Larry W. Esposito

Enceladus' water vapor plume.

The Cassini spacecraft flew close to Saturns small moon Enceladus three times in 2005. Cassinis UltraViolet Imaging Spectrograph observed stellar occultations on two flybys and confirmed the existence, composition, and regionally confined nature of a water vapor plume in the south polar region of Enceladus. This plume provides an adequate amount of water to resupply losses from Saturns E ring and to be the dominant source of the neutral OH and atomic oxygen that fill the Saturnian system.

Photopolarimetry from voyager 2; preliminary results on saturn, titan, and the rings.

The Voyager 2 photopolarimeter was reprogrammed prior to the August 1981 Saturn encounter to perform orthogonal-polarization, two-color measurements on Saturn, Titan, and the rings. Saturns atmosphere has ultraviolet limb brightening in the mid-latitudes and pronounced polar darkening north of 65�N. Titans opaque atmosphere shows strong positive polarization at all phase angles (2.7� to 154�), and no single-size spherical particle model appears to fit the data. A single radial stellar occultation of the darkened, shadowed rings indicated a ring thickness of less than 200 meters at several locations and clear evidence for density waves caused by satellite resonances. Multiple, very narrow strands of material were found in the Encke division and within the brightest single strand of the F ring.

Saturn from Cassini-Huygens

This book comprehensively reviews our current knowledge of Saturn featuring the latest results obtained by the Cassini-Huygens mission.

Imaging photopolarimeter on pioneer saturn.

An imaging photopolarimeter aboard Pioneer 11, including a 2.5-centimeter telescope, was used for 2 weeks continuously in August and September 1979 for imaging, photometry, and polarimetry observations of Saturn, its rings, and Titan. A new ring of optical depth < 2 x 10–3 was discovered at 2.33 Saturn radii and is provisionally named the F ring; it is separated from the A ring by the provisionally named Pioneer division. A division between the B and C rings, a gap near the center of the Cassini division, and detail in the A, B, and C rings have been seen; the nomenclature of divisions and gaps is redefined. The width of the Encke gap is 876 � 35 kilometers. The intensity profile and colors are given for the light transmitted by the rings. A mean particle size ≲ 15 meters is indicated; this estimate is model-dependent. The D ring was not seen in any viewing geometry and its existence is doubtful. A satellite, 1979 S 1, was found at 2.53 � 0.01 Saturn radii; the same object was observed ∼ 16 hours later by other experiments on Pioneer 11. The equatorial radius of Saturn is 60,000 � 500 kilometers, and the ratio of the polar to the equatorial radius is 0.912 � 0.006. A sample of polarimetric data is compared with models of the vertical structure of Saturns atmosphere. The variation of the polarization from the center of the disk to the limb in blue light at 88� phase indicates that the density of cloud particles decreases as a function of altitude with a scale height about one-fourth that of the gas. The pressure level at which an optical depth of 1 is reached in the clouds depends on the single-scattering polarizing properties of the clouds; a value similar to that found for the Jovian clouds yields an optical depth of 1 at about 750 millibars.

Ultraviolet spectroscopy of venus: initial results from the pioneer venus orbiter.

Ultraviolet spectroscopy of the Venus cloud tops reveals absorption features attributed to sulfur dioxide in the atmosphere above the cloud tops. Measurements of scattered sunlight at 2663 angstroms show evidence for horizontal and vertical inhomogeneities in cloud structure. Images of the planet at SO2 absorption wavelengths show albedo features similar to those seen at 3650 angstroms from Mariner 10. Airglowv emissions are consistent with an exospheric temperatuire of ∼275 K, and a night airglows emission has been detected, indicating the precipitation of energy into the dark thermosphere.

Water vapour jets inside the plume of gas leaving Enceladus

A plume of water vapour escapes from fissures crossing the south polar region of the Saturnian moon Enceladus. Tidal deformation of a thin surface crust above an internal ocean could result in tensile and compressive stresses that would affect the width of the fissures; therefore, the quantity of water vapour released at different locations in Enceladus’ eccentric orbit is a crucial measurement of tidal control of venting. Here we report observations of an occultation of a star by the plume on 24 October 2007 that revealed four high-density gas jets superimposed on the background plume. The gas jet positions coincide with those of dust jets reported elsewhere inside the plume. The maximum water column density in the plume is about twice the density reported earlier. The density ratio does not agree with predictions—we should have seen less water than was observed in 2005. The ratio of the jets’ bulk vertical velocities to their thermal velocities is 1.5 ± 0.2, which supports the hypothesis that the source of the plume is liquid water, with gas accelerated to supersonic velocity in nozzle-like channels.

Sulfur Dioxide: Episodic Injection Shows Evidence for Active Venus Volcanism

Pioneer Venus ultraviolet spectra from the first 5 years of operation show a decline (by more than a factor of 10) in sulfur dioxide abundance at the cloud tops and in the amount of submicron haze above the clouds. At the time of the Pioneer Venus encounter, the values for both parameters greatly exceeded earlier upper limits. However, Venus had a similar appearance in the late 1950s, implying the episodic injection of sulfur dioxide possibly caused by episodic volcanism. The amount of haze in the Venus middle atmosphere is about ten times that found in Earths stratosphere after the most recent major volcanic eruptions, and the thermal energy required for this injection on Venus is greater by about an order of magnitude than the largest of these recent Earth eruptions and about as large as the Krakatoa eruption of 1883. The episodic behavior of sulfur dioxide implies that steady-state models of the chemistry and dynamics of cloud-top regions may be of limited use.

The structure of Saturn's rings: Implications from the Voyager stellar occultation

The Voyager 2 photopolarimeter observed the star δ Scorpii as it was occulted by Saturns rings, giving ∼ 100 m resolution in the radial direction across the entire ring system. Radial structure can be seen down to the resolution limit and considerable new structure is seen. In the outer B ring none of this structure is due to imbedded large particles (moonlets) in the ring system. An automatic search with finite Fourier transforms located 13 density waves exicted by resonances with Saturns satellites. More waves were found in searches by eye of predicted resonance locations. Although strong density waves can be located quite easily, no waves have been identified having predicted torques per surface mass density less than 4 × 1016 cm4/sec2. This puts a limit of less than 100 waves likely to be found in Saturns rings, mostly in the A ring. Unresolved or overlapping waves do not play a major role in creating the most obvious radial structure. The total mass of Saturns rings is estimated as 5 × 10−8 of the mass of Saturn. Measurements of wave damping imply a ring thickness of ∼ 30 m in the outer A ring. The power spectrum for the rings shows no dominant individual wavelengths: A preferred size for radial structure in the rings is not seen. At wavelengths less than ∼ 15 km the ring power spectrum drops below the power of noise in the data. The majority of the variance in the ring system is characterized by distance scales greater than 20 km. It is concluded that the majority of ring structures and the majority of variance in ring optical depth are not explained by currently proposed physical mechanisms.

Satellite 'wakes' and the orbit of the Encke Gap moonlet

Abstract Quasiperiodic optical depth variations have been observed in the Voyager stellar (PPS) and radio occultation profiles near the Encke Gap of Saturns rings, and have also been detected in one Voyager image. These fluctuations are believed to be the gravitational “wakes” of a moonlet orbiting within the gap. The existence of such a body had already been proposed by J. N. Cuzzi and J. D. Scargle (1985, Astrophys. J. 292, 276–290) , based on radial “wavy edges” visible in numerous Voyager images of the gap. We develop a general model for these wakes, and use the results to estimate the moonlets orbit and mass from the occultation data; this model may have broader applications to planetary rings. The moonlet longitude is best determined from the PPS scan interior to the gap, and is estimated to trail the observed profile by 32°. Considering the uncertainty caused by our neglect of particle collisions and self-gravity, the longitude is consistent with other estimates obtained from a second portion of the PPS scan exterior to the gap, and also from the radio profile. The moonlet orbits close to the center of the gap at an estimated semimajor axis of 133,603 ± 10 km; it has a mass of 5–10 × 10−12 Saturn masses, which corresponds to a radius of ∼10 km if it is composed primarily of water ice. The consistency of the orbit parameters inferred from the PPS, radio, and wavy edge data virtually guarantees that a single dominant moonlet orbits within the gap.

An Evolving View of Saturn’s Dynamic Rings

Saturns Secrets Probed The Cassini spacecraft was launched on 15 October 1997. It took it almost 7 years to reach Saturn, the second-largest planet in the solar system. After almost 6 years of observations of the series of interacting moons, rings, and magnetospheric plasmas, known as the Kronian system, Cuzzi et al. (p. 1470) review our current understanding of Saturns rings—the most extensive and complex in the solar system—and draw parallels with circumstellar disks. Gombosi and Ingersoll (p. 1476; see the cover) review what is known about Saturns atmosphere, ionosphere, and magnetosphere. We review our understanding of Saturn’s rings after nearly 6 years of observations by the Cassini spacecraft. Saturn’s rings are composed mostly of water ice but also contain an undetermined reddish contaminant. The rings exhibit a range of structure across many spatial scales; some of this involves the interplay of the fluid nature and the self-gravity of innumerable orbiting centimeter- to meter-sized particles, and the effects of several peripheral and embedded moonlets, but much remains unexplained. A few aspects of ring structure change on time scales as short as days. It remains unclear whether the vigorous evolutionary processes to which the rings are subject imply a much younger age than that of the solar system. Processes on view at Saturn have parallels in circumstellar disks.