The Pioneer and Voyager spacecraft made close-up measurements of Saturn’s ionosphere and upper atmosphere in the 1970s and 1980s that suggested a chemical interaction between the rings and atmosphere. Exploring this interaction provides information on ring composition and the influence on Saturn’s atmosphere from infalling material. The Cassini Ion Neutral Mass Spectrometer sampled in situ the region between the D ring and Saturn during the spacecraft’s Grand Finale phase. We used these measurements to characterize the atmospheric structure and material influx from the rings. The atmospheric He/H2 ratio is 10 to 16%. Volatile compounds from the rings (methane; carbon monoxide and/or molecular nitrogen), as well as larger organic-bearing grains, are flowing inward at a rate of 4800 to 45,000 kilograms per second.

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Thermally-driven atmospheric escape evolves from an organized outflow (hydrodynamic escape) to escape on a molecule by molecules basis (Jeans escape) with increasing Jeans parameter, the ratio of the gravitational to thermal energy of molecules in a planet's atmosphere.This transition is described here using the direct simulation Monte Carlo method for a single component spherically symmetric atmosphere. When the heating is predominantly below the lower boundary of the simulation region, R 0 , and well below the exobase, this transition is shown to occur over a surprisingly narrow range of Jeans parameters evaluated at R 0 : λ 0 ~ 2-3. The Jeans parameter λ 0 ~ 2.1 roughly corresponds to the upper limit for isentropic, supersonic outflow and for λ 0 >3 escape occurs on a molecule by molecule basis. For λ 0 > ~6, it is shown that the escape rate does not deviate significantly from the familiar Jeans rate evaluated at the nominal exobase, contrary to what has been suggested. Scaling by the Jeans parameter and the Knudsen number, escape calculations for Pluto and an early Earth's atmosphere are evaluated, and the results presented here can be applied to thermally-induced escape from a number of solar and extrasolar planetary bodies.

On 14 July 2005, Cassini passed within 175 km of Enceladus’ surface enabling a direct in situ measurement of water escaping from the surface by the Ion and Neutral Mass Spectrometer (INMS) and the observation of a stellar occultation by the Ultraviolet Spectrometer (UVIS). We have developed a three‐dimensional, Monte Carlo neutral model to simultaneously model the INMS and UVIS measurements of water gas density and column density, respectively. The data are consistent with a two‐component atmosphere; the first with a weak, distributed source on the surface which, if global, has a source rate of ∼8 × 1025 H2O/s, and the second with a much larger source localized at the south pole with a source rate ∼1028 H2O/s. This latter source is possibly coincident with the “tiger stripe” series of fractures revealed by the Imaging Science Subsystem instrument where the ice was measured to be warmer than the surrounding regions by the Composite Infrared Spectrometer instrument. We estimate the plasma mass loading rate due to interaction between the plume and magnetospheric plasma is 2–3 kg/s for a plume source of 1028 H2O/s. Pickup of water group ions in the plume slows down the plasma to ∼10 km/s relative to Enceladus in the region of, and downstream of, the south polar plume. This is consistent with the mass loading rate inferred from magnetic field perturbations detected during the Cassini flyby and suggests an additional source may be needed to explain the plasma flow deflections detected by the Cassini Plasma Spectrometer.

9 Phone Number: 1-434-924-3244 10 11 problems. Finally, the recent discovery of CO at high altitudes, the 35 effect of Charon and the conditions at the New Horizon encounter 36 are briefly considered. 37

The solar wind implants protons into the top 20–30 nm of lunar regolith grains, and the implanted hydrogen will diffuse out of the regolith but also interact with oxygen in the regolith oxides. We apply a statistical approach to estimate the diffusion of hydrogen in the regolith hindered by forming temporary bonds with regolith oxygen atoms. A Monte Carlo simulation was used to track the temporal evolution of bound OH surface content and the H2 exosphere. The model results are consistent with the interpretation of the Chandrayaan‐1 M3 observations of infrared absorption spectra by surface hydroxyls as discussed in Li and Milliken (2017, https://doi.org/10.1126/sciadv.1701471). The model reproduced the latitudinal concentration of OH by using a Gaussian energy distribution of f(U0 = 0.5 eV, UW = 0.078–0.1 eV) to characterize the activation energy barrier to the diffusion of hydrogen in space weathered regolith. In addition, the model results of the exospheric content of H2 are consistent with observations by the Lyman Alpha Mapping Project on the Lunar Reconnaissance Orbiter. Therefore, we provide support for hydroxyl formation by chemically trapped solar wind protons.

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