Showalter, M. R.
Showalter, M. R., Ballard, L., French, R., Gordon, M., Tiscareno, M.
Objectives: We propose to continue serving as a Discipline Node of the Planetary Data System, but now with an expanded scope that more accurately reflects the role that the Rings Node has long played within the organization. By adopting the name "Ring- Moon Systems Node", we will be better able to emphasize our support not just for studies of ring systems, but also for studies of any planetary system composed of multiple interacting bodies, small or large. From the viewpoint of the user community, the new name provides continuity with our past, but also emphasizes our extensive support for studies of planetary satellites in addition to rings. Our expanded emphasis on the dynamics and observing geometry of moons and rings continues to complement the existing PDS Node structure. Our focus on discoverability, and on developing new ways to describe data products, has potential benefits for all of PDS.
Methods/Techniques: Our plans build upon our long experience in addressing the needs of diverse users. Among these plans is the continued enhancement of OPUS, our signature "Outer Planets Unified Search" engine, which addresses the challenging problem of how to search for data products across a disparate collection of missions and instruments. Our primary goals for the next five years are to:
To these challenges we will apply our extensive experience in building state-of-the-art software tools and data pipelines, in understanding the workings of instruments and missions, and in conducting our own scientific research using the same data sets that we curate. Having been key players in the design of the new PDS4 standards, we look forward to leveraging the capabilities of the new system. Perceived Significance: We bring a unique set of skills to the PDS. We are keenly focused on providing users with new ways to discover, access and analyze data. We are also seeking new ways to streamline the archiving process for data sets new and old. Not only will these contributions be of immediate benefit to PDS users and providers, but they can be spun off to benefit the other Discipline Nodes as well.
Verbiscer, A., Buie, M., Helfenstein, P., Showalter, M.
Following the New Horizons flyby in 2015, we propose a two-cycle program to observe Pluto and its five moons in the post-encounter era, building on the rich legacy of observations obtained during and prior to the historic flyby. At opposition in Cycles 25-26, the Pluto system is visible at the smallest solar phase angle in 87 years. The system will be at true opposition when it crosses the line of nodes in July 2018, and as seen from Pluto, Earth will transit the solar disk. Such rare planetary alignments enable the characterization of small-scale surface texture and porosity as well as the direct measurement of the geometric albedo, rather than an estimation of its value from photometric models. Any variation among the regolith properties of Pluto's moons will test the long-standing hypothesis that ejecta exchange between the moons has altered their surfaces. We will also follow up on the surprising result from New Horizons and HST that the small moons are spinning rapidly and with high obliquities. Styx, Nix, and Hydra show hints of being in strong spin-orbit couplings with Charon, but confirmation requires the additional precision in measurements of their spin rates and polar precession rates proposed here. In addition, we will obtain new astrometry of the small moons, making it possible to determine their masses and bulk densities with much higher precision. Results from this program will enhance the scientific return from the New Horizons mission, providing images complementary to those obtained by the spacecraft on approach and achieving science objectives that cannot be met by either HST or New Horizons alone.
Showalter, M. R. and Pilorz S.
Showalter, M. R., Gordon, M., Kolokolova, L., and Tiscareno M.
The Mikulski Archive for Space Telescopes (MAST) currently contains 29,000 solar system observations. However, because the Hubble Space Telescope (HST) is nominally an astrophysics mission, using MAST for planetary research poses challenges to the user. For example, MAST enforces no standard over how targets are named, forcing planetary scientists to make educated guesses and risk overlooking key data. Searches based on other constraints like phase angle and rotational phase are not possible. Data products, when found, do not contain the types of geometric metadata necessary for straightforward planetary analyses. Historically, several Discipline Nodes of the PDS have attempted to bridge this gap. However, products enter the HST archive continuously, and calibration procedures are updated often, so it has never been possible for the PDS to ensure that its HST holdings are complete or up to date.
We propose to solve this problem once and for all, by fully integrating HST's solar system data products into the Planetary Data System. We will develop a pipeline that supports the frequent and nearly automatic ingestion of HST's planetary data products into the PDS registry, where they will be immediately visible to the users of any of the PDS Discipline Nodes. This task will include the generation of key metadata that supports rich and robust searches of the archive, so that users can constrain searches based on fundamental parameters like time, viewing and lighting geometry, rotation, resolution, etc. It will also identify secondary targets, such as moons and rings, that fall within each field of view.
Methodology: This project leverages the ongoing development of the new PDS4 standard. PDS4 provides a flexible, XML-based mechanism for bundling products and their metadata, and for performing automatic conversions between file formats. In this project, we will develop a pipeline to support tasks such as: (1) automatically querying MAST for solar system products; (2) standardizing the target name(s); (3) defining and generating all of PDS's discipline-specific metadata according to PDS4 standards; (4) creating SPICE kernels; and (5) constructing PDS-compliant product bundles and entering them into the Registry. The project will also ensure that any calibrated or derived products retrieved through the PDS4 Registry employ the latest version of the MAST calibration pipeline, but that previous versions remain accessible.
This is a collaboration between the PDS Rings Node housed at the SETI Institute and the PDS Small Bodies Node at U. Maryland. It builds on APIs that are already available for interacting with MAST. Other PDS Nodes will provide support and perform validation. The Rings Node will take primary responsibility for project coordination and for developing the key software tools according to existing PDS4 APIs. Upon completion, we anticipate that the working pipeline can continue to be maintained within the baseline funding of the PDS Nodes.
Relevance: This proposal will open up what is arguably the single most valuable Earth-based data set to the widest possible use within the planetary community. It therefore supports the overall objective of the PDART program to "increase the amount and quality of digital information and data products available for planetary science research and exploration." This work could later be extended to encompass additional data at MAST, including IUE, FUSE, GALEX, and soon JWST; it could also potentially support other NASA missions such as Spitzer.
Showalter, M. R., de Pater, I., and Lissauer, J. J.
The Cassini data sets have provided remarkable new insights about the processes at work among the rings and small moons of Saturn. Guided by these discoveries, we will seek out and investigate related phenomena in the ring-moon systems orbiting Jupiter, Uranus, Neptune and Pluto. We will employ data from Voyager, Cassini and New Horizons, complemented by the best publicly available data from HST and from the W. M. Keck Telescope. We will apply new and powerful image analysis techniques that should enable us to obtain significant new results from old data. Using these techniques we will: (1) search for possible moons and rings that are significantly fainter than the detection thresholds from previous studies; (2) expand and refine our collection of astrometry and photometry of the small satellites, including newly discovered S/2004 N 1; (3) investigate the orbital interactions among the moons of Uranus, deriving the first constraints on the masses of these bodies; (4) study the apparently chaotic rotation of Pluto's small moons; (5) seek out subtle edge modes in the rings of Uranus, which might hint at the effects of unseen shepherd moons or other external forces; and (6) make sense of the conflicting reports about clumps and asymmetries in the faint Jovian ring.
Showalter, M. R., de Pater, I., and Lissauer, J. J.
This program will address three key science goals related to the rings and inner moons of Neptune. (1) We will recover S/2004 N 1, Neptune's 14th moon, whose discovery was announced by STScI in 2013. This 8-10 km moon orbits between the much larger moons Larissa and Proteus, raising questions about its origin and orbital stability. Measurements from Cycle 23 will supplement the only existing measurements from 2004-2005 and 2009, giving us necessary confirmation of the discovery and providing much stricter constraints on the moon's orbital elements. (2) We will locate Naiad, the innermost known moon, whose orbital position appears to have jumped by 81 degrees in orbital longitude between 2002 and 2004. New measurements will determine whether (a) the 2002 measurements were erroneous, or (b) the moon follows a highly perturbed orbit. If the latter, our measurements will place strong constraints on the nature of these unknown perturbations. (3) We will revisit the arc system embedded in the Adams ring of Neptune. The leading arcs have disappeared since the 1989 Voyager flyby; the trailing arcs have been more stable, although they appear to be fading slowly. We will investigate this unexplained trend, and also seek out evidence for other subtle changes in the rings, by obtaining the first rotationally complete, visual-band profile of the ring-arcs since 1989.
[ADS 2015hst..prop14217S] [STScI 14217]
Showalter, M. R.
Full Report: Ice Giants: A Pre-Decadal Survey Mission Study Report
Showalter, M. R. and Pilorz S.
Showalter, M. R., French, R. G., and Gordon, M. K.
The objective of this proposal is to generate several higher-order data products that will substantially reduce the level of effort required to analyze Cassini's optical remote sensing data sets (ISS, VIMS, CIRS and UVIS), by eliminating the need for most scientists to perform their own navigation and geometric reconstruction. The key product to be delivered to the PDS will be a complete instrument pointing reconstruction with precision suitable for the analysis of most narrow-angle images. In addition, we will deliver a comprehensive set of "backplanes," which describe the geometric content of each sample within each data product from the remote sensing instruments. This will enable most scientists to focus on their research rather than on software development and potentially tedious data analysis steps. This project therefore has the potential to open up the Cassini data sets to much broader participation and also to reduce the overall cost of Cassini data analysis to NASA. This effort will build upon new software developed at the PDS Rings Node, and already in use for "OPUS" (http://pds-rings.seti.org/search), the geometric search engine funded by PDS and the Cassini Project. Initial tests demonstrate the feasibility of this ambitious goal. Key tasks supported by this project will be to develop additional ways to determine robust pointing corrections, and to automate the entire procedure. Extensive quality-control tests and a peer review will be performed to ensure the accuracy of the final products. We will deliver an initial set of products as quickly as possible, so that users can benefit from our work and so that their feedback can enable us to make further refinements. When the project is completed, we will also publish all details of our methods in an appropriate scientific journal.
Buie, M. W., Showalter, M. R., Grundy, W. M., Binzel, R. P., Jacobson, R., Brozovic, M., Weaver, H. A., Spencer, J. R., and Stern, S. A.
We propose a comprehensive set of observations of the Pluto system that will leverage upon the contemporaneous New Horizons flyby to study the surfaces of all the bodies in the system, rotation states of the outer satellites, and provide improved masses. These observations comprise a tightly integrated set of WFC3 imaging and grism data that span from Feb 2015 to Oct 2015, critically providing a broader temporal context for most fully understanding the once-in-a-lifetime encounter snapshot. A small portion of the data collected will also be directly useful for cross-calibrating HST and ground-based data with the New Horizons data. Our science objectives address both the past formation and current evolution of the Pluto system, thereby setting this system into the larger context of the Kuiper Belt. For example, the rotation state measurements anchored with the New Horizons data represent a truly unique dataset that is unlikely to ever be achievable for any similar-sized satellites or Kuiper Belt objects for many decades to come. As such, this proposal will provide a legacy dataset for understanding the diverse members Pluto system and by extension, analogous members of the Kuiper Belt.
Showalter, M. R., Gordon, M. K., and Devore, E.
We propose to continue in our role as the Rings Node for the Planetary Data System. Our future plans build upon our long experience in addressing the needs of diverse users from both within and outside the ring science community. Our most recent innovation is a new on-line search engine, which addresses the otherwise intractable problem of how to search for data products across a disparate collection of missions and instruments. Our primary goals for the next five years are (1) to improve and simplify the experiences of our users in finding and using planetary data, (2) to help integrate the Discipline Nodes of the PDS into a coherent and more effective data system, (3) to preserve and enhance the remarkable scientific legacy of the Cassini mission, and (4) to ensure that historical data sets remain accessible well into the future. In addition, we propose to undertake new E/PO work to capitalize upon our position among the top-ranked web sites related to planetary rings. By combining our visibility, knowledge and technical savvy with the educational expertise already in-house at the SETI Institute, we propose to develop an interactive web site, high-quality animations, diagrams and images, and museum display materials that engage and inspire the public about the nature, diversity, and significance of planetary ring systems.
Showalter, M. R. and Hedman, M. M.
Images from the Galileo spacecraft were recently re-interpreted to reveal a subtle pattern of vertical "ripples" in the Jovian ring. These were shown to have been triggered during in mid-1994, and were probably associated with the impact of SL9 into Jupiter (Showalter et al., 2011, Science 332, 711–713). Additional patterns imaged by Galileo and also New Horizons indicate that these are common features of the ring; four different spiral patterns have been detected in the two data sets. Because any given pattern winds tighter at a known rate, these patterns can be used to infer the approximate the date on which the impact occurred. In addition, the vertical amplitude of the pattern constrains the impactor's mass. In Cycle 21, the rings of Jupiter are open to Earth by a small angle of 1.6 degrees, making this an ideal opportunity to detect similar patterns using HST. We will be able to detect any pattern in the ring that has an amplitude of > 1 km and that was triggered within the last 8–10 years. This information will provide valuable new constraints on the population of small bodies in the outer solar system.
[ADS 2013hst..prop13414S] [STScI 13414]
Showalter, M. R., and Gordon M. K.
The Voyager flybys remain our best source of data for the Uranus and Neptune systems, and provide key reference points for continuing studies of Jupiter and Saturn. However, because the Voyager mission pre-dates NASA's focus on archiving, the data sets remain difficult to use. We propose to modernize the Voyager data sets by (a) producing calibrated and geometrically corrected versions of all the images; (b) generating accurate, continuous "C kernels" describing the pointing of the instruments during each Voyager flyby; and (c) using this information to generate detailed geometric indices describing the contents of the Voyager images and spectra. The resulting volumes of derived products will be delivered to the Planetary Data System. This work will ensure that the Voyager data sets remain easily usable by the scientific community for many decades to come.
Showalter, M. R., Lissauer, J. J., de Pater I., and French R. G.
Nine densely-packed inner moons of Uranus show signs of chaos and orbital instability over a variety of time scales. Many moons show measureable orbital changes within a decade or less. Long-term integrations predict that some moons could collide in less than one million years. One faint ring embedded in the system may, in fact, be the debris left behind from an earlier such collision. Meanwhile, the nearby moon Mab falls well outside the influence of the others but nevertheless shows rapid, as yet unexplained, changes in its orbit. It is embedded within a dust ring that also shows surprising variability. A highly optimized series of observations with WFC3 over the next three cycles will address some of the fundamental open questions about this dynamically active system: Do the orbits truly show evidence of chaos? If so, over what time scales? What can we say about the masses of the moons involved? What is the nature of the variations in Mab's orbit? Is Mab's motion predictable or random? Astrometry will enable us to derive the orbital elements of these moons with 10-km precision. This will be sufficient to study the year-by-year changes and, combined with other data from 2003–2007, the decadal evolution of the orbits. The pairing of precise astrometry with numerical integrations will enable us to derive new dynamical constraints on the masses of these moons. Mass is the fundamental unknown quantity currently limiting our ability to reproduce the interactions within this system. This program will also capitalize upon our best opportunity for nearly 40 years to study the unexplained variations in Uranus's faint outer rings.
[ADS 2010hst..prop12245S, 2011hst..prop12665S, 2012hst..prop13055S] [STScI 12245, 12665, 13055]
Weaver, H. A., Stern, S. A., Showalter, M. R., Buie, M. W., Throop, H., Merline, W. J., Steffl, A., Soummer, R., Mutchler, M.
Our team's discovery last summer of a faint (V~26.1) new satellite (provisionally called "P4") around Pluto demonstrated that the Pluto system is even richer with orbiting material than previously suspected. Moreover, P4's location near yet another Charon mean motion resonance (MMR), as Nix and Hydra also are, suggests that still more Charon MMRs could be populated with small, as yet undetected, satellites. Further still, there is now some evidence from HST for a debris ring inside Charon's orbit. While these are exciting developments scientifically, they have, more importantly, exposed a potentially catastrophic threat to NASA's New Horizons (NH) spacecraft as it passes through the Pluto system in July 2015—i.e., debris ejected from Pluto's small satellitesmay be creating hazards to flight. We propose a Director's Discretionary Time (DDT) program comprised of 34 orbits of Hubble observations for our time-critical need in 2012 to investigate potential hazards to NASA's New Horizons spacecraft as it flies through the Pluto system in July 2015, and to certify potential new trajectory options for the NH mission if it must "bail out" to a safe haven. The results from this Hubble DDT program will be unique and will have profound implications for the New Horizons mission, a top scientific priority of the last NASA Planetary Decadal Survey.
[ADS 2011hst..prop12801W] [STScI 12801]
Showalter, M. R.
[See pds-rings.seti.org/search]Showalter, M. R.
We propose to revisit the data from Voyager and other public-domain archives to better understand the latest problems in planetary ring science. We show that, but careful processing and co-adding of many images, one can achieve hundred-fold improvements over what the eye can see; this makes it possible address numerous fundamental questions that are otherwise beyond our observational capabilities. (a) We will search for the unseen parent bodies that produce the prevalent dust in the Uranian rings. (b) We will investigate why some clumps in the Uranian mu ring that have persisted for years although they should have sheared out in weeks. (c) We will search for faint, outer rings of Neptune that might point to the presence of unseen moons. (d) We will study the evolution of clumps in the F ring as they orbit, which provides important clues about how they form and evolve; ironically, Cassini has not obtained comparable data. (e) We will search for resonant patterns in the rings of Uranus and Neptune, which could indicate the presence of unseen "shepherd" moons nearby. (f) Finally, we will apply numerical simulations and analytic models to explore the orbital variations of several Uranian moons, to estimate their masses and to understand their long-term dynamics. Although each problem is tied to a particular data set and a particular ring system, the phenomena to be studied have relevance to all ring-moons systems and, more broadly, to the dynamics of astrophysical disks. This work supports NASA's strategic goal to advance scientific knowledge of the origin and history of the Solar System, and the PGG program's goal to support analysis of data from planetary missions that are in public archives for investigations into the dynamical evolution of the planets, satellites, small Solar System bodies, and ring systems.
Showalter, M. R. and Hamilton, D. P.
Our key data sets on the Jovian ring system---from Voyager, Galileo, Cassini, HST and now New Horizons, span a time baseline of 28 years. Using this combined information, we will investigate the spatial and temporal variations within the rings to understand the dynamics, evolution and origin of the system. (a) We will study the longitudinal asymmetries in the ring, which were seen clearly by Voyager and Galileo but not by Cassini or New Horizons. We will quantify the level of the changes observed and try to understand the cause. (b) We will study the evidence for vertical "ripples" in the main ring, seen in a few of the finest Galileo images but not before or since. We will determine whether their non-detection by New Horizons is due to instrumental limitations or genuine changes in the ring, and will explore a variety of possible explanations. (c) We will examine the radial structure and thickness of the main ring to understand the the influence of the ring-moons Metis and Adrastea, the dynamics of the parent bodies, and the relationship between the parents and the prevalent dust. (d) Astrometry and orbit modeling of the two moons will clarify their connection to the ring; also, any sign of interactions between them would have implications for the long-term stability of the ring. (e) We will study the 3-D structure of the inner "halo" and outer "gossamer rings" to understand the effects of both gravitational and non-gravitational forces on their orbital motion. This work supports NASA's strategic goal to "advance scientific knowledge of the origin and history of the solar system..." and the JDAP program's goal to "to enhance the scientific return of the Jupiter science data obtained by the New Horizons spacecraft, as well as the Voyager, Galileo, and Cassini spacecraft."
Hedman, M. M. and Showalter, M. R.
We propose to investigate the structure, composition and dynamics of Saturn's faint, dusty rings using the remote sensing data from the Cassini Spacecraft. We will study rings D, E, F and G, plus dusty material in several ring gaps and debris found in the orbits of several small moons. Such rings operate in distinctive dynamical regimes, responding to a variety of gravitational and non-gravitational forces. They can change on time scales ranging from weeks to years, enabling us to study their evolution in real time. This project will build off of work we initiated under our predecessor CDAP grant, and will enable us explore recent discoveries that reveal these rings to be even more complex and dynamic than anyone had expected. For example, some ring features maintain a systematic orientation relative to the Sun; other regions contain structures that appear to be associated with Saturn's rotation rate. The proposed work will also allow us to refine our techniques for deriving model-independent constraints on particle sizes, as well as to develop various analytical and numerical models to describe the observed structures. The proposed work will enhance the scientific return of the Cassini mission by using all available remote sensing data to obtain a better understanding of Saturn's faint rings, which will in turn yield new insights into the dynamics of other dusty disks around other planets and in other astrophysical environments; this is related to Subgoal 3C of NASA's Strategic Goals and Research Objectives.
Showalter, M. R.
Weaver, H. A., Stern, S. A., Showalter, M. R., Hamilton, D., Buie, M. W., Young, L. A., Steffl, A., Jacobson, R., Broxovic, M., and Owen, W.
We propose a simple, 9-orbit (3 orbits at each of 3 epochs) DD program with two objectives: (1) as a safety-of-flight issue, perform a deep search (V=25–26) for satellites in the 0.9 arcsec radius region between Pluto and Charon where NASA's New Horizons mission will fly, and (2) confirm or reject the candidate P5 and P6 satellites seen in recent HST imaging, while performing an even deeper search (V=27) for satellites in the region between Charon and Hydra so observations of them can be planned before the New Horizons mission planning is frozen prior to the on-spacecraft rehearsal. Hubble is uniquely qualified to achieve these time-critical objectives, which we hope to achieve before the Pluto system enters the Hubble solar exclusion zone in early November.
[ADS 2010hst..prop12436S] [STScI 12725]
Showalter, M. R. and Hamilton, D. P.
With the discoveries of Hydra and Nix, dwarf planet Pluto centers a more extensive satellite system than any terrestrial planet. Small moons are usually accompanied by rings of faint dust, which arise from impacts into their surfaces. Such rings show interesting dynamics including the influence of non-gravitational forces such as radiation pressure and Poynting-Robertson drag. A previous search for rings of Pluto in HST images was negative, but was limited by the scattered light from Pluto and Charon. Our more optimized plan will enable us to model and subtract out the scattered light pattern, yielding a detection threshold 10–30 times fainter than the prior limit. If Pluto sports a ring comparable to the major dust rings of the giant planets, our observing plan should detect it. This work supports the New Horizons mission by potentially revealing a dust hazard that might endanger the spacecraft during its 2015 flyby. It could also enhance the science return by allowing planners to target a known feature rather than conducting a more resource-intensive general search for rings. We request Director's Discretionary time because the Pluto observing sequence will be finalized in 2011; thus the observing window in June represents our last chance to obtain results that could influence the mission.
[ADS 2010hst..prop12436S] [STScI 12436]
Showalter, M. R., Cuzzi, J. N., and Gordon, M. K.
We propose to continue serving as the Rings Node of the Planetary Data System, building upon our experience of the last 14 years. We archive, catalog and distribute data related to the rings and inner satellite systems of all four giant planets. The major emphasis in our next five years will be on supporting the Cassini Mission to Saturn. This mission will return volumes of data well beyond what has been previously available to ring scientists; it will make it necessary for us to develop new cataloging tools so that users can quickly select and download the data products relevant to a particular scientific question, crossing boundaries of spacecraft, instrument and data type. We will also support the planning, archiving and analysis of ring observations by New Horizons during its Jupiter flyby in early 2007, and by Earth-based observers during a rare Uranian ring plane crossing in 2007. We will continue to enhance our support for older data sets from Voyager, Galileo, Hubble, and ground-based observatories. Data types to be supported include images, occultation profiles, spectra, image cubes, radar maps, charged particle absorption data, and higher-level data products derived from all of the above. We will also revise and enhance our popular web site, containing educational information and ephemeris tools, as well as continue to answer questions and support special requests from users.
Showalter, M. R., de Pater, I., and Lissauer, J. J.
We will use a subarray of the WFC3/UVIS to study the inner rings, arcs and moons of Neptune with a sensitivity that exceeds that achieved by any previous observations, including Voyager 2 during its 1989 flyby. Our study will reveal any inner moons down to V magnitude 25, corresponding to a radius 20 km (assuming 9% albedo), to address a peculiar, apparent truncation in the size distribution of inner moons and to look for the "shepherds" and source bodies for Neptune's dusty rings. (For comparison, the radius of Neptune's smallest known regular moon, Naiad, is 33 km.) Monitoring of the arcs at fine resolution and sensitivity will reveal their ongoing evolution more clearly and will enable us to assess the role of Galatea, whose resonant perturbations are widely believed to confine the arcs. Our study will also reveal any broad, faint rings with optical depth 10-6, comparable to those now known to encircle all of the other giant planets.
[ADS 2009hst..prop11656S] [STScI 11656]
Showalter, M. R. and Hedman, M. M.
We propose to investigate the properties, dynamics and evolution of Saturn's faint, dusty rings, combining Cassini ISS, VIMS and CIRS data. Such rings reveal a variety of physical processes that are masked within denser rings: non-gravitational forces deflect dust grains from their Keplerian paths and can sweep grains out of the system, requiring embedded (and often unseen) "parent" bodies to replenish the visible dust. Faint rings are prevalent throughout the Solar System and also serve as analogs for stages in the formation of planetary systems. Cassini data provide a unique new opportunity to understand this particular class of dynamical system. Because grains are small in size and number, modest events can trigger large visible changes. Hence, these are the most time-variable rings known. Comparison with Voyager data will reveal processes at work for the last 25 years, and Cassini data alone will show us changes in real time. Saturn has the Solar System's most diverse set of faint rings. The innermost D ring contains several narrow ringlets surrounded by broader belts. The F ring, just outside the main rings, is dominated by clumps and "braids," some of which come and go on time scales of weeks. Its surrounding strands form a spiral, suggesting that they were generated by a large impact several years ago. The E ring is outermost and broadest; it peaks at the orbit of Enceladus, but encompasses the entire region from Mimas to Dione, implying strong non-gravitational perturbations. The G ring orbits at an isolated location; it was recently found to contain a bright arc. Gaps in the main rings also contain dusty ringlets. The Encke Gap contains three, one of which shares its orbit with the moon Pan. Some clumps in this ring hve changed noticeably during the tour so far. Extracting faint ring signals from the data requires special techniques to cope with instrumental artifacts such as stray light and low signal. Detailed photometric and spectroscopic modeling will quantify the grains' properties, including composition and size distribution. This information constrains the production, transport and loss processes. For example, collisional ejecta should exhibit a broad size distribution, but Saturn's magnetic field and solar radiation pressure can selectively disperse certain grain sizes. We will also monitor longitudinal features to understand their origin and evolution. Finally, we will compare our results to existing dynamical models and will develop new models as needed. This project will enhance Cassini science by combining data from multiple instruments in a synergistic way. Faint rings can pose a significant hazard to exploration, so our results will also be useful for planning future planetary missions.
Showalter, M. R.
In this three-year proposal we will address some of the fundamental open questions related to the dynamics and origins of planetary ring systems. We will employ a variety of state-of-the-art techniques in image analysis and photometric modeling to glean untapped new information from the best existing spacecraft- and Earth-based data sets. The three systems to be studied encompass the full range of physical processes at work in planetary rings. (1) Continuing studies of the Voyager images of Saturn's F Ring will illuminate the dynamical processes behind its clumps, kinks and so-called "braids," providing context for recent Cassini results. We will study recently-identified brightness variations that appear to be an indicator of the clumps' collisional origins. We will also study periodicities and kinks in the ring to better define the role of nearby Prometheus and to search more thoroughly for the effects of Pandora and perhaps other nearby bodies. (2) We will take a new look at the question of whether unseen "shepherding" moons confine the rings of Uranus. We will use the best Voyager images to search for evidence of moons down to ~ 5 km in size, well below Voyager's widely quoted detection limit. We will also seek rotating modes and patterns in the rings, which could provide additional evidence for the resonant effects of nearby shepherds. (3) We will study the dynamics of dust in the Jovian ring system, using image analysis techniques that reveal more clearly the three-dimensional structure and photometric properties of the system. Analysis of Galileo, Voyager and Earth-based data, combined with dynamical simulations, will reveal the roles of non-gravitational processes including Poynting-Robertson drag and Lorentz resonances, and will help us to distinguish between rival models for dust evolution through the system. Key observations to explain are a newly-discovered ringlet of dust sharing its orbit with Amalthea and a discrepancy between the locations of dust and source bodies in the main ring.
Showalter, M., R. and Hamilton D. P.
It has been long suspected that Mars is encircled by two faint rings of dust, one originating from each of its moons Phobos and Deimos. Similar dust rings are associated with many of the small, inner moons orbiting Jupiter, Saturn, Uranus and Neptune. On December 31, 2007, Earth will pass through Mars' equatorial plane just a week after its December 24 opposition, providing an exceedingly rare opportunity to image the rings under nearly ideal viewing geometry. The next equivalent viewing opportunity occurs in 2022. Using the Wide Fields of WFPC2 and a highly optimized observing plan, we expect to be able to detect rings with edge-on reflectivities of ~ 10-8, which is at or below the level where most dynamicists expect rings to be visible. This is a factor of 10–30 more sensitive than the detection limit we achieved during a slightly inferior viewing opportunity in 2001. The rings have been predicted to show some interesting dynamical properties, including large asymmetries and inclinations. A positive detection will test these predictions, serving as an effective test of dynamical models developed to account for the properties of other faint planetary rings as well. With such a stringent limit, even a negative result will be of considerable interest, challenging dynamicists to explain the remarkably low density of dust within the Martian system.
[ADS 2007hst..prop11292S] [STScI 11187]
Showalter, M. R., Lissauer, J. J., French, R. G., Hamilton, D., and Nicholson, P.
The rings of Uranus turn edge-on to Earth in May and August 2007. In between, we will have a rare opportunity to see the unlit face of the rings. With the nine optically thick rings essentialy invisible, we will observe features and phenomena that are normally lost in their glare. We will use this opportunity to search thoroughly for the embedded "shepherd" moons long believed to confine the edges of the rings, setting a mass limit roughly 10 times smaller than that of the smallest shepherd currently known, Cordelia. We will measure the vertical thicknesses of the rings and study the faint dust belts only known to exist from a single Voyager image. We will also study the colors of the newly-discovered faint, outer rings; recent evidence suggests that one ring is red and the other blue, implying that each ring is dominated by a different set of physical processes. We will employ near-edge-on photometry from 2006 and 2007 to derive the particle filling factor within the rings, to observe how ring epsilon responds to the "traffic jam" as particles pass through its narrowest point, and to test the latest models for preserving eccentricities and apse alignment within the rings. Moreover, this data set will allow us to continue monitoring the motions of the inner moons, which have been found to show possibly chaotic orbital variations; by nearly doubling the time span of the existing ACS astrometry, the details of the variations will become much clearer.
[ADS 2006hst..prop10870S, 2007hst..prop11292S] [STScI 10870, 11292]
Hammel, H., de Pater, I., and Showalter, M.
We request a single half night in 2008 to observe the Uranus system as it moves past ring plane crossing. We still require data at ring opening angles of ~2° and ~4.7°. We request the 2° window here (4.7° is covered in a UC proposal). These 2008 observations are the last opportunity to view the rings at these angles for ~40 years. We will compare observed I/F changes with models to determine the rings' physical characteristics. We will also observe the uranian atmosphere, assessing the nature of atmospheric features, and tracking discrete cloud features in order to study zonal winds.
[ADS 2008koa..prop..238H]
de Pater, I., Hammel, H., and Showalter, M.
[ADS 2008koa..prop..206D]
Showalter, M. R., de Pater, I., and Hammel, H.
We request one full night and two half-nights to observe Uranus during its ring plane crossing. The edge-on main rings will be nearly invisible in August–September, allowing a clear view of Uranus's smallest moons and faintest rings. This proposal is one component of a program to study diverse aspects of the Uranian ring-moon system near equinox: (a) Where are the "shepherd" moons believed to confine Uranus's narrow rings? (b) What is the thickness and packing density of ring (epsilon)? Can it explain the ring's large eccentricity? (c) What is the nature of the dust rings scattered throughout the system, some only seen in Voyager data? (d) What is the time scale of the chaotic interactions recently identified among Uranus's inner moons? (e) What perturbs Mab, whose orbit is more irregular than that of any other known moon? (f) Why is Mab's dust ring blue whereas other rings are red? It will be 42 years before we have another opportunity to address most of these questions using Earth-based data.
[ADS 2007noao.prop....9S]
Hammel, H., de Pater, I., and Showalter, M. R.
We request 2.5 nights to observe the Uranus system during its ring plane crossing (RPX), an event that occurs once every 42 years. Near the August RPX by the Earth, images of the unlit side of the nearly edge-on rings allow multi-wavelength measurements of faint moons and the new outer rings, searches for material near the epsilon ring, and dust sheet characterization. Near the December RPX by the Sun, the rings are dark yet slightly open, permitting studies of longitudinal asymmetries and shepherding moons. There may well be serendipitous discoveries, as has occurred for other Solar System RPXs.[ADS 2007koa..prop..192H]
de Pater, I., Hammel, H., and Showalter, M. R.
[ADS 2007koa..prop..199D]
Showalter, M. R.
Showalter, M. R.
Uranus has a family of thirteen satellites orbiting interior to the innermost classical moon, Miranda. Nine of these comprise the Portia group, a closely-packed dynamical system that has recently been found to show significant orbital variations over time scales of 1–2 decades. This result supports inferences that the system is chaotic, with collisions expected over time scales of less than one million years. No analogous orbital system has been seen elsewhere in the Solar System. With these new results, it becomes much more important to understand the orbital history of the inner moons of Uranus. The HST archive contains numerous detections of these moons, from WFPC2, NICMOS and ACS, that have never been used for orbital determinations. Many observations fill a gap between 1994 and 2003, during which the orbits have never been measured. This is a proposal to use all the available data from the HST archive to derive the orbital variations of the larger moons in the Portia group from 1994 to 2005. This investigation will provide unique new information about the time scales over which the variations occur and the nature of the hypothesized chaos. For example, this study will enable us to test the prediction that the two adjacent moons Cressida and Desdemona have closely coupled variations, and it may reveal whether the surprisingly large orbital deviations of Belinda are related to its resonance with the nearby, but much smaller moon Perdita.
[ADS 2006hst..prop10977S] [STScI 10977]
Showalter, M. R.
In this three-year proposal we will continue to study a variety of timely topics related to the dynamics of planetary ring systems, via the analysis of Voyager, Galileo and Earth-based data combined with theoretical modeling. (1) Continuing studies of Saturn's F Ring will reveal the dynamical processes behind its clumps, kinks and "braids." We will study newly-identified brightness variations that appear to be an indicator of the clumps' collisional origins. We will also study periodicities and kinks in the ring to better define the role of nearby Prometheus and to search more thoroughly for the effects of Pandora. We will investigate whether the ring itself may be driving the orbital wanderings of these two moons. (2) We will study the dynamics of dust in the Jovian ring system, using image analysis techniques that reveal more clearly the three-dimensional structure and photometric properties of this system. Combined with dynamical simulations, this will reveal the strengths of non-gravitational processes including Poynting-Robertson drag and Lorentz resonances, and will help to distinguish rival models for dust evolution in the system. Key observations to explain are a newly-discovered ringlet of dust sharing its orbit with Amalthea and the discrepancy between the locations of dust and source bodies in the main ring. (3) After reconstructing the geometry of the Voyager IRIS data, we will map out the thermal emission from Saturn's three main rings. The goal is to employ thermal emission as a tracer of dynamical properties such as particle rotation, ring thickness and vertical mixing.
Showalter, M. R.
Showalter, M. R., de Pater, I., and Lissauer, J. J.
We will use the High Resolution Channel (HRC) of ACS to study the inner rings, arcs and moons of Neptune with a sensitivity that exceeds that achieved by Voyager 2 during its 1989 flyby. Our study will reveal any moons down to V magnitude 25.5, to address a peculiar truncation in the size distribution of inner moons and to look for the "shepherds" and source bodies for Neptune's dusty rings. (For comparison, Neptune's smallest known moon is Naiad, at magnitude 23.9). Recent ground-based studies show that the mysterious arcs in the Adams Ring continue to shift and change, and may be fading away entirely. We will obtain the visual-band data uniquely necessary to determine whether the arcs are fading. Long-term monitoring of the arcs at high resolution and sensitivity will reveal their gradual changes more clearly and enable us to assess the role of Galatea, whose resonances are widely believed to confine the arcs.
[ADS 2004hst..prop10398S] [STScI 10398]
Showalter, M. R. and Lissauer J. J.
We propose to image the rings and small inner satellites of Uranus using the High Resolution Channel of the ACS. The revolutionary capabilities of the ACS will allow us to address a variety of important questions relating to ring properties and ring-moon interactions. Observations at a range of wavelengths and phase angles will reveal the opposition surges of these rings and moons, providing information on color and surface structure. Measurements of the ring in front of the planet will provide complementary information on optical depth; any variations of optical depth with wavelength will reveal the rings' poorly-constrained population of embedded dust. The rings of Uranus are closing rapidly as the planet approaches equinox in 2007, an event that takes place only every 42 years. Using this opportunity, our observations will be repeated at different solar and terrestrial tilt angles; this sequence of images will be particularly valuable for constraining the physical thickness and packing density of the rings. We will place particular emphasis on the rotational variations of the Epsilon Ring, whose radial width (and therefore its packing density) varies by a factor of five. In addition, deep exposures through the CLEAR filter will enable us to detect and recover 4–5 km moons in or near the ring system. Dynamicists invoke numerous such moons to "shepherd" the many sharp ring boundaries, so this will serve as a definitive test of the theory.
[ADS 2003hst..prop.9823S, 2004hst..prop10102S, 2005hst..prop10473S] [STScI 9426, 10102, 10473]
Showalter, M. R., Bollinger, K. J., Cuzzi, J. N., and Nicholson, P. D.
Showalter, M., Burns, J., de Pater, I., Hamilton, D., and Horanyi, M.
Jupiter's ring system consists of three components: the main band, the vertically-extended inner halo and the exterior gossamer rings. Each component illustrates aspects of dust dynamics within Jupiter's inclined magnetic field and its strong plasma environment. We will image all three components with ACS during an unusual, extended period of edge-on viewing, December 2002 through February 2003. For faint planetary rings, this geometry improves the signal-to-noise ratio considerably and permits an unambiguous decoupling of radial and vertical structure.
Although the Jovian rings have been examined by four spacecraft and from the ground, we are still lacking in a systematic set of data that can distinguish between the ring's prominent dust population and its embedded macroscopic source bodies. We also do not know the size distribution of dust with sufficient accuracy to test rival theories of ring origin. Observations of the system at a range of wavelengths and phase angles with ACS will finally make this determination possible. Coordinated observations at the W. M. Keck Telescope will extend our wavelength coverage well into the IR.
[ADS 2002hst..prop.9426S] [STScI 9426]
Showalter, M., Hamilton, D., and Nicholson P.
It has been long believed that Mars should be encircled by two faint rings of dust, one originating from each of its moons Phobos and Deimos. Similar dust rings have recently been associated with all the inner small moons of Jupiter. On May 28, 2001, Earth will pass through Mars' equatorial plane within weeks of its opposition, providing a unique opportunity to detect these rings via direct imaging. Using WFPC2, we will be able to detect rings with normal optical depths of ~10-8, which is well within the range of the Martian rings' predicted densities and 10–100 times fainter than the known Jovian rings. The rings have been predicted to show some interesting dynamical properties, including large asymmetries and inclinations. A positive detection will enable us to test these predictions, serving as an effective test of models developed to account for the faint rings of Jupiter and Saturn as well. It will also provide both photometric and dynamical constraints on the dust size distribution, enabling us to distinguish between several models of the rings' dynamics and evolution.
[ADS 2000hst..prop.8579S] [STScI 8579]
Showalter, M. R., and Jacoby R.
The volume of data returned by NASA’s latest generation of planetary missions continues to expand rapidly. The interest in these data, both in the community of professional astronomers and in the general public, has kept pace. But one thing has not kept pace—the sophistication of the data analysis tools we apply to these remarkable data. The Internet is now the route by which most of these data reach their receptive audience, but when it comes to what we actually do with the data, the Internet remains more often than not an afterthought. The reason for this is that the Internet has rarely if ever been integrated into our data analysis applications. In this proposal, we present a variety of related approaches intended to increase the scientific return on NASA’s investment in planetary exploration. Using recent advances in software technology, and most specifically the ability of software to integrate seamlessly with the Internet, we will develop tools that streamline the rapid, collaborative visualization and analysis of planetary images.
Showalter, M. R., Bollinger, K. J., Cuzzi, J. N., and Nicholson, P. D.
We propose to continue serving as the Rings Node for the Planetary Data System. As in the past, we will be based at NASA Ames Research Center but will maintain close collaborations with colleagues at Stanford University and Cornell University.
The Cassini Mission to Saturn is on the horizon, and promises to provide a vast increase in our knowledge of Saturn's remarkable ring system. Hence, supporting this mission will serve as a major focus for the Rings Node over the next five years. Earth’s crossings through Saturn’s ring plane in 1995 and 1996 provide unique observing opportunities from the ground and the data returned will be a valu- able resource for Cassini planning.
Jupiter’s ring system will provide a secondary focus. The Galileo mission will arrive there shortly, although its crippled antenna will substantially reduce the total volume of data returned. In addition, Earth-based observers have developed new technologies that make the rings far easier to detect from the ground. With the telescopes of the world trained on Jupiter for the recent Shoemaker/Levy 9 impacts, a large new supply of data on the rings has been obtained. Observers will continue to monitor the ring system for possible changes caused by comet dust impacts; these changes have been predicted by some theorists.
The ring systems of Uranus and Neptune will not be neglected, of course. After completing the archiving of the Voyager data, the Rings Node will continue to support the restoration of Earth-based data sets at a steady pace.
Showalter, M. R.
We propose to unite two current technologies, ALFIE ("ALgorithms for Image Examination") and the JavaTM Programming Language, into a powerful "workbench" for analyzing planetary images. Specific innovations of ALFIE 2.0 will include: (1) built-in Internet access, so that widely-scattered collaborators can exchange results as if they were sharing files on a single workstation; (2) an open, object-oriented architecture, so that users may readily augment ALFIE 2.0's capabilities; (3) a rich selection of image models and geometric transforms, supporting multi-disciplinary studies of atmospheres, surfaces and rings using diverse data sets; (4) support for all the widely used astronomical image formats, including FITS, VICAR, PDS, TIFF and GIF; (5) powerful tools and scripting capabilities to automate many repetitious tasks that normally require human intervention; (6) automatic preservation of intermediate processing steps, so that a user can always recover and share previous results quickly and efficiently; (7) genuine platform independence, so the same application can run without modification on workstations, PCs and Macintoshes. Once complete, the Rings Node of the Planetary Data System will distribute ALFIE 2.0 and will provide ongoing maintenance and support.
Nicholson P., Danielson, G. E., French, R. G., Larson, S. M., Lissauer, J. J., Seitzer, P. O., Showalter, M. R., and Sicardy B.
The plane of Saturn's rings passes through the earth on 10 August 1995, and through the sun on 17–21 November 1995, providing a rare opportunity to determine the thickness, vertical distortions, and pole orientation of the rings. A similar opportunity will not recur until the year 2038. We will obtain time series of images using the Wide Field Camera and methane filter at each crossing time, in order to measure the radial profile of apparent ring thickness, to determine the moment of the earth's crossing to within a few minutes, and to look for the expected warp in the ring plane due to satellite perturbations. In addition, the Planetary Camera will be used to recover the small satellites Pan and Atlas and thus refine their orbital periods. A series of multi-color WFC observations will be used to probe the structure and particle size distribution within the faint E and G rings.
[ADS 1995hst..prop.5836N] [STScI 5836]
Cuzzi, J. N., Pollack, J. B., Showalter, M. R., Marouf, E. A., Tyler, G. L., and Nicholson, P. D.
We propose to provide the planetary rings component of the PDS Small Bodies Discipline Node. We will to operate as a subordinate, but relatively independent, subnode reporting to the Discipline Node Manager and being represented on the Advisory Council. We believe that this mode of operation provides an optimal combination of the needed discipline expertise, and the efficiency obtained by locating more generic Node operations at a single site. Although our group is not affiliated directly with any of the Discipline Node proposals, we understand that NASA and the PDS reserve the right to select the optimum internal makeup of Nodes and Subnodes from elements contained in different proposals.
We describe our past and current activities which are of greatest interest to the PDS program, and the resources we will bring to the PDS endeavor. Our team provides an excellent mix of in-depth experience with all of the appropriate data sets, maintains an active and well-respected program of research in planetary rings, and has the ability and interest to provide the leadership and community focus desired of a PDS Subnode. Our existing facilities are more than adequate to serve the needs of a subnode for at least the next few years. In fact, we believe that the enclosed proposal demonstrates that our team has been acting as the de facto Planetary Rings Discipline Subnode for nearly a decade.
We describe the multifaceted ways in which we will support the functions and operations of PDS. We also sketch the research program that we will maintain as part of this activity, giving specific examples of long-term research which will at once provide significant insight into planetary rings and motivate a deeper understanding of the data sets.