arXiv subjects

Joseph A. Burns

Publications and source records attributed to Joseph A. Burns.

At least 19 recordsLinked to original sources

arXiv2017

What confines the rings of Saturn?

The viscous spreading of planetary rings is believed to be counteracted by satellite torques, either through an individual resonance or through overlapping resonances. For the A ring of Saturn, it has been commonly believed that the satellite Janus alone can prevent the ring from spreading via its 7:6 Lindblad resonance. We discuss this common misconception and show that, in reality, the A ring is confined by the contributions from the group of satellites Pan, Atlas, Prometheus, Pandora, Janus, Epimetheus, and Mimas, whose cumulative torques from various resonances gradually decrease the angular momentum flux transported outward through the ring via density and bending waves. We further argue that this decrease in angular momentum flux occurs through 'flux reversal'. Furthermore, we use the magnitude of the satellites' resonance torques to estimate the effective viscosity profile across the A ring, showing that it decreases with radius from ~50 cm2 s-1 to less than ~10 cm2 s-1. The gradual estimated decrease of the angular momentum flux and effective viscosity are roughly consistent with results obtained by balancing the shepherding torques from Pan and Daphnis with the viscous torque at the edges of the Encke and Keeler gaps, as well as the edge of the A ring. On the other hand, the Mimas 2:1 Lindblad resonance alone seems to be capable of confining the edge of the B ring, and contrary to the situation in the A ring, we show that the effective viscosity across the B ring is relatively constant at ~24-30 cm2 s-1.

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arXiv2017

Dynamical phenomena at the inner edge of the Keeler gap

We analyze several thousand Cassini ISS images in order to study the inner edge of the Keeler gap in Saturn's outer A ring. We find strong evidence for an m=32 perturbation with a mean amplitude of radial variation of 4.5 km. Phase analysis yields a pattern speed consistent with the mean motion of Prometheus, indicating that this pattern is generated by the 32:31 Inner Lindblad resonance with Prometheus. In addition, we find evidence of 18-lobed and 20-lobed patterns with amplitudes of ~1.5 km. These patterns, whose rotation rates correspond to resonance locations ~4 km interior to the gap edge, are believed to be normal modes. The former is probably related to the nearby 18:17 (m=18) resonance with Pandora. In addition to these resonant and normal mode patterns, we also observe multiple localized features that appear to move at the local keplerian rate and that persist for only a few months. One hypothesis is that different groups of ring particles at the inner edge of the gap may be reacting differently to the resonance with Prometheus, with local variations in the forced eccentricity and/or pericenter; an alternative hypothesis is the existence of several unseen objects embedded at or near the inner edge of the Keeler gap, similar to those suspected to exist at the outer edges of the A and B rings (Spitale and Porco 2009, 2010). In either case, observations of the ring edge at opposite ansae demonstrate that the localized features must be on eccentric orbits.

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arXiv2017

True Polar Wander of Enceladus From Topographic Data

Many obsects in the solar system are suspected to have experience reorientation of their spin axes. As their rotation rates are slow and their shapes are nearly spherical, the formation of mass anomalies, by either endogenic of exogenic processes, can change objects' moments of inertia. Therefore, the objects reorient to align their largest moment of inertia with their spin axis. Such phenomenon is called True Polar Wander (TPW). Here we report the discovery of a global series of topographic lows on Saturn's satellite Enceladus that we interpret to show that this synchronously locked moon has undergone TPW by ~55° about the tidal axis. We use improved topographic data from the spherical harmonic expansion of Cassini limb and stereogrammetric measurements to characterize regional topography over the surface of Enceladus. We identify a group of nearly antipodal basins orthogonal to a topographic basin chain tracing a non-equatorial circumglobal belt across Enceladus' surface. We argue that the belt and the antipodal regions are fossil remnants of an earlier equator and poles, respectively. We argue that these lows arise from isostasic compensation and that their pattern reflects spatial variations in internal dynamics of the ice shell. Our hypothesis is consistent with a variety of geological features visible in Cassini images.

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arXiv2016

Radial profiles of the Phoebe ring, a vast debris disk around Saturn

We present observations at optical wavelengths with the Cassini Spacecraft's Imaging Science System of the Phoebe ring, a vast debris disk around Saturn that seems to be collisionally generated by its irregular satellites. The analysis reveals a radial profile from 80-260 Saturn radii ($R_S$) that changes behavior interior to $\approx 110 R_S$, which we attribute to either the moon Iapetus sweeping up small particles, or to orbital instabilities that cause the ring to flare up vertically. Our study yields an integrated I/F at 0.635 $μ$m along Saturn's shadow in the Phoebe ring's midplane from 80-250 $R_S$ of $2.7^{+0.9}_{-0.3} \times 10^{-9}$. We develop an analytical model for the size-dependent secular dynamics of retrograde Phoebe ring grains, and compare this model to the observations. This analysis implies that 1) the "Phoebe" ring is partially sourced by debris from irregular satellites beyond Phoebe's orbit and 2) the scattered light signal is dominated by small grains ($\lesssim 20μ$m in size). If we assume that the Phoebe ring is generated through steady-state micrometeoroid bombardment, this implies a power-law size distribution with index $> 4$, which is unusually steep among solar system rings. This suggests either a steep size distribution of ejecta when material is initially released, or a subsequent process that preferentially breaks up large grains.

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arXiv2015

Observing Planetary Rings with JWST: Science Justification and Observation Requirements

The James Webb Space Telescope (JWST) will provide unprecedented opportunities to observe the rings and small satellites in our solar system, accomplishing three primary objectives: 1) discovering new rings and moons, 2) unprecedented spectroscopy, and 3) time-domain observations. We give details on these science objectives and describe requirements that JWST must fulfill in order to accomplish the science objectives

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arXiv2015

How Janus' Orbital Swap Affects the Edge of Saturn's A Ring?

We present a study of the behavior of Saturn's A ring outer edge, using images and occultation data obtained by the Cassini spacecraft over a period of 8 years from 2006 to 2014. More than 5000 images and 170 occultations of the A ring outer edge are analyzed. Our fits confirm the expected response to the Janus 7:6 Inner Lindblad resonance (ILR) between 2006 and 2010, when Janus was on the inner leg of its regular orbit swap with Epimetheus. During this period, the edge exhibits a regular 7-lobed pattern with an amplitude of 12.8 km and one minimum aligned with the orbital longitude of Janus, as has been found by previous investigators. However, between 2010 and 2014, the Janus/Epimetheus orbit swap moves the Janus 7:6 LR away from the A ring outer edge, and the 7-lobed pattern disappears. In addition to several smaller-amplitudes modes, indeed, we found a variety of pattern speeds with different azimuthal wave numbers, and many of them may arise from resonant cavities between the ILR and the ring edge; also we found some other signatures consistent with tesseral resonances that could be associated with inhomogeneities in Saturn's gravity field. Moreover, these signatures do not have a fixed pattern speed. We present an analysis of these data and suggest a possible dynamical model for the behavior of the A ring's outer edge after 2010.

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arXiv2014

First observations of the Phoebe ring in optical light

The Phoebe ring, Saturn's largest and faintest ring, lies far beyond the planet's well-known main rings. It is primarily sourced by collisions with Saturn's largest irregular satellite Phoebe, perhaps through stochastic macroscopic collisions, or through more steady micrometeoroid bombardment. The ring was discovered with the Spitzer Space Telescope at 24 $μ$m and has a normal optical depth of $\sim 2 \times 10^{-8}$ (Verbiscer et al. 2009). We report the first observations of sunlight scattered off the Phoebe ring using the Cassini spacecraft's ISS camera at optical wavelengths. We find that material between $\approx 130-210$ Saturnian radii ($R_S$) from the planet produces an I/F of $1.7 \pm 0.1 \times 10^{-11}$ per $R_S$ of the line-of-sight distance through the disk. Combining our measurements with the Spitzer infrared data, we can place constraints on the ring-particles' light-scattering properties. Depending on the particles' assumed phase function, the derived single-scattering albedo can match either photometric models of Phoebe's dark regolith or brighter sub-surface material excavated by macroscopic impacts on Phoebe.

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arXiv2013

Chaotic dust dynamics and implications for the hemispherical color asymmetries of the Uranian satellites

Dust grains generated by the Uranian irregular satellites will undergo chaotic large-amplitude eccentricity oscillations under the simultaneous action of radiation forces and the highly misaligned quadrupole potentials of the oblate planet and distant Sun. From a suite of orbital histories, we estimate collision proba- bilities of dust particles with the regular satellites and argue that this process may explain the observed hemispherical color asymmetries of the outermost four regular satellites of Uranus.

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arXiv2013

Probing the inner boundaries of Saturn's A ring with the Iapetus -1:0 nodal bending wave

The Iapetus -1:0 nodal bending wave, the first spiral wave ever described in Saturn's rings, has been seen again for the first time in 29 years. We demonstrate that it is in fact the nodal bending wave, not the 1:0 apsidal density wave as previously reported. We use wavelet analysis to determine the wavelength profile, thus deriving the surface density at every point in the region covered by the bending wave. This profile is consistent with surface densities measured from more localized spiral density waves in the outer Cassini Division and the inner and mid-A Ring, varying smoothly from the low values of the former to the higher values of the latter. Most remarkably, our analysis indicates that there is no significant change in surface density across the boundary between the outer Cassini Division and the inner-A ring, despite the very abrupt increase in optical depth and reflected brightness at this location. We consider anew the nature of the classically identified "inner edge of the A ring," given that it does not appear to be correlated with any abrupt increase in surface density. There is an abrupt increase in surface density at the Pandora 5:4 density wave, ~300 km outward of the A ring's inner edge. Further study is needed to robustly interpret our findings in terms of particle properties and abundances, much less to explain the origins of the implied structure.

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arXiv2013

Compositions and origins of outer planet systems: Insights from the Roche critical density

We consider the Roche critical density (rho_Roche), the minimum density of an orbiting object that, at a given distance from its planet, is able to hold itself together by self-gravity. It is directly related to the more familiar "Roche limit," the distance from a planet at which a strengthless orbiting object of given density is pulled apart by tides. The presence of a substantial ring requires that transient clumps have an internal density less than rho_Roche. Conversely, in the presence of abundant material for accretion, an orbiting object with density greater than rho_Roche will grow. Comparing the rho_Roche values at which the Saturn and Uranus systems transition rapidly from disruption-dominated (rings) to accretion-dominated (moons), we infer that the material composing Uranus' rings is likely more rocky, as well as less porous, than that composing Saturn's rings. From the high values of rho_Roche at the innermost ring-moons of Jupiter and Neptune, we infer that those moons may be composed of denser material than expected, or more likely that they are interlopers that formed farther from their planets and have since migrated inward, now being held together by internal material strength. Finally, the "Portia group" of eight closely-packed Uranian moons has an overall surface density similar to that of Saturn's A ring. Thus, it can be seen as an accretion-dominated ring system, of similar character to the standard ring systems except that its material has a characteristic density greater than the local rho_Roche.

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arXiv2012

Dynamical Instabilities in High-Obliquity Systems

High-inclination circumplanetary orbits that are gravitationally perturbed by the central star can undergo Kozai oscillations---large-amplitude, coupled variations in the orbital eccentricity and inclination. We first study how this effect is modified by incorporating perturbations from the planetary oblateness. Tremaine et al. (2009) found that, for planets with obliquities > 68.875 degrees, orbits in the equilibrium local Laplace plane are unstable to eccentricity perturbations over a finite radial range, and execute large-amplitude chaotic oscillations in eccentricity and inclination. In the hope of making that treatment more easily understandable, we analyze the problem using orbital elements, confirming this threshold obliquity. Furthermore, we find that orbits inclined to the Laplace plane will be unstable over a broader radial range, and that such orbits can go unstable for obliquities less than 68.875 degrees. Finally, we analyze the added effects of radiation pressure, which are important for dust grains and provide a natural mechanism for particle semimajor axes to sweep via Poynting-Robertson drag through any unstable range. We find that generally the effect persists; however, the unstable radial range is shifted and small retrograde particles can avoid the instability altogether. We argue that this is occurs because radiation pressure modifies the equilibrium Laplace plane.

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arXiv2011

Finding the trigger to Iapetus' odd global albedo pattern: Dynamics of dust from Saturn's irregular satellites

The leading face of Saturn's moon Iapetus, Cassini Regio, has an albedo only one tenth that on its trailing side. The origin of this enigmatic dichotomy has been debated for over forty years, but with new data, a clearer picture is emerging. Motivated by Cassini radar and imaging observations, we investigate Soter's model of dark exogenous dust striking an originally brighter Iapetus by modeling the dynamics of the dark dust from the ring of the exterior retrograde satellite Phoebe under the relevant perturbations. In particular, we study the particles' probabilities of striking Iapetus, as well as their expected spatial distribution on the Iapetian surface. We find that, of the long-lived particles (greater than about 5 microns), most particle sizes (greater than about 10 microns) are virtually certain to strike Iapetus, and their calculated distribution on the surface matches up well with Cassini Regio's extent in its longitudinal span. The satellite's polar regions are observed to be bright, presumably because ice is deposited there. Thus, in the latitudinal direction we estimate polar dust deposition rates to help constrain models of thermal migration invoked to explain the bright poles (Spencer & Denk 2010). We also analyze dust originating from other irregular outer moons, determining that a significant fraction of that material will eventually coat Iapetus--perhaps explaining why the spectrum of Iapetus' dark material differs somewhat from that of Phoebe. Finally we track the dust particles that do not strike Iapetus, and find that most land on Titan, with a smaller fraction hitting Hyperion. As has been previously conjectured, such exogenous dust, coupled with Hyperion's chaotic rotation, could produce Hyperion's roughly isotropic, moderate-albedo surface.

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arXiv2010

Cassini imaging search rules out rings around Rhea

We have conducted an intensive search for any material that may orbit Rhea, using images obtained by the Cassini ISS narrow-angle camera. We find no evidence for any such material, contradicting an earlier and surprising inference that Rhea, the second-largest moon of Saturn, possesses a system of narrow rings embedded in a broad circum-satellite disk or cloud (Jones et al. 2008, Science). If one accepts the calculations of the previous authors, our results would require any narrow rings around Rhea to be composed of objects no smaller than 10 meters in radius, in order to be massive enough to account for the observed charged-particle absorptions but diffuse enough to evade detection in our images. Such a particle-size distribution is unrealistic given that erosion processes will break down 10-meter objects to smaller sizes, which would then have been seen in our images. Furthermore, Jones et al. (2008) assumed that the absorption of electrons by hypothetical circum-Rhea particles is proportional to the volume of the particles, even when they are much larger than the electron penetration depth (van Allen 1983, 1987, JGR). If we assume instead that large particles can only absorb electrons efficiently near their surfaces, then our results rule out any solid material orbiting Rhea as the explanation for the absorptions detected by Jones et al. (2008), regardless of particle size, by a margin of four (for narrow rings) or two (for a broad diffuse cloud) orders of magnitude. We conclude that the anomalous electron absorptions reported by Jones et al. (2008) are the signature of a new and hitherto unknown magnetospheric process.

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arXiv2010

Physical characteristics and non-keplerian orbital motion of "propeller" moons embedded in Saturn's rings

We report the discovery of several large "propeller" moons in the outer part of Saturn's A ring, objects large enough to be followed over the 5-year duration of the Cassini mission. These are the first objects ever discovered that can be tracked as individual moons, but do not orbit in empty space. We infer sizes up to 1--2 km for the unseen moonlets at the center of the propeller-shaped structures, though many structural and photometric properties of propeller structures remain unclear. Finally, we demonstrate that some propellers undergo sustained non-keplerian orbit motion. (Note: This arXiv version of the paper contains supplementary tables that were left out of the ApJL version due to lack of space).

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arXiv2009

An analytic parameterization of self-gravity wakes in Saturn's rings, with application to occultations and propellers

We have developed a semi-analytic method of parameterizing N-body simulations of self-gravity wakes in Saturn's rings, describing their properties by means of only 6 numbers: 3 optical depths and 3 weighting factors. These numbers are obtained using a density-estimation procedure that finds the frequencies of various values of local density within a simulated ring patch. Application of our parameterization to a suite of N-body simulations implies that the distribution of local optical depths is trimodal, rather than bimodal as previous authors have assumed. Rings dominated by self-gravity wakes appear to be mostly empty space. The implications of this result for the analysis of occultation data are more conceptual than practical. The only adjustment needed is that the model parameter $τ_{gap}$ should be interpreted as representing the area-weighted average optical depth within the gaps (or inter-wake regions). The most significant consequence of our results applies to the question of why "propeller" structures observed in the mid-A ring are seen as relative-bright features, even though the most prominent features of simulated propellers are regions of relatively low density. We find preliminary quantitative support for the hypothesis that propellers would be bright if they involve a local and temporary disruption of self-gravity wakes, flooding the region with more "photometrically active" material (i.e., material that can contribute to the rings' local optical depth) even though the overall density is lower, and thus raising their apparent brightnesses in agreement with observations. We suggest that this mechanism be tested by future detailed numerical models.

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arXiv2009

The Rotation of Janus and Epimetheus

Epimetheus, a small moon of Saturn, has a rotational libration (an oscillation about synchronous rotation) of 5.9 +- 1.2 degrees, placing Epimetheus in the company of Earth's Moon and Mars' Phobos as the only natural satellites for which forced rotational libration has been detected. The forced libration is caused by the satellite's slightly eccentric orbit and non-spherical shape. Detection of a moon's forced libration allows us to probe its interior by comparing the measured amplitude to that predicted by a shape model assuming constant density. A discrepancy between the two would indicate internal density asymmetries. For Epimetheus, the uncertainties in the shape model are large enough to account for the measured libration amplitude. For Janus, on the other hand, although we cannot rule out synchronous rotation, a permanent offset of several degrees between Janus' minimum moment of inertia (long axis) and the equilibrium sub-Saturn point may indicate that Janus does have modest internal density asymmetries. The rotation states of Janus and Epimetheus experience a perturbation every four years, as the two moons "swap" orbits. The sudden change in the orbital periods produces a free libration about synchronous rotation that is subsequently damped by internal friction. We calculate that this free libration is small in amplitude (<0.1 degree) and decays quickly (a few weeks, at most), and is thus below the current limits for detection using Cassini images.

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arXiv2007

The population of propellers in Saturn's A Ring

We present an extensive data set of ~150 localized features from Cassini images of Saturn's Ring A, a third of which are demonstrated to be persistent by their appearance in multiple images, and half of which are resolved well enough to reveal a characteristic "propeller" shape. We interpret these features as the signatures of small moonlets embedded within the ring, with diameters between 40 and 500 meters. The lack of significant brightening at high phase angle indicates that they are likely composed primarily of macroscopic particles, rather than dust. With the exception of two features found exterior to the Encke Gap, these objects are concentrated entirely within three narrow (~1000 km) bands in the mid-A Ring that happen to be free from local disturbances from strong density waves. However, other nearby regions are similarly free of major disturbances but contain no propellers. It is unclear whether these bands are due to specific events in which a parent body or bodies broke up into the current moonlets, or whether a larger initial moonlet population has been sculpted into bands by other ring processes.

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arXiv2007

Cassini Imaging of Saturn's Rings II: A Wavelet Technique for Analysis of Density Waves and Other Radial Structure in the Rings

We describe a powerful signal processing method, the continuous wavelet transform, and use it to analyze radial structure in Cassini ISS images of Saturn's rings. Wavelet analysis locally separates signal components in frequency space, causing many structures to become evident that are difficult to observe with the naked eye. Density waves, generated at resonances with saturnian satellites orbiting outside (or within) the rings, are particularly amenable to such analysis. We identify a number of previously unobserved weak waves, and demonstrate the wavelet transform's ability to isolate multiple waves superimposed on top of one another. We also present two wave-like structures that we are unable to conclusively identify. In a multi-step semi-automated process, we recover four parameters from clearly observed weak spiral density waves: the local ring surface density, the local ring viscosity, the precise resonance location (useful for pointing images, and potentially for refining saturnian astrometry), and the wave amplitude (potentially providing new constraints upon the masses of the perturbing moons). Our derived surface densities have less scatter than previous measurements that were derived from stronger non-linear waves, and suggest a gentle linear increase in surface density from the inner to the mid-A Ring. We show that ring viscosity consistently increases from the Cassini Division outward to the Encke Gap. Meaningful upper limits on ring thickness can be placed on the Cassini Division (3.0 m at r~118,800 km, 4.5 m at r~120,700 km) and the inner A Ring (10 to 15 m for r<127,000 km).

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