Cassini thermal observations of Saturn's main rings: Implications for particle rotation and vertical mixing

In late 2004 and 2005 the Cassini composite infrared spectrometer (CIRS) obtained spatially resolved thermal infrared radial scans of Saturn's main rings (A, B and C, and Cassini Division) that show ring temperatures decreasing with increasing solar phase angle, α, on both the lit and unlit faces of the ring plane. These temperature differences suggest that Saturn's main rings include a population of ring particles that spin slowly, with a spin period greater than 3.6 h, given their low thermal inertia. The A ring shows the smallest temperature variation with α, and this variation decreases with distance from the planet. This suggests an increasing number of smaller, and/or more rapidly rotating ring particles with more uniform temperatures, resulting perhaps from stirring by the density waves in the outer A ring and/or self-gravity wakes.The temperatures of the A and B rings are correlated with their optical depth, τ, when viewed from the lit face, and anti-correlated when viewed from the unlit face. On the unlit face of the B ring, not only do the lowest temperatures correlate with the largest τ, these temperatures are also the same at both low and high α, suggesting that little sunlight is penetrating these regions.The temperature differential from the lit to the unlit side of the rings is a strong, nearly linear, function of optical depth. This is consistent with the expectation that little sunlight penetrates to the dark side of the densest rings, but also suggests that little vertical mixing of ring particles is taking place in the A and B rings.     

Topographic modeling of Phoebe using Cassini images

High-resolution Cassini stereo images of Saturn's moon Phoebe have been used to derive a regional digital terrain model (DTM) and an orthoimage mosaic of the surface. For DTM-control a network of 130 points measured in 14 images (70-390 m/pixel resolution) was established which was simultaneously used to determine the orientation of the spin-axis. The J2000 spin-axis was found at Dec=78.0°±0.1° and RA=356.6°±0.3°, substantially different from the former Voyager solution. The control points yield a mean figure radius of 107.2 km with RMS residuals of 6.2 km demonstrating the irregular shape of this body. The DTM was computed from densely spaced conjugate image points determined by methods of digital image correlation. It has a horizontal resolution of 1-2 km and vertical accuracies in the range 50-100 m. It is limited in coverage, but higher in resolution than the previously derived global shape model of Phoebe [Porco et al., 2005. Cassini imaging science: initial results on Phoebe and Iapetus. Science 307, 1237-1242] and allows us to study the morphology of the surface in more detail. There is evidence for unconsolidated material from a steep and smooth slope at the rim of a 100 km impact feature. There are several conically shaped craters on Phoebe, which may hint at highly porous and low compaction material on the surface.     

The calibration of the Cassini-Huygens CAPS Electron Spectrometer

We present the two-stage method used to calibrate the electron spectrometer (ELS), part of the plasma spectrometer (CAPS) on board the Cassini spacecraft currently in orbit around Saturn. The CAPS-ELS is a top-hat electrostatic analyser designed to measure electron fluxes between 0.5 eV and 26 keV. The on-ground calibration method described here includes the production of photoelectrons, which are energised and passed into the CAPS-ELS in a purpose designed calibration facility. Knowledge of the intensity of these incident electrons and the subsequent instrument output provides an on-ground calibrated geometric factor. Comparative studies of physical quantities such as plasma density and electron differential flux calculated using on-ground calibration factor with the quantities deduced from the wave experiment and high energy electron detector provide in-flight calibration. The results of this are presented together with a comparison of the experimentally calibrated values with simulated calibration values.     

Titan's highly dynamic magnetic environment: A systematic survey of Cassini magnetometer observations from flybys TA-T62

We analyze the variability of the ambient magnetic field near Titan during Cassini encounters TA-T62 (October 2004-October 2009). Cassini magnetometer (MAG) data show that the moon's magnetic environment is strongly affected by its proximity to Saturn's warped and highly dynamic magnetodisk. In the nightside sector of Saturn's magnetosphere, the magnetic field near Titan is controlled by intense vertical flapping motions of the magnetodisk current sheet, alternately exposing the moon to radially stretched lobe-type fields and to more dipolar, but highly distorted current sheet fields. In southern summer, when most of the Cassini encounters took place, the magnetodisk current sheet was on average located above Titan's orbital plane. However, around equinox in August 2009, the distortions of Titan's magnetic environment due to the rapidly moving current sheet reached a maximum, thus suggesting that the equilibrium position of the sheet at that time was significantly closer to the moon's orbital plane. In the dayside magnetosphere, the formation of the magnetodisk lobes is partially suppressed due to the proximity of the magnetopause. Therefore, during most encounters that took place near noon, Titan was embedded in highly distorted current sheet fields. Within the framework of this study, we not only provide a systematic classification of all Titan flybys between October 2004 and October 2009 as lobe-type or current sheet scenarios, but we also calculate the magnetospheric background field near Titan's orbit whenever possible. Our results show that so far, there is not a single Cassini flyby that matches the frequently applied picture of Titan's plasma interaction from the pre-Cassini era (background field homogeneous, stationary and perpendicular to the moon's orbital plane). The time scales upon which the ambient magnetospheric field close to Titan undergoes significant changes range between only a few minutes and up to several hours. The implications for the development of numerical models for Titan's local plasma interaction are discussed as well.     

The Spin-Orbit Resonant Rotation of Mercury: A Two Degree of Freedom Hamiltonian Model

The paper develops a hamiltonian formulation describing the coupled orbital and spin motions of a rigid Mercury rotation about its axis of maximum moment of inertia in the frame of a 3:2 spin orbit resonance; the (ecliptic) obliquity is not constant, the gravitational potential of mercury is developed up to the second degree terms (the only ones for which an approximate numerical value can be given) and is reduced to a two degree of freedom model in the absence of planetary perturbations. Four equilibria can be calculated, corresponding to four different values of the (ecliptic) obliquity. The present situation of Mercury corresponds to one of them, which is proved to be stable. We introduce action-angle variables in the neighborhood of this stable equilibrium, by several successive canonical transformations, so to get two constant frequencies, the first one for the free spin-orbit libration, the other one for the 1:1 resonant precession of both nodes (orbital and rotational) on the ecliptic plane. The numerical values obtained by this simplified model are in perfect agreement with those obtained by Rambaux and Bois [Astron. Astrophys. 413, 381–393]. This revised version was published online in July 2006 with corrections to the Cover Date.     

Investigation of energetic proton penetration in Titan's atmosphere using the Cassini INCA instrument

Saturn's largest moon, Titan, provides an interesting opportunity to study how dense atmospheres interact with the surrounding plasma environment. Without an intrinsic magnetic field, this satellite's nitrogen-rich atmosphere is relatively unprotected from plasma interactions. Therefore, the energy-deposition rate is important for understanding chemistry and dynamics in Titan's atmosphere. Since the plasma environment can vary significantly we focus here on the T18 Titan encounter using in-situ data from instruments on board the Cassini spacecraft. These instruments cannot provide in-situ information below the spacecraft closest approach altitude (∼>960 km) so we use the Cassini magnetospheric imaging instrument (MIMI) ion-neutral camera (INCA) to remotely image energetic hydrogen particle fluxes (20-80 keV) at altitudes below Titan closest approach. We also use the MIMI low-energy magnetosphere measurements system (LEMMS) to measure the incident ion fluxes as the spacecraft approaches Titan and combine these data sets with an atmospheric model to first reproduce INCA images. We then use this model to calculate the energy-deposition profiles for the observed incident proton flux. Our model is able to reproduce the INCA observations and give the energy density deposited vs. altitude in Titan's atmosphere; however, we find that the incident fluxes and energy-deposition profiles vary significantly during the encounter.     

High-resolution CASSINI-VIMS mosaics of Titan and the icy Saturnian satellites

The Visual Infrared Mapping Spectrometer (VIMS) onboard the CASSINI spacecraft obtained new spectral data of the icy satellites of Saturn after its arrival at Saturn in June 2004. VIMS operates in a spectral range from 0.35 to 5.2 μm, generating image cubes in which each pixel represents a spectrum consisting of 352 contiguous wavebands.As an imaging spectrometer VIMS combines the characteristics of both a spectrometer and an imaging instrument. This makes it possible to analyze the spectrum of each pixel separately and to map the spectral characteristics spatially, which is important to study the relationships between spectral information and geological and geomorphologic surface features.The spatial analysis of the spectral data requires the determination of the exact geographic position of each pixel on the specific surface and that all 352 spectral elements of each pixel show the same region of the target. We developed a method to reproject each pixel geometrically and to convert the spectral data into map projected image cubes. This method can also be applied to mosaic different VIMS observations. Based on these mosaics, maps of the spectral properties for each Saturnian satellite can be derived and attributed to geographic positions as well as to geological and geomorphologic surface features. These map-projected mosaics are the basis for all further investigations.     

On the discovery of CO nighttime emissions on Titan by Cassini/VIMS: Derived stratospheric abundances and geological implications

We present a quantitative analysis of CO thermal emissions discovered on the nightside of Titan by Baines et al. [2005. The atmospheres of Saturn and Titan in the near-infrared: First results of Cassini/VIMS. Earth, Moon, and Planets, 96, 119–147]. in Cassini/VIMS spectral imagery. We identify these emission features as the P and R branches of the 1-0 vibrational band of carbon monoxide (CO) near 4.65 μm. For CH₃D, the prominent Q branch of the ν₂ fundamental band of CH₃D near 4.55 μm is apparent. CO₂ emissions from the strong v₃ vibrational band are virtually absent, indicating a CO₂ abundance several orders of magnitude less than CO, in agreement with previous investigations. Analysis of CO emission spectra obtained over a variety of altitudes on Titan's nightside limb indicates that the stratospheric abundance of CO is 32±15 ppm, and together with other recent determinations, suggests a vertical distribution of CO nearly constant at this value from the surface throughout the troposphere to at least the stratopause near 300 km altitude. The corresponding total atmospheric content of CO in Titan is 2.9±1.5×10¹⁴ kg. Given the long lifetime of CO in the oxygen-poor Titan atmosphere (0.5–1.0 Gyr), we find a mean CO atmospheric production rate of 6±3×10⁵ kg yr⁻¹. Given the lack of primordial heavy noble gases observed by Huygens [Niemann et al., 2005. The abundances of constituents of Titan's atmosphere from the GCMS on the Huygens probe. Nature, 438, 779–784], the primary source of atmospheric CO is likely surface emissions. The implied CO/CH₄ mixing ratio of near-surface material is 1.8±0.9×10⁻⁴, based on an average methane surface emission rate over the past 0.5 Gyr of 1.3×10⁻¹³ gm cm⁻² s⁻¹ as required to balance hydrocarbon haze production via methane photolysis [Wilson and Atreya, 2004. Current state of modeling the photochemistry of Titan's mutually dependent atmosphere and ionosphere. J. Geophys. Res. 109, E06002 Doi:10.1029/2003JE002181]. This low CO/CH₄ ratio is much lower than expected for the sub-nebular formation region of Titan and supports the hypothesis [e.g., Atreya et al., 2005. Methane on Titan: photochemical-meteorological-hydrogeochemical cycle. Bull. Am. Astron. Soc. 37, 735] that the conversion of primordial CO and other carbon-bearing materials into CH₄-enriched clathrate-hydrates occurs within the deep interior of Titan via the release of hydrogen through the serpentinization process followed by Fischer–Tropsch catalysis. The time-averaged predicted emission rate of methane-rich surface materials is 0.02 km³ yr⁻¹, a value significantly lower than the rate of silicate lava production for the Earth and Venus, but nonetheless indicative of significant active geological processes reshaping the surface of Titan.     

CASSINI CIRS OBSERVATIONS OF A ROLL-OFF IN SATURN RING SPECTRA AT SUBMILLIMETER WAVELENGTHS

The Cassini Composite Infrared Spectrometer (CIRS) spatially resolved Saturn’s main rings in the far-infrared, measuring the spectrum from 20 to 400 wavenumbers (cm⁻¹) (tens of microns to submillimeter wavelengths). We find a spectral roll-off below 50 cm⁻¹ (200 μm) for each of the A, B and C rings. From these data we derive temperatures and emissivities for each ring. Mie calculations of individual water ice particles show a natural variation in the optical properties of the rings similar to the roll-off we observe in our data. A simple radiative transfer model placing a distribution of water ice particles randomly in a layer provides a good fit to the data and illustrates one possible interpretation of the results. This is most likely only part of the explanation for the roll-off effect as the impact of shape, surface, and composition variations have been left for future analysis.     

Relaxation Oscillations in Tidally Evolving Satellites

We study the rotational evolution under tidal torques of axisymmetric natural satellites in inclined, precessing orbits. In the spin- and orbit-averaged equations of motion, we find that a global limit cycle exists for parameter values near the stability limit of Cassini state . The limit cycle involves an alternation between states of near-synchronous spin at low obliquity, and strongly subsynchronous spin at an obliquity near 90°. This dynamical feature is characterized as a relaxation oscillation, arising as the system slowly traverses two saddle-node bifurcations in a reduced system. This slow timescale is controlled by ε, the nondimensional tidal dissipation rate. Unfortunately, a straightforward expansion of the governing equations for small ε is shown to be insufficient for understanding the underlying structure of the system. Rather, the dynamical equations of motion possess a singular term, multiplied by ε, which vanishes in the unperturbed system. We thus provide a demonstration that a dissipatively perturbed conservative system can behave qualitatively differently from the unperturbed system. This revised version was published online in July 2006 with corrections to the Cover Date.