Chemical interactions between Saturn’s atmosphere and its rings

Abstract

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.

Saturn's Ionosphere: Electron Density Altitude Profiles and D‐Ring Interaction From The Cassini Grand Finale

“…The southern hemisphere (Figure b) is observed to be much more variable than the northern one regardless of the magnetic L ‐shell values and is shown to be connected to magnetic L ‐shells that cross the D‐ring ( L ≥1.11 R s ). Moreover, since at those L values, the ionosphere is mainly dominated by H + (Wahlund et al, ); it implies the dominance of electrodynamical processes via flux tubes, which traverse the electrically conductive D‐ring over chemical processes (Waite et al, ). In fact, using the RPWS radio measurements, Sulaiman, Kurth, Hospodarsky, et al () show the presence of clear signatures of whistler waves (classified as auroral hiss and very low frequency saucers) in the southern hemisphere at similar altitudes, directly linked to the planet on field lines connected to the D‐ring, hence suggesting as well an electrodynamic coupling with the ring.…”

“…Assuming the dominance of H + and T i ≈ T e ≈1,160K (Wahlund et al, ), we obtain H theo ≈1,917km. This is significantly larger than the values of 840 and 260 km derived in regions D and C , respectively, possibly related to the thermal equilibrium assumption we have made ( T i ≈ T e ) and/or hinting toward a significant component of molecular ions and a mean positive ion mass, in region D closer to 2–3 amu (mainly H + and ) and in region C 5–10 amu (Morooka et al, ; Waite et al, ).…”

Currents Associated With Saturn's Intra‐D Ring Azimuthal Field Perturbations

“…The physical reason for the variability thus remains to be elucidated. However, the evidence points toward a significant role for variability in thermospheric winds, likely in combination with ionospheric conductivities (Hadid, Morooka, Wahlund, Moore, et al, ,Hadid et al, ; Wahlund et al, ), and possibly influenced by the variable inflow of ~10 4 kg/s of volatiles and dust into equatorial atmosphere (Hsu et al, ; Mitchell et al, ; Perry et al, ; Waite et al, ). Significant nonconjugacy in the field and current profiles is also often present on smaller spatial scales.…”

Variability of Intra–D Ring Azimuthal Magnetic Field Profiles Observed on Cassini's Proximal Periapsis Passes

“…In situ Cassini measurements on the proximal passes indeed show the presence of strong variability in the upper equatorial ionospheric plasma from pass to pass, indicative of significant dynamical interactions with D ring material as well as ring shadow effects (Hadid et al, ; Wahlund et al, ). Observations on these passes also demonstrate the influx of ~10 4 kg s −1 of volatiles and dust into the equatorial atmosphere, captured from the inner D ring by atmospheric drag (Hsu et al, ; Mitchell et al, ; Perry et al, ; Waite et al, ). Tentative evidence for rotational modulation of such influx due to azimuthally structured components of the D ring (e.g., Hedman et al, ), specifically from the D68 ringlet lying at a radial distance of ~67,600 km (~1.122 R S ), was also presented by Waite et al ().…”

“…Observations on these passes also demonstrate the influx of ~10 4 kg s −1 of volatiles and dust into the equatorial atmosphere, captured from the inner D ring by atmospheric drag (Hsu et al, ; Mitchell et al, ; Perry et al, ; Waite et al, ). Tentative evidence for rotational modulation of such influx due to azimuthally structured components of the D ring (e.g., Hedman et al, ), specifically from the D68 ringlet lying at a radial distance of ~67,600 km (~1.122 R S ), was also presented by Waite et al (). Considering that such time dependence might conceivably relate to the variable intra–D ring fields discussed here, we have also examined whether the field signatures are organized in a phase system rotating with the D68 ringlet at a rate of 1751.7° per day (equivalent to the Kepler orbit period of 4.932 hr).…”