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Journal Cover Journal of Geophysical Research : Planets
  [123 followers]  Follow
    
   Full-text available via subscription Subscription journal
   ISSN (Online) 2169-9100
   Published by AGU Homepage  [17 journals]
  • Spectral Behavior of Irradiated Sodium Chloride Crystals Under Europa-Like
           Conditions
    • Authors: Michael J. Poston; Robert W. Carlson, Kevin P. Hand
      Abstract: F- and M-color center formation (decay) was observed during (after) irradiation of sodium chloride crystal grains with 10 keV electrons as a function of temperature, radiation dose rate, and radiation dose. The F-centers (peak center: 460 nm) were found to form and decay at a faster rate than the M-centers (peak center: 720 nm). These effects were influenced by temperature, and possibly by irradiation dose rate. Tracking the band depth ratio of the color center features during irradiation could enable age determination of geologically very-young features on the surface of Europa and other icy ocean worlds.
      PubDate: 2017-11-22T13:55:26.064247-05:
      DOI: 10.1002/2017JE005429
       
  • A full-Maxwell approach for large angle polar wander of viscoelastic
           bodies
    • Authors: H. Hu; W. van der Wal, L.L.A. Vermeersen
      Abstract: For large-angle long-term true polar wander (TPW) there are currently two types of non-linear methods which give approximated solutions: those assuming that the rotational axis coincides with the axis of maximum moment of inertia (MoI), e.g. 'Nakada2007_iteration (), which simplifies the Liouville equation and those based on the quasi-fluid approximation (e.g. ('Ricard1993polarwander)) which approximates the Love number. Recent studies show that both can have a significant bias for certain models ('Cambiotti_assumption; 'Hu2017). Therefore, we still lack an (semi-)analytical method which can give exact solutions for large-angle TPW for a model based on Maxwell rheology. This paper provides a method which analytically solves the MoI equation and adopts an extended iterative procedure introduced in 'Hu2017 () to obtain a time-dependent solution. The new method can be used to simulate the effect of a remnant bulge or models in different hydrostatic states. We show the effect of the viscosity of the lithosphere on long-term, large-angle TPW. We also simulate models without hydrostatic equilibrium and show that the choice of the initial stress-free shape for the elastic (or highly-viscous) lithosphere of a given model is as important as its thickness for obtaining a correct TPW behaviour. The initial shape of the lithosphere can be an alternative explanation to mantle convection for the difference between the observed and model predicted flattening. Finally, it is concluded that, based on the quasi-fluid approximation, TPW speed on Earth and Mars is underestimated while the speed of the rotational axis approaching the end position on Venus is overestimated.
      PubDate: 2017-11-22T13:55:22.080198-05:
      DOI: 10.1002/2017JE005365
       
  • Recent basal melting of a mid-latitude glacier on Mars
    • Authors: Frances E. G. Butcher; M. R. Balme, C. Gallagher, N. S. Arnold, S. J. Conway, A. Hagermann, S. R. Lewis
      Abstract: Evidence for past basal melting of young (late Amazonian), debris-covered glaciers in Mars’ mid-latitudes is extremely rare. Thus, it is widely thought that these viscous flow features (VFFs) have been perennially frozen to their beds. We identify an instance of recent, localized wet-based mid-latitude glaciation, evidenced by a candidate esker emerging from a VFF in a tectonic rift in Tempe Terra. Eskers are sedimentary ridges deposited in ice-walled meltwater conduits and are indicative of glacial melting. We compare the candidate esker to terrestrial analogues, present a geomorphic map of landforms in the rift, and develop a landsystem model to explain their formation. We propose that the candidate esker formed during a transient phase of wet-based glaciation. We then consider the similarity between the geologic setting of the new candidate esker and that of the only other candidate esker to be identified in association with an existing mid-latitude VFF; both are within tectonic graben/rifts proximal to volcanic provinces. Finally, we calculate potential basal temperatures for a range of VFF thicknesses, driving stresses, mean annual surface temperatures, and geothermal heat fluxes, which unlike previous studies, include the possible role of internal strain heating. Strain heating can form an important additional heat source, especially in flow convergence zones, or where ice is warmer due to elevated surface temperatures or geothermal heat flux. Elevated geothermal heat flux within rifts, perhaps combined with locally-elevated strain heating, may have permitted wet-based glaciation during the late Amazonian, when cold climates precluded more extensive wet-based glaciation on Mars.
      PubDate: 2017-11-22T10:00:02.726059-05:
      DOI: 10.1002/2017JE005434
       
  • Investigation of Charon's Craters with Abrupt Terminus Ejecta, Comparisons
           with Other Icy Bodies, and Formation Implications
    • Authors: Stuart J. Robbins; Kirby Runyon, Kelsi N. Singer, Veronica J. Bray, Ross A. Beyer, Paul Schenk, William B. McKinnon, William M. Grundy, Francis Nimmo, Jeffrey M. Moore, John R. Spencer, Oliver L. White, Richard P. Binzel, Marc W. Buie, Bonnie J. Buratti, Andrew F. Cheng, Ivan R. Linscott, Harold J. Reitsema, Dennis C. Reuter, Mark R. Showalter, G. Len Tyler, Leslie A. Young, Catherine B. Olkin, Kimberly S. Ennico, Harold A. Weaver, S. Alan Stern
      Abstract: On the moon and other airless bodies, ballistically emplaced ejecta transitions from a thinning, continuous inner deposit to become discontinuous beyond approximately one crater radius from the crater rim and can further break into discrete rays and secondary craters. In contrast, on Mars, ejecta often form continuous, distinct, and sometimes thick deposits that transition to a low ridge or escarpment that may be circular or lobate. The martian ejecta type has been variously termed pancake, rampart, lobate, or layered, and in this work we refer to it as "abrupt termini" ejecta (ATE). Two main formation mechanisms have been proposed, one requiring atmospheric interaction and the other mobilization of near-surface volatiles. ATE morphologies are also unambiguously seen on Ganymede, Europa, Dione, and Tethys, but they are not as common as on Mars. We have identified up to 38 craters on Charon that show signs of ATE, including possible distal ramparts and lobate margins. These ejecta show morphologic and morphometric similarities with other moons in the Solar System which are a sub-set of the properties observed on Mars. From comparison of these ejecta on Charon and other Solar System bodies, we find the strongest support for sub-surface volatile mobilization and ejecta fluidization as the main formation mechanism for the ATE, at least on airless, icy worlds. This conclusion comes from the bodies on which they are found, an apparent preference for certain terrains, and the observation that craters with ATE can be near to similarly sized craters that only have gradational ejecta.
      PubDate: 2017-11-21T15:26:29.629648-05:
      DOI: 10.1002/2017JE005287
       
  • Electrical Investigation of Metal-Olivine Systems and Application to the
           Deep Interior of Mercury
    • Authors: Zhou Zhang; Anne Pommier
      Abstract: We report electrical conductivity measurements on metal–olivine systems at 5 and 7 GPa and up to 1675°C in order to investigate the electrical properties of Core-Mantle Boundary (CMB) systems. Electrical experiments were conducted in the multi-anvil apparatus using the impedance spectroscopy technique. The samples are composed of one metal layer (Fe, FeS, FeSi2, or Fe-Ni-S-Si) and one polycrystalline olivine layer, with the metal:olivine ratio ranging from 1:0.7 to 1:9.2. For all samples, we observe that the bulk electrical conductivity increases with temperature from 10-2.5 to 101.8 S/m, which is higher than the conductivity of polycrystalline olivine but lower than the conductivity of the pure metal phase at similar conditions. In some experiments, a conductivity jump is observed at the temperature corresponding to the melting temperature of the metallic phase. Both the metal:olivine ratio and the metal phase geometry control the electrical conductivity of the two-layer samples. By combining electrical results, textural analyses of the samples, and previous studies of the structure and composition of Mercury's interior, we propose an electrical profile of the deep interior of the planet that accounts for a layered CMB-outer core structure. The electrical model agrees with existing conductivity estimates of Mercury's lower mantle and CMB using magnetic observations and thermodynamic calculations, and thus, supports the hypothesis of a layered CMB-outermost core structure in the present-day interior of Mercury. We propose that the layered CMB-outer core structure is possibly electrically insulating, which may influence the planet's structure and cooling history.
      PubDate: 2017-11-21T15:25:31.195399-05:
      DOI: 10.1002/2017JE005390
       
  • Oxidative alteration of ferrous smectites and implications for the redox
           evolution of early Mars
    • Authors: Steven M. Chemtob; Ryan D. Nickerson, Richard V. Morris, David G. Agresti, Jeffrey G. Catalano
      Abstract: Surface conditions on early Mars were likely anoxic, similar to early Earth, but the timing of the evolution to oxic conditions characteristic of contemporary Mars is unresolved. Ferrous trioctahedral smectites are the thermodynamically predicted products of anoxic basalt weathering, but orbital analyses of Noachian-aged terrains find primarily Fe3+-bearing clay minerals. Rover-based detection of Fe2+-bearing trioctahedral smectites at Gale Crater suggest that ferrous smectites are the unoxidized progenitors of orbitally-detected ferric smectites. To assess this pathway, we conducted ambient-temperature oxidative alteration experiments on four synthetic ferrous smectites having molar Fe/(Mg+Fe) from 1.00 to 0.33. Smectite suspension in air-saturated solutions produced incomplete oxidation (24–38% Fe3+/ΣFe). Additional smectite oxidation occurred upon re-exposure to air-saturated solutions after anoxic hydrothermal recrystallization, which accelerated cation and charge redistribution in the octahedral sheet. Oxidation was accompanied by contraction of the octahedral sheet (d(060) decreased from 1.53-1.56 Å to 1.52 Å), consistent with a shift towards dioctahedral structure. Ferrous smectite oxidation by aqueous hydrogen peroxide solutions resulted in nearly complete Fe2+ oxidation but also led to partial Fe3+ ejection from the structure, producing nanoparticulate hematite. Reflectance spectra of oxidized smectites were characterized by (Fe3+,Mg)2-OH bands at 2.28-2.30 μm, consistent with oxidative formation of dioctahedral nontronite. Accordingly, ferrous smectites are plausible precursors to observed ferric smectites on Mars, and their presence in late-Noachian sedimentary units suggests that anoxic conditions may have persisted on Mars beyond the Noachian.
      PubDate: 2017-11-21T15:25:25.02082-05:0
      DOI: 10.1002/2017JE005331
       
  • Iron abundances in lunar impact basin melt sheets from orbital magnetic
           field data
    • Authors: Joana S. Oliveira; Mark A. Wieczorek, Gunther Kletetschka
      Abstract: Magnetic field data acquired from orbit shows that the Moon possesses many magnetic anomalies. Though most of these are not associated with known geologic structures, some are found within large impact basins within the interior peak ring. The primary magnetic carrier in lunar rocks is metallic iron, but indigenous lunar rocks are metal poor and can not account easily for the observed field strengths. The projectiles that formed the largest impact basins must have contained a significant quantity of metallic iron, and a portion of this iron would have been retained on the Moon's surface within the impact melt sheet. Here, we use orbital magnetic field data to invert for the magnetization within large impact basins using the assumption that the crust is unidirectionally magnetized. We develop a technique based on laboratory thermoremanent magnetization acquisition to quantify the relationship between the strength of the magnetic field at the time the rock cooled and the abundance of metal in the rock. If we assume that the magnetized portion of the impact melt sheet is 1 km thick, we find average abundances of metallic iron ranging from 0.11% to 0.45 wt.%, with an uncertainty of a factor of about three. This abundance is consistent with the metallic iron abundances in sampled lunar impact melts and the abundance of projectile contamination in terrestrial impact melts. These results help constrain the composition of the projectile, the impact process, and the time evolution of the lunar dynamo.
      PubDate: 2017-11-20T14:55:24.792345-05:
      DOI: 10.1002/2017JE005397
       
  • A geophysical perspective on the bulk composition of Mars
    • Authors: A. Khan; C. Liebske, A. Rozel, A. Rivoldini, F. Nimmo, J. A. D. Connolly, A.-C. Plesa, D. Giardini
      Abstract: We invert the Martian tidal response and mean mass and moment of inertia for chemical composition, thermal state, and interior structure. The inversion combines phase equilibrium computations with a laboratory-based viscoelastic dissipation model. The rheological model, which is based on measurements of anhydrous and melt-free olivine, is both temperature and grain size sensitive and imposes strong constraints on interior structure. The bottom of the lithosphere, defined as the location where the conductive geotherm meets the mantle adiabat, occurs deep within the upper mantle (∼250–500 km depth) resulting in apparent upper mantle low-velocity zones. Assuming an Fe-FeS core, our results indicate: 1) a Mantle with a Mg# (molar Mg/Mg+Fe) of ∼0.75 in agreement with earlier geochemical estimates based on analysis of Martian meteorites; 2) absence of bridgmanite- and ferropericlase-dominated basal layer; 3) core compositions (13.5–16 wt% S), core radii (1640–1740 km), and core-mantle-boundary temperatures (1560–1660 ∘ C) that, together with the eutectic-like core compositions, suggest the core is liquid; and 4) bulk Martian compositions that are overall chondritic with a Fe/Si (wt ratio) of 1.63–1.68. We show that the inversion results can be used in tandem with geodynamic simulations to identify plausible geodynamic scenarios and parameters. Specifically, we find that the inversion results are reproduced by stagnant lid convection models for a range of initial viscosities (∼1019–1020 Pa·s) and radioactive element partitioning between crust and mantle around 0.001. The geodynamic models predict a mean surface heat flow between 15–25 mW/m2.
      PubDate: 2017-11-14T12:10:48.85665-05:0
      DOI: 10.1002/2017JE005371
       
  • On the Origins of Mars' Exospheric Non-Thermal Oxygen Component as
           observed by MAVEN and modeled by HELIOSARES
    • Authors: F. Leblanc; J. Y. Chaufray, R. Modolo, L. Leclercq, S. Curry, J. Luhmann, R. Lillis, T. Hara, J. McFadden, J. Halekas, N. Schneider, J. Deighan, P. R. Mahaffy, M. Benna, R. E. Johnson, F. Gonzalez-Galindo, F. Forget, M. A. Lopez-Valverde, F. G. Eparvier, B. Jakosky
      Abstract: The first measurements of the emission brightness of the oxygen atomic exosphere by Mars Atmosphere and Volatile EvolutioN (MAVEN) mission have clearly shown that it is composed of a thermal component produced by the extension of the upper atmosphere and of a non-thermal component. Modeling these measurements allows us to constrain the origins of the exospheric O and, as a consequence, to estimate Mars' present oxygen escape rate. We here propose an analysis of three periods of MAVEN observations based on a set of three coupled models: a hybrid magnetospheric model (LatHyS), an Exospheric General Model (EGM) and the Global Martian Circulation model of the Laboratoire de Météorologie Dynamique (LMD-GCM), which provide a description of Mars' environment from the surface up to the solar wind. The simulated magnetosphere by LatHyS is in good agreement with MAVEN Plasma and Field Package instruments data. The LMD-GCM modeled upper atmospheric profiles for the main neutral and ion species are compared to NGIMS/MAVEN data showing that the LMD-GCM can provide a satisfactory global view of Mars' upper atmosphere. Finally, we were able to reconstruct the expected emission brightness intensity from the oxygen exosphere using EGM. The good agreement with the averaged measured profiles by IUVS during these three periods suggests that Mars' exospheric non-thermal component can be fully explained by the reactions of dissociative recombination of the O2+ ion in Mars' ionosphere, limiting significantly our ability to extract information from MAVEN observations of the O exosphere on other non-thermal processes, such as sputtering.
      PubDate: 2017-11-08T21:41:10.978086-05:
      DOI: 10.1002/2017JE005336
       
  • The Vertical Dust Profile over Gale Crater, Mars
    • Authors: Scott D. Guzewich; C. E. Newman, M. D. Smith, J. E. Moores, C. L. Smith, C. Moore, M. I. Richardson, D. Kass, A. Kleinböhl, M. Mischna, F. J. Martín-Torres, M.-P. Zorzano-Mier, M. Battalio
      Abstract: We create a vertically coarse, but complete, vertical profile of dust mixing ratio from the surface to the upper atmosphere over Gale Crater, Mars, using the frequent joint atmospheric observations of the orbiting Mars Climate Sounder (MCS) and the Mars Science Laboratory (MSL) Curiosity rover. Using these data and an estimate of planetary boundary layer (PBL) depth from the MarsWRF general circulation model, we divide the vertical column into three regions. The first region is the Gale Crater PBL, the second is the MCS-sampled region, and the third is between these first two. We solve for a well-mixed dust mixing ratio within this third (middle) layer of atmosphere to complete the profile.We identify a unique seasonal cycle of dust within each atmospheric layer. Within the Gale PBL, dust mixing ratio maximizes near southern hemisphere summer solstice (Ls = 270°) and minimizes near winter solstice (Ls = 90-100°) with a smooth sinusoidal transition between them. However, the layer above Gale Crater and below the MCS-sampled region more closely follows the global opacity cycle and has a maximum in opacity near Ls = 240° and exhibits a local minimum (associated with the “solsticial pause” in dust storm activity) near Ls = 270°. With knowledge of the complete vertical dust profile, we can also assess the frequency of high-altitude dust layers over Gale. We determine that 36% of MCS profiles near Gale Crater contain an “absolute” high-altitude dust layer wherein the dust mixing ratio is the maximum in the entire vertical column.
      PubDate: 2017-11-08T21:41:07.628694-05:
      DOI: 10.1002/2017JE005420
       
  • Dioctahedral phyllosilicates versus zeolites and carbonates versus
           zeolites competitions as constraints to understanding early Mars
           alteration conditions
    • Authors: Jean-Christophe Viennet; Benjamin Bultel, Lucie Riu, Stephanie C. Werner
      Abstract: Widespread occurrence of Fe,Mg-phyllosilicates have been observed on Noachian Martian terrains. Therefore, the study of Fe,Mg-phyllosilicates formation, in order to characterize early Martian environmental conditions, is of particular interest to the Martian community. Previous studies have shown that the investigation of Fe,Mg-smectite formation alone helps to describe early Mars environmental conditions, but there are still large uncertainties in terms of pH range, oxic/anoxic conditions, etc... Interestingly, carbonates and/or zeolites have also been observed on Noachian surfaces in association with the Fe,Mg-phyllosilicates.Consequently, the present study focuses on the di/trioctahedral phyllosilicate/carbonate/zeolite formation as a function of various CO2 contents (100% N2, 10% CO2 / 90% N2, 100% CO2), from a combined approach including closed system laboratory experiments for 3 weeks at 120°C and geochemical simulations. The experimental results show that as the CO2 content decreases, the amount of dioctahedral clay minerals decreases in favour of trioctahedral minerals. Carbonates and dioctahedral clay minerals are formed during the experiments with CO2. When Ca-zeolites are formed, no carbonates and dioctahedral minerals are observed. Geochemical simulation aided in establishing pH as a key parameter in determining mineral formation patterns. Indeed, under acidic conditions dioctahedral clay minerals and carbonate minerals are formed, while trioctahedral clay minerals are formed in basic conditions with a neutral pH value of 5.98 at 120°C. Zeolites are favoured from pH>~7.2. The results obtained shed new light on the importance of dioctahedral clay minerals versus zeolites and carbonates versus zeolites competitions, to better define the aqueous alteration processes throughout early Mars history.
      PubDate: 2017-11-06T21:35:37.173521-05:
      DOI: 10.1002/2017JE005343
       
  • Comment on “Radiative transfer in CO2-rich atmospheres: 1. Collisional
           line mixing implies a colder early Mars”
    • Authors: M. Turbet; H. Tran
      Abstract: Ozak et al. [2016] claimed that explicitly including the effect of CO2 collisional line mixing (LM) in their radiative transfer calculations yield CO2 atmospheres that are more transparent to infrared radiation than when spectra calculations are made using sub-Lorentzian line shapes. This would in particular imply significantly colder surface temperatures (up to 15 K) for early Mars than estimated in previous studies. Here we show that the relative cooling that Ozak et al. [2016] associated to the effect of collisional line mixing is in fact due to a wrong choice of broadening species (air instead of CO2). We then calculated Line-by-Line spectra of pure CO2 atmospheres using a line-mixing model developed for self-broadened CO2. Using the LMD Generic model (in 1D radiative-convective mode), we find that calculations made with the proper collisional line mixing model and with sub-Lorentzian line shapes lead to differences between early Mars surface temperatures smaller than 2 Kelvins only.
      PubDate: 2017-11-06T15:15:23.549042-05:
      DOI: 10.1002/2017JE005373
       
  • Response to comment on “Radiative transfer in CO2-rich atmospheres:
           1.Collisional line mixing implies a colder early Mars”
    • Authors: N. Ozak; Oded Aharonson, Itay Halevy Halevy
      Abstract: Turbet and Tran [2017] have identified that we mistakenly used air-broadened CO2 absorption spectra to generate coefficients for a rapid radiative transfer code, instead of self-broadened spectra. The resulting underestimation of absorption by CO2 led us to suggest that when the effects of CO2 line mixing are taken into account, surface temperatures on early Mars are up to 15 K colder than those calculated in previous studies, in which empirical corrections of the CO2 spectrum were used (χ-factors). Using the correct CO2 absorption spectra, including the effects of line mixing, Turbet and Tran [2017] found that the surface temperature is colder by only 2 K than temperatures calculated with the χ-factor approach. While we acknowledge the mistake, and thank Turbet and Tran [2017] for finding and correcting it, we note that our main conclusions hold, that surface temperatures are overestimated when empirical χ-factors are used instead of a full account of CO2 line mixing in radiative transfer calculations in CO2-rich planetary atmospheres. Additionally, we emphasize that the rapid radiative transfer model we developed and tested is useful for studies of planetary climate, provided the correct absorption spectra are used to generate new k-coefficients.
      PubDate: 2017-11-06T15:15:20.721266-05:
      DOI: 10.1002/2017JE005389
       
  • Direct Measurement of Interparticle Forces of Titan Aerosol Analogs
           (‘Tholin’) Using Atomic Force Microscopy
    • Authors: Xinting Yu; Sarah M. Hörst, Chao He, Patricia McGuiggan, Nathan T. Bridges
      Abstract: To understand the origin of the dunes on Titan, it is important to investigate the material properties of Titan's organic sand particles on Titan. The organic sand may behave distinctively compared to the quartz/basaltic sand on terrestrial planets (Earth, Venus, Mars) due to differences in interparticle forces. We measured the surface energy (through contact angle measurements) and elastic modulus (through Atomic Force Microscopy, AFM) of the Titan aerosol analog (tholin). We find the surface energy of a tholin thin film is about 70.9 mN/m and its elastic modulus is about 3.0 GPa (similar to hard polymers like PMMA and polystyrene). For two 20 μm diameter particles, the theoretical cohesion force is therefore 3.3 μN. We directly measured interparticle forces for relevant materials: tholin particles are 0.8±0.6 μN, while the interparticle cohesion between walnut shell particles (a typical model materials for the Titan Wind Tunnel, TWT) is only 0.4±0.1 μN. The interparticle cohesion forces are much larger for tholins and presumably Titan sand particles than materials used in the TWT. This suggests we should increase the interparticle force in both analog experiments (TWT) and threshold models to correctly translate the results to real Titan conditions. The strong cohesion of tholins may also inform us how the small aerosol particles (∼1 μm) in Titan's atmosphere are transformed into large sand particles (∼200 μm). It may also support the cohesive sand formation mechanism suggested by Rubin and Hesp (2009), where only unidirectional wind is needed to form linear dunes on Titan.
      PubDate: 2017-11-02T18:05:26.241812-05:
      DOI: 10.1002/2017JE005437
       
  • The geologic exploration of the Bagnold Dune Field at Gale crater by the
           Curiosity Rover
    • Authors: Matthew Chojnacki; Lori K. Fenton
      Abstract: The Mars Science Laboratory rover Curiosity engaged in a month-long campaign investigating the Bagnold dune field in Gale crater. What represents the first in situ investigation of a dune field on another planet has resulted in a number of discoveries. Collectively, the Curiosity rover team has compiled the most comprehensive survey of any extra-terrestrial aeolian system visited to date with results that yield important insights into a number of processes, including sediment transport, bedform morphology and structure, chemical and physical composition of aeolian sand, and wind regime characteristics. These findings and more are provided in detail by the JGR-Planets Special Issue Curiosity's Bagnold Dunes Campaign, Phase I.
      PubDate: 2017-11-01T17:45:46.799409-05:
      DOI: 10.1002/2017JE005455
       
  • Global regolith thermophysical properties of the Moon from the Diviner
           Lunar Radiometer Experiment
    • Authors: Paul O. Hayne; Joshua L. Bandfield, Matthew A. Siegler, Ashwin R. Vasavada, Rebecca R. Ghent, Jean-Pierre Williams, Benjamin T. Greenhagen, Oded Aharonson, Catherine M. Elder, Paul G. Lucey, David A. Paige
      Abstract: We used infrared data from the Lunar Reconnaissance Orbiter (LRO) Diviner Lunar Radiometer Experiment to globally map thermophysical properties of the Moon's regolith fines layer. Thermal conductivity varies from 7.4×10-4 W m-1 K-1 at the surface, to 3.4×10-3 W m-1 K-1 at depths of ~1 m, given density values of 1100 kg m-3 at the surface, to 1800 kg m-3 at 1-m depth. On average, the scale height of these profiles is ~7 cm, corresponding to a thermal inertia of 55 ±2 J m-2 K-1 s-1/2 at 273 K, relevant to the diurnally active near-surface layer, ~4-7 cm. The temperature-dependence of thermal conductivity and heat capacity leads to a ~2x diurnal variation in thermal inertia at the equator. On global scales, the regolith fines are remarkably uniform, implying rapid homogenization by impact gardening of this layer on timescales < 1 Gyr. Regional and local scale variations show prominent impact features < 1 Gyr old, including higher thermal inertia (> 100 J m-2 K-1 s-1/2) in the interiors and ejecta of Copernican-aged impact craters, and lower thermal inertia (< 50 J m-2 K-1 s-1/2) within the lunar cold spots identified by Bandfield et al. (2014). Observed trends in ejecta thermal inertia provide a potential tool for age-dating craters of previously unknown age, complementary to the approach suggested by Ghent et al. (2014). Several anomalous regions are identified in the global 128 pixels-per-degree maps presented here, including a high-thermal inertia deposit near the antipode of Tycho crater.
      PubDate: 2017-11-01T17:40:32.781559-05:
      DOI: 10.1002/2017JE005387
       
  • Winds in the middle cloud deck from the near-IR imaging by the Venus
           Monitoring Camera onboard Venus Express
    • Authors: I. V. Khatuntsev; M. V. Patsaeva, D. V. Titov, N. I. Ignatiev, A. V. Turin, A. A. Fedorova, W. J. Markiewicz
      Abstract: For more than eight years the Venus Monitoring Camera (VMC) on-board the Venus Express orbiter performed continuous imaging of the Venus cloud layer in UV, visible and near-IR filters. We applied the correlation approach to sequences of the near-IR images at 965 nm to track cloud features and determine the wind field in the middle and lower cloud (49-57km). From the VMC images that spanned from December of 2006 through August of 2013 we derived zonal and meridional components of the wind field. In low-to-middle latitudes (5-65°S) the velocity of the retrograde zonal wind was found to be 68-70 m/s. The meridional wind velocity slowly decreases from peak value of +5.8±1.2 m/s at 15°S to 0 at 65-70°S. The mean meridional speed has a positive sign at 5-65°S suggesting equatorward flow. This result, together with the earlier measurements of the poleward flow at the cloud tops indicate the presence of a closed Hadley cell in the altitude range 55-65km. Long term variations of zonal and meridional velocity components were found during 1200 earth days of observation. At 20°±5°S the zonal wind speed increases from -67.18±1.81 m/s to -77.30±2.49 m/s. The meridional wind gradually increases from +1.30±1.82 m/s to +8.53±2.14 m/s. Following Bertaux et al. [2016] we attribute this long-term trend to the influence from the surface topography on the dynamical process in the atmosphere via the upward propagation of gravity waves that became apparent in the VMC observations due to slow drift of the Venus Express orbit over Aphrodite Terra.
      PubDate: 2017-10-23T21:11:51.08643-05:0
      DOI: 10.1002/2017JE005355
       
  • Constraints on Ceres' internal structure and evolution from its shape and
           gravity measured by the Dawn spacecraft
    • Authors: A. I. Ermakov; R. R. Fu, J. C. Castillo-Rogez, C. A. Raymond, R. S. Park, F. Preusker, C. T. Russell, D. E. Smith, M. T. Zuber
      Abstract: Ceres is the largest body in the asteroid belt with a radius of approximately 470 km. In part due to its large mass, Ceres more closely approaches hydrostatic equilibrium than major asteroids. Pre-Dawn mission shape observations of Ceres revealed a shape consistent with a hydrostatic ellipsoid of revolution. The Dawn spacecraft Framing Camera has been imaging Ceres since March 2015, which has led to high-resolution shape models of the dwarf planet, while the gravity field has been globally determined to a spherical harmonic degree 14 (equivalent to a spatial wavelength of 211 km) and locally to 18 (a wavelength of 164 km). We use these shape and gravity models to constrain Ceres' internal structure. We find a negative correlation and admittance between topography and gravity at degree 2 and order 2. Low admittances between spherical harmonic degrees 3 and 16 are well explained by Airy isostatic compensation mechanism. Different models of isostasy give crustal densities between 1200 and 1400 kg/m3 with our preferred model giving a crustal density of 1287+70−87kg/m3. The mantle density is constrained to be 2434+5−8kg/m3. We compute isostatic gravity anomaly and find evidence for mascon-like structures in the two biggest basins. The topographic power spectrum of Ceres and its latitude dependence suggest that viscous relaxation occurred at the long wavelengths (>246 km). Our density constraints combined with finite element modeling of viscous relaxation suggests that the rheology and density of the shallow surface are most consistent with a rock, ice, salt and clathrate mixture.
      PubDate: 2017-10-18T09:00:02.350807-05:
      DOI: 10.1002/2017JE005302
       
  • Seasonal Slumps in Juventae Chasma, Mars
    • Authors: Lujendra Ojha; Matt Chojnacki, George D. McDonald, Andrew Shumway, Michael J. Wolff, Michael D. Smith, Alfred S. McEwen, Ken Ferrier, Christian Huber, James J. Wray, Anthony Toigo
      Abstract: Dark topographic slumps several meters wide, tens of meters in length and up to a meter in depth are observed on the slopes of Juventae Chasma (JC), Valles Marineris (VM), Mars. These slumps usually originate near the terminal points of recurring slope lineae (RSL). Near their initiation points, the slumps have topographic depressions due to the removal of materials; near their lowermost reaches, new materials are deposited in lobes. Over the course of three Mars years, ten active slumps have been observed in JC, all of which formed in or near the same season (areocentric longitude: Ls 0°–120°). Mars Color Imager (MARCI) observations show low-altitude atmospheric obscurations confined within the topography of the VM and JC in the seasons when the slumps form. In one instance, data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and MARCI show evidence of H2O ice in the atmospheric obscuration, likely due to the formation of a low-level afternoon cloud above a dust storm, or mixing of condensate clouds with a diffuse dust cloud. The presence of atmospheric obscurations with H2O ice near times when the slumps form is intriguing, but no direct evidence currently exists to support that they aid in slump formation. Further monitoring of this site will help establish if RSL and/or atmospheric events play a role in the creation of contemporary slumps.
      PubDate: 2017-10-12T18:22:24.927073-05:
      DOI: 10.1002/2017JE005375
       
  • Preservation of Mid-Latitude Ice Sheets on Mars
    • Authors: A. M. Bramson; S. Byrne, J. Bapst
      Abstract: Excess ice with a minimum age of tens of millions of years is widespread in Arcadia Planitia on Mars, and a similar deposit has been found in Utopia Planitia. The conditions that led to the formation and preservation of these mid-latitude ice sheets hold clues to past climate and subsurface structure on Mars. We simulate the thermal stability and retreat of buried excess ice sheets over 21 Myr of Martian orbital solutions and find that the ice sheets can be orders of magnitude older than the obliquity cycles that are typically thought to drive mid-latitude ice deposition and sublimation. Retreat of this ice in the last 4 Myr could have contributed ~6% of the volume of the North Polar Layered Deposits (NPLD) and more than 10% if the NPLD are older than 4 Myr. Matching the measured dielectric constants of the Arcadia and Utopia Planitia deposits requires ice porosities of ~25–35%. We model geothermally driven vapor migration through porous ice under Martian temperatures and find that Martian firn may be able to maintain porosity for timescales longer than we predict for retreat of the ice.
      PubDate: 2017-10-11T15:02:21.538299-05:
      DOI: 10.1002/2017JE005357
       
  • The Total Electron Content of the Martian Ionosphere from MRO/SHARAD
           observations
    • Authors: Michael Mendillo; Clara Narvaez, Bruce Campbell
      Abstract: The Shallow Radar (SHARAD) on the Mars Reconnaissance Orbiter (MRO), used to search for sub-surface water ice, requires corrections to range delays and image distortions caused by radiowave propagation through the martian ionosphere. These corrections yield a phase parameter that is linearly correlated with values of the total electron content (TEC), defined as the integral of the electron density profile Ne(h). This new database has been validated using previous observational patterns of TEC magnitudes and variability versus latitude, local time and season. The SHARAD TEC data span the years 2007 to 2014, providing the first opportunity to study solar cycle effects upon daytime TEC magnitudes. A parameterization of TEC versus solar zenith angle and solar flux, within the context of photo-chemical-equilibrium theory, provides a flexible TEC module for the Mars Initial Reference Ionosphere (MIRI) model. A high spatial resolution study of TEC variability in the southern hemisphere confirmed a previously tentative conclusion about daytime TEC morphology controlled by the local inclination angles of crustal magnetic fields.
      PubDate: 2017-10-10T15:47:12.802643-05:
      DOI: 10.1002/2017JE005391
       
  • The Evolution of Juventae Chasma, Valles Marineris, Mars: Progressive
           collapse and sedimentation
    • Authors: F. Fueten; N. Novakovic, R. Stesky, J. Flahaut, E. Hauber, A. P. Rossi
      Abstract: Juventae Chasma is an isolated chasm located north of the interconnected chasms within Valles Marineris. It contains four separate interior layered deposit (ILD) mounds. We have combined layer measurements, as well as mineralogical data of the mounds with topographic data of the remaining chasm and the surrounding plateau. Our observations indicate that the chasm underwent a complex geological history. We suggest that individual ILD mounds are not contemporaneous with each other and a series of progressive collapses enlarged an initially smaller chasm to its current shape. ILDs were deposited in a lacustrine setting and the time of ILD deposition was limited. Basin collapse and chasm enlargement continued beyond the time of ILD deposition and thus were not driven by sediment load. Glacial processes appear to have played a role in the late, post-lacustrine history of the chasm. We present a simplified model that highlights some of the complexity of the chasm evolution, but acknowledge that the true history is undoubtedly more complicated.
      PubDate: 2017-10-10T15:42:13.082406-05:
      DOI: 10.1002/2017JE005334
       
  • Constraining the date of the Martian dynamo shutdown by means of crater
           magnetization signatures
    • Authors: Foteini Vervelidou; Vincent Lesur, Matthias Grott, Achim Morschhauser, Robert J. Lillis
      Abstract: Mars is believed to have possessed a dynamo that ceased operating approximately 4 Ga ago, although the exact time is still under debate. The scope of this study is to constrain the possible timing of its cessation by studying the magnetization signatures of craters. The study uses the latest available model of the lithospheric magnetic field of Mars, which is based on Mars Global Surveyor data. We tackle the problem of non-uniqueness that characterises the inversion of magnetic field data for the magnetization by inferring only the visible part of the magnetization, i.e., the part of the magnetization that gives rise to the observed magnetic field. Further on, we demonstrate that a zero visible magnetization is a valid proxy for the entire magnetization being zero under the assumption of a magnetization distribution of induced geometry. This assumption holds for craters whose thermoremanent magnetization has not been significantly altered since its acquisition. Our results show that the dynamo shut off after the impacts that created the Acidalia and SE Elysium basins and before the crust within the Utopia basin cooled below its magnetic blocking temperature. Accounting for the age uncertainties in the dating of these craters, we estimate that the dynamo shut off at an N(300) crater retention age of 2.5-3.2 or an absolute model age of 4.12 - 4.14 Ga. Moreover, the Martian dynamo may have been weaker in its early stage, which if true implies that the driving mechanism of the Martian dynamo was not the same throughout its history.
      PubDate: 2017-10-09T18:21:11.490147-05:
      DOI: 10.1002/2017JE005410
       
  • Evidence for a Localised Source of the Argon in the Lunar Exosphere
    • Authors: Jacob A. Kegerreis; Vincent R. Eke, Richard J. Massey, Simon K. Beaumont, Rick C. Elphic, Luís F. Teodoro
      Abstract: We perform the first tests of various proposed explanations for observed features of the Moon's argon exosphere, including models of: spatially varying surface interactions; a source that reflects the lunar near-surface potassium distribution; and temporally varying cold trap areas. Measurements from the Lunar Atmosphere and Dust Environment Explorer (LADEE) and the Lunar Atmosphere Composition Experiment (LACE) are used to test whether these models can reproduce the data. The spatially varying surface interactions hypothesized in previous work cannot reproduce the persistent argon enhancement observed over the western maria. They also fail to match the observed local time of the near-sunrise peak in argon density, which is the same for the highland and mare regions, and is well reproduced by simple surface interactions with a ubiquitous desorption energy of 28 kJ mol−1. A localised source can explain the observations, with a trade-off between an unexpectedly localised source or an unexpectedly brief lifetime of argon atoms in the exosphere. To match the observations, a point-like source requires source and loss rates of ∼1.9 × 1021 atoms s−1. A more diffuse source, weighted by the near-surface potassium, requires much higher rates of ∼1.1 × 1022 atoms s−1, corresponding to a mean lifetime of just 1.4 lunar days. We do not address the mechanism for producing a localised source, but demonstrate that this appears to be the only model that can reproduce the observations. Large, seasonally varying cold traps could explain the long-term fluctuation in the global argon density observed by LADEE, but not that by LACE.
      PubDate: 2017-10-02T17:21:34.537785-05:
      DOI: 10.1002/2017JE005352
       
  • A low O/Si ratio on the surface of Mercury: Evidence for silicon
           smelting'
    • Authors: Francis M. McCubbin; Kathleen E. Vander Kaaden, Patrick N. Peplowski, Aaron S. Bell, Larry R. Nittler, Jeremy W. Boyce, Larry G. Evans, Lindsay P. Keller, Stephen M. Elardo, Timothy J. McCoy
      Abstract: Data from the Gamma-Ray Spectrometer (GRS) that flew on the MESSENGER spacecraft indicate that the O/Si weight ratio of Mercury's surface is 1.2 ± 0.1. This value is lower than any other celestial surface that has been measured by GRS and suggests that 12–20% of the surface materials on Mercury are composed of Si-rich, Si-Fe alloys. The origin of the metal is best explained by a combination of space weathering and graphite-induced smelting. The smelting process would have been facilitated by interaction of graphite with boninitic and komatiitic parental liquids. Graphite entrained at depth would have reacted with FeO components dissolved in silicate melt, resulting in the production of up to 0.4–0.9 wt.% CO from the reduction of FeO to Fe0—CO production that could have facilitated explosive volcanic processes on Mercury. Once the graphite-entrained magmas erupted, the tenuous atmosphere on Mercury prevented the buildup of CO over the lavas. The partial pressure of CO would have been sufficiently low to facilitate reaction between graphite and SiO2 components in silicate melts to produce CO and metallic Si. Although exotic, Si-rich metal as a primary smelting product is hypothesized on Mercury for three primary reasons: (1) low FeO abundances of parental magmas, (2) elevated abundances of graphite in the crust and regolith, and (3) the presence of only a tenuous atmosphere at the surface of the planet within the 3.5–4.1 Ga timespan over which the planet was resurfaced through volcanic processes.
      PubDate: 2017-09-29T18:25:28.695676-05:
      DOI: 10.1002/2017JE005367
       
  • The Mars Science Laboratory (MSL) Bagnold Dunes campaign, Phase I:
           Overview and introduction to the special issue
    • Authors: Nathan T. Bridges; Bethany L. Ehlmann
      Abstract: The Bagnold dunes in Gale Crater, Mars are the first active aeolian dune field explored in situ on another planet. The Curiosity rover visited the Bagnold dune field to understand modern winds and modern aeolian processes, rates, and structures; to constrain dune material composition, provenance, and the extent and type of compositional sorting of materials; and to collect knowledge that informs the interpretation of past aeolian processes that are preserved in the Martian sedimentary rock record. The Curiosity rover conducted a coordinated campaign of activities lasting four months, interspersed with other rover activities, and employing all of the rover's science instruments and several engineering capabilities. Described in thirteen manuscripts and summarized here, the major findings of the Bagnold Dunes Campaign, phase I include: the characterization of and explanation for a distinctive, meter-scale size of sinuous aeolian bedform formed in the high kinetic viscosity regime of Mars’ thin atmosphere; articulation and evaluation of a grain splash model that successfully explains the occurrence of saltation even at wind speeds below the fluid threshold; determination of the dune sands’ basaltic mineralogy and crystal chemistry in comparison with other soils and sedimentary rocks; and characterization of chemically distinctive volatile reservoirs in sand-sized versus dust-sized fractions of Mars soil, including the importance of amorphous phase(s) as volatile carriers.
      PubDate: 2017-09-18T18:26:38.359278-05:
      DOI: 10.1002/2017JE005401
       
  • Investigation of Lunar Spinels at Sinus Aestuum
    • Authors: Catherine M. Weitz; Matthew I. Staid, Lisa R. Gaddis, Sebastian Besse, Jessica M. Sunshine
      Abstract: Sinus Aestuum is the only known location on the Moon where orbital data has detected Fe- and/or Cr-spinel. We analyzed Moon Mineralogy Mapper (M3) visible to near-infrared data of the largest and strongest spinel signatures and determined these locations always correspond to impact craters. M3 spectra show at least three types of spinels may be present, all of which exhibit a strong and broad absorption at ~2100 nm, and also one of the following: (1) a narrow 700-750 nm absorption; (2) a broad 600-900 nm absorption; or (3) both a weaker 700 nm and stronger 1000 nm absorption. All the spinel detections occur on either larger highland massifs that make up Sinus Aestuum east and west, or smaller highland kīpukas and buried highlands within the mare. Almost all of the spinel signatures occur within the mapped pyroclastic dark mantle deposit (DMD). The strong correlation between spinel and DMD distribution on the highlands at Sinus Aestuum is best explained if the spinels were emplaced during the same explosive eruption(s) that deposited the pyroclastics in the Sinus Aestuum DMD. Our observations are most consistent with models of melt-rock reactions in the anorthositic lunar crust that produce contaminated (high-Al) regions within a volcanic dike or magmatic reservoir that was capable of erupting pyroclastic glass beads containing pleonaste spinel [Mg,Fe]Al2O4. Over billions of years, this surface layer of spinels and pyroclastics became heterogeneously mixed into and partially buried within the highland regolith where younger impact craters may sometimes expose it.
      PubDate: 2017-09-18T18:26:35.199458-05:
      DOI: 10.1002/2017JE005309
       
  • New insights into lithology distribution across the Moon
    • Authors: Xianmin Wang; Siyuan Zhao
      Abstract: Lithology distribution across the Moon is pivotal for understanding lunar evolution. However, so far, the distribution of lunar rock suites is still uncertain; as a result, many related core issues on lunar evolution have long been in dispute. This work reports on a new lithology distribution map across the Moon and discusses some critical issues of the lunar evolutionary process. The oxide abundances derived from Chang’E-1 Interference Imaging Spectrometer imagery and the Th contents inferred by Lunar Prospector Gamma Ray and Neutron Spectrometer data are employed to generate the lithological map by using the decision tree C5.0 algorithm. The following conclusions are inferred from this new map. (1) Magnesian suite is widely distributed across the Feldspathic Highlands Terrane and in the periphery of the South Pole-Aitken Terrane. Thus, the viewpoints have been validated that the early magnesian magmatism may be a global phenomenon and that KREEP basalt is not necessary for the petrogenesis of Mg-rich rocks. Moreover, the regions of Dryden, Chaffee S, Theophilus and Moscoviense are confirmed as magnesian suite exposures. (2) The observation that the Feldspathic Highlands Terrane has a higher Mg/(Mg+Fe) value than the maria is related to the flood of the Mg-suite across the Feldspathic Highlands Terrane. (3) The specific exposed locations of the alkali suite across the Moon has long been unsolved, and this work discloses that the alkali suite prevails in the outskirts of the Procellarum KREEP Terrane, the center of the South Pole-Aitken Terrane and some isolated locales. (4) Focusing on distinguishing mare basalts from other mafic rocks, 7 geochemical indices are proposed to determine the potential exposures of mare basalts across the Moon. (5) In Mare Insularum, a series of KREEP volcanisms may have occurred and lasted longer than the mare volcanism.
      PubDate: 2017-09-18T18:25:38.116609-05:
      DOI: 10.1002/2017JE005394
       
  • Conditions for Sublimating Water Ice to Supply Ceres' Exosphere
    • Authors: M. E. Landis; S. Byrne, N. Schörghofer, B. E. Schmidt, P. O. Hayne, J. Castillo-Rogez, M. V. Sykes, J.-P. Combe, A. I. Ermakov, T. H. Prettyman, C. Raymond, C. T. Russell
      Abstract: Observations of a water vapor exosphere around Ceres suggest the dwarf planet may be episodically outgassing at a rate of ~6 kg s-1 from unknown sources. With data from the Dawn mission as constraints, we use a coupled thermal and vapor diffusion model to explore three different configurations of water ice (global buried pore-filling ice, global buried excess ice, and local exposed surface ice) that could be present on Ceres. We conclude that a buried ice table cannot alone explain the vapor production rates previously measured, but newly exposed surface ice, given the right conditions, can exceed that vapor production rate. Sublimation lag deposits form that bury and darken this surface ice over a large range of timescales (from
      PubDate: 2017-09-05T18:46:01.672493-05:
      DOI: 10.1002/2017JE005335
       
  • Water Ice Radiolytic O2, H2 and H2O2 Yields for any Projectile Species,
           Energy or Temperature: A Model for Icy Astrophysical Bodies
    • Authors: B. D. Teolis; C. Plainaki, T. A. Cassidy, U. Raut
      Abstract: O2, H2 and H2O2 radiolysis from water ice is pervasive on icy astrophysical bodies, but the lack of a self-consistent, quantitative model of the yields of these water products versus irradiation projectile species and energy has been an obstacle to estimating the radiolytic oxidant sources to the surfaces and exospheres of these objects. A major challenge is the wide variation of O2 radiolysis yields between laboratory experiments, ranging over four orders of magnitude from 5×10-7 to 5×10-3 molecules per eV for different particles and energies. We revisit decades of laboratory data to solve this long standing puzzle, finding an inverse projectile range dependence in the O2 yields, due to preferential O2 formation from an ~30 Å thick oxygenated surface layer. Highly penetrating projectile ions and electrons with ranges ≳30 Å are therefore less efficient at producing O2 than slow/heavy ions and low-energy electrons (≲ 400 eV) which deposit most energy near the surface. Unlike O2 the H2O2 yields from penetrating projectiles fall within a comparatively narrow range of (0.1-6)×10-3 molecules per eV, and do not depend on range, suggesting H2O2 forms deep in the ice uniformly along the projectile track, e.g. by reactions of OH radicals. We develop an analytical model for O2, H2 and H2O2 yields from pure water ice for electrons and singly-charged ions of any mass and energy, and apply the model to estimate possible O2 source rates on several icy satellites. The yields are upper limits for icy bodies on which surface impurities may be present.
      PubDate: 2017-09-05T18:45:53.90552-05:0
      DOI: 10.1002/2017JE005285
       
  • Impact characteristics of different rocks in a pulsed-laser irradiation
           experiment: Simulation of micrometeorite bombardment on the Moon
    • Authors: Yanxue Wu; Xiongyao Li, Wenqing Yao, Shijie Wang
      Abstract: Without the protection of the atmosphere, the soils on lunar surfaces undergo a series of optical, physical, and chemical changes during micrometeorite bombardment. To simulate the micrometeorite-bombardment process and analyze the impact characteristics, four types of rocks, including terrestrial basalt and anorthosite supposed to represent lunar rock, an H-type chondrite (the Huaxi ordinary chondrite) and an iron meteorite (the Gebel Kamil iron meteorite) supposed to represent micrometeorite impactors, are irradiated by a nanosecond-pulse laser in a high vacuum chamber. Based on laser irradiation experiments, the laser pits are found to be of different shapes and sizes which vary with the rock type. Many melt and vapor deposits are found on the mineral surfaces of all the samples, and nanophase iron (npFe) or Fe-Ni alloy particles are typically distributed on the surfaces of ilmenite, kamacite or other minerals near kamacite. By analyzing the focused ion beam (FIB) ultrathin slices of laser pits with a transmission electron microscope (TEM), the results show that the subsurface structures can be divided into three classes and that npFe can be easily found in Fe-bearing minerals. These differences in impact characteristics will help determine the source material of npFe and infer the type of micrometeorite impactors. During micrometeorite bombardment, in the mare regions, the npFe are probably produced simultaneously from lunar basalt and micrometeorites with iron-rich minerals, while the npFe in the highlands regions mainly come from micrometeorites.
      PubDate: 2017-09-05T18:45:50.456597-05:
      DOI: 10.1002/2016JE005220
       
  • Solar wind sputtering rates of small bodies and ion mass spectrometry
           detection of secondary ions
    • Authors: M. J. Schaible; C. A. Dukes, A. C. Hutcherson, P. Lee, M. R. Collier, R. E. Johnson
      Abstract: Solar wind interactions with the surfaces of asteroids and small moons eject atoms and molecules from the uppermost several nm of regolith grains through a process called sputtering. A small fraction of the sputtered species, called secondary ions, leave the surface in an ionized state, and these are diagnostic of the surface composition. Detection of secondary ions using ion mass spectrometry (IMS) provides a useful method of analysis due to low backgrounds and high instrument sensitivities. However, the sputtered secondary ion yield and the atomic composition of the surface are not 1-to-1 correlated. Thus, relative yield fractions based on experimental measurements are needed to convert measured spectra to surface composition. Here, available experimental results are combined with computationally derived solar wind sputtering yields to estimate secondary ion fluxes from asteroid-sized bodies in the Solar System. The Monte Carlo simulation code SDTrimSP is used to estimate the total sputtering yield due to solar wind ion bombardment for a diverse suite of meteorite and lunar soil compositions. Experimentally measured relative secondary ion yields are then used to determine the abundance of refractory species (Mg+,Al+,Ca+,Fe+) relative to Si+, and it is shown that relative abundances can be used to distinguish whether a body is primitive or has undergone significant geologic reprocessing. Estimates of the sputtered secondary ion fluxes are used to determine the IMS sensitivity required to adequately resolve major element ratios for nominal orbital geometries.
      PubDate: 2017-09-05T18:45:38.985677-05:
      DOI: 10.1002/2017JE005359
       
  • Evolved Gas Analyses of Sedimentary Rocks and Eolian Sediment in Gale
           Crater, Mars: Results of the Curiosity Rover's Sample Analysis at Mars
           (SAM) Instrument from Yellowknife Bay to the Namib Dune
    • Authors: B. Sutter; A. C. McAdam, P. R. Mahaffy, D. W. Ming, K. S. Edgett, E. B. Rampe, J. L. Eigenbrode, H. B. Franz, C. Freissinet, J. P. Grotzinger, A. Steele, C. H. House, P. D. Archer, C. A. Malespin, R. Navarro-González, J. C. Stern, J. F. Bell, F. J. Calef, R. Gellert, D. P. Glavin, L. M. Thompson, A. S. Yen
      Abstract: The Sample Analysis at Mars instrument evolved gas analyzer (SAM-EGA) has detected evolved water, H2, SO2, H2S, NO, CO2, CO, O2 and HCl from two eolian sediments and nine sedimentary rocks from Gale Crater, Mars. These evolved gas detections indicate nitrates, organics, oxychlorine phase, and sulfates are widespread with phyllosilicates and carbonates occurring in select Gale Crater materials. Coevolved CO2 (160 ± 248 - 2373 ± 820 μgC(CO2)/g), and CO (11 ± 3 - 320 ± 130 μgC(CO)/g) suggest organic-C is present in Gale Crater materials. Five samples evolved CO2 at temperatures consistent with carbonate (0.32± 0.05 - 0.70± 0.1 wt.% CO3). Evolved NO amounts to 0.002 ± 0.007 - 0.06 ± 0.03 wt.% NO3. Evolution of O2 suggests oxychlorine phases (chlorate/perchlorate) (0.05 ± 0.025 - 1.05 ± 0.44wt. % ClO4) are present while SO2 evolution indicates the presence of crystalline and/or poorly crystalline Fe- and Mg-sulfate and possibly sulfide. Evolved H2O (0.9 ± 0.3 - 2.5 ± 1.6 wt.% H2O) is consistent with the presence of adsorbed water, hydrated salts, interlayer/structural water from phyllosilicates, and possible inclusion water in mineral/amorphous phases. Evolved H2 and H2S suggest reduced phases occur despite the presence of oxidized phases (nitrate, oxychlorine, sulfate, carbonate). SAM results coupled with CheMin mineralogical and APXS elemental analyses indicate that Gale Crater sedimentary rocks have experienced a complex authigenetic/diagenetic history involving fluids with varying pH, redox, and salt composition. The inferred geochemical conditions were favorable for microbial habitability and if life ever existed, there was likely sufficient organic-C to support a small microbial population.
      PubDate: 2017-07-12T16:21:16.414574-05:
      DOI: 10.1002/2016JE005225
       
  • “Sedimentary processes of the Bagnold Dunes: Implications for the eolian
           rock record of Mars”
    • Authors: R. C. Ewing; M. G. A. Lapotre, K. W. Lewis, M. Day, N. Stein, D. M. Rubin, R. Sullivan, S. Banham, M. P. Lamb, N. T. Bridges, S. Gupta, W. W. Fischer
      Abstract: The Mars Science Laboratory rover Curiosity visited two active wind-blown sand dunes within Gale crater, Mars, which provided the first ground-based opportunity to compare martian and terrestrial eolian dune sedimentary processes and study a modern analog for the martian eolian rock record. Orbital and rover images of these dunes reveal terrestrial-like and uniquely martian processes. The presence of grainfall, grainflow, and impact ripples resembled terrestrial dunes. Impact ripples were present on all dune slopes and had a size and shape similar to their terrestrial counterpart. Grainfall and grainflow occurred on dune and large ripple lee slopes. Lee slopes were ~29° where grainflows were present and ~33° where grainfall was present. These slopes are interpreted as the dynamic and static angles of repose, respectively. Grainsize measured on an undisturbed impact ripple ranges between 50 μm and 350 μm with an intermediate axis mean size of 113 μm (median: 103 μm). Dissimilar to dune eolian processes on Earth, large, meter-scale ripples were present on all dune slopes. Large ripples had nearly symmetric to strongly asymmetric topographic profiles and heights ranging between 12 cm and 28 cm. The composite observations of the modern sedimentary processes highlight that the martian eolian rock record is likely different from its terrestrial counterpart because of the large ripples, which are expected to engender a unique scale of cross-stratification. More broadly, however, in the Bagnold Dune Field as on Earth, dune-field pattern dynamics and basin-scale boundary conditions will dictate the style and distribution of sedimentary processes.
      PubDate: 2017-07-05T17:28:15.41185-05:0
      DOI: 10.1002/2017JE005324
       
  • Aeolian Saltation on Mars at Low Wind Speeds
    • Authors: R. Sullivan; J. F. Kok
      Abstract: Laboratory experiments indicate that the fluid threshold friction speed, u*tf, required to initiate fully developed aeolian saltation is much higher on Mars than on Earth. A discrepancy exists between Mars climate models that do not predict winds this strong, and observations that sand-sized particles are indeed moving. This paper describes how wind friction speeds well below u*tf, but above the impact threshold, u*ti, required to sustain saltation, can initiate sustained saltation on Mars, but at relatively low flux. Numerical experiments indicate that a sand grain on Mars mobilized sporadically between u*ti and u*tf will develop, over fetch lengths longer than generally available within low-pressure wind tunnels, trajectories capable of splashing grains that propagate saltation and collectively form a cluster of saltating grains that migrate downwind together. The passage of a saltation cluster should leave behind a narrow zone of affected surface grains. The cumulative effect of many clusters represents a low-flux phenomenon that should produce slow changes to aeolian bedforms over periods in which winds remain close to u*ti and never or rarely reach u*tf. Field evidence from small impact ripples along rover traverses is consistent with effects of saltation at these low friction speeds, without obvious evidence for events ≥ u*tf. The potential utility of this grain mobility process is that it can operate entirely at more common winds well below u*tf, and so help explain widespread sand movements observed on Mars wherever evidence might be mostly absent for u*tf being exceeded.
      PubDate: 2017-07-05T17:28:11.227443-05:
      DOI: 10.1002/2017JE005275
       
  • Geochemistry of the Bagnold dune field as observed by ChemCam and
           comparison with other aeolian deposits at Gale crater
    • Authors: Cousin Agnes; Erwin Dehouck, Pierre-Yves Meslin, Olivier Forni, Amy J. Williams, Nathan Stein, Olivier Gasnault, Nathan Bridges, Bethany Ehlmann, Susanne Schröder, Valérie Payré, William Rapin, Patrick Pinet, Violaine Sautter, Nina Lanza, Jérémie Lasue, Sylvestre Maurice, Roger C. Wiens
      Abstract: The Curiosity rover conducted the first field investigation of an active extraterrestrial dune. This study of the Bagnold dunes focuses on the ChemCam chemical results and also presents findings on the grain size distributions based on the ChemCam RMI and MAHLI images. These active dunes are composed of grains that are mostly
      PubDate: 2017-06-27T19:20:36.980025-05:
      DOI: 10.1002/2017JE005261
       
  • Mineralogy of an Active Eolian Sediment from the Namib Dune, Gale Crater,
           Mars
    • Authors: C. N. Achilles; R. T. Downs, D. W. Ming, E. B. Rampe, R. V. Morris, A. H. Treiman, S. M. Morrison, D. F. Blake, D. T. Vaniman, R. C. Ewing, S. J. Chipera, A. S. Yen, T. F. Bristow, B. L. Ehlmann, R. Gellert, R. M. Hazen, K. V. Fendrich, P. I. Craig, J. P. Grotzinger, D. J. Des Marais, J. D. Farmer, P. C. Sarrazin, J. M. Morookian
      Abstract: The Mars Science Laboratory rover, Curiosity, is using a comprehensive scientific payload to explore rocks and soils in Gale crater, Mars. Recent investigations of the Bagnold Dune Field provided the first in situ assessment of an active dune on Mars. The CheMin X-ray diffraction instrument on Curiosity performed quantitative mineralogical analyses of the
      PubDate: 2017-06-15T16:05:20.927638-05:
      DOI: 10.1002/2017JE005262
       
  • Chemistry, Mineralogy, and Grain Properties at Namib and High Dunes,
           Bagnold Dune Field, Gale Crater, Mars: A Synthesis of Curiosity Rover
           Observations
    • Authors: B. L. Ehlmann; K. S. Edgett, B. Sutter, C. N. Achilles, M. L. Litvak, M. G. A. Lapotre, R. Sullivan, A. A. Fraeman, R. E. Arvidson, D. F. Blake, N. T. Bridges, P. G. Conrad, A. Cousin, R. T. Downs, T. S. J. Gabriel, R. Gellert, V. E. Hamilton, C. Hardgrove, J. R. Johnson, S. Kuhn, P. R. Mahaffy, S. Maurice, M. McHenry, P.-Y. Meslin, D. W. Ming, M. E. Minitti, J. M. Morookian, R. V. Morris, C. D. O'Connell-Cooper, P. C. Pinet, S. K. Rowland, S. Schröder, K. L. Siebach, N. T. Stein, L. M. Thompson, D. T. Vaniman, A. R. Vasavada, D. F. Wellington, R. C. Wiens, A. S. Yen
      Abstract: The Mars Science Laboratory Curiosity rover performed coordinated measurements to examine the textures and compositions of aeolian sands in the active Bagnold dune field. The Bagnold sands are rounded to subrounded, very fine- to medium- sized (~45-500 µm) with ≥6 distinct grain colors. In contrast to sands examined by Curiosity in a dust-covered, inactive bedform called Rocknest and soils at other landing sites, Bagnold sands are darker, less red, better sorted, have fewer silt-sized or smaller grains, and show no evidence for cohesion. Nevertheless, Bagnold mineralogy and Rocknest mineralogy are similar with plagioclase, olivine, and pyroxenes in similar proportions comprising>90% of crystalline phases, along with a substantial amorphous component (35% ± 15%). Yet, Bagnold and Rocknest bulk chemistry differ. Bagnold sands are Si-enriched relative to other soils at Gale crater, and H2O, S, and Cl are lower relative to all previously measured martian soils and most Gale crater rocks. Mg, Ni, Fe, and Mn are enriched in the coarse-sieved fraction of Bagnold sands, corroborated by VNIR spectra that suggest enrichment of olivine. Collectively, patterns in major element chemistry and volatile release data indicate two distinctive volatile reservoirs in martian soils: (1) amorphous components in the sand-sized fraction (represented by Bagnold) that are Si-enriched, hydroxylated alteration products and/or impact or volcanic glasses; and (2) amorphous components in the fine fraction (
      PubDate: 2017-06-12T11:10:21.15296-05:0
      DOI: 10.1002/2017JE005267
       
  • APXS-derived chemistry of the Bagnold dune sands: Comparisons with Gale
           crater soils and the global martian average
    • Authors: C. D. O'Connell-Cooper; J. G. Spray, L. M. Thompson, R. Gellert, J. A. Berger, N. I. Boyd, E. D. Desouza, G. M. Perrett, M. Schmidt, S. J. VanBommel
      Abstract: We present APXS data for the active Bagnold dune field within the Gale impact crater (MSL mission). We derive an APXS-based Average Basaltic Soil (ABS) composition for Mars based on past and recent data from the MSL and MER missions. This represents an update to the Taylor and McLennan (2009) average martian soil, and facilitates comparison across martian datasets.The active Bagnold dune field is compositionally distinct from the ABS, with elevated Mg, Ni and Fe, suggesting mafic mineral enrichment, and uniformly low levels of S, Cl and Zn, indicating only a minimal dust component. A relationship between decreasing grain size and increasing felsic content is revealed. The Bagnold Sands possess the lowest S/Cl of all martian unconsolidated materials..Gale soils exhibit relatively uniform major element compositions, similar to Meridiani Planum and Gusev Crater basaltic soils (MER missions). However, they show minor enrichments in K, Cr, Mn and Fe, which may signify a local contribution.The lithified eolian Stimson Formation within the Gale impact crater is compositionally similar to the ABS and Bagnold sands, which provide a modern analogue for these ancient eolian deposits. Compilation of APXS-derived soil data reveals a generally homogenous global composition for martian soils, but one that can be locally modified due to past or extant geologic processes that are limited in both space and time.
      PubDate: 2017-06-05T14:25:21.155559-05:
      DOI: 10.1002/2017JE005268
       
  • Visible/near-infrared spectral diversity from in situ observations of the
           Bagnold Dune Field sands in Gale Crater, Mars
    • Authors: Jeffrey R. Johnson; Cherie Achilles, James F. Bell, Steve Bender, Edward Cloutis, Bethany Ehlmann, Abigail Fraeman, Olivier Gasnault, Victoria E. Hamilton, Stéphane Le Mouélic, Sylvestre Maurice, Patrick Pinet, Lucy Thompson, Danika Wellington, Roger C. Wiens
      Abstract: As part of the Bagnold Dune campaign conducted by Mars Science Laboratory rover Curiosity, visible/near-infrared reflectance spectra of dune sands were acquired using Mast Camera (Mastcam) multispectral imaging (445-1013 nm) and Chemistry and Camera (ChemCam) passive point spectroscopy (400-840 nm). By comparing spectra from pristine and rover-disturbed ripple crests and troughs within the dune field, and through analysis of sieved grain size fractions, constraints on mineral segregation from grain sorting could be determined. In general, the dune areas exhibited low relative reflectance, a weak ~530 nm absorption band, an absorption band near 620 nm, and a spectral downturn after ~685 nm consistent with olivine-bearing sands. The finest grain size fractions occurred within ripple troughs and in the subsurface, and typically exhibited the strongest ~530 nm bands, highest relative reflectances, and weakest red/near-infrared ratios, consistent with a combination of crystalline and amorphous ferric materials. Coarser-grained samples were the darkest and bluest, and exhibited weaker ~530 nm bands, lower relative reflectances, and stronger downturns in the near-infrared, consistent with greater proportions of mafic minerals such as olivine and pyroxene. These grains were typically segregated along ripple crests and among the upper surfaces of grain flows in disturbed sands. Sieved dune sands exhibited progressive decreases in reflectance with increasing grain size, as observed in laboratory spectra of olivine size separates. The continuum of spectral features observed between the coarse- and fine-grained dune sands suggests that mafic grains, ferric materials, and airfall dust mix in variable proportions depending on aeolian activity and grain sorting.
      PubDate: 2017-04-25T12:45:53.991918-05:
      DOI: 10.1002/2016JE005187
       
  • Compositional Variations in Sands of the Bagnold Dunes, Gale Crater, Mars,
           from Visible-Shortwave Infrared Spectroscopy and Comparison to
           Ground-Truth from the Curiosity Rover
    • Authors: M. G. A. Lapotre; B. L. Ehlmann, S. E. Minson, R. E. Arvidson, F. Ayoub, A. A. Fraeman, R. C. Ewing, N. T. Bridges
      Abstract: During its ascent up Mount Sharp, the Mars Science Laboratory Curiosity rover traversed the Bagnold Dune Field. We model sand modal mineralogy and grain size at four locations near the rover traverse, using orbital shortwave infrared single scattering albedo spectra and a Markov-Chain Monte Carlo implementation of Hapke's radiative transfer theory to fully constrain uncertainties and permitted solutions. These predictions, evaluated against in situ measurements at one site from the Curiosity rover, show that XRD-measured mineralogy of the basaltic sands is within the 95% confidence interval of model predictions. However, predictions are relatively insensitive to grain size and are non-unique, especially when modeling the composition of minerals with solid solutions. We find an overall basaltic mineralogy and show subtle spatial variations in composition in and around the Bagnold dunes, consistent with a mafic enrichment of sands with cumulative transport distance by sorting of olivine, pyroxene, and plagioclase grains during aeolian saltation. Furthermore, the large variations in Fe and Mg abundances (~20 wt%) at the Bagnold Dunes suggest that compositional variability induced by wind sorting may be enhanced by local mixing with proximal sand sources. Our estimates demonstrate a method for orbital quantification of composition with rigorous uncertainty determination and provide key constraints for interpreting in situ measurements of compositional variability within martian aeolian sandstones.
      PubDate: 2017-04-14T14:30:31.935201-05:
      DOI: 10.1002/2016JE005133
       
  • Issue Information
    • Pages: 1955 - 1955
      Abstract: No abstract is available for this article.
      PubDate: 2017-11-15T16:02:00.027557-05:
      DOI: 10.1002/jgre.20584
       
 
 
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