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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.05.020 Byline: Y.-C. Wang (a), J. Mueller (b), U. Motschmann (b)(d), W.-H. Ip (a)(c) Keywords: Magnetospheres; Mercury; Terrestrial planets Abstract: The latest measurements from the two encounters of the MESSENGER spacecraft in year 2008 have discovered several interesting features of the magnetosphere of Mercury. We have performed high-resolution 3D hybrid model calculations to simulate the solar wind interaction with the Hermean magnetosphere during the first two Mercury encounters of the MESSENGER spacecraft in 2008. It is found that the global structure of the Hermean magnetosphere is significantly controlled by the direction of the interplanetary magnetic field. The bow shock size and shape and the magnetotail configuration have very large differences in these two encounters with northward-pointing and southward-pointing interplanetary magnetic field, respectively. Comparisons are also given with the observed magnetic field profiles and the computational results. In general, good agreement can be found including the interesting feature of the relatively thick magnetopause current layer at outbound measurements. Our work shows that 3D hybrid simulation is a promising method to study in detail the Hermean magnetosphere in parallel with the post-MOI observations of the MESSENGER spacecraft and the Bepi-Colombo mission in future. Author Affiliation: (a) Institute of Astronomy, National Central University, No. 300, Jhongda Rd, Jhongli City, Taoyuan County 32001, Taiwan, ROC (b) Institut fuer Theoretische Physik, Technische Universitaet Braunschweig, Mendelssohnstrase 3, 38106 Braunschweig, Germany (c) Institute of Space Science, National Central University, No. 300, Jhongda Rd, Jhongli City, Taoyuan County 32001, Taiwan, ROC (d) DLR Institut fuer Planetenforschung, German Aerospace Center, Rutherfordstrasse 2, 12489 Berlin, Germany Article History: Received 22 August 2009; Revised 21 May 2010; Accepted 25 May 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.05.019 Byline: W.-L. Tseng (a), W.-H. Ip (a), R.E. Johnson (b), T.A. Cassidy (b), M.K. Elrod (b) Keywords: Saturn; Rings; Atmosphere; Magnetosphere Abstract: The saturnian system is subject to constant bombardment by interplanetary meteoroids and irradiation by solar UV photons. Both effects release neutral molecules from the icy ring particles either in the form of impact water vapor or gas emission in the form of H.sub.2O, O.sub.2 and H.sub.2. The observations of the Cassini spacecraft during its orbit insertion have shown the existence of molecular and atomic oxygen ions. Subsequent modeling efforts have led to the picture that an exospheric population of neutral oxygen molecules is probably maintained in the vicinity of the rings via photolytic-decomposition of ice and surface reactions. At the same time, ionized products O.sup.+ and O.sub.2.sup.+ ions move along the magnetic field lines and, depending on the optical local thickness rings, can thread through the ring plane or impact a ring particle, the ion principal sink. In addition, collisional interactions between the ions and neutrals will change the scale height of the ions and produce a scattered component of O.sub.2 molecules and O atoms which can be injected into Saturn's upper atmosphere or the inner magnetosphere. The ring atmosphere, therefore, serves as a source of O.sub.2.sup.+ ions throughout Saturn's magnetosphere. If photolysis of ice is the dominant source of O.sub.2, then the complex structure of the ring atmosphere/ionosphere and the injection rate of neutral O.sub.2 will be subject to modulation by the seasonal variation of Saturn along its orbit. In this work, we show how the physical properties of the ring oxygen atmosphere, the scattered component, and the magnetospheric O.sub.2.sup.+ ion source rate vary as the ring system goes through the cycle of solar insolation. In particular, it is shown that the magnetopheric O.sub.2.sup.+ ions should be nearly depleted at Saturn's equinox if O.sub.2 is produced mainly by photolysis of the ring material. Author Affiliation: (a) Institute of Astronomy, National Central University, No. 300, Jhongda Rd., Jhongli 320, Taiwan (b) Department of Materials Science & Engineering, University of Virginia, Charlottesville, VA 22903, USA Article History: Received 5 February 2009; Revised 20 May 2009; Accepted 25 May 2009

A one-dimensional photochemical steady state model is used to study the vertical distribution of different gas species in the neutral atmosphere of Pluto. The major species is supposed to be [N.sub.2] with C[H.sub.4] as the leading minor species with a mixing ratio between 3.7 x [10.sup.-3] and 7.4 x [10.sup.-3], depending on the pressure and temperature on the surface. As a result of photolytic reactions, a number of hydrocarbons and nitriles are mainly produced in the 50-300 km region. Because of the uncertainty in the atmospheric thermal structure of Pluto, photochemical models with three different T(p) models have been developed. The turbulent processes have been parameterized, as in previous atmospheric models, with an eddy diffusion coefficient K = [K.sub.o][([n.sub.o]/n).sup.0.5] placing the homopause at an assumed altitude of [approximately]150 km. For the case we call nominal ([T.sub.surf] = 37 K and [p.sub.surf] = 10 [Mu]bar) parent molecules such as [N.sub.2], C[H.sub.4], and CO must be supplied from the surface with rates of 1.85 x [10.sup.7], 2.87 x [10.sup.8], and 6.83 x [10.sup.3] [cm.sup.-2] [sec.sup.-1], respectively. As a consequence of photolysis, H and [H.sub.2] escaping fluxes, scaled to the surface, are 3.50 x [10.sup.8] and 2.12 x [10.sup.8] [cm.sup.-2] [sec.sup.-1], respectively. Condensation of light hydrocarbons and nitriles occur between 3 and 5 km, depending on the compound and the surface temperature. Scaling the downward flux of these compounds to the amount of ice that would be deposited on the surface, we have deduced that the most abundant surface ices would be [C.sub.2] [H.sub.6], [C.sub.2] [H.sub.2], HCN, and [C.sub.2] [H.sub.4], in that order.

Dust measurements on the Vega and Giotto probes to Comet Halley have shown that dust particles of very small sizes (10-100 A) might be very abundant in cometary comas. A scenario of self-generation of such population of very small grains (VSGs) via collisional interaction with the cometary dust coma is considered here. If the VSGs can be charged to an electrostatic potential of [greater than or equal to] 10 V, they would be effectively trapped by electromagnetic interaction with the cometary plasma and the collisional erosion of the cometary dust coma would be facilitated. In the case that a small fraction of the impact kinetic energy can be converted to soft X-ray emission, the resultant X-ray luminosity might reach 5-10% of the value observed by ROSAT thus providing a second component of the source mechanism.

Recent spectroscopic observations of the saturnian system by the Hubble Space Telescope have shown the surprising results that the icy satellites and the rings are copious sources of neutral gas as a result of magnetospheric particle sputtering and meteoroid bombardment. Not only are the composition and structure of the saturnian magnetosphere significantly influenced by the continuous injection of water-group neutrals and ions, but the dynamics and mass distribution of the main ring as well as the tenuous E ring are also subject to the physical processes occurring in this unique plasma-dust-gas complex. By coupling the orbital motion of the neutral gas cloud with the photochemical effects and plasma chemistry, a numerical scheme is developed to analyze the possible contributions from different source mechanisms to the OH emission detected by the Hubble Space Telescope. It is found that, even if the maximum ion sputtering rate estimated by M. Shi, A. Baragiola, D. E. Grosjean, R. E. Johnson, S. Jurac, and J. Schou (1995, J. Geophys. Res. 100, 26387-26395) is used, the number density of the hydroxyl molecules still falls short by a factor of about 2. The discrepency can be partially elevated if additional source from meteoroid-impact vapor production at Enceladus is introduced. The collisional interaction of Enceladus with the E-ring particles of micron size first proposed by D. P. Hamilton and J. A. Burns (1993, Nature, 365, 498) is of particular promise. One corollary of the present model consideration is that the impact of the E-ring particles in highly eccentric orbits with the A ring could lead to a dense ring atmosphere.

The newly detected oxygen atmosphere of Europa is modeled by invoking charged particle sputtering with [H.sub.2]O and [O.sub.2] molecules as the main ejecta. The magnetospheric corotating ions could provide the required source strength ([approximately]3 x [10.sup.26] [sec.sup.-1]) of [O.sub.2] molecules if a fraction ([approximately]20%) of the exospheric ions were recycled to Europa's surface where they produce additional sputtering product. Two exospheric components are expected to form: an extended corona with a size of a few satellite radii which is composed of sputtered molecules in ballistic motion, and a thermal population with a surface density of [10.sup.8]-[10.sup.9] [cm.sup.-3] and a scale height of about 20 km. The electron impact ionization of this exosphere would lead to an Io-like interaction with the jovian magnetosphere with a field-aligned Birkeland current of about 5 x [10.sup.5] A.

Strong collisional interactions occur between interplanetary meteoroids and Saturn's rings. Mass and angular momentum transport of the ring system occurs mostly through a continuous process of impact fragmentation and erosion. Laboratory experiments show that hypervelocity impacts on ice-silicate mixtures result in a tenuous dust halo above and below the ring plane. Half thickness of the dust halo could be about 1500 km. Good observational data can be obtained during edge-on viewing condition in 1995 and 1996.

The structure of Saturn's neutral gas cloud, which is the product of collisions between meteoroids and the planet's ring system, was examined with respect to the asymmetric impact geometry of the ring-meteoroid interaction. The production rate of gas generated by meteoroid-impact was estimated to be 5 x 10(super 27) molecules per second. Water and oxygen molecule density around the ring was placed at 100 per cubic centimeter and 3000 per cubic centimeter, respectively. Oxygen atoms was hypothesized to reach icy satellites' orbital region through photodissociation.

Experimental results on the spectral identification of new infrared absorption features and the changes of their absorbances produced through vacuum ultraviolet–extreme ultraviolet (VUV–EUV) photon-induced chemical reactions in the C2H2–H2O mixed ices at 10 K are obtained. To the best of our knowledge, this is the first time that EUV photons have been employed in the study of the photolysis of ice analogues. Two different compositions, i.e., C2H2:H2O=1:4 and 1:1, were investigated in this work. A tunable intense synchrotron radiation light source available at the Synchrotron Radiation Research Center, Hsinchu, Taiwan, was employed to provide the required VUV–EUV photons. In this study, the photon wavelengths selected to irradiate the icy samples corresponded to the prominent solar hydrogen, helium, and helium ion lines at 121.6 nm, 58.4 nm, and 30.4 nm, respectively. The photon dosages used were typically in the range of 1×1015 to 2×1017 photons. Molecular species produced and identified in the ice samples at 10 K resulting from VUV–EUV photon irradiation are mainly CO, CO2, CH4, C2H6, CH3OH, and H2CO. In addition to several unidentified features, we have tentatively assigned several absorption features to HCO, C3H8, and C2H5OH. While new molecular species were formed, the original reactants, i.e., H2O and C2H2, were detectably depleted due to their conversion to other species. The new chemical species produced by irradiation of photons at 30.4 nm and 58.4 nm can be different from those produced by the 121.6-nm photolysis. In general, the product column density of CO reaches saturation at a lower photon dosage than that of CO2. Furthermore, the production yield of CO is higher than that of CO2 in the photon irradiation. In the present study, we also observe that the photon-induced chemical reaction yields are high using photons at 30.4 and 58.4 nm. The results presented in this work are essential to our understanding of chemical synthesis in ice analogues, e.g., the cometary-type ices and icy satellites of planetary systems. [Copyright &y& Elsevier]