for He rainout and in turn internal heat sources. The Galileo Probe used two independent techniques to determine the He/H2 ratio at Jupiter. However, determining the He/H2 ratio at Saturn has proven to be more elusive with values in the well mixed atmosphere ranging from highly depleted He to solar He abundance. This talk will concentrate on recent efforts to put together INMS mass spectrometry measurements of He/H2 in the upper atmosphere during the Cassini Grand Finale with infrared (CIRS) and ultraviolet (UVIS) observations in the well mixed atmosphere to provide a consistent pressure temperature profile that can be stitched together with the help of a new shape model to provide a consistent picture of the He/H2 ratio throughout the atmosphere.
The magnetic fields of the Giant Planets: more differences than similarities
Professor Jeremy Bloxham FRS, Harvard, USA
Jupiter and Saturn are ostensibly similar as are Uranus and Neptune, yet their magnetic fields differ considerably. In particular, the magnetic fields of Jupiter and Saturn, as recently revealed in detail by the Juno and Cassini spacecraft, are quite dissimilar, suggesting that their magnetic fields are sensitive markers of the interior dynamics of these planets. We examine recent magnetic field observations from the Juno spacecraft, which is currently in a polar orbit around Jupiter. From the first phase of the Juno mission we find a magnetic field that is quite unlike any other: the field in Jupiter’s northern hemisphere is non-dipolar, with flux concentrated in a single band at mid-latitudes; in the southern hemisphere the field is nearly dipolar. In addition, we see a single, isolated intense flux spot at the equator. We consider possible explanations for this field morphology in terms of the interior of Jupiter, and contrast its magnetic field with that of Saturn.
Giant planet formation
Professor Ravit Helled, University of Zurich, Switzerland
Giant planets are thought to have cores in their deep interiors, and the division into a heavy-element core and hydrogen-helium envelope is used in both formation and structure models.
Proffesr Helled will briefly discuss the standard model for giant planet formation, and will show that the primordial internal structure of giant planets depends on their formation location and growth history. She will present a formation scenario for Jupiter that is consistent with cosmochemical constraints, and discuss the expected primordial internal structure of Jupiter from recent formation and evolution models (fuzzy core, inhomogeneous interior, planetesimal/pebble accretion). Professor Helled will also discuss mechanisms for heavy-element enrichments, and the challenges linked to enriched outer envelopes. Finally, she will discuss the importance of these theoretical results for interpreting the measurements of the Juno and Cassini missions.
Lessons from Juno & Cassini: linking atmosphere and interior of Jupiter and Saturn
Professor Tristan Guillot, Observatoire de la Côte d'Azur, France
In orbit since July 2016, Juno is changing the way we see Jupiter but also the other giant planets. The measurements of the gravity field of the planet, two orders of magnitude better than previous measurements (Folkner et al. 2017, Iess et al. 2018) have allowed to probe the deep interior in several ways. First it allowed for the first time to constrain the depth of the planet’s zonal jets to about 3000km below the clouds (Kaspi et al. 2018, Guillot et al. 2018). Second, it led to new interior models including the presence of a dilute core (Wahl et al. 2017) and a puzzling, still unsolved interior structure (Debras & Chabrier 2019). In parallel, similar measurements during the Cassini Grande Finale orbits led to a constraint on the depth of Saturn’s zonal flow, about 9000km (Iess et al. 2019, Galanti et al. 2019), in agreement with the Juno results for Jupiter. This tells us that differential rotation in the interior is suppressed where hydrogen becomes conductive and is dragged by the giant planet's powerful magnetic fields into a nearly-uniform rotation. Similarly, the complex interior structure of Jupiter is to be related to the likely presence of a deep extended stable region in Saturn which is required to explain the planets’ oscillations (Fuller 2014).
Adding to this complexity, measurements of Jupiter’s deep atmosphere by Juno’s microwave radiometer (Janssen et al. 2017) show that ammonia, but probably also temperature are not as uniform as one expected, down to pressures of tens of bars (Li et al. 2017). I will show that this may be explained by the interaction of water storms with ammonia, leading to a non-uniform and intrinsically variable distribution of abundances and temperatures both vertically and latitudinally. This appears to be a feature of hydrogen-dominated atmospheres, both due to the absence of a surface and to the fact that contrary to the Earth, condensates are much heavier than the surrounding air (see Guillot 1995, Li & Ingersoll 2015). The implications could be far-reaching for the understanding of giant planets dynamics and interiors.