Journal of Geophysical Research: Space Physics
Fate of Ice Grains in Saturn’s Ionosphere
, N. L. Reedy
, and S. Sakai
Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA,
Gateway STEM High School, St. Louis,
Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Tokyo, Japan
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.
A linkage between Saturn’s rings and upper atmosphere has long been suspected (Nagy et al., 2009).
Connerney and Waite (1984) invoked a ﬂux of water from the rings into the ionosphere in order to recon-
cile modeled electron densities with radio occultation measurements. Ionospheric
densities, and hence
the electron densities, in theoretical models are much too high unless water-related species are present that
ions producing chemically short-lived water ions. More recently, the importance of ionospheric
water chemistry has been conﬁrmed by Moore et al. (2015). The water ﬂux into the low-latitude ionosphere
required by the ionospheric models is about
F = 10
. Evidence for water group species
was found using infrared observations of
(see O’Donoghue, Stallard, Melin, Cowley,
et al., 2013). These Earth-based observations showed latitudinal structures that apparently connect with
Another issue, possibly relevant to ring-ionosphere linkage, is the upper atmosphere energy “crisis.” Stellar
occultation measurements (Koskinen et al., 2015) have shown that the thermospheric temperature at low to
middle latitudes at Saturn is about 400 K, yet both one-dimensional models and three-dimensional global
thermospheric general circulation models produce temperatures less than 200 K (Müller-Wodarg et al., 2006).
Explanations for the discrepancy could include gravity wave heating (Nagy et al., 2009), dynamical pro-
cesses not accounted for in the models, or ring-atmospheric coupling. For example, if the ring ﬂux into the
atmosphere was high enough, with enough energy, perhaps this could supply suﬃcient heat to remedy
In addition to water group molecules and ions, grains from the rings (produced, e.g., by micrometeoroid
sputtering) could also travel to Saturn carrying material and energy. Grains and dust associated with the rings
are expected to be mainly water ice. Ip et al. (2016) undertook test particle calculations of grain trajectories
and showed that nanograins become electrically charged and can travel to the atmosphere with the details
depending on where and how their energy and material are deposited in the atmosphere. The fate of such
ice grains, as they impact the upper atmosphere and deposit energy and/or water mass, is the topic of the
The predicted inﬂux of water grains into the Saturn atmosphere can have two eﬀects: friction from in-falling
particles can deposit heat energy into the environment, and the particles can deposit water mass at varying
altitudes as they vaporize from frictional heat.
• Nanosized ice grains impacting
Saturn’s upper atmosphere deposit
energy and/or water mass which may
account for observation
• We model the fate of in-falling ice
in the upper atmosphere of Saturn,
tracking the deposited energy/mass
from individual grains
• With Cassini data, we can potentially
determine the possible impact of
“ring rain” on the energy and water
mass discrepancy at Saturn
• Supporting Information S1
Hamil, O., Cravens, T. E., Reedy, N. L.,
& Sakai, S. (2018). Fate of ice grains
in Saturn’s ionosphere. Journal of
Geophysical Research: Space
Physics, 123, 1429–1440.
Received 21 JUL 2017
Accepted 22 DEC 2017
Accepted article online 25 JAN 2018
Published online 8 FEB 2018
©2018. American Geophysical Union.
All Rights Reserved.
HAMIL ET AL.