The plasma plumes of Europa and Callisto

We investigate the proposition that Europa and Callisto emit plasma plumes, i.e., a contiguous body of ionospheric plasma, extended in the direction of the corotation flow, analogous to the plume of smoke emitted in the downwind direction from a smokestack. Such plumes were seen by Voyager 1 to be emitted by Titan. We find support for this proposition in published data from Galileo Plasma Science and Plasma Wave observations taken in the corotation wakes of both moons and from magnetometer measurements reported from near the orbit of, but away from, Europa itself. This lends credence to the hypothesis that the plumes escaping from the ionospheres of Europa and Callisto are wrapped around Jupiter by corotation, survive against dispersion for a fairly long time and are convected radially by magnetospheric motions. We present simple models of plume acceleration and compare the plumes of the Europa and Callisto to the known plumes of Titan.     

Thermal state and complex geology of a heterogeneous salty crust of Jupiter's satellite, Europa

The complex geology of Europa is evidenced by many tectonic and cryomagmatic resurfacing structures, some of which are “painted” into a more visible expression by exogenic alteration processes acting on the principal endogenic cryopetrology. The surface materials emplaced and affected by this activity are mainly composed of water ice in some areas, but in other places there are other minerals involved. Non-ice minerals are visually recognized by their low albedo and reddish color either when first emplaced or, more likely, after alteration by Europan weathering processes, especially sublimation and alteration by ionizing radiation. While red chromophoric material could be due to endogenic production of solid sulfur allotropes or other compounds, most likely the red substance is an impurity produced by radiation alteration of hydrated sulfate salts or sulphuric acid of mainly internal origin. If the non-ice red materials or their precursors have a source in the satellite interior, and if they are not merely trace contaminants, then they can play an important role in the evolution of the icy crust, including structural differentiation and the internal dynamics. Here we assume that these substances are major components of Europa's cryo/hydrosphere, as some models have predicted they should be. If this is an accurate assumption, then these substances should not be neglected in physical, chemical, and biological models of Europa, even if major uncertainties remain as to the exact identity, abundance, and distribution of the non-ice materials. The physical chemical properties of the ice-associated materials will contribute to the physical state of the crust today and in the geological past. In order to model the influence of them on the thermal state and the geology, we have determined the thermal properties of the hydrated salts. Our new lab data reveal very low thermal conductivities for hydrated salts compared to water ice. Lower conductivities of salty ice would produce steeper thermal gradients than in pure ice. If there are salt-rich layers inside the crust, forming salt beds over the seafloor or a briny eutectic crust, for instance, the high thermal gradients may promote endogenic geological activity. On the seafloor, bedded salt accumulations may exhibit high thermochemical gradients. Metamorphic and magmatic processes and possible niches for thermophilic life at shallow suboceanic depths result from the calculated thermal profiles, even if the ocean is very cold.     

Statistics of the variations of the high-energy electron population between 7 and 28 jovian radii as measured by the Galileo spacecraft

Recent measurements of the high-energy, omni-directional electron environment by the Galileo spacecraft Energetic Particle Detector (EPD) have been analyzed in the range from 7 to 28 Jupiter radii. 10-min averages of these data between Jupiter orbit insertion in 1995 to the end of the mission have been analyzed to provide estimates of the electron differential fluxes at 1.5, 2, and 11 MeV in the jovian equatorial plane as a function of radial distance. These data provide a long term picture of the variations in the high-energy electron environment over the ∼8 years of the Galileo mission. This paper reviews those measurements and the statistics associated with them for the 8 year period. In general, the data variations are well behaved with variations being within a factor of ∼2 of a median value at a given distance from Jupiter. These results are analyzed in detail and the orbit variations discussed in the context of the overall data set. The results of this analysis of the long-term statistical variations in high-energy electron fluxes are directly applicable to models that estimate the effects of the radiation environment on Jupiter's moons and their atmospheres as they permit estimates of the possible range of radiation effects that might be expected.     

Europa's atmosphere, gas tori, and magnetospheric implications

A two-dimensional kinetic model calculation for the water group species (H₂O, H₂, O₂, OH, O, H) in Europa's atmosphere is undertaken to determine its basic compositional structure, gas escape rates, and velocity distribution information to initialize neutral cloud model calculations for the most important gas tori. The dominant atmospheric species is O₂ at low altitudes and H₂ at higher altitudes with average day-night column densities of 4.5×¹⁰¹⁴ and 7.7×¹⁰¹³ cm⁻², respectively. H₂ forms the most important gas torus with an escape rate of ∼2×¹⁰²⁷ s⁻¹ followed by O with an escape rate of ∼5×¹⁰²⁶ s⁻¹, created primarily as exothermic O products from O₂ dissociation by magnetospheric electrons. The circumplanetary distributions of H₂ and O are highly peaked about the satellite location and asymmetrically distributed near Europa's orbit about Jupiter, have substantial forward clouds extending radially inward to Io's orbit, and have spatially integrated cloud populations of 4.2×¹⁰³³ molecules for H₂ and 4.0×¹⁰³² atoms for O that are larger than their corresponding populations in Europa's local atmosphere by a factor of ∼200 and ∼1000, respectively. The cloud population for H₂ is a factor of ∼3 times larger than that for the combined cloud population of Io's O and S neutral clouds and provides the dominant neutral population beyond the so-called ramp region at 7.4-7.8 RJ in the plasma torus. The calculated brightness of Europa's O cloud on the sky plane is very dim at the sub-Rayleigh level. The H₂ and O tori provide a new source of europagenic molecular and atomic pickup ions for the thermal plasma and introduce a neutral barrier in which new plasma sinks are created for the cooler iogenic plasma as it is transported radially outward and in which new sinks are created to alter the population and pitch angle distribution of the energetic plasma as it is transported radially inward. The europagenic instantaneous pickup ion rates are peaked at Europa's orbit, dominate the iogenic pickup ion rates beyond the ramp region, and introduce new secondary plasma source peaks in the solution of the plasma transport problem. The H₂ torus is identified as the unknown Europa gas torus that creates both the observed loss of energetic H⁺ ions at Europa's orbit and the corresponding measured ENA production rate for H.     

Heat flow and thickness of a convective ice shell on Europa for grain size-dependent rheologies

Although it is mostly accepted that the lower part of the ice shell of Europa is actively convective, there is still much uncertainty about the flow mechanism dominating the rheology of this convective layer, which largely depends on the grain size of the ice. In this work, we examined thermal equilibrium states in a tidally heated and strained convective shell, for two rheologies sensitive to grain size, grain boundary sliding and diffusion creep. If we take a lower limit of 70 mW m⁻² for the surface heat flow, according to some geological features observed, the ice grain size should be less than 2 or 0.2 mm for grain boundary sliding or diffusion creep respectively. If in addition the thickness of the ice shell is constrained to a few tens of kilometers and it is assumed that the thickness of the convective layer is related to lenticulae spacing, then grain sizes between 0.2 and 2 mm for grain boundary sliding, and between 0.1 and 0.2 mm for diffusion creep are obtained. Also, local convective layer thicknesses deduced from lenticulae spacing are more similar to those here derived for grain boundary sliding. Our results thus favor grain boundary sliding as the dominant rheology for the water ice in Europa's convective layer, since this flow mechanism is able to satisfy the imposed constraints for a wider range of grain sizes.     

Morphology and origin of palimpsests on Ganymede based on Galileo observations

Palimpsests are large, circular, low-relief impact scars on Ganymede and Callisto. These structures were poorly understood based on Voyager-era analysis, but high-resolution Galileo images allow more detailed inspection. We analyze images of four Ganymedean palimpsests targeted by Galileo: Memphis and Buto Faculae, Epigeus, and Zakar. Ganymedean craters and Europan ring structures are used as tools to help better understand palimpsests, based on morphologic similarities. From analysis of Galileo images, palimpsests consist of four surface units: central plains, an unoriented massif facies, a concentric massif facies, and outer deposits. Using as a tie point the location in these structures where secondary craters begin to appear, outer deposits of palimpsests are analogous to the outer ejecta facies of craters; the concentric massif facies of palimpsests are analogous to the pedestal facies of craters; and the unoriented massif facies and central plains are analogous to crater interiors. These analogies are supported by the presence of buried preexisting structure beneath the outer two and absence of buried structure beneath the inner two units. Our observations indicate that palimpsest deposits represent fluidized impact ejecta, rather than cryovolcanic deposits or ancient crater interiors.     

A Melt-through Model for Chaos Formation on Europa

The character of chaotic terrain on Europa is consistent with its formation by the melting of a thin conducting ice shell from below. Tidal dissipation can provide adequate energy for such a process. For example, only a few percent of Europa's predicted tidal heat, spread over a region 200 km in diameter, can lead to large melt regions within a few tens of thousands of years. Stronger, more localized concentrations result in melt-through in significantly shorter times (i.e., a few hundred years). The time scale for melt-through is shorter than the time scale for the solid-state viscous inflow of ice by several orders of magnitude. In general, modest concentrations of tidal heat can melt ice away faster than viscous inflow, leading to melt-through. A mechanism to transmit these heat concentrations through the ocean is required for this model. Such heat transport could be the result of convective plumes in the ocean driven by seafloor volcanism or by the destabilization of a stratified ocean.     

Nonsynchronous Rotation Evidence and Fracture History in the Bright Plains Region, Europa

A geologic map for the Bright Plains in the Conamara Chaos region of Europa is presented and is used to unravel a detailed fracture sequence using cross-cutting relationships and fracture mechanics principles. Fracture orientations in the Bright Plains region rotated with time, consistently in a clockwise sense. This conclusion agrees with the observations of other researchers' northern Europan hemisphere investigations and points strongly toward the fracture sequence being controlled by the effect of nonsynchronous rotation, whereby the outer ice crust of Europa rotates slightly faster than the satellite's interior. This is convincing evidence that Europa's crust has been decoupled from the interior, possibly due to the presence of a liquid ocean beneath the crust.Tidal stresses induced in the ice crust by the combined effects of nonsynchronous rotation and diurnal tidal flexing can be calculated using the assumption that the crust behaves elastically over relatively short time scales (i.e., no viscous relaxation of stresses). The fracture orientations in the Bright Plains area were compared to a global scale tidal stress field to determine the longitudes at which each fracture set developed. The fracture sequence points strongly to the Bright Plains region of the crust having rotated at least 720° (and perhaps up to 900°) with respect to the satellite's interior during the visible fracture history. This amount exceeds previously published estimates of nonsynchronous rotation. The orientations of the most recent surface fractures are incompatible with the current state of stress in the Bright Plains region, implying a period of a few thousand years since the most recent fracturing events based on existing nonsynchronous rotation rate estimates.     

Neutral particle release from Europa’s surface

In this paper, we look at space weathering processes on the icy surface of Jupiter’s 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 Europa’s 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 2.0 × 10²⁷ H₂O s⁻¹; and (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 Europa’s surface total net 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 favors the scenario of the presence of O₂ produced by radiolysis and photolysis.