Investigations of the variability of dust particle sizes in the martian atmosphere using the NASA Ames General Circulation Model

We present a Mars General Circulation Model (GCM) numerical investigation of the physical processes (i.e., wind stress and dust devil dust lifting and atmospheric transport) responsible for temporal and spatial variability of suspended dust particle sizes. Measurements of spatial and temporal variations in airborne dust particles sizes in the martian atmosphere have been derived from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) spectral and emission phase function data [Wolff, M.J., Clancy, R.T., 2003. J. Geophys. Res. (Planets) 108 (E9), doi:10.1029/2003JE002057. 1-1; Clancy, R.T., Wolff, M.J., Christensen, P.R., 2003. J. Geophys. Res. (Planets) 108 (E9), doi:10.1029/2003JE002058. 2-1]. The range of dust particle sizes simulated by the NASA Ames GCM is qualitatively consistent with TES-derived observations of effective dust particle size variability. Model results suggest that the wind stress dust lifting scheme (which produces regionally confined dust lifting) is the process responsible for the majority of the dust particle size variability in the martian atmosphere. Additionally, model results suggest that atmospheric transport processes play an important role in the evolution of atmospheric dust particles sizes during substantial dust storms on Mars. Finally, we show that including the radiative effects of a spatially variable particle size distribution significantly influences thermal and dynamical fields during the dissipation phase of the simulated global dust storm.     

Subsurface structure of Planum Boreum from Mars Reconnaissance Orbiter Shallow Radar soundings

We map the subsurface structure of Planum Boreum using sounding data from the Shallow Radar (SHARAD) instrument onboard the Mars Reconnaissance Orbiter. Radar coverage throughout the 1,000,000-km² area reveals widespread reflections from basal and internal interfaces of the north polar layered deposits (NPLD). A dome-shaped zone of diffuse reflectivity up to 12 μs (∼1-km thick) underlies two-thirds of the NPLD, predominantly in the main lobe but also extending into the Gemina Lingula lobe across Chasma Boreale. We equate this zone with a basal unit identified in image data as Amazonian sand-rich layered deposits [Byrne, S., Murray, B.C., 2002. J. Geophys. Res. 107, 5044, 12 pp. doi:10.1029/2001JE001615; Fishbaugh, K.E., Head, J.W., 2005. Icarus 174, 444-474; Tanaka, K.L., Rodriguez, J.A.P., Skinner, J.A., Bourke, M.C., Fortezzo, C.M., Herkenhoff, K.E., Kolb, E.J., Okubo, C.H., 2008. Icarus 196, 318-358]. Elsewhere, the NPLD base is remarkably flat-lying and co-planar with the exposed surface of the surrounding Vastitas Borealis materials. Within the NPLD, we delineate and map four units based on the radar-layer packets of Phillips et al. [Phillips, R.J., and 26 colleagues, 2008. Science 320, 1182-1185] that extend throughout the deposits and a fifth unit confined to eastern Gemina Lingula. We estimate the volume of each internal unit and of the entire NPLD stack (821,000 km³), exclusive of the basal unit. Correlation of these units to models of insolation cycles and polar deposition [Laskar, J., Levrard, B., Mustard, J.F., 2002. Nature 419, 375-377; Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2007. J. Geophys. Res. 112, E06012, 18 pp. doi:10.1029/2006JE002772] is consistent with the 4.2-Ma age of the oldest preserved NPLD obtained by Levrard et al. [Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2007. J. Geophys. Res. 112, E06012, 18 pp. doi:10.1029/2006JE002772]. We suggest a dominant layering mechanism of dust-content variation during accumulation rather than one of lag production during periods of sublimation.     

Kinetic Alfven wave in the presence of general loss-cone distribution function in inhomogeneous magnetoplasma—particle aspect analysis

Particle aspect analysis is extended for kinetic Alfven waves in an inhomogeneous magnetoplasma in the presence of a general loss-cone distribution function. The effect of finite Larmor radius is incorporated in the finite temperature anisotropic plasma. Expressions for the field-aligned current, perpendicular current (to B), dispersion relation, particle energy and growth rate are derived and effects of steepness of loss-cone distribution and plasma density inhomogeneity are discussed. The treatment of the kinetic Alfven wave instability is based on the assumption that the plasma consists of resonant and non-resonant particles. It is assumed that resonant particles support the oscillatory nature of the wave. The excitation of the wave is treated by the wave particle energy exchange method. The applicability of the investigation is discussed for auroral acceleration phenomena. © 1999 Elsevier Science Ltd. All rights reserved.     

Observation of the hydrogen corona with SPICAM on Mars Express

A series of seven dedicated Lyman-α observations of exospheric atomic hydrogen in the martian corona were performed in March 2005 with the ultraviolet spectrometer SPICAM on board Mars Express. Two types of observations are analyzed, observations at high illumination (for a solar zenith angle SZA equal to 30°) and observations at low illumination (for a solar zenith angle equal to 90° (terminator), and near the south pole). The measured Lyman-α emission is interpreted as purely resonant scattering of solar photons. Because the Lyman-α emission is optically thick, we use a forward model to analyze this data. Below the exobase, the hydrogen density is described by a diffusive model and above the exobase, it follows Chamberlain's approach without satellite particles. For different hydrogen density profiles between 80 and 50,000 km, the volume emission rates are computed by solving the radiative transfer equation. Such an approach has been used to analyze the Mariner 6, 7 exospheric Lyman-α data during the late 1960s. A reasonable fit of the set of observations is obtained assuming an exobase temperature between 200 and 250 K and an exobase density of ∼1-4 × 10⁵ cm⁻³ in good agreement with photochemical models. However, when considering the average exospheric temperature of 200 K measured by other methods [Leblanc, F., Chaufray, J.Y., Witasse, O., Lilensten, J., Bertaux, J.-L., 2006a. J. Geophys. Res. 111 (E9), doi:10.1029/2005JE002664. E09S11; Leblanc, F., Chaufray, J.-Y., Bertaux, J.-L., 2007. Geophys. Res. Lett. 34, doi:10.1029/2006GL028437. L02206; Bougher, S.W., Engel, S., Roble, R.G., Foster, B., 2000. J. Geophys. Res. 105, 17669-17692; Mazarico, E., Zuber, M.T., Lemoine, F.G., Smith, D.E., 2007. J. Geophys. Res. 112, doi:10.1029/2006JE002734. E05014] a supplementary hot population is needed above the exobase to reconcile Lyman-α measurements with these previous measurements, particularly for the observations at low SZA. In this case, the densities and temperatures at the exobase for the two populations are 1.0±0.2×¹⁰⁵ cm⁻³ and T=200 K and 1.9±0.5×¹⁰⁴ cm⁻³ and T>500 K for the cold and hot populations respectively at low SZA. At high SZA, the densities and temperatures are equal to 2±0.4×¹⁰⁵ cm⁻³ and T=200 K and n=1.2±0.5×¹⁰⁴ cm⁻³ and T>500 K. These high values of the hot hydrogen component are not presently supported by the theory. Moreover, it is important to underline that the two population model remains relatively poorly constrained by the limited range of altitude covered by the present set of SPICAM measurements and cannot be unambiguously identified because of the global uncertainty of our model and of SPICAM measurements. For a one population solution, an average water escape rate of 7.5 × 10⁻⁴ precipitable μm/yr is estimated, yielding a lifetime of 13,000 years for the average present water vapor content of 10 precipitable microns.     

Stratigraphical evidence of late Amazonian periglaciation and glaciation in the Astapus Colles region of Mars

Recent modeling of the meteorological conditions during and following times of high obliquity suggests that an icy mantle could have been emplaced in western Utopia Planitia by atmospheric deposition during the late Amazonian period [Costard, F.M., Forget, F., Madeleine, J.B., Soare, R.J., Kargel, J.S., 2008. Lunar Planet. Sci. 39. Abstract 1274; Madeleine, B., Forget, F., Head, J.W., Levrard, B., Montmessin, F., 2007. Lunar Planet. Sci. 38. Abstract 1778]. Astapus Colles (ABa) is a late Amazonian geological unit — located in this hypothesized area of accumulation — that comprises an icy mantle tens of meters thick [Tanaka, K.L., Skinner, J.A., Hare, T.M., 2005. US Geol. Surv. Sci. Invest., Map 2888]. For the most part, this unit drapes the early Amazonian Vastitas Borealis interior unit (ABv_i); to a lesser degree it overlies the early Amazonian Vastitas Borealis marginal unit (ABv_m) and the early to late Hesperian UP plains unit HBu₂ [Tanaka, K.L., Skinner, J.A., Hare, T.M., 2005. US Geol. Surv. Sci. Invest., Map 2888]. Landscapes possibly modified by late-Amazonian periglacial processes [Costard, F.M., Kargel, J.S., 1995. Icarus 114, 93-112; McBride, S.A., Allen, C.C., Bell, M.S., 2005. Lunar Planet. Sci. 36. Abstract 1090; Morgenstern, A., Hauber, E., Reiss, D., van Gasselt, S., Grosse, G., Schirrmeister, L., 2007. J. Geophys. Res. 112, doi:10.1029/2006JE002869. E06010; Seibert, N.M., Kargel, J.S., 2001. Geophys. Res. Lett. 28, 899-902; Soare, R.J., Kargel, J.S., Osinski, G.R., Costard, F., 2007. Icarus 191, 95-112; Soare, R.J., Osinski, G.R., Roehm, C.L., 2008. Earth Planet. Sci. Lett. 272, 382-393] and glacial processes [Milliken, R.E., Mustard, J.F., Goldsby, D.L., 2003. J. Geophys. Res. 108 (E6), doi:10.1029/2002JE002005. 5057; Mustard, J.F., Cooper, C.D., Rifkin, M.K., 2001. Nature 412, 411-414; Tanaka, K.L., Skinner, J.A., Hare, T.M., 2005. US Geol. Surv. Sci. Invest., Map 2888] have been reported within the region. Researchers have assumed that the periglacial and glacial landscapes occur within the same geological unit, the ABa [i.e., Morgenstern, A., Hauber, E., Reiss, D., van Gasselt, S., Grosse, G., Schirrmeister, L., 2007. J. Geophys. Res. 112; doi:10.1029/2006JE002869. E06010; Tanaka, K.L., Skinner, J.A., Hare, T.M., 2005. US Geol. Surv. Sci. Invest., Map 2888]. In this study we use HiRISE (High Resolution Image Science Experiment, Mars Reconnaissance Orbiter) imagery to identify the stratigraphical separation of the two landscapes and show that periglacial landscape modification has occurred in the geological units that underlie the ABa, not in the ABa itself. Moreover, we suggest that the periglacial landscape extends well beyond the perimeter of the ABa and could be the product of “wet” cold-climate processes. These processes involve freeze-thaw cycles and intermittently stable liquid-water at or near the surface. By contrast, we propose that the ABa is a very recent late-Amazonian geological unit formed principally by “dry” cold-climate processes. These processes comprise accumulation (by atmospheric deposition) and ablation (by sublimation).     

Effect of general loss-cone distribution function on the shear-driven electrostatic ion-cyclotron instability

The shear-driven electrostatic ion-cyclotron instability (EICI) is studied using the loss-cone distribution function by particle aspect analysis. The effect of the loss-cone distribution on the dispersion relation and growth rate of weak shear-driven EICI is studied. The whole plasma is considered to consist of resonant and non-resonant particles. The wave is assumed to propagate obliquely to the static magnetic field. It is found that the frequency of the EICI is Doppler shifted due to the transverse inhomogeneous flow in the direction of the magnetic field. It is also found that for anisotropic plasma the critical velocity shear needed to excite EICI depends upon the loss-cone distribution index (J). With the increasing values the loss-cone distribution indices (J), the critical value of normalized velocity shear needed to generate EICI in anisotropic plasma, decreases and is of the order of the weak shear. The loss-cone distribution acts as a source of free energy and generates the weak shear-driven EICI at longer perpendicular perturbations. It also lowers the transverse and parallel energy of the resonant ions. The study may explain the frequently observed EICI in the auroral acceleration region.     

Suprathermal electron fluxes on the nightside of Mars: ASPERA-3 observations

Recently aurora-type UV emissions were discovered on the nightside of Mars [Bertaux, J.-L., Leblanc, F., Witasse, O., et al., 2005. Discovery of an aurora on Mars. Nature 439, doi:10.1038/nature03603]. It was suggested that these emissions are produced by suprathermal electrons with energies of tens of eV, rather than by the electrons with spectra peaked above 100 eV [Leblanc, F., Witasse, O., Winningham J., et al., 2006. Origin of the martian aurora observed by spectroscopy for investigation of characteristics of the atmosphere of Mars (SPICAM) onboard Mars Express. J. Geophys. Res. 111, A09313, doi:10.1029/2006JA011763]. In this paper we present observations of fluxes of suprathermal electrons (E_e≈30-100 eV) on the Martian nightside by the ASPERA-3 experiment onboard the Mars Express spacecraft. Narrow spikes of suprathermal electrons are often observed in energy-time spectrograms of electron fluxes at altitudes between 250 and 600 km. These spikes are spatially organized and form narrow strips in regions with strong upward or downward crustal magnetic field. The values of electron fluxes in such events generally could explain the observed auroral UV emissions although a question of their origin (transport from the dayside or local precipitation) remains open.     

O2 1.27 μm emission maps as derived from OMEGA/MEx data

We present a method to derive the 2D maps of the O₂ (a¹Δ_g) airglow emission at 1.27 μm from the OMEGA/MEx nadir observations. The OMEGA imaging capabilities allow monitoring the 2D distribution, daily and seasonal variation of the O₂ emission intensities with a detection limit of 4 MR. The highest values, of the order of ∼31 MR, are found on the south pole for 11 h < LT < 13 h, during the early spring (186° < Ls < 192°) of martian year (MY) 27, according to the Mars Year numbering scheme of Clancy et al. [Clancy, R.T., Wolff, M.J., Christensen, P.R., 2003. Mars aerosol studies with the MGS TES emission phase function observations: Optical depths, particle sizes, and ice cloud types versus latitude and solar longitude. J. Geophys. Res. 108. doi: 10.1029/2003JE002058]. In the polar regions the day-by-day variability, associated with polar vortex turbulences, is obtained of the order of 30-50% as predicted by the model [Lefévre, F., Lebonnois, S., Montmessin, F., Forget, F., 2004. Three-dimensional modeling of ozone on Mars. J. Geophys. Res. 109, E07004. doi: 10.1029/2004JE002268] and found by SPICAM [Perrier, S., Bertaux, J.-L., Lebonnois, S., Korablev, O., Fedorova, A., 2006. Global distribution of total ozone on Mars from SPICAM/MEX UV measurements. J. Geophys. Res. 111, E09S06. doi: 10.1029/2006JE002681]. In the considered set of data a maximum of the O₂ emission is observed between 11 h and 15 h LT in the latitude range 70-85° during early spring on both hemispheres, while for the southern autumn-winter season a maximum is found between 50° and 60° in the southern hemisphere for MY28. Increase of intensity of the O₂ emission observed from Ls 130° to 160° at southern high latitudes may be explained by increase of solar illumination conditions in the maps acquired during the considered period.Atmospheric waves crossing the terminator on the southern polar regions are observed for the first time during the MY28 early spring. The spatial scale of the waves ranges from 100 to 130 km, and the intensity fluctuations are of the order of 4MR.This study confirms the high potentiality of O₂ (a¹Δ_g) day glow as a passive tracer of the martian atmosphere dynamics at high latitudes.     

Ancient melting of mid-latitude snowpacks on Mars as a water source for gullies

We hypothesize that during past epochs of high obliquity seasonal snowfields at mid-latitudes melted to produce springtime sediment-rich surface flows resulting in gully formation. Significant seasonal mid-latitude snowfall does not occur on Mars today. General Circulation Model (GCM) results, however, suggest that under past climate conditions there may have been centimeters of seasonal mid-latitude snowfall [Mischna, M.A., Richardson, M.I., Wilson, R.J., McCleese, D.J., 2003. J. Geophys. Res. Planets 108, doi:10.1029/2003JE002051. 5062]. Gully locations have been tabulated by several researchers (e.g. [Heldmann, J.L., Mellon, M.T., 2004. Icarus 168, 285–304; Heldmann, J.L., Carlsson, E., Johansson, H., Mellon, M.T., Toon, O.B., 2007. Icarus 188, 324–344; Malin, M.C., Edgett, K.S., 2000. Science 288, 2330–2335]) and found to correspond to mid-latitude bands. A natural question is whether the latitudinal bands where the gullies are located correspond to areas where the ancient snowfalls may have melted, producing runoff which may have incised gullies. In this study we model thin snowpacks with thicknesses similar to those predicted by [Mischna, M.A., Richardson, M.I., Wilson, R.J., McCleese, D.J., 2003. J. Geophys. Res. Planets 108, doi:10.1029/2003JE002051. 5062]. We model these snowpacks under past climate regimes in order to determine whether snowmelt runoff could have occurred, and whether significant amounts of warm soil (T>273 K) existed on both poleward and equatorward slopes in the regions where gullies exist. Both warm soil and water amounts are modeled because soil and water may have mixed to form a sediment-rich flow. We begin by applying the snowpack model of Williams et al. [Williams, K.E., Toon, O.B., Heldmann, J.E., Mellon, M., 2008. Icarus 196, 565–577] to past climate regimes characterized by obliquities of 35° (600 ka before present) and 45° (5.5 ma before present), and to all latitudes between 70° N and 70° S. We find that the regions containing significant snowmelt runoff correspond to the regions identified by Heldmann and Mellon [Heldmann, J.L., Mellon, M.T., 2004. Icarus 168, 285–304], Heldmann et al. [Heldmann, J.L., Carlsson, E., Johansson, H., Mellon, M.T., Toon, O.B., 2007. Icarus 188, 324–344] and Malin and Edgett [Malin, M.C., Edgett, K.S., 2000. Science 288, 2330–2335] as containing large numbers of gullies. We find that the snowmelt runoff (>1 mm, with equivalent rainfall rates of 0.25 mm/h) and warm soil (>1 cm depth) would have occurred on slopes within the gullied latitudinal bands. The snowfall amounts modeled are predicted to be seasonal [Mischna, M.A., Richardson, M.I., Wilson, R.J., McCleese, D.J., 2003. J. Geophys. Res. Planets 108, doi:10.1029/2003JE002051. 5062], and our modeling finds that under the previous climate regimes there would have been meltwater present on the slopes in question for brief periods of time, on the order of days, each year. Our model provides a simple explanation for the latitudinal distribution of the gullies, and also suggests that the gullies date to times when water migrated away from the present poles to the mid-latitudes.     

Towards self-consistent modeling of the martian dichotomy: The influence of one-ridge convection on crustal thickness distribution

In order to find an explanation for the origin of the martian crustal dichotomy, a number of recent papers have examined the effect of layered viscosity on the evolution of a degree-1 mantle convection, e.g. Roberts and Zhong [Roberts, J.H., Zhong, S., 2006. J. Geophys. Res. 111. E06013] and Yoshida and Kageyama [Yoshida, M., Kageyama, A., 2006. J. Geophys. Res. 111, doi:10.1029/2005JB003905. B03412]. It was found that a mid-mantle viscosity jump, combined with highly temperature- and depth-dependent rheology, are effective in developing a degree-1 convection within a short timescale. Such a layered viscosity profile could be justified by martian mineralogy. However, the effect of a degree-1 convective planform on the crustal thickness distribution has not yet been demonstrated. It is not obvious whether a thinner crust, due to sublithospheric erosion and crustal thinning, or a thicker crust, due to enhanced crustal production, would form above the hemisphere of mantle upwelling. Also, the general shape of the dichotomy, which is not strictly hemispherical, has not yet been fully investigated. Here we propose a model of the crustal patterns produced by numerical simulations of martian mantle convection, using the finite-volume multigrid code StagYY [Tackley, P.J., 2008. Phys. Earth Planet. Int. 107, 7-18, doi:10.1016/j.pepi.2008.08.005] A self-consistent treatment of melting, crustal formation and chemical differentiation has been added to models of three-dimensional thermal convection. This allows us to obtain global maps of the crustal thickness distribution as it evolves with time. The obtained results demonstrate that it is indeed possible to form a crustal dichotomy as a consequence of near degree-1 mantle convection early in Mars' history. We find that some of the observed patterns show intriguing first order similarities to the elliptical shape of the martian dichotomy. In all models, the region of thick crust is located over the region of mantle upwelling, which itself is a ridge-like structure spread over roughly one half of the planet, a planform we describe as “one-ridge convection.”     

Shear Driven Kinetic Alfven Wave with General Loss-Cone Distribution Function in the Plasma Sheet Boundary Layer

Expressions for the dispersion relation and growth rate of the KAW are derived for weak and strong shear regimes using the kinetic approach in view of the simultaneous observations of the large earthward Alfvenic Poynting flux, small-scale kinetic Alfven wave (KAW), earthward flowing electrons and upward flowing ions, at the substorm event in the plasma sheet boundary layer (PSBL). General loss-cone distribution function is adopted to describe the velocity distribution of the plasma particles. The results explain the generation of the observed KAW in the PSBL by the weak shear at the substorm onset. It is found that during the substorm expansion phase the cyclotron damping of KAW may lead to the upward flowing ion. Whereas, it’s Landau damping that may lead to the parallel energisation of the electrons that explains the observed loss of Alfvenic Poynting flux. It is also noted that the loss-cone distribution index changes the profiles of the frequency and growth rate plots of the shear-driven KAW. The loss-cone distribution function is therefore, an important factor for the excitation of KAW in the active region of the magnetosphere at the PSBL. Results are consistent with the finding of Wu and Seyler (J Geophys Res 108A6:1236, 2003) concerning kinetic Alfven wave generation and its stabilization by the sheared flow.     

Valley network-fed, open-basin lakes on Mars: Distribution and implications for Noachian surface and subsurface hydrology

A new catalog of 210 open-basin lakes (lakes with outlet valleys) fed by valley networks shows that they are widely distributed in the Noachian uplands of Mars. In order for an outlet valley to form, water must have ponded in the basin to at least the level of the outlet. We use this relationship and the present topography to directly estimate the minimum amount of water necessary to flood these basins in the past. The volumes derived for the largest lakes (∼3×¹⁰⁴ to ∼2×¹⁰⁵ km³) are comparable to the largest lakes and small seas on modern Earth, such as the Caspian Sea, Black Sea, and Lake Baikal. We determine a variety of other morphometric properties of these lakes and their catchments (lake area, mean depth, volume, shoreline development, outlet elevation, and watershed area). Most candidate lakes have volumes proportional to and commensurate with their watershed area, consistent with precipitation as their primary source. However, other lakes have volumes that are anomalously large relative to their watershed areas, implying that groundwater may have been important in their filling. Candidate groundwater-sourced lakes are generally concentrated in the Arabia Terra region but also include the Eridania basin [Irwin, R.P., Howard, A.D., Maxwell, T.A., 2004a. J. Geophys. Res. 109, doi: 10.1029/2004JE002287. E12009; Irwin, R.P., Watters, T.R., Howard, A.D. Zimbelman, J.R., 2004b. J. Geophys. Res. 109, doi: 10.1029/2004JE002248. E09011] and several lakes near the dichotomy boundary. This areal distribution is broadly consistent with where groundwater should have reached the surface as predicted by current models. Both surface runoff and groundwater flow appear to have been important sources for lakes and lake chains, suggesting a vertically integrated hydrological system, the absence of a global cryosphere, and direct communication between the surface and subsurface hydrosphere of early Mars.     

Different eigenproblem models for field line resonances in cold plasma: Effect on magnetospheric density estimates

Magnetospheric plasma density can be remotely sensed through ground-based magnetometer data using a suitable model for field line resonances (FLRs) formed by standing shear Alfvén wave on closed geomagnetic field lines. The simplest type of FLR model, which is also the most relevant for magnetometer data inversion purposes, is based on solving a certain eigenvalue problem. Over the years a number of such models have been developed [Singer, H.J., Southwood, D.J., Walker, R.J., Kivelson, M.G., 1981. Alfvén wave resonances in a realistic magnetospheric magnetic field geometry. J. Geophys. Res. 86, 4589–4596; Rankin, R., Fenrich, F., Tikhonchuk, V.T., 2000. Shear Alfvén waves on stretched magnetic field lines near midnight in Earth's magnetosphere. Geophys. Res. Lett. 27, 3265–3268; Rankin, R., Kabin, K., Marchand, R., 2006. Alfvénic field line resonances in arbitrary magnetic field topology. Adv. Space Res. 38, 1720–1729]. In this paper we summarize the properties of these models and investigate the effect of using these different models on the magnetospheric density inferred from the ground-based magnetometer measurements. We also formulate a simple criterion which can be used to determine which one of these models should be used for a particular field line.     

Effect of parallel electric field on electrostatic ion-cyclotron instability in anisotropic plasma in the presence of ion beam and general distribution function—Particle aspect analysis

Dispersion relation, resonant energy transferred, growth rate and marginal instability criteria for the electrostatic ion-cyclotron wave with general loss-cone distribution in low-β anisotropic, homogeneous plasma in the auroral acceleration region are discussed by investigating the trajectories of the charged particles. Effects of the parallel electric field, ion beam velocity, steepness of the loss-cone distribution and temperature anisotropy on resonant energy transferred and growth rate of the instability are discussed. It is found that the effect of the parallel electric field is to stabilize the wave and enhance the transverse acceleration of ions whereas the effect of steepness of loss-cone, ion beam velocity and the temperature anisotropy is to enhance the growth rate and decrease the transverse acceleration of ions. The steepness of the loss-cone also introduces a peak in the growth rate which shifts towards the lower side of the perpendicular wave number with the increasing steepness of the loss-cone.     

Digging deep for ice in Isidis Planitia—New constraints on subsurface volatile concentrations from thermal modeling

The Isidis Planitia region on Mars usually is regarded as a comparably attractive site for landing missions based on engineering constraints such as elevation and smooth regional topography. The Mars Express landed element Beagle 2 was deployed to this area, and the southern margin of the basin was selected as one of the backup landing sites for the NASA Mars Exploration Rovers.Especially in the context of the Beagle 2 mission, Isidis Planitia has been discussed as a place which might have experienced a volatile-rich history with associated potential for biological activity [e.g. Bridges et al., 2003. Selection of the landing site in Isidis Planitia of Mars Probe Beagle 2. J. Geophys. Res. 108(E1), 5001, doi: 10.1029/2001JE001820]. However the measurements of by the GRS instrument on Mars Odyssey indicate a maximum inferred water abundance of only 3 wt% in the upper few meters of the surface [Feldman et al., 2004. Global distribution of near-surface hydrogen on Mars. J. Geophys. Res. 109, E09006, doi: 10.1029/2003JE002160]. Based on these measurements this area seems to be one of the driest spots in the equatorial region of Mars.To support future landing site selections we took a more detailed look at the minimum burial depth of stable ice deposits in this area, focusing as an example on the planned Beagle 2 landing site. We are especially interested in the likelihood of ground ice deposits within the range of proposed subsurface sampling tools as drills or ‘mole’-like devices [Richter et al., 2002. Development and testing of subsurface sampling devices for the Beagle 2 Lander. Planet. Space Sci. 50, 903-913] given reasonable physical constraints for the surface and near surface material.For a mission like ExoMars [Kminek, G., Vago, J.L., 2005. The Aurora Exploration Program—The ExoMars Mission. In: Proceedings of the 35th Lunar and Planetary Science Conference, abstract no. 1111, 15-19 March 2004, League City, TX] with a focus on finding traces of fossil life the area might be of potential interest, because these traces would be better conserved in the dry soil. Modeling and measurement indicate that Isidis Planitia is indeed a dry place and any hypothetical ground ice deposits in this region are out of range of currently proposed sampling devices.     

Numerical modeling of protocore destabilization during planetary accretion: Methodology and results

We developed and tested an efficient 2D numerical methodology for modeling gravitational redistribution processes in a quasi spherical planetary body based on a simple Cartesian grid. This methodology allows one to implement large viscosity contrasts and to handle properly a free surface and self-gravitation. With this novel method we investigated in a simplified way the evolution of gravitationally unstable global three-layer structures in the interiors of large metal-silicate planetary bodies like those suggested by previous models of cold accretion [Sasaki, S., Nakazawa, K., 1986. J. Geophys. Res. 91, 9231-9238; Karato, S., Murthy, V.R., 1997. Phys. Earth Planet Interios 100, 61-79; Senshu, H., Kuramoto, K., Matsui, T., 2002. J. Geophys. Res. 107 (E12), 5118. 10.1029/2001JE001819]: an innermost solid protocore (either undifferentiated or partly differentiated), an intermediate metal-rich layer (either continuous or disrupted), and an outermost silicate-rich layer. Long-wavelength (degree-one) instability of this three-layer structure may strongly contribute to core formation dynamics by triggering planetary-scale gravitational redistribution processes. We studied possible geometrical modes of the resulting planetary reshaping using scaled 2D numerical experiments for self-gravitating planetary bodies with Mercury-, Mars- and Earth-size. In our simplified model the viscosity of each material remains constant during the experiment and rheological effects of gravitational energy dissipation are not taken into account. However, in contrast to a previously conducted numerical study [Honda, R., Mizutani, H., Yamamoto, T., 1993. J. Geophys. Res. 98, 2075-2089] we explored a freely deformable planetary surface and a broad range of viscosity ratios between the metallic layer and the protocore (0.001-1000) as well as between the silicate layer and the protocore (0.001-1000). An important new prediction from our study is that realistic modes of planetary reshaping characterized by a high viscosity protocore and low viscosity molten silicate and metal [Senshu, H., Kuramoto, K., Matsui, T., 2002. J. Geophys. Res. 107 (E12), 5118. 10.1029/2001JE001819] may result in the transient exposure of the protocore to the planetary surface and a strongly (up to 8% of the planetary diameter) aspherical deviation of the planetary shape during the early stages of core formation. Exposure of the protocore might happen in the early stages of iron core formation. This process may conceivably convert a large amount of potential energy into temperature increase and a transient strongly non-uniform depth of the magma ocean around the protoplanet. Our simplified model also predicts that the time for metallic core formation out of the metal-rich layer depends mainly on the dynamics of the deformation of the solid strong protocore. In nature this dynamics will be strongly dependent on the effective viscosity of the protocore, which should generally have non-Newtonian pressure-, temperature-, and stress-dependent rheology with strong thermomechanical feedbacks from gravitational energy dissipation.     

Analysis of thermal emission from the nightside of Venus at 1.51 and 1.55 μm

We present radiative transfer modelling of thermal emission from the nightside of Venus in two ‘spectral window’ regions at 1.51 and 1.55 μm. The first discovery of these windows, reported by Erard et al. [Erard, S., Drossart, P., Piccioni, G., 2009. J. Geophys. Res. Planets 114, doi:10.1029/2008JE003116. E00B27], was achieved using a principal component analysis of data from the VIRTIS instrument on Venus Express. These windows are spectrally narrow, with a full-width at half-maximum of ∼20 nm, and less bright than the well-known 1.7 and 2.3 μm spectral windows by two orders of magnitude.In this note we present the first radiative transfer analysis of these windows. We conclude that the radiation in these windows originates at an altitude of 20-35 km. As is the case for the other infrared window regions, the brightness of the windows is affected primarily by the optical depth of the overlying clouds; in addition, the 1.51 μm radiance shows a very weak sensitivity to water vapour abundance.     

Variability of CO concentrations in the Venus troposphere from Venus Express/VIRTIS using a Band Ratio Technique

A fast method is presented for deriving the tropospheric CO concentrations in the Venus atmosphere from near-infrared spectra using the night side 2.3 μm window. This is validated using the spectral fitting techniques of Tsang et al. [Tsang, C.C.C., Irwin, P.G.J., Taylor, F.W., Wilson, C.F., Drossart, P., Piccioni, G., de Kok, R., Lee, C., Calcutt, S.B., and the Venus Express/VIRTIS Team, 2008a. Tropospheric carbon monoxide concentrations and variability on Venus with Venus Express/VIRTIS-M observations. J. Geophys. Res. 113, doi: 10.1029/2008JE003089. E00B08] to show that monitoring CO in the deep atmosphere can be done quickly using large numbers of observations, with minimal effect from cloud and temperature variations. The new method is applied to produce some 1450 zonal mean CO profiles using data from the first eighteen months of operation from the Visible and Infrared Thermal Imaging Spectrometer infrared mapping subsystem (VIRTIS-M-IR) on Venus Express. These results show many significant long- and short-term variations from the mean equator-to-pole increasing trend previously found from earlier Earth- and space-based observations, including a possible North-South dichotomy, with interesting implications for the dynamics and chemistry of the lower atmosphere of Venus.     

The effect of surface topography on the lunar photoelectron sheath and electrostatic dust transport

The dayside near-surface lunar plasma environment is electrostatically complex, due to the interaction between solar UV-induced photoemission, the collection of ambient ions and electrons, and the presence of micron and sub-micron sized dust grains. Further complicating this environment, although less well understood in effect, is the presence of surface relief, typically in the form of craters and/or boulders. It has been suggested that such non-trivial surface topography can lead to complex electrostatic potentials and fields, including “mini-wakes” behind small obstacles to the solar wind flow and “supercharging” near sunlit-shadowed boundaries (Criswell, D.R., De, B.R. [1977]. J. Geophys. Res. 82 (7); De, B.R., Criswell, D.R. [1977]. J. Geophys. Res. 82 (7); Farrell, W.M., Stubbs, T.J., Vondrak, R.R., Delory, G.T., Halekas, J.S. [2007]. Geophys. Res. Lett. 34; Wang, X., Horányi, M., Sternovsky, Z., Robertson, S., Morfill, G.E. [2007]. Geophys. Res. Lett. 34, L16104). In this paper, we present results from a three-dimensional, self-consistent, electrostatic particle-in-cell code used to model the dayside near-surface lunar plasma environment over a variety of local times with the presence of a crater. Additionally, we use the particle-in-cell model output to study the effect of surface topography on the dynamics of electrostatic dust transport, with the goal of understanding previous observations of dust dynamics on the Moon and dust ponding on various asteroids.