P53B-1235
The Hydrostatic Shape of Europa and Implications for the Satellite's Libration
As a result of rotation and tides, Europa takes a triaxial ellipsoidal form with the longest axis in the direction to Jupiter and the rotation axis as the shortest axis. The solid interior, subsurface ocean, and shell have different polar and equatorial flattenings. In the hydrostatic approximation, the internal flattening profile can be determined from the density profile of the interior structure by using Clairaut theory. We calculated interior structure models of Europa and the shape of the internal layers by using the mass, radius, and mean moment of inertia as constraints. To obtain the moment of inertia, we extended the classical Radau equation, which relates the rotational flattening of the surface to the mean moment of inertia, to include also tidal deformation. Because of the equatorial flattening of the icy shell and the solid interior, misalignment of the principal axes of the icy shell and the solid interior due to differential rotation results in gravitational coupling between both layers. We derived an analytical expression for the strength of this coupling by using an approach that has been developed for the gravitational coupling between the mantle and the solid inner core of the Earth. The shell-interior gravitational coupling implies that rotation variations (librations) of the surface of Europa cannot be studied separately from the librations of the interior. We calculated the coupled librations of the shell and solid interior for a set of models of the interior structure of Europa. The presence of an ocean is shown to increase the amplitude of libration by about 7%, depending mainly on the thickness of the icy shell. Therefore, libration observations offer the possibility of detection of a subsurface ocean in Europa and estimation of the thickness of its overlying icy shell. For very thin shells of about 1 km, our results show that the shell libration is resonantly amplified.
P53B-1236
Detection of Solid Tides on Europa Through Ground-Tracking of a Low-Altitude, Altimeter- Equipped Orbiter
The possibility of detecting a global liquid ocean beneath the icy crust of Europa without the use of landers or ice penetrators rests on the measurement of the Love numbers h2 and k2. These are respectively related to the radial deformation of the surface and the consequent tidally-induced variation of the gravitational field of this icy satellite. Depending on the rigidity of the icy crust, the response of the Europan surface to the tidal forces gives an indication of the depth of a possible subsurface ocean. Previous studies in this area have addressed the detection of tidal surface deformations through the analysis of the tidally induced orbital perturbations of a Europan orbiter. As a preliminary study in preparation for future missions to Europa, as in the LAPLACE proposal to the European Space Agency, the approach followed here is to introduce the presence of an onboard altimeter. In this study we then generate synthetic measurements taken from an altimeter-equipped, low-altitude orbiter, supplemented with Earth-based tracking of the orbiter. For simplicity, ground-tracking is simulated as a range data-type. Altimeter measurements are simulated using parameters based on available models for the interior of Europa derived from Galileo mission data. Reference orbits were obtained by numerical investigations of the dynamically unstable near-Europa environment. Orbits were found to be stable over periods of approximately one to three months at altitudes of 100 km and inclinations varying from 75 degrees to 105 degrees. The measurements are consequently simulated over a period of one to two months. Under the hypothesis that Europan gravity field information of sufficient accuracy has been obtained in the first phase of the mission, the simulations address the detection of the solid tide related Love parameters h2 and k2. Results of this sensitivity study will be presented for a variety of orbital configurations with the aim to help in the design of future Europa exploration missions.
P53B-1237
The Conditions at Europa's Silicate-Water Interface
The data returned by the Galileo spacecraft strongly suggest that a liquid layer, several tens of kilometers thick, is squeezed between an outer ice shell and an inner silicate core. Compared to the other icy satellites, the relative small amount of H2O does not allow for the presence of a high-pressure ice layer between the ocean and the silicate core. The (P,T) conditions at the silicate-ocean boundary are quite similar to those existing on the Earth's sea floor where organisms live without solar energy. The study of Europa is therefore essential to answer questions such as: does life exist everywhere water is present? Can life arise and develop in an environment where there is no sunlight? Present models suggest a major difference between the Earth's sea floor and Europa's: plate tectonics allow for melting of silicates at 50 km depth and magmatic activity at mid-ocean ridges. Colonies of living species have been observed at places where the heat flux is not as high as it is at mid-ocean ridges but the heat flux is still much higher than that predicted by thermal evolution models of Europa's silicate core. One possibility would be that tidal heating resulting from Europa's eccentric orbit around Jupiter heats up the outer silicate layers resulting in volcanism similar to the one observed on Io. However, recent models (Tobie et al., JGR, 2003) suggest that most of tidal heating is dissipated in the outer icy crust and not in the silicate layer. 3D spherical models describing the thermal evolution of Europa's silicate shell are being run. These models investigate the possibility for Europa's silicate core to reach a state where tidal heating could be important and would allow for active volcanism during long periods of time. Determining the presence of an ocean and its characteristics are a major objective of the ‘LAPLACE' proposal that was submitted to the European Space Agency in response to the Cosmic Vision AO. This paper will describe the proposed payload that will acquire the data necessary to answer the questions described above.
P53B-1238
Hydrothermal Convection in Europa's Silicate Mantle
Observations by the Galileo spacecraft have shown that Europa likely possesses a liquid water ocean beneath a solid icy outer shell although the depth to the top of the ocean and its thickness remain unknown. The ice shell thickness has important implications for the formation of chaotic terrain and the accessibility of the ocean to future exploration. Hydrothermal plumes have been suggested as a possible mechanism for thinning the ice shell and facilitating the formation of chaotic terrain. The plumes could transport heat from Europa's silicate mantle to the ice shell and possibly melt through the ice. We present new numerical simulations of hydrothermal circulation in the silicate mantle and ocean of Europa that allow us to quantify its impact on overall heat transport and the global ice shell thickness. The processes in our models are hydrothermal convection in the mantle and ocean, thermal diffusion in the core and mantle, parameterized thermal diffusion to account for enhanced heat transfer in the ocean and ice layer, radiogenic heating in the rocky mantle, latent heat of melting/freezing, and tidal heating in the ice shell. Our models start with a differentiated Europa. We select a reasonable value of 750 km for the radius of Europa's metallic core. The silicate mantle extends from the top of the metallic core to a radius of 1465 km. The ocean is approximately 100 km thick and is surrounded by a thin ice shell. The metallic core and silicate mantle have an initial conductive temperature profile appropriate for Europa immediately after differentiation. Our model assumes that tidal heating starts much later than hydrothermal convection. Consequently, hydrothermal convection is not initially aided by tidal heating. The convective pattern is established within a few tens of million years; thereafter a fairly steady state is reached for a few billion years. During this time the ocean thickness is reduced to roughly 50-60 km. However, as a consequence of hydrothermal convection, heating at the base of the ice is spatially heterogeneous and variations in ice shell thickness that directly correlate with the hydrothermal plumes are maintained. Tidal dissipative heating starts around 4 Gyr when the Laplace resonance between Io, Europa, and Ganymede likely formed. The ice shell thins to about 20 km, corresponding to an ocean thickness of 80 km. This process is complete in 20-30 Myr. Additionally, the topographic variations initially present in the ice shell decrease and the ice becomes much more uniform once tidal heating starts. Over time, there is a gradual loss of energy, but the interior is very warm and has high heat content, so the overall dynamical activity decreases only very gradually. Our model indicates that it would be difficult to completely melt through the ice shell. However, even without tidal heating, hydrothermal convection is able to maintain an ocean through most of Europa's history. Additionally, hydrothermal convection promotes the transport of salt from the silicate mantle to the ice shell. The transport of salt through convective flow leads to the formation of a brine layer at the bottom of the ice shell due to exclusion of salt upon freezing. The average salt concentration at the base of the ice is significantly higher than in the bulk of the ocean. Convection within the ice shell will likely be affected by the presence of a brine layer at its base.
P53B-1239
The Thermo-Chemical Evolution of Europa: Constraints on Silicate Volcanism at Europa's Ocean Floor
One of the key factors for Europa's habitability is the level of internal activity and its possible consequences, e.g. volcanism, at the ocean floor. Tidal dissipation in the silicate mantle would affect Europa's thermal state with consequences for the long-term existence of an ocean and -in case of volcanism- for ocean chemistry. In parameterized thermo-chemical evolution models we consider the thermal coupling between an iron-rich core, a silicate mantle and a water/ice layer. The models further include tidal heating in both the silicate and the ice. Models without tidal heating already suggest that partial melt in the silicate mantle can be maintained up to a few billion years. In that case, tidal heating in the silicate shell will become important, prolonging the lifetime of partially molten reservoirs and associated ocean-floor volcanism. Heating of the silicate mantle would also affect the thermal state and thickness of the ice shell. Additionally, tidal heating in the mantle may impede the generation of a self-sustained magnetic field in Europa's core. We will discuss consequences of time-dependent orbital parameters due to the resonance locking with Io and Ganymede on Europa's thermal state.
P53B-1240
Europan Cycloidal Rift Densities and Io Volcano Distribution: Implications for Tidal Activity
Previous works have described cycloidal rifts and how they form (Hoppa, 1999), but none have gone into detail on where they occur and what rift density may mean to the system as a whole. In this study, GIS software is used in conjunction with high-resolution images to map cycloidal rifts on Europa's surface. Total lengths of rifts within varying latitudinal/longitudinal areas are calculated and plotted to display which regions of the planet are most active in cyclodial rifting processes. GIS is also used to create contour maps displaying rift density. Finally, rift length plots are compared to volcano counts on Io using the same longitude latitude intervals. The initial results show clear minimum values at around ±90° longitude, and maxima near 0° and 180° longitude (the sub-Jovian point and its antipode). Maxima on latitude are clustered around the equator and taper off towards the poles. These values correspond well to expected areas of extension and compression in Europa's icy crust. There is also a fairly strong correlation between the positions of these minimum and maximum values and the minimum and maximum volcano counts on Io, further supporting this assertion.
P53B-1241
Global Geology and Stratigraphy of Europa
Previous geologic mapping of Europa, using hemisphere-wide low-resolution Voyager coverage, as well as local and regional higher resolution Galileo coverage, has now been synthesized and augmented as a global map and linea stratigraphy produced in ArcGIS using a photomosaic of the best Galileo and Voyager resolutions and a custom linea-stratigraphy sorting tool. A stratigraphic sequence from an older plains-building period (60 mya according the crater age estimates), through a linea-building period to a chaos-forming period is seen, with minor contribution from impact cratering. Variable photometry and resolution in the base map makes interpretations necessarily tentative, underscoring the need for future missions to conduct a global mapping at higher resolution. This work is being published as a forthcoming USGS map product, and as a chapter in the UoA Space Science Series volume on Europa.
P53B-1242
Tidally-driven Fractures on Europa: Historical Overview and New Modeling Techniques
Cycloids are made up of linked arcuate segments and were observed in both Voyager and Galileo imagery. It has been proposed that cycloids are fractures that propagate in response to the tidal stress field, which changes throughout each orbit due to Europa's eccentricity. Several studies have tested tidal models by generating cycloidal features and comparing them to actual cycloids, resulting in compelling evidence for non-synchronous rotation. Also, recent modeling of cycloids in equatorial regions has shown that the obliquity of Europa is large enough to affect the formation of surface features. In addition to constraining the orbital and rotational parameters that control Europa's tidal stress field, modeling of cycloidal fractures can provide constraints on the mechanical properties of the ice shell. Despite the successes of cycloid modeling, the methodology for generating cycloids and comparing them with actual data could be greatly improved and more widely applied. Only five cycloids have been modeled although dozens of global-scale features can be identified in the Galileo images. And although equatorial cycloids provide the best constraints on the amount of obliquity and direction of the spin pole, only one cycloid in this region has been successfully modeled, mostly due to the increased parameter space and lack of symmetry in the stress field. Searching for best fits by hand is no longer feasible, especially for the large number of cycloids needed to precisely constrain Europa's orbital and rotational parameters. In addition, past cycloid modeling has not relied on a quantitative measure of goodness of fit when matching hypothetical cycloids to the observed features. While this approach may have been satisfactory for early work, as fits improve, it becomes increasingly important to have a consistent and quantitative measure with which to evaluate modeled cycloids. Moreover, a quantitative measure of misfit can be translated into uncertainties for model parameters. We are currently developing a new method of cycloid modeling that combines the standard tidal model (including obliquity) with an automated and quantitative fitting program and a Markov Chain Monte Carlo algorithm to search the parameter space. Our preliminary work has shown that this method is effective when applied to a synthetic crack; the algorithm does indeed "find" the parameters with which the crack was generated. Our ongoing work involves mapping and modeling large-scale equatorial cycloids, using this upgraded method, in order to constrain the amount of obliquity, direction of the spin pole, non-synchronous rotation rate, and amount of stress generated by non-synchronous rotation.
P53B-1243
Formation of Ridge-Type Strike-Slip Faults on Europa
Europa, Jupiter's fourth largest moon, has been imaged by both NASA's Voyager and Galileo spacecraft. The most common lineaments found on Europa's pervasively fractured surface are ridges, many of which also appear to be strike-slip faults in that they offset other crosscutting lineaments. Recent formation models have proposed that ridges may be created through shearing, although earlier models characterized them as tension fractures. We developed a technique to determine the dominant deformation characteristics along ridges having apparent lateral offsets. We show evidence for both lateral shearing and convergence along ridges, indicating that apparent lateral offsets are not purely the result of strike-slip motions. Using ISIS software, Galileo images were reprojected using both orthogonal and transverse mercator projections, to conserve line lengths and preserve angular relationships, respectively. These projections are needed to accurately differentiate shearing-related offsets from convergence-related offsets. However, there are caveats to our developed technique for determining these relative offsets and thus the formation mechanisms behind ridge development. Apparent offsets must be large enough to overcome image resolution constraints. It is also important that several offset lineaments crosscut the ridge with a wide range of orientations relative to the ridge in order to most accurately determine the relative motions across the ridge. Relative orientations are defined by angle alpha, measured clockwise from the ridge to the crosscut feature. At both small and large alpha angles (0 to 30 degrees and 160 to 180 degrees), apparent offsets due to convergence across the ridge may become relatively large compared to where alpha angles are closer to 90 degrees (for which pure ridge-orthogonal motion gives an apparent offset of zero). Causes of apparent offsets are quantified using plots of normalized separations of offset features versus alpha, which produce unique curves depending on the relative amounts of lateral motion and convergence. This analysis of offsets across ridges that resemble strike-slip faults will highlight the processes that drive ridge development (i.e., dilation, contraction, lateral shearing, or a combination thereof) and current ridge formation models may ultimately be refined.
P53B-1244
Implications of Viscous Relaxation in Europa's Icy Shell for Interpreting Global Tectonic Features
We calculate tidally induced stress at the surface of an icy satellite based on the gravitational potential. The ice shell is treated as a viscoelastic Maxwell solid, and assumed to float atop a global ocean. The shell is thus capable of differential rotation relative to the silicate core, which is presumed to rotate synchronously. If the shell experiences non-synchronous rotation (NSR), stresses result from the time-varying potential. In a Maxwell solid both the magnitude and time-variability of stress induced by a forcing depend on the frequency of the forcing. Viscous relaxation allows stress to dissipate, and the non-instantaneous response of the material creates a phase lag between the forcing function and the stresses resulting from it. The importance of viscous effects in an ice shell undergoing NSR will be determined by its viscosity (η), shear modulus (μ), and the rotation period (T), and can be parameterized by the quantity: Δ \equiv \frac{T}{2π~τM} = \frac{μ}{η ω}} Where ω is the forcing frequency, and τM is the Maxwell time. If Δ \gg 1 the response is fluid; no shear stress is supported. If Δ \ll 1 the response is elastic; shear stresses may be large, and will be in phase with the forcing function. For Δ ≈ 1 the response is viscous; shear stresses may be large or small, have a phase lag relative to the forcing function, and are very sensitive to Δ. Choosing values appropriate to Europa and its ice shell (η ~ 1022 Pa sec, T ~ 107 yr, μ ~ 109 Pa) yields Δ ≈ 1. Between Δ=10-1 and Δ=101 the stresses due to NSR shift ~ 40° in longitude and relax from 3.5 MPa to 0.5 MPa. Because of this sensitivity, and because our knowledge of the shell's viscosity and rotation rate is limited, it is impossible to make confident predictions of stress magnitude or longitudinal dependence. We demonstrate this uncertainty through a study of the global arcuate lineaments on Europa that have previously been interpreted as tensile fractures due to NSR stresses. If a variable rotation rate is considered, it becomes difficult to use a lineament's apparent longitude of formation as a proxy for time of formation relative to other lineaments, since the stress field will sweep across the surface as ω changes. We demonstrate that a small reorientation of an otherwise synchronously rotating shell can produce lineaments spread across up to 45° in longitude.
P53B-1245
Determining the roles of active, passive, and compositional support mechanisms in the formation of bands and complex ridges on Europa utilizing topography
It has been suggested that band topography observed in limited stereo coverage of Europa could result from lateral density variations created through active thermal (e.g., shear heating), passive thermal (e.g., thermal buoyancy), or compositional (e.g., compositional buoyancy) mechanisms. More extensive topographic data has been produced using stereo photogrammetry and ‘2d' photoclinometry and is being used to test the applicability of these support mechanisms to explain the formation of bands and complex ridges. Previously, three regions on Europa were examined (i.e., E4. E12 Wedges, and Rhadamanthys) and, in the latter two regions, topography indicates that young bands appear to be elevated above older ones, possibly suggesting that passive support mechanisms (i.e., thermal buoyancy) play a role in the formation of the most recent bands. We are currently examining additional regions of the surface utilizing DEMs and topographic profiles of Europa's surface to further characterize the likelihood of passive support mechanisms (e.g., thermal buoyancy) as a means of accounting for age relationships of bands with respect to each other and the surrounding terrain. We are also looking in more detail at complex ridges to determine their relationship, if any, to bands and to the surrounding terrain.
P53B-1246
Terrestrial Diapirs as Analogs to Europa's Lenticulae and Chaos, and Implications for Europa Exploration
Ice diapirism has been cited to explain Europa's pits, spots, and domes (commonly collectively referred to as "lenticulae") as well as the satellite's larger chaos terrains. Europa's diapirs have been modeled as thermal- compositional in origin, rising within an ice shell greater than ~20 km thick. The morphologies and characteristics of terrestrial diapirs shed light on possible diapiric processes within Europa. Diapirs commonly rise in fields of similarly sized subcircular features which can intrude into the shallow subsurface or extrude onto the surface. Rim synclines (peripheral depressions) may form in response to withdrawal of diapiric material from a diapir's surroundings, and peripheral and crestal faults are predicted above intrusive diapirs. The heads of neighboring synchronously active diapirs can flatten against one another. Each of these terrestrial characteristics is consistent with the morphologies of some Europan lenticulae. Terrestrial diapiric heads can merge into a broad canopy, potentially analogous to the formation of some Europan chaos terrains. Xenoliths can be carried upward within terrestrial diapirs, suggesting that diapirism within Europa's ice shell can dredge deep material up toward the surface on the timescale of diapir rise. The deepest strata rise into the central axes of terrestrial diapirs, implying that materials from greatest depth in Europa's ice shell may be exposed in the centers of individual extrusive lenticulae and in discrete locations within chaos regions. Lenticulae and chaos are high priority locations to explore for materials that have risen from near Europa's ice-ocean interface to the surface.
P53B-1247
CO2, SO, Hydrates, and Other Constituents on the Trailing and Anti-Jovian Hemispheres of Europa From Reprocessed Galileo/NIMS Measurements
The Near Infrared Mapping Spectrometer observations at wavelengths 0.7-5.3 μm of the Jovian moon Europa are being reprocessed using information gained throughout and beyond the Galileo mission. Early analysis of these reprocessed data (a global observation of the anti-Jovian and trailing hemispheres at 47 km spatial resolution) show evidence of spectral features not or only weakly apparent before (e.g., Geophys. Res. Lett., submitted). These include strong absorption bands attributed to CO2 and SO (centered near 4.25 and 4.0 μm, respectively), as well as weaker bands, such as one attributed to H2O2 (near 3.5 μm). The strong hydrate bands, including at near 1.5 and 1.95 μm, are more clearly defined. The band depth distribution of the CO2 and the most well defined hydrate bands are strongly associated with the (endogenic) dark reddish regions on the surface. The SO is not strongly correlated with the CO2, but has a similar sparse distribution. We will continue this study looking at higher spatial resolution observations and higher spectral resolution observations of the same area from the same and other orbits of Galileo.
P53B-1248
Consequence of Electron Mobility in Icy Grains on Solar System Objects
Solar system ices have been shown to contain organic molecules, whether in the ice on Mars, comets such as Tempel-1 (from the Deep Impact mission) or on the surfaces of Europa, Ganymede, and Callisto. Sub-surface oceans containing ionic salts have been proposed to interpret the induced components of the local magnetic fields at these Galilean moons. Presence of liquid water is thought to be a requirement for potential astrobiological habilitability, particularly on Europa where the putative subsurface ocean is likely closest to the outer surface. Recent laboratory studies have shown that radiation processing of water-rich ices containing aromatic organic impurities readily ionizes organic molecules imbedded in an ice matrix. As a result, transient charge separation is produced more efficiently in ices containing organic impurities. This charge separation is partially stabilized by electron trapping. This could have important consequences since the icy moons of the giant planets are imbedded in both the magnetic field and trapped particle radiation environments of the planetary magnetospheres. Internal discharges of accumulated free charges (i.e. ice lightning) could significantly affect molecular chemistry of the irradiated outer layer beyond the direct effects of irradiation. Here we present new experimental results and theoretical modelling that deals with mobility of electrons produced by photoionization of PAHs (polycyclic aromatic hydrocarbons) in an ice matrix. We find that a small portion of the electrons (about 5% of the originally generated) are weakly trapped in the impurity-containing ices and can be made mobile at temperatures between 50 K and 125 K. Current flow of these mobile electrons could affect electrical conductivity of the irradiated surfaces and contribute to induced magnetic fields. This solid-state micro-ionospheric environment, comparable to a thin metallic conducting shell, may then need to be taken into account, along with the above-surface ionosphere, in modelling background variations affecting detection of induced magnetic fields from the sub-surface oceans. References: 1. M. S. Gudipati, L. J. Allamandola, J. F. Cooper, S. Sturner, R. E. Johnson (in preparation) 2. J. F. Cooper, R. E. Johnson, B. H. Mauk, H. B. Garrett, N. Gehrels, Icarus 149, 133 (2001). 3. M. S. Gudipati, Journal of Physical Chemistry A 108, 4412 (2004). 4. M. S. Gudipati, L. J. Allamandola, Astrophysical Journal Letters 615, L177 (2004). 5. M. S. Gudipati, L. J. Allamandola, Astrophysical Journal 638, 286 (2006). 6. M. S. Gudipati, L. J. Allamandola, Journal of Physical Chemistry A 110, 9020 (2006).
P53B-1249
Modeling Minor Constituents of Europa's Atmosphere
A spacecraft orbiting Jupiter's moon Europa, of the sort considered by both ESA and NASA, would provide an opportunity to determine the composition and morphology of its tenuous atmosphere. Europa's atmosphere, though tenuous, has been detected by Earth-based telescopes. Its O2 atmosphere was detected from Earth orbit and its much thinner alkali atmosphere was detected by ground-based telescopes. Many other species are expected based on surface reflectance spectra, such as H2O, Sn, SO2, CO2, H2O2. I will discuss the issues involved in the modeling of these as-yet-undetected components. Previous theoretical studies and observations of the atmosphere produced important conclusions about the surface and its interaction with the Jovian magnetosphere. The modeling and detection of minor components could reveal much more. Of particular interest is the detectability of these species with an orbiting mass spectrometer or more distant light spectrometer.
P53B-1250
Radar Spectrometry for the Europa Explorer
We present the concept of using a lightweight, millimeter and submillimeter radar system, in orbit around Europa, to study its surface chemistry through measurements of its tenuous atmosphere. This technique adapts conventional radar concepts to the shorter wavelengths where molecular absorptions are common, easily resolved and well understood. A tunable mW-level solid-state transmitter on board the Europa Explorer will actively illuminate a spot on the surface and the back-scattered radiation will be detected by a heterodyne receiver system, enabling spectrometry of the exosphere. All molecules in the exosphere with an electric or magnetic dipole will absorb radiation in the millimeter and submillimeter wavelengths. Physical constraints including: practical mass and power of the instrument, two separate frequency bands, an altitude of 100-200 km and the scattering properties of the Europan surface were used to determine sensitivities to interesting species. Our calculations show that polar species present at ppm levels in the ice will be detectable during the sputter induced ballistic transits of the exosphere with roughly ten seconds or less of integration time from orbit. This will enable us to determine the chemical composition of surface species including salts and organic molecules, thus providing information on Europa's astrobiological potential. Additionally, measurements of water and oxygen will be routine and allow physical aspects of the exosphere, such as the sputtered velocity profiles and the local magnetic environment, to be probed remotely. The instrument will also be useful for measurements of surface temperatures, surface roughness and dielectric properties and surface to satellite ranging.
P53B-1251
Sodium Recycling at Europa : What do we Learn From the Sodium Cloud Variability ?
Discovery and further observations of Europa's thin atmosphere of sodium have been carried out by M.E. Brown (Brown and Hill 1996, Brown 2001, Brown 2004) and A.E. Potter and co-workers (Leblanc et al, 2005). The resonant scattering emission of sodium around Europa has been successfully modelled and compared to the compilation of such observations by Leblanc at al 2002; Leblanc et al 2005). Such an analysis confirmed that the cloud morphology is dominated by the production of Na from the trailing hemisphere. The influence of Europa's centrifugal latitude as well as the contribution of Io's sodium source at Europa orbit were also estimated. These studies concluded that the observed sodium atmosphere should be largely endogenic to Europa. However, significant variations of the total emission intensity along Europa's orbit around Jupiter were reported that were difficult to explain without adhoc assumptions on the variability of the sodium ejecta rate with respect to Europa position in Jupiter magnetosphere. In the present study, we investigate the redistribution of the ejected sodium atoms on the surface of the moon during its orbit around Jupiter following the suggestion by Leblanc et al (2005). In our model, the redistribution of sodium atoms at Europa's surface occurs from a set of ejection and absorption of the sodium atoms. Ejection processes are sputtering induced by energetic jovian particles, as well as photo-stimulated and thermal desorptions from the surface. Absorption mainly depends on the surface temperature and porosity. We will present comparisons of the newly calculated sodium emission with the observations, as well as density distributions of sodium at Europa's surface. Consequences of those calculations on the sodium cloud morphology will also be discussed.
P53B-1252
Motions of Energetic Electrons and Ions in the Vicinity of Europa
Because of the increasng interest in a new mission to Europa in the form of Europa Orbiter, the issue of the radiation environment near this Jovian moon has attracted renewed attention. This is because a better description of the fluxes of energetic charged particles will be important to understanding of the radiolysis of the surface icy materials which holds the key to the chemical compositions in different regions of Europa's surface and the production of the oxygen atmosphere. In addition, such study should be of use to mission design concerning the radiation dosage to be endured by the spacecraft. It is also known that the motions of energetic electrons and ions are significantly influenced by the magnetic and electric fields near Europa which are modified by its interaction with the Jovian magnetosphere. For this reason, resistive MHD model simulations of Europa's plasma and field environment have been carried out taking into account the induced dipole field generated by the time-variation of the planetary magnetic field as a result of the flapping of the Jovian current sheet. The viability of this set of model calculations will be examined by comparison to the magnetic field measurements from the Galileo magnetometer observations during several of the Europa encounters. Results of some test runs of particle trajectory calculations for MeV electrons and ions will be reported.
P53B-1253
The survival of large organic molecules during hypervelocity impacts with water ice: implications for sampling the icy surfaces of moons
There are a number of measurements relevant to planetary geology that can only be adequately performed by physically contacting a sample. This necessitates landing on the surface of a moon or planetary body or returning samples to earth. The need to physically contact a sample is particularly important in the case of measurements that could detect medium to low concentrations of large organic molecules present in surface materials. Large organic molecules, although a trace component of many meteoritic materials and rocks on the surface of earth, carry crucial information concerning the processing of meteoritic material in the surface and subsurface environments, and can be crucial indicators for the presence of life. Unfortunately landing on the surface of a small planetary body or moon is complicated, particularly if surface topography is only poorly characterised and the atmosphere thin thus requiring a propulsion system for a soft landing. One alternative to a surface landing may be to use an impactor launched from an orbiting spacecraft to launch material from the planets surface and shallow sub-surface into orbit. Ejected material could then be collected by a follow-up spacecraft and analyzed. The mission scenario considered in the Europa-Ice Clipper mission proposal included both sample return and the analysis of captured particles. Employing such a sampling procedure to analyse large organic molecules is only viable if large organic molecules present in ices survive hypervelocity impacts (HVIs). To investigate the survival of large organic molecules in HVIs with icy bodies a two stage light air gas gun was used to fire steel projectiles (1-1.5 mm diameter) at samples of water ice containing large organic molecules (amino acids, anthracene and beta-carotene a biological pigment) at velocities > 4.8 km/s.UV-VIS spectroscopy of ejected material detected beta-carotene indicating large organic molecules can survive hypervelocity impacts. These preliminary results are yet to be scaled up to a point where they can be accurately interpreted in the context of a likely mission scenario. However, they strongly indicate that in a low mass payload mission scenario where a lander has been considered unfeasible, such a sampling strategy merits further consideration.
P53B-1254
Astrobiology Science and Technology for Exploring Planets: Programmatic Preparations for Europa Exploration
A new era of planetary exploration - and the investigation of whether life exists or has existed beyond Earth - requires the development of biologically relevant instruments capable of extensive, autonomous operations on and below planetary surfaces. In collaboration with other agencies, NASA's Astrobiology Science and Technology for Exploring Planets (ASTEP) Program sponsors investigations focused on exploring the Earth's extreme environments through use of terrestrial field campaigns to develop a sound technical and scientific basis upon which to conduct a search for life or life-related molecules on other planets. The ASTEP Program is a science-driven exploration program exercising new science and operational/technological capabilities to enable this next generation of planetary exploration. One focus of ASTEP has been preparing for future missions to Europa, where the challenges of mass, power, volume are acute in accomplishing remote exploration, ice penetration, and following the lure of potential ocean-bottom features (~100km down!) such as hydrothermal venting. This poster will discuss the advantages of extreme-environment expeditions in preparing for the exploration of other worlds, and review current and future ASTEP-funded activities that contribute to Europa exploration. In addition, future mission concepts for Earth-based Europa exploration that might be proposed to a future ASTEP opportunity will be envisioned.
P53B-1255
Planetary Protection for future missions to Europa and other icy moons: the more things change...
NASA maintains a planetary protection policy regarding contamination of extraterrestrial bodies by terrestrial microorganisms and organic compounds, and sets limits intended to minimize or prevent contamination resulting from spaceflight missions. Europa continues to be a high priority target for astrobiological investigations, and other icy moons of the outer planets are becoming increasingly interesting as data are returned from current missions. In 2000, a study was released by the NRC that provided recommendations on preventing the forward contamination of Europa. This study addressed a number of issues, including cleaning and sterilization requirements, the applicability of protocols derived from Viking and other missions to Mars, and the need to supplement spore based culture methods in assessing spacecraft bioload. The committee also identified a number of future studies that would improve knowledge of Europa and better define issues related to forward contamination of that body. The standard recommended by the 2000 study and adopted by NASA uses a probabilistic approach, such that spacecraft sent to Europa must demonstrate a probability less than 10-4 per mission of contaminating an europan ocean with one viable terrestrial organism. A number of factors enter into the equation for calculating this probability, including at least bioload at launch, probability of survival during flight, probability of reaching the surface of Europa, and probability of reaching an europan ocean. Recently, the NASA Planetary Protection Subcommittee of the NASA Advisory Council has recommended that the probabilistic approach recommended for Europa be applied to all outer planet icy moons, until another NRC study can be convened to reevaluate the issues in light of recent data. This presentation will discuss the status of current and anticipated planetary protection considerations for missions to Europa and other icy moons.