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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.04.009 Byline: E. Sciamma-O'Brien (a), N. Carrasco (a), C. Szopa (a), A. Buch (b), G. Cernogora (a) Keywords: Atmospheres, Chemistry; Satellites, Atmospheres; Saturn, Satellites; Titan; Experimental techniques Abstract: Here we present the first quantitative study of the gas to solid particle conversion in a Radio Frequency dusty plasma experiment simulating the complex atmospheric reactivity on Titan. Analogs of Titan's aerosols have been produced in different N.sub.2-CH.sub.4 gas mixtures. Using in situ mass spectrometry, it has been found that, by varying the initial methane concentration, aerosols could be produced in methane steady state concentrations similar to Titan's atmospheric conditions. In our experiment, an initial [approximately equal to]5% methane concentration is necessary to ensure a [approximately equal to]1.5% methane steady state concentration in the plasma. The tholin mass production rate has been quantified as a function of the initial methane concentration. A maximum was found for a steady state CH.sub.4 concentration in agreement with Titan's atmospheric CH.sub.4 concentrations. At this maximum, the tholin C/N ratio is about 1.45 and the carbon gas to solid conversion yield is about 35%. We have modeled the mass production rate by a parabolic function, highlighting two competitive chemical regimes controlling the tholin production efficiency: an efficient growth process which is proportional to the methane consumption, and an inhibiting process which opposes the growth process and dominates it for initial methane concentrations higher than [approximately equal to]5%. To explain these two opposite effects, we propose two mechanisms: one involving HCN patterns in the tholins for the growth process, and one involving the increasing amount of atomic hydrogen in the plasma as well as the increase in aliphatic contributions in the tholins for the inhibiting process. This study highlights new routes for understanding the chemical growth of the organic aerosols in Titan's atmosphere. Author Affiliation: (a) Universite Versailles St.-Quentin, UPMC Univ. Paris 06, CNRS, LATMOS, 11 Blvd d'Alembert, 78280 Guyancourt, France (b) Ecole Centrale de Paris, Grande voie des Vignes, F-92295 ChAcentstenay-Malabry Cedex, France Article History: Received 8 December 2009; Revised 28 March 2010; Accepted 12 April 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.03.005 Byline: M.O. Fillingim (a), L.M. Peticolas (a), R.J. Lillis (a), D.A. Brain (a), J.S. Halekas (a), D. Lummerzheim (b), S.W. Bougher (c) Keywords: Mars, atmosphere; Ionospheres; Magnetic fields Abstract: Using an electron transport model, we calculate the electron density of the electron impact-produced nighttime ionosphere of Mars and its spatial structure. As input we use Mars Global Surveyor electron measurements, including an interval when accelerated electrons were observed. Our calculations show that regions of enhanced ionization are localized and occur near magnetic cusps. Horizontal gradients in the calculated ionospheric electron density on the night side of Mars can exceed 10.sup.4 cm.sup.-3 over a distance of a few tens of km; the largest gradients produced by the model are over 600cm.sup.-3 km.sup.-1. Such large gradients in the plasma density have several important consequences. These large pressure gradients will lead to localized plasma transport perpendicular to the ambient magnetic field which will generate horizontal currents and electric fields. We calculate the magnitude of these currents to be up to 10 nA/m.sup.2. Additionally, transport of ionospheric plasma by neutral winds, which vary in strength and direction as a function of local time and season, can generate large (up to 1000 nA/m.sup.2) and spatially structured horizontal currents where the ions are collisionally coupled to the neutral atmosphere while electrons are not. These currents may contribute to localized Joule heating. In addition, closure of the horizontal currents and electric fields may require the presence of vertical, field-aligned currents and fields which may play a role in high altitude acceleration processes. Author Affiliation: (a) Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450, USA (b) Geophysical Institute, University of Alaska, Fairbanks, AK 99775-7320, USA (c) Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI 48109-2143, USA Article History: Received 25 September 2008; Revised 5 February 2009; Accepted 2 March 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.07.012 Byline: Eric Quirico (a), Gilles Montagnac (b), Victoria Lees (c), Paul F. McMillan (c), Cyril Szopa (d), Guy Cernogora (d), Jean-NoA'l Rouzaud (e), Patrick Simon (f)(g), Jean-Michel Bernard (a), Patrice Coll (h), Nicolas Fray (h), Robert D. Minard (i), Francois Raulin (h), Bruno Reynard (b), Bernard Schmitt (a) Keywords: Titan; Spectroscopy; Atmospheres; chemistry Abstract: Powdered samples of carbon-nitrogen-hydrogen 'tholins' that mimic Titan's atmosphere aerosols were produced under levitation conditions in the laboratory with a dusty plasma (PAMPRE experiment) using different initial N.sub.2:CH.sub.4 gas mixtures and studied using UV Raman and infrared spectroscopy, X-ray diffraction and high resolution transmission electron microscopy (HRTEM). Comparison between the tholins produced in the PAMPRE experiments and samples prepared by other techniques reveal that they form a fairly homogeneous family of hydrogenated carbon nitride materials. Wall effects during the PAMPRE deposition experiments and other were found to have little effect on the chemical structure of tholins. The first-order UV Raman bands of the disordered carbonaceous materials point to a large contribution of sp.sup.2 clusters present compared with olefinic CN or CC groupings, whereas features at 690 and 980 cm.sup.-1 suggest C.sub.3N.sub.3 rings are present as a species inserted in the macromolecular network. Diffraction techniques do not indicate the presence of large polyaromatic species in any of the tholins studied, whatever their nitrogen concentration, in disagreement with certain previous observations. This precludes the idea that the nature and degree of absorption in the visible range is controlled by the size of polyaromatic species, as has been observed in series of carbon-based materials obtained via thermal processing. Infrared spectroscopy analysis of the tholins has confirmed earlier identifications of chemical groups present including primary amines, nitriles, and alkyl moieties such as CH.sub.2/CH.sub.3, but has ruled out CH.sub.2/CH.sub.3 branches appearing on secondary or tertiary amines. Similar analyses were also performed on a polymeric (HCN).sub.x material, which was found to present several similarities with the tholins, hence suggesting similar polymerization processes. Author Affiliation: (a) Universite Joseph Fourier, CNRS/INSU, Laboratoire de Planetologie de Grenoble UMR 5109, BAcentstiment D de Physique, BP 53, 38041 Grenoble Cedex 9, France (b) Laboratoire de Sciences de la Terre, Ecole Normale Superieure de Lyon, 46 Allee d'Italie, 69364 Lyon Cedex 7, France (c) Department of Chemistry and Materials Chemistry Centre, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom (d) Service d'Aeronomie (SA), UMR CNRS 7620, Universite Pierre et Marie Curie-Paris 6 et Universite de Versailles, Reduit de Verrieres, Route des Gatines, 91371 Verrieres le Buisson, France (e) Laboratoire de Geologie, Ecole Normale Superieure de Paris, 24 Rue Lhomond, 75231 Paris Cedex 5, France (f) CNRS, UPR3079 CEMHTI, 45071 Orleans Cedex 2, France (g) Universite d'Orleans, BP 6749, 45067 Orleans Cedex 2, France (h) Laboratoire Interuniversitaire des Systemes Atmospheriques, Universite Paris XII-Paris VII, 61 Av. du Gal de Gaulle, 94010 Creteil Cedex, France (i) Chemistry Department, Pennsylvania State University, 104 Chemistry Building, University Park, PA 16802, USA Article History: Received 9 November 2007; Revised 8 July 2008