ABSTRACT: TandEM was proposed as an L-class (large) mission in response to ESA’s Cosmic Vision 2015–2025 Call, and accepted for further
studies, with the goal of exploring Titan and Enceladus. The mission concept is to perform in situ investigations of two worlds
tied together by location and properties, whose remarkable natures have been partly revealed by the ongoing Cassini–Huygens
mission. These bodies still hold mysteries requiring a complete exploration using a variety of vehicles and instruments. TandEM
is an ambitious mission because its targets are two of the most exciting and challenging bodies in the Solar System. It is
designed to build on but exceed the scientific and technological accomplishments of the Cassini–Huygens mission, exploring
Titan and Enceladus in ways that are not currently possible (full close-up and in situ coverage over long periods of time).
In the current mission architecture, TandEM proposes to deliver two medium-sized spacecraft to the Saturnian system. One spacecraft
would be an orbiter with a large host of instruments which would perform several Enceladus flybys and deliver penetrators
to its surface before going into a dedicated orbit around Titan alone, while the other spacecraft would carry the Titan in
situ investigation components, i.e. a hot-air balloon (Montgolfière) and possibly several landing probes to be delivered through
Experimental Astronomy 04/2012; 23(3):893-946. · 1.82 Impact Factor
ABSTRACT: Theoretical and observational progress in studies of Saturn's ring system since the mid-1980s is reviewed, focussing on advances
in configuration and dynamics, composition and size distribution, dust and meteoroids, interactions of the rings with the
planet and the magnetosphere, and relationships between the rings and various satellites. The Cassini instrument suite of
greatest relevance to ring studies is also summarized, emphasizing how the individual instruments might work together to solve
outstanding problems. The Cassini tour is described from the standpoint of ring studies, and major ring science goals are
Space Science Reviews 04/2012; 104(1):209-251. · 3.61 Impact Factor
Nature Geoscience 11/2011; 4:750-752. · 11.75 Impact Factor
ABSTRACT: Lightning discharges in Saturn's atmosphere emit radio waves with intensities about 10,000 times stronger than those of their terrestrial counterparts. These radio waves are the characteristic features of lightning from thunderstorms on Saturn, which last for days to months. Convective storms about 2,000 kilometres in size have been observed in recent years at planetocentric latitude 35° south (corresponding to a planetographic latitude of 41° south). Here we report observations of a giant thunderstorm at planetocentric latitude 35° north that reached a latitudinal extension of 10,000 kilometres-comparable in size to a 'Great White Spot'-about three weeks after it started in early December 2010. The visible plume consists of high-altitude clouds that overshoot the outermost ammonia cloud layer owing to strong vertical convection, as is typical for thunderstorms. The flash rates of this storm are about an order of magnitude higher than previous ones, and peak rates larger than ten per second were recorded. This main storm developed an elongated eastward tail with additional but weaker storm cells that wrapped around the whole planet by February 2011. Unlike storms on Earth, the total power of this storm is comparable to Saturn's total emitted power. The appearance of such storms in the northern hemisphere could be related to the change of seasons, given that Saturn experienced vernal equinox in August 2009.
Nature 07/2011; 475(7354):75-7. · 36.28 Impact Factor
ABSTRACT: In August 2009 the Sun illuminated Saturn’s rings from almost exactly edge-on, revealing a subtle corrugation that extends
across the entire C ring. This corrugation’s amplitude is 2 to 20 meters and its wavelength is 30 to 80 kilometers. Radial
trends in the corrugation’s wavelength indicate that this structure—like a similar corrugation previously identified in the
D ring—results from differential nodal regression within a ring that became tilted relative to Saturn’s equator plane in 1983.
We suggest that this initial tilt arose because interplanetary debris struck the rings. The corrugation’s radial extent implies
that the impacting material was a dispersed cloud of debris instead of a single object, and the corrugation’s amplitude indicates
that the debris’ total mass was ~1011 to 1013 kilograms.
Science 05/2011; 332(6030):708-711. · 31.20 Impact Factor
ABSTRACT: In August 2009 the Sun illuminated Saturn's rings from almost exactly edge-on, revealing a subtle corrugation that extends across the entire C ring. This corrugation's amplitude is 2 to 20 meters and its wavelength is 30 to 80 kilometers. Radial trends in the corrugation's wavelength indicate that this structure--like a similar corrugation previously identified in the D ring--results from differential nodal regression within a ring that became tilted relative to Saturn's equator plane in 1983. We suggest that this initial tilt arose because interplanetary debris struck the rings. The corrugation's radial extent implies that the impacting material was a dispersed cloud of debris instead of a single object, and the corrugation's amplitude indicates that the debris' total mass was ~10(11) to 10(13) kilograms.
Science 03/2011; 332(6030):708-11. · 31.20 Impact Factor
ABSTRACT: In this paper we discuss geology, stratigraphy and ages of bright ray
craters on the saturnian satellites Rhea and Dione and constrain the
origin of potential impactors.
ABSTRACT: Voyager images and Cassini occultation data have previously shown that the behavior of the outer edge of Saturn's massive B ring is determined only in part by a static response to the 2:1 inner Lindblad resonance with Mimas. In Cassini images of this region, we find, in addition to the expected wavenumber-2 forced distortion, evidence for unforced self-excited wavenumber-3, wavenumber-2, and wavenumber-1 normal modes. These are the first observations to suggest substantial wave amplification in Saturn's broad rings. Moreover, the presence of these free modes strongly implicates viscous overstability as their underlying cause and, by inference, the cause for most, if not all, of the unforced structures throughout the high-mass-density B ring and in other high-mass-density regions in Saturn's rings. Analysis of each of the inferred waves reveals a consistent lower bound on the average surface mass density of ~44 g cm–2 for the outer 250 km of the ring, though the true surface density could be as high as 100 g cm–2 or higher. Interference between the forced and free wavenumber-2 modes yields a total wavenumber-2 pattern that varies in amplitude and orientation with a characteristic period of ~5.5 years. We also find localized disturbances, including 3.5 km tall vertical structures, that provide circumstantial evidence for embedded massive bodies in the Mimas resonance zone. The presence of such bodies is supported by the presence of a shadow-casting moonlet ~0.3 km wide near the ring's edge.
The Astronomical Journal 10/2010; 140(6):1747. · 4.03 Impact Factor
ABSTRACT: The Imaging Science Subsystem (ISS) on Cassini has been observing the
plumes of Enceladus intensively for the past several years over a broad
range of phase angles (up to 178 degrees), image scales (down to 80 m
per pixel) and wavelengths (from 338 nm to 918 nm). Cassini's February
2010 Enceladus flyby has returned the highest spatial resolution yet,
ten times better than that reported in Porco et al. (2006). From this
one set of images, we have derived profiles of brightness and plume
width vs. altitude. From the former, we can get a better estimate of the
vertical velocity distribution. From the latter, we can estimate the
horizontal spreading rate and interaction of particles with the gas. The
higher resolution allows us to study individual jets close to the vent,
revealing the slow-moving particles that are falling back to the
surface. Eventually, from the phase angle and filter coverage over the
entire data set and the inclusion of VIMS spectral data we will be able
to estimate particle size, number density, and temporal variability.
Comparing with UVIS observations of the gas phase, we can estimate the
solid-to-vapor ratio. Together these data provide important constraints
on conditions below the surface. We will report on the progress made so
far in the analysis of this extensive data set and implications for or
against liquid water and what depth it might occur.
ABSTRACT: 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.
Science 03/2010; 327(5972):1470-1475. · 31.20 Impact Factor
ABSTRACT: 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.
Science 03/2010; 327(5972):1470-5. · 31.20 Impact Factor
ABSTRACT: We examine the outer edge of Saturn's A-ring, whose shape is strongly influenced by the co-orbital satellites Janus and Epimetheus, during the period from day 2005-121 to day 2009-036. Twenty-four Cassini imaging data sets are used, each one giving a picture of the ring during a short interval, allowing us to explore its time variability in detail for the first time. We find that the ring experienced a period of adjustment within ~8 months of the 2006 January co-orbital swap, corresponding to the interval during which the two satellites were within about 60° of one another. Outside that adjustment period, the ring is dominated by an m = 7 pattern, as expected near a 7:6 inner Lindblad resonance, but the alignment is opposite in phase to that predicted for isolated test particles, and the amplitude of the radial distortion varies with time. We find that the amplitude variation corresponds to a beat pattern between the perturbations from the two satellites as would be expected if the responses add linearly. However, we also find deviations of limited azimuthal extent from the simple m = 7 pattern. Some of the additional structure may arise from coupling between the two excited modes in the ring, but the origin of these features is still under investigation.
The Astronomical Journal 10/2009; 138(5):1520. · 4.03 Impact Factor
Astronomical Journal. 07/2009; 138:272-286.
ABSTRACT: Propeller-shaped disturbances occur in Saturn's rings as a result of
small moonlets, tens to hundreds of meters in diameter, embedded in the
rings (Tiscareno et al. 2006, Nature). These propellers are most
abundant in a 3,000-km-wide belt in the mid-A Ring (Sremcevic et al.
2007, Nature). A detailed analysis of this population was given by
Tiscareno et al (2008, AJ).
Here we report observations of five new propellers, all of which are
larger than the largest previously known. All five of these are found
in multiple, widely-separated apparitions, allowing us for the first
time to investigate the longevity and orbital stability of propellers.
These new propellers all occur exterior to the Encke Gap, farther from
Saturn than the population in the propeller-rich belt, and the larger
sizes are likely in part enabled by the larger Hill sphere of any
individual seed. The larger sizes also permit a more detailed view of
propeller morphology; for the first time, both dark and bright
components of the propeller structure are discernible.
Note: This presentation will actually be given as an oral talk, in place
of #21.06 by Tiscareno et al. A poster containing the latter
presentation will appear here, in the Late Poster session.
ABSTRACT: The Cassini spacecraft executed a close flyby of Enceladus on August 11
(altitude: 50km); two more are planned for October 9 (altitude: 25 km),
and October 31 (altitude: 196 km). High resolution (as fine as
7m/pixel) images of known geologically active features in the South
Polar Terrain (SPT) have been returned to investigate how plume
eruptions, tectonism, and seismicity alter the surface and to reveal how
the SPT has evolved over time. We examined six known eruption sites
(Spitale and Porco 2007, Nature 449, 695-697) along Cairo, Baghdad, and
Damascus Sulci, as well as inactive portions of the "tiger stripes" and
bright fractured terrain in adjacent areas. We also obtained contiguous
ISS broadband multi-spectral mosaics of the entire SPT region to refine
our geological and digital terrain maps and to search for volcanically
and tectonically driven temporal changes. The highest-resolution images
show ice blocks up to tens of meters in size that are widely but
non-uniformly distributed over a variety of terrain units. The upraised
flanks and valley walls of active tiger stripes are mantled in places by
smooth fluffy-looking deposits, most likely accumulations of
coarse-grained plume fallout. With increasing lateral distance from the
stripes, the smooth upraised flank deposits grade into rounded,
platy-textured, elongate hills and a conspicuous system of
quasi-parallel knobby ridges and grooves that have spacings and
dimensions comparable to the tiger stripe flanks themselves. Peculiar
narrow lenticular ridges, perhaps emplaced by extrusion or as icy
pyroclastic deposits, rise from tens to hundreds of meters along the
medial fissures of some tiger stripes. On regional scales, the ends of
the tiger stripes are bounded by a complex network of fractured
terrain, within which can be found numerous transform faults that lie at
high angles relative to the trends of the tiger stripes. Observed
offsets along these transforms and an absence of lateral symmetry of
the displaced terrains suggest that tiger stripes are not exact analogs
to classic terrestrial oceanic rifts. Instead, any possible tectonic
divergence is more likely a result of the superposition of many
regionally and temporally distributed spreading centers.
AGU Fall Meeting Abstracts. 11/2008; -1:02.
ABSTRACT: High-resolution images taken in December 2007 by Cassini's Imaging
Science Subsystem (ISS) reveal parallel sets of grooves in the southern
hemisphere of Epimetheus. Earlier images had shown grooves in the
northern portion of Pandora. On both satellites, these grooves are
typically 600 m across, less than 200 m deep, and less than 10 km in
length. These features are interpreted as regolith expressions of
fractures or faults in the consolidated portion of the satellites. The
major group of grooves on Epimetheus suggests expression of planes
perpendicular to the Saturn direction, similar to the pattern of one of
the prominent groove sets on Phobos. Pandora's grooves suggest a pattern
of planes rotated slightly off the perpendicular to the Saturn
direction. Other small Saturn satellites with good image coverage--
Telesto, Hyperion, and Phoebe --do not have such organized patterns of
grooves. Grooves on asteroids (Eros, Gaspra, Ida) are likely associated
with multiple cratering events and are distinct in pattern from
Epimetheus’ and Phobos’ grooves. Repetitive tidal stressing
may explain the differences between satellite and asteroidal grooves.
Owing to orbital eccentricities and any non-synchronous spins or
librations, tidal stresses vary. For the current orbits of Epimetheus
and Pandora, the former produces stress variations that are very modest
(103 dyne/cm2) while those due to the latter are larger,
perhaps 105 dyne/cm2. If these are to account for the
observed grooves, the small moons must be very fragile, perhaps not
surprising in view of their high porosities.
ABSTRACT: Saturn's dense A and B rings are pervaded by a microstructure dubbed
"self-gravity wakes," which arise due to a rough balance between the
clumping together of particles under their mutual self-gravity and their
shearing apart again due to tidal forces (Julian and Toomre 1966; Salo
1995). This effect causes azimuthal variations in the rings' brightness
as seen in images (Franklin et al. 1987; Dones and Porco 1989; Salo et
al. 2004; Porco et al. 2008) and in the optical depth as probed by
occultations (Colwell et al. 2006; Hedman et al. 2007).
The latter papers explain the occultation observations with models that
assume widely separated elongated structures that have an optical-depth
dichotomy, with nearly-opaque wakes (with optical depth
κwake) and a low but relatively constant optical depth
in the spaces between the wakes (κgap). However, it is
not known whether simulated wakes (not to mention real ones) can be so
characterized, nor, if they can, how κwake and
κgap respond to environmental parameters such as
optical depth and coefficient of restitution. What do observed values of
κgap (Colwell et al. 2006; Hedman et al. 2007) tell us
about the conditions under which wakes occur?
To this end, we determine the distribution of densities in simulated
wake cells. Our method uses an adaptive bin size to simultaneously
accommodate low-density regions, where particles are sparse (large bins
required), and the sharp boundaries between high- and low-density
regions (small bins required). The result is a histogram of the local
densities within simulated patches of the ring. We apply this method to
a suite of simulated wake cells, and will present our results. We
further plan to use our results to address the question of whether local
disruption of self-gravity wakes can explain the observed brightness of
"propeller" structures (Tiscareno et al. 2008, AJ).
ABSTRACT: Deciphering the mechanisms of Enceladus’ plumes is one of the most
important and challenging tasks for planetary science. Cassini has
provided a wealth of data by remote and in-situ data collection, but
fundamental details of the vents and their context remain elusive. Three
flybys of Enceladus by Cassini in 2008, on August 11 (altitude: 50km),
October 9 (30km), and October 31 (200 km) are designed to further our
knowledge of Enceladus’ geology and geophysics. Anticipated data
include images as good as 7 m/pixel of parts of the geologically active
South Polar Terrain (SPT). We targeted six different known eruption
sites (Spitale and Porco 2007, Nature 449, 695-697) along Cairo Sulcus,
Baghdad Suclus, and Damascus Sulcus, as well as non-active portions of
the the "tiger stripes" and bright grooved terrain in between. On each
of the three flybys we also plan contiguous ISS broadband multi-spectral
mosaics of the entire SPT region so that we can search for volcanically
and tectonically driven temporal changes and construct detailed digital
terrain maps. Previous images of the tiger stripes and other rift
systems on Enceladus resolve geomorphic structures on hundred meter
scales or larger. Within those resolution limits, tiger stripes are
morphologically distinguished most strongly from comparably sized young
looking rifts elsewhere on Enceladus by their prominent upturned flanks,
the muted appearance of their surface relief, and their relative absence
of distinct cliff faces, probably of solid ice along scarps. The
anticipated new high-resolution images will provide critical structural
details needed to identify the extent to which unique attributes of
tiger stripes are caused by mantling by plume fallout, tectonic
deformation, seismic disruption, or perhaps thermal processes. Here, we
present a first analysis of the August 11 close flyby images.
ABSTRACT: Moons embedded in planetary-ring gaps generate radial waves on the
gap-edges. The scale and morphology of these waves can be used to deduce
properties of the moons, particularly their masses. However, existing
analytic theory describing the relationship between the moon's mass and
the edge-wave amplitudes neglects non-linear effects during encounters
as well as possible effects of pre-encounter inclinations and
eccentricities of the moon and particles. We will present our numerical
integrations of ring-satellite encounters in which we explore the
effects of the non-linearity, eccentricities, and inclinations.
We find that in the Saturnian system, Pan's mass can accurately be
deduced using the existing analytic theory. Daphnis is more dynamically
interesting, being more inclined, more eccentric, and closer to its
gap-edge than Pan. The proximity of Daphnis to the Keeler gap's edges
results in significant non-linear effects, leading to the analytic
calculation over-estimating the mass of Daphnis by 30%.
The eccentricity and inclination both cause time-variable structure on
the gap-edge, meaning single observations are not sufficient for
determining the mass of the satellite. Additionally, Daphnis'
inclination induces an inclination in the gap-edge which conversely act
to decrease Daphnis' inclination with a damping timescale of several
thousands of years.
ABSTRACT: The edges of the Keeler and Encke gaps of Saturn's A Ring are
significantly disturbed as the embedded moons (Daphnis and Pan,
respectively) interact with the adjacent dense rings. Cassini images of
these edges reveal the structure to be much more complex than predicted
by analytical perturbation theory (Tiscareno et al 2005, DPS). In the
Encke Gap, we see both amplitude and frequency modulations of the
expected monochromatic sinusoid, as well as some sharper
"glitches” in the pattern. In the Keeler Gap, the expected
32-lobed pattern in the inner edge due to a resonance with Prometheus is
"lumpy” and asymmetric, while the outer edge features sharp-edged
Much of the unexpected structure may be due to the superposition of
multiple patterns, each moving with its own frequency. Determination of
these pattern speeds will help us to identify the source of the
perturbations, and may help us better appreciate the transfer of angular
momentum between dense particle disks and embedded moons. We will
present high-resolution maps of these edges covering all longitudes and
at many points in time.