Description: Publication date: April 2015 Source: Icarus, Volume 250 Author(s): C. Mejía , A.L.F. de Barros , E. Seperuelo Duarte , E.F. da Silveira , E. Dartois , A. Domaracka , H. Rothard , P. Boduch Porous water ice and water ice mixtures H 2 O:X (X = CO, CO 2 and CH 4 ) produced at 15 K, with film thicknesses in the 0.5–1 μm range, were irradiated by swift ions and monitored by mid-infrared spectroscopy (FTIR). The analysis of the evolution of the pure water ice infrared absorption on ion beam dose reveals a strong correlation among three quantities: (i) the absorbance of the most intense band (3250 cm −1 ), (ii) the wavelength of the maximum absorbance of this band and (iii) the absorbance of the OH-dangling bonds. This correlation is interpreted as indications of the water ice compaction by irradiation: as the beam fluence increases, the ice porosity decreases, the dangling bond peaks collapse and the area and position of the 3250 cm −1 band vary exponentially, all of them evolving with the same compaction cross section ( σ c ). The linear dependence σ c ∝ S e ( S e being the electronic stopping power) is observed for both pure and mixed water ices, confirming previous results. We suggests that the infrared absorption A -value varies with dose as ( 1 - ζ e - D / D 0 ) during the compaction process ( D 0 = 0.2 eV/molec being the effective energy density to eliminate the OH-db, and ζ is a parameter characterizing the porosity). These findings may be used as a diagnostic tool to probe the morphology of water ices occurring in the outer Solar System and in the ISM.

Description: Publication date: April 2015 Source: Icarus, Volume 250 Author(s): Rosemary M. Killen , Joseph M. Hahn Mercury’s calcium exosphere varies in a periodic way with that planet’s true anomaly. We show that this pattern can be explained by impact vaporization from interplanetary dust with variations being due to Mercury’s radial and vertical excursions through an interplanetary dust disk having an inclination within 5 degrees of the plane of Mercury’s orbit. Both a highly inclined dust disk and a two-disk model (where the two disks have a mutual inclination) fail to reproduce the observed variation in calcium exospheric abundance with Mercury true anomaly angle. However, an additional source of impacting dust beyond the nominal dust disk is required near Mercury’s true anomaly ( ν ) 25° ± 5°. This is close to but not coincident with Mercury’s true anomaly ( ν = 45°) when it crosses Comet 2P/Encke’s present day orbital plane. Interestingly, the Taurid meteor storms at Earth, which are also due to Comet Encke, are observed to occur when Earth’s true anomaly is ±20 or so degrees before and after the position where Earth and Encke orbital planes cross. The lack of exact correspondence with the present day orbit of Encke may indicate the width of the potential stream along Mercury’s orbit or a previous cometary orbit. The extreme energy of the escaping calcium, estimated to have a temperature >50,000 K if the source is thermal, cannot be due to the impact process itself but must be imparted by an additional mechanism such as dissociation of a calcium-bearing molecule or ionization followed by recombination.

Description: Publication date: April 2015 Source: Icarus, Volume 250 Author(s): Patrick L. Harner , Martha S. Gilmore We present visible–near infrared (VNIR, 0.35–5 μm) spectra for a suite of hydrous carbonates that may be relevant to the surface of Mars. This includes VNIR spectra for ikaite, nesquehonite, synthetic monohydrocalcite and lansfordite over the 0.35–2.5 μm range that are new to the literature. The spectral features of the hydrous carbonates are dominated by absorptions at ∼1.0, 1.2, 1.4–1.5, 1.9 and 2.8 μm that are due to overtones and combinations of fundamental water and hydroxyl vibrations. Absorptions due to (CO 3 ) 2 − , Mg-OH, Fe-OH, and/or water are seen at ∼2.3–2.5, 3.4, and 3.9 μm in hydrous Mg and Mg–Fe 3+ carbonates containing hydroxyl groups, but are weaker than in the common anhydrous carbonates. When present in the hydrous carbonates, the positions of the centers of the 2.3 μm and/or 2.5 μm absorptions are often shifted relative to the anhydrous carbonates, which may be diagnostic. Some or all of the (CO 3 ) 2 − absorptions typical of anhydrous carbonates are weak to absent in the hydrous carbonates, and thus this group may be difficult to distinguish from other hydrous minerals like sulfates, phyllosilicates or chlorides in Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data using standard spectral search parameters for anhydrous carbonates. We present strategies for recognizing hydrous carbonates in CRISM data using combinations of spectral parameters that measure the intensity and shape of the water-related absorptions in these minerals.

Description: Publication date: April 2015 Source: Icarus, Volume 250 Author(s): T. Kadono , A.I. Suzuki , K. Wada , N.K. Mitani , S. Yamamoto , M. Arakawa , S. Sugita , J. Haruyama , A.M. Nakamura We performed impact experiments with granular targets to reveal the formation process of crater “rays”, the non-uniform ejecta distributions around some fresh craters on the Moon and planets. We found mesh patterns, loosely woven with spaces like a net, as ejecta. A characteristic length of spaces between meshes was evaluated, and an angle, defined as the ratio of the characteristic length to the distance from the ejection point, was obtained as ∼a few degrees. These features are similar to the results of the analyses of the ray patterns around two lunar craters, Glushko and Kepler. Numerical simulations of granular material showed that clear mesh pattern appeared at lower coefficients of restitution between particles but was less clear at larger one, suggesting that the inelastic collisions between particles cause the clear mesh-pattern formation of impact ejecta.

Description: Publication date: April 2015 Source: Icarus, Volume 250 Author(s): C. Pilorget , J. Fernando , B.L. Ehlmann , S. Douté The amplitude and angular distribution of the light scattered by planetary surfaces give essential information about their physical and compositional properties. In particular, the angular variation of the bidirectional reflectance, characterized through the phase curve, is directly related to the grain size, shape and internal structure. We use a new radiative transfer model that allows specifying the photometric parameters of each grain individually to study the evolution of the phase curve for various kinds of mixtures (spatial, intimate and layered), mimicking different situations encountered for natural surfaces. Results show that the phase curve evolution is driven by the most abundant/brightest/highly anisotropic scattering grains within the mixture. Both spatial and intimate mixtures show similar trends in the phase curves when varying the photometric parameters of the grains. Simple laws have been produced to quantify the evolution of these variations. Layered mixtures have also been investigated and are generally very sensitive to the photometric properties of the top monolayer. Implications for the interpretation of photometric data and their link with the phases identified by spectroscopy are examined. The photometric properties of a few planetary bodies are also discussed over a couple of examples. These different results constitute a new support for the interpretation of orbital/in situ photometric datasets.

Description: Publication date: April 2015 Source: Icarus, Volume 250 Author(s): Timothy A. Goudge , John F. Mustard , James W. Head , Mark R. Salvatore , Sandra M. Wiseman We present morphologic observations and spectral modeling results of a large, kaolin-group mineral-bearing deposit within Kashira crater in the southern highlands of Mars. We employ both non-linear unmixing of Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) visible to near-infrared (VNIR) reflectance data and linear unmixing of Thermal Emission Spectrometer (TES) thermal infrared (TIR) emissivity data to quantitatively analyze the kaolin-group mineral within this deposit. We use a novel approach for quantitative analysis of CRISM data through non-linear unmixing with in-scene, orbitally-derived endmembers combined with laboratory measured endmembers. Results from this approach indicate that the deposit within Kashira crater is best modeled as a combination of surrounding spectral units (i.e., in-scene derived endmembers) with the addition of the kaolin-group mineral halloysite. Linear unmixing of TES data suggest that the deposit contains ∼30% halloysite, a result that is supported by a sensitivity analysis. Potential formation mechanisms for this deposit include hydrothermal alteration, arid-environment pedogenic weathering of a basaltic mound deposit, or pedogenic weathering of a volcanic ash deposit. Our modeling results offer a quantitative reconciliation of the CRISM and TES datasets, and provide a consistent mineralogy from spectral unmixing for an aqueous alteration mineral-bearing deposit on Mars using a combined analysis of both VNIR and TIR hyperspectral data.

Description: Publication date: April 2015 Source: Icarus, Volume 250 Author(s): Dong Wang , Peter J. Gierasch , Jonathan I. Lunine , Olivier Mousis The bulk water abundance on Jupiter potentially constrains the planet’s formation conditions. We improve the chemical constraints on Jupiter’s deep water abundance in this paper. The eddy diffusion coefficient is used to model vertical mixing in planetary atmosphere, and based on laboratory studies dedicated to turbulent rotating convection, we propose a new formulation of the eddy diffusion coefficient for the troposphere of giant planets. The new formulation predicts a smooth transition from the slow rotation regime (near the equator) to the rapid rotation regime (near the pole). We estimate an uncertainty for the newly derived coefficient of less than 25%, which is much better than the one order of magnitude uncertainty used in the literature. We then reevaluate the water constraint provided by CO, using the newer eddy diffusion coefficient. We considered two updated CO kinetic models, one model constrains the water enrichment (relative to solar) between 0.1 and 0.75, while the other constrains the water enrichment between 3 and 11.

Description: Publication date: April 2015 Source: Icarus, Volume 250 Author(s): Leigh N. Fletcher , P.G.J. Irwin , J.A. Sinclair , G.S. Orton , R.S. Giles , J. Hurley , N. Gorius , R.K. Achterberg , B.E. Hesman , G.L. Bjoraker The seasonal evolution of Saturn’s polar atmospheric temperatures and hydrocarbon composition is derived from a decade of Cassini Composite Infrared Spectrometer (CIRS) 7–16 μm thermal infrared spectroscopy. We construct a near-continuous record of atmospheric variability poleward of 60° from northern winter/southern summer (2004, L s = 293 ° ) through the equinox (2009, L s = 0 ° ) to northern spring/southern autumn (2014, L s = 56 ° ). The hot tropospheric polar cyclones that are entrained by prograde jets within 2–3° of each pole, and the hexagonal shape of the north polar belt, are both persistent features throughout the decade of observations. The hexagon vertices rotated westward by ≈30° longitude between March 2007 and April 2013, confirming that they are not stationary in the Voyager-defined System III longitude system as previously thought. Tropospheric temperature contrasts between the cool polar zones (near 80–85°) and warm polar belts (near 75–80°) have varied in both hemispheres, resulting in changes to the vertical windshear on the zonal jets in the upper troposphere and lower stratosphere. The extended region of south polar stratospheric emission has cooled dramatically poleward of the sharp temperature gradient near 75°S (by approximately −5 K/yr), coinciding with a depletion in the abundances of acetylene ( 0.030 ± 0.005 ppm/yr) and ethane ( 0.35 ± 0.1 ppm/yr), and suggestive of stratospheric upwelling with vertical wind speeds of w ≈ + 0.1 mm/s. The upwelling appears most intense within 5° latitude of the south pole. This is mirrored by a general warming of the northern polar stratosphere (+5 K/yr) and an enhancement in acetylene ( 0.030 ± 0.003 ppm/yr) and ethane ( 0.45 ± 0.1 ppm/yr) abundances that appears to be most intense poleward of 75°N, suggesting subsidence at w ≈ - 0.15 mm/s. However, the sharp gradient in stratospheric emission expected to form near 75°N by northern summer solstice (2017, L s = 90 ° ) has not yet been observed, so we continue to await the development of a northern summer stratospheric vortex. The peak stratospheric warming in the north occurs at lower pressure levels ( p 〈 1 mbar) than the peak stratospheric cooling in the south ( p > 1 mbar). Vertical motions are derived from both the temperature field (using the measured rates of temperature change and the deviations from the expectations of radiative equilibrium models) and hydrocarbon distributions (solving the continuity equation). Vertical velocities tend towards zero in the upper troposphere where seasonal temperature contrasts are smaller, except within the tropospheric polar cyclones where w ≈ ± 0.02 mm/s. North polar minima in tropospheric and stratospheric temperatures were detected in 2008–2010 (lagging one season, or 6–8 years, behind winter solstice); south polar maxima appear to have occurred before the start of the Cassini observations (1–2 years after summer solstice), consistent with the expectations of radiative climate models. The influence of dynamics implies that the coldest winter temperatures occur in the 75–80° region in the stratosphere, and in the cool polar zones in the troposphere, rather than at the poles themselves. In addition to vertical motions, we propose that the UV-absorbent polar stratospheric aerosols entrained within Saturn’s vortices contribute significantly to the radiative budget at the poles, adding to the localised enhancement in the south polar cooling and north polar warming poleward of ±75°.

Description: Publication date: April 2015 Source: Icarus, Volume 250 Author(s): Bruce A. Campbell , Donald B. Campbell , Gareth A. Morgan , Lynn M. Carter , Michael C. Nolan , John F. Chandler We combine Earth-based radar maps of Venus from the 1988 and 2012 inferior conjunctions, which had similar viewing geometries. Processing of both datasets with better image focusing and co-registration techniques, and summing over multiple looks, yields maps with 1–2 km spatial resolution and improved signal to noise ratio, especially in the weaker same-sense circular (SC) polarization. The SC maps are unique to Earth-based observations, and offer a different view of surface properties from orbital mapping using same-sense linear (HH or VV) polarization. Highland or tessera terrains on Venus, which may retain a record of crustal differentiation and processes occurring prior to the loss of water, are of great interest for future spacecraft landings. The Earth-based radar images reveal multiple examples of tessera mantling by impact “parabolas” or “haloes”, and can extend mapping of locally thick material from Magellan data by revealing thinner deposits over much larger areas. Of particular interest is an ejecta deposit from Stuart crater that we infer to mantle much of eastern Alpha Regio. Some radar-dark tessera occurrences may indicate sediments that are trapped for longer periods than in the plains. We suggest that such radar information is important for interpretation of orbital infrared data and selection of future tessera landing sites.

Description: Publication date: April 2015 Source: Icarus, Volume 250 Author(s): G. Peterson , F. Nimmo , P. Schenk The surface of Ariel, an icy satellite orbiting Uranus, shows extensional tectonic features suggesting an episode of endogenic heating in the satellite’s past. Using topography derived from stereo-photoclinometry, we identified flexural uplift at a rift zone suggesting elastic thickness values in the range 3.8–4.4 km. We estimate the temperature at the base of the lithosphere to be in the range 99–146 K, depending on the strain rate assumed, with corresponding heat fluxes of 28–92 mW/m 2 . Neither tidal heating, assuming Ariel’s current eccentricity, nor radiogenic heat production from the silicate core are enough to cause the inferred heat fluxes. None of three proposed ancient mean-motion resonances produce equilibrium tidal heating values in excess of 4.3 mW/m 2 . Thus, the origin of the inferred high heat fluxes is currently mysterious.