Dr. Mark Showalter

Planetary astronomer Mark Showalter is rabid about rings. While everyone knows about Saturn’s spectacular ring system, it’s often forgotten that Jupiter, Uranus, and Neptune are also encircled by fainter and narrower rings. Each of these systems interacts closely with a family of small, inner moons. Showalter works on some of NASA’s highest-profile missions to the outer planets, including Cassini, now orbiting Saturn, and New Horizons, which recently flew past Jupiter en route to its 2015 encounter with Pluto. Known for his persistence in planetary image analysis, Mark's work on the earlier Voyager mission led to his discovery of Jupiter’s faint, outer “gossamer” rings and Saturn’s tiny ring-moon, Pan.

At the end of 2007, Uranus will experience an equinox, when the Sun crosses from the south side of its equator to the north. This is the first equinox on Uranus since 1965, an event that will give Earth-based astronomers their first opportunity to study the planet’s rings while they are edge-on. This viewing geometry is ideal for finding the faintest rings and smallest moons embedded in the system. Since 2002, Mark has been leading a team of astronomers using the Hubble Space Telescope to study Uranus, and his work has already led to the discovery of two small moons and two faint rings. During the upcoming equinox, he hopes to espy the as-yet unseen “shepherd” moons that are believed to keep Uranus’s rings in place.

Throughout 2007, he will also be traveling regularly to the ten-meter Keck Telescope in Hawaii for ground-based observations. Using one of the world's largest telescope dishes and sophisticated new imaging cameras, Keck can often acquire images that rival the quality of the Hubble images.

Rings and the faint moons that interact with them are more than just local anomalies. They serve as dynamical laboratories where we can observe some of the same processes that operate, albeit on much larger scales, in galaxies and during the formation of planetary systems.

Adopt a Scientist Opportunity

The best part of Mark’s job is that he can come to work in the morning not knowing what new discovery might be awaiting him in the latest data. He welcomes the opportunity to share this spirit of discovery with interested individuals or small groups. Watch over his shoulder as he processes the latest data and be among the first to see features that have never before been revealed to human eyes.

However, Mark isn’t just rabid about rings. As an avid scuba diver, amateur naturalist and award-winning photographer, he spends his vacations exploring the diversity of life on Earth in its most distant and exotic and underwater environments. He has dived everywhere from Alaska to Australia, the Galapagos Islands, the Red Sea, and throughout the Caribbean and South Pacific. As a different kind of journey of discovery, we invite experienced scuba divers on an expedition to a destination of their choice. Work with Mark to understand more about environments and life forms as we prepare for the trip, and then compare notes after each dive. Such a trip would also afford ample “down time” to explore Mark’s other passion, so we can examine the latest images from the heavens above when we’re not focused on the oceans below.

Projects

"Rings of Uranus: Dynamics, Properties and Shepherding Moons"

HST–GO–10102.03

This project consists of two observing programs running on the Hubble Space Telescope, focusing on the ring systems of Uranus and Neptune. Uranus has a set of narrow rings and a family of 13 nearby moons. Our Uranus observations in August 2003 discovered the two smallest of these moons, currently designated S/2003 U 1 and S/2003 U 2. Both bodies are only a few miles across, perhaps the size of San Francisco. It is a testament to the remarkable capabilities of the Hubble telescope that we are able to see bodies from Earth that were too small to be detected by Voyager's cameras during the 1986 flyby. Our work on Neptune's ring system is just beginning but our first observations clearly show its mysterious arcs, incomplete rings that have held together for at least 20 years now.

“Transcending Voyager: A Deeper Look at Neptune’s Ring-Moon System”

HST–GO–10398.01

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.

"A Proposal to Relocate the Planetary Data System Rings Node to the SETI Institute"

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The Planetary Rings Node is devoted to archiving and distributing scientific data sets relevant to planetary ring systems. The two major classes of ring data are images and occultation profiles, although a variety of additional data types (e.g. spectra, particle absorption signatures, etc.) are also of interest. A large fraction of our data sets are from the Voyager missions to the outer planets, but Earth-based and HST data sets are also represented. The Rings Node also performs a variety of services to support research into these data sets. These services include developing on-line catalogs and information systems, filling orders for data, developing software tools, and coordinating special observing campaigns.

The Planetary Rings Node supports research into all aspects of planetary ring systems, including their physical properties (particle sizes, composition, ring geometry), dynamical processes (including interactions with satellites) and origins. Indirectly, the Node also supports studies of other dynamical analogs such as the solar nebula and the asteroid belt, where similar physical processes are (or were) at work. In addition to supporting research, the Rings Node is committed to NASA's broader goal of educating the public about science and technology. The Rings Node is a cooperative project of SETI Institute, NASA Ames Research Center and the Center for Radar Astronomy at Stanford University. The PDS Rings Node has been relocated from NASA Ames Research Center to the SETI Institute in order to improve its on-line availability to the outside world. The PDS Rings Node had been located at NASA Ames since the PDS was first established in 1989. The PI has managed it since the very beginning and was recently re-selected to serve for another five years, 2004–2009.

“Planetary Rings: Observation and Interpretation”

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This project addresses some of the fundamental open questions related to the dynamics and origins of planetary ring systems. We 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.