Fate of Ice Grains in Saturn's Ionosphere

Abstract

It has been proposed that the rings of Saturn can contribute both material (i.e., water) and energy to its upper atmosphere and ionosphere. Ionospheric models require the presence of molecular species such as water that can chemically remove ionospheric protons, which otherwise are associated with electron densities that greatly exceed those from observation. These models adopt topside fluxes of water molecules. Other models have shown that ice grains from Saturn's rings can impact the atmosphere, but the effects of these grains have not been previously studied. In the current paper, we model how ice grains deposit both material and energy in Saturn's upper atmosphere as a function of grain size, initial velocity (at the “top” of the atmosphere, defined at an altitude above the cloud tops of 3,000 km), and incident angle. Typical grain speeds are expected to be roughly 15–25 km/s. Grains with radii on the order of 1–10 nm deposit most of their energy in the altitude range of 1,700–1,900 km, and can vaporize, depending on initial velocity and impact angle, contributing water mass to the upper atmosphere. We show that grains in this radius range do not significantly vaporize in our model at initial velocities lower than about 20 km/s.

In situ collection of dust grains falling from Saturn’s rings into its atmosphere

“…This estimate is more than an order of magnitude higher than the 3 to 20 kg s −1 of water required to explain the observed H þ 3 emission (17). Nevertheless, given the uncertainties in the processes related to the ablation and mass deposition [e.g., (45)], our results agree reasonably well with the ring rain mass infall estimation, implying that the observed nanograin population is the cause of the ring rain effect (19,21,22).…”

“…These additional neutrals reduce the local electron density, leading to a local accumulation of H þ 3 . Models of the atmospheric capture of 10-nm ice particles predict their mass deposition near an altitude of 2000 km above the level of 1-bar pressure in Saturn's atmosphere (45). The deposited nanograin material would then diffuse downward and interact with the ionosphere, prolonging the lifetime of H þ 3 (46,47).…”

Saturn's Dusty Ionosphere

“…The precipitating nanometer‐sized ice dust of the “ring rain” chemically interact with the upper ionosphere create the layer of H + and H 3 + , further undergo sublimation and fragmentation when they impact the deeper atmospheric layers. Grains on the order of 1–10 nm deposit their energy in the altitude range of 1,700–1,900 km, and the accumulation of such particles may form a grain‐rich layer in the atmosphere (Hamil et al, ). The observations in this study indicated that dusty plasma effect that are observed in the terrestrial mesosphere can exist permanently in the equator region of Saturn's upper atmosphere.…”

“…High concentration of negatively charged dust may cause electron acceleration within their potential wells (Farrell et al, ). Energy deposition may also be the result of a nanometer‐sized particle from the D ring (Hamil et al, ; Mitchell et al, ), where at low altitudes the nanometer‐sized grains impact the atmosphere, break apart and sublimate, slow down, and accumulate near the ionospheric peak. Last, the dust grains are illuminated by the solar EUV radiation and subject to photoelectron emissions with energies of about 2 eV.…”

The Ion Composition of Saturn's Equatorial Ionosphere as Observed by Cassini

“…But given that altitude and latitude are at least partially confounded variables for the proximal orbits, one could also state that species R is more highly confined to the equatorial region than M. Perhaps the source of R is heavy neutrals generated by ablation of grains entering the atmosphere from the rings (Hamil et al, 2018; But given that altitude and latitude are at least partially confounded variables for the proximal orbits, one could also state that species R is more highly confined to the equatorial region than M. Perhaps the source of R is heavy neutrals generated by ablation of grains entering the atmosphere from the rings (Hamil et al, 2018; …”