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In this paper, we look at space weathering processes on the icy surface of Jupiters moon Europa. The heavy energetic ions of the Jovian plasma (H+, O+, S+, C+) can erode the surface of Europa via ion sputtering (IS), ejecting up to 1000 H2O molecules per ion. UV Photons impinging the Europas surface can also result in neutral atom release via photon-stimulated desorption (PSD) and chemical change (photolysis). In this work, we study the efficiency of the IS and PSD processes for ejecting water molecules, simulating the resulting neutral H2O density. We also estimate the contribution to the total neutral atom release by the Ion Backscattering (IBS) process. Moreover, we estimate the possibility of detecting the sputtered high energy atoms, in order to distinguish the action of the IS process from other surface release mechanisms. Our main results are: 1) The most significant sputtered-particle flux and the largest contribution to the neutral H2O-density come from the incident S+ ions; 2) The H2O density produced via PSD is lower than that due to sputtering by ~1.5 orders of magnitude; 3) In the energy range below 1 keV, the IBS can be considered negligible for the production of neutrals, whereas in the higher energy range it becomes the dominant neutral emission mechanism; 4) the total sputtering rate for Europa is 2.0cdot 1027 H2O s-1; 5) the fraction of escaping H2O via IS is 22% of the total sputtered population, while the escape fraction for H2O produced by PSD is 30% of the total PSD population. Since the PSD exosphere is lower than the IS one, the major agent for Europas surface erosion is IS on both the non-illuminated and illuminated side. Lastly, the exospheric neutral density, estimated from the Galileo electron density measurements appears to be higher than that calculated for H2O alone; this favours the scenario of the presence of O2 produced by radiolysis and photolysis.
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Physical processing of Europan surface water ice by thermal relaxation, charged particle bombardment, and possible cryovolcanic activity can alter the percentage of the crystalline form of water ice compared to that of the amorphous form of water ice (the crystallinity) on Europas surface. The timescales over which amorphous water ice is thermally transformed to crystalline water ice at Europan surface temperatures suggests that the water ice there should be primarily in the crystalline form, however, surface bombardment by charged particles induced by Jupiters magnetic field, and vapor deposition of water ice from Europan plumes, can produce amorphous water ice surface deposits on short timescales. The purpose of this investigation is to determine whether the Europan surface water ice crystallinity derived from ground-based spectroscopic measurements is in agreement with the crystallinity expected based on temperature and radiation modeling. Using a 1D thermophysical model of Europas surface, we calculate a full-disk crystallinity of Europas leading hemisphere by incorporating the thermal relaxation of amorphous to crystalline water ice and the degradation of crystalline to amorphous water ice by irradiation. Concurrently, we derive the full-disk crystallinity of Europas leading hemisphere using a comparison of near-infrared ground-based spectral observations from Grundy et al. (1999), Busarev et al. (2018), and the Apache Point Observatory with laboratory spectra from Mastrapa et al. (2018) and the Ice Spectroscopy Lab at the Jet Propulsion Laboratory. We calculate a modeled crystallinity significantly higher than crystallinities derived from ground-based observations and laboratory spectra. This discrepancy may be a result of geophysical processes, such as by vapor-deposited plume material, or it may arise from assumptions and uncertainties in the crystallinity calculations.
Much of the geologic activity preserved on Europas icy surface has been attributed to tidal deformation, mainly due to Europas eccentric orbit. Although the surface is geologically young (30 - 80 Myr), there is little information as to whether tidally-driven surface processes are ongoing. However, a recent detection of water vapor near Europas south pole suggests that it may be geologically active. Initial observations indicated that Europas plume eruptions are time-variable and may be linked to its tidal cycle. Saturns moon, Enceladus, which shares many similar traits with Europa, displays tidally-modulated plume eruptions, which bolstered this interpretation. However, additional observations of Europa at the same time in its orbit failed to yield a plume detection, casting doubt on the tidal control hypothesis. The purpose of this study is to analyze the timing of plume eruptions within the context of Europas tidal cycle to determine whether such a link exists and examine the inferred similarities and differences between plume activity on Europa and Enceladus.
We performed numerical simulations of impact crater formation on Europa to infer the thickness and structure of its ice shell. The simulations were performed using iSALE to test both the conductive ice shell over ocean and the conductive lid over warm convective ice scenarios for a variety of conditions. The modeled crater depth-diameter is strongly dependent on thermal gradient and temperature of the warm convective ice. Our results indicate that both a fully conductive (thin) shell and a conductive-convective (thick) shell can reproduce the observed crater depth-diameter and morphologies. For the conductive ice shell over ocean, the best fit is an approximately 8 km thick conductive ice shell. Depending on the temperature (255 - 265 K) and therefore strength of warm convective ice, the thickness of the conductive ice lid is estimated at 5 - 7 km. If central features within the crater, such as pits and domes, form during crater collapse, our simulations are in better agreement with the fully conductive shell (thin shell). If central features form well after the impact, however, our simulations suggest a conductive-convective shell (thick shell) is more likely. Although our study does not provide firm conclusion regarding the thickness of Europas ice shell, our work indicates that Valhalla-class multiring basins on Europa may provide robust constraints on the thickness of Europas ice shell.
We report results of a Hubble Space Telescope (HST) campaign with the Advanced Camera for Surveys to observe Europa at eastern elongation, i.e. Europas leading side, on 2008 June 29. With five consecutive HST orbits, we constrain Europas atmospheric ion{O}{1} 1304 A and ion{O}{1} 1356 A emissions using the prism PR130L. The total emissions of both oxygen multiplets range between 132 $pm$ 14 and 226 $pm$ 14 Rayleigh. An additional systematic error with values on the same order as the statistical errors may be due to uncertainties in modelling the reflected light from Europas surface. The total emission also shows a clear dependence of Europas position with respect to Jupiters magnetospheric plasma sheet. We derive a lower limit for the O$_2$ column density of 6 $times$ 10$^{18}$ m$^{-2}$. Previous observations of Europas atmosphere with STIS in 1999 of Europas trailing side show an enigmatic surplus of radiation on the anti-Jovian side within the disk of Europa. With emission from a radially symmetric atmosphere as a reference, we searched for an anti-Jovian vs sub-Jovian asymmetry with respect to the central meridian on the leading side, and found none. Likewise, we searched for departures from a radially symmetric atmospheric emission and found an emission surplus centered around 90 degree west longitude, for which plausible mechanisms exist. Previous work about the possibility of plumes on Europa due to tidally-driven shear heating found longitudes with strongest local strain rates which might be consistent with the longitudes of maximum UV emissions. Alternatively, asymmetries in Europas UV emission can also be caused by inhomogeneous surface properties, inhomogeneous solar illuminations, and/or by Europas complex plasma interaction with Jupiters magnetosphere.
In this paper we demonstrate a technique of utilizing magnetically trapped neutral Rb-87 atoms to measure the magnitude and direction of stray electric fields emanating from surface contaminants. We apply an alternating external electric field that adds to (or subtracts from) the stray field in such a way as to resonantly drive the trapped atoms into a mechanical dipole oscillation. The growth rate of the oscillations amplitude provides information about the magnitude and sign of the stray field gradient. Using this measurement technique, we are able to reconstruct the vector electric field produced by surface contaminants. In addition, we can accurately measure the electric fields generated from adsorbed atoms purposely placed onto the surface and account for their systematic effects, which can plague a precision surface-force measurement. We show that baking the substrate can reduce the electric fields emanating from adsorbate, and that the mechanism for reduction is likely surface diffusion, not desorption.
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