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Joseph N. Spitale

I. Report on Research
 
I.a. Ongoing Projects / Research Funding Sources
 
Huygens ringlet; collaborators: J Hahn, P. D. Nicholson. R. French
Saturn's Huygens ringlet, located ~250 km exterior to the outer edge of Saturn's B ring, has for some time been known to be non-circular, based on images and occultations obtained by the Voyager spacecraft.  Using Cassini ISS imaging data, we have shown that the ringlet is influenced by a strong resonance with Saturn’s satellite Mimas resonance, but primarily on the inner edge.  The outer edge shows a similar pattern, but the speed of that pattern indicates that it is a free oscillation of the ringlet.  The amplitudes of the inner and outer m=2 modes, ~1.6 and 3.0 km respectively, are far too small to have been detected by Voyager.
 
The relation between Keplerian modes of the inner and outer edges of the ringlet is complicated, with two distinct behaviors apparent: the earliest data set, taken in 2005, shows a steep eccentricity gradient, a mean width of ~50 km, and an apsidal offset (outer minus inner) of ~45 deg; later data sets show a nearly flat eccentricity gradient, mean widths decreasing with time to ~20 km, and near apsidal alignment.  
 
Differential precession would produce a negative apsidal offset (at least initially), not the positive offset seen in the earliest observation.  In order to reach the observed configuration, the ring would have to be either oscillating about an apse-aligned state with an amplitude of at least 45 deg, or the inner edge would need to have overtaken the outer edge shortly before the 2005 observations.  In order for the inner apoapse to shear past the outer periapse, the ring elements would need to adjust to avoid streamline crossing.  The large mean width and eccentricity gradient seen in the earliest data set may be consistent with such an adjustment.  The decreasing width seen in the subsequent observations suggests an active confinement mechanism that has yet to be explained.
 
Another explanation for the variation in width is that we are fitting entirely the wrong sort of model.  The imaging data sets appear to also be consistent with heliotropic behavior, i.e., it may follow the Sun.    We have had not had much success in resolving this issue, though some recent developments from collaborators Nicholson and French may help.  I am in the process of preparing a manuscript to report our findings, though I would prefer to answer this question of the width variation.
Enceladus’ jets; collaborator: T. Hurford
Saturn’s satellite Enceladus emits a plume of particles and gas from fissures near its south pole.  The physical mechanism driving the jets must be tied to the behavior of the vents.  By comparing  the timing of the jet activity as a function of location with tidal stresses at those locations, we hope to understand how the cracks behave under the influence of  the diurnal stress field.  I published the first look at this in 2007 (Spitale and Porco 2007; Nature 449): that work was able to pinpoint the locations of 8 regions, which lie on the south-pole cracks, and which appear to be generally active, but the observations were not adequate for determining time variability at any single location.  Since that time, Cassini has performed a number of close flybys of the satellite and I incorporated observations that have give us much better resolution, and parallax.  Those observations allow us to resolve the physical locations as well as the actual timing of the activity with much greater precision that the earlier observations, and I had hoped that would allow us to make the direct comparison between stresses and activity that we have been after.  However, we encountered a number of difficulties with that approach, causing me to look for a better solution, which I found.  
 
The new approach involves simulating how the jet activity ought to look in an image for various assumptions about the mechanical properties of the cracks, and comparing with the activity observed in the actual image.  This approach removes the ambiguities associated with identifying individual jets, and distinguishing real jets from broad sheets of emission.  In the process, we discovered that a number of prominent features previously identified as jets can be explained as simply the result of looking through a continuous sheet of material emanating from a sinuous fissure, analogous to looking through a gathered shear curtain.  In other words, we (and presumably others) had been measuring things that may not actually exist.  The new procedure appears to be ready to use and the stresses have been computed along all of the fissures for several hundred images, so we can now move forward and look for the correlations that we have been seeking.  Some preliminary tests are promising, but I don’t know what we will find.  Hopefully well have something very interesting to write up in the next month or two.  This project is my primary focus at the moment.
 
 
 
 
 
 
 
Ring photometry; collaborator/PI: L. Dones
I worked on a small project with Luke Dones at SWRI to map out the reflectivity of Saturn’s main rings as a function of incidence, emission, and phase angle (and to a lesser extent, longitude) that can be compared with model ray-tracing calculations that incorporate realistic spatial and size distributions for the ring particles. Special geometries such as near zero phase angle, very high phase angles, and low tilt angles are of special interest. The ultimate goal is to determine the physical properties of the ring particles such as their internal densities and coefficients of restitution.  I provided the requested products and I hope to continue with this at some level, but this is a long-term background project for Luke and I don’t know how, or whether, we will proceed.
 
 
 
 
Cassini imaging team associate; PI: J. A. Burns
After leaving Carolyn Porco’s group in October of 2011, I was able to become re-associated with the Cassini imaging team through Joe Burns’ group at Cornell.  My main contribution is to continue to assist in the planning and execution of the Cassini Solstice Mission, but this will also facilitate collaboration on some science projects.  Funding for about 2 months of salary per year for two years was approved in the latter part of 2012, but we ave not yet received and funds.
 
 
B-ring Propeller; collaborator M. Tiscareno
In August 2009, near Saturn equinox, a Cassini ISS azimuthal scan of the B-ring captured 3.5-km-high structures near the ring edge (Spitale and Porco 2010).  Later in that same scan, an isolated shadow-casting object appeared at an orbital radius of ~116914 km.  The object was provisionally designated S/2009 S1.  The shadow length implied a height of ~150m out of the ringplane (Spitale and Porco 2010), or ~300-m diameter  if the object is spherical.  However: if the feature is a solid body, then where is the propeller, i.e., the gravitational disturbance formed by the body’s self gravity?  That quandary has since been explained by Michikoshi and Kokubo (2011), who showed that under certain circumstances, a propeller will not be formed by a B-ring moonlet. Therefore it may be possible to observe a sizable object with no discernible propeller.  So is that what we’re seeing here?
 
To better understand the geometry of the observed object, we started by simulating the shadow using a simple Monte-Carlo ray-tracing algorithm, assuming bi-axial ellipsoids with various aspect ratios.  To determine the most likely shape, we compared contrasts of the simulated and observed shadows, to find that a spherical body indeed gives the best match to the shadow.  
 
 
 
 
 
Although the shadow simulation showed that the shadow-casting body is not resolved (it’s width of ~300 m is much smaller than the image pixel scale of ~2500 m), the bright feature at the base of the shadow is resolved.  Moreover, the bright feature is not perfectly circular, but has a slight asymmetry whose tilt is consistent with Keplerian shear.  We therefore proposed that the bright resolved feature is the propeller created by the central body, which is not resolved.  If that hypothesis is correct, then measurements of the propeller dimensions may be used to probe the local ring properties using some standard methods from the literature.  Those comparisons do support the conclusion that the imaged object is a propeller, but before publication we will need to evaluate the competing hypothesis that the object is instead an impact plume.  
 
 
 
 
 
 
Ring Seismology; collaborators M. Marley and J. Fortney, J. Colwell
It has long been known that resonances with satellites can excite waves and open gaps in Saturn’s rings (Goldreich and Tremaine, 1982).  Many of the disturbances seen in the rings have been associated with such resonances, but Saturn’s rings contain numerous features that have yet to be definitively explained.  Various authors have proposed Saturn itself as the source of some of these unexplained features.   
 
Marley and Porco (1993) suggested that low-order  seismic modes of Saturn, which perturb the interior density profile of the planet, could periodically perturb the external gravitational field of the planet and, like a satellite, induce bending and density waves in the rings.  Planet-driven density waves would propagate towards the planet at outer Lindblad resonances instead of away from the planet at inner Lindblad resonances, as do those excited by external satellites.  They identified about half a dozen wave features in Saturn’s C ring that propagate towards Saturn, were in the expected location in the rings, and thus were candidates to be formed by this process.  However, they recognized that because bending waves propagate in the opposite direction than density waves, unidentified waves that had been observed in the Voyager Radio Science occultation profile of the C ring by Rosen (1991b) could be either bending waves produced at an inner resonance with an unknown satellite or density waves produced at an outer resonance by the oscillations.   
 
If specific ring features can be connected to specific oscillation modes of the planet, then the rings will serve as a seismograph, providing a precise measure of the integrated sound speed inside Saturn.  As shown by Marley (1991), such constraints (especially when available for several modes) would characterize both the size and mass of Saturn’s core and the sound speed through the region of the hydrogen phase transition. Given the Voyager data available at the time, little more could be done by MP93 other than to identify specific ring features that might be associated with oscillation modes of the planet.  Indeed, they concluded that, in the foreseeable future, only the Cassini mission could obtain the observations necessary to fully characterize the unexplained waves, which is necessary in order to produce an improved model of Saturn’s interior. 
 
The purpose of this project is to use Cassini occultation measurements to resolve the ambiguitues in these C-ring features in order to determine whether they are consistent with seismic forcing.  If so, then we will use our measured properties of these waves to constrain interior models of the planet.  This project is slowly coming online.  I have put together the software needed to analyze the waves, but I have had trouble obtaining the data sets.  That should be resolved soon.
I.b. Proposals
During 2012, I submitted a proposal to the CDAPS program (Co-I T. Hurford, see Enceladus’ Jets in Sec. I.a above), which  included a request to become a Cassini Participating Scientist.  The proposal and the Participating Scientist request were both selected.  
 
The OPR proposal (Co-I: M. Marley, collaborators: J. Colwell, J. Fortney, see Ring Seismology in Sec. I.a above) that I submitted in 2011 was selected.
 
Along with D. O’Brien and S. Byrne, I had been working on a ROSES supplement to develop a field class for TUSD teachers.  Unfortunately that supplement program was suspended indefinitely by NASA, so we put the project on hold.
 
Funding for my work with the Cornell group (see Cassini Imaging Team Associate in Sec. I.a above) was approved by the Cassini project.
 
 
II. Publications
 
II.a. Peer-reviewed Publications
 
Mitchell, C. J., Porco, C. C., Dones, H. L., Spitale, J. N.  he Behavior of Spokes in Saturn’s B Ring.  Submitted to Icarus.
 
Hurford, T., Helfenstein, P., Spitale, J. N.  Tidal Control of Jet Eruptions Observed by Cassini ISS Netween 2005 and 2007.  Icarus 220 896-903
 
 
 
II.B. Conference presentations
 
Hahn, J. M., Spitale. J. N. Results from N-body Simulations of the Outer Edge of Saturn's B Ring
.  DPS meeting 2012.
 
Spitale. J. N., Tiscareno, M. Cassini Images A Propeller In Saturn's B-ring
.  DPS meeting 2012
 
Hahn, J. M., Spitale. J. N.  Global N-body Simulations of Broad Planetary Rings and Narrow Ringlets
.  DDA meeting 2012
 
Spitale. J. N., Tiscareno, M.  A Propeller In Saturn's B-ring
.  DDA meeting 2012
 
 
 
 
 
 
 
III. Service to the Science Community
 
III.a. Peer Reviews
I reviewed a manuscript that was submitted to Advances in Space Research by Yang et al. regarding a model for spoke formation in Saturn’s rings.
 
I provided external reviews for 1 PGG proposal and 2 Origins proposals.
 
I participated in the DAWN data review.
 
I served on the JUICE review panel.
 
I am chairing the OPR 2012 dynamics panel, which has not yet met.
  
 
III.b  Other Service
Since leaving Carolyn Porco’s group, I have been conferring with the Jet Propulsion Laboratory on arranging the public release of MINAS, the software that I developed during my time with Cassini.  I began distributing a beta version of the software, renamed OMINAS (i.e., open-MINAS) to a small number of scientists.  Although the software will be free and open, my goal with the release of OMINAS is to generate future funding through a) subcontracting with other groups to assist in the development of applications for MINAS, and b) creating new professional connections that will lead to collaboration on future proposals. 
 
 I gave a phone interview to the BBC for a program about asteroid hazard mitigation.
 
 
IV. Teaching Activities / Public Lectures
  
LPL Fieldtrip class
I participated in the planning and execution of two graduate field studies classes (PTYS 594), run through the U of A Planetary Sciences department.  In the spring we looked at the geology of Death Valley, and in the fall we visited a number of sites around within 50 miles of Tucson to give a picture of the local geology.
 
 
 
 
Research Year: 
2012

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