Publication year: 2012 Source:Icarus Kevin J. Walsh, Derek C. Richardson, Patrick Michel We present results from numerical experiments testing the behavior of cohesionless gravitational aggregates experiencing a gradual increase of angular momentum. The test bodies used in these numerical simulations are gravitational aggregates of different construction, distinguished by the size distribution of the particles constituting them, parameterized in terms of the angle of friction (ϕ). Shape change and mass loss are found to depend strongly on ϕ, with results ranging from oblate spheroids forming binary systems to near-fluid behavior characterized by mass shedding bursts and no binary formation. Bodies with the highest angle of friction, ϕ∼40°, evolve to shapes with average axis ratios of c/a ∼0.70 and b/a ∼0.90 (a ≥b ≥c), and are efficient at forming satellites. Bodies with lower angle of friction, ϕ∼20°, evolve to shapes with average axis ratios of c/a ∼0.61 and b/a ∼0.83, and are less efficient at forming satellites. The most fluid-like bodies tested, with ϕnear zero, become very elongated, with average axis ratios c/a ∼0.40 and b/a ∼0.56, and do not form satellites in any simulation. In all but 2 fluid-like cases out of 360, no more than 5% of the total mass was ejected in a single event. Bodies with substantial cores were also tested under slow spin-up, and cases with cores larger than ∼30% of the total mass were successful at forming binaries. The binary systems created in all simulations are analysed and compared against observed binary near-Earth asteroids and small Main Belt asteroids. The shape and rotation period of the primary, orbital and rotational period of the secondary, and the orbital semi-major axis and eccentricity are found to closely match the observed population.
Highlights
► We model the slow spin-up of rubble pile asteroids. ► Asteroids with high angles of friction (>40 deg) can maintain spherical/oblate shapes at critical Rotation. ► Asteroids that can maintain spherical/oblate shapes can build satellites in orbit via mass lost from their equators
Publication year: 2012 Source:Icarus P.G.J. Irwin, C. de Bergh, R. Courtin, B. Bézard, N.A. Teanby, G.R. Davis, L.N. Fletcher, G.S. Orton, S.B. Calcutt, D. Tice, J. Hurley New line data describing the absorption of CH4 and CH3D from 1.26 – 1.71 μm (Campargue et al. 2012, building upon previous papers by Campargue et al. 2010; Wang et al. 2010, 2011) have been applied to the analysis of Gemini-N/NIFS observations of Uranus made in 2010 and compared with earlier disc-averaged observations made by KPNO/FTS in 1982. The new line data are found to improve greatly the fit to the observed spectra and present a huge advance over previous methane absorption tables by allowing us to determine the CH3D/CH4 ratio and also start to break the degeneracy between methane abundance and cloud top height. The best fits are obtained if the cloud particles in the main cloud deck at the 2-3 bar level become less scattering with wavelength across the 1.4 – 1.6 μm region and we have modelled this variation here by varying the extinction cross-section and single-scattering albedo of the particles.Applying the new line data to the NIFS spectra of Uranus, we determine a new estimate of the CH3D/CH4 ratio of, which is consistent with the estimate of de Bergh et al. (1986) of, made by fitting a disc-averaged KPNO/FTS spectrum measured in 1982, but much better constrained. The NIFS observations made in 2010 have been disc-averaged and compared with the 1982 KPNO/FTS spectrum and found to be in excellent agreement.Using k-tables fitted to the new line data, the central meridian observations of Uranus H-band spectrum (1.49 – 1.64 μm) made by Gemini-N/NIFS in 2010 have been reanalyzed. The use of the new methane absorption coefficients and the modified scattering properties of the cloud particles in the main cloud deck appears to break the degeneracy between cloud height and methane abundance immediately above it in this spectral region and we find that both vary with latitude across Uranus’ disc. Overall, we find that the main cloud deck becomes higher, but thinner from equator to poles, with a local maximum in cloud top height in the circumpolar zones at 45° N and 45°S. At the same time, using the ‘D’ temperature pressure profile of Lindal et al. (1987) and a deep methane abundance of 1.6% (Baines et al., 1995) we find that the relative humidity of methane is high near the equator (∼60%) and decreases sharply towards the poles, except near the circumpolar zone at 45° N, which has brightened steadily since 2007, and where there is a local maximum in methane relative humidity. In tests conducted with the warmer ‘F1’ profile of Sromovsky et al. (2011) we find a similar variation of methane abundance above the main cloud, although for this warmer temperature profile this abundance is dependent mostly on the fitted deep methane mole fraction.
Highlights
► Revised methane line data give very good fit to Uranus H-band spectra. ► Reflectivity of particles in main cloud deck is found to decrease with wavelength from 1.4 – 1.6 μm. ► Uranus’ CH3D/CH4 ratio is determined to be
. ► New line data allow differentiation between methane humidity and cloud top height. ► Methane humidity is found to be high in the tropics, but to decrease rapidly polewards of 45° N and S.
Publication year: 2012 Source:Icarus Jodi Gaeman, Saswata Hier-Majumder, James H. Roberts We present a study of coupled thermal and structural evolution of Neptune’s moon, Triton, driven by tidal dissipation and radiogenic heating. Triton’s orbital history likely involves capture from a binary system by Neptune, followed by a period of circularization. This work investigates Triton’s evolution past its circularization. We examine the rate of ice shell growth as a function of different orbital eccentricities, in the presence of radiogenic heating. Tidal dissipation in the ice shell, proportional to orbital eccentricity squared, concentrates heating near the base, reducing the basal heat flux. As the growth of the ice shell is proportional to the basal heat flux, increased tidal heating creates a blanketing effect, reducing the rate of ice shell growth. Radiogenic heating from Triton’s core is the other, more dominant, source of heat to the shell. Despite being several orders of magnitude higher than the tidal dissipation, radiogenic heating alone fails to sustain an ocean within Triton over 4.5 Ga. For orbital eccentricities of
and
it takes approximately 2 Ga and 3 Ga, respectively, to completely freeze the ocean. For higher values of orbital eccentricities, an ocean can be sustained in Triton’s interior over 4.5 Ga. If Triton’s history past circularization involves a slow decrease in orbital eccentricity to the current value, a thin, possibly
Publication year: 2012 Source:Icarus Daniel Jontof-Hutter, Douglas P. Hamilton We study the radial and vertical stability of dust grains launched with all charge-to-mass ratios at arbitrary distances from rotating planets with complex magnetic fields. We show that the aligned dipole magnetic field model analyzed by Jontof-Hutter and Hamilton (2012) is an excellent approximation in most cases, but that fundamentally new physics arises with the inclusion of non-axisymmetric magnetic field terms. In particular, large numbers of distant negatively-charged dust grains, stable in a magnetic dipole, can be driven to escape by a more complex field. We trace the origin of the instability to overlapping Lorentz resonances which are extremely powerful when the gravitational and electromagnetic forces on a dust grain are comparable. These resonances enable a dust grain to tap the spin energy of the planet to power its escape. We also explore the relatively minor influence of different launch speeds and the far more important effects of variable grain charge. Only the latter are capable of significantly affecting the micron-sized grains that dominate visible and infrared images of faint dust rings. Finally, we present full stability maps for Earth, Jupiter, Saturn, Uranus, and Neptune with magnetic fields modeled out to octupole order. Not surprisingly, dust in the tortured magnetic fields of Uranus and Neptune show the greatest instability.
Highlights
► Planetary magnetic fields affect the orbits of Kepler-launched dust grains. ► We explore the dynamics of grains at Jupiter, Saturn, Uranus, Neptune and Earth. Non-zero launch impulses have little effect on orbital stability. ► Lorentz resonances can destabilize motions causing negative grains to escape. ► We explore the sensitivity of dust grain orbits to time-variable charging.
Publication year: 2012 Source:Icarus Zhiyong Xiao, Robert G. Strom AbstractThe small crater populations (diameter smaller than 1 km) are widely used to date planetary surfaces. The reliability of small crater counts is tested by counting small craters at several young and old lunar surfaces, including Mare Nubium and craters Alphonsus, Tycho and Giordano Bruno. Based on high-resolution images from both the Lunar Reconnaissance Orbiter Camera and Kaguya Terrain Camera, small craters in two different diameter ranges are counted for each counting area. Large discrepancies exist in both the cumulative (absolute model ages) and relative plots for the two different size ranges of the same counting areas. The results indicate that dating planetary surfaces using small crater populations is highly unreliable because the contamination of secondaries may invalidate the results of small crater counts. A comparison of the size-frequency distributions of the small crater populations and impact ejected boulders around fresh lunar craters shows the same upturn as typical Martian secondaries, which supports the argument that secondaries dominate the small crater populations on the Moon and Mars. Also, the size-frequency distributions of small rayed lunar and Martian craters of probable primary origin are similar to that of the Population 2 craters on the inner solar system bodies post-dating Late Heavy Bombardment. Dating planetary surfaces using the small crater populations requires the separation of primaries from secondaries which is extremely difficult. The results also show that other factors, such as different target properties and the subjective identification of impact craters by different crater counters, may also affect crater counting results. We suggest that dating planetary surfaces using small crater populations should be with highly cautious.
Highlights
► We count small craters on both young and old surfaces using LROC and Kaguya data. ► Craters in 2 diameter ranges are counted for each counting area and then compared. ► Discrepancies occur in the results and the contamination of secondaries is one reason. ► Steep upturns in size distributions are caused by secondaries but not primaries. ► Lunar and Martian small primaries have flat slopes like the Population 2 craters.
Publication year: 2012 Source:Icarus Lucy A. McFadden, Fabienne A. Bastien, Max Mutchler, Carolyn A. Crow, Heather Weir, Jian-Yang Li, Douglas P. Hamilton We imaged the region around asteroid (4) Vesta in nine long exposures using the Wide Field Planetary Camera 2 on the Hubble Space Telescope on May 14 and 16, 2007 to conduct a deep search for satellites in support of NASA’s Dawn mission that orbited (4) Vesta in 2011-2012. Several previous search efforts have been undertaken, but no satellites were detected. Our search covered distances from 14 to 260 Vesta radii and searched to a limiting magnitude of 22.5 ± 0.4 in HST’s wide-band red filter (F702W). Our upper limit for possible satellites corresponds to a satellite just 22 ± 4m in radius, assuming the same optical properties as Vesta. Our upper limit is ∼10 times smaller than the best limit of previous searches. In situ satellite searches by NASA’s Dawn spacecraft will probe regions closer to Vesta than our effort reported here.
Highlights
► Vestan satellite upper limit is 22 ±4 m, ∼10 times smaller than previous searches. ► Volunteer searchers found almost all objects brighter than 22.5 ±0.4 magnitude. ► With albedo at 700 nm of 0.47 (Vesta’s), this magnitude corresponds to a radius of 22 ±4 meters. ► The region searched does not extend to the altitude of the Dawn mission’s spacecraft. ► There remains scientific reason to search for satellites with the Dawn spacecraft.
Publication year: 2012 Source:Icarus Andrew C. Walker, Chris H. Moore, David B. Goldstein, Philip L. Varghese, Laurence M. Trafton Io’s sublimation atmosphere is inextricably linked to the SO2 surface frost temperature distribution which is poorly constrained by observations. We constrain Io’s surface thermal distribution by a parametric study of its thermophysical properties in an attempt to better model the morphology of Io’s sublimation atmosphere. Io’s surface thermal distribution is represented by three thermal units: sulfur dioxide (SO2) frosts/ices, non-frosts (probably sulfur allotropes and/or pyroclastic dusts), and hot spots. The hot spots included in our thermal model are static high temperature surfaces with areas and temperatures based on Keck infrared observations. Elsewhere, over frosts and non-frosts, our thermal model solves the one-dimensional heat conduction equation in depth into Io’s surface and includes the effects of eclipse by Jupiter, radiation from Jupiter, and latent heat of sublimation and condensation. The best fit parameters for the SO2 frost and non-frost units are found by using a least-squares method and fitting to observations of the Hubble Space Telescope’s Space Telescope Imaging Spectrograph (HST STIS) mid- to near-UV reflectance spectra and Galileo PPR brightness temperature. The thermophysical parameters are the frost Bond albedo, αF, and thermal inertia, ΓF, as well as the non-frost surface Bond albedo, αNF, and thermal inertia, ΓNF. The best fit parameters are found to be αF ≈ 0.55 ± 0.02 and ΓF ≈ 200 ± 50 J m-2 K-1 s-1/2 for the SO2 frost surface and αNF ≈ 0.49 ± 0.02 and ΓNF ≈ 20 ± 10 J m-2 K-1 s-1/2 for the non-frost surface.These surface thermophysical parameters are then used as boundary conditions in global atmospheric simulations of Io’s sublimation-driven atmosphere using the direct simulation Monte Carlo (DSMC) method. These simulations are unsteady, three-dimensional, parallelized across 360 processors, and include the following physical effects: inhomogeneous surface frosts, plasma heating, and a temperature-dependent residence time on the non-frost surface. The DSMC simulations show that the sub-Jovian hemisphere is significantly affected by the daily solar eclipse. The simulated SO2 surface frost temperature is found to drop only ∼5 K during eclipse due to the high thermal inertia of SO2 surface frosts but the SO2 gas column density falls by a factor of 20 compared to the pre-eclipse column due to the exponential dependence of the SO2 vapor pressure on the SO2 surface frost temperature. Supersonic winds exist prior to eclipse but become subsonic during eclipse because the collapse of the atmosphere significantly decreases the day-to-night pressure gradient that drives the winds. Prior to eclipse, the supersonic winds condense on and near the cold nightside and form a highly non-equilibrium oblique shock near the dawn terminator. In eclipse, no shock exists since the gas is subsonic and the shock only reestablishes itself an hour or more after egress from eclipse. Furthermore, the excess gas that condenses on the non-frost surface during eclipse leads to an enhancement of the atmosphere near dawn. The dawn atmospheric enhancement drives winds that oppose those that are driven away from the peak pressure region above the warmest area of the SO2 frost surface. These opposing winds meet and are collisional enough to form stagnation point flow.The simulations are compared to Lyman-α observations in an attempt to explain the asymmetry between the dayside atmospheres of the anti-Jovian and sub-Jovian hemispheres. Lyman-α observations indicate that the anti-Jovian hemisphere has higher column densities than the sub-Jovian hemisphere and also has a larger latitudinal extent. A composite “average dayside atmosphere” is formed from a collisionless simulation of Io’s atmosphere throughout an entire orbit. This composite “average dayside” atmosphere without the effect of global winds indicates that the sub-Jovian hemisphere has lower average column densities than the anti-Jovian hemisphere (with the strongest effect at the sub-Jovian point) due primarily to the diurnally averaged effect of eclipse. This is in qualitative agreement with the sub-Jovian / anti-Jovian asymmetry in the Lyman-α observations which were alternatively explained by the bias of volcanic centers on the anti-Jovian hemisphere. Lastly, the column densities in the simulated average dayside atmosphere agree with those inferred from Lyman-α observations despite the thermophysical parameters being constrained by mid- to near UV observations which show much higher instantaneous SO2 gas column densities. This may resolve the apparent discrepancy between the lower “average dayside” column densities observed in the Lyman-α and the higher instantaneous column densities observed in the mid- to near UV.
Highlights
► Io’s surface thermophysical properties are constrained by a parametric study. ► Best fit albedos are αF ≈ 0.55 and αNF ≈ 0.49. ► Best fit thermal inertias are ΓF ≈ 200 J m-2 K-1 s-1/2 and ΓNF ≈ 20 J m-2 K-1 s-1/2. ► Atmospheric structure of the sub-Jovian side is strongly affected by Jovian eclipse. ► Sub-Jovian / anti-Jovian hemisphere asymmetry is likely partially caused by eclipse.
Publication year: 2012 Source:Icarus Alan D. Howard, Jeffrey M. Moore, Paul M. Schenk, Oliver L. White, John Spencer The unique appearance of Hyperion can be explained in part by the loss to space of ballistic ejecta during impact events, as was proposed by Thomas et al (2007a). We conclude that such loss is a partial explanation, accounting for the lack of appreciable intercrater plains on a saturation-cratered surface. In order to create the smooth surfaces and the reticulate, honeycomb pattern of narrow divides between old craters, appreciable subsequent modification of crater morphology must occur through mass-wasting processes accompanied by sublimation, probably facilitated by the loss of co2 as a component of the relief-supporting matrix of the bedrock. During early stages of crater degradation, steep, crenulate bedrock slopes occupy the upper crater walls with abrupt transitions downslope onto smooth slopes near the angle of repose mantled by mass wasting debris, as can be seen within young craters. Long-continued mass wasting eventually results in slopes totally mantled with particulate debris. This mass wasting effectively destroys small craters, at least in part accounting for the paucity of sub-kilometer craters on Hyperion. Surface temperatures measured by Cassini CIRS range from 58°K to 127°K and imply a surface thermal inertia of 11 ± 2 J m-2 K-1 s-1/2 and bolometric albedo ranging from 0.05 to 0.33. Resulting H2O sublimation rates are only tens of cm per billion years for most of the surface, so the evolution of the observed landforms is likely to require sublimation of more volatile species such as CO2.
Highlights
► The lack of inter-crater plains is in part due to loss of fall back ejecta. ► Weathering and mass wasting creates honeycomb pattern of old craters. ► Fresher craters exhibit upper slopes that are still undergoing erosional retreat. ► This style of erosion is consistent with sublimation loss of topography-supporting cement.
Publication year: 2012 Source:Icarus J. Brad Dalton III, Dale P. Cruikshank, Roger N. Clark Compositional mapping of the surface of Hyperion using Cassini Visible and Infrared Mapping Spectrometer (VIMS) observations reveals a heterogeneous surface dominated by water ice accompanied by additional materials. Carbon dioxide, as evidenced by a prominent absorption band centered at 4.26 μm, is distributed over most of the surface, including icy regions. This does not represent exposures of pure CO2 ice, but concentrations of CO2 molecules adsorbed on other materials or complexed in H2O, perhaps as a clathrate (Cruikshank et al. 2010). Localized deposits of low-albedo material in subcircular depressions exhibit spectral absorptions indicative of C-H in aromatic (3.29 μm) and aliphatic (3.35-3.50 μm) hydrocarbons. An absorption band at 2.42 μm that is also seen on other Saturnian satellites, tentatively identified as H2 (Clark et al., 2011, 2012) adsorbed on dark material grains, is also prominent. Our best spectral models included H2O and CO2 ice, with small amounts of nanophase Fe and Fe2O3. Weaker and more spatially scattered absorption features are also found at 4.48, 4.60, and 4.89 μm, although no clear molecular identifications have yet been made. While strongest in the low-albedo deposits, the CO2, hydrocarbon and putative H2 bands vary in strength throughout the icy regions, as do the 4.48-, 4.60- and 4.89-μm bands, suggesting that this background ice is laced with a complex mixture of non-ice compounds.
Highlights
► We examine Cassini visible and near-infrared imaging spectroscopy of Hyperion. ► Surface mostly H2O ice together with CO2 and both aliphatic and aromatic hydrocarbons. ► CO2, H2, carbon-and nanophase Fe-bearing material concentrated in dark lag deposits. ► Carbon dioxide and other contaminants are mixed into the water ice everywhere. ► Hyperion bears primordial material and was probably formed elsewhere prior to capture.
Publication year: 2012 Source:Icarus M. Andriopoulou, E. Roussos, N. Krupp, C. Paranicas, M. Thomsen, S. Krimigis, M.K. Dougherty, K.-H. Glassmeier We have created a new, updated catalogue of energetic electron microsignature events caused by the moons Tethys and Dione. We used electron data of the MIMI-LEMMS detector that is onboard the Cassini spacecraft, in the energy range 20-300 keV and for the period from July 2004 to January 2011. The present study looks at how the location of a moon’s wake deviates from the nearly circular orbital path of the body. The radial deviation of the wake from the moon’s orbit is a very sensitive tracer of plasma motion in the magnetosphere including its small radial components. The positions of the dropouts the spacecraft detects when it flies through the wakes, or microsignatures, cannot be explained in our study by asymmetric magnetic fields in the inner magnetosphere. Instead, we hypothesize a uniform electric field of around 0.11 to 0.18 mV/m within 4.4-7.0 Rs approximately, oriented roughly from noon to midnight, to explain the persistent radial offsets of the microsignatures from their expected positions. This corresponds to a radial speed that is at most a few percent of rigid corotation and therefore very difficult to measure by direct means. We additionally report a tendency for microsignatures with non-monotonic energy dispersion to have drifted across the post-midnight sector more than those with zero or monotonic energy dispersion.
Highlights
► We report systematic asymmetries of the microsignature radial displacements with respect to local time in Saturn, inward in the nightside and outward in the dayside. ► Magnetic field asymmetries are much smaller than the observed microsignature displacements. ► We require a uniform electric field, pointing towards noon to midnight and with strength of not more than 1.0 mV/m to account for these displacements. ► A flow with a dusk-to-dawn orientation and radial velocity of few km/sec must exist in the inner magnetosphere of Saturn.
