arXiv subjects

M. M. Hedman

Publications and source records attributed to M. M. Hedman.

At least 37 records · Page 2Linked to original sources

arXiv2019

Axisymmetric density waves in Saturn's rings

Density waves in Saturn's rings are usually tightly wrapped spiral patterns generated by resonances with either Saturn's moons or structures inside the planet. However, between the Barnard and Bessel Gaps in the Cassini Division (i.e. between 120,240 and 120,300 km from Saturn's spin axis), there are density variations that appear to form an axisymmetric density wave consisting of concentric zones of varying densities that propagate radially through the rings. Axisymmetric waves cannot be generated directly by a satellite resonance, but instead appear to be excited by interference between a nearby satellite resonance and normal mode oscillations on the inner edge of the Barnard Gap. Similar axisymmetric waves may exist just interior to other resonantly confined edges that exhibit a large number of normal modes, including the Dawes ringlet in the outer C ring and the outermost part of the B ring.

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arXiv2019

Bright clumps in the D68 ringlet near the end of the Cassini Mission

The D68 ringlet is the innermost narrow feature in Saturn's rings. Prior to 2014, the brightness of this ringlet did not vary much with longitude, but sometime in 2014 or 2015 a series of bright clumps appeared within D68. These clumps were up to four times brighter than the typical ringlet, occurred within a span of ~ 120 degrees in corotating longitude, and moved at an average rate of 1751.7 degrees/day during the last year of the Cassini mission. The slow evolution and relative motions of these clumps suggest that they are composed of particles with a narrow (sub-kilometer) spread in semi-major axis. The clumps therefore probably consist of fine material released by collisions among larger (up to 20 meters wide) objects orbiting close to D68. The event that triggered the formation of these bright clumps is still unclear, but it could have some connection to the material observed when the Cassini spacecraft passed between the planet and the rings.

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arXiv2018

Kronoseismology IV. Six previously unidentified waves in Saturn's middle C ring

Recent studies of stellar occultations observed by the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft have demonstrated that multiple spiral wave structures in Saturn's rings are probably generated by normal-mode oscillations inside the planet. Wavelet-based analyses have been able to unambiguously determine both the number of spiral arms and the rotation rate of many of these patterns. However, there are many more planetary normal modes that should have resonances in the rings, implying that many normal modes do not have sufficiently large amplitudes to generate obvious ring waves. Fortunately, recent advances in wavelet analysis allow weaker wave signals to be uncovered by combining data from multiple occultations. These new analytical tools reveal that a pattern previously identified as a single spiral wave actually consists of two superimposed waves, one with 5 spiral arms rotating at 1593.6 degrees/day and one with 11 spiral arms rotating at 1450.5 degrees/day. Furthermore, a broad search for new waves revealed four previously unknown wave patterns with 6, 7, 8 and 9 spiral arms rotating around the planet at 1538.2 degrees/day, 1492.5 degrees/day, 1454.2 degrees/day and 1421.8 degrees/day, respectively. These six patterns provide precise frequencies for another six fundamental normal modes inside Saturn, yielding what is now a complete sequence of fundamental sectoral normal modes with azimuthal wavenumbers from 2 to 10. These frequencies should place strong constraints on Saturn's interior structure and rotation rate, while the relative amplitudes of these waves should help clarify how the corresponding normal modes are excited inside the planet.

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arXiv2018

Spatial Variations in the Dust-to-Gas Ratio of Enceladus' Plume

On day 138 of 2010, the plume of dust and gas emerging from Enceladus' South Polar Terrain passed between the Sun and the Cassini spacecraft. This solar occultation enabled Cassini's Ultraviolet Imaging Spectrograph (UVIS) and the Visual and Infrared Mapping Spectrometer (VIMS) to obtain simultaneous measurements of the plume's gas and dust components along the same lines of sight. The UVIS measurements of the plume's gas content are described in Hansen et al. (2011, GRL 38:11202) , while this paper describes the VIMS data and the information they provide about the plume's particle content. Together, the VIMS and UVIS measurements reveal that the plume material above Baghdad and Damascus sulci has a dust-to-gas mass ratio that is roughly an order of magnitude higher than the material above Alexandria and Cairo sulci. Similar trends in the plume's dust-to-gas ratio are also found in data obtained when Cassini flew through the plume in 2009, during which time the Ion and Neutral Mass Spectrometer (INMS), Radio and Plasma Wave Science instrument (RPWS) and Cosmic Dust Analyzer (CDA) instruments made in-situ measurements of the plume's gas and dust densities (Dong et al. 2015 JGR 120:915-937). These and other previously-published systematic differences in the material erupting from different fissures likely reflect variations in subsurface conditions across Encealdus' South Polar Terrain.

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arXiv2016

Are there moonlets near the Uranian alpha and beta rings?

The Voyager 2 Radio Science Subsystem occultations of the Uranian alpha and beta rings exhibit quasi-periodic optical depth variations with radial wavelengths that vary with longitude. These patterns may be wakes from small moonlets orbiting exterior to these rings. Based on the observed structures in the rings, we estimate that the moonlets would need to be located about 100 km exterior to the rings' semimajor axes and be 2-7 km in radius. Such moonlets could help keep the rings confined. Due to their small radii and presumed low albedo, the expected brightness of these moonlets is on the order of the noise in Voyager 2 images.

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arXiv2016

A curious ringlet that shares Prometheus' orbit but precesses like the F ring

Images obtained by the Cassini spacecraft of the region just beyond Saturn's main rings reveal a previously unreported narrow and dusty ringlet that has dynamical connections with both Saturn's small satellite Prometheus and the F ring. The radial position of this ringlet is observed to vary with time and longitude, indicating that it is eccentric with an eccentricity of 0.0012 and that its mean orbital radius varies between 139,300 km and 139,400 km. These mean radii are consistent with material trapped in a co-orbital 1:1 resonance with Prometheus. However, the apsidal precession rate of this ringlet is not that expected for material close to Prometheus' orbit (2.76 degrees/day). Instead, the ringlet appears to be precessing at the same rate as the F ring (2.70 degrees/day). This ringlet therefore appears to consist of material co-rotating with Prometheus whose apsidal precession rates have been modified by interactions with F-ring material. This ringlet may therefore provide new insights into how rings can maintain organized eccentric structures over a range of semi-major axes

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arXiv2016

The B-ring's surface mass density from hidden density waves: Less than meets the eye?

Saturn's B ring is the most opaque ring in our solar system, but many of its fundamental parameters, including its total mass, are not well constrained. Spiral density waves generated by mean-motion resonances with Saturn's moons provide some of the best constraints on the rings' mass density, but detecting and quantifying such waves in the B ring has been challenging because of this ring's high opacity and abundant fine-scale structure. Using a wavelet-based analyses of 17 occultations of the star gamma Crucis observed by the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft, we are able to examine five density waves in the B ring. Two of these waves are generated by the Janus 2:1 and Mimas 5:2 Inner Lindblad Resonances at 96,427 km and 101,311 km from Saturn's center, respectively. Both of these waves can be detected in individual occultation profiles, but the multi-profile wavelet analysis reveals unexpected variations in the pattern speed of the Janus 2:1 wave that might arise from the periodic changes in Janus' orbit. The other three wave signatures are associated with the Janus 3:2, Enceladus 3:1 and Pandora 3:2 Inner Lindblad Resonances at 115,959 km, 115,207 km and 108,546 km. These waves are not visible in individual profiles, but structures with the correct pattern speeds can be detected in appropriately phase-corrected average wavelets. Estimates of the ring's surface mass density derived from these five waves fall between 40 and 140 g/cm^2, even though the ring's optical depth in these regions ranges from 1.5 to almost 5. This suggests that the total mass of the B ring is most likely between one-third and two-thirds the mass of Saturn's moon Mimas.

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arXiv2015

The weather report from IRC+10216: Evolving irregular clouds envelop carbon star

High angular resolution images of IRC+10216 are presented in several near infrared wavelengths spanning more than 8 years. These maps have been reconstructed from interferometric observations obtained at both Keck and the VLT, and also from stellar occultations by the rings of Saturn observed with the Cassini spacecraft. The dynamic inner regions of the circumstellar environment are monitored over eight epochs ranging between January 2000 and July 2008. The system is shown to experience substantial evolution within this period including the fading of many previously reported persistent features, some of which had been identified as the stellar photosphere. These changes are discussed in context of existing models for the nature of the underlying star and the circumstellar environment. With access to these new images, we are able to report that none of the previously identified bright spots in fact contain the star, which is buried in its own dust and not directly visible in the near infrared.

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arXiv2015

A new pattern in Saturn's D ring created in late 2011

Images obtained by the Cassini spacecraft between 2012 and 2015 reveal a periodic brightness variation in a region of Saturn's D ring that previously appeared to be rather featureless. Furthermore, the intensity and radial wavenumber of this pattern have decreased steadily with time since it was first observed. Based on analogies with similar structures elsewhere in the D ring, we propose that this structure was created by some event that disturbed the orbital motions of the ring particles, giving them finite orbital eccentricities and initially aligned pericenters. Differential orbital precession then transformed this structure into a spiral pattern in the ring's optical depth that became increasingly tightly wound over time. The observed trends in the pattern's radial wavenumber are roughly consistent with this basic model, and also indicate that the ring-disturbing event occurred in early December 2011. Similar events in 1979 may have generated the periodic patterns seen in this same region by the Voyager spacecraft. The 2011 event could have been caused by debris striking the rings, or by a disturbance in the planet's electromagnetic environment. The rapid reduction in the intensity of the brightness variations over the course of just a few years indicates that some process is either damping orbital eccentricities in this region or causing the orbital pericenters of particles with the same semi-major axis to become misaligned.

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arXiv2015

Saturn's G and D rings provide nearly complete measured scattering/phase functions of nearby debris disks

The appearance of debris disks around distant stars depends upon the scattering/phase function (SPF) of the material in the disk. However, characterizing the SPFs of these extrasolar debris disks is challenging because only a limited range of scattering angles are visible to Earth-based observers. By contrast, Saturn's tenuous rings can be observed over a much broader range of geometries, so their SPFs can be much better constrained. Since these rings are composed of small particles released from the surfaces of larger bodies, they are reasonable analogs to debris disks and so their SPFs can provide insights into the plausible scattering properties of debris disks. This work examines two of Saturn's dusty rings: the G ring (at 167,500 km from Saturn's center) and the D68 ringlet (at 67,600 km). Using data from the cameras onboard the Cassini spacecraft, we are able to estimate the rings' brightnesses at scattering angles ranging from 170 to 0.5 degrees. We find that both of the rings exhibit extremely strong forward-scattering peaks, but for scattering angles above 60 degrees their brightnesses are nearly constant. These SPFs can be well approximated by a linear combination of three Henyey-Greenstein functions, and are roughly consistent with the SPFs of irregular particles from laboratory measurements. Comparing these data to Fraunhofer and Mie models highlights several challenges involved in extracting information about particle compositions and size distributions from SPFs alone. The SPFs of these rings also indicate that the degree of forward scattering in debris disks may be greatly underestimated.

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arXiv2015

Why are dense planetary rings only found between 8 and 20 AU?

The recent discovery of dense rings around the Centaur Chariklo (and possibly Chiron) reveals that complete dense planetary rings are not only found around Saturn and Uranus, but also around small bodies orbiting in the vicinity of those giant planets. This report examines whether there could be a physical process that would make rings more likely to form or persist in this particular part of the outer Solar System. Specifically, the ring material orbiting Saturn and Uranus appears to be much weaker than the material forming the innermost moons of Jupiter and Neptune. Also, the mean surface temperatures of Saturn's, Uranus' and Chariklo's rings are all close to 70 K. Thus the restricted distribution of dense rings in our Solar System may arise because icy materials are particularly weak around that temperature.

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arXiv2014

Titan's atmosphere as observed by Cassini/VIMS solar occultations: CH$_4$, CO and evidence for C$_2$H$_6$ absorption

We present an analysis of the VIMS solar occultations dataset, which allows us to extract vertically resolved information on the characteristics of Titan's atmosphere between 100-700 km with a characteristic vertical resolution of 10 km. After a series of data treatment procedures, 4 occultations out of 10 are retained. This sample covers different seasons and latitudes of Titan. The transmittances show clearly the evolution of the haze and detect the detached layer at 310 km in Sept. 2011 at mid-northern latitudes. Through the inversion of the transmission spectra with a line-by-line radiative transfer code we retrieve the vertical distribution of CH$_4$ and CO mixing ratio. The two methane bands at 1.4 and 1.7 μm are always in good agreement and yield an average stratospheric abundance of $1.28\pm0.08$%. This is significantly less than the value of 1.48% obtained by the GCMS/Huygens instrument. The analysis of the residual spectra after the inversion shows that there are additional absorptions which affect a great part of the VIMS wavelength range. We attribute many of these additional bands to gaseous ethane, whose near-infrared spectrum is not well modeled yet. Ethane contributes significantly to the strong absorption between 3.2-3.5 μm that was previously attributed only to C-H stretching bands from aerosols. Ethane bands may affect the surface windows too, especially at 2.7 μm. Other residual bands are generated by stretching modes of C-H, C-C and C-N bonds. In addition to the C-H stretch from aliphatic hydrocarbons at 3.4 μm, we detect a strong and narrow absorption at 3.28 μm which we tentatively attribute to the presence of PAHs in the stratosphere. C-C and C-N stretching bands are possibly present between 4.3-4.5 μm. Finally, we obtain the CO mixing ratio between 70-170 km. The average result of $46\pm16$ ppm is in good agreement with previous studies.

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arXiv2014

Corrugations and eccentric spirals in Saturn's D ring: New insights into what happened at Saturn in 1983

Previous investigations of Saturn's outer D ring (73,200-74,000 km from Saturn's center) identified periodic brightness variations whose radial wavenumber increased linearly over time. This pattern was attributed to a vertical corrugation, and its temporal variability implied that some event --possibly an impact with interplanetary debris-- caused the ring to become tilted out the planet's equatorial plane in 1983. This work examines these patterns in greater detail using a more extensive set of Cassini images in order to obtain additional insights into the 1983 event. These additional data reveal that the D ring is not only corrugated, but also contains a time-variable periodic modulation in its optical depth that probably represents organized eccentric motions of the D-ring's particles. This second pattern suggests that whatever event tilted the rings also disturbed the radial or azimuthal velocities of the ring particles. Furthermore, the relative amplitudes of the two patterns indicate that the vertical motions induced by the 1983 event were 2.3+/-0.5 times larger than the corresponding in-plane motions. If these structures were indeed produced by an impact, material would need to strike the ring at a steep angle (more than 60 degrees from the ring plane) to produce such motions. Meanwhile, the corrugation wavelengths in the D ring are about 0.7% shorter than one would predict based on extrapolations from similar structures in the nearby C ring. This could indicate that the D-ring was tilted/disturbed about 60 days before the C ring. Such a timing difference could be explained if the material that struck the ring was derived from debris released when some object broke up near Saturn some months earlier. This debris would need to have a substantial initial velocity dispersion and have its orbital properties perturbed by solar tides prior to its collision with the rings.

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arXiv2014

More Kronoseismology with Saturn's rings

In a previous paper (Hedman and Nicholson 2013), we developed tools that allowed us to confirm that several of the waves in Saturn's rings were likely generated by resonances with fundamental sectoral normal modes inside Saturn itself. Here we use these same tools to examine eight additional waves that are probably generated by structures inside the planet. One of these waves appears to be generated by a resonance with a fundamental sectoral normal mode with azimuthal harmonic number m=10. If this attribution is correct, then the m=10 mode must have a larger amplitude than the modes with m=5-9, since the latter do not appear to generate strong waves. We also identify five waves with pattern speeds between 807 degrees/day and 834 degrees/day. Since these pattern speeds are close to the planet's rotation rate, they probably are due to persistent gravitational anomalies within the planet. These waves are all found in regions of enhanced optical depth known as plateaux, but surprisingly the surface mass densities they yield are comparable to the surface mass densities of the background C ring. Finally, one wave appears to be a one-armed spiral pattern whose rotation rate suggests it is generated by a resonance with a structure inside Saturn, but the nature of this perturbing structure remains unclear. Strangely, the resonant radius for this wave seems to be drifting inwards at an average rate of 0.8 km/year over the last thirty years, implying that the relevant planetary oscillation frequency has been steadily increasing.

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arXiv2014

Exploring overstabilities in Saturn's A ring using two stellar occultations

Certain regions of Saturn's rings exhibit periodic opacity variations with characteristic radial wavelengths of up to a few hundred meters that have been attributed to viscous overstabilities. The Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft observed two stellar occultations of the star gamma Crucis that had sufficient resolution to discern a sub-set of these periodic patterns in a portion of the A ring between 124,000 and 125,000 km from Saturn center. These data reveal that the wavelengths and intensities of the patterns vary systematically across this region, but that these parameters are not strictly determined by the ring's average optical depth. Furthermore, our observations indicate that these opacity variations have an azimuthal coherence scale of around 3000 km.

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arXiv2014

Scientific rationale of Saturn's in situ exploration

Remote sensing observations meet some limitations when used to study the bulk atmospheric composition of the giant planets of our solar system. A remarkable example of the superiority of in situ probe measurements is illustrated by the exploration of Jupiter, where key measurements such as the determination of the noble gases abundances and the precise measurement of the helium mixing ratio have only been made available through in situ measurements by the Galileo probe. This paper describes the main scientific goals to be addressed by the future in situ exploration of Saturn placing the Galileo probe exploration of Jupiter in a broader context and before the future probe exploration of the more remote ice giants. In situ exploration of Saturn's atmosphere addresses two broad themes that are discussed throughout this paper: first, the formation history of our solar system and second, the processes at play in planetary atmospheres. In this context, we detail the reasons why measurements of Saturn's bulk elemental and isotopic composition would place important constraints on the volatile reservoirs in the protosolar nebula. We also show that the in situ measurement of CO (or any other disequilibrium species that is depleted by reaction with water) in Saturn's upper troposphere would constrain its bulk O/H ratio. We highlight the key measurements required to distinguish competing theories to shed light on giant planet formation as a common process in planetary systems with potential applications to most extrasolar systems. In situ measurements of Saturn's stratospheric and tropospheric dynamics, chemistry and cloud-forming processes will provide access to phenomena unreachable to remote sensing studies. Different mission architectures are envisaged, which would benefit from strong international collaborations.

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arXiv2014

Non-circular features in Saturn's D ring: D68

D68 is a narrow ringlet located only 67,627 km (1.12 planetary radii) from Saturn's spin axis. Images of this ringlet obtained by the Cassini spacecraft reveal that this ringlet exhibits persistent longitudinal brightness variations and a substantial eccentricity (ae=25+/-1 km). By comparing observations made at different times, we confirm that the brightness variations revolve around the planet at approximately the local orbital rate (1751.6 degrees/day), and that the ringlet's pericenter precesses at 38.243+/-0.008 degrees/day, consistent with the expected apsidal precession rate at this location due to Saturn's higher-order gravitational harmonics. Surprisingly, we also find that the ringlet's semi-major axis appears to be decreasing with time at a rate of 2.4+/-0.4 km/year between 2005 and 2013. A closer look at these measurements, along with a consideration of earlier Voyager observations of this same ringlet, suggests that the mean radius of D68 moves back and forth, perhaps with a period of around 15 Earth years or about half a Saturn year. These observations could place important constraints on both the ringlet's local dynamical environment and the planet's gravitational field.

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arXiv2013

Efficiently Extracting Energy from Cosmological Neutrinos

Detecting the extremely low-energy neutrinos that form the Cosmic Neutrino Background (CNB) presents many experimental challenges, but pursuing this elusive goal is still worthwhile because these weakly-interacting particles could provide a new window into the structure and composition of the early universe. This report examines whether cosmological neutrinos can deposit sufficient energy into a target system to be detectable with plausible extensions of current bolometric technologies. While the macroscopic wavelengths of cosmological neutrinos can greatly enhance their cross sections with dense targets, such interactions can only be detectable if they transfer a significant fraction of each neutrino's kinetic energy into the detector system. We find that a large array of dense target masses coupled to suitable motion-sensitive circuits could potentially satisfy both of these conditions and thus might be able to serve as the basis for a more practical cosmological neutrino detector.

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