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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.07.013 Byline: J. Zendejas (a), A. Segura (b), A.C. Raga (b) Keywords: Astrobiology; Atmospheres, Evolution; Extrasolar planets; Solar wind Abstract: We present an analytic model for the interaction between planetary atmospheres and stellar winds from main sequence M stars, with the purpose of obtaining a quick test-model that estimates the timescale for total atmospheric mass loss due to this interaction. Planets in the habitable zone of M dwarfs may be tidally locked and may have weak magnetic fields, because of this we consider the extreme case of planets with no magnetic field. The model gives the planetary atmosphere mass loss rate as a function of the stellar wind and planetary properties (mass, atmospheric pressure and orbital distance) and an entrainment efficiency coefficient [alpha]. We use a mixing layer model to explore two different cases: a time-independent stellar mass loss and a stellar mass loss rate that decreases with time. For both cases we consider planetary masses within the range of 1[right arrow]10M.sub.[direct sum] and atmospheric pressures with values of 1, 5 and 10atm. We apply our model to Venus by estimating its atmospheric mass loss rate by the interaction with the solar wind and compare our model with more detailed simulations. We find a good agreement between our results and the atmospheric mass loss obtained by more detailed models, and it is therefore appropriate for carrying out an exploration of the broad parameter space of exoplanetary systems. For the time-dependent case, planets without magnetic field in the habitable zone of M dwarfs with initial stellar mass losses of M.sub.w Author Affiliation: (a) Instituto de Astronomia, Universidad Nacional Autonoma de Mexico, A.P. 70-264, 04510 D.F., Mexico (b) Instituto de Ciencias Nucleares, Universidad Nacional Autonoma de Mexico, A.P. 70-543, 04510 D.F., Mexico Article History: Received 3 November 2009; Revised 13 July 2010; Accepted 14 July 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.01.004 Byline: C. Moore (a), K. Miki (a), D.B. Goldstein (a), K. Stapelfeldt (a)(b), P.L. Varghese (a), L.M. Trafton (a), R.W. Evans (a)(b) Keywords: Io; Aurorae; Jupiter, Satellites; Satellites, Atmospheres; Collisional physics Abstract: We present a Monte Carlo (MC) model of [OI] 6300A and [SII] 6716A emission from Io entering eclipse. The simulation accounts for the 3-D distribution of SO.sub.2, O, SO, S, and O.sub.2 in Io's atmosphere, several volcanic plumes, and the magnetic field around Io. Thermal electrons from the jovian plasma torus are input along the simulation domain boundaries and move along the magnetic field lines distorted by Io, occasionally participating in collisions with neutrals. We find that the atmospheric asymmetry resulting from varying degrees of atmospheric collapse across Io (due to eclipse ingress) and the presence of volcanoes contributes significantly to the unique morphology of the [OI] 6300A emission. The [OI] radiation lifetime of [approximately equal to]134s limits the emission to regions that have a sufficiently low neutral density so that intermolecular collisions are rare. We find that at low altitudes (typically 4x10.sup.9 cm.sup.-3) to collisionally quench nearly all (>95%) of the excited oxygen for reasonable quenching efficiencies. Upstream (relative to the plasma flow), Io's perturbation of the jovian magnetic field mirrors electrons with high pitch angles, while downstream collisions can trap the electrons. This magnetic field perturbation is one of the main physical mechanisms that results in the upstream/downstream brightness asymmetry in [OI] emission seen in the observation by Trauger et al. (Trauger, J.T., Stapelfeldt, K.R., Ballester, G.E., Clarke, J.I., 1997. HST observations of [OI] emissions from Io in eclipse. AAS-DPS Abstract (1997DPS29.1802T)). There are two other main causes for the observed brightness asymmetry. First, the observation's viewing geometry of the wake spot crosses the dayside atmosphere and therefore the wake's observational field of view includes higher oxygen column density than the upstream side. Second, the phased entry into eclipse results in less atmospheric collapse and thus higher collisional quenching on the upstream side relative to the wake. We compute a location (both in altitude and latitude) for the intense wake emission feature that agrees reasonably well with this observation. Furthermore, the peak intensity of the simulated wake feature is less than that observed by a factor of [approximately equal to]3, most likely because our model does not include direct dissociation-excitation of SO.sub.2 and SO. We find that the latitudinal location of the emission feature depends not so much on the tilt of the magnetic field as on the relative north/south flux tube depletion that occurs due to Io's changing magnetic latitude in the plasma torus. From 1-D simulations, we also find that the intensity of [SII] 6716 and 6731A emission is much weaker than that of [OI] even if the [SII] excitation cross section is 10.sup.3 times larger than excitation to [OI]. This is because the density of S.sup.+ is much less than that of O and because the Einstein-A coefficient of the [SII] emission is a factor of [approximately equal to]10 smaller than that of [OI]. Author Affiliation: (a) The University of Texas at Austin, Austin, TX 78712, USA (b) JPL, Caltech, USA Article History: Received 28 July 2009; Revised 2 January 2010; Accepted 3 January 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.03.016 Byline: Michael E. Purucker Keywords: Moon; interior; Moon; surface; Magnetic fields; Magnetospheres Abstract: A preliminary model of the internal magnetic field of the Moon is developed using a novel, correlative technique on the low-altitude Lunar Prospector magnetic field observations. Subsequent to the removal of a simple model of the external field, an internal dipole model is developed for each pole-to-pole half-orbit. This internal dipole model exploits Lunar Prospector's orbit geometry and incorporates radial and theta vector component data from immediately adjacent passes into the model. These adjacent passes are closely separated in space and time and are thus characteristic of a particular lunar regime (wake, solar wind, magnetotail, magnetosheath) or regimes. Each dipole model thus represents the correlative parts of three adjacent passes, and provides an analytic means of continuing the data to a constant surface of 30 km above the mean lunar radius. The altitude-normalized radial field from the wake and tail regimes is used to build a model in which 99.2% of the 360 by 360 bins covering the lunar surface are filled. This global model of the radial magnetic field is used to construct a degree 178 spherical harmonic model of the field via the Driscoll and Healy sampling theorem. Terms below about degree 150 are robust, and polar regions are considered to be the least reliable. The model resolves additional detail in the low magnetic field regions of the Imbrium and Orientale basins, and also in the four anomaly clusters antipodal to the large lunar basins. The model will be of use in understanding the sources of the internal field, and as a first step in modeling the interaction of the internal field with the solar wind. Author Affiliation: Raytheon at Planetary Geodynamics Lab, Code 698, Goddard Space Flight Center, Greenbelt, MD 20771, USA Article History: Received 1 October 2007; Revised 16 March 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.02.013 Byline: Ulrich R. Christensen, Johannes Wicht Keywords: Magnetic fields; Mercury; Saturn Abstract: A substantial part of Mercury's iron core may be stably stratified because the temperature gradient is subadiabatic. A dynamo would operate only in a deep sublayer. We show that such a situation arises for a wide range of values for the heat flow and the sulfur content in the core. In Saturn the upper part of the metallic hydrogen core could be stably stratified because of helium depletion. The magnetic field is unusually weak in the case of Mercury and unusually axisymmetric at Saturn. We study numerical dynamo models in rotating spherical shells with a stable outer region. The control parameters are chosen such that the magnetic Reynolds number is in the range of expected Mercury values. Because of its slow rotation, Mercury may be in a regime where the dipole contribution to the internal magnetic field is weak. Most of our models are in this regime, where the dynamo field consists mainly of rapidly varying higher multipole components. They can hardly pass the stable conducting layer because of the skin effect. The weak low-degree components vary more slowly and control the structure of the field outside the core, whose strength matches the observed field strength at Mercury. In some models the axial dipole dominates at the planet's surface and in others the axial quadrupole is dominant. Differential rotation in the stable layer, representing a thermal wind, is important for attenuating non-axisymmetric components in the exterior field. In some models that we relate to Saturn the axial dipole is intrinsically strong inside the dynamo. The surface field strength is much larger than in the other cases, but the stable layer eliminates non-axisymmetric modes. The Messenger and Bepi Colombo space missions can test our predictions that Mercury's field is large-scaled, fairly axisymmetric, and shows no secular variations on the decadal time scale. Author Affiliation: Max-Planck-Institut fur Sonnensystemforschung, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany Article History: Received 14 August 2007; Revised 18 February 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.05.005 Byline: Sabine Stanley, Jeremy Bloxham Keywords: Uranus; Neptune; Magnetic fields Abstract: The non-axisymmetric, non-dipolar magnetic fields of Uranus and Neptune are markedly different from the axially-dipolar dominated fields of the other planets in our Solar System with active dynamos. Stanley and Bloxham [Stanley, S., Bloxham, J., 2004. Nature 428, 151-153] used numerical modeling to demonstrate that Uranus' and Neptune's unusual fields could be the result of a different convective region geometry in these planets. They found that a numerical dynamo operating in a thin shell surrounding a stably-stratified fluid interior produces magnetic field morphologies similar to those of Uranus and Neptune. This geometry for the convective region was initially proposed by Hubbard et al. [Hubbard, W.B., Podolak, M., Stevenson, D.J., 1995. In: Cruickshank, D. (Ed.), Neptune and Triton. Univ. of Arizona Press, Tucson, pp. 109-138] to explain both the magnetic field morphology as well as the low intrinsic heat flows from these planets. Here we examine the influence of varying the stable layer radius in numerical models and compare the results to thin shell models surrounding solid inner cores. We find that a limited range of stable-layer shell thicknesses exist in which Uranus/Neptune-like field morphologies result. This allows us to put constraints on the size of the convective layers in Uranus and Neptune. Author Affiliation: Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA Article History: Received 3 October 2005; Revised 3 April 2006

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.09.018 Byline: E. Kallio (a), A. Fedorov (b), E. Budnik (b), T. Sales (a), P. Janhunen (a), W. Schmidt (a), H. Koskinen (a)(q), P. Riihela (a), S. Barabash (c), R. Lundin (c), M. Holmstrom (c), H. Gunell (c), K. Brinkfeldt (c), Y. Futaana (c), H. Andersson (c), M. Yamauchi (c), A. Grigoriev (c), J.-A. Sauvaud (b), J.-J. Thocaven (b), J.D. Winningham (d), R.A. Frahm (d), J.R. Sharber (d), J.R. Scherrer (d), A.J. Coates (e), D.R. Linder (e), D.O. Kataria (e), J. Kozyra (f), J.G. Luhmann (g), E. Roelof (h), D. Williams (h), S. Livi (h), C.C. Curtis (i), K.C. Hsieh (i), B.R. Sandel (i), M. Grande (j), M. Carter (j), S. McKenna-Lawler (k), S. Orsini (l), R. Cerulli-Irelli (l), M. Maggi (l), P. Wurz (m), P. Bochsler (m), N. Krupp (n), J. Woch (n), M. Franz (n), K. Asamura (o), C. Dierker (p) Keywords: Mars; atmosphere; Ionospheres Abstract: We have analysed ion escape at Mars by comparing ASPERA-3/Mars Express ion measurements and a 3-D quasi-neutral hybrid model. As Mars Express does not have a magnetometer onboard, the analysed IMA data are from an orbit when the IMF clock angle was possible to determine from the magnetic field measurements of Mars Global Surveyor. We found that fast escaping planetary ions were observed at the place which, according to the 3-D model, is anticipated to contain accelerated heavy ions originating from the martian ionosphere. The direction of the interplanetary magnetic field was found to affect noticeably which regions can be magnetically connected to Mars Express and to the overall 3-D Mars-solar wind interaction. Author Affiliation: (a) Space Research Unit, Finnish Meteorological Institute, Erik Palmenin aukio 1, P.O. Box 503, FIN-00101 Helsinki, Finland (b) Centre d'Etude Spatiale des Rayonnements, BP-4346, F-31028 Toulouse, France (c) Swedish Institute of Space Physics, Box 812, S-98 128, Kiruna, Sweden (d) Southwest Research Institute, San Antonio, TX 7228-0510, USA (e) Mullard Space Science Laboratory, University College London, Surrey RH5 6NT, UK (f) Space Physics Research Laboratory, University of Michigan, Ann Arbor, MI 48109-2143, USA (g) Space Science Laboratory, University of California in Berkeley, Berkeley, CA 94720-7450, USA (h) Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723-6099, USA (i) University of Arizona, Tucson, AZ 85721, USA (j) Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK (k) Space Technology Ltd., National University of Ireland, Maynooth, Co. Kildare, Ireland (l) Istituto di Fisica dello Spazio Interplanetari, I-00133 Rome, Italy (m) Physikalisches Institut, University of Bern, CH-3012 Bern, Switzerland (n) Max-Planck-Institut fur Aeronomie, D-37191 Katlenburg-Lindau, German (o) Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamichara, Japan (p) Technical University of Braunschweig, Hans-Sommer-Strasse 66, D-38106 Braunschweig, Germany (q) Department of Physical Sciences, University of Helsinki, P.O. Box 64, FIN-00014 Helsinki, Finland Article History: Received 19 April 2005; Revised 12 September 2005