Cassini Grand Finale Research Articles

A joint investigation of Saturn’s deep zonal flow via its gravitational field and Ohmic dissipation

ABSTRACT The Cassini Grand Finale provided a unique opportunity to study Saturn’s deep zonal flow. In this paper, we present a comprehensive deep zonal flow model for Saturn using a joint inversion of observed gravity and zonal flow-induced Ohmic dissipation in the semi-conducting region, under the assumption that the planet’s cloud-level wind is limited to a shallow weather layer. Our model unveils a strong equatorially symmetric zonal flow (O(100) m s−1) and a weaker antisymmetric zonal flow (O(1) m s−1) beneath the cloud-level winds. Furthermore, we show that the maximum depth of the deep zonal flow is around 7800 km, surpassing previous results derived from gravity alone and with the assumption that the rapid cloud-level winds extend deep into the planet’s interior. The meridional profile of the deep zonal flow differs significantly from the cloud-level zonal winds and predicts a strong westward zonal flow in the region with latitude around ±23°, where the observed cloud-top winds remain eastward. We also demonstrate that the zonal flow inside and outside the tangent cylinder exhibits significant differences in speed and scale. Moreover, our findings suggest that the coupling between the deep zonal flow and cloud-level winds varies across latitudes, with the shallow-wind model applicable to polar regions within the tangent cylinder and the deep-wind model more relevant to equatorial regions outside the tangent cylinder. Our findings highlight the importance of accounting for the planet’s deep zonal flow in future studies of Saturn’s atmospheric dynamics.

The Case for Future Gravity Science Investigations at Saturn with a Planetary Orbiter

The next phase in the robotic exploration of the Saturnian system will target unresolved questions about the gas giant’s atmosphere, interior, magnetosphere, and rings that were left open by investigations on board the Cassini mission. Among these, we find gravity science objectives, which relate mainly to explaining the origin of the large wind-induced gravity signal and reconciling existing incongruities regarding the internal density distribution. Here we attempt to identify which future observations may yield additional information about Saturn’s internal mechanisms. We report on the results of precise numerical simulations of a gravity science experiment carried out with a planetary orbiter in the 2040s and characterized by varying periapsis altitude over the 1 bar planetary surface. The goal is to identify achievable measurement requirements that can advance the current understanding of Saturn’s interior and atmospheric dynamics. We find that inclined, elliptical orbits with low-altitude closest approaches over the northern hemisphere can improve the present determination of the gravity field coefficients by up to 1 order of magnitude, provided that at least a dozen pericenters are devoted to gravity science. Furthermore, if future observations are combined with the available Cassini Grand Finale data in the southern hemisphere, a significant improvement in terms of maximum measurable gravity field degree is observed.

Large mass inflow rates in Saturn’s rings due to ballistic transport and mass loading

The Cassini mission provided key measurements needed to determine the absolute age of Saturn’s rings, including the extrinsic micrometeoroid flux at Saturn, the volume fraction of non-icy pollutants in the rings, and the total ring mass. These three factors constrain the ring age to be no more than a few 100 Myr (Kempf et al., 2022). Observations during the Cassini Grand Finale also showed that the rings are losing mass to the planet at a prodigious rate. Some of the mass flux falls as “ring rain” at high latitudes. However, the influx in ring rain is considerably less than the total measured mass influx of 4800 to 45000 kg s−1 at lower latitudes (Waite et al., 2018).In addition to polluting the rings, micrometeoroid impacts lead to ballistic transport, the mass and angular momentum transport due to net exchanges of meteoroid impact ejecta. Because the ejecta are predominantly prograde, they carry net angular momentum outward. As a result, ring material drifts inward toward the planet. Here, for the first time, we use a simple model to quantify this radial mass inflow rate for dense rings and find that, for plausible choices of parameters, ballistic transport and mass loading by meteoroids can produce a total inward flux of material in the inner B ring and in the C ring that is on the order of a few ⋅103 to a few ⋅104 kg s−1, in agreement with measurements during the Cassini Grand Finale. From these mass inflow rates, we estimate that the remaining ring lifetime is ∼ 15 to 400 Myr. Combining this with a revised pollution age of ∼ 120 Myr, we conclude that Saturn’s rings are not only young but ephemeral and probably started their evolution on a similar timescale to their pollution age with an initial mass of one to a few Mimas masses.In addition to showing that meteoroid impacts can produce a large sustained mass inflow through the B and C rings, this paper addresses various uncertainties and considers possible contributions by additional transport mechanisms and by external torques. We also map out a set of future research projects, including global simulations.

The Enigmatic Abundance of Atomic Hydrogen in Saturn’s Upper Atmosphere

A planet’s Lyα emission is sensitive to its thermospheric structure. Here we report joint Hubble Space Telescope and Cassini cross-calibration observations of the Saturn Lyα emission made 2 weeks before the Cassini grand finale. To investigate the long-term Saturn Lyα airglow observed by different ultraviolet instruments, we cross-correlate their calibration, finding that while the official Cassini/UVIS sensitivity should be lowered by ∼75%, the Voyager 1/UVS sensitivities should be enhanced by ∼20% at the Lyα channels. This comparison also allowed us to discover a permanent feature of the Saturn disk Lyα brightness that appears at all longitudes as a brightness excess (Lyα bulge) of ∼30% (∼12σ) extending over the latitude range ∼5°–35° N compared to the regions at equator and ∼60° N. This feature is confirmed by three distinct instruments between 1980 and 2017 in the Saturn north hemisphere. To analyze the Lyα observations, we use a radiation transfer model of resonant scattering of solar and interplanetary Lyα photons and a latitude-dependent photochemistry model of the upper atmosphere constrained by occultation and remote-sensing observations. For each latitude, we show that the Lyα observations are sensitive to the temperature profile in the upper stratosphere and lower thermosphere, thus providing useful information in a region of the atmosphere that is difficult to probe by other means. In the Saturn Lyα bulge region, at latitudes between ∼5° and ∼35°, the observed brightening and line broadening support seasonal effects, variation of the temperature vertical profile, and potential superthermal atoms that require confirmation.

A Global Simulation of the Dynamo, Zonal Jets, and Vortices on Saturn

Abstract The fluid dynamics planet Saturn gives rise to alternating east–west jet streams, large cyclonic and anticyclonic vortices, and a dipole-dominant magnetic field that is highly axisymmetric about the planetary rotation axis. Modeling these features in a self-consistent manner is crucial for understanding the dynamics of Saturn’s interior and atmosphere. Here we report a turbulent high-resolution dynamo simulation in a spherical shell that produces these features simultaneously for the first time. A crucial model ingredient is a long-hypothesized stably stratified layer (SSL), sandwiched between a deep metallic hydrogen layer and an outer low-conductivity molecular layer, born out of the limited solubility of helium inside metallic hydrogen at certain depths. The model spontaneously produces polar cyclones and significant low-latitude and midlatitude jet stream activity in the molecular layer. The off-equatorial low-latitude jet streams partially penetrate into the SSL and interact with the magnetic field. This helps to axisymmetrize the magnetic field about the rotation axis and convert some of the poloidal magnetic field to a toroidal field, which appears as two global magnetic energy rings surrounding the deeper dynamo region. The simulation also mimics a distinctive dip in the fifth spherical harmonic in Saturn’s magnetic energy spectrum as inferred from the Cassini Grand Finale measurements. Our model highlights the role of an SSL in shaping the fluid dynamical and magnetic features of giant planets, as exemplified at Saturn.

Observations of the First Harmonic of Saturn Kilometric Radiation During Cassini's Grand Finale

AbstractClear first harmonic emissions of Saturn Kilometric Radiation are discovered during the Cassini Grand Finale orbits. Both ordinary (O) and extraordinary (X) mode fundamental emissions accompanied by X mode harmonics are observed. Analysis shows that the frequency ratio between the fundamental and harmonic emissions is 2.01 ± 0.08, and the harmonic emissions display weaker intensities than the fundamental, by 30–40 dB for the X‐X (fundamental‐harmonic) type harmonic and 10–30 dB for the O‐X type harmonic. The intensity relations between the two types of harmonics, that is, O‐X and X‐X show different patterns that we attribute to different conditions of emission at the source. Direction‐finding results shows that the fundamental and harmonic emissions are plausibly generated in the same source region. In agreement with previous studies at Earth, the generation of the two types of harmonics can be attributed to the cyclotron maser instability operating with different plasma density and electron energy distributions in the source region.

Implications from secondary emission from neutral impact on Cassini plasma and dust measurements

ABSTRACT We investigate the role of secondary electron and ion emission from impact of gas molecules on the Cassini Langmuir probe (RPWS-LP or LP) measurements in the ionosphere of Saturn. We add a model of the emission currents, based on laboratory measurements and data from comet 1P/Halley, to the equations used to derive plasma parameters from LP bias voltage sweeps. Reanalysing several hundred sweeps from the Cassini Grand Finale orbits, we find reasonable explanations for three open conundrums from previous LP studies of the Saturn ionosphere. We find an explanation for the observed positive charging of the Cassini spacecraft, the possibly overestimated ionospheric electron temperatures, and the excess ion current reported. For the sweeps analysed in detail, we do not find (indirect or direct) evidence of dust having a significant charge-carrying role in Saturn’s ionosphere. We also produce an estimate of H2O number density from the last six revolutions of Cassini through Saturn’s ionosphere in greater detail than reported by the Ion and Neutral Mass Spectrometer. Our analysis reveals an ionosphere that is highly structured in latitude across all six final revolutions, with mixing ratios varying with two orders of magnitude in latitude and one order of magnitude between revolutions and altitude. The result is generally consistent with an empirical photochemistry model balancing the production of H+ ions with the H+ loss through charge transfer with e.g. H2O, CH4, and CO2, for which water vapour appears as the likeliest dominant source of the signal in terms of yield and concentration.

Compositional Measurements of Saturn's Upper Atmosphere and Rings From Cassini INMS: An Extended Analysis of Measurements From Cassini's Grand Finale Orbits

AbstractThe Cassini spacecraft's final orbits sampled Saturn's atmosphere and returned surprisingly complex mass spectra from the Ion and Neutral Mass Spectrometer. Signal returned from the instrument included native Saturn species, as expected, as well as a significant amount of signal attributed to vaporized ices and higher mass organics believed to be flowing into Saturn's atmosphere from the rings. In this paper, we present an in‐depth compositional analysis of the mass spectra returned from Cassini's last few orbits. We use a mass spectral deconvolution algorithm designed specifically to handle the complexities involved with unit resolution spaceflight mass spectrometry data to determine the relative abundance of species detected in the observations. We calculate the downward external flux and mass deposition rates of ring volatile species into Saturn's atmosphere and conclude that during these observations ring material was being deposited into Saturn's equatorial region at a rate on the order of 104 kg/s.

Recipe for a Saturn‐Like Dynamo

AbstractThe Cassini Grand Finale orbits provided detailed observations of Saturn’s internal magnetic field. Unique characteristics of the observed field, such as its axisymmetry and power spectrum, provide constraints on dynamo processes deep in Saturn’s interior. Here, we use numerical dynamo simulations to explore what ingredients are necessary in a dynamo in order to produce the unique “Saturn‐like” surface magnetic field. We find that characteristics of Saturn’s magnetic field can be sensitive to properties of a stably stratified helium rain‐out layer and thermal perturbations at the top of the layer. Our best models suggest that a relatively thick stably stratified helium rain‐out layer may extend out to 70% of Saturn’s radius, likely imprinted with weak heat flux out of the equatorial region and strong heat flux out of high latitudes. These results may provide new constraints on stable stratification inside Saturn and its thermal evolution.

Saturn's Nightside Ring Current During Cassini's Grand Finale

AbstractDuring Cassini's Grand Finale proximal orbits, the spacecraft traversed the nightside magnetotail to ∼21 Saturn radii. Clear signatures of Saturn's equatorial current sheet are observed in the magnetic field data. An axisymmetric model of the ring current is fitted to these data, amended to take into account the tilt of the current layer by solar wind forcing, its teardrop‐shaped nature and the magnetotail and magnetopause fringing fields. Variations in ring current parameters are examined in relation to external driving of the magnetosphere by the solar wind and internal driving by the two planetary period oscillations (PPOs), and compared with previous dawn and dayside observations. We find that the relative phasing of the PPOs determines the ring current's response to solar wind conditions. During solar wind compressions when the PPOs are in antiphase, a thick partial ring current is formed on the nightside, dominated by hot plasma injected by tail reconnection. This partial ring current should close partly via magnetopause currents and possibly via field‐aligned currents into the ionosphere. However, during solar wind compressions when the PPOs are in phase, this partial ring current is not detected. During solar wind rarefactions an equatorial “magnetodisc” configuration is observed in the dayside/dawn/nightside regions, with similar total currents flowing at these local times. During very quiet intervals of prolonged solar wind rarefaction, a thin current sheet with an enhanced current density is formed, indicative of a ring current dominated by cool, dense, Enceladus water group ions.

Towards a solution to the energy crisis

The upper atmospheres of all the giant planets are hotter than models predict. Analysis of Cassini Grand Finale observations of Saturn provide evidence that heat generated by the aurora is responsible.

A pole-to-pole pressure–temperature map of Saturn’s thermosphere from Cassini Grand Finale data

Temperatures of the outer planet thermospheres exceed those predicted by solar heating alone by several hundred degrees. Enough energy is deposited at auroral regions to heat the entire thermosphere, but models predict that equatorward distribution is inhibited by strong Coriolis forces and ion drag. A better understanding of auroral energy deposition and circulation are critical to solving this so-called energy crisis. Stellar occultations observed by the Ultraviolet Imaging Spectrograph instrument during the Cassini Grand Finale were designed to map the thermosphere from pole to pole. We analyze these observations, together with earlier observations from 2016 and 2017, to create a two-dimensional map of densities and temperatures in Saturns thermosphere as a function of latitude and depth. The observed temperatures at auroral latitudes are cooler and peak at higher altitudes and lower latitudes than predicted by models, leading to a shallower meridional pressure gradient. Under modified geostrophy, we infer slower westward zonal winds that extend to lower latitudes than predicted, supporting equatorward flow from approximately 70 to 30 degrees latitude in both hemispheres. We also show evidence of atmospheric waves in the data that can contribute to equatorward redistribution of energy through zonal drag.

Saturn’s Probable Interior: An Exploration of Saturn’s Potential Interior Density Structures

Abstract The gravity field of a giant planet is typically our best window into its interior structure and composition. Through comparison of a model planet’s calculated gravitational potential with the observed potential, inferences can be made about interior quantities, including possible composition and the existence of a core. Necessarily, a host of assumptions go into such calculations, making every inference about a giant planet’s structure strongly model dependent. In this work, we present a more general picture by setting Saturn’s gravity field, as measured during the Cassini Grand Finale, as a likelihood function driving a Markov Chain Monte Carlo exploration of the possible interior density profiles. The result is a posterior distribution of the interior structure that is not tied to assumed composition, thermal state, or material equations of state. Constraints on interior structure derived in this Bayesian framework are necessarily less informative, but are also less biased and more general. These empirical and probabilistic constraints on the density structure are our main data product, which we archive for continued analysis. We find that the outer half of Saturn’s radius is relatively well constrained, and we interpret our findings as suggesting a significant metal enrichment, in line with atmospheric abundances from remote sensing. As expected, the inner half of Saturn’s radius is less well constrained by gravity, but we generally find solutions that include a significant density enhancement, which can be interpreted as a core, although this core is often lower in density and larger in radial extent than typically found by standard models. This is consistent with a dilute core and/or composition gradients.

Interpreting the Equatorially Antisymmetric Gravitational Field of Saturn Measured by the Cassini Grand Finale

Abstract The equatorially antisymmetric gravitational field of Saturn, represented by the four gravitational coefficients J 3, J 5, J 7, and J 9, has been measured by the Cassini Grand Finale. Since the rotational distortion and the internal structure do not contribute to the asymmetric gravity field, the spectrum of the odd coefficients offers a direct window into the structure and amplitude of fluid motion in Saturn. We show that the observed cloud-top asymmetric winds extending into Saturn’s interior cannot produce the measured gravity field, suggesting that the cloud-top winds are confined within a thin weather layer and hence do not contribute to the observed gravity. We interpret the asymmetric gravitational field by searching for an a priori unknown zonal circulation taking place in the molecular envelope. We find that a zonal circulation confined in the 20,000 km thick molecular layer produces the coefficients J 3, J 5, J 7, J 9 in agreement with measurements. The depth of the circulation is consistent with that inferred from the equatorially symmetric gravitational field of Saturn.

The Morphology of Saturn's Aurorae Observed During the Cassini Grand Finale

AbstractCassini's mission exploring the Saturn system ended with the Grand Finale, a series of orbits bringing the spacecraft closer to the planet than ever before and providing unique opportunities for observations of the ultraviolet aurorae. This study presents a selection of high‐resolution imagery showing the aurorae's small‐scale structure in unprecedented detail. We find the main arc to vary between a smooth and a rippled structure, likely indicating quiet and disturbed magnetospheric conditions, respectively. It is usually accompanied by a diffuse and dim outer emission on its equatorward side which appears to be driven by wave scattering of hot electrons from the inner ring current into the loss cone. The duskside is characterized by highly dynamic structures which may be signatures of radial plasma injections. This image set will be the only high‐resolution data for the foreseeable future and hence forms an important basis for future auroral research on Saturn.

Cassini INMS constraints on the composition and latitudinal fractionation of Saturn ring rain material

During Cassini's Grand Finale, the spacecraft flew between Saturn's upper atmosphere and its innermost ring, the D ring. Throughout these final orbits, Cassini encountered material flowing from the D ring into Saturn's atmosphere. Here, we present a compositional analysis of this material using data collected by Cassini's Ion Neutral Mass Spectrometer. We confirm the identification of CH4, CO2, CO, N2, H2O, NH3, and organics in the ring material, and provide upper limits for organic compounds of interest. The H/C, O/C, N/C, and S/C ratios of ring material are constrained using three different kinds of model spectra: automated fits, hand fits of inorganics + hydrocarbons, and hand fits of inorganics + organics with priority given to O-, N-, and S-bearing organics. Additionally, we compare data from the final plunge to earlier orbits, and find that ring material can be divided into gas and dust constituents, with CO2 at the volatility boundary between the two phases. At increasing distance above the equatorial plane, the gas/dust molar ratio increases.

The landscape of Saturn’s internal magnetic field from the Cassini Grand Finale

The Cassini mission entered the Grand Finale phase in April 2017 and executed 22.5 highly inclined, close-in orbits around Saturn before diving into the planet on September 15th 2017. Here we present our analysis of the Cassini Grand Finale magnetometer (MAG) dataset, focusing on Saturn’s internal magnetic field. These measurements demonstrate that Saturn’s internal magnetic field is exceptionally axisymmetric, with a dipole tilt less than 0.007 degrees (25.2 arcsecs). Saturn’s magnetic equator was directly measured to be shifted northward by ∼ 0.0468 ± 0.00043 (1σ) RS, 2820±26 km, at cylindrical radial distances between 1.034 and 1.069 RS from the spin-axis. Although almost perfectly axisymmetric, Saturn’s internal magnetic field exhibits features on many characteristic length scales in the latitudinal direction. Examining Br at the a=0.75RS, c=0.6993RS isobaric surface, the degree 4 to 11 contributions correspond to latitudinally banded magnetic perturbations with characteristic width ∼15∘, similar to that of the off-equatorial zonal jets observed in the atmosphere of Saturn. Saturn’s internal magnetic field beyond 60∘, in particular the small-scale features, are less well constrained by the available measurements, mainly due to incomplete spatial coverage in the polar region. Magnetic fields associated with the ionospheric Hall currents were estimated and found to contribute less than 2.5 nT to Gauss coefficients beyond degree 3. The magneto-disk field features orbit-to-orbit variations between 12 nT and 15.4 nT along the close-in part of Grand Finale orbits, offering an opportunity to measure the electromagnetic induction response from the interior of Saturn. A stably stratified layer thicker than 2500 km likely exists above Saturn’s deep dynamo to filter out the non-axisymmetric internal magnetic field. A heat transport mechanism other than pure conduction, e.g. double diffusive convection, must be operating within this layer to be compatible with Saturn’s observed luminosity. The latitudinally banded magnetic perturbations likely arise from a shallow secondary dynamo action with latitudinally banded differential rotation in the semi-conducting layer.

Saturn's Ionosphere: Electron Density Altitude Profiles and D‐Ring Interaction From The Cassini Grand Finale

AbstractWe present the electron density (ne) altitude profiles of Saturn's ionosphere at near‐equatorial latitudes from all 23 orbits of Cassini's Grand Finale. The data are collected by the Langmuir probe part of the Radio and Plasma Wave Science investigation. A high degree of variability in the electron density profiles is observed. However, organizing them by consecutive altitude ranges revealed clear differences between the southern and northern hemispheres. The ne profiles are shown to be more variable and connected to the D‐ring below 5,000 km in the southern hemisphere compared to the northern hemisphere. This observed variability is explained to be a consequence of an electrodynamic interaction with the D‐ring. Moreover, a density altitude profile is constructed for the northern hemisphere indicating the presence of three different ionospheric layers. Similar properties were observed during Cassini's final plunge, where the main ionospheric peak is crossed at ∼1,550‐km altitude.

Depth of the dynamo region and zonal circulation of the molecular layer in Saturn inferred from its equatorially symmetric gravitational field

ABSTRACT The high-precision equatorially symmetric gravitational field of Saturn (the even gravitational coefficients J2, J4, …, J12) measured by the Cassini Grand Finale reflects its internal structure, its non-spherical shape caused by rotation and its strong zonal circulation whose location is controlled by the depth of its dynamo. We construct a four-layer, non-spheroidal (i.e. its shape is irregular) model of Saturn comprised of an inner core, a metallic dynamo region, an outer molecular envelope and a thin transition layer between the metallic and molecular regions. The model produces the even zonal gravitational coefficients that are in agreement with those measured by the Cassini Grand Finale within the error bars. Our Saturnian model reveals that (i) the observed cloud-top winds extending to any depth on cylinders cannot explain the measured coefficients J2, J4, …, J12; (ii) a deep zonal flow confined in the 20 000 km thick molecular layer is required to interpret them; (iii) the profile of the zonal flow – whose direction is sufficiently alternating with several retrograde peaks – significantly differs from that of the surface winds, implying that the observed winds are confined to a shallow layer, do not extend deeply into Saturn and do not contribute to the observed gravity; and (iv) the Saturnian dynamo can substantially affect the structure of its equatorially symmetric gravitational field by stopping the zonal-flow penetration and by changing, because of the boundary condition at the metallic and molecular interface, the distribution of the dynamic density anomalies.

ALMA Spectral Imaging of Titan Contemporaneous with Cassini's Grand Finale

Abstract The Cassini mission performed 127 targeted flybys of Titan during its 13 yr mission to Saturn, culminating in the Grand Finale between 2017 April and September. Here we demonstrate the use of the Atacama Large Millimeter/submillimeter Array (ALMA) to continue Cassini’s legacy for chemical and climatological studies of Titan’s atmosphere. Whole-hemisphere, interferometric spectral maps of HCN, HNC, HC3N, CH3CN, C2H3CN, C2H5CN, and C3H8 were obtained using ALMA in 2017 May at moderate (≈0.″2, or ≈1300 km) spatial resolution, revealing the effects of seasonally variable chemistry and dynamics on the distribution of each species. The ALMA submillimeter observations of HCN and HC3N are consistent with Cassini infrared data on these species, obtained in the same month. Chemical/dynamical lifetimes of a few years are inferred for C2H3CN and C2H5CN, in reasonably close agreement with the latest chemical models incorporating the sticking of C2H5CN to stratospheric aerosol particles. ALMA radial limb flux profiles provide column density information as a function of altitude, revealing maximum abundances in the thermosphere (above 600 km) for HCN, HNC, HC3N, and C2H5CN. This constitutes the first detailed measurement of the spatial distribution of HNC, which is found to be confined predominantly to altitudes above 730 ± 60 km. The HNC emission map shows an east–west hemispheric asymmetry of 13% ± 3%. These results are consistent with very rapid production (and loss) of HNC in Titan’s uppermost atmosphere, making this molecule an effective probe of short-timescale (diurnal) ionospheric processes.