Cassini Data Research Articles

Coupled ion and neutral rotating model of Titan's upper atmosphere

A one-dimensional composition model of Titan's upper atmosphere is constructed, coupling 36 neutral species and 47 ions. Energy inputs from the Sun and from Saturn's magnetosphere and updated temperature and eddy coefficient parameters are taken into account. A rotating technique at constant latitude and varying local-time is proposed to account for the diurnal variation of solar inputs. The contributions of photodissocation, neutral chemistry, ion–neutral chemistry, and electron recombination to neutral production are presented as a function of altitude and local time. Local time-dependent mixing ratio and density profiles are presented in the context of the TA and T5 Cassini data and are compared in detail to previous models. An independent and simplified ion and neutral scheme (19-species) is also proposed for future 3D-purposes. The model results demonstrate that a complete understanding of the chemistry of Titan's upper atmosphere requires an understanding of the coupled ion and neutral chemistry. In particular, the ionospheric chemistry makes significant contributions to production rates of several important neutral species.

Saturn kilometric radiation as a monitor for the solar wind?

Since the Voyager mission it is known that Saturn Kilometric Radiation (SKR) is strongly influenced by external forces, i.e., the solar wind and in particular the solar wind ram pressure. Recent studies using Cassini data essentially confirmed these findings for particular periods during the first Cassini orbit of Saturn. The data coverage of SKR by the Cassini/RPWS experiment for the period of six months prior to Saturn Orbit Insertion (July 1, 2004) is rather continuous, whereas there are gaps in the solar wind plasma data. The strong correlation of SKR with the solar wind may provide an indication on the variations of the solar wind plasma, specifically during the gap periods. These periods lacking solar wind data are substituted by Ulysses solar wind data which have been propagated over ∼4 AU, applying magnetohydrodynamic propagation models. Cross correlation studies showed that Ulysses solar wind data can be taken as a substitute for missing Cassini data. The use of SKR as monitor for solar wind variations is discussed. With the present set of observations the SKR proxy lacks significant reliability.

Magnetic field structure of Saturn's dayside magnetosphere and its mapping to the ionosphere: Results from ring current modeling

Ring current modeling in Saturn's magnetosphere using Pioneer‐11, Voyager, and Cassini data has shown that the size and strength of the current system grows with the extension of the magnetosphere, governed by the solar wind dynamic pressure. Here we show that the consequent middle magnetosphere field is quasi‐dipolar in form when the magnetosphere is strongly compressed, but extends into a magnetodisc when it is strongly expanded. We also show that the region occupied by the modeled ring current corresponds to an essentially fixed shell of field lines that expands and contracts with the size of the system, thus mapping to an almost fixed co‐latitude range in Saturn's ionosphere, between ∼14° and ∼20° in the northern hemisphere, and ∼16° and ∼22° in the southern hemisphere. The median dayside UV auroral oval is found to map from near the poleward edge of the modeled ring current toward the boundary of open field lines at smaller co‐latitudes. In the equatorial plane this corresponds to a layer ∼2–5 Saturn radius wide (depending on magnetosphere size), extending from near the outer edge of the ring current to the vicinity of the magnetopause.

VLA observations of synchrotron radiation at 15 GHz

We present observations of the synchrotron radiation from Jupiter obtained in July 2004 with the Very Large Array at a frequency of 15 GHz. The array was in its most compact (D) configuration and the distance to Jupiter was 6.077 AU, making the apparent size of Jupiter relatively small, and favorable for such observations. We measured a total synchrotron radiation flux density of 1.26 ± 0.12 Jy , scaled to a distance of 4.04 AU, which was 2.4% of the total flux density from the planet. Our results agree, within the uncertainties, with a previous VLA measurement ( 1.5 ± 0.15 Jy in March 1991 [de Pater, I., Dunn, D., 2003. Icarus 163, 449–455]); both values are a factor of 3–3.5 higher than the flux density reported from Cassini data in 2001 [Bolton, S.J., and 20 colleagues, 2002. Nature 415, 987–991].

Cassini observations of the variation of Saturn's ring current parameters with system size

We have examined residual magnetic field vectors observed in Saturn's magnetosphere during the first 2 years of the Cassini mission and have fit them to a simple axisymmetric model of the ring current in the middle magnetosphere. We then examine the variations of the ring current parameters with size of the magnetosphere. In addition, we obtain secondary parameters, including the value of the axial field at the center of the ring (equivalently Saturn's Dst) Bz0, the total current IT flowing in the modeled ring current region, and the ratio of the ring current magnetic moment relative to the magnetic moment of Saturn's dipole field, kRC. Results show that the derived parameters increase significantly with system size, due principally to the increasing radius of the outer edge of the ring. We consider the implications of the response of the magnetic moment of the ring current to changing magnetospheric size, by theoretical consideration of the magnetic moment of individual particles in the ring current. The strong positive correlation of the ring current magnetic moment with system size suggests a system in which the ring current is dominated by inertia currents, rather than by thermal effects as in the case of the Earth, with magnetosphere‐ionosphere coupling maintaining the angular velocity of the plasma. The variations of Saturn's ring current parameters with system size found in this study are shown to be closely compatible with the size variations in response to the solar wind dynamic pressure recently determined from Cassini data.

Mass and interior of Enceladus from Cassini data analysis

Gravity results are available from radio Doppler data acquired by the Deep Space Network during the encounter of the Cassini spacecraft with Enceladus in February 2005. We report the mass of Enceladus to be ( 1.0798 ± 0.0016 ) × 10 20 kg , which implies a density of 1608.9 ± 4 kg m −3 . For a core made of hydrated silicates with a density of 2500 kg m −3 the core radius is ∼ 190 km and the quadrupole moment C 22 ∼ 1.4 × 10 −3 . If Enceladus is in hydrostatic equilibrium, the larger than previously anticipated density implies that the recently proposed secondary spin–orbit resonance cannot be present. Therefore, the source of endogenic activity of Enceladus remains unexplained.

Gravity field and interior of Rhea from Cassini data analysis

The Cassini spacecraft encountered Rhea on November 26, 2005. Analysis of the Doppler data acquired at and around closest approach yields the mass of Rhea and the quadrupole moments of its gravity field with unprecedented accuracy. We obtained G M = 153.9395 ± 0.0018 km 3 s −2 which corresponds to a density of 1232.8 ± 5.4 kg m −3 . Our results for J 2 and C 22 are ( 7.947 ± 0.892 ) × 10 −4 and ( 2.3526 ± 0.0476 ) × 10 −4 , respectively. These values are consistent with hydrostatic equilibrium. From the value of C 22 , we infer the non-dimensional moment of inertia C / M R 2 = 0.3721 ± 0.0036 . Our models of Rhea's interior based on the gravity data favor an almost undifferentiated satellite. A discontinuity between a core and a mantle is possible but not required by the data. Models with a constant silicate mass fraction throughout the body cannot account for the determined quadrupole coefficients. The data exclude fully differentiated models in which the core would be composed of unhydrated silicates and the mantle would be composed of pure ice. If the mantle contains 10% in mass of silicates, the core extends to 630 km in radius and has a silicate mass fraction of 40%. A continuous model in which the silicates are more concentrated toward the center of the body than in the outer layers is allowed by the gravity data but excluded by thermal evolution considerations. The one model that fits the gravity data and is self-consistent when energy transport and ice melting are qualitatively considered is an “almost undifferentiated” Rhea, in which a very large uniform core is surrounded by a relatively thin ice shell containing no rock at all.

Gravimagnetic effect of the barycentric motion of the Sun and determination of the post-Newtonian parameter γ in the Cassini experiment

The most precise test of the post-Newtonian γ parameter in the solar system has been achieved in measurement of the frequency shift of radio waves to and from the Cassini spacecraft as they passed near the Sun. The test relies upon the JPL model of radiowave propagation that includes, but does not explicitly parametrize, the impact of the non-stationary component of the gravitational field of the Sun, generated by its barycentric orbital motion, on the Shapiro delay. This non-stationary gravitational field of the Sun is associated with the Lorentz transformation of the metric tensor and the affine connection from the heliocentric to the barycentric frame of the solar system and can be treated as gravimagnetic field. The gravimagnetic field perturbs the propagation of a radio wave and contributes to its frequency shift at the level up to 4×10−13 that may affect the precise measurement of the parameter γ in the Cassini experiment to about one part in 10 000. Our analysis suggests that the translational gravimagnetic field of the Sun can be extracted from the Cassini data, and its effect is separable from the space curvature characterized by the parameter γ.

Saturn eddy momentum fluxes and convection: First estimates from Cassini images

We apply an automated cloud feature tracking algorithm to estimate eddy momentum fluxes in Saturn's southern hemisphere from Cassini Imaging Science Subsystem near-infrared continuum image sequences. Voyager Saturn manually tracked images had suggested no conversion of eddy to mean flow kinetic energy, but this was based on a small sample of <1000 wind vectors. The automated procedure we use for the Cassini data produces an order of magnitude more usable wind vectors with relatively unbiased sampling. Automated tracking is successful in and around the westward jet latitudes on Saturn but not in the vicinity of most eastward jets, where the linearity and non-discrete nature of cloud features produces ambiguous results. For the regions we are able to track, we find peak eddy fluxes ∼ 10 m 2 s −2 and a clear positive correlation between eddy momentum fluxes and meridional shear of the mean zonal wind, implying that eddies supply momentum to eastward jets and remove momentum from westward jets at a rate ∼ 5 × 10 −6 m s −2 . The behavior we observe is similar to that seen on Jupiter, though with smaller eddy-mean kinetic energy conversion rates per unit mass of atmosphere ( 3.3 × 10 −5 m 2 s −3 ). We also use the appearance and rapid evolution of small bright features at continuum wavelengths, in combination with evidence from weak methane band images where possible, to diagnose the occurrence of moist convective storms on Saturn. Areal expansion rates imply updraft speeds of ∼ 1 m s −1 over the convective anvil cloud area. As on Jupiter, convection preferentially occurs in cyclonic shear regions on Saturn, but unlike Jupiter, convection is also observed in eastward jet regions. With one possible exception, the large eddy fluxes seen in the cyclonic shear latitudes do not seem to be associated with convective events.

Wind variations in Jupiter's equatorial atmosphere: A QQO counterpart?

Jupiter's equatorial atmosphere, much like the Earth's, is known to show quasi-periodic variations in temperature, particularly in the stratosphere, but variations in other jovian atmospheric tracers have not been studied for any correlations to these oscillations. Data taken at NASA's Infrared Telescope Facility (IRTF) from 1979 to 2000 were used to obtain temperatures at two levels in the atmosphere, corresponding to the upper troposphere (250 mbar) and to the stratosphere (20 mbar). We find that the data show periodic signals at latitudes corresponding to the troposphere zonal wind jets, with periods ranging from 4.4 (stratosphere, 95% confidence at 4° S planetographic latitude) to 7.7 years (troposphere, 97% confidence at 6° N). We also discuss evidence that at some latitudes the troposphere temperature variations are out of phase from the stratosphere variations, even where no periodicity is evident. Hubble Space Telescope images were used, in conjunction with Voyager and Cassini data, to track small changes in the troposphere zonal winds from 20° N to 20° S latitude over the 1994–2000 time period. Oscillations with a period of 4.5 years are found near 7°–8° S, with 80–85% significance. Further, the strongest evidence for a QQO-induced tropospheric wind change tied to stratospheric temperature change occurs near these latitudes, though tropospheric temperatures show little periodicity here. Comparison of thermal winds and measured zonal winds for three dates indicate that cloud features at other latitudes are likely tracked at pressures that can vary by up to a few hundred millibar, but the cloud altitude change required is too large to explain the wind changes measured at 7° S.

The Orbits of Saturn's Small Satellites Derived from Combined Historic andCassiniImaging Observations

We report on the orbits of the small, inner Saturnian satellites, either recovered or newly discovered in recent Cassini imaging observations. The orbits presented here reflect improvements over our previously published values in that the time base of Cassini observations has been extended, and numerical orbital integrations have been performed in those cases in which simple precessing elliptical, inclined orbit solutions were found to be inadequate. Using combined Cassini and Voyager observations, we obtain an eccentricity for Pan 7 times smaller than previously reported because of the predominance of higher quality Cassini data in the fit. The orbit of the small satellite (S/2005 S1 [Daphnis]) discovered by Cassini in the Keeler gap in the outer A ring appears to be circular and coplanar; no external perturbations are apparent. Refined orbits of Atlas, Prometheus, Pandora, Janus, and Epimetheus are based on Cassini , Voyager, Hubble Space Telescope, and Earth-based data and a numerical integration perturbed by all the massive satellites and each other. Atlas is significantly perturbed by Prometheus, and to a lesser extent by Pandora, through high-wavenumber mean-motion resonances. Orbital integrations involving Atlas yield a mass of GMAtlas = (0.44 ± 0.04) × 10-3 km3 s -2, 3 times larger than reported previously (GM is the product of the Newtonian constant of gravitation G and the satellite mass M). Orbital integrations show that Methone is perturbed by Mimas, Pallene is perturbed by Enceladus, and Polydeuces librates around Dione's L5 point with a period of about 791 days. We report on the nature and orbits of bodies sighted in the F ring, two of which may have persisted for a year or more.

Jupiter's atmospheric temperatures: From Voyager IRIS to Cassini CIRS

Retrievals performed on Cassini Composite Infrared Spectrometer data obtained during the distant Jupiter flyby in 2000/2001 have been used to generate global temperature maps of the planet in the troposphere and stratosphere, but to higher latitudes than were shown previously by Flasar et al. [Flasar, F.M., 39 colleagues, 2004a. Nature 427, 132–135; Flasar, F.M., 44 colleagues, 2004b. Space Sci. Rev. 115, 169–297]. Similar retrievals were performed on Voyager 1 IRIS data to provide the first detailed IRIS map of the stratosphere, and high latitudes in the troposphere. Thermal winds were calculated for each data set and show strong vertical shears in the zonal winds at low latitudes, and meridional temperature gradients indicate the presence of circumpolar jets, as well. The temperatures retrieved from the two spacecraft were also compared with yearly ground-based data obtained over the intervening two decades. Tropospheric temperatures reveal gradual changes at low latitudes, with little obvious seasonal or short-term variation [Orton et al., 1994. Science 265, 625–631]. Stratospheric temperatures show much more complicated behavior over short timescales, consistent with quasi-quadrennial oscillations at low latitudes, as suggested in prior analyses of shorter intervals of ground-based data [Orton et al., 1991. Science 252, 537–542; Friedson, A.J., 1999. Icarus 137, 34–55]. A scaling analysis indicates that meridional motions, mechanically forced by wave or eddy convergence, play an important role in modulating the temperatures and winds in the upper troposphere and stratosphere on seasonal and shorter timescales. At latitudes away from the equator, the mechanical forcing can be derived simply from a temporal record of temperature and its vertical derivative. Ground-based observations with improved vertical resolution and/or long-term monitoring from spacecraft are required for this purpose, though the Voyager and Cassini data given indications that the magnitude of the forcing is very small.

Plutonium-238 and Cassini

Science is to be commended for the Cassini series of articles (25 Feb., pp. [1222-1276][1]). Plutonium-238-powered isotopic generators supplied the electricity that allowed Cassini's data gathering and transmission. Without Pu-238 sources, there would have been no Cassini. Yet, Cassini was almost

Comparative Study of the Dynamics of the Outer Planets

The two classes of outer planets, Gas Giants and Ice Giants, have distinctly different global circulation patterns and internal structure. Ongoing ground-based observations of the Ice Giants provide clues to better understanding and Galileo and Cassini data will generate constraints for Gas Giant modeling. The composition below the cloud levels, the depths to which the winds penetrate and the processes that sustain the zonal winds and weather systems are not understood. Basic questions concerning the structure, composition and atmospheric dynamics that are sustained on the four giants could be answered by a combination of orbiters and probes. Future missions that could answer these questions are not currently under development.

Observations of two complete substorm cycles during the Cassini Earth swing‐by: Cassini magnetometer data in a global context

During the Earth swing‐by of the Cassini spacecraft, a worldwide program of data‐gathering was undertaken to define the prevailing interplanetary and geophysical conditions. This included observations of the interplanetary medium, outer magnetosphere, geostationary orbit, UV aurora, geomagnetic disturbance, and ionospheric flow. These data show that during the Cassini outbound passage through the geomagnetic tail the magnetosphere underwent two complete “classic” substorm cycles. The global data are used to set the Cassini data into a context which allows a much fuller interpretation. The pass took place through the dawn tail, where previous Geotail observations indicate that the plasma sheet usually remains ‘stationary’ at expansion phase onset. Reconnection and plasmoid formation are typically dusk and midnight phenomena. The Cassini observations nevertheless show a marked response of the plasma sheet both to the growth phase (thinning), and expansion phase onset (expansion without heating). Subsequent impulsive expansion of the substorm into the Cassini sector, however, resulted in the prompt appearance of reconnection‐related phenomena (earthward flowing plasma and strongly disturbed fields), though the spacecraft footprint remained poleward of the intense UV auroras. Due to continuing strong southward‐directed IMF during the expansion phases, a quasi‐equilibrium appears to have formed between dayside and near‐Earth nightside reconnection for ∼30 min after onset. Auroral zone recovery began about ∼10 min after a northward turn of the interplanetary magnetic field (IMF) to nearer‐zero values in each case. A net closure of open flux then ensued, leading to deflation of the tail lobe field, ‘dipolarization’ of the near‐Earth tail plasma sheet field, simultaneous reduction in the earthward plasma sheet flow and the flow in the nightside ionosphere, and displacement of the ground‐based disturbance to high latitudes.

Looking for Changes in the Saturnian System Between Voyager and Cassini

This paper reviews a number of time-dependent phenomena that are relevant to our understanding of the dynamics of planetary rings and that will be investigated using Voyager and Cassini data. A long time baseline may help us decipher the physics of the spokes, understand better the morphology of the F ring and the rigid precession of non-circular ringlets, measure more precisely than has been done so far the satellites' torques and the viscosity of the A ring, and discover small satellites in the Saturnian ring system. Two exciting possibilities are those of determining the recession rates of the small satellites that border the rings, and of observing changes due to viscous diffusion in the irregular structures of the B ring.