Description: Publication date: Available online 4 December 2015 Source: Planetary and Space Science Author(s): M. Connors, C.T. Russell, H.R. Lai Potentially hazardous asteroid Oljato has a very eccentric low-inclination orbit of semimajor axis a 2.17 au, placing it just outside 4:1 resonance with Jupiter. Its association with magnetic field anomalies known as Interplanetary Field Enhancements (IFEs) in the solar wind led to speculation of a cometary nature and origin. Spectroscopic work showed that it was instead of silicate E-type typical of the inner asteroid belt or Hungarias. We have investigated the region potentially subject to 4:1 resonant effects and find that resonant pumping of eccentricity e takes place due to the outer planets, with moderate increases in inclination i in non-ejected cases. The outer planets do not, however, cause a change sufficient to move Oljato to its present location from the resonance. With inner planet effects included, the increase in e and i is in most cases reduced, however a diffusion increases, so that such a pumping/scattering mechanism can explain the present orbit of Oljato. IFEs may plausibly be related to a debris cascade involving secondary material along Oljato's orbit. We investigate the dynamics of such inferred meteoroids, finding that planetary encounters cause gaps in their distribution along the orbit. The control case of Eros confirms that encounters are needed to cause the gaps, with slow diffusion of secondary material in their absence.

Description: Publication date: Available online 4 December 2015 Source: Planetary and Space Science Author(s): Piotr A. Dybczyński, Małgorzata Królikowska We report the current status of the on-going project aimed at advancing our understanding of a source or sources of the actual long-period comets (LPCs). For the last several years we have developed several new methods and numerical packages to study in detail well-observed LPCs motion. Our main goal is to increase the thoroughness and precision of each step, starting from a sophisticated astrometric observation treatment through osculating orbit determination with different dynamical models, including different formulations of non-gravitational (NG) forces and, if necessary, adjusting the observational interval to obtain the most accurate past (original) and future orbits. Then we trace LPCs motion for one orbital period backward and forward. In this last step, we fully take into account Galactic perturbations as well as the gravitational influence of all known potential stellar perturbers acting during the relevant time interval around present time. At each step, we carefully propagate observational uncertainties by means of replacing each comet with a swarm of thousands of randomly generated virtual comets, all fully satisfying observational constraints. At the current stage of the project, we have determined osculating orbits for over 100 LPCs, some of them in several different variants. We carefully chose an appropriate osculating orbit variant for past and future motion studies and follow numerically LPCs motion up to a distance of 250 au from the Sun, obtaining original and future orbits. To study their motion further, we selected over 90 stars as potential perturbers and included their influence during the numerical integration of cometary motion. Our computer tools are fully prepared to use more stellar data, e.g. from the Gaia mission. We already noticed several important facts: (1) Including NG effects in the process of osculating orbit determination improves significantly our knowledge on cometary past and future motion. (2) In the case of well-observed comets with long periods covered with astrometric data it can be fruitful to obtain original or future orbits not from the whole set of observations but from shorter arcs, e.g. to exclude observations close to the perihelion, where violent NG effects can disturb a comet motion. (3) Taking into account the observational uncertainties for 1/a-distribution of original/future orbits, we produce a detailed shape of the famous ’Oort spike’ that fully reflects observational constraints. (4) We found that the significant percentage of LPCs have their previous perihelia deep in the planetary region – as a result one cannot treat them as ‘new comets’ since they experienced both planetary perturbations and solar radiation heating at least during their previous perihelion passage. (5) The widely used concept of the Jupiter-Saturn barrier should be revised since significant number of LPCs can migrate through it without any significant orbital changes. (6) None of the known stars have changed dynamical status of any of studied orbits of LPCs significantly.

Description: Publication date: Available online 3 December 2015 Source: Planetary and Space Science Author(s): J. Varela, F. Pantellini, M. Moncuquet The aim of this study is to simulate the interaction of the solar wind with the Hermean magnetosphere when the interplanetary magnetic field is weak, performing a parametric study for all the range of hydrodynamic values of the solar wind predicted on Mercury for the ENLIL + GONG WSA + Cone SWRC model: density from 12 to 180 cm −3 , velocity from 200 to 500 km/s and temperatures from 2 · 10 4 to 18 · 10 4 K, and compare the results with a real MESSENGER orbit as reference case. We use the code PLUTO in spherical coordinates and an asymmetric multipolar expansion for the Hermean magnetic field. The study shows for all simulations a stand off distance larger than the Mercury radius and the presence of close magnetic field lines on the day side of the planet, so the dynamic pressure of the solar wind is not high enough to push the magnetopause on the planet surface if the interplanetary magnetic field is weak. The simulations with large dynamic pressure lead to a large compression of the Hermean magnetic field modifying its topology in the inner magnetosphere as well as the plasma flows from the magnetosheath towards the planet surface.

Description: Publication date: Available online 3 December 2015 Source: Planetary and Space Science Author(s): L.H. Regoli, E. Roussos, M. Feyerabend, G.H. Jones, N. Krupp, A.J. Coates, S. Simon, U. Motschmann, M.K. Dougherty We study how the local electromagnetic disturbances introduced by Titan affect the ionization rates of the atmosphere. For this, we model the precipitation of energetic particles, specifically hydrogen and oxygen ions with energies between 1 keV and 1 MeV, into Titan's exobase for the specific magnetospheric configuration of the T9 flyby. For the study, a particle tracing software package is used which consists of an integration of the single particle Lorentz force equation using a 4 th order Runge-Kutta numerical method. For the electromagnetic disturbances, the output of the A.I.K.E.F. hybrid code (kinetic ions, fluid electrons) is used, allowing the possibility of analyzing the disturbances and asymmetries in the access of energetic particles originated by their large gyroradii. By combining these methods, 2D maps showing the access of each set of particles were produced. We show that the access of different particles is largely dominated by their gyroradii, with the complexity of the maps increasing with decreasing gyroradius, due to the larger effect that local disturbances introduced by the presence of the moon have in the trajectory of the particles with lower energies. We also show that for particles with gyroradii much larger than the moon's radius, simpler descriptions of the electromagnetic environment can reproduce similar results to those obtained when using the full hybrid simulation description, with simple north-south fields being sufficient to reproduce the hybrid code results for O + ions with energies larger than 10 keV but not enough to reproduce those for H + ions at any of the energies covered in the present study. Finally, by combining the maps created with upstream plasma flow measurements by the MIMI/CHEMS instrument, we are able to estimate normalized fluxes arriving at different selected positions of the moon's exobase. We then use these fluxes to calculate energy deposition and non-dissociative N 2 ionization rates for precipitating O + and H + ions and find differences in the ion production rates of up to almost 80% at the selected positions. All these results combined show that the electromagnetic field disturbances present in the vicinity of Titan significantly affect the contribution of energetic ions to local ionization profiles.

Description: Publication date: Available online 3 December 2015 Source: Planetary and Space Science Author(s): Dean C. Hines, Anny-Chantal Levasseur-Regourd Polarimetric observations of comets have provided crucial insight into the composition and evolution of cometary dust particles. Herein, we present a brief overview of the polarization properties observed in comets, and some possible interpretations. We also discuss recent imaging polarimetry observations of C/2012 S1 (ISON) and 67 P/Churyumov-Gerasimenko using the Hubble Space Telescope . The observations of 67 P/Churyumov-Gerasimenko are of particular interest, as they were timed to be contemporary with the initial rendezvous of Rosetta and the subsequent landing of the probe Philae. We also outline some unanswered questions and future developments that will greatly enhance our ability to further leverage the power of polarimetry for cometary research.

Description: Publication date: Available online 3 December 2015 Source: Planetary and Space Science Author(s): E Kallio, S. Dyadechkin, S. Fatemi, M. Holmström, Y. Futaana, P. Wurz, V.A. Fernandes, F. Álvarez, J. Heilimo, R. Jarvinen, W. Schmidt, A.-M. Harri, S. Barabash, J. Mäkelä, N. Porjo, M. Alho The study of dust above the lunar surface is important for both science and technology. Dust particles are electrically charged due to impact of the solar radiation and the solar wind plasma and, therefore, they affect the plasma above the lunar surface. Dust is also a health hazard for crewed missions because micron and sub-micron sized dust particles can be toxic and harmful to the human body. Dust also causes malfunctions in mechanical devices and is therefore a risk for spacecraft and instruments on the lunar surface. Properties of dust particles above the lunar surface are not fully known. However, it can be stated that their large surface area to volume ratio due to their irregular shape, broken chemical bonds on the surface of each dust particle, together with the reduced lunar environment cause the dust particles to be chemically very reactive. One critical unknown factor is the electric field and the electric potential near the lunar surface. We have developed a modelling suite, Dusty Plasma Environments: near-surface characterization and Modelling (DPEM), to study globally and locally dust environments of the Moon and other airless bodies. The DPEM model combines three independent kinetic models: (1) a 3D hybrid model, where ions are modelled as particles and electrons are modelled as a charged neutralizing fluid, (2) a 2D electrostatic Particle-in-Cell (PIC) model where both ions and electrons are treated as particles, and (3) a 3D Monte Carlo (MC) model where dust particles are modelled as test particles. The three models are linked to each other unidirectionally; the hybrid model provides upstream plasma parameters to be used as boundary conditions for the PIC model which generates the surface potential for the MC model. We have used the DPEM model to study properties of dust particles injected from the surface of airless objects such as the Moon, the Martian moon Phobos and the asteroid RQ36. We have performed a (vo, m/q)-phase space study where the property of dust particles at different initial velocity (vo) and initial mass per charge (m/q) ratio were analysed. The study especially identifies regions in the phase space where the electric field within a non-quasineutral plasma region above the surface of the object, the Debye layer, becomes important compared with the gravitational force. Properties of the dust particles in the phase space region where the electric field plays an important role are studied by a 3D Monte Carlo model. The current DPEM modelling suite does not include models of how dust particles are initially injected from the surface. Therefore, the presented phase space study cannot give absolute 3D dust density distributions around the analysed airless objects. For that, an additional emission model is necessary, which determines how many dust particles are emitted at various places on the analysed (vo, m/q)-phase space. However, this study identifies phase space regions where the electric field within the Debye layer plays an important role for dust particles. Overall, the initial results indicate that when a realistic dust emission model is available, the unified lunar based DPEM modelling suite is a powerful tool to study globally and locally the dust environments of airless bodies such as planetary moons, Mercury, asteroids and non-active comets far from the Sun.

Description: Publication date: Available online 3 December 2015 Source: Planetary and Space Science Author(s): Timothy A. Kral, Timothy H. Goodhart, Joshua D. Harpool, Christopher E. Hearnsberger, Graham L. McCracken, Stanley W. McSpadden In 2008, the Mars Phoenix Lander discovered perchlorate at its landing site, and in 2012, the Curiosity Rover confirmed the presence of perchlorate on Mars. The research reported here was designed to determine if certain methanogens could grow in the presence of three different perchlorate salt solutions. The methanogens tested were Methanothermobacter wolfeii , Methanosarcina barkeri , Methanobacterium formicicum and Methanococcus maripaludis . Media were prepared containing 0, 0.5%, 1.0%, 2%, 5% and 10% wt/vol magnesium perchlorate, sodium perchlorate, or calcium perchlorate. Organisms were inoculated into their respective media followed by incubation at each organism’s growth temperature. Methane production, commonly used to measure methanogen growth, was measured by gas chromatography of headspace gas samples. Methane concentrations varied with species and perchlorate salt tested. However, all four methanogens produced substantial levels of methane in the presence of up to 1.0% perchlorate, but not higher. The standard procedure for growing methanogens typically includes sodium sulfide, a reducing agent, to reduce residual molecular oxygen. However, the sodium sulfide may have been reducing the perchlorate, thus allowing for growth of the methanogens. To investigate this possibility, experiments were conducted where stainless steel nails were used instead of sodium sulfide as the reducing agent. Prior to the addition of perchlorate and inoculation, the nails were removed from the liquid medium. Just as in the prior experiments, the methanogens produced methane at comparable levels to those seen with sodium sulfide as the reductant, indicating that sodium sulfide did not reduce the perchlorate to any significant extent. Additionally, cells metabolizing in 1% perchlorate were transferred to 2%, cells metabolizing in 2% were transferred to 5%, and finally cells metabolizing in 5% were transferred to 10%. All four species produced methane at 2% and 5%, but not 10% indicating some success in adapting cells to concentrations higher than 1%. The results reported here indicate that the presence of perchlorate on Mars does not rule out the possible existence of methanogens.

Description: Publication date: Available online 2 December 2015 Source: Planetary and Space Science Author(s): Vasilij G. Shevchenko, Irina N. Belskaya, Karri Muinonen, Antti Penttilä, Yurij N. Krugly, Feodor P. Velichko, Vasilij G. Chiorny, Ivan G. Slyusarev, Ninel M. Gaftonyuk, Igor A. Tereschenko We present new observational data for selected main-belt asteroids of different compositional types. The detailed magnitude-phase dependencies including small phase angles (〈1 deg) were obtained for these asteroids, namely: (10) Hygiea (down to the phase angle of 0.3 deg, C-type), (176) Iduna (0.2 deg, G-type), (214) Aschera (0.2 deg, E-type), (218) Bianca (0.3 deg, S-type), (250) Bettina (0.3 deg, M-type), (419) Aurelia (0.1 deg, F-type), (596) Scheila (0.2 deg, D-type), (635) Vundtia (0.2 deg, B-type), (671) Carnegia (0.2 deg, P-type), (717) Wisibada (0.1 deg, T-type), (1021) Flammario (0.6 deg, B-type), and (1279) Uganda (0.5 deg, E-type). For several asteroids, the dependencies of brightness on the phase angle were investigated in the BVRI bands. We found a great diversity in the opposition-effect behavior both in the magnitude and the width of the opposition surges, especially for low-albedo asteroids. Some low-albedo asteroids (e.g., (10) Hygiea) display a broad opposition effect with an amplitude of 0.15–0.20 mag relative to the extrapolation of the linear part of the phase curve. Other asteroids (e.g., (596) Scheila, (1021) Flammario) show linear magnitude-phase dependencies down to small phase angles (0.1–0.2 deg). Using numerous data sets on the magnitude-phase dependencies with extensive phase-angle coverage, we examined in more detail the new three-parameter H , G 1 , G 2 magnitude system. We determined the values of the G 1 and G 2 parameters for magnitude phase dependences of individual asteroids and obtained the average parameters for main asteroid compositional types. The values obtained can be used for the estimation of the absolute magnitude of an asteroid from a single observed magnitude when the magnitude-phase dependency is unknown and/or to calculate a visible magnitude for the ephemerides.

Description: Publication date: Available online 27 November 2015 Source: Planetary and Space Science Author(s): Paolo Tanga, François Mignard, Aldo Dell'Oro, Karri Muinonen, Thierry Pauwels, William Thuillot, Jérôme Berthier, Alberto Cellino, Daniel Hestroffer, Jean-Marc Petit, Benoit Carry, Pedro David, Marco Delbo', Grigori Fedorets, Laurent Galluccio, Mikael Granvik, Christophe Ordenovic, Hanna Pentikäinen The Gaia mission started its regular observing program in the summer of 2014, and since then it is regularly obtaining observations of asteroids. This paper draws the outline of the data processing for Solar System objects, and in particular on the daily “short-term” processing, from the on-board data acquisition to the ground-based processing. We illustrate the tools developed to compute predictions of asteroid observations, we discuss the procedures implemented by the daily processing, and we illustrate some tests and validations of the processing of the asteroid observations. Our findings are overall consistent with the expectations concerning the performances of Gaia and the effectiveness of the developed software for data reduction.

Description: Publication date: Available online 28 November 2015 Source: Planetary and Space Science Author(s): Jialong Lai, Yi Xu, Xiaoping Zhang, Zesheng Tang Geological structure of the subsurface of the Moon provides valuable information on lunar evolution. Recently, Chang׳E-3 has utilized lunar penetrating radar (LPR), which is equipped on the lunar rover named as Yutu, to detect the lunar geological structure in Northern Imbrium (44.1260N, 19.5014W) for the first time. As an in situ detector, Chang׳E-3 LPR has relative higher horizontal and vertical resolution and less clutter impact compared to spaceborne radars and earth-based radars. In this work, we analyze the LPR data at 500 MHz transmission frequency to obtain the shallow subsurface structure of the landing area of Chang׳E-3 in Mare Imbrium. Filter method and amplitude recovery algorithms are utilized to alleviate the adverse effects of environment and system noises and compensate the amplitude losses during signal propagation. Based on the processed radar image, we observe numerous diffraction hyperbolae, which may be caused by discrete reflectors beneath the lunar surface. Hyperbolae fitting method is utilized to reverse the average dielectric constant to certain depth ( ε ¯ ). Overall, the estimated ε ¯ increases with the depth and ε ¯ could be classified into three categories. Average ε ¯ of each category is 2.47, 3.40 and 6.16, respectively. Because of the large gap between the values of ε ¯ of neighboring categories, we speculate a three-layered structure of the shallow surface of LPR exploration region. One possible geological picture of the speculated three-layered structure is presented as follows. The top layer is weathered layer of ejecta blanket with its average thickness and bound on error is 0.95±0.02 m. The second layer is the ejecta blanket of the nearby impact crater, and the corresponding average thickness is about 2.30±0.07 m, which is in good agreement with the two primary models of ejecta blanket thickness as a function of distance from the crater center. The third layer is regarded as a mixture of stones and soil. The echoes below the third layer are in the same magnitude as the noises, which may indicate that the fourth layer, if it exists, is uniform (no clear reflector) and its thickness is beyond the detection limit of LPR. Hence, we infer the fourth layer is a basalt layer.