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During Cassini's final, spectacular months, in situ instruments made the first direct measurements of nanoparticles, finding an exceptionally large flow from the rings into Saturn's atmosphere. Cassini's Ion and Neutral Mass Spectrometer measured material in three altitude bands and found a global‐integrated flux of 2–20 × 104 kg/s that is dominated by hydrocarbon material <104u. Ranging from clusters of a few molecules to radii of several nanometers, nanoparticles are ubiquitous throughout Saturn's rings but embedded in the regolith of larger particles and not detectable as independent particles using remote observations. The smallest nanoparticles are susceptible to atmosphere drag by Saturn's tenuous exosphere that reaches the inner edge of the D ring. The unsustainable large flux suggests a recent disturbance of Saturn's inner ring material, possibly associated with the clumping that appeared in the D68 ringlet in 2015. Plain Language Summary: For 40 years, calculations based on remote observations indicated that Saturn's magnetic field carries ions and charged particles from the rings to the midlatitudes of Saturn. In Cassini's last few months of life, direct, in situ measurements found that 10 tons/s of molecules and particles smaller than two nanometers are streaming along the plane of the rings into Saturn's atmosphere by another process: atmospheric drag. Saturn's extended atmosphere reaches the inner edge of Saturn's rings and extracts neutral particles less than one thousandth the thickness of a human hair by slowing them down until they fall into Saturn. Surprisingly, the flux is a hundred times larger than past predictions, and at least half of the material is hydrocarbon, which comprises less than 5% of the water ice‐dominated rings. Cassini's data also show that the influx varies at least a factor of 4 and may be linked to clumps that appeared in 2015 on D68, the ringlet on the inner edge of the rings. These newly discovered particles and processes alter the evolutionary landscape of the rings and provide an exciting, rich field for future research aimed at understanding the origin and history of the rings. Key Points: Atmospheric drag, an interaction newly applied to Saturn and its rings, extracts >104 kg/s of neutral nanoparticles from Saturn's ringsMolecules and particles <2 nm in radius are preferentially transported by atmospheric dragNeutral, nanometer‐sized hydrocarbon material is currently the largest mass loss from Saturn's rings [ABSTRACT FROM AUTHOR]

The middle atmospheres of planets are driven by a combination of radiative heating and cooling, mean meridional motions, and vertically propagating waves (which originate in the deep troposphere). It is very difficult to model these effects and, therefore, observations are essential to advancing our understanding of atmospheres. The equatorial stratospheres of Earth and Jupiter oscillate quasi-periodically on timescales of about two and four years, respectively, driven by wave-induced momentum transport. On Venus and Titan, waves originating from surface–atmosphere interaction and inertial instability are thought to drive the atmosphere to rotate more rapidly than the surface (superrotation). However, the relevant wave modes have not yet been precisely identified. Here we report infrared observations showing that Saturn has an equatorial oscillation like those found on Earth and Jupiter, as well as a mid-latitude subsidence that may be associated with the equatorial motion. The latitudinal extent of Saturn’s oscillation shows that it obeys the same basic physics as do those on Earth and Jupiter. Future highly resolved observations of the temperature profile together with modelling of these three different atmospheres will allow us determine the wave mode, the wavelength and the wave amplitude that lead to middle atmosphere oscillation. [ABSTRACT FROM AUTHOR]

The New Horizons (NH) spacecraft observed Io's aurora in eclipse on four occasions during spring 2007. NH Alice ultraviolet spectroscopy and concurrent Hubble Space Telescope ultraviolet imaging in eclipse investigate the relative contribution of volcanoes to Io's atmosphere and its interaction with Jupiter's-magnetosphere. Auroral brightness and morphology variations after eclipse ingress and egress reveal changes in the relative contribution of sublimation and volcanic sources to the atmosphere. Brightnesses viewed at different geometries are best explained by a dramatic difference between the dayside and nightside atmospheric density. Far-ultraviolet aurora morphology reveals the influence of plumes on Io's electrodynamic interaction with Jupiter's magnetosphere. Comparisons to detailed simulations of to's aurora indicate that volcanoes supply 1 to 3% of the dayside atmosphere. [ABSTRACT FROM AUTHOR]

The Composite Infrared Spectrometer observed Jupiter in the thermal infrared during the swing-by of the Cassini spacecraft. Results include the detection of two new stratospheric species, the methyl radical and diacetylene, gaseous species present in the north and south auroral infrared hot spots; determination of the variations with latitude of acetylene and ethane, the latter a tracer of atmospheric motion; observations of unexpected spatial distributions of carbon dioxide and hydrogen cyanide, both considered to be products of comet Shoemaker-Levy 9 impacts; characterization of the morphology of the auroral infrared hot spot acetylene emission; and a new evaluation of the energetics of the northern auroral infrared hot spot. [ABSTRACT FROM AUTHOR]

The atmosphere of Jupiter's satellite Io is extremely tenuous, time variable and spatially heterogeneous. Only a few molecules -- SO[SUB2], SO and S[SUB2] -- have previously been identified as constituents of this atmosphere, and possible sources[SUP1-4] include frost sublimation, surface sputtering and active volcanism. Io has been known[SUP5,6] for almost 30 years to be surrounded by a cloud of Na, which requires an as yet unidentified atmospheric source of sodium. Sodium chloride has been recently proposed as an important atmospheric constituent, based on the detection of chlorine in Io's plasma torus[SUP7,8] and models of Io's volcanic gases[SUP9]. Here we report the detection of NaCl in Io's atmosphere; it constitutes only ∼0.3% when averaged over the entire disk, but is probably restricted to smaller regions than SO[SUB2] because of its rapid photolysis and surface condensation[SUP10]. Although the inferred abundance of NaCl in volcanic gases is lower than predicted[SUP9], those volcanic emissions provide an important source of Na and Cl in Io's neutral clouds and plasma torus. [ABSTRACT FROM AUTHOR]

A review is presented of the article "Pluto's interaction with its space environment: Solar wind, energetic particles, and dust" by F. Bagenal, F. Strobel, A. Young and others concerning data from the New Horizons space probe determining the interaction between Pluto's atmosphere and the surrounding space environment.

Jupiter's moon Europa has a predominantly water-ice surface that is modified by exposure to its space environment. Charged particles break molecular bonds in surface ice, thus dissociating the water to ultimately produce H 2 and O 2 , which provides a potential oxygenation mechanism for Europa's subsurface ocean. These species are understood to form Europa's primary atmospheric constituents. Although remote observations provide important global constraints on Europa's atmosphere, the molecular O 2 abundance has been inferred from atomic O emissions. Europa's atmospheric composition had never been directly sampled and model-derived oxygen production estimates ranged over several orders of magnitude. Here, we report direct observations of H 2 + and O 2 + pickup ions from the dissociation of Europa's water-ice surface and confirm these species are primary atmospheric constituents. In contrast to expectations, we find the H 2 neutral atmosphere is dominated by a non-thermal, escaping population. We find 12 ± 6 kg s -1 (2.2 ± 1.2 × 10 26  s -1 ) O 2 are produced within Europa's surface, less than previously thought, with a narrower range to support habitability in Europa's ocean. This process is found to be Europa's dominant exogenic surface erosion mechanism over meteoroid bombardment., Competing Interests: Competing interestsThe authors declare no competing interests., (© The Author(s) 2024.)

The Kuiper Belt is a distant region of the outer Solar System. On 1 January 2019, the New Horizons spacecraft flew close to (486958) 2014 MU 69 , a cold classical Kuiper Belt object approximately 30 kilometers in diameter. Such objects have never been substantially heated by the Sun and are therefore well preserved since their formation. We describe initial results from these encounter observations. MU 69 is a bilobed contact binary with a flattened shape, discrete geological units, and noticeable albedo heterogeneity. However, there is little surface color or compositional heterogeneity. No evidence for satellites, rings or other dust structures, a gas coma, or solar wind interactions was detected. MU 69 's origin appears consistent with pebble cloud collapse followed by a low-velocity merger of its two lobes., (Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

Saturn's moon Rhea had been considered massive enough to retain a thin, externally generated atmosphere capable of locally affecting Saturn's magnetosphere. The Cassini spacecraft's in situ observations reveal that energetic electrons are depleted in the moon's vicinity. The absence of a substantial exosphere implies that Rhea's magnetospheric interaction region, rather than being exclusively induced by sputtered gas and its products, likely contains solid material that can absorb magnetospheric particles. Combined observations from several instruments suggest that this material is in the form of grains and boulders up to several decimetres in size and orbits Rhea as an equatorial debris disk. Within this disk may reside denser, discrete rings or arcs of material.

Temperatures obtained from early Cassini infrared observations of Titan show a stratopause at an altitude of 310 kilometers (and 186 kelvin at 15 degrees S). Stratospheric temperatures are coldest in the winter northern hemisphere, with zonal winds reaching 160 meters per second. The concentrations of several stratospheric organic compounds are enhanced at mid- and high northern latitudes, and the strong zonal winds may inhibit mixing between these latitudes and the rest of Titan. Above the south pole, temperatures in the stratosphere are 4 to 5 kelvin cooler than at the equator. The stratospheric mole fractions of methane and carbon monoxide are (1.6 +/- 0.5) x 10(-2) and (4.5 +/- 1.5) x 10(-5), respectively.

Stratospheric temperatures on Saturn imply a strong decay of the equatorial winds with altitude. If the decrease in winds reported from recent Hubble Space Telescope images is not a temporal change, then the features tracked must have been at least 130 kilometers higher than in earlier studies. Saturn's south polar stratosphere is warmer than predicted from simple radiative models. The C/H ratio on Saturn is seven times solar, twice Jupiter's. Saturn's ring temperatures have radial variations down to the smallest scale resolved (100 kilometers). Diurnal surface temperature variations on Phoebe suggest a more porous regolith than on the jovian satellites.

Well-resolved far-ultraviolet spectroscopic images of O I, S I, and previously undetected H ILyman-alpha emission from Io were obtained with the Hubble space telescope imaging spectrograph (STIS). Detected O I and S I lines (1250 to 1500 angstroms) have bright equatorial spots (up to 2.5 kilorayleighs) that shift position with jovian magnetic field orientation; limb glow that is brighter on the hemisphere facing the jovian magnetic equator; and faint diffuse emission extending to approximately 20 Io radii. All O I and S I features brightened by approximately 50 percent in the last two images, concurrently with a ground-based observation of increased iogenic [O I] 6300-angstrom emission. The H ILyman-alpha emission, consisting of a small, approximately 2-kilorayleigh patch near each pole, has a different morphology and time variation.

Europa, the second large satellite out from Jupiter, is roughly the size of Earth's Moon, but unlike the Moon, it has water ice on its surface. There have been suggestions that an oxygen atmosphere should accumulate around such a body, through reactions which break up the water molecules and form molecular hydrogen and oxygen. The lighter H2 molecules would escape from Europa relatively easily, leaving behind an atmosphere rich in oxygen. Here we report the detection of atomic oxygen emission from Europa, which we interpret as being produced by the simultaneous dissociation and excitation of atmospheric O2 by electrons from Jupiter's magnetosphere. Europa's molecular oxygen atmosphere is very tenuous, with a surface pressure about 10(-11) that of the Earth's atmosphere at sea level.