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  • 1. Brain, D.
    et al.
    Barabash, S.
    Boesswetter, A.
    Bougher, S.
    Brecht, S.
    Chanteur, G.
    Hurley, D.
    Dubinin, E.
    Fang, X.
    Fraenz, M.
    Halekas, J.
    Harnett, E.
    Holmström, M.
    Kallio, E.
    Lammer, H.
    Ledvina, S.
    Liemohn, M.
    Liu, K.
    Luhmann, J.
    Ma, Y.
    Modolo, Ronan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Nagy, A.
    Motschmann, U.
    Nilsson, H.
    Shinagawa, H.
    Simon, S.
    Terada, N.
    A comparison of global models for the solar wind interaction with Mars2010In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 206, no 1, p. 139-151Article in journal (Refereed)
    Abstract [en]

    We present initial results from the first community-wide effort to compare global plasma interaction model results for Mars. Seven modeling groups participated in this activity, using MHD, multi-fluid, and hybrid assumptions in their simulations. Moderate solar wind and solar EUV conditions were chosen, and the conditions were implemented in the models and run to steady state. Model output was compared in three ways to determine how pressure was partitioned and conserved in each model, the location and asymmetry of plasma boundaries and pathways for planetary ion escape, and the total escape flux of planetary oxygen ions. The two participating MHD models provided similar results, while the five sets of multi-fluid and hybrid results were different in many ways. All hybrid results, however, showed two main channels for oxygen ion escape (a pickup ion 'plume' in the hemisphere toward which the solar wind convection electric field is directed, and a channel in the opposite hemisphere of the central magnetotail), while the MHD models showed one (a roughly symmetric channel in the central magnetotail). Most models showed a transition from an upstream region dominated by plasma dynamic pressure to a magnetosheath region dominated by thermal pressure to a low altitude region dominated by magnetic pressure. However, calculated escape rates for a single ion species varied by roughly an order of magnitude for similar input conditions, suggesting that the uncertainties in both the current and integrated escape over martian history as determined by models are large. These uncertainties are in addition to those associated with the evolution of the Sun, the martian dynamo, and the early atmosphere, highlighting the challenges we face in constructing Mars' past using models.

  • 2. Cravens, T. E.
    et al.
    Robertson, I. P.
    Waite, J. H., Jr.
    Yelle, R. V.
    Vuitton, V.
    Coates, A. J.
    Wahlund, Jan Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Ågren, Karin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Richard, M. S.
    De La Haye, V.
    Wellbrock, A.
    Neubauer, F. M.
    Model-data comparisons for Titan's nightside ionosphere2009In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 199, no 1, p. 174-188Article in journal (Refereed)
    Abstract [en]

    Solar and X-ray radiation and energetic plasma from Saturn's magnetosphere interact with the upper atmosphere producing an ionosphere at Titan. The highly coupled ionosphere and upper atmosphere system mediates the interaction between Titan and the external environment. A model of Titan's nightside ionosphere will be described and the results compared with data from the Ion and Neutral Mass Spectrometer (INMS) and the Langmuir probe (LP) part of the Radio and Plasma Wave (RPWS) experiment for the T5 and T21 nightside encounters of the Cassini Orbiter with Titan. Electron impact ionization associated with the precipitation of magnetospheric electrons into the upper atmosphere is assumed to be the source of the nightside ionosphere, at least for altitudes above 1000 km. Magnetospheric electron fluxes measured by the Cassini electron spectrometer (CAPS ELS) are used as an input for the model. The model is used to interpret the observed composition and structure of the T5 and T21 ionospheres. The densities of many ion species (e.g., CH5+ and C2H5+) measured during T5 exhibit temporal and/or spatial variations apparently associated with variations in the fluxes of energetic electrons that precipitate into the atmosphere from Saturn's magnetosphere.

  • 3.
    Davidsson, Björn J. R.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Gulkis, Samuel
    Alexander, Claudia
    von Allmen, Paul
    Kamp, Lucas
    Lee, Seungwon
    Warell, Johan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Gas kinetics and dust dynamics in low-density comet comae2010In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 210, no 1, p. 455-471Article in journal (Refereed)
    Abstract [en]

    Extensive regions of low-density cometary comae are characterized by important deviations from the Maxwell-Boltzmann velocity distribution, i.e. breakdown of thermodynamic equilibrium. The consequences of this on the shapes of emission and absorption lines, and for the acceleration of solid bodies due to gas drag, have rarely been investigated. These problems are studied here to aid in the development of future coma models, and in preparation for observations of Comet 67P/Churyumov-Gerasimenko from the ESA Rosetta spacecraft. Two topics in particular, related to Rosetta, are preparation for in situ observations of water, carbon monoxide, ammonia, and methanol emission lines by the mm/sub-mm spectrometer MIRO, as well as gas drag forces on dust grains and on the Rosetta spacecraft itself. Direct Simulation Monte Carlo (DSMC) modeling of H2O/CO mixtures in spherically symmetric geometries at various heliocentric distances are used to study the evolution of the (generally non-Maxwellian) velocity distribution function throughout the coma. Such distribution functions are then used to calculate Doppler broadening profiles and drag forces. It is found that deviation from thermodynamic equilibrium indeed is commonplace, and already at 2.5 AU from the Sun the entire comet coma displays manifestations of such breakdown, e.g., non-equal partitioning of energy between kinetic and rotational modes, causing substantial differences between translational and rotational temperatures. We exemplify how deviations from thermodynamic equilibrium affect the properties of Doppler broadened line profiles. Upper limits on the size of liftable dust grains as well as terminal grain velocities are presented. Furthermore, it is demonstrated that the drag-to-gravity force ratio is likely to decrease with decreasing cometocentric distance, which may be of relevance both for Rosetta and for the lander probe Philae.

  • 4.
    Davidsson, Björn J. R.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Gutierrez, J
    Rickman, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Physical properties of morphological units on Comet 9P/Tempel 1 derived from near-IR Deep Impact spectra2009In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 201, no 1, p. 335-357Article in journal (Refereed)
    Abstract [en]

    In this paper we analyze near-infrared thermal emission spectra of the spatially resolved nucleus Of Comet 9P/Tempel 1 obtained by the NASA spacecraft Deep Impact. Maps of spectral reddening. the product X' between the beaming function and directional emissivity, as well as Suit ace temperature are constructed. Thermophysical modeling is used to estimate the degree of small scale surface toughness and thermal inertia by detailed reproduction of the empirical temperature map. Mie and Hapke theories are Used in combination with numerically Calculated beaming functions to analyze the X' trial and place constraints oil composition and grain size of the Surface material. We show that it is absolutely mandatory to include small scale Surface roughness in thermophysical modeling of this object, since the resulting self treating is vital for reproducing the measured temperatures. A small scale self heating parameter in the range 0.6 <= xi <= 0.75 is common, but smoother areas where 0.2 <= xi <= 0.3 are also found. Contrary to models neglecting small scale surface roughness, we find that the thermal inertia of Comet 9P/Tempel 1 generally is high (1000-3000 J m(-1) K-1 s(-1/2)). although it may be substantially lower (40-380 Jm(-2) K-1 s(-1/2)) in specific areas. We obtain a disk-averaged reddening of 3.5% kA(-1), with statistically significant local variations around that value on a +/- 1.0% kA(-1) level. vast regions appear covered by small (similar to 0.1 mu m) highly absorbing grains such as carbon or iron-rich silicates. Other regions appear dominated by somewhat larger (similar to 0.5 mu m) and/or less absorbing grains such as troilite or magnesium-rich silicates. Surface variations in reddening, roughness, thermal inertia, composition and/or grain size are moderately to strongly correlated to the locations of morphological units oil the surface. The existence of morphological units with differing physical properties may be primordial. hence reflecting a diversity in the building block cometesimals, or resulting front evolutionary processes. (c) 2009 Elsevier Inc. All rights reserved.

  • 5.
    Davidsson, Björn J. R.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Gutierrez, Pedro J.
    Groussin, Olivier
    A'Hearn, Michael F.
    Farnham, Tony
    Feaga, Lori M.
    Kelley, Michael S.
    Klaasen, Kenneth P.
    Merlin, Frederic
    Protopapa, Silvia
    Rickman, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Sunshine, Jessica M.
    Thomas, Peter C.
    Thermal inertia and surface roughness of Comet 9P/Tempel 12013In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 224, no 1, p. 154-171Article in journal (Refereed)
    Abstract [en]

    Re-calibrated near-infrared spectroscopy of the resolved nucleus of Comet 9P/Tempel 1 acquired by the Deep Impact spacecraft has been analyzed by utilizing the post-Stardust-NExT nucleus shape model and spin pole solution, as well as a novel thermophysical model that explicitly accounts for small-scale surface roughness and thermal inertia. We find that the thermal inertia varies measurably across the surface, and that thermal emission from certain regions only can be reproduced satisfactory if surface roughness is accounted for. Particularly, a scarped/pitted terrain that experienced morning sunrise during the flyby is measurably rough (Hapke mean slope angle similar to 45 degrees) and has a thermal inertia of at most 50J m(-2) K-1 s(-1/2), but probably much lower. However, thick layered terrain and thin layered terrain experiencing local noon during the flyby have a substantially larger thermal inertia, reaching 150J m(-2) K-1 s(-1/2) if the surface is as rough as the scarped/pitted terrain, but 200J m(-2) K-1 s(-1/2) if the terrain is considered locally flat. Furthermore, the reddening of the nucleus near-infrared 1.5-2.2 gm spectrum varies between morphological units, being reddest for thick layered terrain (median value 3.4% k angstrom(-1)) and most neutral for the smooth terrain known to contain surface water ice (median value 3.1% k angstrom(-1)). Thus, Comet 9P/Tempel 1 is heterogeneous in terms of both thermophysical and optical properties, due to formation conditions and/or post-formation processing. 

  • 6.
    Davidsson, Björn J. R.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics.
    Gutiérrez, Pedro J.
    Rickman, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics.
    Nucleus properties of Comet 9P/Tempel 1 estimated from non-gravitational force modeling2007In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 187, no 1, p. 306-320Article in journal (Refereed)
    Abstract [en]

    The nucleus mass and bulk density of Comet 9P/Tempel 1 have been estimated by utilizing the non-gravitational force modeling technique. Here, the water production rates and non-gravitational perturbations of the orbit are calculated for a large number of model nuclei with different surface ice distribution patterns. By requiring that the empirical water production rate curve is reproduced, a subset of model nuclei are selected, for which masses are calculated by demanding that empirical non-gravitational changes of the orbital period and in the longitude of perihelion (per revolution) are reproduced. We obtain a mass M=5.8(±1.6)×1013 kg, and a bulk density , which compares very well with measurements made by the Deep Impact Science Team. The main goal of the current work is therefore to demonstrate functionality of an indirect method, i.e., mass estimation through non-gravitational force modeling, by comparing such results to ground truth data. Furthermore, the thermal inertia of active areas is estimated as 30–100 MKS, using a comparatively realistic thermophysical model (although a value in the range 100–350 MKS is obtained with a more simple model). An active area fraction of 3% is predicted, and these areas are probably confined to the northern hemisphere, being located close to the cometary equator.

  • 7.
    Davidsson, Björn J. R.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Rickman, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Surface roughness and three-dimensional heat conduction in thermophysical models2014In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 243, p. 58-77Article in journal (Refereed)
    Abstract [en]

    A thermophysical model is presented that considers surface roughness, cast shadows, multiple or single scattering of radiation, visual and thermal infrared self heating, as well as heat conduction in one or three dimensions. The code is suitable for calculating infrared spectral energy distributions for spatially resolved or unresolved minor Solar System bodies without significant atmospheres or sublimation, such as the Moon, Mercury, asteroids, irregular satellites or inactive regions on comet nuclei. It is here used to explore the effects of surface roughness on spatial scales small enough for heat conduction to erase lateral temperature gradients. Analytically derived corrections to one-dimensional models that reproduce the results of three-dimensional modeling are presented. We find that the temperature of terrains with such small-scale roughness is identical to that of smooth surfaces for certain types of topographies and non-scattering material. However, systematic differences between smooth and rough terrains are found for scattering materials, or topographies with prominent positive relief. Contrary to common beliefs, the roughness on small spatial scales may therefore affect the thermal emission of Solar System bodies.

  • 8.
    Davidsson, Björn J. R.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Rickman, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Bandfield, Joshua L.
    Groussin, Olivier
    Gutierrez, Pedro J.
    Wilska, Magdalena
    Capria, Maria Teresa
    Emery, Joshua P.
    Helbert, Joern
    Jorda, Laurent
    Maturilli, Alessandro
    Mueller, Thomas G.
    Interpretation of thermal emission. I. The effect of roughness for spatially resolved atmosphereless bodies2015In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 252, p. 1-21Article in journal (Refereed)
    Abstract [en]

    Spacecraft observations of atmosphereless Solar System bodies, combined with thermophysical modeling, provide important information about the thermal inertia and degree of surface roughness of these bodies. The thermophysical models rely on various methods of generating topography, the most common being the concave spherical segment. We here compare the properties of thermal emission for a number of different topographies - concave spherical segments, random Gaussians, fractals and parallel sinusoidal trenches - for various illumination and viewing geometries, degrees of surface roughness and wavelengths. We find that the thermal emission is strongly dependent on roughness type, even when the degrees of roughness are identical, for certain illumination and viewing geometries. The systematic usage of any single topography model may therefore bias determinations of thermal inertia and level of roughness. We outline strategies that may be employed during spacecraft observations to disentangle thermal inertia, level of roughness and type of topography. We also compare the numerically complex and time consuming full-scale thermophysical models with a simplified statistical approach, which is fairly easy to implement and quick to run. We conclude that the simplified statistical approach is similar to thermophysical models for cases tested here, which enables the user to analyze huge amounts of spectral data at a low numerical cost.

  • 9. DeMeo, Francesca E.
    et al.
    Binzel, Richard P.
    Lockhart, Matthew
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Mars encounters cause fresh surfaces on some near-Earth asteroids2014In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 227, p. 112-122Article in journal (Refereed)
    Abstract [en]

    All airless bodies are subject to the space environment, and spectral differences between asteroids and meteorites suggest many asteroids become weathered on very short (<1 Myr) timescales. The spectra of some asteroids, particularly Q-types, indicate surfaces that appear young and fresh, implying they have been recently been exposed. Previous work found that Earth encounters were the dominant freshening mechanism and could be responsible for all near-Earth object (NEO) Q-types. In this work we increase the known NEO Q-type sample of by a factor of three. We present the orbital distributions of 64 Q-type near-Earth asteroids, and seek to determine the dominant mechanisms for refreshing their surfaces. Our sample reveals two important results: (i) the relatively steady fraction of Q-types with increasing semi-major axis and (ii) the existence of Q-type near-Earth asteroids with Minimum Orbit Intersection Distances (MOID) that do not have orbit solutions that cross Earth. Both of these are evidence that Earth-crossing is not the only scenario by which NEO Q-types are freshened. The high Earth-MOID asteroids represent 10% of the Q-type population and all are in Amor orbits. While surface refreshing could also be caused by Main Belt collisions or mass shedding from YORP spinup, all high Earth-MOID Q-types have the possibility of encounters with Mars indicating Mars could be responsible for a significant fraction of NEOs with fresh surfaces.

  • 10. Domingue, Deborah L.
    et al.
    Vilas, Faith
    Holsclaw, Gregory M.
    Warell, Johan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Izenberg, Noam R.
    Murchie, Scott L.
    Denevi, Brett W.
    Blewett, David T.
    McClintock, William E.
    Anderson, Brian J.
    Sarantos, Menelaos
    Whole-disk spectrophotometric properties of Mercury: Synthesis of MESSENGER and ground-based observations2010In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 209, no 1, p. 101-124Article in journal (Refereed)
    Abstract [en]

    Disk-integrated and disk-resolved measurements of Mercury's surface obtained by both the Mercury Dual Imaging System (MDIS) and the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) onboard the MErcury Surface. Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft were analyzed and compared with previous ground-based observations of Mercury at 11 wavelengths The spectra show no definitive absorption features and display a red spectral slope (increasing reflectance with increasing wavelength) typical of space-weathered rocky sui faces The MDIS spectra show evidence of phase reddening, which is not observed in the MASCS spectra The MDIS spectra are commensurate with ground-based observations to within 10%, whereas the MASCS spectra display greater discrepancies with ground-based observations at near-infrared wavelengths The derived photometric calibrations provide corrections within 10% for observations taken at phase angles less than similar to 100 degrees The derived photometric properties are indicative of a more compact regolith than that of the lunar surface or of average S-type asteroids The photometric roughness of the cur face is also much smoother than the Moon's The calculated geometric albedo (reflectance at zero phase) is higher than lunar values The lower reflectance of immature units on Mercury compared with immature units Oil the Moon, in conjunction with the higher geometric albedo, is indicative of more complicated grain structures within Mercury's regolith.

  • 11. Farrell, W. M.
    et al.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Morooka, Michiko
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Gurnett, D. A.
    Kurth, W. S.
    MacDowall, R. J.
    An estimate of the dust pickup current at Enceladus2014In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 239, p. 217-221Article in journal (Refereed)
    Abstract [en]

    We demonstrate that the acceleration of submicron dust originating at Enceladus by a reduced co-rotating E-field is capable of creating a dust pickup current perpendicular to the magnetic field with values ranging from 3 to 15 kA (depending upon the effective grain charge). Such a current represents a new contribution to the total pickup current in the region. As such, we suggest that dust pickup currents, along with ion and electron pickup currents, are all active within the plume.

  • 12. Farrell, William M.
    et al.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Morooka, Michiko
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Gurnett, Donald A.
    Kurth, William S.
    MacDowall, Robert J.
    The electromagnetic pickup of submicron-sized dust above Enceladus's northern hemisphere2012In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 219, no 1, p. 498-501Article in journal (Refereed)
    Abstract [en]

    As the saturnian magnetoplasma sweeps past Enceladus, it experiences both a decrease in electron content and sharp slowdown in the northern hemisphere region within similar to 5 Enceladus Radii (R-e). This slowdown is observed by Cassini in regions not obviously associated with the southern directed plume-originating ions. We suggest herein that the decrease in northern hemisphere electron content and plasma slowdown could both be related to the presence of fine dust grains that are being accelerated by the Lorentz force created within the saturnian magnetic field system.

  • 13. Fouchard, M.
    et al.
    Froeschle, Ch
    Rickman, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Valsecchi, G. B.
    The key role of massive stars in Oort cloud comet dynamics2011In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 214, no 1, p. 334-347Article in journal (Refereed)
    Abstract [en]

    The effects of a sample of 1300 individual stellar encounters spanning a wide range of parameter values (mass, velocity and encounter distance) are investigated. Power law fits for the number of injected comets demonstrate the long range effect of massive stars, whereas light stars affect comets mainly along their tracks. Similarly, we show that the efficiency of a star to fill the phase space region of the Oort cloud where the Galactic tides are able to inject comets into the observable region - the so-called "tidally active zone" (TAZ) - is also strongly dependent on the stellar mass. Power laws similar to those for direct injection are obtained for the efficiency of stars to fill the TAZ. This filling of the "tidally active zone is crucial for the long term flux of comets from the Oort cloud. Based on long-term Monte Carlo simulations using a constant Galactic tide and a constant flux of stellar encounters, but neglecting the detailed effects of planetary perturbations, we show that this flux essentially results from a two step mechanism: (i) the stellar injection of comets into the TAZ; and (ii) the tidal injection of TAZ comets into the loss cone. We find that single massive stars are able to induce "comet drizzles" - corresponding to an increase of the cometary flux of about 40% - which may last for more than 100 Myr by filling the TAZ to a higher degree than normal. It appears that the stars involved in this process are the same that cause comet showers.

  • 14.
    Fouchard, M.
    et al.
    Univ Lille, UPMC Univ Paris 06, Sorbonne Univ, PSL Res Univ,LAL IMCCE,Observ Paris,CNRS, 1 Impasse Observ, F-59000 Lille, France..
    Rickman, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy. PAS Space Res Ctr, Bartycka 18A, PL-00716 Warsaw, Poland..
    Froeschle, Ch.
    Observ Cote Azur, UMR 7293, Blvd Observ,BP 4229, F-06304 Nice 4, France..
    Valsecchi, G. B.
    IAPS INAF, Via Fosso Cavaliere 100, I-00133 Rome, Italy.;IFAC CNR, Via Madonna del Piano 10, I-50019 Sesto Fiorentino, FI, Italy..
    On the present shape of the Oort cloud and the flux of "new" comets2017In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 292, p. 218-233Article in journal (Refereed)
    Abstract [en]

    Long term evolution of an initial set of 10(7) Oort cloud comets is performed for the age of the solar system taking into account the action of passing stars using 10 different sequences of stellar encounters, Galactic tides and the gravity of the giant planets. The initial conditions refer to a disk-shaped Oort cloud precursor, concentrated toward the ecliptic with perihelia in the region of Uranus and Neptune. Our results show that the shape of the Oort cloud quickly reach a kind of steady state beyond a semi-major axis greater than about 2000 AU (this threshold depending on the evolution time-span), with a Boltzmann distribution of the orbital energy. The stars act in an opposite way to what was found in previous papers, that is they emptied an initial Tidal Active Zone that is overfilled with respect to the isotropic case. Consequently, the inclusion of stellar perturbations strongly affect the shape of the Oort spike. On the contrary, the Oort spike shape appears to be poorly dependent on the stellar sequences used, whereas the total flux of observable comets and the proportion of retrograde comets for the inner part of the spike are significantly dependent of it. Then it has been highlighted that the total flux, the shape of the Oort spike and the shape of the final Oort cloud are almost independent of the initial distribution of orbital energy considered.

  • 15. Fouchard, M.
    et al.
    Rickman, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Froeschle, Ch
    Valsecchi, G. B.
    Planetary perturbations for Oort Cloud comets. I. Distributions and dynamics2013In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 222, no 1, p. 20-31Article in journal (Refereed)
    Abstract [en]

    This paper is the first in a series, where we aim to model the injection of comets from the Oort Cloud so well that the shape of the energy distribution of long-period comets (i.e., the distribution of reciprocal semi-major axis) together with the observed rate of perihelion passages can be used to make serious inferences about the population size and energy distribution of the cloud. Here we explore the energy perturbations caused by the giant planets on long-period comets with perihelia inside or near the planetary system. We use a simplified dynamical model to integrate such perturbations for large samples of fictitious comets and analyse the statistics of the outcomes. After demonstrating the sensitivity of derived parameters to the sample size, when close encounters are involved, we derive a map of the RMS energy perturbation as a function of perihelion distance (q) and the cosine of the inclination (i), which compares well with the results of previous papers. We perform a critical analysis of the loss cone concept by deriving the "opacity" (chance of leaving the Oort spike by planetary perturbations per perihelion passage) as a function of q and cos i, concluding that the often made assumption of full opacity for q < 15 AU is seriously in error. While such a conclusion may also have been drawn from earlier studies, we provide the first full, quantitative picture. Moreover, we make a preliminary investigation of the long-term evolution of long-period comet orbits under the influence of planetary perturbations, neglecting the external effects of Galactic tides and stellar encounters. This allows us to make predictions about the production of decoupled objects like Halley-type comets and Centaurs from the injection of Oort Cloud comets, as well as of a related population of transneptunians deriving from the Oort Cloud with perihelia well detached from the planets.

  • 16. Fouchard, M.
    et al.
    Rickman, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Froeschle, Ch.
    Valsecchi, G. B.
    Planetary perturbations for Oort cloud comets: II. Implications for the origin of observable comets2014In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 231, p. 110-121Article in journal (Refereed)
    Abstract [en]

    We present Monte Carlo simulations of the dynamical history of the Oort cloud, where in addition to the main external perturbers (Galactic tides and stellar encounters) we include, as done in a companion paper (Fouchard, M., Rickman, H., Froeschle, Ch., Valsecchi, G.B. [2013b] Icarus, in press), the planetary perturbations experienced each time the comets penetrate to within 50 AU of the Sun. Each simulation involves an initial sample of four million comets and extends over a maximum of 5 Gyr. For better understanding of the outcomes, we supplement the full dynamical model by others, where one or more of the effects are left out. We concentrate on the production of observable comets, reaching for the first time a perihelion within 5 AU of the Sun. We distinguish between four categories, depending on whether the comet jumps across, or creeps through, the Jupiter-Saturn barrier (perihelion distances between 5 and 15 AU), and whether the orbit leading to the observable perihelion is preceded by a major planetary perturbation or not. For reasons explained in the paper, we call the strongly perturbed comets "Kaib-Quinn comets". We thus derive a synthetic picture of the Oort spike, from which we draw two main conclusions regarding the full dynamical model. One is that 2/3 of the observable comets are injected with the aid of a planetary perturbation at the previous perihelion passage, and about half of the observable comets are of the Kaib-Quinn type. The other is that the creepers dominate over the jumpers. Due to this fact, the spike peaks at only 31000 AU, and the majority of new comets have semi-major axes less than this value. The creepers show a clear preference for retrograde orbits as a consequence of the need to avoid untimely, planetary ejection before becoming observable. Thus, the new comets should have a 60/40 preference for retrograde against prograde orbits in apparent conflict with observations. However, both these and other results depend on our model assumptions regarding the initial structure of the Oort cloud, which is isotropic in shape and has a relatively steep energy distribution. We also find that they depend on the details of the past history of external perturbations including GMC encounters, and we provide special discussions of those issues.

  • 17. Fouchard, M.
    et al.
    Rickman, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Froeschle, Ch.
    Valsecchi, G. B.
    Planetary perturbations for Oort cloud comets: III. Evolution of the cloud and production of centaurs and Halley type comets2014In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 231, p. 99-109Article in journal (Refereed)
    Abstract [en]

    We present Monte Carlo simulations of the dynamical history of the Oort cloud, where in addition to the main external perturbers (Galactic tides and stellar encounters) we include, as done in a companion paper (Fouchard, M., Rickman, H., Froeschle, Ch., Valsecchi, G.B. [2013b] Icarus, in press), the planetary perturbations experienced each time the comets penetrate to within 50 AU of the Sun. Each simulation involves an initial sample of four million comets and extends over a maximum of 5 Gyr. For better understanding of the outcomes, we supplement the full dynamical model by others, where one or more of the effects are left out. In the companion paper we studied in detail how observable comets are injected from the Oort cloud, when account is taken of the planetary perturbations. In the present paper we concentrate on how the cloud may evolve in the long term and also on the production of decoupled comets, which evolve into semi-major axes less than 1000 AU. Concerning the long-term evolution, we find that the largest stellar perturbations that may statistically be expected during the age of the Solar System induce a large scale migration of comets within the cloud. Thus, comets leave the inner parts, but the losses from the outer parts are even larger, so at the end of our simulations the Oort cloud is more centrally condensed than at the beginning. The decoupled comets, which form a source of centaurs and Halley type comets (roughly in the proportions of 70% and 30%, respectively), are mainly produced by planetary perturbations, Jupiter and Saturn being the most efficient. This effect is dependent on synergies with the Galactic tide and stellar encounters, bringing the perihelia of Oort cloud comets into the planetary region. The star-planet synergy has a large contribution due to the strong encounters that produce major comet showers. However, outside these showers a large majority of decouplings may be attributed to the tide-planet synergy.

  • 18. Gutierrez, P
    et al.
    Davidsson, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics.
    Non-gravitational force modeling of Comet 81P/Wild 2: II. Rotational evolution2007In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 191, no 2, p. 651-664Article in journal (Refereed)
    Abstract [en]

    In this paper, we have studied both the dynamical and the rotational evolution of an 81P/Wild 2-like comet under the effects of the outgassing-induced force and torque. The main aim is to study if it is possible to reproduce the non-gravitational orbital changes observed in this comet, and to establish the likely evolution of both orbital and rotational parameters. To perform this study, a simple thermophysical model has been used to estimate the torque acting on the nucleus. Once the torque is calculated, Euler equations are solved numerically considering a nucleus mass directly estimated from the changes in the orbital elements (as determined from astrometry). According to these simulations, when the water production rate and changes in orbital parameters for 1997, as well as observational rotational parameters for 2004 are imposed as constraints, the change in the orbital period of 81P/Wild 2, ΔP=P˙, will decrease so that P¨=‑5 to ‑1minorbit, which is similar to the actual tendency observed from 1988 up to 1997. This nearly constant decreasing can be explained as due to a slight drift of the spin axis orientation towards larger ecliptic longitudes. After studying the possible spin axis orientations proposed for 1997, simulations suggest that the spin obliquity and argument (I,Φ)=(56°,167°) is the most likely. As for rotational evolution, changes per orbit smaller than 10% of the actual spin velocity are probable, while the most likely value corresponds to a change between 2 and 7% of the spin velocity. Equally, net changes in the spin axis orientation of 4° 8° per orbit are highly expected.

  • 19.
    Johnson, R. E.
    et al.
    Univ Virginia, Mat Sci & Engn, Charlottesville, VA 22902 USA;NYU, Dept Phys, 4 Washington Pl, New York, NY 10003 USA.
    Sundqvist, B. U. R.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Sputtering and detection of large organic molecules from Europa2018In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 309, p. 338-344Article in journal (Refereed)
    Abstract [en]

    Mass spectroscopy of bio-molecules by heavy ion induced sputtering, which became a practical laboratory procedure, was also suggested as a potential tool for spacecraft studies of targets of interest in astrobiology. With the planning of new missions to Europa, there is renewed interest in the possibility of detecting organic molecules that might have originated in its subsurface ocean and can be sputtered from its surface often intact by impacting energetic heavy ions trapped in Jupiter's magnetosphere. Here we review the laboratory data and modeling bearing on this issue. We then give estimates of the ejection into the gas-phase of trace organic species embedded in an ice matrix on Europa's surface and their possible detection during a flyby mission. 

  • 20.
    Jorda, L.
    et al.
    Univ Aix Marseille, Lab Astrophys Marseille, UMR7326, CNRS, 38 Rue Frederic Joliot Curie, F-13388 Marseille 13, France..
    Gaskell, R.
    Planetary Sci Inst, 1700 East Ft Lowell,Suite 106, Tucson, AZ 85719 USA..
    Capanna, C.
    Univ Aix Marseille, Lab Astrophys Marseille, UMR7326, CNRS, 38 Rue Frederic Joliot Curie, F-13388 Marseille 13, France..
    Hviid, S.
    Deutsch Zentrum Luft & Raumfahrt DLR, Inst Planetary Res, Rutherfordstr 2, D-12489 Berlin, Germany..
    Lamy, P.
    Univ Aix Marseille, Lab Astrophys Marseille, UMR7326, CNRS, 38 Rue Frederic Joliot Curie, F-13388 Marseille 13, France..
    Durech, J.
    Charles Univ Prague, Fac Math & Phys, Astron Inst, V Holesovickch 2, CR-18000 Prague, Czech Republic..
    Faury, G.
    AKKA Technol, 6 Rue Roger Camboulives, F-31100 Toulouse, France..
    Groussin, O.
    Univ Aix Marseille, Lab Astrophys Marseille, UMR7326, CNRS, 38 Rue Frederic Joliot Curie, F-13388 Marseille 13, France..
    Gutierrez, P.
    CSIC, Inst Astrofis Andalucia, Aptd 3004, E-18080 Granada, Spain..
    Jackman, C.
    KinetX Aerosp Inc, 21 W Easy St, Simi Valley, CA 93065 USA..
    Keihm, S. J.
    CALTECH, Jet Prop Lab, 4800 Oak Grove Dr, Pasadena, CA 91109 USA..
    Keller, H. U.
    Tech Univ Carolo Wilhelmina Braunschweig, Inst Geophys & Extraterr Phys, D-38106 Braunschweig, Germany..
    Knollenberg, J.
    Deutsch Zentrum Luft & Raumfahrt DLR, Inst Planetary Res, Rutherfordstr 2, D-12489 Berlin, Germany..
    Kuehrt, E.
    Deutsch Zentrum Luft & Raumfahrt DLR, Inst Planetary Res, Rutherfordstr 2, D-12489 Berlin, Germany..
    Marchi, S.
    Southwest Res Inst, 1050 Walnut St, Boulder, CO 80302 USA..
    Mottola, S.
    Deutsch Zentrum Luft & Raumfahrt DLR, Inst Planetary Res, Rutherfordstr 2, D-12489 Berlin, Germany..
    Palmer, E.
    Planetary Sci Inst, 1700 East Ft Lowell,Suite 106, Tucson, AZ 85719 USA..
    Schloerb, F. P.
    Univ Massachusetts, 619 Lederle Grad Res Tower, Amherst, MA 01003 USA..
    Sierks, H.
    Max Planck Inst Sonnensyst Forsch, Justus von Liebig Weg 3, D-37077 Gottingen, Germany..
    Vincent, J. -B
    A'Hearn, M. F.
    Univ Maryland, Dept Astron, College Pk, MD 20742 USA..
    Barbieri, C.
    Univ Padua, Dept Phys & Astron, Vicolo Osservatorio 3, I-35122 Padua, Italy..
    Rodrigo, R.
    CSIC INTA, Ctr Astrobiol, Madrid 28850, Spain.;Int Space Sci Inst, Hallerstr 6, CH-3012 Bern, Switzerland..
    Koschny, D.
    ESA RSSD, Keplerlaan 1,Postbus 299, NL-2201 AZ Noordwijk, Netherlands..
    Rickman, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics. PAS Space Res Ctr, Bartycka 18A, PL-00716 Warsaw, Poland..
    Barucci, M. A.
    Univ Paris Diderot, Univ Paris 06, CNRS, LESIA,Obs Paris, 5 Pl J Janssen, F-92195 Meudon, France..
    Bertaux, J. L.
    CNRS UVSQ IPSL, LATMOS, 11 Blvd Alembert, F-78280 Guyancourt, France..
    Bertini, I.
    Univ Padua, Ctr Ateneo Studi & Attivita Spaziali Giuseppe Col, Via Venezia 15, I-35131 Padua, Italy..
    Cremonese, G.
    Osserv Astron Padova, INAF, Vicolo Osservatorio 5, I-35122 Padua, Italy..
    Da Deppo, V.
    CNR IFN UOS Padova LUXOR, Via Trasea 7, I-35131 Padua, Italy..
    Davidsson, B.
    PAS Space Res Ctr, Bartycka 18A, PL-00716 Warsaw, Poland..
    Debei, S.
    Univ Padua, Dept Ind Engn, Via Venezia 1, I-35131 Padua, Italy..
    De Cecco, M.
    Univ Trento, Via Mesiano 77, I-38100 Trento, Italy..
    Fornasier, S.
    Univ Paris Diderot, Univ Paris 06, CNRS, LESIA,Obs Paris, 5 Pl J Janssen, F-92195 Meudon, France..
    Fulle, M.
    INAF, Osservatorio Astron, Via Tiepolo 11, I-34014 Trieste, Italy..
    Guettler, C.
    Max Planck Inst Sonnensyst Forsch, Justus von Liebig Weg 3, D-37077 Gottingen, Germany..
    Ip, W. -H
    Natl Cent Univ, Inst Space Sci, Chungli 32054, Taiwan.
    Kramm, J. R.
    Max Planck Inst Sonnensyst Forsch, Justus von Liebig Weg 3, D-37077 Gottingen, Germany..
    Kueppers, M.
    ESA ESAC, POB 78, Villanueva De La Caliada 28691, Spain..
    Lara, L. M.
    CSIC, Inst Astrofis Andalucia, Aptd 3004, E-18080 Granada, Spain..
    Lazzarin, M.
    Univ Padua, Dept Phys & Astron, Vicolo Osservatorio 3, I-35122 Padua, Italy..
    Lopez Moreno, J. J.
    CSIC, Inst Astrofis Andalucia, Aptd 3004, E-18080 Granada, Spain..
    Marzari, F.
    Univ Padua, Dept Phys & Astron, Vicolo Osservatorio 3, I-35122 Padua, Italy..
    Naletto, G.
    Univ Padua, Ctr Ateneo Studi & Attivita Spaziali Giuseppe Col, Via Venezia 15, I-35131 Padua, Italy.;CNR IFN UOS Padova LUXOR, Via Trasea 7, I-35131 Padua, Italy.;Univ Padua, Dept Informat Engn, Via Gradenigo 6-B, I-35131 Padua, Italy..
    Oklay, N.
    Max Planck Inst Sonnensyst Forsch, Justus von Liebig Weg 3, D-37077 Gottingen, Germany..
    Thomas, N.
    Univ Bern, Phys Inst, Sidlerstr 5, CH-3012 Bern, Switzerland..
    Tubiana, C.
    Max Planck Inst Sonnensyst Forsch, Justus von Liebig Weg 3, D-37077 Gottingen, Germany..
    Wenzel, K-P
    ESA SSO, Keplerlaan 1,Postbus 299, NL-2201 AZ Noordwijk, Netherlands.
    The global shape, density and rotation of Comet 67P/Churyumov-Gerasimenko from preperihelion Rosetta/OSIRIS observations2016In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 277, p. 257-278Article in journal (Refereed)
    Abstract [en]

    The Rosetta spacecraft reached Comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) in August 2014 at an heliocentric distance of 3.6 a.u. and was then put in orbit around its nucleus to perform detailed observations. Among the collected data are the images acquired by the OSIRIS instrument up to the perihelion passage of the comet in August 2015, which allowed us to map the entire nucleus surface at high-resolution in the visible. Stereophotoclinometry methods have been used to reconstruct a global high-resolution shape model and to monitor its rotational parameters using data collected up to perihelion. The nucleus has a conspicuous bilobate shape with overall dimensions along its principal axes of (4.34 +/- 0.02) x (2.60 +/- 0.02) x (2.12 +/- 0.06) km. The best-fit ellipsoid dimensions of the individual lobes along their principal axes of inertia are found to be 4.10 x 3.52 x 1.63 km and 2.50 x 2.14 x 1.641cm. Their volume amounts to 66% and 27% of the total volume of the nucleus. The two lobes are connected by a "neck" whose volume has been estimated to represent similar to 7% of the total volume of the comet. Combining the derived volume of 18.8 +/- 0.3 km(3) with the mass of 9.982 +/- 0.003 x 10(12) kg determined by the Rosetta/RSI experiment, we obtained a bulk density of the nucleus of 532 +/- 7 kg m(-3). Together with the companion value of 535 35 kg m-3 deduced from the stereophotogrammetry shape model of the nucleus (Preusker et al. [2015] Astron. Astrophys. 583, A33), these constitute the first reliable and most accurate determination of the density of a cometary nucleus to date. The calculated porosity is quite large, ranging approximately from 70% to 75% depending upon the assumed density of the dust grains and the dust-to-ice mass ratio. The nature of the porosity, either micro or macro or both, remains unconstrained. The coordinates of the center of gravity are not compatible with a uniform nucleus density. The direction of the offset between the center of gravity and the center of figure suggests that the big lobe has a slightly higher bulk density compared to the small one. the center of mass position cannot be explained by different, but homogenous densities in the two lobes. The initial rotational period of 12.4041 +/- 0.0001 h of the nucleus persisted until October 2014. It then slightly increased to a maximum of 12.4304h reached on 19 May 2015 and finally dropped to 12.305 h just before perihelion on August 10, 2015. A periodogram analysis of the (RA, Dec) direction of the Z-axis of the comet obtained in parallel with the shape reconstruction exhibits a highly significant minima at 11.5 +/- 0.5 day clearly indicating an excited rotational state with an amplitude of 0.15 +/- 0.03 degrees.

  • 21.
    Karlsson, Ola
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Lagerkvist, Claes-Ingvar
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Davidsson, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    (U)BVRI photometry of Trojan L5 asteroids2009In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 199, no 1, p. 106-118Article in journal (Refereed)
  • 22.
    Keller, H.
    et al.
    Max-Planck-Institut für Sonnensystemforschung.
    Rickman, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics.
    Davidsson, Björn J. R.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics.
    Wenzel, Klaus-Peter
    Observations of Comet 9P/Tempel 1 around the Deep Impact event by the OSIRIS cameras onboard Rosetta2007In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 187, no 1, p. 87-103Article in journal (Refereed)
    Abstract [en]

    The OSIRIS cameras on the Rosetta spacecraft observed Comet 9P/Tempel 1 from 5 days before to 10 days after it was hit by the Deep Impactprojectile. The Narrow Angle Camera (NAC) monitored the cometary dust in 5 different filters. The Wide Angle Camera (WAC) observed through filters sensitive to emissions from OH, CN, Na, and OI together with the associated continuum. Before and after the impact the comet showedregular variations in intensity. The period of the brightness changes is consistent with the rotation period of Tempel 1. The overall brightness ofTempel 1 decreased by about 10% during the OSIRIS observations. The analysis of the impact ejecta shows that no new permanent coma structureswere created by the impact. Most of the material moved with ∼200 m s−1 . Much of it left the comet in the form of icy grains which sublimatedand fragmented within the first hour after the impact. The light curve of the comet after the impact and the amount of material leaving the comet(4.5–9 × 106 kg of water ice and a presumably larger amount of dust) suggest that the impact ejecta were quickly accelerated by collisions withgas molecules. Therefore, the motion of the bulk of the ejecta cannot be described by ballistic trajectories, and the validity of determinations ofthe density and tensile strength of the nucleus of Tempel 1 with models using ballistic ejection of particles is uncertain.

  • 23.
    Konôpková, Zuzana
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology.
    Lazor, Peter
    Uppsala University, Disciplinary Domain of Science and Technology.
    On the thermal state and dynamo in the core of MercuryIn: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643Article in journal (Other academic)
  • 24. Lee, Seungwon
    et al.
    von Allmen, Paul
    Kamp, Lucas
    Gulkis, Samuel
    Davidsson, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Non-LTE radiative transfer for sub-millimeter water lines in Comet 67P/Churyumov-Gerasimenko2011In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 215, no 2, p. 721-731Article in journal (Refereed)
    Abstract [en]

    The European Space Agency (ESA) Rosetta spacecraft (Schulz, R., Alexander, C., Boehnhardt, H., Glassmeier, K.H. (Eds.) [2009]. "ROSETTA - ESA") will encounter Comet 67P/Churyumov-Gerasimenko in 2014 and spend the next 18 months in the vicinity of the comet, permitting very high spatial and spectral resolution observations of the coma and nucleus. During this time, the heliocentric distance of the comet will change from similar to 3.5 AU to similar to 1.3 AU, accompanied by an increasing temperature of the nucleus and the development of the coma. The Microwave Instrument for the Rosetta Orbiter (MIRO) will observe the ground-state rotational transition (1(10)-1(01)) of H(2) (16)O at 556.936 GHz, the two isotopologues H(2)(17)O and H(2) (18)O and other molecular transitions in the coma during this time (Gulkis, S. et al., [2007]. MIRO: Microwave Instrument for Rosetta Orbiter. Space Sci. Rev, 128, 561-597).

    The aim of this study is to simulate the water line spectra that could be obtained with the MIRO instrument and to understand how the observed line spectra with various viewing geometries can be used to study the physical conditions of the coma and the water excitation processes throughout the coma. We applied an accelerated Monte Carlo method to compute the excitations of the seven lowest rotational levels (1(01), 1(10), 2(12), 2(21), 3(03), 3(12), and 3(21)) of ortho-water using a comet model with spherically symmetric water outgassing, density, temperature and expansion velocity at three different heliocentric distances 1.3 AU, 2.5 AU, and 3.5 AU. Mechanisms for the water excitation include water-water collisions, water-electron collisions, and infrared pumping by solar radiation.

    Synthetic line spectra are calculated at various observational locations and directions using the MIRO instrument parameters. We show that observations at varying viewing distances from the nucleus and directions have the potential to give diagnostic information on the continuum temperature and water outgassing rates at the surface of the nucleus, and the gas density, expansion velocity, and temperature of the coma as a function of distance from the nucleus. The gas expansion velocity and temperature affect the spectral line width and frequency shift of the line from the rest frequency, while the gas density (which is directly related to the outgassing rate) and the line excitation temperature determine the antenna temperature of the absorption and emission signal in the line profile.

  • 25. Luhmann, J. G.
    et al.
    Ulusen, D.
    Ledvina, S. A.
    Mandt, K.
    Magee, B.
    Waite, J. H.
    Westlake, J.
    Cravens, T. E.
    Robertson, I.
    Edberg, Niklas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Ågren, Karin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Ma, Y. -J
    Wei, H.
    Russell, C. T.
    Dougherty, M. K.
    Investigating magnetospheric interaction effects on Titan's ionosphere with the Cassini orbiter Ion Neutral Mass Spectrometer, Langmuir Probe and magnetometer observations during targeted flybys2012In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 219, no 2, p. 534-555Article in journal (Refereed)
    Abstract [en]

    In the similar to 6 years since the Cassini spacecraft went into orbit around Saturn in 2004, roughly a dozen Titan flybys have occurred for which the Ion Neutral Mass Spectrometer (INMS) measured that moon's ionospheric density and composition. For these, and for the majority of the similar to 60 close flybys probing to altitudes down to similar to 950 km, Langmuir Probe electron densities were also obtained. These were all complemented by Cassini magnetometer observations of the magnetic fields affected by the Titan plasma interaction. Titan's ionosphere was expected to differ from those of other unmagnetized planetary bodies because of significant contributions from particle impact due to its magnetospheric environment. However, previous analyses of these data clearly showed the dominance of the solar photon source, with the possible exception of the nightside. This paper describes the collected ionospheric data obtained in the period between Cassini's Saturn Orbit Insertion in 2004 and 2009, and examines some of their basic characteristics with the goal of searching for magnetospheric influences. These influences might include effects on the altitude profiles of impact ionization by magnetospheric particles at the Titan orbit location, or by locally produced pickup ions freshly created in Titan's upper atmosphere. The effects of forces on the ionosphere associated with both the draped and penetrating external magnetic fields might also be discernable. A number of challenges arise in such investigations given both the observed order of magnitude variations in the magnetospheric particle sources and the unsteadiness of the magnetospheric magnetic field and plasma flows at Titan's (similar to 20Rs (Saturn Radius)) orbit. Transterminator flow of ionospheric plasma from the dayside may also supply some of the nightside ionosphere, complicating determination of the magnetospheric contribution. Moreover, we are limited by the sparse sampling of the ionosphere during the mission as the Titan interaction also depends on Saturn Local Time as well as possible intrinsic asymmetries and variations of Titan's neutral atmosphere. We use organizations of the data by key coordinate systems of the plasma interaction with Titan's ionosphere to help interpret the observations. The present analysis does not find clear characteristics of the magnetosphere's role in defining Titan's ionosphere. The observations confirm the presence of an ionosphere produced mainly by sunlight, and an absence of expected ionospheric field signatures in the data. Further investigation of the latter, in particular, may benefit from numerical experiments on the inner boundary conditions of 3D models including the plasma interaction and features such as neutral winds.

  • 26. Nilsson, Hans
    et al.
    Edberg, Niklas J. T.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Stenberg, Gabriella
    Barabash, Stas
    Holmström, Mats
    Futaana, Yoshifumi
    Lundin, Rickard
    Fedorov, Andrei
    Heavy ion escape from Mars, influence from solar wind conditions and crustal magnetic fields2011In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 215, no 2, p. 475-484Article in journal (Refereed)
    Abstract [en]

    We have used more than 4 years of Mars Express ion data to estimate the escape of heavy ions (O(1)(+) O(2)(+) and CO(2)(+)) from Mars. To take the limited field of view of the instrument into account, the data has been binned into spatial bins and angular bins to create average distribution functions for different positions in the near Mars space. The net escape flux for the studied low solar activity period, between May 2007 and May 2011, is 2.0 +/- 0.2 x 10(24) s(-1). The escape has been calculated independently for four different quadrants in the Y(MSO) - Zmso plane, south, dusk, north and dawn. Escape is highest from the northern and dusk quadrants, 0.6 +/- 0.1 x 10(24) s(-1), and smallest from the south and dawn quadrants, 0.4 +/- 0.1 x 10(24) s(-1). The flux ratio of molecular (O(2)(+) and CO(2)())(+) to 0 ions is 0.9 +/- 0.1, averaged over all quadrants. The flux difference between the north and south quadrants is statistically significant, and is presumed to be due to the presence of significant crustal magnetic fields in the southern hemisphere, reducing the outflow. The difference between the dawn and dusk quadrants is likely due to the magnetic tension associated with the nominal Parker angle spiral, which should lead to higher average magnetic tension on the dusk side. The escape increases during periods of high solar wind flux and during times when co-rotating interaction regions (CIR) affect Mars. In the latter case the increase is a factor 2.4-2.9 as compared to average conditions.

  • 27. Roussos, E.
    et al.
    Kollmann, P.
    Krupp, N.
    Paranicas, C.
    Krimigis, S. M.
    Mitchell, D. G.
    Persoon, A. M.
    Gurnett, D. A.
    Kurth, W. S.
    Kriegel, H.
    Simon, S.
    Khurana, K. K.
    Jones, G. H.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Holmberg, Madeleine K. G.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Space Plasma Physics.
    Energetic electron observations of Rhea's magnetospheric interaction2012In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 221, no 1, p. 116-134Article in journal (Refereed)
    Abstract [en]

    Saturn's moon Rhea is thought to be a simple plasma absorber, however, energetic particle observations in its vicinity show a variety of unexpected and complex interaction features that do not conform with our current understanding about plasma absorbing interactions. Energetic electron data are especially interesting, as they contain a series of broad and narrow flux depletions on either side of the moon's wake. The association of these dropouts with absorption by dust and boulders orbiting within Rhea's Hill sphere was suggested but subsequently not confirmed, so in this study we review data from all four Cassini flybys of Rhea to date seeking evidence for alternative processes operating within the moon's interaction region. We focus on energetic electron observations, which we put in context with magnetometer, cold plasma density and energetic ion data. All flybys have unique features, but here we only focus on several structures that are consistently observed. The most interesting common feature is that of narrow dropouts in energetic electron fluxes, visible near the wake flanks. These are typically seen together with narrow flux enhancements inside the wake. A phase-space-density analysis for these structures from the first Rhea flyby (R1) shows that Liouville's theorem holds, suggesting that they may be forming due to rapid transport of energetic electrons from the magnetosphere to the wake, through narrow channels. A series of possibilities are considered to explain this transport process. We examined whether complex energetic electron drifts in the interaction region of a plasma absorbing moon (modeled through a hybrid simulation code) may allow such a transport. With the exception of several features (e.g. broadening of the central wake with increasing electron energy), most of the commonly observed interaction signatures in energetic electrons (including the narrow structures) were not reproduced. Additional dynamical processes, not simulated by the hybrid code, should be considered in order to explain the data. For the small scale features, the possibility that a flute (interchange) instability acts on the electrons is discussed. This instability is probably driven by strong gradients in the plasma pressure and the magnetic field magnitude: magnetometer observations show clearly signatures consistent with the (expected) plasma pressure loss due to ion absorption at Rhea. Another potential driver of the instability could have been gradients in the cold plasma density, which are, however, surprisingly absent from most crossings of Rhea's plasma wake. The lack of a density depletion in Rhea's wake suggests the presence of a local cold plasma source region. Hybrid plasma simulations show that this source cannot be the ionized component of Rhea's weak exosphere. It is probably related to accelerated photoelectrons from the moon's negatively charged surface, indicating that surface charging may play a very important role in shaping Rhea's magnetospheric interaction region. (C) 2012 Elsevier Inc. All rights reserved.

  • 28. Ulusen, D.
    et al.
    Luhmann, J. G.
    Ma, Y. -J
    Ledvina, S.
    Cravens, T. E.
    Mandt, K.
    Waite, J. H.
    Wahlund, Jan Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Investigation of the force balance in the Titan ionosphere: Cassini T5 flyby model/data comparisons2010In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 210, no 2, p. 867-880Article in journal (Refereed)
    Abstract [en]

    Cassini's Titan flyby on 16 April, 2005 (T5) is the only encounter when the two main ionizing sources of the moon's atmosphere, solar radiation and corotating plasma, align almost anti-parallel. In this paper a single-fluid multi-species 3D MHD model of the magnetospheric plasma interaction for T5 conditions is analyzed. Model results are compared to observations to investigate the ionospheric dynamics at Titan as well as to understand the deviations from a typical solar wind interaction, such as Venus' interaction with the solar wind. Model results suggest that for the T5 interaction configuration, corotating plasma is the dominant driver determining the global interaction features at high altitudes. In the lower ionosphere below similar to 1500 km altitude - where the control of the ionospheric composition transfers from dynamic to chemical processes - magnetic and thermal pressure gradients oppose each other locally, complicating the ionospheric dynamics. Model results also imply that the nightside ionosphere - produced only by the impact ionization in the model - does not provide enough thermal pressure to balance the incident plasma dynamic pressure. As a result, the induced magnetic barrier penetrates into the ionosphere by plasma convection down to similar to 1000 km altitude and by magnetic diffusion below this altitude. Moreover, strong horizontal drag forces due to ion-neutral collisions and comparable drag forces estimated from possible neutral winds in the lower ionosphere below similar to 1400 km altitude oppose over local regions, implying that the Titan interaction must be treated as a 3D problem. Ion and electron densities calculated from the model generally agree with the Cassini Ion Neutral Mass Spectrometer and Langmuir probe measurements; however, there are significant differences between the calculated and measured magnetic fields. We discuss possible explanations for the discrepancy in the magnetic field predictions.

  • 29. Ulusen, D.
    et al.
    Luhmann, J. G.
    Ma, Y. J.
    Mandt, K. E.
    Waite, J. H.
    Dougherty, M. K.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Russell, C. T.
    Cravens, T. E.
    Edberg, Niklas J. T.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Ågren, Karin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Comparisons of Cassini flybys of the Titan magnetospheric interaction with an MHD model: Evidence for organized behavior at high altitudes2012In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 217, no 1, p. 43-54Article in journal (Refereed)
    Abstract [en]

    Recent papers suggest the significant variability of conditions in Saturn's magnetosphere at the orbit of Titan. Because of this variability, it was expected that models would generally have a difficult time regularly comparing to data from the Titan flybys. However, we find that in contrast to this expectation, it appears that there is underlying organization of the interaction features roughly above similar to 1800 km (1.7 Rt) altitude by the average external field due to Saturn's dipole moment. In this study, we analyze Cassini's plasma and magnetic field data collected at 9 Titan encounters during which the external field is close to the ideal southward direction and compare these observations to the results from a 2-fluid (1 ion, 1 electron) 7-species MHD model simulations obtained under noon SLT conditions. Our comparative analysis shows that under noon SLT conditions the Titan plasma interaction can be viewed in two layers: an outer layer between 6400 and 1800 km where interaction features observed in the magnetic field are in basic agreement with a purely southward external field interaction and an inner layer below 1800 km where the magnetic field measurements show strong variations and deviate from the model predictions. Thus the basic features inferred from the Voyager 1 flyby seem to be generally present above similar to 1800 km in spite of the ongoing external variations from SLT excursions, time variability and magnetospheric current systems as long as a significant southward external field component is present. At around similar to 1800 km kinetic effects (such as mass loading and heavy ion pickup) and below 1800 km ionospheric effects (such as drag of ionospheric plasma due to coupling with neutral winds and/or magnetic memory of Titan's ionosphere) complicate what is observed.

  • 30. Vigren, E.
    et al.
    Galand, M.
    Yelle, R. V.
    Cui, J.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Ågren, Karin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Space Plasma Physics.
    Lavvas, P. P.
    Mueller-Wodarg, I. C. F.
    Strobel, D. F.
    Vuitton, V.
    Bazin, A.
    On the thermal electron balance in Titan's sunlit upper atmosphere2013In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 223, no 1, p. 234-251Article in journal (Refereed)
    Abstract [en]

    The Cassini mission has investigated Titan's upper atmosphere in detail and found that, under solar irradiation, it has a well-developed ionosphere, which peaks between 1000 and 1200km. In this paper we focus on the T40, T41, T42 and T48 Titan flybys by the Cassini spacecraft and use in situ measurements of N2 and CH4 densities by the Ion Neutral Mass Spectrometer (INMS) as input into a solar energy deposition model to determine electron production rates. We combine these electron production rates with estimates of the effective recombination coefficient based on available laboratory data for Titan ions' dissociative recombination rates and electron temperatures derived from the Langmuir probe (LP) to predict electron number densities in Titan's upper atmosphere, assuming photochemical equilibrium and loss of electrons exclusively through dissociative recombination with molecular ions. We then compare these predicted electron number densities with those observed in Titan's upper atmosphere by the LP. The assumption of photochemical equilibrium is supported by a reasonable agreement between the altitudes where the electron densities are observed to peak and where the electron production rates are calculated to peak (roughly corresponding to the unit optical depth for HeII photons at 30.38nm). We find, however, that the predicted electron number densities are nearly a factor of two higher than those observed throughout the altitude range between 1050 and 1200km (where we have made estimates of the effective recombination coefficient). There are different possible reasons for this discrepancy; one possibility is that there may be important loss processes of free electrons other than dissociative recombination in Titan's upper atmosphere.

  • 31.
    Vigren, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Galand, M.
    Yelle, R. V.
    Wellbrock, A.
    Coates, A. J.
    Snowden, D.
    Cui, J.
    Lavvas, P.
    Edberg, Niklas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Shebanits, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Wahlund, Jan Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Vuitton, V.
    Mandt, K.
    Ionization balance in Titan's nightside ionosphere2015In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 248, p. 539-546Article in journal (Refereed)
    Abstract [en]

    Based on a multi-instrumental Cassini dataset we make model versus observation comparisons of plasma number densities, n(p) = (n(e)n(1))(1/2) (n(e) and n(1) being the electron number density and total positive ion number density, respectively) and short-lived ion number densities (N+, CH2+, CH3+, CH4+) in the southern hemisphere of Titan's nightside ionosphere over altitudes ranging from 1100 and 1200 km and from 1100 to 1350 km, respectively. The n(p) model assumes photochemical equilibrium, ion-electron pair production driven by magnetospheric electron precipitation and dissociative recombination as the principal plasma neutralization process. The model to derive short-lived-ion number densities assumes photochemical equilibrium for the short-lived ions, primary ion production by electron-impact ionization of N-2 and CH4 and removal of the short-lived ions through reactions with CH4. It is shown that the models reasonably reproduce the observations, both with regards to tip and the number densities of the short-lived ions. This is contrasted by the difficulties in accurately reproducing ion and electron number densities in Titan's sunlit ionosphere. (C) 2014 Elsevier Inc. All rights reserved.

  • 32.
    Warell, Johan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Davidsson, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    A Hapke model implementation for compositional analysis of VNIR spectra of Mercury2010In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 209, no 1, p. 164-178Article in journal (Refereed)
    Abstract [en]

    An implementation of Hapke's radiative transfer-based photometric model for light scattering in semi-transparent porous media is presented with special emphasis on the analysis of reflectance spectra of Mercury The model allows intimate mixing of an arbitrary number of regolith components with varying modal abundances, modal chemistries and grain sizes, matured by microphase iron Reflectance spectra of suites of silicates of varying grain sizes and chemistries are used to calculate the imaginary coefficient of the complex index of refraction as a function of chemistry, thus limiting the modeling effects of chemically atypical laboratory samples, and allowing controlled modeling of minerals with varying chemical compositions The performance of the model in the visual to near-infrared wavelength range is evaluated for a range of chemically characterized silicate mixtures of terrestrial powders, meteorite powders, matured lunar return samples, and remotely sensed lunar spectra.

  • 33.
    Warell, Johan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Sprague, A.
    Kozlowski, R.
    Rothery, D. A.
    Lewis, N.
    Helbert, J.
    Cloutis, E.
    Constraints on Mercury's surface composition from MESSENGER and ground-based spectroscopy2010In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 209, no 1, p. 138-163Article in journal (Refereed)
    Abstract [en]

    The composition and chemistry of Mercury's regolith has beets calculated from MESSENGER MASCS 0 3-1 3 mu m spectra from the first flyby, using an implementation of Hapke's radiative transfer-based photometric model for light scattering in semi-transparent porous media, and a linear spectral mixing algorithm We combine this investigation with linear spectral fitting results from mid-infrared spectra and compare derived oxide abundances with mercurian formation models and lunar samples Hapke modeling results indicate a regolith that is optically dominated by finely comminuted particles with average area weighted grain size near 20 pin Mercury shows lunar-style space weathering, with maturation-produced microphase iron present at similar to 0 065 wt% abundance, with only small variations between mature and immature sites, the amount of which is unable to explain Mercury's low brightness relative to the Moon The average modal mineralogies for the flyby 1 spectra derived from Hapke modeling are 35-70% Na-rich plagioclase or orthoclase, up to 30% Mg-rich clinopyroxene, <5% Mg-rich orthopyroxene, minute olivine, similar to 20-45% low-Fe, low-Ti agglutinitic glass, and <10% of one or more lunar-like opaque minerals Mercurian average oxide abundances derived from Hapke models and mid-infrared linear fitting include 40-50 wt% SiO2, 10-35 wt% Al2O3, 1-8 wt% FeO, and <25 wt% TiO2, the inferred rock type is basalt Lunar-like opaques or glasses with high Fe and/or Ti abundances cannot on their own, or in combination, explain Mercury's low brightness The linear mixing results indicate the presence of clinopyroxenes that contain up to 21 wt% MnO and the presence of a Mn-rich hedenbergite Mn in M1 crystalline lattice sites of hedenbergite suppresses the strong 1 and 2 mu m crystal field absorption bands and may thus act as a strong darkening agent on Mercury Also, one or more of thermally darkened silicates, Fe-poor opaques and matured glasses, or Mercury-unique Ostwald-ripened microphase iron nickel may lower the albedo A major part of the total microphase Iron present in Mercury's regolith is likely derived from FeO that is not intrinsic to the crust but has been subsequently delivered by exogenic sources.

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