Europa’s atmospheric neutral escape: Importance of symmetrical O 2 charge exchange

Abstract We model the interaction of the jovian magnetospheric plasma with the atmosphere of Europa using a multi-species chemistry model where the atmospheric distributions of H2 and O2 are prescribed. The plasma flow is idealized as an incompressible flow around a conducting obstacle. We compute changes in plasma composition resulting from this interaction as well as the reaction rates integrated over the simulation domain for several upstream plasma conditions (ion density, ion temperature and flow velocity). We show that for all cases, the main atmospheric loss process is a cascade of symmetrical charge exchanges on O2, which results in the ejection of neutrals. The production rate of ejected neutrals is about an order of magnitude larger than the production of ions. This conclusion is relevant to future missions to Europa that aim to detect fast neutrals. The neutral ejection resulting from this charge exchange creates an oxygen cloud around the orbit of the moon that is very extended radially but also very tenuous, and has not yet been directly detected.

[1]  W. Paterson,et al.  Galileo plasma observations at Europa: Ion energy spectra and moments , 1999 .

[2]  F. M. Neubauer,et al.  Nonlinear standing Alfvén wave current system at Io: Theory , 1980 .

[3]  Yukikazu Itikawa,et al.  Cross Sections for Electron Collisions with Oxygen Molecules , 2009 .

[4]  Robert E. Johnson.  Plasma-induced sputtering of an atmosphere , 1994 .

[5]  Anna Milillo,et al.  Loss rates of Europa׳s tenuous atmosphere , 2016 .

[6]  A. Milillo,et al.  Neutral particle release from Europa’s surface , 2010 .

[7]  Paul D. Feldman,et al.  Transient Water Vapor at Europa’s South Pole , 2014, Science.

[8]  M. Yousfi,et al.  Ar+/Ar, O2+/O2 and N2+/N2 Elastic Momentum Collision Cross Sections: Calculation and Validation Using the Semi-Classical Model , 2014 .

[9]  Anna Milillo,et al.  The role of sputtering and radiolysis in the generation of Europa exosphere , 2012 .

[10]  M. Yoshino,et al.  Cross Sections and Related Data for Electron Collisions with Hydrogen Molecules and Molecular Ions , 1990 .

[11]  W. Huntress,et al.  Ion cyclotron resonance studies on the reaction of H2+ and D2+ ions with various simple molecules and hydrocarbons , 1975 .

[12]  N. Thomas,et al.  Particle Environment Package (PEP) , 2013 .

[13]  Francesco Lazzarotto,et al.  Energetic neutral particles detection in the environment of Jupiter's icy moons: Ganymede's and Europa's neutral imaging experiment (GENIE) , 2013 .

[14]  J. Cooper,et al.  Jovian plasma torus interaction with Europa: 3D hybrid kinetic simulation. First results , 2010 .

[15]  W. Heiland,et al.  Energy loss of highly charged ions on an aluminum surface , 1996 .

[16]  William S. Kurth,et al.  Magnetospheric interactions with satellites , 2004 .

[17]  Amanda R. Hendrix,et al.  Cassini UVIS observations of Europa's oxygen atmosphere and torus , 2005 .

[18]  Jung-Sik Yoon,et al.  Cross Sections for Electron Collisions with Hydrogen Molecules , 2008 .

[19]  Fritz M. Neubauer,et al.  Interaction of the Jovian magnetosphere with Europa: Constraints on the neutral atmosphere , 1998 .

[20]  R. Baragiola,et al.  Sputtering of ice by low-energy ions , 2008 .

[21]  Paul D. Feldman,et al.  The Far-Ultraviolet Oxygen Airglow of Europa and Ganymede , 1998 .

[22]  F. Neubauer,et al.  Influence of the internally induced magnetic field on the plasma interaction of Europa , 2008 .

[23]  A. Fitzsimmons,et al.  Sputtering of oxygen ice by low energy ions , 2015, 1507.07906.

[24]  V. Anicich,et al.  Ion-molecule chemistry in Titan's ionosphere , 1997 .

[25]  L. Esposito,et al.  A NEW UNDERSTANDING OF THE EUROPA ATMOSPHERE AND LIMITS ON GEOPHYSICAL ACTIVITY , 2014 .

[26]  P. Banks.  Collision frequencies and energy transfer ions , 1966 .

[27]  F. Leblanc,et al.  Composition and Detection of Europa's Sputter-Induced Atmosphere , 2007 .

[28]  H. Tawara,et al.  Cross sections for electron capture and loss by positive ions in collisions with atomic and molecular hydrogen , 1985 .

[29]  F. Flasar,et al.  The ionosphere of Europa from Galileo radio occultations. , 1997, Science.

[30]  R. Jaumann,et al.  Effects of the External Environment on Icy Satellites , 2010 .

[31]  Andrew J. Steffl,et al.  Cassini UVIS observations of the Io plasma torus. II. Radial variations , 2004 .

[32]  On Europa's magnetospheric interaction : A MHD simulation of the E4 flyby , 1999 .

[33]  T. L. Bailey,et al.  Inelastic Collisions of H+2 and N+2 Ions with Hydrogen Molecules , 1966 .

[34]  R. Stebbings,et al.  Charge Transfer in Oxygen, Nitrogen, and Nitric Oxide , 1963 .

[35]  Robert E. Johnson,et al.  Europa's neutral cloud: morphology and comparisons to Io , 2004 .

[36]  F. Bagenal,et al.  A multispecies chemistry model of Io's local interaction with the Plasma Torus , 2008 .

[37]  D. Williams,et al.  In‐situ observations of a neutral gas torus at Europa , 2003 .

[38]  Andrew J. Steffl,et al.  Cassini UVIS observations of the Io plasma torus. I. Initial results , 2004 .

[39]  D. G. Mitchell,et al.  Energetic neutral atoms from a trans-Europa gas torus at Jupiter , 2003, Nature.

[40]  P. D. Feldman,et al.  Detection of an oxygen atmosphere on Jupiter's moon Europa , 1995, Nature.

[41]  Peter A. Delamere,et al.  Plasma conditions at Europa’s orbit , 2015 .

[42]  Rosaly M. C. Lopes,et al.  Atmospheric/Exospheric Characteristics of Icy Satellites , 2010 .

[43]  Peter A. Delamere,et al.  Radial Variations in the Io Plasma Torus during the Cassini Era , 2005 .

[44]  A. Barnett.  In situ measurements of the plasma bulk velocity near the Io flux tube , 1986 .

[45]  W. Smyth,et al.  Europa's atmosphere, gas tori, and magnetospheric implications , 2006 .

[46]  Robert E. Johnson,et al.  Charge exchange cross sections for the Io plasma torus , 1989 .

[47]  Edward A. Mason,et al.  Transport Properties of Gaseous Ions over a Wide Energy Range , 1976 .

[48]  Wing-Huen Ip,et al.  Europa's Oxygen Exosphere and Its Magnetospheric Interaction , 1996 .

[49]  D. Strobel,et al.  Three‐dimensional plasma simulation of Io's interaction with the Io plasma torus: Asymmetric plasma flow , 1999 .

[50]  J. B. Dalton,et al.  Magnetospheric ion sputtering and water ice grain size at Europa , 2013 .

[51]  Anna Milillo,et al.  Exospheric O2 densities at Europa during different orbital phases , 2013 .

[52]  Peter A. Delamere,et al.  Modeling Variability of Plasma Conditions in the Io Torus , 2003 .

[53]  Joachim Saur,et al.  Orbital apocenter is not a sufficient condition for HST/STIS detection of Europa’s water vapor aurora , 2014, Proceedings of the National Academy of Sciences.

[54]  J. Cooper,et al.  Surface-bounded atmosphere of Europa , 2005 .

[55]  Robert E. Johnson,et al.  Trace constituents of Europa's atmosphere , 2009 .

[56]  W. Kurth,et al.  Asymmetry of Io's outer atmosphere: Constraints from five Galileo flybys , 2012 .