The domination of Saturn's low-latitude ionosphere by ring 'rain'

Cassini Exploration of the Planet Saturn: A Comprehensive Review

Before Cassini, scientists viewed Saturn's unique features only from Earth and from three spacecraft flying by. During more than a decade orbiting the gas giant, Cassini studied the planet from its interior to the top of the atmosphere. It observed the changing seasons, provided up-close observations of Saturn's exotic storms and jet streams, and heard Saturn's lightning, which cannot be detected from Earth. During the Grand Finale orbits, it dove through the gap between the planet and its rings and gathered valuable data on Saturn's interior structure and rotation. Key discoveries and events include: watching the eruption of a planet-encircling storm, which is a 20- or 30-year event, detection of gravity perturbations from winds 9000 km below the tops of the clouds, demonstration that eddies are supplying energy to the zonal jets, which are remarkably steady over the 25-year interval since the Voyager encounters, re-discovery of the north polar hexagon after 25 years, determination of elemental abundance ratios He/H, C/H, N/H, P/H, and As/H, which are clues to planet formation and evolution, characterization of the semiannual oscillation of the equatorial stratosphere, documentation of the mysteriously high temperatures of the thermosphere outside the auroral zone, and seeing the strange intermittency of lightning, which typically ceases to exist on the planet between outbursts every 1-2 years. These results and results from the Jupiter flyby are all discussed in this review.

Saturn's near-equatorial ionospheric conductivities from in situ measurements

Cassini's Grand Finale orbits provided for the first time in-situ measurements of Saturn's topside ionosphere. We present the Pedersen and Hall conductivities of the top near-equatorial dayside ionosphere, derived from the in-situ measurements by the Cassini Radio and Wave Plasma Science Langmuir Probe, the Ion and Neutral Mass Spectrometer and the fluxgate magnetometer. The Pedersen and Hall conductivities are constrained to at least 10^-5-10^-4 S/m at (or close to) the ionospheric peak, a factor 10-100 higher than estimated previously. We show that this is due to the presence of dusty plasma in the near-equatorial ionosphere. We also show the conductive ionospheric region to be extensive, with thickness of 300-800 km. Furthermore, our results suggest a temporal variation (decrease) of the plasma densities, mean ion masses and consequently the conductivities from orbit 288 to 292.

Cassini-Huygens' exploration of the Saturn system: 13 years of discovery

The Cassini-Huygens mission to Saturn provided a close-up study of the gas giant planet, as well as its rings, moons, and magnetosphere. The Cassini spacecraft arrived at Saturn in 2004, dropped the Huygens probe to study the atmosphere and surface of Saturn's planet-sized moon Titan, and orbited Saturn for the next 13 years. In 2017, when it was running low on fuel, Cassini was intentionally vaporized in Saturn's atmosphere to protect the ocean moons, Enceladus and Titan, where it had discovered habitats potentially suitable for life. Mission findings include Enceladus' south polar geysers, the source of Saturn's E ring; Titan's methane cycle, including rain that creates hydrocarbon lakes; dynamic rings containing ice, silicates, and organics; and Saturn's differential rotation. This Review discusses highlights of Cassini's investigations, including the mission's final year.

Local-time averaged maps of H3+ emission, temperature and ion winds

We present Keck-NIRSPEC observations of Saturn's [Formula: see text] aurora taken over a period of a month, in support of the Cassini mission's 'Grand Finale'. These observations produce two-dimensional maps of Saturn's [Formula: see text] temperature and ion winds for the first time. These maps show surprising complexity, with different morphologies seen in each night. The [Formula: see text] ion winds reveal multiple arcs of 0.5-1 km s^-1 ion flows inside the main auroral emission. Although these arcs of flow occur in different locations each night, they show intricate structures, including mirrored flows on the dawn and dusk of the planet. These flows do not match with the predicted flows from models of either axisymmetric currents driven by the Solar Wind or outer magnetosphere, or the planetary periodic currents associated with Saturn's variable rotation rate. The average of the ion wind flows across all the nights reveals a single narrow and focused approximately 0.3 km s^-1 flow on the dawn side and broader and more extensive 1-2 km s^-1 sub-corotation, spilt into multiple arcs, on the dusk side. The temperature maps reveal sharp gradients in ionospheric temperatures, varying between 300 and 600 K across the auroral region. These temperature changes are localized, resulting in hot and cold spots across the auroral region. These appear to be somewhat stable over several nights, but change significantly over longer periods. The position of these temperature extremes is not well organized by the planetary period and there is no evidence for a thermospheric driver of the planetary period current system. Since no past magnetospheric or thermospheric models explain the rich complexity observed here, these measurements represent a fantastic new resource, revealing the complexity of the interaction between Saturn's thermosphere, ionosphere and magnetosphere. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H₃⁺, H₅⁺ and beyond'.

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

Saturn's main rings are composed of >95% water ice, and the nature of the remaining few percent has remained unclear. The Cassini spacecraft's traversals between Saturn and its innermost D ring allowed its cosmic dust analyzer (CDA) to collect material released from the main rings and to characterize the ring material infall into Saturn. We report the direct in situ detection of material from Saturn's dense rings by the CDA impact mass spectrometer. Most detected grains are a few tens of nanometers in size and dynamically associated with the previously inferred "ring rain." Silicate and water-ice grains were identified, in proportions that vary with latitude. Silicate grains constitute up to 30% of infalling grains, a higher percentage than the bulk silicate content of the rings.

Chemical interactions between Saturn’s atmosphere and its rings

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.

In situ measurements of Saturn's ionosphere show that it is dynamic and interacts with the rings

The ionized upper layer of Saturn's atmosphere, its ionosphere, provides a closure of currents mediated by the magnetic field to other electrically charged regions (for example, rings) and hosts ion-molecule chemistry. In 2017, the Cassini spacecraft passed inside the planet's rings, allowing in situ measurements of the ionosphere. The Radio and Plasma Wave Science instrument detected a cold, dense, and dynamic ionosphere at Saturn that interacts with the rings. Plasma densities reached up to 1000 cubic centimeters, and electron temperatures were below 1160 kelvin near closest approach. The density varied between orbits by up to two orders of magnitude. Saturn's A- and B-rings cast a shadow on the planet that reduced ionization in the upper atmosphere, causing a north-south asymmetry.

The Great Cold Spot in Jupiter's upper atmosphere

Past observations and modeling of Jupiter's thermosphere have, due to their limited resolution, suggested that heat generated by the aurora near the poles results in a smooth thermal gradient away from these aurorae, indicating a quiescent and diffuse flow of energy within the subauroral thermosphere. Here we discuss Very Large Telescope-Cryogenic High-Resolution IR Echelle Spectrometer observations that reveal a small-scale localized cooling of ~200 K within the nonauroral thermosphere. Using Infrared Telescope Facility NSFCam images, this feature is revealed to be quasi-stable over at least a 15 year period, fixed in magnetic latitude and longitude. The size and shape of this "Great Cold Spot" vary significantly with time, strongly suggesting that it is produced by an aurorally generated weather system: the first direct evidence of a long-term thermospheric vortex in the solar system. We discuss the implications of this spot, comparing it with short-term temperature and density variations at Earth.