Cassini Spacecraft Research Articles

Phoebe’s differentiated interior from refined shape analysis

Context.Phoebe is an irregular satellite of Saturn, and its origin, from either between the orbits of the giant planets or the Kuiper Belt, is still uncertain. The extent of differentiation of its interior can potentially help inform its formation location because it is mainly determined by heat from 26-aluminum. The internal structure is reflected in the shape, assuming the body is relaxed to hydrostatic equilibrium. Although previous data analysis indicates Phoebe is close to hydrostatic equilibrium, its heavily cratered surface makes it difficult to tease out its low-order shape characteristics.Aims.This paper aims to extract Phoebe’s global shape from the observations returned by the Cassini mission for comparison with uniform and stratified interior models under the assumption of hydrostatic equilibrium.Methods.The global shape is derived from fitting spherical harmonics and keeping only the low-degree harmonics that represent the shape underneath the heavily cratered surface. The hydrostatic theoretical model for shape interpretation is based on the Clairaut equation developed to the third order (although the second order is sufficient in this case).Results.We show that Phoebe is differentiated with a mantle density between 1900 and 2400 kg m−3. The presence of a porous surface layer further restricts the fit with the observed shape. This result confirms the earlier suggestion that Phoebe accreted with sufficient 26-aluminium to drive at least partial differentiation, favoring an origin with C-type asteroids.

Vacuum Brans–Dicke theory in the Jordan and Einstein frames: Can they be distinguished by lensing?

Vacuum Brans-Dicke (BD) theory continues to receive widespread attention since it is consistent with solar and cosmological experiments. The theory can be self-consistently described in two frames, the Jordan frame (JF) and the conformally rescaled Einstein frame (EF), the transformations providing an easy passage from one frame to the other at the level of actions and solutions. While coordinate transformations do not change curvature properties, conformal transformations do change them leading to corresponding changes in the numerical values of observables. A previous article by Bhadra et al.[Formula: see text] did exemplify this change between JF and EF using the diagnostic of second-order light deflection. This important work leaves room for further improvements on two points, which we do here. First, the measurement of second-order effect faced technically unsurmountable difficulties even around the Sun, hence actually abandoned. Second, the comparison of quantitative values between JF and EF should be based on a common value of [Formula: see text] connecting the two frames. Keeping these in mind, we investigate a technically easier diagnostic, viz., the weak field lensing (WFL) and compare the quantitative changes at common [Formula: see text] to show that the two frames can indeed be distinguished by lensing experiments. Specifically, the predictions of light deflection, image position, total magnification and magnification factor are computed in the EF and compared with those recently obtained (by Gao et al.[Formula: see text]) directly in the JF BD class I solution. The use of the value of BD coupling constant [Formula: see text], suggested by the Cassini spacecraft solar experiment, reveals that an exceptionally high degree of accuracy is needed to experimentally rule out one or the other frame by means of WFL measurements.

Changes in a Dusty Ringlet in the Cassini Division after 2010

Abstract A dusty ringlet designated R/2006 S3, also known as the “Charming Ringlet,” is located around 119,940 km from the center of Saturn within the Laplace Gap in the Cassini Division. Prior to 2010, the ringlet had a simple radial profile and a predictable eccentric shape with two components, one forced by solar radiation pressure and the other freely precessing around the planet. However, observations made by the Cassini spacecraft since late 2012 revealed a shelf of material extending inwards from the ringlet that was not present in the earlier observations. Closer inspection of images obtained after 2012 shows that sometime between 2010 and 2012, the freely precessing component of the ringlet’s eccentricity increased by over 50%, and that for at least 3 yr after 2012, the ringlet had longitudinal brightness variations that rotated around the planet at a range of rates corresponding to ∼60 km in orbital semimajor axis. Some event therefore disturbed this ringlet between 2010 and late 2012.

Nondetection of Radio Emissions From Titan Lightning by Cassini RPWS

AbstractThe Saturn‐orbiting Cassini spacecraft completed 126 close Titan flybys from 2004 until 2017. During almost all of them the Cassini Radio and Plasma Wave Science (RPWS) instrument was turned on to search for radio emissions attributed to Titan lightning. Here we report about their nondetection after close inspection of all Titan flybys throughout the Cassini mission. We also infer new and strong constraints on the permissible flash energy and flash rate of potential Titan lightning. The nondetection of lightning flashes by Cassini observations implies that any lightning on Titan must be either very weak, very rare, or does not exist at all, and the latter could be due to cloud electric fields being too low to initiate a discharge. This finding holds important implications for the prebiotic chemistry of Titan and also implies that lightning will not be a significant hazard to the upcoming Dragonfly mission.

Inflow Speed Analysis of Interchange Injections in Saturn's Magnetosphere

AbstractDuring its more than 13 years in orbit, the Cassini spacecraft detected a large number of plasma and energetic charged particle injections in Saturn's inner magnetosphere. In the corotating frame of the planet, the plasma contained within an injection moves radially inward with the component particles gaining energy. The highest energy particles in the injection experience stronger gradient‐curvature drifts in the longitudinal direction and can drift out of the main body of the injection. We have used these drift‐out effects to estimate the inflow speed of 20 injections by surveying cases from the available plasma data. We find that the average inflow speed from our sample is 15 km/s, and the values are well distributed between 0 and 50 km/s, with a few higher estimates. We have also computed the radial travel distance of interchange events and found that these are typically one to two Saturn radii. We discuss the implications of these quantifications on our understanding of transport.

Saturn's Nightside Dynamics During Cassini's F Ring and Proximal Orbits: Response to Solar Wind and Planetary Period Oscillation Modulations

AbstractWe examine the final 44 orbits of the Cassini spacecraft traversing the midnight sector of Saturn's magnetosphere to distances of ~21 Saturn radii, to investigate responses to heliospheric conditions inferred from model solar wind and Cassini galactic cosmic ray flux data. Clear storm responses to anticipated magnetospheric compressions are observed in magnetic field and energetic particle data, together with Saturn kilometric radiation (SKR), auroral hiss, and ultraviolet auroral emissions. Most compression events are associated with corotating interaction regions, producing ~2–3.5 day intervals of magnetospheric activity that are recurrent with the ~26 day solar rotation period (one or two such events per rotation), though one on the final pass is related to a nonrecurrent interplanetary shock possibly associated with an earlier X‐class solar flare. The response to compressions is modulated by the concurrent relative phasing of the northern and southern planetary period oscillation (PPO) systems, with long (>1 planetary rotation) SKR low‐frequency extension (LFE) intervals associated with strong field‐aligned coupling currents being favored when the two PPO systems act together to thin and thicken the tail plasma sheet during each PPO cycle. LFE onsets/intensifications are then favored at thin plasma sheet phases most unstable to reconnection, producing energetic nightside particle injections and poleward contractions of dawn‐brightened auroras. Correspondingly, solar rotation recurrent intervals of magnetospheric quiet conditions also occur with weak energetic particle fluxes and auroral emissions, associated with extended solar wind rarefactions. Overall, the results emphasize how strongly activity in Saturn's magnetosphere is modulated by concurrent heliospheric conditions.

Matching of Models of the Internal Structure and Thermal Regime of Partially Differentiated Titan with Gravity Field

The problem of matching models of the internal structure of partially differentiated Titan with experimentally measured values of Love number k2 (Iess et al., 2012; Durante et al., 2019) according to results of studying the gravity field of Titan based on flights of the Cassini spacecraft is discussed. The values of k2 obtained presumably point to the presence of large masses of liquid (ocean) in the interior of Titan. However, there are no reliable data about the thickness of the outer ice crust and internal (underice) ocean. In this work, constraints on the heat flow, structure of the water-ice shell, and composition of the Titan ocean which are necessary for matching the calculated (model) and experimental Love numbers are considered. The energy release due to ice crystallization in the ocean is estimated. Estimates for the model Love numbers and maximal surface heat flows for the L/LL chondritic composition of the rock–iron component of Titan are obtained with allowance for the radiogenic energy, as well as for the energy of ice crystallization in the ocean due to the satellite cooling: F ~ 5.8 mW/m2, k2 = 0.53, the thickness of the ice Ih shell $${{H}_{{{{{\text{I}}}_{h}}}}}$$ ~ 100 km, and the ocean depth HW ~ 280 km. Model Love numbers k2 agree with experimental ones (Iess et al., 2012) in the presence of the ocean. To match the model (k2 ≥ 0.55) and recently refined values of Love numbers (k2)D = 0.616 ± 0.067 (Durante et al., 2019), it is necessary to satisfy the constraints on the magnitude of the surface heat flow F ≥ 6.3 mW/m2, which corresponds to a water ocean thickness HW ≥ 310 km under the ice Ih shell with a thickness $${{H}_{{{{{\text{I}}}_{h}}}}}$$ ≤ 90 km. The effect of heat flow variations, ice crust thickness, and water–ammonia ocean density on model values of Love numbers is analyzed and the negligible influence of the NH3 admixture in the ocean on the k2 value is demonstrated. The moment of inertia for models of partially differentiated Titan has constraints: I* ≤ 0.342 at k2 ≥ 0.56.

Ray-tracing thermal modelling of Saturn's main rings

The observations of the Cassini spacecraft gave new perspectives to our understanding of the rings of Saturn and their thermal studies due to their large variety of observation geometries. However, we still know little about the ring particles' physical and, particularly, thermal properties, which is the reason why a number of observations are not easy to explain. Nevertheless, the observed temperature profiles of the rings can be reproduced by considering the rings' general structural properties like local optical depth, τ, and, for the optically-thick rings, wake properties like the ratios H/W or wake thickness to wake width, and S/W or wake separation to wake width; as well as their main energy input sources (e. g., direct and reflected solar energy and Saturn's infrared energy).In this work, we assume that the main rings (C, B, Cassini Division and A rings) are monolayers and study the temperature variations with solar elevation angle, B′, of ten of their regions with a simple thermal balance equation that depends on the rings' shadowing behaviour due to the illumination geometry (B′) and the arrangement of particles across the rings (τ). The shadowing is a key feature, which we reproduce with a numerical function obtained via ray tracing from arrays of spherical particles (C(B′,τ)). Using the former model, we fit Cassini Composite Infrared Spectrometer (CIRS) temperature data with the Bond albedo, AV, and the so-called rotation parameter, f (which represents the rotation status of the ring particles) as only parameters. These parameters are two of the most important physical properties that define the rings' temperatures.With our method, we are able to reproduce the CIRS data temperature trends of the selected regions reasonably well. For evaluation purposes we compare the derived Bond albedo values with those obtained with the Cassini Imaging Science Subsystem (ISS) at wavelengths in the range 451 ≲ λ ≲ 650nm. The comparison suggests a dominance of slow rotators. Additionally, we evaluate the differences obtained when using either a numerical or an analytical shadowing function (in particular, that by (Altobelli et al., 2008)) in the thermal model. The differences between these two approaches reflect some details of the structure of the ring regions we study.

Returning Samples From Enceladus for Life Detection

Evidence suggests that Saturn’s icy moon Enceladus has a subsurface ocean that sources plumes of water vapor and ice vented to space from its south pole. In situ analyses of this material by the Cassini spacecraft have shown that the ocean contains key ingredients for life (elements H, C, N, O and possibly S; simple and complex organic compounds; chemical disequilibria at water-rock interfaces; clement temperature, pressure, and pH). The Cassini discoveries make Enceladus’ interior a prime locale for life detection beyond Earth. Scant material exchange with the inner Solar System makes it likely that such life would have emerged independently of life on Earth. Thus, its discovery would illuminate life’s universal characteristics. The alternative result of an upper bound on a detectable biosphere in an otherwise habitable environment would likewise considerably advance our understanding of the prevalence of life beyond Earth. Here we outline the rationale for returning vented ocean samples, accessible from Enceladus’ surface or low altitudes, to Earth for life detection. Returning samples allows analyses using laboratory instruments that cannot be flown, with decades or more to adapt and repeat analyses. We describe an example set of measurements to estimate the amount of sample to be returned and discuss possible mission architectures and collection approaches. We then turn to the challenges of preserving sample integrity and implementing planetary protection policy. We conclude by placing such a mission in the broader context of Solar System exploration.

Potential vorticity structure of Titan’s polar vortices from Cassini CIRS observations

The Cassini mission has provided the best opportunity to date to extensively study the seasonal variation in Titan’s atmosphere, with observations spanning almost half a Titan year (Ls=293−93°). An important feature in the Titan middle-atmosphere is the formation of a polar vortex. Observations have shown that an initially well-developed northern vortex enriched with trace gas species gradually breaks down after spring equinox as a new vortex emerges in southern winter. Here we use Cassini CIRS observations to derive the temperature and composition of the middle-atmosphere. We use the gradient wind equation to first estimate the mean zonal winds, and then the Potential Vorticity (PV) throughout Titan’s atmosphere over the timespan of the Cassini mission. PV is a useful diagnostic quantity for studying the dynamics of polar vortices because it is materially conserved for adiabatic and frictionless flows, and can be inverted to find all other dynamical fields. Our results show the formation of a strong zonal jet in the winter hemisphere, with wind velocities reaching 220 ms−1, which is consistent with previous studies. An annular PV structure is also observed over the winter poles, whereby a ring of PV encircles a local minima over the pole. Such distributions are often found to be unstable without a restoring force, yet they are seen here in numerous observations in both the northern and southern hemispheres. A comparison with the annular Martian vortices shows that latent heat release from condensation or subsidence-induced adiabatic heating may explain the origin and stability of the annulus. Finally, we investigate the evolution of the size of the vortices and the role of strong PV gradients as a dynamical mixing barrier for trace gas species across the vortex edge. We find that longer lived gases are less confined to the vortex than those with shorter photochemical lifetimes.

A GLOBAL SHAPE MODEL FOR SATURN’S MOON ENCELADUS FROM A DENSE PHOTOGRAMMETRIC CONTROL NETWORK

Abstract. A planetary body’s global shape provides both insight into its geologic evolution, and a key element of any Planetary Spatial Data Infrastructure (PSDI). NASA’s Cassini mission to Saturn acquired more than 600 moderate- to high-resolution images (<500 m/pixel) of the small, geologically active moon Enceladus. The moon’s internal global ocean and intriguing geology mark it as a candidate for future exploration and motivates the development of a PSDI. Recently, two PSDI foundational data sets were created: geodetic control and orthoimages. To provide the third foundational data set, we generate a new shape model for Enceladus from Cassini images and a dense photogrammetric control network (nearly 1 million tie points) using the U.S. Geological Survey’s Integrated Software for Imagers and Spectrometers (ISIS) and the Ames Stereo Pipeline (ASP). The new shape model is near-global in extent and gridded to 2.2 km/pixel, ∼50 times better resolution than previous global models. Our calculated triaxial shape, rotation rate, and pole orientation for Enceladus is consistent with current International Astronomical Union (IAU) values to within the error; however, we determined a new prime meridian offset (Wo) of 7.063°. We calculate Enceladus’ long-wavelength topography by subtracting the best-fit triaxial ellipsoid from our shape model. The result is comparable to previous global models but can resolve topographic features as small as 5–7 km across in certain areas. To evaluate the spatially varying quality of the model, we calculate the point density (variable from 5 to more than 50 points per pixel), normalized median absolute deviation of the points within each pixel (typically less than 100 m), and the minimum expected vertical precision of each point (ranging from 29 m to 2 km).

Compositional Measurements of Saturn's Upper Atmosphere and Rings from Cassini INMS

AbstractThe Cassini spacecraft's last orbits directly sampled Saturn's thermosphere and revealed a much more chemically complex environment than previously believed. Observations from the Ion and Neutral Mass Spectrometer (INMS) aboard Cassini provided compositional measurements of this region and found an influx of material raining into Saturn's upper atmosphere from the rings. We present here an in‐depth analysis of the CH4, H2O, and NH3 signal from INMS and provide further evidence of external material entering Saturn's atmosphere from the rings. We use a new mass spectral deconvolution algorithm to determine the amount of each species observed in the spectrum and use these values to determine the influx and mass deposition rate for these species.

Updated Equipotential Shapes of Jupiter and Saturn Using Juno and Cassini Grand Finale Gravity Science Measurements

AbstractA commonly used shape model for the giant plants of Jupiter and Saturn is an oblate ellipsoid, a simplified model of the equipotential shape. The ellipsoidal shape models were originally derived from radio occultation data and gravity data after the Voyager flybys in 1979. Through precise Doppler tracking of NASA's Juno and Cassini spacecraft telecommunications links, zonal coefficients in a spherical harmonic expansion of the gravity field of Jupiter and Saturn have been resolved to degree 10, including the detection of nonzero odd zonal harmonics, which have been interpreted as differential rotation in the atmosphere for both planets. In this work, we construct the equipotential surfaces of Jupiter and Saturn using the recently measured gravity fields determined by Juno through perijove 8 and Cassini through end of mission. For both planets, even zonal harmonics dominate the equipotential shape, differing from the reference ellipsoid up to ~32 km at Jupiter and ~125 km at Saturn in the midlatitude regions. Saturn's internal rotation period estimated with ring seismology produces a shape that is fully consistent with the Pioneer and Voyager radio occultation measurements, and Jupiter's shape is fully consistent with the Pioneer and Voyager radio occultation measurements. With current equipotential theory, the recent analysis of the depth of the deep zonal flow on Jupiter and Saturn cannot fully be explained by the shape from current radio occultation measurements; additional occultation measurements and reanalysis of the Pioneer and Voyager radio occultations and will be useful for further constraining the shape of the planets.

Long waves in oceanic shallow water: Symbolic computation on the bilinear forms and Bäcklund transformations for the Whitham–Broer–Kaup system

For the Earth, water is at the core of sustainable development and at the heart of adaptation to climate change. For the Enceladus, the sixth-largest moon of the Saturn, Cassini spacecraft discovers a possible global ocean of liquid water beneath an icy crust. Oceanic water waves are one of the most common phenomena in nature. Hereby, on the Whitham–Broer–Kaup system for the dispersive long waves in the oceanic shallow water, with respect to the horizontal velocity of the water wave and height of the deviation from the equilibrium position of the water, our binary Bell polynomials and symbolic computation lead to two sets of the bilinear forms, two sets of the N-soliton solutions and two sets of the auto-Backlund transformations with the sample solitons, where N is a positive integer. Our bilinear forms and auto-Backlund transformations are different from those reported in the existing literatures. All of our results are dependent on the coefficients in the system which represent different oceanic-water-wave dispersion/diffusion powers.

Saturn’s Atmospheric Helium Abundance from Cassini Composite Infrared Spectrometer Data

Abstract We use thermal infrared data from the Composite Infrared Spectrometer, which was on board the Cassini orbiter, to retrieve the helium abundance in Saturn’s visible atmosphere. We find that the data is most consistent with a helium-to-hydrogen mole ratio in the range between 0.04 and 0.075, near the lower end of previous measurements, and implying a significant sequestration of helium in Saturn’s interior. The primary sources of uncertainty are in the spectroscopic parameters for H2–H2 collision induced absorption and the nonuniqueness of the spectral inverse problem.

Does Dione have a liquid water ocean?

Icy Moons Several icy moons orbiting the giant planets have oceans of liquid water beneath their solid ice surfaces. It has been suggested that Dione, the fourth largest moon of Saturn, could have a similar subsurface ocean. Zannoni et al. analyzed radio-tracking data taken when the Cassini spacecraft (named after the astronomer who discovered Dione in 1684) flew past the moon on three occasions between 2011 and 2015. They reconstructed Dione's gravity field, combined it with measurements of topography, and then matched the geophysical properties with models of the interior structure. Their analysis supports the interpretation that Dione has a small, deep, subsurface liquid ocean, but they caution that the models are not fully constrained. Icarus 345 , 113713 (2020).

Preliminarily study of Saturn’s upper atmosphere density by observing Cassini plunging via China’s deep space station

When the Cassini spacecraft finally plunged into the Saturnian atmosphere on 2017 September 15, China’s deep space telescope pointed to Saturn to observe Cassini and study the Saturnian upper neutral atmosphere. In this first Chinese Saturnian radio science experiment, X band Doppler velocity radio science data between the deep space telescope and the Cassini spacecraft were obtained. After removing Saturnian and solar gravity effects, Earth rotation effect, the remaining Saturnian atmosphere drag information was retrieved in the Cassini final plunge progress. Saturn’s upper neutral atmosphere mass density profile is approximately estimated based on atmosphere mass density derived principally by real orbit measurement data. Saturn’s upper neutral atmospheremass density from76 000 km to 1400 km is estimated fromthe orbit measurement data, the mass density results are about from 1.4 × 10−15 kg cm−3 to 2.5 × 10−14 kg cm−3.

Incorporating Physical Knowledge Into Machine Learning for Planetary Space Physics.

Recent improvements in data collection volume from planetary and space physics missions have allowed the application of novel data science techniques. The Cassini mission for example collected over 600 gigabytes of scientific data from 2004 to 2017. This represents a surge of data on the Saturn system. In comparison, the previous mission to Saturn, Voyager over 20 years earlier, had onboard a ~70 kB 8-track storage ability. Machine learning can help scientists work with data on this larger scale. Unlike many applications of machine learning, a primary use in planetary space physics applications is to infer behavior about the system itself. This raises three concerns: first, the performance of the machine learning model, second, the need for interpretable applications to answer scientific questions, and third, how characteristics of spacecraft data change these applications. In comparison to these concerns, uses of “black box” or un-interpretable machine learning methods tend toward evaluations of performance only either ignoring the underlying physical process or, less often, providing misleading explanations for it. The present work uses Cassini data as a case study as these data are similar to space physics and planetary missions at Earth and other solar system objects. We build off a previous effort applying a semi-supervised physics-based classification of plasma instabilities in Saturn’s magnetic environment, or magnetosphere. We then use this previous effort in comparison to other machine learning classifiers with varying data size access, and physical information access. We show that incorporating knowledge of these orbiting spacecraft data characteristics improves the performance and interpretability of machine leaning methods, which is essential for deriving scientific meaning. Building on these findings, we present a framework on incorporating physics knowledge into machine learning problems targeting semi-supervised classification for space physics data in planetary environments. These findings present a path forward for incorporating physical knowledge into space physics and planetary mission data analyses for scientific discovery.

Comparison of the Deep Atmospheric Dynamics of Jupiter and Saturn in Light of the Juno and Cassini Gravity Measurements

The nature and structure of the observed east-west flows on Jupiter and Saturn have been a long-standing mystery in planetary science. This mystery has been recently unraveled by the accurate gravity measurements provided by the Juno mission to Jupiter and the Grand Finale of the Cassini mission to Saturn. These two experiments, which coincidentally happened around the same time, allowed the determination of the overall vertical and meridional profiles of the zonal flows on both planets. This paper reviews the topic of zonal jets on the gas giants in light of the new data from these two experiments. The gravity measurements not only allow the depth of the jets to be constrained, yielding the inference that the jets extend to roughly 3000 and 9000 km below the observed clouds on Jupiter and Saturn, respectively, but also provide insights into the mechanisms controlling these zonal flows. Specifically, for both planets this depth corresponds to the depth where electrical conductivity is within an order of magnitude of 1 S m−1, implying that the magnetic field likely plays a key role in damping the zonal flows. An intrinsic characteristic of any gravity inversion, as discussed here, is that the solutions might not be unique. We analyze the robustness of the solutions and present several independent lines of evidence supporting the results presented here.

The root of anomalously specular reflections from solid surfaces on Saturn\u2019s moon Titan

Saturn’s moon Titan has a methane cycle with clouds, rain, rivers, lakes, and seas; it is the only world known to presently have a volatile cycle akin to Earth’s tropospheric water cycle. Anomalously specular radar reflections (ASRR) from Titan’s tropical region were observed with the Arecibo Observatory (AO) and Green Bank Telescope (GBT) and interpreted as evidence for liquid surfaces. The Cassini spacecraft discovered lakes/seas on Titan, however, it did not observe lakes/seas at the AO/GBT anomalously specular locations. A satisfactory explanation for the ASRR has been elusive for more than a decade. Here we show that the ASRR originate from one terrain unit, likely paleolakes/paleoseas. Titan observations provide ground-truth in the search for oceans on exoearths and an important lesson is that identifying liquid surfaces by specular reflections requires a stringent definition of specular; we propose a definition for this purpose.