Europa's atmosphere, gas tori, and magnetospheric implications
Introduction
The nature of Europa's bound atmosphere and escaping extended neutral clouds (i.e., or gas tori) and their interactions with and impact on the plasma torus has been a problem of growing scientific interest for over two decades. This problem involves a fascinating set of coupled and complex neutral–plasma interactions and exothermic processes that are interconnected over a significant range of space and time scales. These interactions take place because the satellite, with its locally bound and escaping circumplanetary atmosphere, is overtaken at sub-Alfvénic speeds of ∼100 km s−1 by the heavy-ion (O+, O2+, S+, S2+, S3+, S4+) corotating (thermal, suprathermal, and energetic) iogenic plasma as it undergoes slow radial transport across Europa's orbit which is located well within Jupiter's strong magnetic field. In this paper, the plasma torus (rather than, as may be more appropriate, the Io–Europa plasma torus) will refer to plasma that is located radially from somewhat within Io's orbit to somewhat beyond Europa's orbit.
The formation of a tenuous bound atmosphere for Europa with O2 and O column densities of and of , respectively, and with a total gas escape rate of was suggested from early estimates for the ion sputtering rates from Europa's icy surface of H2O and its dissociative products (Johnson et al., 1981, Johnson et al., 1982, Johnson et al., 1983). The discovery in 1994 of an O2 atmosphere with a column density of was determined from the analysis of Hubble Space Telescope (HST) observations of O ultraviolet emissions attributed primarily to electron impact dissociative excitation of O2 (Hall et al., 1995, Hall et al., 1998). Europa's O2 atmosphere with a comparable column density has been independently confirmed from emission observations acquired in 2001 by the Ultraviolet Imaging Spectrograph (UVIS) on the Cassini spacecraft during its flyby of Jupiter (Hansen et al., 2005). Early models for Europa's O2 column density (Ip, 1996), based upon a surface sputtering rate of and an assumption that thermal ion sputtering dominated over energetic ion sputtering, were a factor of ∼10 too low. Later estimates (Shi et al., 1995, Ip et al., 1998) showed, however, that the suprathermal ions dominated the thermal ion sputtering rate, with new estimated sputtering rates in the range of , sufficient to support the HST observed O2 column density. For the higher neutral source rates, an important model for the three-dimensional sub-Alfvénic electrodynamic interaction (Wolf-Gladrow et al., 1987, Neubauer, 1980, Neubauer, 1998) of the plasma and Europa was developed by Saur et al. (1998). This model was based upon a representative two-species (electron and one ion) plasma and an empirical model for a one-species O2 atmosphere and was used to calculate the electric field, including Pedersen and Hall conductivities, for an assumed straight-line undisturbed planetary magnetic field. Saur et al. (1998) determined a preferred O2 column density of by matching the HST measured O emission brightness and also balancing the net-mass source to the atmosphere with a net-mass loss of ∼50 kg s−1. This atmospheric mass loss, equivalent to an O2 escape rate of , was dominated by the loss of fast neutrals () rather than the loss of ionospheric pickup ions (). In their model, the fast neutrals were created by knock-on and charge exchange collisions of O2 mostly with ionospheric ions having calculated densities of , similar to measured values (Kliore et al., 1997), and characteristic ion velocities of ∼20 km s−1 (Strobel, D.F., private communication, 1999) slowed near the satellite by the polarization electric field. These ionospheric ions, together with an equal population of ionospheric cold (i.e., chemically inactive, except for recombination) electrons, were produced by the impact of magnetospheric electrons with the atmosphere.
Kinetic theory models (Wong et al., 2000, Shematovich and Johnson, 2001, Marconi, 2003a, Shematovich et al., 2005) have been developed more recently for Europa's tenuous atmosphere and are able to include more than one gas species, to calculate explicitly the non-equilibrium nature of gases, and to treat kinetically various gas chemistry, such as molecular dissociation with exothermic (i.e., hot kinetic) products, not considered by Saur et al. (1998). The non-equilibrium nature of Europa's atmosphere was studied in the one-dimensional kinetic model of Shematovich and Johnson (2001) for O2 and in the improved one-dimensional kinetic model of Shematovich et al. (2005) including surface sources for both O2 and H2O. They showed that the O2 velocity distribution near the surface, although having a small suprathermal tail, is somewhat near equilibrium due to the frequent non-sticking but thermalizing collisions of O2 with the surface (producing near-surface small atmospheric scale heights). At higher altitudes, however, the O2 velocity distribution becomes significantly more non-thermal due to upward velocities of the surface-layer source and momentum exchange with hot dissociative products. In contrast, they showed that the H2O velocity distribution is similar near the surface and at higher altitudes and is determined essentially by its upward surface-sputtered velocity distribution, since downward moving H2O sticks upon impact with the surface. The small H2O non-thermal escape component of (Shematovich et al., 2005) then creates a neutral cloud, while the non-escape component of H2O is present as a species of only very minor abundance in the bound atmosphere. The 1-D kinetic models of Shematovich et al. (2005) also included photo and electron impact dissociation of O2 with exothermic products considered and provide an additional avenue for O escape of . The most complete kinetic model for Europa's atmosphere to date is the two-dimensional multi-species model of Marconi (2003a), which produces qualitatively similar results to those of Shematovich et al. (2005). This two-dimensional model includes all the water group species (H2O, H2, O2, OH, O, H), their important ionization and dissociation reactions including exothermic neutral products for magnetospheric electron chemistry and photochemistry, and collisional and charge exchange processes for magnetospheric and ionospheric ions in the atmosphere, which have been implemented more recently. In this model, O2 is the most abundant species in the atmosphere and is the dominant species near the surface while H2 is somewhat less abundant but is the dominant species at higher altitudes and has by far the largest escape rate. An improved version of the Marconi (2003a) model is adopted in this paper to investigate and calculate the basic spatial nature and gas escape rates for the water group species in Europa's tenuous atmosphere.
The existence of neutral clouds for Europa was suspected from possible evidence for plasma sources near Europa's magnetic L shell from the analysis of ion spectra from the Pioneer 10 spacecraft acquired in December 1973 (Intriligator and Miller, 1982). The analysis of Voyager plasma science data acquired in 1979 provided much more compelling evidence for possible sources of new ions in the vicinity of Europa's orbit because of enhanced temperatures, anisotropic ion distributions, and a sustained O2+ ion density (Bagenal, 1989, Bagenal et al., 1992) as well as the unusual and different radial behaviors of the O+, S+, and S2+ density profiles well inside and also near Europa's orbit (Bagenal, 1994). Studies for impact on the plasma torus of direct sputtered neutrals (H2O, O2, H2, and H) from Europa's surface were undertaken by Schreier et al. (1993) using a simple one-box model to calculate the ions and neutral densities for a set of plasma–neutral chemistry. To match representative ion densities at Europa's orbit for a total sputtered neutral source of with specified surface sputtering species yields but no consideration for species ejection kinetics, surface sticking, or accumulation of diatomic species in Europa's gravitational field, they concluded (1) that, in addition to iogenic plasma, a Europa neutral cloud density of was required, with neutral densities dominated by H, and (2) that Europa's contributions to the plasma torus content, although small compared to the iogenic source, can cause local density enhancements and compositional changes.
The first gases to be discovered in Europa's extended neutral clouds as well as in the outer portion of its corona (i.e., within an upper portion of Europa's Lagrange sphere of radius 8.7 satellite radii ) were Na in 1995 (Brown and Hill, 1996) and K in 1998 (Brown, 2001), both observed in solar resonance scattered emissions from ground-based telescopes. The inferred speeds of these minor atmospheric species are indeed consistent, being comparable to or less than Europa's surface escape speed (∼2 km s−1) as indicated by the observed Na and K column density radial power-law behaviors (Brown and Hill, 1996, Brown, 2001), Na Doppler line profiles (Potter, A.E., private communication, 1999), and deduced surface sputtering velocity distributions (Leblanc et al., 2002, Leblanc et al., 2005) with an average Na escape rate of . More recently, the existence of much more abundant neutral clouds (of unknown composition) for Europa was reported by Mauk et al. (2003) based upon the detection of copious energetic neutral atoms (ENAs) imaged by the ion and neutral camera (INCA) aboard the Cassini spacecraft in late 2000 and early 2001. The ENAs are thought to be produced by charge exchange of energetic magnetospheric H+ ions with Europa's neutral clouds. To produce the ENA flux observed, a neutral cloud population was estimated to be atoms (number density of ∼40 cm−3) for a pure H or mixed-species neutral cloud (Mauk et al., 2003, Mauk et al., 2004). Additional compositional analysis of these data (Mitchell et al., 2004) has also indicated the existence of a heavy (presumably O) ENA flux about one-half of the light (H) ENA flux. Analysis of energetic-ion depletion and pitch-angle signatures measured by the Energetic Particle Detector (EPD) aboard the Galileo spacecraft also provides estimated average number densities of ∼20 to 50 cm−3 for Europa's neutral clouds (Lagg et al., 2003). In addition, the dramatic decrease in the energetic ion population for decreasing radial distance from outside of Europa's orbit to Io's orbit (Paranicus et al., 2003) and the large drop in energetic H+ at Europa's orbit (Mauk et al., 2004) also generally reinforce this picture. To develop a framework to study these new observations, models for Europa's neutral clouds have recently been developed for the water group (O2, H2, O, H, OH, and H2O) gases (Smyth and Marconi, 2003a, Smyth and Marconi, 2004) and will be used in this paper to explore the circumplanetary distribution, sky-plane brightness, and pickup ion sources created for the most important gas species.
The paper is organized as follows. In Section 2, the kinetic model for Europa's tenuous atmosphere, which is utilized to provide initialization information for the neutral cloud model for the circumplanetary gas distribution, is briefly described, and a model calculation is presented for the water group species to illustrate their basic spatial behaviors and to determine their escape rates. In Section 3, the neutral cloud model for Europa is discussed, and model calculations are undertaken for the circumplanetary distributions of the most important species, H2 and O, and for their pickup ion sources for the plasma torus. The implications of Europa's neutral clouds are discussed in Section 4, and a summary is given in Section 5.
Section snippets
DSMC model description
Since the neutral cloud model is valid only above the exobase of a planetary atmosphere, it is necessary to have a description of a planetary atmosphere below the exobase in order to be able to initialize the calculation for the circumplanetary gas distribution. The atmosphere of Europa below the exobase is tenuous, as indicated by the estimated column densities for the major atmospheric species O2 of only , and, consequently, the atmospheric species will generally have
Neutral cloud model description
The density and circumplanetary spatial distribution of Europa's neutral clouds and their created europagenic plasma sources will be calculated by using the well-developed three-dimensional neutral cloud model for Io (Smyth and Combi, 1988a, Smyth and Combi, 1988b) with more recent improvements (see Smyth and Marconi, 2003b), which has been adapted in the last few years to Europa's water group neutral clouds (Smyth and Marconi, 2003a, Smyth and Marconi, 2004). The modeling for Europa is similar
Discussion
With the basic nature of Europa's most important neutral clouds for H2 and O characterized in Section 3, two important questions arise. First, what are the relative abundances and populations of the neutral clouds of Europa and Io? Second, what are the expected impacts of Europa's neutral clouds on the properties of the thermal plasma and the energetic plasma in Jupiter's inner magnetosphere? The first question is directly addressed below. For the second more complex question, various factors
Summary
A kinetic model calculation for the water group species in Europa's atmosphere is performed to determine its basic compositional structure and the corresponding gas escape rates and velocity distribution information required to initialize neutral cloud model calculations for the circumplanetary distributions of the most important gas tori. O2 is the dominant atmospheric species at low altitudes with the largest atmospheric column density of and is sustained by a surface sputtering
Acknowledgements
The research was sponsored by the National Aeronautical and Space Administration through the Jupiter System Data Analysis Program under contract NASW-02003 and through the Planetary Atmospheres Program and Geospace Space Science Program under the joint contract NASW-02036, both to AER. Dr. Marconi acknowledges support from the NASA Planetary Atmospheres Program under grant NAG5-8558. This analysis was also partially supported through the Planetary Atmospheres Program under contract NNG04GQ56G
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