Ed. Board

Publication date: 1 September 2015 Source:Icarus, Volume 257







Impact and cratering rates onto Pluto

Publication date: 15 September 2015 Source:Icarus, Volume 258

Author(s): Sarah Greenstreet , Brett Gladman , William B. McKinnon

The New Horizons spacecraft fly-through of the Pluto system in July 2015 will provide humanity’s first data for the crater populations on Pluto and its binary companion, Charon. In principle, these surfaces could be dated in an absolute sense, using the observed surface crater density (# craters/km2 larger than some threshold crater diameter D). Success, however, requires an understanding of both the cratering physics and absolute impactor flux. The Canada-France Ecliptic Plane Survey (CFEPS) L7 synthetic model of classical and resonant Kuiper belt populations (Petit, J.M. et al. [2011]. Astron. J. 142, 131–155; Gladman, B. et al. [2012]. Astron. J. 144, 23–47) and the scattering object model of Kaib et al. (Kaib, N., Roškar, R., Quinn, T. [2011]. Icarus 215, 491–507) calibrated by Shankman et al. (Shankman, C. et al. [2013]. Astrophys. J. 764, L2–L5) provide such impact fluxes and thus current primary cratering rates for each dynamical sub-population. We find that four sub-populations (the
q < 42 AU

hot and stirred main classicals, the classical outers, and the plutinos) dominate Pluto’s impact flux, each providing
15 25 %

of the total rate. Due to the uncertainty in how the well-characterized size distribution for Kuiper belt objects (with impactor diameter
d > 100 km

) connects to smaller projectiles, we compute cratering rates using five model impactor size distributions: a single power-law, a power-law with a knee, a power-law with a divot, as well as the “wavy” size distributions described in Minton et al. (Minton, D.A. et al. [2012]. Asteroids Comets Meteors Conf. 1667, 6348) and Schlichting et al. (Schlichting, H.E., Fuentes, C.I., Trilling, D.E. [2013]. Astron. J. 146, 36–42). We find that there is only a small chance that Pluto has been hit in the past 4Gyr by even one impactor with a diameter larger than the known break in the projectile size distribution (
d 100 km

) which would create a basin on Pluto (
D 400 km

in diameter). We show that due to present uncertainties in the impactor size distribution between
d = 1 100 km

, computing absolute ages for the surface of Pluto is entirely dependent on the extrapolation to small sizes and thus fraught with uncertainty. We show, however, what the ages would be for several cases and illustrate the relative importance of each Kuiper belt sub-population to the cratering rate, both now and integrated into the past. In addition, we compute the largest “fresh” crater expected to have formed in 1Gyr on the surface of Pluto and in 3Gyr on Charon (to 95% confidence) and use the “wavy” size distribution models to predict whether these largest “fresh” craters will provide surfaces for which portions of the crater production function can be measured should most of the target’s surface appear saturated. The fly-through results coupled with telescopic surveys that bridge current uncertainties in the
d = 10 100 km

regime should eventually result in the population estimate uncertainties for the Kuiper belt sub-populations, and thus the impact fluxes onto Pluto and Charon, dipping to
< 30 %

. We also compute “disruption timescales” (to a factor of three accuracy) for Pluto’s smaller satellites: Styx, Nix, Kerberos, and Hydra. We find that none of the four satellites have likely undergone a catastrophic disruption and reassembly event in the past
4 Gyr

. In addition, we find that for a knee size distribution with
α faint 0.4

(down to sub-km diameters), satellites of all sizes are able to survive catastrophic disruption over the past 4Gyr.





Tidal Love numbers of membrane worlds: Europa, Titan, and Co.

Publication date: 15 September 2015 Source:Icarus, Volume 258

Author(s): Mikael Beuthe

Under tidal forcing, icy satellites with subsurface oceans deform as if the surface were a membrane stretched around a fluid layer. ‘Membrane worlds’ is thus a fitting name for these bodies and membrane theory provides the perfect toolbox to predict tidal effects. I describe here a new membrane approach to tidal perturbations based on the general theory of viscoelastic–gravitational deformations of spherically symmetric bodies. The massive membrane approach leads to explicit formulas for viscoelastic tidal Love numbers which are exact in the limit of zero crust thickness. Formulas for load Love numbers come as a bonus. The accuracy on
k 2

and
h 2

is better than one percent if the crust thickness is less than five percents of the surface radius, which is probably the case for Europa and Titan. The new approach allows for density differences between crust and ocean and correctly includes crust compressibility. This last feature makes it more accurate than the incompressible propagator matrix method. Membrane formulas factorize shallow and deep interior contributions, the latter affecting Love numbers mainly through density stratification. I show that a screening effect explains why ocean stratification typically increases Love numbers instead of reducing them. For Titan, a thin and dense liquid layer at the bottom of a light ocean can raise
k 2

by more than ten percents. The membrane approach can also deal with dynamical tides in a non-rotating body. I show that a dynamical resonance significantly decreases the tilt factor and may thus lead to underestimating Europa’s crust thickness. Finally, the dynamical resonance increases tidal deformations and tidal heating in the crust if the ocean thickness is of the order of a few hundred meters.





Seasonal changes in Saturn’s stratosphere inferred from Cassini/CIRS limb observations

Publication date: 15 September 2015 Source:Icarus, Volume 258

Author(s): M. Sylvestre , S. Guerlet , T. Fouchet , A. Spiga , F.M. Flasar , B. Hesman , G.L. Bjoraker

We present temperature and hydrocarbons abundances (C2H6, C2H2, C3H8) retrieved from Cassini/CIRS limb spectra, acquired during northern spring in 2010 (
L S = 12 °

) and 2012 (
L S = 31 °

). We compare them to the previous limb measurements performed by Guerlet et al. (Guerlet, S. et al. [2009]. Icarus 203, 214–232) during northern winter. The latitudinal coverage (from 79°N to 70°S) and the sensitivity of our observations to a broad range of pressure levels (from 20hPa to 0.003hPa) allow us to probe the meridional and vertical structure of Saturn’s stratosphere during northern spring. Our results show that in the northern hemisphere, the lower stratosphere (1hPa) has experienced the strongest warming from northern winter to spring (
11 ± 0.9 1.1

K), while the southern hemisphere exhibits weak variations of temperature at the same pressure level. We investigate the radiative contribution in the thermal seasonal evolution by comparing these results to the radiative–convective model of Guerlet et al. (Guerlet, S. et al. [2014]. Icarus 238, 110–124). We show that radiative heating and cooling by atmospheric minor constituents is not always sufficient to reproduce the measured variations of temperature (depending on the pressure level). The measurements of the hydrocarbons abundances and their comparison with the predictions of the 1D photochemical model of Moses and Greathouse (Moses, J.I., Greathouse, T.K. [2005]. J. Geophys. Res. (Planets) 110, 9007) give insights into large scale atmospheric dynamics. At 1hPa, C2H6, C2H2, and C3H8 abundances are remarkably constant from northern winter to spring. At the same pressure level, C2H6 and C3H8 exhibit homogeneous meridional distributions unpredicted by this photochemical model, unlike C2H2. This is consistent with the existence of a meridional circulation at 1hPa, as suggested by previous studies.





High S/N Keck and Gemini AO imaging of Uranus during 2012–2014: New cloud patterns, increasing activity, and improved wind measurements

Publication date: 15 September 2015 Source:Icarus, Volume 258

Author(s): L.A. Sromovsky , I. de Pater , P.M. Fry , H.B. Hammel , P. Marcus

We imaged Uranus in the near infrared from 2012 into 2014, using the Keck/NIRC2 camera and Gemini/NIRI camera, both with adaptive optics. We obtained exceptional signal to noise ratios by averaging 8–16 individual exposures in a planet-fixed coordinate system. These noise-reduced images revealed many low-contrast discrete features and large scale cloud patterns not seen before, including scalloped waveforms just south of the equator, and an associated transverse ribbon wave near 6°S. In all three years numerous small (600–700km wide) and mainly bright discrete features were seen within the north polar region (north of about55°N). Two small dark spots with bright companions were seen at middle latitudes. Over 850 wind measurements were made, the vast majority of which were in the northern hemisphere. Winds at high latitudes were measured with great precision, revealing an extended region of solid body rotation between 62°N and at least 83°N, at a rate of 4.08±0.015°/h westward relative to the planet’s interior (radio) rotation of 20.88°/h westward. Near-equatorial speeds measured with high accuracy give different results for waves and small discrete features, with eastward drift rates of 0.4°/h and 0.1°/h respectively. The region of polar solid body rotation is a close match to the region of small-scale polar cloud features, suggesting a dynamical relationship. The winds from prior years and those from 2012–2014 are consistent with a mainly symmetric wind profile up to middle latitudes, with a small asymmetric component of ∼0.09°/h peaking near ±30°, and about 60% greater amplitude if only prior years are included, suggesting a declining mid-latitude asymmetry. While winds at high southern latitudes(50–90°S) are unconstrained by groundbased observations, a recent reanalysis of 1986 Voyager 2 observations by Karkoschka (Karkoschka [2015]. Icarus 250, 294–307) has revealed an extremely large north–south asymmetry in this region, which might be seasonal. Greatly increased activity was seen in 2014, including the brightest ever feature seen in K′ images (de Pater et al. [2015]. Icarus 252, 121–128), as well as other significant features, some of which had long lives. Over the 2012–2014 period we identified six persistent discrete features. Three were tracked for more than 2years, two more for more than 1year, and one for at least 5months and continuing. Several drifted in latitude towards the equator, and others appeared to exhibit latitudinal oscillations with long periods. We found two pairs of long-lived features that survived multiple passages within their own diameters of each other. Zonally averaged cloud patterns were found to persist over 2012–2014. When averaged over longitude, there is a brightness variation with latitude from 55°N to the pole that is similar to effective methane mixing ratio variations with latitude derived from 2012 STIS observations (Sromovsky et al. [2014]. Icarus 238, 137–155).

Graphical abstract

image




Giant-planet chemistry: Ammonium hydrosulfide (NH4SH), its IR spectra and thermal and radiolytic stabilities

Publication date: 15 September 2015 Source:Icarus, Volume 258

Author(s): Mark J. Loeffler , Reggie L. Hudson , Nancy J. Chanover , Amy A. Simon

Here we present our recent studies of proton-irradiated and unirradiated ammonium hydrosulfide, NH4SH, a compound predicted to be an important tropospheric cloud component of Jupiter and other giant planets. We irradiated both crystalline and amorphous NH4SH at 10–160K and used IR spectroscopy to observe and identify reaction products in the ice, specifically NH3 and long-chained sulfur-containing ions. Crystalline NH4SH was amorphized during irradiation at all temperatures studied with the rate being the fastest at the lowest temperatures. Irradiation of amorphous NH4SH at ∼10–75K showed that 60–80% of the NH4 + remained when equilibrium was reached, and that NH4SH destruction rates were relatively constant within this temperature range. Irradiations at higher temperatures produced different dose dependence and were accompanied by pressure outbursts that, in some cases, fractured the ice. The thermal stability of irradiated NH4SH was found to be greater than that of unirradiated NH4SH, suggesting that an irradiated giant-planet cloud precipitate can exist at temperatures and altitudes not previously considered.





Constraints on the depths of origin of peak rings on the Moon from Moon Mineralogy Mapper data

Publication date: 15 September 2015 Source:Icarus, Volume 258

Author(s): David M.H. Baker , James W. Head

Important to understanding the process of basin formation on planetary bodies are constraints on the mineralogy and depths of origin of interior ring structures. We summarize previous analyses of the mineralogy of basin materials on the Moon and use hyperspectral image-cubes from the Chandrayaan-1’s Moon Mineralogy Mapper (M3) to determine the mineralogy of interior rings in lunar protobasins and peak-ring basins. Nearly all peak rings outside of South Pole-Aitken (SPA) basin have extensive outcrops of pure anorthosite (⩾99% plagioclase) on the order of several square kilometers in areal dimensions. No obvious mantle components were identified. Outcrops spectrally dominated by pyroxene occur within SPA and other areas of thinner crust, such as regions within large ancient impact basins. In addition, many outcrops of candidate shocked plagioclase are observed within the same peak rings containing crystalline plagioclase. These spectral observations strongly support a crustal origin for peak rings on the Moon. Recent analyses of the Orientale basin and other lunar basins show that the inner rings of multi-ring basins are also anorthosite-rich and therefore derived from the lunar crust. To further constrain the depths of origin of materials forming peak rings, we compare the pre-impact crustal thickness for each basin with calculated vertical reference points, including: (1) maximum depth of excavation, which is the deepest point at which the crater will excavate material, (2) maximum depth of melting, which is deeper than the maximum depth of excavation and represents the maximum extent of impact-induced melting beneath the sub-impact point, and (3) maximum depth of the transient cavity, which is deepest part of the growing transient cavity that is formed of both excavated and displaced target material. Taken together with the observed mineralogy, the origin of peak-ring lithologies is constrained to stratigraphic levels near the maximum depth of excavation and likely shallower than this if the lower crust is comprised of noritic materials. The maximum depth of melting for peak-ring basins extends far into the mantle and is therefore not a valid proxy for estimating the depth of origin of materials forming peak rings. We find that our estimates of the depths of origin of peak-ring materials are consistent with current models of peak-ring formation, including predictions by hydrocode simulations and conceptual models emphasizing the role of interior impact melting and centro-symmetric collapse of the walls of the transient cavity. Firmer constraints on the depths of origin of peak rings on the Moon await an improved understanding of the crustal compositional structure, particularly that of the lower crust, and improved model predictions on the sampling depths and shock pressures experienced by uplifted peak-ring materials.





Saturn’s upper atmosphere during the Voyager era: Reanalysis and modeling of the UVS occultations

Publication date: 15 September 2015 Source:Icarus, Volume 258

Author(s): Ronald J. Vervack Jr. , Julianne I. Moses

The Voyager 1 and 2 Ultraviolet Spectrometer (UVS) solar and stellar occultation dataset represents one of the primary, pre-Cassini sources of information that we have on the neutral upper atmosphere of Saturn. Despite its importance, however, the full set of occultations has never received a consistent, nor complete, analysis, and the results derived from the initial analyses over thirty years ago left questions about the temperature and density profiles unanswered. We have reanalyzed all six of the UVS occultations (three solar and three stellar) to provide an up-to-date, pre-Cassini view of Saturn’s upper atmosphere. From the Voyager UVS data, we have determined vertical profiles for H2, H, CH4, C2H2, C2H4, and C2H6, as well as temperature. Our analysis also provides explanations for the two different thermospheric temperatures derived in earlier analyses (400–450K versus 800K) and for the unusual shape of the total density profile noted by Hubbard et al. (1997). Aside from inverting the occultation data to retrieve densities and temperatures, we have investigated the atmospheric structure through a series of photochemical models to infer the strength of atmospheric mixing and other physical and chemical properties of Saturn’s mesopause region during the Voyager flybys. We find that the data exhibit considerable variability in the vertical profiles for methane, suggesting variations in vertical winds or the eddy diffusion coefficient as a function of latitude and/or time in Saturn’s upper atmosphere. The results of our reanalysis will provide a useful baseline for interpreting new data from Cassini, particularly in the context of change over the past three decades.





Detection of new olivine-rich locations on Vesta

Publication date: 15 September 2015 Source:Icarus, Volume 258

Author(s): Ernesto Palomba , Andrea Longobardo , Maria Cristina De Sanctis , Angelo Zinzi , Eleonora Ammannito , Simone Marchi , Federico Tosi , Francesca Zambon , Maria Teresa Capria , Christopher T. Russell , Carol A. Raymond , Edward A. Cloutis

The discovery of olivine on Vesta’s surface by the VIR imaging spectrometer onboard the Dawn space mission has forced us to reconsider our views of Vestan petrogenetic models. Olivines were expected to be present in the interior of Vesta: in the mantle of a vertically layered body as invoked by the magma ocean models, or at the base (or within) the mantle–crust boundary as proposed by fractionation models. Olivines have been detected by VIR-Dawn in two wide areas near Arruntia and Bellicia, regions located in the northern hemisphere. Interestingly, these olivine-rich terrains are far from the Rheasilvia and the more ancient Veneneia basins, which are expected to have excavated the crust down to reach the mantle. In this work we present our attempts to identify other undetected olivine rich areas on Vesta by using spectral parameters sensitive to olivine such as the Band Area Ratio (BAR) and other specific parameters created for the detection of olivines on Mars (forsterite, fayalite and a generic olivine index). As a preliminary step we calibrated these parameters by means of VIS–IR spectra of different HED meteorite samples: behaviors versus sample grain size and albedo were analyzed and discussed. We selected the BAR and the Forsterite Index as the best parameters that can be used on Vesta. A cross-correlation analysis has been applied in order to detect olivine signature on the VIR hyperspectral cubes. These detections have then been confirmed by an anti-correlation analysis between the BAR and one of the olivine parameters, independent of the first method applied. In agreement with the recent discovery, Arruntia and Bellicia were found to be as the most olivine-rich areas, i.e. where the parameter values are strongest. In addition we detected 6 new regions, all but one located in the Vesta north hemisphere. This result confirms again that the old petrogenetic models cannot be straightforwardly applied to Vesta and should be reshaped in the view of these new detections. An alternative and very recent option can be represented by the model according to which surface “eruption” of material from the mantle, including olivine can reach the surface of Vesta.





Electron-molecule chemistry and charging processes on organic ices and Titan’s icy aerosol surrogates

Publication date: 15 September 2015 Source:Icarus, Volume 258

Author(s): C. Pirim , R.D. Gann , J.L. McLain , T.M. Orlando

Electron-induced polymerization processes and charging events that can occur within Titan’s atmosphere or on its surface were simulated using electron irradiation and dissociative electron attachment (DEA) studies of nitrogen-containing organic condensates. The DEA studies probe the desorption of H from hydrogen cyanide (HCN), acetonitrile (CH3CN), and aminoacetonitrile (NH2CH2CN) ices, as well as from synthesized tholin materials condensed or deposited onto a graphite substrate maintained at low temperature (90–130K). The peak cross sections for H desorption during low-energy (3–15eV) electron irradiation were measured and range from 3×10 21 to 2×10−18 cm2. Chemical and structural transformations of HCN ice upon 2keV electron irradiation were investigated using X-ray photoelectron and Fourier-transform infrared spectroscopy techniques. The electron-beam processed materials displayed optical properties very similar to tholins produced by conventional discharge methods. Electron and negative ion trapping lead to 1011 chargescm−2 on a flat surface which, assuming a radius of 0.05μm for Titan aerosols, is ∼628charges/radius (in μm). The facile charge trapping indicates that electron interactions with nitriles and complex tholin-like molecules could affect the conductivity of Titan’s atmosphere due to the formation of large negative ion complexes. These negatively charged complexes can also precipitate onto Titan’s surface and possibly contribute to surface reactions and the formation of dunes.