In this special issue of Science, Gas Giant Planets, six Research Articles report results of some of the Cassini spacecraft's final transmissions to Earth. Cassini's 20-year mission culminated in a series of wild orbits. First, it grazed the outer rim of Saturn's rings and then, in the final phase -- Cassini's Grand Finale -- the spacecraft dove through the narrow gap between the planet and its icy rings before plunging into and disintegrating into the planet's upper atmosphere. Each article in this collection provides new observations and insights into this previously unexplored region of the Saturn system.
In one Research Article, Jack Waite and colleagues used data from Cassini's "Grand Finale" passage to directly measure the composition of Saturn's atmosphere and of the material falling into the atmosphere from its rings. Exploring the chemical interaction between the two provides information about the chemical makeup of the rings, and about modifications to the planet's atmosphere from infalling material. Using Cassini's onboard Ion Neutral Mass Spectrometer to take in situ measurements from the region between the planet and rings, the authors discovered that water, methane, ammonia, carbon monoxide and/or molecular nitrogen and carbon dioxide regularly enter Saturn's atmosphere as influx from the D-ring (the innermost ring). Unexpectedly, the results indicate that chemically complex organic-rich material also falls into the planet's atmosphere, perhaps as larger nanoparticles, further modifying the composition and structure of the ionosphere and thermosphere. According to the authors, relatively recent disturbances to the D-ring, such as a passing comet, may have altered the rate of materials falling onto the planet. This variability in the transfer of chemicals could affect the observed carbon and oxygen content of the atmosphere over long timescales.
In another Research Article, Hsiang-Wen Hsu and colleagues determined the composition of dust particles falling from the rings into the planet. While Saturn's main rings are comprised of over 95% water ice, the materials that make up the rings' remainder have remained elusive. Cassini's inner passage allowed for the analysis of "ring rain," and identified both silicate and water ice grains up to tens of nanometers in size. According to Hsu et al., the percentage of silicate falling on the planet as dust is higher than the bulk silicate content of the rings. Falling dust grains were also the subject of an article by Donald Mitchell and colleagues. Mitchell et al. discuss the atmospheric physics that causes these grains to fall out of orbit and into Saturn's atmosphere.
Cassini's close passage also allowed for the study of Saturn's magnetic field, as explored in a Research Article by Michele Dougherty and colleagues. They measured the planet's internal and external magnetic fields and identified small-scale magnetic structures that together suggest a complex multi-layer dynamo process inside the planet. Elias Russos and colleagues present their results in yet another Research Article, which describes the discovery of an additional radiation belt trapped within the rings. According to Russos et al., strong proton depletions correlate with features on the D-ring suggesting that the dust environment near Saturn is diverse and highly structured. Finally, Laurent Lamy and colleagues present measurements taken as Cassini flew through regions of Saturn's magnetosphere that generate planetary auroral radio emissions, a characteristic of all the magnetized planets yet known. According to Lamy et al., understanding these radio emissions required in situ measurements from within their source region, as obtained here. The measurements could help in the hunt for exoplanets and other astronomical phenomena.