Benjamin D. Teolis

Cassini Finds an Oxygen–Carbon Dioxide Atmosphere at Saturn’s Icy Moon Rhea

Extraterrestrial Atmosphere The detection of oxygen in the atmospheres of Jupiters icy moons, Europa and Ganymede, and the presence of this gas as the main constituent of the atmosphere that surrounds Saturns rings, has suggested the possibility of oxygen atmospheres around the icy moons that orbit inside Saturns magnetosphere. Using the Ion Neutral Mass Spectrometer onboard the Cassini spacecraft, Teolis et al. (p. 1813, published online 25 November; see the Perspective by Cruikshank) report the detection of a very tenuous oxygen and carbon dioxide atmosphere around Saturns icy moon Rhea. As with other icy satellites, this atmosphere is maintained through the dissociation of surface molecules and ejection into the atmosphere as a result of Saturns magnetospheric radiation. Rhea’s atmosphere is maintained by chemical decomposition of surface water ice under irradiation from Saturn’s magnetosphere. The flyby measurements of the Cassini spacecraft at Saturn’s moon Rhea reveal a tenuous oxygen (O2)–carbon dioxide (CO2) atmosphere. The atmosphere appears to be sustained by chemical decomposition of the surface water ice under irradiation from Saturn’s magnetospheric plasma. This in situ detection of an oxidizing atmosphere is consistent with remote observations of other icy bodies, such as Jupiter’s moons Europa and Ganymede, and suggestive of a reservoir of radiolytic O2 locked within Rhea’s ice. The presence of CO2 suggests radiolysis reactions between surface oxidants and organics or sputtering and/or outgassing of CO2 endogenic to Rhea’s ice. Observations of outflowing positive and negative ions give evidence for pickup ionization as a major atmospheric loss mechanism.

THE 12C/13C RATIO ON TITAN FROM CASSINI INMS MEASUREMENTS AND IMPLICATIONS FOR THE EVOLUTION OF METHANE

We have re-evaluated the Cassini Ion Neutral Mass Spectrometer (INMS) 12 C/ 13 C ratios in the upper atmosphere of Titan based on new calibration sensitivities and an improved model for the NH3 background in the 13 CH4 mass channel. The INMS measurements extrapolated to the surface give a 12 C/ 13 Ci n CH4 of 88.5 ± 1.4. We compare the results to a revised ratio of 91.1 ± 1.4 provided by the Huygens Gas Chromatograph Mass Spectrometer and 86.5 ± 7.9 provided by the Cassini Infrared Spectrometer and determine implications of the revised ratios for the evolution of methane in Titan’s atmosphere. Because the measured 12 C/ 13 C is within the probable range of primordial values, we can only determine an upper boundary for the length of time since methane began outgassing from the interior, assuming that outgassing of methane (e.g., cryovolcanic activity) has been continuous ever since. We find that three factors play a crucial role in this timescale: (1) the escape rate of methane, (2) the difference between the current and initial ratios and the rate of methane, and (3) production or resupply due to cryovolcanic activity. We estimate an upper limit for the outgassing timescale of 470 Myr. This duration can be extended to 940 Myr if production rates are large enough to counteract the fractionation due to escape and photochemistry. There is no lower limit to the timescale because the current ratios are within the range of possible primordial values.

Surface current balance and thermoelectric whistler wings at airless astrophysical bodies: Cassini at Rhea

Sharp magnetic perturbations found by the Cassini spacecraft at the edge of the Rhea flux tube are consistent with field-aligned flux tube currents. The current system results from the difference of ion and electron gyroradii and the requirement to balance currents on the sharp Rhea surface. Differential-type hybrid codes that solve for ion velocity and magnetic field have an intrinsic difficulty modeling the plasma absorbers sharp surface. We overcome this problem by instead using integral equations to solve for ion and electron currents and obtain agreement with the magnetic perturbations at Rheas flux tube edge. An analysis of the plasma dispersion relations and Cassini data reveals that field-guided whistler waves initiated by (1) the electron velocity anisotropy in the flux tube and (2) interaction with surface sheath electrostatic waves on topographic scales may facilitate propagation of the current system to large distances from Rhea. Current systems like those at Rhea should occur generally, for plasma absorbers of any size such as spacecraft or planetary bodies, in a wide range of space plasma environments. Motion through the plasma is not essential since the current system is thermodynamic in origin, excited by heat flow into the object. The requirements are a difference of ion and electron gyroradii and a sharp surface, i.e., without a significant thick atmosphere. Key Points Surface current balance condition yields a current system at astronomical bodies Current system possible for sharp (airless) objects of any size Current system is thermoelectric and motion through the plasma nonessential

Chemical interactions between Saturn’s atmosphere and its rings

Cassinis final phase of exploration The Cassini spacecraft spent 13 years orbiting Saturn; as it ran low on fuel, the trajectory was changed to sample regions it had not yet visited. A series of orbits close to the rings was followed by a Grand Finale orbit, which took the spacecraft through the gap between Saturn and its rings before the spacecraft was destroyed when it entered the planets upper atmosphere. Six papers in this issue report results from these final phases of the Cassini mission. Dougherty et al. measured the magnetic field close to Saturn, which implies a complex multilayer dynamo process inside the planet. Roussos et al. detected an additional radiation belt trapped within the rings, sustained by the radioactive decay of free neutrons. Lamy et al. present plasma measurements taken as Cassini flew through regions emitting kilometric radiation, connected to the planets aurorae. Hsu et al. determined the composition of large, solid dust particles falling from the rings into the planet, whereas Mitchell et al. investigated the smaller dust nanograins and show how they interact with the planets upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturns atmosphere. Science, this issue p. eaat5434, p. eaat1962, p. eaat2027, p. eaat3185, p. eaat2236, p. eaat2382 INTRODUCTION Past remote observations of Saturn by Pioneer 11, Voyager 1 and 2, Earth-based observatories, and the Cassini prime and solstice missions suggested an inflow of water from the rings to the atmosphere. This would modify the chemistry of Saturn’s upper atmosphere and ionosphere. In situ observations during the Cassini Grand Finale provided an opportunity to study this chemical interaction. RATIONALE The Cassini Grand Finale consisted of 22 orbital revolutions (revs), with the closest approach to Saturn between the inner D ring and the equatorial atmosphere. The Cassini Ion Neutral Mass Spectrometer (INMS) measured the composition of Saturn’s upper atmosphere and its chemical interactions with material originating in the rings. RESULTS Molecular hydrogen was the most abundant constituent at all altitudes sampled. Analysis of the atmospheric structure of H2 indicates a scale height with a temperature of 340 ± 20 K below 4000 km, at the altitudes and near-equatorial latitudes sampled by INMS. Water infall from the rings was observed, along with substantial amounts of methane, ammonia, molecular nitrogen, carbon monoxide, carbon dioxide, and impact fragments of organic nanoparticles. The infalling mass flux was calculated to be between 4800 and 45,000 kg s−1 in a latitude band of 8° near the equator. The interpretation of this spectrum is complicated by the Cassini spacecraft’s high velocity of 31 km s−1 relative to Saturn’s atmosphere. At this speed, molecules and particles have 5 eV per nucleon of energy and could have fragmented upon impact within the INMS antechamber of the closed ion source. As a result, the many organic compounds detected by INMS are very likely fragments of larger nanoparticles. Evidence from INMS indicates the presence of molecular volatiles and organic fragments in the infalling material. Methane, carbon monoxide, and nitrogen make up the volatile inflow, whereas ammonia, water, carbon dioxide, and organic compound fragments are attributed to fragmentation inside the instrument’s antechamber of icy, organic-rich grains. The observations also show evidence for orbit-to-orbit variations in the mixing ratios of infalling material; this suggests that the source region of the material is temporally and/or longitudinally variable, possibly corresponding to localized source regions in the D ring. CONCLUSION The large mass of infalling material has implications for ring evolution, likely requiring transfer of material from the C ring to the D ring in a repeatable manner. The infalling material can affect the atmospheric chemistry and the carbon content of Saturn’s ionosphere and atmosphere. INMS mass spectra from the Grand Finale. The graphic depicts the Cassini spacecraft as it passes from north to south between Saturn and its rings. The inset spectrum shows the mass deconvolution of compounds measured by INMS on rev 290. The x axis is in units of mass per charge (u) and extends over the full mass range of INMS (1 to 99 u). The y axis is in counts per measurement cycle integrated over the closest-approach data. The mass influx rate for rev 290, derived from mass deconvolution of the rev-integrated spectrum, is shown as embedded text in the spectrum. The side panel gives the average of the mass deconvolution of revs 290, 291, and 292 in mass density units (g cm–3). The composition of the ring-derived compounds in terms of percentage mass density is also shown. IMAGE COURTESY OF NASA/JPL-CALTECH/SWRI 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.

Cassini CAPS Identification of Pickup Ion Compositions at Rhea

Saturns largest icy moon, Rhea, hosts a tenuous surface-sputtered exosphere composed primarily of molecular oxygen and carbon dioxide. In this Letter, we examine Cassini Plasma Spectrometer velocity space distributions near Rhea and confirm that Cassini detected nongyrotropic fluxes of outflowing CO+_2 during both the R1 and R1.5 encounters. Accounting for this nongyrotropy, we show that these possess comparable along-track densities of ∼2 × 10^(−3) cm^(−3). Negatively charged pickup ions, also detected during R1, are surprisingly shown as consistent with mass 26 ± 3 u which we suggest are carbon-based compounds, such as CN−, C_2H-, C-_2, or HCO−, sputtered from carbonaceous material on the moons surface. The negative ions are calculated to possess along-track densities of ∼5 × 10^(−4) cm^(−3) and are suggested to derive from exogenic compounds, a finding consistent with the existence of Rheas dynamic CO_2 exosphere and surprisingly low O_2 sputtering yields. These pickup ions provide important context for understanding the exospheric and surface ice composition of Rhea and of other icy moons which exhibit similar characteristics.