S.-Y. Ye

Characteristics of ice grains in the Enceladus plume from Cassini observations

Ice grains in the Enceladus plume have been observed by several Cassini instruments during many Enceladus encounters. In this paper we study the ice grains ranging from less than one nanometer to micrometers in size based on multiple instrument observations. We have analyzed the nanograin data from the E17 and E18 Cassini Plasma Spectrometer (CAPS) energy spectra using the approach of Hill et al. (2012) and studied the charging of the grains using the E3–E6 Radio and Plasma Wave Spectrometer (RPWS)-Langmuir Probe (LP) data presented by Morooka et al. (2011). To bridge the gap between CAPS nanograin observations and Cosmic Dust Analyzer and RPWS micrograin observations, we propose a composite size distribution and fit it to the E3, E5, E17, and E18 CAPS and RPWS data. The resulting size distribution peaks at ~2 nm and provides a total grain mass density ~20% that of the water vapor measured by Ion and Neutral Mass Spectrometer at the densest part of the plume and a total grain charge density much smaller than that inferred from the RPWS-LP plasma data. Charge balance with the RPWS-LP plasma data would require many more grains than provided by the best fit composite distribution to the CAPS and RPWS data and would provide a grain mass density comparable to that of the water vapor. On the basis of these results, we study the subsequent motion of the ice grains and estimate the resulting grain current (~104 to ~105A) and grain mass production rate (~15–65 kg/s).

Properties of dust particles near Saturn inferred from voltage pulses induced by dust impacts on Cassini spacecraft

The Cassini Radio and Plasma Wave Science (RPWS) instrument can detect dust particles when voltage pulses induced by the dust impacts are observed in the wideband receiver. The size of the voltage pulse is proportional to the mass of the impacting dust particle. For the first time, the dust impacts signals measured by dipole and monopole electric antennas are compared, from which the effective impact area of the spacecraft is estimated to be 4 m2. In the monopole mode, the polarity of the dust impact signal is determined by the spacecraft potential and the location of the impact (on the spacecraft body or the antenna), which can be used to statistically infer the charge state of the spacecraft. It is shown that the differential number density of the dust particles near Saturn can be characterized as a power law dn/dr ∝ rμ, where μ ~ − 4 and r is the particle size. No peak is observed in the size distribution, contrary to the narrow size distribution found by previous studies. The RPWS cumulative dust density is compared with the Cosmic Dust Analyzer High Rate Detector measurement. The differences between the two instruments are within the range of uncertainty estimated for RPWS measurement. The RPWS onboard dust recorder and counter data are used to map the dust density and spacecraft charging state within Saturns magnetosphere.

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

Cassini enters Saturns ionosphere The upper reaches of most planetary atmospheres contain a layer that is ionized by incoming solar radiation—the ionosphere. As it went through its final orbits around Saturn, the Cassini spacecraft dipped close enough to the planet to pass directly through the ionosphere. Wahlund et al. examined the plasma data collected in situ and found that Saturns ionosphere is highly variable and interacts with the planets inner ring. They also observed decreases in ionization within regions shaded from the Sun by the rings. Science, this issue p. 66 The Cassini spacecraft has flown through Saturn’s ionosphere, which is highly variable and affected by the planet’s 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.

Rotational modulation of Saturn's radio emissions after equinox

Saturn kilometric radiation (SKR), narrowband emission, and auroral hiss are periodically modulated due to Saturns rotation, and the periods were found to vary with time. We analyze Cassini observations of Saturns radio emissions with the main focus on the four years 2012–2015. It is shown that the rotation rates of SKR north and south were different since mid-2012 with SKR north being faster until autumn 2013, followed by a 1 year interval of similar north and south rotation rates and phases, before the northern SKR component finally became slower than the southern SKR in late 2014. The dual rotation rates of 5 kHz narrowband emissions reappeared for slightly longer than 1 year after a long break since equinox. Auroral hiss provides an unambiguous way of tracking the rotation signals from each hemisphere because the whistler mode waves cannot cross the equator. Rotation rates of auroral hiss and narrowband emissions are consistent with each other and those of SKR when they are observed at high latitudes in early 2013. The phase difference between SKR and auroral hiss and the intensity of auroral hiss are local time dependent.

Intense Harmonic Emissions Observed in Saturn's Ionosphere

The Cassini spacecrafts first Grand Finale orbit was carried out in April 2017. This set of 22 orbits had an inclination of 63 degrees with a periapsis grazing Saturns ionosphere, thus providing ...

Dust detection in space using the monopole and dipole electric field antennas

During the grand finale of the Cassini mission, the Radio and Plasma Wave Science instrument will be used to assess the risk involved in exposing the instruments to the dusty environment around the F and D rings. More specifically, the slope of the size distribution and the dust density will be determined based on the signals induced on the electric antennas by dust impacts. To reduce the uncertainties in the generation mechanism of the dust impact signals and the resulting dust properties based on the interpretation of data, we designed and carried out experiments in late 2015, when we switched antenna mode from monopole to dipole at the ring plane crossings. Comparison of the data collected with these two antenna setups provides valuable hints on how the dust impact signals are generated in each antennamode.

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

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 During the Cassini spacecraft’s Grand Finale mission in 2017, it performed 22 traversals of the 2000-km-wide region between Saturn and its innermost D ring. During these traversals, the onboard cosmic dust analyzer (CDA) sought to collect material released from the main rings. The science goals were to measure the composition of ring material and determine whether it is falling into the planet’s atmosphere. RATIONALE Clues about the origin of Saturn’s massive main rings may lie in their composition. Remote observations have shown that they are formed primarily of water ice, with small amounts of other materials such as silicates, complex organics, and nanophase hematite. Fine-grain ejecta generated by hypervelocity collisions of interplanetary dust particles (IDPs) on the main rings serve as microscopic samples. These grains could be examined in situ by the Cassini spacecraft during its final orbits. Deposition of ring ejecta into Saturn’s atmosphere has been suggested as an explanation for the pattern of ionospheric H3+ infrared emission, a phenomenon known as ring rain. Dynamical studies have suggested a preferential transport of charged ring particles toward the planet’s southern hemisphere because of the northward offset of Saturn’s internal magnetic field. However, the deposition flux and its form (ions or charged grains) remained unclear. In situ characterization of the ring ejecta by the Cassini CDA was planned to provide observational constraints on the composition of Saturn’s ring system and test the ring rain hypothesis. RESULTS The region within Saturn’s D ring is populated predominantly by grains tens of nanometers in radius. Larger grains (hundreds of nanometers) dominate the mass density but are narrowly confined within a few hundred kilometers around the ring plane. The measured flux profiles vary with the CDA pointing configurations. The highest dust flux was registered during the ring plane crossings when the CDA was sensitive to the prograde dust populations (Kepler ram pointing) (see the figure). When the CDA was pointed toward the retrograde direction (plasma ram pointing), two additional flux enhancements appeared on both sides of the rings at roughly the same magnetic latitude. The south dust peak is stronger and wider, indicating the dominance of Saturn’s magnetic field in the dynamics of charged nanograins. These grains are likely fast ejecta released from the main rings and falling into Saturn, producing the observed ionospheric signature of ring rain. We estimate that a few tons of nanometer-sized ejecta is produced each second across the main rings. Although this constitutes only a small fraction (<0.1%) of the total ring ejecta production, it is sufficient to supply the observed ring rain effect. Two distinct grain compositional types were identified: water ice and silicate. The silicate-to-ice ratio varies with latitude; the global average ranges from 1:11 to 1:2, higher than that inferred from remote observations of the rings. CONCLUSION Our observations illustrate the interactions between Saturn and its main rings through charged, nanometer-sized ejecta particles. The dominance of nanograins between Saturn and its rings is a dynamical selection effect, stemming from the grains’ high ejection speeds (hundreds of meters per second and higher) and Saturn’s offset magnetic field. The presence of the main rings modifies the effects of the IDP infall to Saturn’s atmosphere. The rings do this asymmetrically, leading to the distribution of the ring rain phenomenon. Confirmed ring constituents include water ice and silicates, whose ratio is likely shaped by processes associated with ring erosion processes and ring-planet interactions. Schematic view of the nanometer-sized ring ejecta environment in the vicinity of Saturn. CDA measurements were taken during Cassini’s Grand Finale mission. The measured dust flux profiles, presented by the histograms along the spacecraft trajectory, show different patterns depending on the instrument pointing configuration. The highest dust flux occurred at the ring plane under Kepler ram pointing (yellow). The profiles registered with plasma ram pointing (green) show two additional, mid-latitude peaks at both sides of the rings with substantial north-south asymmetry. This signature in the vertical profiles indicates that the measured nanograins in fact originate from the rings and are whirling into Saturn under the dynamical influence of the planet’s offset magnetic field. Blue and orange dots represent the two grain composition types identified in the mass spectra, water ice and silicate, respectively. 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.

The Dusty Plasma Disk Around the Janus/Epimetheus Ring

We report on the electron, ion, and dust number densities and the electron temperatures obtained by the Radio and Plasma Wave Science instruments onboard Cassini during the Ring-Grazing orbits. The ...

Cassini RPWS Dust Observation Near the Janus/Epimetheus Orbit

During the Ring Grazing orbits near the end of Cassini mission, the spacecraft crossed the equatorial plane near the orbit of Janus/Epimetheus (~2.5 Rs). This region is populated with dust particles that can be detected by the Radio and Plasma Wave Science (RPWS) instrument via an electric field antenna signal. Analysis of the voltage waveforms recorded on the RPWS antennas provides estimations of the density and size distribution of the dust particles. Measured RPWS profiles, fitted with Lorentzian functions, are shown to be mostly consistent with the Cosmic Dust Analyzer, the dedicated dust instrument on board Cassini. The thickness of the dusty ring varies between 600 and 1,000 km. The peak location shifts north and south within 100 km of the ring plane, likely a function of the precession phase of Janus orbit.

Enceladus Auroral Hiss Emissions During Cassini's Grand Finale

Cassini’s Radio and Plasma Wave Science (RPWS) instrument detected intense auroral hiss emissions during one of its perikrone passes of the Grand Finale orbits. The emissions were detected when Cassini traversed a flux tube connected to Enceladus’ orbit (L-shell = 4) and at a time when both the spacecraft and the icy moon were in similar longitudes. Previous observations of auroral hiss related to Enceladus were made only during close flybys and here we present the first observation of such emissions close to Saturn. Further, ray-tracing analysis shows the source location at a latitude of 63°, in excellent agreement with earlier UVIS observations of Enceladus’ auroral footprint by Pryor et al. (2011, https://doi.org/10.1038/nature09928). The detection has been afforded exclusively by the Grand Finale phase, which enabled sampling of Enceladus’ high-latitude flux tube near Saturn. This result provides new insight into the spatial extent of the electrodynamic interaction between Saturn and Enceladus. Plain Language Summary Cassini’s high-inclination Grand Finale orbits brought the spacecraft closer to Saturn than ever before, with the closest approach between the cloud tops and the inner edge of the D ring. This unprecedented set of orbits introduced a new view of Saturn’s system by enabling direct measurements of high-latitude Enceladus flux tubes close to Saturn. Here we present evidence of communication between Saturn’s ionosphere and Enceladus during the Grand Finale orbits, revealing the vast spatial extent of their coupling via plasma waves.