General circulation and transport in Saturn’s upper troposphere and stratosphere

We present a model for the general circulation and dynamical transport in Saturn’s upper troposphere and stratosphere and derive the effective advective circulation and eddy transport coefficients required for use in two-dimensional (latitude–altitude) photochemistry–transport models. A three-dimensional Outer-Planet General Circulation Model (OPGCM) is used to generate the transport data. We find that the OPGCM adequately captures the global-scale, pole-to pole temperature contrast, but overestimates mid- and high-latitude temperatures in the summer hemisphere by ～5 K. In addition, the model reproduces the local temperature minimum seen at the equator in Cassini Composite Infrared Spectrometer (CIRS) 0.1-mbar data but not the local maximum in 1-mbar temperatures, suggesting that it is capturing the phase of Saturn’s Semiannual Oscillation associated with a temperature minimum at the equator but not the opposite phase. The meridional circulation at low latitudes is found to be dominated by a seasonally reversing Hadley circulation, characterized by upwelling near the equator, cross-equatorial flow from summer to winter hemisphere, and strong subsidence centered near 25° latitude in the winter hemisphere. The cross-equatorial flow induces an asymmetry in which the equatorial jet is found to be stronger in the winter than in the summer stratosphere. The location of the subsidence near 25°N for L_s ～ 310° coincides with local maxima in acetylene, diacetylene, and methylacetylene mixing ratios measured by Cassini/CIRS (Guerlet, S., Fouchet, T., Bézard, B., Moses, J.I., Fletcher, L.N., Simon-Miller, A.A., Flasar, F.M. [2010]. Icarus 209, 682–695). This result supports the suggestion by Guerlet et al. (2010) that the hydrocarbon abundances are enhanced at this latitude by pronounced downward transport of hydrocarbon-rich air from above. The lateral eddy diffusion coefficient is found to typically be ～10⁵–10⁶ m² s^?1 at mid-latitudes, implying meridional eddy transport time scales of order 100–1000 years.     

Small-scale variability in Saturn’s lower ionosphere

We perform and present a wavelet analysis on all 31 Cassini electron density profiles published to date (Nagy, A.F. et al. [2006]. J. Geophys. Res. 111 (A6), CiteID A06310; Kliore, A.J. et al. [2009]. J. Geophys. Res. 114 (A4), CiteID A04315). We detect several discrete scales of variability present in the observations. Small-scale variability (S < 700 km) is observed in almost all data sets at different latitudes, both at dawn and dusk conditions. The most typical scale of variability is 300 km with scales between 200 km and 450 km being commonly present in the vast majority of the profiles. A low latitude dawn/dusk asymmetry is noted in the prevalent scales with the spectrum peaking sharply at the 300 km scale at dusk conditions and being broader at dawn conditions. Compared to dawn conditions the dusk ionosphere also shows more significant variability at the 100 km scale. The 300 km vertical scale is also present in the few available profiles from the northern hemisphere. Early observations from 2005 show a dominant scale at 350 km whereas later in 2007–2008 the spectrum shifts to the shorter scales with the most prominent scale being 300 km. The performed wavelet analysis and the obtained results are independent of assumptions about the nature of the layers and do not require a definition for a “background” electron density profile.In the second part of the paper we present a gravity wave propagation/dissipation model for Saturn’s upper atmosphere and compare the wave properties to the characteristics of the observed electron density variability at different scales. The general features observed in the data are consistent with gravity waves being present in the lower ionosphere and causing layering of the ions and the electrons. The wave-driving mechanism provides a simultaneous explanation for several of the properties of the observed variability: (i) lack of variability in the electron density above the predicted region of wave dissipation; (ii) in most cases the peak amplitude of variability occurs within the altitude range for dissipation of gravity waves or below; (iii) shorter scales have smaller amplitudes than the longer scales; (iv) shorter scales are present at lower altitudes whereas longer scales persist to higher altitudes; and (v) several layers often form a system of equally spaced maxima and minima that can be traced over a large altitude range.     

Processes forming and sustaining Saturn’s proton radiation belts

Saturn’s proton radiation belts extend over the orbits of several moons that split this region of intense radiation into several distinct belts. Understanding their distribution requires to understand how their particles are created and evolve. High-energy protons are thought to be dominantly produced by cosmic ray albedo neutron decay (CRAND). The source of the lower energies and the role of other effects such as charge exchange with the gas originating from Enceladus is still an open question. There is also no certainty so far if the belts exist independently from each other and the rest of the magnetosphere or if and how particles are exchanged between these regions. We approach these problems by using measurements acquired by the MIMI/LEMMS instrument onboard the Cassini spacecraft. Protons in the range from 500 keV to 40 MeV are considered. Their intensities are averaged over 7 years of the mission and converted to phase space densities at constant first and second adiabatic invariant. We reproduce the resulting radial profiles with a numerical model that includes radial diffusion, losses from moons and interactions with gas, and a phenomenological source. Our results show that the dominating effects away from the moon sweeping corridors are diffusion and the source, while interactions with gas are secondary. Based on a GEANT4 simulation of the interaction of cosmic rays with Saturn’s rings, we conclude that secondary particles produced within the rings can only account for the high-energy part of the source. A comparison with the equivalent processes within Earth’s atmosphere shows that Saturn’s atmosphere can contribute to the production of the lower energies and might be even dominating at the higher energies. Other possibilities to supply the belts and exchange particles between them, as diffusion and injections from outside the belts, or stripping of ENAs, can be excluded.     

Solar Cycle Effects on the Dynamics of Jupiter’s and Saturn’s Magnetospheres

The giant planetary magnetospheres surrounding Jupiter and Saturn respond in quite different ways, compared to Earth, to changes in upstream solar wind conditions. Spacecraft have visited Jupiter and Saturn during both solar cycle minima and maxima. In this paper we explore the large-scale structure of the interplanetary magnetic field (IMF) upstream of Saturn and Jupiter as a function of solar cycle, deduced from solar wind observations by spacecraft and from models. We show the distributions of solar wind dynamic pressure and IMF azimuthal and meridional angles over the changing solar cycle conditions, detailing how they compare to Parker predictions and to our general understanding of expected heliospheric structure at 5 and 9 AU. We explore how Jupiter’s and Saturn’s magnetospheric dynamics respond to varying solar wind driving over a solar cycle under varying Mach number regimes, and consider how changing dayside coupling can have a direct effect on the nightside magnetospheric response. We also address how solar UV flux variability over a solar cycle influences the plasma and neutral tori in the inner magnetospheres of Jupiter and Saturn, and estimate the solar cycle effects on internally driven magnetospheric dynamics. We conclude by commenting on the effects of the solar cycle in the release of heavy ion plasma into the heliosphere, ultimately derived from the moons of Jupiter and Saturn.     

On the thermal history of Saturn’s satellites Titan and Enceladus

The thermal histories of two geologically active satellites of Saturn—Titan and Enceladus—are discussed. During the Cassini mission, it was found that there are both nitrogen-containing compounds—NH₃ and N₂-and CO₂ and CH₄ in the water plumes of Enceladus; at that, ammonia is the prevailing form. This may testify that during evolution, the material of the satellite was warmed up to T ∼ 500–600 K, when NH₃ (the form of nitrogen capable of being accreted) could only be partly converted into N₂. Contrary to Enceladus, the temperature inside Titan probably reached values higher than 800 K or even higher than 1000 K, since the process of the chemical dissociation of ammonia was completely finished on this satellite and its atmosphere contains only molecular nitrogen. While the internal heating of Titan up to high temperatures can be explained by its large mass, the heating source for Enceladus’ interior is far from evident. Such traditional heating sources as the energy of gravitational differentiation and the radiogenic heating due to shortliving ²⁶Al and ⁶⁰Fe could not be effective. The first one is because of the small size of Enceladus (RE ≈ 250 km), and the inefficiency of the second one is caused by the fact that the satellite was formed not earlier than 8–10 Myr after the formation of calcium and aluminum-enriched inclusions in carbonaceous chondrites (CAI), i.e., after ²⁶Al had completely decayed. In the present paper, we propose other heating mechanisms-the heat of long-living radioactive elements and tidal heat, which could provide the observed chemical composition of the water plumes of Enceladus rather than only the differentiation of its protomatter into the ironstone core and the ice mantle.     

Changes in the activity of Jupiter’s hemispheres

Solar-induced changes in the reflective properties of the visible disk of Jupiter mostly depend on variations in the Earth’s jovimagnetic latitude. Since the orbit of Jupiter is eccentric (the eccentricity is e = 0.04845) and the planet passes perihelion at the time close to the summer solstice, the atmosphere receives 21% more solar energy in the northern hemisphere than in the southern one. According to the results of our studies, the ratio of the brightness values for the northern and southern tropical and temperate zones is a clear indicator of photometric activity of the processes in the atmosphere of Jupiter. From the analysis of the observational data for the period from 1962 to 2017, the cyclicity in changes of the activity factor of the hemispheres of the planet with a period of 11.87 years was found. This suggests that the atmosphere of Jupiter experiences seasonal restructuring.     

The origin and evolution of Saturn’s 2011–2012 stratospheric vortex

The planet-encircling springtime storm in Saturn’s troposphere (December 2010–July 2011, Fletcher, L.N. et al. [2011]. Science 332, 1413–1414; Sánchez-Lavega, A. et al. [2011]. Nature 475, 71–74; Fischer, G. et al. [2011]. Nature 475, 75–77) produced dramatic perturbations to stratospheric temperatures, winds and composition at mbar pressures that persisted long after the tropospheric disturbance had abated. Thermal infrared (IR) spectroscopy from the Cassini Composite Infrared Spectrometer (CIRS), supported by ground-based IR imaging from the VISIR instrument on the Very Large Telescope and the MIRSI instrument on NASA’s IRTF, is used to track the evolution of a large, hot stratospheric anticyclone between January 2011 and March 2012. The evolutionary sequence can be divided into three phases: (I) the formation and intensification of two distinct warm airmasses near 0.5 mbar between 25 and 35°N (B1 and B2) between January–April 2011, moving westward with different zonal velocities, B1 residing directly above the convective tropospheric storm head; (II) the merging of the warm airmasses to form the large single ‘stratospheric beacon’ near 40°N (B0) between April and June 2011, disassociated from the storm head and at a higher pressure (2 mbar) than the original beacons, a downward shift of 1.4 scale heights (approximately 85 km) post-merger; and (III) the mature phase characterised by slow cooling (0.11 ± 0.01 K/day) and longitudinal shrinkage of the anticyclone since July 2011. Peak temperatures of 221.6 ± 1.4 K at 2 mbar were measured on May 5th 2011 immediately after the merger, some 80 K warmer than the quiescent surroundings. From July 2011 to the time of writing, B0 remained as a long-lived stable stratospheric phenomenon at 2 mbar, moving west with a near-constant velocity of 2.70 ± 0.04 deg/day (?24.5 ± 0.4 m/s at 40°N relative to System III longitudes). No perturbations to visible clouds and hazes were detected during this period.With no direct tracers of motion in the stratosphere, we use thermal windshear calculations to estimate clockwise peripheral velocities of 200–400 m/s at 2 mbar around B0. The peripheral velocities of the two original airmasses were smaller (70–140 m/s). In August 2011, the size of the vortex as defined by the peripheral collar was 65° longitude (50,000 km in diameter) and 25° latitude. Stratospheric acetylene (C₂H₂) was uniformly enhanced by a factor of three within the vortex, whereas ethane (C₂H₆) remained unaffected. The passage of B0 generated a new band of warm stratospheric emission at 0.5 mbar at its northern edge, and there are hints of warm stratospheric structures associated with the beacons at higher altitudes (p < 0.1 mbar) than can be reliably observed by CIRS nadir spectroscopy. Analysis of the zonal windshear suggests that Rossby wave perturbations from the convective storm could have propagated vertically into the stratosphere at this point in Saturn’s seasonal cycle, one possible source of energy for the formation of these stratospheric anticyclones.     

A predator–prey model for moon-triggered clumping in Saturn’s rings

UVIS occultation data show clumping in Saturn’s F ring and at the B ring outer edge, indicating aggregation and disaggregation at these locations that are perturbed by Prometheus and by Mimas. The inferred timescales range from hours to months. Occultation profiles of the edge show wide variability, indicating perturbations by local mass aggregations. Structure near the B ring edge is seen in power spectral analysis at scales 200–2000 m. Similar structure is also seen at the strongest density waves, with significance increasing with resonance strength. For the B ring outer edge, the strongest structure is seen at longitudes 90° and 270° relative to Mimas. This indicates a direct relation between the moon and the ring clumping. We propose that the collective behavior of the ring particles resembles a predator–prey system: the mean aggregate size is the prey, which feeds the velocity dispersion; conversely, increasing dispersion breaks up the aggregates. Moons may trigger clumping by streamline crowding, which reduces the relative velocity, leading to more aggregation and more clumping. Disaggregation may follow from disruptive collisions or tidal shedding as the clumps stir the relative velocity. For realistic values of the parameters this yields a limit cycle behavior, as for the ecology of foxes and hares or the “boom-bust” economic cycle. Solving for the long-term behavior of this forced system gives a periodic response at the perturbing frequency, with a phase lag roughly consistent with the UVIS occultation measurements. We conclude that the agitation by the moons in the F ring and at the B ring outer edge drives aggregation and disaggregation in the forcing frame. This agitation of the ring material may also allow fortuitous formation of solid objects from the temporary clumps, via stochastic processes like compaction, adhesion, sintering or reorganization that drives the denser parts of the aggregate to the center or ejects the lighter elements. Any of these more persistent objects would then orbit at the Kepler rate. We would also expect the formation of clumps and some more permanent objects at the other perturbed regions in the rings… including satellite resonances, shepherded ring edges, and near embedded objects like Pan and Daphnis (where the aggregation/disaggregation cycles are forced similar to Prometheus forcing of the F ring).     

Geophysical evolution of Saturn’s satellite Phoebe,a large planetesimal in the outer Solar System

Saturn’s satellite Phoebe is the best-characterized representative of large outer Solar System planetesimals, thanks to the close flyby by the Cassini spacecraft in June 2004. We explore the information contained in Phoebe’s physical properties, density and shape, which are significantly different from those of other icy objects in its size range. Phoebe’s higher density has been interpreted as evidence that it was captured, probably from the proto-Kuiper-Belt. First, we demonstrate that Phoebe’s shape is globally relaxed and consistent with a spheroid in hydrostatic equilibrium with its rotation period. This distinguishes the satellite from ‘rubble-piles’ that are thought to result from the disruption of larger proto-satellites. We numerically model the geophysical evolution of Phoebe, accounting for the feedback between porosity and thermal state. We compare thermal evolution models for different assumptions on the formation of Phoebe, in particular the state of its water, amorphous or crystalline. We track the evolution of porosity and thermal conductivity as well as the destabilization of amorphous ice or clathrate hydrates. While rubble-piles may never reach temperatures suitable for porous ice to creep and relax, we argue that Phoebe’s shape could have relaxed due to heat from the decay of ²⁶Al, provided that this object formed less than 3 Myr after the production of the calcium–aluminum inclusions. This is consistent with the idea that Phoebe could be an exemplar of planetesimals that formed in the transneptunian region and later accreted onto outer planet satellites, either during the satellite’s formation stage, or still later, during the late heavy bombardment.     

Saturn’s F ring as seen by Cassini UVIS: Kinematics and statistics

We present a new orbital model of Saturn’s F ring core based on 93 occultations by the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Voyager radio and stellar occultations. We demonstrate that the core, despite its intrinsic variability, is well-described as an inclined, freely precessing ellipse. We find that post-fit residuals with a root-mean-square of 24 km are genuine, representing the well-known non-Keplerian features observed in the ring. Over the nearly 4 years of UVIS observations we find the residual variance to increase, coincident with the apse anti-alignment of Prometheus and F ring core in December 2009. This increase in dynamical F ring core temperature most likely reflects the ever-stronger perturbations by Prometheus. Our results are in good agreement with Earth-based and HST observations as well as Voyager imaging.Cassini UVIS stellar occultations resolve the F ring at unprecedented resolutions of a few meters and we identify the F ring core and inner and outer strands. We infer their normal optical depth and full width at half maximum (FWHM) and show that core and strands form distinct morphological groups. Typically, a strand is about ten times wider than the core (average FWHM is ～10 km) while having a ten times smaller optical depth. Unlike in pre-Cassini occultations the F ring core displays significant optical depth with in some cases >3. In many cases we find a narrow optically thick component (～ few km and τ > 0.5) embedded in the F ring core. Entertaining the possibility that this is the actual, “true” F ring core then UVIS results suggest that this “true” core is highly non-continuous. In addition, we report the detection of a previously unknown structure – dubbed the “secondary” as it visually resembles the F ring core. Its morphology is similar to that of the core in optical depth and FWHM and it displays individual opaque features. Despite its core-like appearance, we show that its kinematics is consistent with that of strands. We conclude that it is the most prominent strand seen to date. It represents a striking example of strand creation resulting in what could be called a morphological “small-scale” version of the F ring core. This extraordinary object should be one of the prime targets of future F ring studies.     

Diurnal variation of electron density in Saturn’s ionosphere: Model comparisons with Saturn Electrostatic Discharge (SED) observations

Using the Saturn Thermosphere Ionosphere Model (STIM), we present a study of the diurnal variation of electron density, with a focus on comparisons with peak electron densities (N_MAX) inferred from the low-frequency cutoff of radio emission due to lightning in the lower atmosphere, called Saturn Electrostatic Discharges (SEDs). It is demonstrated that photochemistry in Saturn’s ionosphere cannot reproduce the SED-inferred diurnal variation in N_MAX unless additional production and loss sources outside of the current best estimates are considered. Additional explanations of the SED-inferred diurnal variation of N_MAX are presented and analyzed, such as the possibility that the low-frequency cutoff seen in SEDs is due to the presence of sharp low-altitude layers of plasma, as frequently seen in radio occultation measurements. Finally, we outline the observational constraints that must be fulfilled by any candidate explanations of the SED-inferred diurnal variation of N_MAX.     

Discrete ULF modes in the Earth’s magnetosphere near the Alfven frequency minimum

The equation of small oscillations of ULF waves in the Earth’s magnetosphere is derived accounting for a fast magnetosonic wave. The spectrum of discrete Alfven modes near the Alfven frequency minimum is studied on the basis of this equation.     

Further seasonal changes in Uranus’ cloud structure observed by Gemini-North and UKIRT

Near-infrared observations of Uranus were made in October/November 2010 with the Gemini-North telescope in Hawaii, using NIFS, an integral field spectrograph, and the NIRI instrument in imaging mode. Observations were acquired using adaptive optics and have a spatial resolution of approximately 0.1–0.2″.The observed spectra along Uranus’ central meridian were analysed using a multiple-scattering retrieval algorithm to infer the vertical/latitudinal variation in cloud optical depth, which we compare with previous observations made by Gemini-North/NIFS in 2009 and UKIRT/UIST observations made between 2006 and 2008. Assuming a continuous distribution of small particles (r ～ 1 μm, and refractive index of 1.4 + 0i) with the single scattering albedo set to 0.75 and using a Henyey–Greenstein phase function with asymmetry parameter set to 0.7 at all wavelengths and latitudes, the retrieved cloud density profiles show that the north polar zone at 45°N has continued to steadily brighten while the south polar zone at 45°S has continued to fade. As with our previous analyses we find that, assuming that the methane vertical profile is the same at all latitudes, the clouds forming these polar zones at 45°N and 45°S lie at slightly lower pressures than the clouds at more equatorial latitudes. However, we also find that the Gemini data can be reproduced by assuming that the main cloud remains fixed at ～2 bar at all latitudes and adjusting the relative humidity of methane instead. In this case we find that the deep cloud is still more opaque at the equator and at the zones at 45°N and 45°S and shows the same seasonal trends as when the methane humidity remain fixed. However, with this approach the relative humidity of methane is seen to rise sharply from approximately 20% at polar latitudes to values closer to 80% for latitudes equatorward of 45°S and 45°N, consistent with the analysis of 2002 HST observations by Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2009]. Icarus 202, 287–302), with a possible indication of seasonal variability. Overall, Uranus appeared to be less convectively active in 2010 than in the previous 4 years, supporting the conclusion that now the northern spring equinox (which occurred in 2007) has passed, the atmosphere is settling back into the more quiescent state seen by Voyager 2 in 1986.     

Ohm’s law for plasma in general relativity and Cowling’s theorem

The general-relativistic Ohm’s law for a two-component plasma which includes the gravitomagnetic force terms even in the case of quasi-neutrality has been derived. The equations that describe the electromagnetic processes in a plasma surrounding a neutron star are obtained by using the general relativistic form of Maxwell equations in a geometry of slow rotating gravitational object. In addition to the general-relativistic effect first discussed by Khanna and Camenzind (Astron. Astrophys. 307:665, 1996) we predict a mechanism of the generation of azimuthal current under the general relativistic effect of dragging of inertial frames on radial current in a plasma around neutron star. The azimuthal current being proportional to the angular velocity ω of the dragging of inertial frames can give valuable contribution on the evolution of the stellar magnetic field if ω exceeds 2.7×10¹⁷(n/σ) s⁻¹ (n is the number density of the charged particles, σ is the conductivity of plasma). Thus in general relativity a rotating neutron star, embedded in plasma, can in principle generate axial-symmetric magnetic fields even in axisymmetry. However, classical Cowling’s antidynamo theorem, according to which a stationary axial-symmetric magnetic field can not be sustained against ohmic diffusion, has to be hold in the general-relativistic case for the typical plasma being responsible for the rotating neutron star.     

Photographic observations of Saturn’s moons at the MAO NAS of Ukraine in 1961–1990

A catalog of 1385 astrometric positions of Saturn’s moons S2–S9 has been compiled with Tycho-2 as a reference frame from photographic observations obtained at the Main Astronomical Observatory, National Academy of Sciences of Ukraine, in 1961–1990. Astronegatives have been digitized with an Epson Expression 10000XL commercial scanner in 16-bit grayscale with a resolution of 1200 dpi. Reduction has been performed in the LINUX-MIDAS-ROMAFOT software supplemented with additional modules. The internal positional accuracy of the reduction is 0.09…0.23′′ for both coordinates and 0.27…0.37^m for the photographic magnitudes of the Tycho-2 catalog. The calculated topocentric positions of the moons are compared online with the IMCCE ephemeris data (DE405 + TASS1,7). Moon-minus-moon differential coordinates are found for most of the moons and compared with theoretical data (http://lnfm1.sai.msu.ru/neb/nss/nssephmr.htm).     

Atronomical CCD observations of the main Saturn’s satellites at Pulkovo Observatory in 2004–2007

The results of astrometric observations of Saturn’s satellites (S1–S8) obtained using a 26-inch refractor and a normal astrograph at Pulkovo Observatory in 2004–2007 are given. High-accuracy equatorial coordinates of Saturn’s satellites in the system of the UCAC2 reference catalog and the relative “satellite-satellite” positions suitable for specifying their motion theories are obtained. The observations are compared with the DE405 + TASS1.7 and INPOP06 + TASS1.7 theories of motion. The root-mean-square errors of the obtained satellite positions lie within the range of 10–50 mas, as far as the intrinsic convergence is concerned, and 20–70 mas, as far as the extrinsic one is concerned. The observation results are included into the astrometrical database of the Pulkovo Observatory (www.puldb.ru).     

Dynamics of the Jupiter Trojans with Saturn’s perturbation when the two planets are in migration

In a previous paper (Hou et al. in Celest Mech Dyn Astron 119:119–142, 2014a), the problem of dynamical symmetry between two Jupiter triangular libration points (TLPs) with Saturn’s perturbation in the present configuration of the two planets was studied. A small short-time scale spatial asymmetry exists but gradually disappears with the time going, so the planar stable regions around the two Jupiter TLPs should be dynamically symmetric from a longtime perspective. In this paper, the symmetry problem is studied when the two planets are in migration. Several mechanisms that can cause asymmetries are discussed. Studies show that three important ones are the large short-time scale spatial asymmetry when Jupiter and Saturn are in resonance, the changing orbits of Jupiter and Saturn in the planet migration process, and the chaotic nature of Trojan orbits during the planet migration process. Their joint effects can cause an observable difference to the two Jupiter Trojan swarms. The thermal Yarkovsky effect is also found to be able to cause dynamical differences to the two TLPs, but generally they are too small to be practically observed.