I am involved in a number of research projects using remote-sensing data obtained by the Imaging Science Subsystem (ISS) and the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini Spacecraft in orbit around Saturn. Most of these projects are dedicated to exploring the structure, composition and dynamics of Saturn's rings, but some involve other aspects of the Saturn system, including Enceladus' plume and the planet's interior. Brief descriptions of some of these projects that have yielded publications are listed below.
Saturn's dense main rings show a diverse array of structures that reflect the gravitational and collisional interactions between the individual ring particles. For example, competition between mutual gravitational attraction and Keplerian shear leads to the clumping of particles and the generation of textures with characteristic sizes well less than one kilometer. VIMS occultation data has allowed allow us to quantify the orientation and shapes of some of these features. In particular, we have found that these parameters change significantly with their location in the ring, which should help us better understand how ring particles clump and interact in different dynamical environments.
On the opposite end of the size scale, there are several gross features in the rings which are still poorly understood. For example, consider the series of eight narrow gaps found in the Cassini Division. While it has been suggested that these gaps were held open by a series of small moons, thus far no-one has reported direct evidence that such moons actually exist. Our recent analyses of VIMS occultation data suggests that the positions of these gaps may instead be determined by a series of resonances involving asymmetries in the outermost part of the nearby B ring. If this idea turns out to be correct, then different regions within massive disks could be coupled to each other in more diverse ways than was previously appreciated.
Saturn's main rings contain a series of spiral patterns known as ''density-waves'' that represent the response of the rings to periodic perturbing forces. Most of these patterns appear to be generated at resonances where the orbit period of the ring particles is close to a whole-number-ratio times the orbit period with one of Saturn's moons. However, there are multiple waves in the inner parts of Saturn's rings that are not near any known resonance with any moon. Early investigations of these features using data from the Voyager spacecraft suggested that they might be generated by resonances with normal-mode oscillations inside the planet itself (Marley and Porco, Icarus 1993), but there were not enough data to prove these associations. Using wavelet-based techniques, we have been able to verify that these patterns are indeed generated by structures inside the planet, and therefore can provide new insights into the planet's interior. Furthermore, we found multiplicities of patterns that were not predicted by standard models of Saturn's interior.M.M. Hedman and P.D. Nicholson. Kronoseismology: Using density waves in Saturn's C ring to probe the planet's interior. The Astronomical Journal 146:12 (2013) [arXiv] [PDF]
The ring that shows the most dramatic examples of variability on short time scales is the D ring. This is the innermost of Saturn's rings, located only some 5,000-15,000 km above Saturn's clouds. When the Voyager spacecraft flew by Saturn in the early 1980s, the brightest feature in this region was a narrow ringlet dubbed D72. When Cassini arrived twenty-five years later, this ringlet had transformed into a broader feature that was no brighter than other ringlets in this region. The cause of this dramatic change is still unknown.
Another example of a time-variable feature in this ring appears as a series of bright and dark bands in the outer part of the D ring. These brightness variations are created in part by a vertical corrugation in the ring. The radial wavelength of this corrugation has been declining steadily over the course of the Cassini mission. Extrapolating backwards in time, we find that this structure likely formed in the early 1980s, when this portion of the D ring somehow became tilted relative to Saturn's equator plane.
The dusty rings that lie just inside and outside Saturn's main rings contain unusual patterns that rotate around the planet every 10-11 hours. This is close to the same periods seen in Saturn's radio emissions, so it would appear that something in Saturn's magnetosphere in perturbing ring particles' orbits at these locations. The tiny dust grains that populate these regions are particularly sensitive to non-gravitational forces and therefore it is not unreasonable that the same electromagnetic disturbances that generate Saturn's radio emissions are also affecting these rings. However, it is not yet clear exactly how the observed structures in the rings are coupled to the periodic signals in Saturn's magnetic environment.
Prior to Cassini's arrival at Saturn, the G ring was one of Saturn's most mysterious rings. The small dust grains that make up most of the visible G ring should be eroded or removed from the Saturn system on relatively short timescales, and unlike other dusty rings, there was no obvious nearby source bodies that could replenish this dust.
During Cassini's first few years at Saturn, the cameras spied a bright arc near the inner edge of the G ring. The motion of this feature indicated that the material in this arc is confined by a resonance with Saturn's moon Mimas. In-situ measurements showed that this arc contains a substantial population of centimeter-to-meter-sized particles, which were likely the source of the dust that makes up most of the visible G ring. More recent follow-up observations of this arc revealed that it contained a tiny moon, now called Aegaeon, which would be the largest object inhabiting this arc.
Just as Aegaeon seems to be a source for the G ring, Saturn's small moons Anthe, Methone and Pallene also seem to be sources for their own extremely faint rings. Interestingly, Aegaeon, Anthe and Methone all appear to be trapped in resonances with Mimas, and all are embedded in arcs of debris that are likely confined by gravitational perturbations from that moon. Comparisons between these different ring-moon systems can therefore help us better understand how efficiently dust is produced and dispersed in different situations.
The widest gaps in Saturn's main rings harbor narrow dusty ringlets. One of these ringlets, found in the Laplace Gap in the outer Cassini Division, appears to be displaced away from the center of Saturn towards the Sun. This displacement manifests itself as systematic variations in the apparent radial position of this ringlet among the various Cassini images (Images taken close to Saturn's shadow show this ringlet to be closer to the inner edge of the Laplace Gap than it is typically observed in images taken of the sunward part of the rings). This "heliotropic" behavior can be explained as the result of solar radiation pressure gently nudging the particles in this ring and perturbing their orbits. Small particles like those found in this ringlet are particularly sensitive to such forces, and the observed magnitude of the displacement is consistent with that predicted for particles roughly 10 microns across. This ringlet thus provides a nice illustration of how non-gravitational forces can sculpt dusty systems
Data from multiple spacecraft missions has now revealed that both Saturn's and Jupiter's rings have been disturbed by impulsive events, probably cometary impacts, within the last few decades.
Images of Saturn's rings taken by the Cassini spacecraft in 2009 revealed strangely periodic brightness variations in the C ring. These patterns were only seen when the Sun illuminated the rings from almost exactly edge-on, and thus represent vertical corrugations similar to those previously found in the D ring (see above). Detailed studies of trends in these corrugations' wavelength demonstrate that they arose from an event in 1983 that slightly titled the rings out of Saturn's equator plane.
Earlier observations of Jupiter's rings by the Galileo spacecraft recorded evidence for similar vertical corrugations. Comparing the wavelengths of these corrugations in images taken by both Galileo and New Horizons showed that this pattern was also changing over time as one would expect if Jupiter's rings had suddenly become tilted sometime in the past. In this case, the tilting event would have occurred in the summer of 1994, around the same time as the comet Shoemaker-Levy 9 was crashing into the planet. This coincidence strongly suggests that Jupiter's rings were tilted material from Shoemaker Levy 9, and subsequent calculations show that sufficient fine debris released when the comet broke up in 1990 could have passed through the rings to produce the observed tilt. Theoretical calculations also demonstrate that Saturn's rings could have been tilted by a collision with a shattered comet as well. If this is correct, then the rings preserve a record of cometary impacts stretching back decades.
The analytical tools that we are developing to determine the particle sizes in faint rings have proven useful to investigations of the particle content in Enceladus' plume. This plume consists of vapor and small ice grains launched from beneath the moon's surface by processes that are still rather obscure. Spatially-resolved spectra of the plume obtained by VIMS provide information about the particle size distribution in the plume, which in turn constrains the velocity distribution of particles launched from Enceladus' surface. These measurements therefore should help efforts to determine what is happening beneath the moon's surface to accelerate these tiny ice grains.
More recently, we have found evidence that the brightness of the plume changes significantly as the moon moves around its eccentric orbit. These variations in plume output are probably due to the changes in the tidal stresses experienced by the moon, sand so provide further information about what is going on inside Encealdus.