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Titan's geology is complex, with a wide range of surface morphology, including fluvial, aeolian, and cryovolcanic and tectonic activity. Surface observations have not yet revealed large-scale and distinct characteristics of strike-slip faulting (i.e., en echelon structures, fault duplexes, and clear offset features) that have been observed on other ocean worlds such as Europa, Ganymede, and Enceladus [1,2]. However, the SW Xanadu region shows offsets in fluvial networks that seem to have been caused by strike-slip faulting [3]optimal shear failure conditions may be present within Titan's shallow subsurface, where the existence of a porous ice cover filled with liquid hydrocarbons can create an environment for areas of frictional instability [4], shear heating [5], and possibly cryovolcanism [6,7]. In this work, we examine Titan’s ability to host shear deformation at identified rectilinear fluvial features that are inferred to be strike-slip faults [3] through a sensitivity analysis guided by Coulomb failure laws [8] and tidal stress mechanisms [9]. Our modeling technique includes considerations for how the presence of near-surface liquid hydrocarbons and the crustal porosity of the ice significantly reduce the resistance to shear failure of strike-slip faults subjected to diurnal tidal stresses through a pore pressure parameter.For this study, we examine failure conditions at proposed example strike-slip faults [3] in SW Xanadu (Figure 1): (A) 9ºS 138ºW. Using the SatStress [9,10] tidal stress model for Titan-appropriate rheology [6], we compute the diurnal tidal stress tensor and resolve shear and normal stresses onto shallow fault planes (100 m depth) with azimuthal orientations consistent with mapped observations [3], as well as for the full range of orientations. We explore candidate coefficients of friction (μf= 0.3-0.5) [11] and hydrostatic pore fluid pressure ratios for Titan (l = 0.67-0.9) [4]. Figure 2 illustrates where the magnitudes of frictional stress and absolute value of shear stress overlap and the Coulomb criterion is met using the mapped azimuth of the suggested fault structure, permitting a finite slip-window. At a shallow fault depth of 100 m and for the inferred oriented faults, our model suggests that shear failure is possible under diurnal tidal stresses subjected to the studied parameters. For an assumed coefficient of friction of μf = 0.4, shear failure can only be achievable with intermediate to high pore fluid pressure ratios (λ > 0.75). At location (A), the resulting left-lateral slip direction from the model agrees with the inferred left-lateral shear direction of the proposed mapped strike-slip fault [3]. However, right- and left-lateral slip windows are achieved for a range of possible azimuths at this location with μf = 0.4 and a pore fluid pressure ratio of l = 0.75 (Figure 2, top). While a variety of factors can control this observed fluvial drainage system on Titan, the general azimuthal orientation of these features in this region suggests that there is a subsurface tectonic fabric trending roughly east-west guiding the flow of these rectilinear fluvial systems. Overall, our findings show that on Titan, the crustal porosity of ice and the inclusion of near-surface liquids will minimize the resistance to shear failure of faults subjected to diurnal tidal stresses. The optimal combination of tensile and shear tractions can allow for finite slip windows where the Coulomb criteria is met.References: [1] Cameron, M.E. et al. (2018) Icarus, 315, 92-114. [2] Martin, E.S. (2016) Geophys. Res. Let., 43, 2456–2464. [3] Matteoni, P. et al. (2020) JGR: Planets, 125(12). [4] Liu, Z. et al. (2016) Icarus, 270, 2-13. [5] Nimmo, F. et al. (2007) Nature, 447, 289. [6] Sohl, F. et al. (2014) JGR: Planets, 119, 1013-1036. [7] Malaska, M.J. et al. (2016) Icarus, 270, 130-161. [8] Byerlee, J. (1978) Pageoph, Vol. 116. [9] Wahr, J. et al. (2009) Icarus, 200, 188-206. [10] Patthoff, A. et al. (2016) AGU, Abstract #P51B-2147. [11] Schulson, E.M. (2016) Int. Material Reviews, 60:8, 451-478.

The outer H2O ice shell of Europa, a Galilean satellite of Jupiter thought to exhibit a global interior water ocean, shows evidence for plate-like surface motions along tectonic boundaries. In this study, we investigate whether forces similar to those that drive plate tectonics on Earth may drive plate motions on icy satellites. We focus on whether the forces of slab pull and ridge push, driven by thermal and compositional buoyancy, can overcome the mechanical forces resisting plate motions to sustain interaction between surface ice and the interior ocean. We find that Earth-like plate tectonics is implausible within the outer ice shell of icy satellites because the forces resisting self-sustaining plate cycling are of significantly greater order than the driving forces. In the case of Ganymede and Triton, if it has a thick shell, plate tectonics would require warm, low-viscosity slabs to remain intact for timescales that are orders of magnitude longer than those of interior convection.

High-resolution Galileo data of Ganymede's complex surface provide strong and ubiquitous evidence of strike-slip motion: en echelon structures, strike-slip duplexes, laterally offset pre-existing features, and strained craters. In a previous study, we performed a detailed mapping of these strike-slip morphologies within nine regions of Ganymede: Dardanus Sulcus, Tiamat Sulcus, Nun Sulci, Byblus Sulcus, Nippur and Philus Sulci, the Transitional Terrain of Northern Marius Regio, Anshar Sulcus, Arbela Sulcus, and Uruk Sulcus. Strike-slip indicators were inferred in various combinations at each site, in addition to extensional processes. However, the prominence of strike-slip indicators suggests shearing has been an important process to the structural development of Ganymede's surface. Moreover, tidal stresses on Ganymede, under particular circumstances, may have been sufficient to induce Coulomb failure and generate strike-slip faulting. Here we investigate the role of both diurnal and non-synchronous rotation (NSR) tidal stresses on Ganymede using the numerical code SatStress. We resolve normal and shear tractions onto discrete fault segments of specified orientation and assess Coulomb failure potential for the nine inferred fault zones. Testing a range of subsurface fault depths (z = 0–5 km) and ice friction (μf = 0.2–0.6), we find that tidal stress models of combined diurnal and NSR stress readily promote Coulomb failure within each studied fault zone. High friction (μf = 0.6) limits failure depths to ∼1 km, while low friction (μf = 0.2) extends failure depths to ∼2 km, consistent with elastic thickness estimates. We also compare each fault zone's predicted shear sense to the inferred shear sense from our structural mapping efforts and find compatible senses of shear among six of the nine regions that exhibit notable fault offset and/or prevalent inferences of en echelon, duplexes, and strained craters. In addition, principal stress orientations and failure feasibility computed on a global scale suggest that combined diurnal and NSR tidal stresses promote Coulomb failure within the shallow (

The 27 satellites of Uranus are enigmatic, with dark surfaces coated by material that could be rich in organics. Voyager 2 imaged the southern hemispheres of Uranus' five largest 'classical' moons Miranda, Ariel, Umbriel, Titania, and Oberon, as well as the largest ring moon Puck, but their northern hemispheres were largely unobservable at the time of the flyby and were not imaged. Additionally, no spatially resolved datasets exist for the other 21 known moons, and their surface properties are essentially unknown. Because Voyager 2 was not equipped with a near-infrared mapping spectrometer, our knowledge of the Uranian moons' surface compositions, and the processes that modify them, is limited to disk-integrated datasets collected by ground- and space-based telescopes. Nevertheless, images collected by the Imaging Science System on Voyager 2 and reflectance spectra collected by telescope facilities indicate that the five classical moons are candidate ocean worlds that might currently have, or had, liquid subsurface layers beneath their icy surfaces. To determine whether these moons are ocean worlds, and investigate Uranus' ring moons and irregular satellites, close-up observations and measurements made by instruments onboard a Uranus orbiter are needed., Accepted in AAS Planetary Science Journal. arXiv admin note: text overlap with arXiv:2007.07284

Several large ridges, termed dorsa, stand over 800 m above their surroundings in a region centered on the trailing hemisphere of Saturn's icy moon Enceladus. In map view, these dorsa are linear to curvilinear, 20 km to more than 50 km long, and display near-orthogonal trends. They cross-cut (are younger than) most other geological features in the region. High-resolution limb profiles show the dorsa to be asymmetric in cross-sectioned profile and 5–6 km in width, and high-resolution images show striations along their crests. The structure and morphology of the dorsa suggest they are thrust blocks, possibly analogous to lobate scarps or wrinkle-ridges found on the terrestrial planets. The low slopes of their backlimbs and steeper forelimbs, suggest the dorsa were formed as wrinkle ridges or lobate scarps overlying thrust faults that penetrate 1–4 km deep to a detachment, most likely at the brittle-ductile transition (BDT). Their near-orthogonal trends are consistent with biaxial horizontal shortening. These relationships suggest that the central trailing hemisphere was recently subjected to a relatively high heat flow at the time that deformation occurred.

The heavily fractured surface of Ganymede displays many morphologically distinctive regions of inferred distributed shear and strike-slip faulting that may be important to the structural development of its surface. To better understand the role of strike-slip tectonism in shaping this complex icy surface, we perform detailed mapping at nine sites using Galileo and Voyager imagery, noting key examples of strike-slip morphologies where present. These four morphological indicators are: en echelon structures, strike-slip duplexes, laterally offset pre-existing features, and possible strained craters. We map sites of both light, grooved terrain (Nun Sulci, Dardanus Sulcus, Tiamat Sulcus, Uruk Sulcus, and Arbela Sulcus), and terrains that are transitional from dark to light terrains (Nippur and Philus Sulci, Byblus Sulcus, Anshar Sulcus, and the Transitional Terrain of Northern Marius Regio). At least one, if not more, of the four strike-slip morphological indicators are observed at every site, suggesting strike-slip tectonism is indeed important to Ganymede's evolutionary history. Byblus Sulcus is the only mapped site where the presence of strike-slip indicators is limited to only a few en echelon structures; every other mapped site displays examples of at least two types, with Arbela Sulcus containing candidate examples of all four. In addition, quantification of morphological characteristics suggests related rotation between sites, as evidenced by the predominant NW/SE trend of mapped features within the light terrain present in five different sites (Nun, Tiamat, Uruk, Nippur/Philus, Byblus, and Anshar Sulcus). Moreover, incorporation of strike-slip tectonism with pre-existing observations of extensional behavior provides an improved, synoptic representation of Ganymede's tectonic history.

NASA’s Cassini spacecraft spent 13 years exploring the Saturn system, including 23 targeted flybys of the small, geologically active moon Enceladus. These flybys provided a wealth of image data from Cassini’s Imaging Science Subsystem. To improve the usability of the Enceladus data set, we created a new, global photogrammetric control network for Enceladus that enabled compilation of a versatile cartographic package to support geologic mapping and other investigations. The network used 586 images in four image filters with a pixel scale generally between 50 and 500 m per pixel and a phase angle less than 120° and consisted of 10,362 tie points and 173,704 individual image measures, averaging nearly 17 measures per tie point. Least squares bundle adjustment resulted in a root-mean-square residual of 0.45 pixel, corresponding to root-mean-square ground point uncertainties of 66, 51, and 46 m in latitude, longitude, and radius, respectively. Using our geodetic control network, we created new global image mosaics, coregistered flyby mosaics to support geologic mapping, and updated pointing kernels for every image used in the solution. These products, including the updated pointing kernels, are available to the community through NASA’s Planetary Data System Imaging Annex. The bundle adjustment solution also yielded independently determined shape information, resulting in radii within the stated uncertainty of International Astronomical Union values. The challenges of the data set, and the technical methodology described here are applicable to bodies imaged during multiple flybys with variable viewing and illumination geometry, including other midsized satellites of Saturn, and the Europa Clipper mission.

The young ( Greeley et al., 2000 )—during the E12 flyby of Europa in Dec. 1997. This dataset is now two decades old, but it has not been analyzed in detail until this work. Despite the largely different viewing and lighting conditions, we mosaic these high-resolution frames into the 220 m/pixel regional context frame. We then perform geomorphologic mapping of the high-resolution image mosaic and the regional image frame, for comparison, and we also create a structural map of the high-resolution image mosaic. The units in the geomorphologic map are defined by surface texture, landform shape (morphology), dimension, and relative albedo. The structural map units include interpretations from the geomorphologic map units and their interpretation implies potential kinematic processes for the formation of particular structures. Our primary mapping observations include the regular spacing and gentle slopes of the ridge-and-trough terrain, the sharp boundaries and preserved structures of the chaos terrain, and the symmetry but irregular size of double ridges. We then evaluate proposed formation mechanisms for these and other mapped features. The high-resolution images also reveal an abundance of small (

Enceladus is the first outer solar system body on which pit chains have been positively identified. We map the global distribution of pit chains and show that pit chains are among the youngest tectonic features on Enceladus's surface, concentrated in the cratered plains centered on Enceladus's Saturnian and anti-Saturnian hemispheres. Pit chains on Enceladus are interpreted as the surface expressions of subsurface dilational fractures underlying a cover of unconsolidated material, which we infer to be a geologically young cover of loose regolith that mantles the surface of Enceladus. A widespread layer of regolith may act to insulate the surface, which has implications for the thermal state of Enceladus's ice shell. The widespread distribution of pit chains across the cratered plains indicates that this ancient surface has recently been tectonically active.

Radar probing of Jovian icy satellites is fundamental for understanding the moons' origin and their thermal evolution as potential habitable environments in our Solar System. Using the current state of knowledge of the geological and geophysical properties of Ganymede, Europa and Callisto, we perform a comprehensive radar detectability study to quantify the exploration depth and the lower limit for subsurface identification of water and key tectonic structural elements. To achieve these objectives, we establish parametric dielectric models that reflect different hypotheses on the formation and thermal evolution of each moon. The models are then used for FDTD radar propagation simulations at the 9-MHz sounding frequency proposed for both ESA JUICE and NASA Europa missions. We investigate the detectability above the galactic noise level of four predominant subsurface features: brittle-ductile interfaces, shallow faults, brine aquifers, and the hypothesized global oceans. For Ganymede, our results suggest that the brittle-ductile interface could be within radar detectability range in the bright terrains, but is more challenging for the dark terrains. Moreover, understanding the slope variation of the brittle-ductile interface is possible after clutter reduction and focusing. For Europa, the detection of shallow subsurface structural elements few kilometers deep (such as fractures, faults and brine lenses) is achievable and not compromised by surface clutter. The objective of detecting the potential deep global ocean on Europa is also doable under both the convective and conductive hypotheses. Finally, for Callisto, radar waves can achieve an average penetration depth of ∼15 km, although the current understanding of Callisto's subsurface dielectric properties does not suggest sufficiently strong contrasts to produce unambiguous radar returns.

Chaos terrains are among the most prominent landforms of Europa, and are generally among the youngest features recorded on the surface. Chaos units were formed by to endogenic activity, maybe related to solid-state convection and thermal diapirism in the ice shell, perhaps aided by melting of salt-rich ice bodies below the surface. In this work, we analyze the different units of chaotic terrain in a portion of Argadnel Regio, a region located on the anti-Jovian hemisphere of Europa, and their possible timing in the general stratigraphic framework of this satellite. Two different chaos units can be differentiated, based on surface texture, morphology, and cross-cutting relationships with other units, and from interpretations based on pre-existing surface restoration through elimination of a low albedo band. The existence of two stratigraphically different chaos units implies that conditions for chaos formation occurred during more than a single discreet time on Europa, at least in Argadnel Regio, and perhaps in other places. The existence of older chaos units on Europa might be related to convective episodes possibly favored by local conditions in the icy shell, such as variations in grain size, abundance of non-water ice-components, or regional thickness of the brittle lithosphere or the entire ice shell.

Many outer solar system bodies are thought to harbor liquid water oceans beneath their ice shells. This article first reviews how such oceans are detected. We then discuss how they are maintained, when they formed, and what the oceans' likely characteristics are. We focus in particular on Europa, Ganymede, Callisto, Titan, and Enceladus, bodies for which there is direct evidence of subsurface oceans. We also consider candidate ocean worlds such as Pluto and Triton.

The ocean in Jupiter's moon Europa may have redox balance similar to Earth's. On Earth, low-temperature hydration of crustal olivine produces substantial hydrogen, comparable to any potential flux from volcanic activity. Here we compare hydrogen and oxygen production rates of the Earth system with fluxes to Europa's ocean. Even without volcanic hydrothermal activity, water-rock alteration in Europa causes hydrogen fluxes 10 times smaller than Earth's. Europa's ocean may have become reducing for a brief epoch, for example, after a thermal-orbital resonance ∼2 Gyr after accretion. Estimated oxidant flux to Europa's ocean is comparable to estimated hydrogen fluxes. Europa's ice delivers oxidants to its ocean at the upper end of these estimates if its ice is geologically active, as evidence of geologic activity and subduction implies.

We present the first mechanical analysis based on realistic rheology and boundary conditions on the formation of evenly spaced strike-slip faults. Two quantitative models employing the stress-shadow concept, widely used for explaining extensional-joint spacing, are proposed in this study: (1) an empirically based stress-rise-function model that simulates the brittle-deformation process during the formation of evenly spaced parallel strike-slip faults, and (2) an elastic plate model that relates fault spacing to the thickness of the fault-hosting elastic medium. When applying the models for the initiation and development of the tiger-stripe fractures (TSF) in the South Polar Terrain (SPT) of Enceladus, the mutually consistent solutions of the two models, as constrained by the mean spacing of the TSF at ∼35 km, requires that the brittle ice-shell thickness be ∼30 km, the elastic thickness be ∼0.7 km, and the cohesive strength of the SPT ice shell be ∼30 kPa. However, if the brittle and elastic models are decoupled and if the ice-shell cohesive strength is on the order of ∼1 MPa, the brittle ice shell would be on the order of ∼10 km.

Europa's young surface implies relatively recent resurfacing. Ridged plains, which make up >50% of Europa's surface, have not yet been fully analyzed for a potential formation mechanism. Because ridged plains dominate Europa's surface, this terrain is key to understanding how Europa has resurfaced and how the resurfacing mechanisms may have evolved through time. In this work, we create a new high-resolution topographic model and a two-layer physical analog model to investigate the formation of ridged plains. We find that the ridged plains most closely resemble the compressional physical analog experiments which generate folds. Specifically, the analog experiments with a brittle layer ~3000 m. Compared to the predicted current ice shell thickness, this would imply that Europa's ice-shell has thickened through the visible surface history.

Despite a decade of intense research the mechanical origin of the tiger-stripe fractures (TSF) and their geologic relationship to the hosting South Polar Terrain (SPT) of Enceladus remain poorly understood. Here we show via systematic photo-geological mapping that the semi-squared SPT is bounded by right-slip, left-slip, extensional, and contractional zones on its four edges. Discrete deformation along the edges in turn accommodates translation of the SPT as a single sheet with its transport direction parallel to the regional topographic gradient. This parallel relationship implies that the gradient of gravitational potential energy drove the SPT motion. In map view, internal deformation of the SPT is expressed by distributed right-slip shear parallel to the SPT transport direction. The broad right-slip shear across the whole SPT was facilitated by left-slip bookshelf faulting along the parallel TSF. We suggest that the flow-like tectonics, to the first approximation across the SPT on Enceladus, is best explained by the occurrence of a transient thermal event, which allowed the release of gravitational potential energy via lateral viscous flow within the thermally weakened ice shell.

Global structural mapping of high-resolution Cassini images of Enceladus reveals a richly varied surface. Most notable are three main regions of deformation each containing multiple structural units. In addition to the well known “South Polar Terrain” (SPT), there are two other large regions of deformation that we term “Leading Hemisphere Terrain” (LHT) and “Trailing Hemisphere Terrain” (THT). Each of these three terrains includes a circumferential belt that encloses one or more other structurally deformed units. Areal extents range from about 80,000 km2 (SPT) to 195,000 km2 (LHT) or 160 to 250 km equivalent circular radius. Based on relative crater densities, the THT is inferred to be older than the LHT; the geologically active SPT is the youngest. The overall similarities in shape and dimension of the three tectonized terrains suggest similar formational processes, plausibly related to broad loading of a thin elastic shell. A viable scenario is that each tectonized terrain formed above a large-scale region of warm upwelling ice, with subsequent downwarping triggered by cooling and/or subsurface melting. However, differences in morphological detail suggest that the specific evolution of each tectonized terrain has been different.

The cameras of New Horizons will provide robust data sets that should be imminently amenable to geological analysis of the Pluto system’s landscapes. In this paper, we begin with a brief discussion of the planned observations by the New Horizons cameras that will bear most directly on geological interpretability. Then we broadly review the major geological processes that could potentially operate on the surfaces of Pluto and its major moon Charon. We first survey exogenic processes (i.e. those for which energy for surface modification is supplied externally to the planetary surface): impact cratering, sedimentary processes (including volatile migration), and the work of wind. We conclude with an assessment of the prospects for endogenic activity in the form of tectonics and cryovolcanism.

Grooved terrain on Ganymede consists of distinct areas of parallel to subparallel ridges and troughs at a variety of spatial scales. Grooved terrain has been interpreted as the product of tectonism in the form of fault-accommodated distributed lithospheric extension. We use physical analog methods to test the formation of grooved terrain by imbricate normal faulting in response to distributed extension. Faults and fault systems produced in the models are geometrically and kinematically similar to patterns inferred for some grooved terrains on Ganymede. The high degree of similarity between model structures and those observed on Ganymede indicates that rotational half-graben brittle block faulting can explain at least some tectonic resurfacing on Ganymede and that 20% extension is sufficient to form structures analogous to grooved terrain.

[1] The long wavelength surface topography of Enceladus shows depressions about 1 km in depth and ∼102 km wide. One possible cause of this topography is spatially variable amounts of compaction of an initially porous ice shell, driven by spatial variations in heat flux. Here, we show that the heat flux variations associated with convection in the shell can quantitatively match the observed features. We develop a simple model of viscous compaction that includes the effect of porosity on thermal conductivity, and find that an initial shell porosity of at least 20–25% is required to develop the observed topography over ∼1 Ga. This mechanism produces topographic depressions, not rises, above convective upwellings, and does not generate detectable gravity anomalies. Unlike transient dynamic topography, it can potentially leave a permanent record of ancient convective processes in the shallow lithospheres of icy satellites.

Jupiter’s icy moon Europa displays a variety of lineament types ranging from arcuate to “wavy” to cycloidal. These features can span 100s of km and reach heights of ~200 m. Here we explore how these features could evolve in a rotating diurnal stress field, with contributions from nonsynchronous rotation (NSR) and obliquity stresses. Previous work has invoked simulations of diurnal and added obliquity stress to explain Europa’s observed cycloidal lineaments. However, these models assumed an elastic ice shell, and neither of these two stress mechanisms alone can simulate Europa’s wavy lineaments. We expand on that previous elastic-shell modeling to demonstrate that diurnal tidal stresses can combine with NSR and obliquity stresses to create cycloidal lineaments or lineaments with a “wavy” planform, as simulated with the viscoelastic model SatStressGUI. If only diurnal tidal stress, or obliquity plus diurnal tidal stresses, are considered, then cycloidal lineaments are formed in response the changing magnitude and direction of the resultant principal stresses. The characteristics of the lineaments are controlled by a variety of parameters mainly propagation speed, ~1–5 m/s, thickness and viscosity of the lower ice layer, with a thicker and more viscous lower ice resulting in a smaller stress magnitude. For NSR, the magnitude of the simulated stress is chiefly dependent on the period of NSR and thickness and viscosity of the upper ice layer, such that a longer NSR period or a thicker ice shell with a low viscosity results in a smaller stress magnitude. When NSR stress is added and is similar in magnitude to the diurnal or obliquity stress, the simulated propagating lineaments can be wavy in planform shape. As the magnitude of the NSR stress is increased such that NSR stress dominates over diurnal and obliquity stress, the simulated lineaments are generally arcuate. We suggest that small amounts of NSR stress might have contributed to the formation of cycloids but that significant NSR was not necessary to account for their planform shape. But NSR may be an important contributing factor to the formation of the Europa’s wavy lineaments.

Orbital remote sensing provides a powerful way to efficiently survey targets such as the Earth and other planets and moons for features of interest. One such feature of astrobiological relevance is the presence of surface sulfur deposits. These deposits have been observed to be associated with microbial activity at the Borup Fiord glacial springs in Canada, a location that may provide an analogue to other icy environments such as Europa. This article evaluates automated classifiers for detecting sulfur in remote sensing observations by the hyperion spectrometer on the EO-1 spacecraft. We determined that a data-driven machine learning solution was needed because the sulfur could not be detected by simply matching observations to sulfur lab spectra. We also evaluated several methods (manual and automated) for identifying the most relevant attributes (spectral wavelengths) needed for successful sulfur detection. Our findings include (1) the Borup Fiord sulfur deposits were best modeled as containing two sub-populations: sulfur on ice and sulfur on rock; (2) as expected, classifiers using Gaussian kernels outperformed those based on linear kernels, and should be adopted when onboard computational constraints permit; and (3) Recursive Feature Elimination selected sensible and effective features for use in the computationally constrained environment onboard EO-1. This study helped guide the selection of algorithm parameters and configuration for the classification system currently operational on EO-1. Finally, we discuss implications for a similar onboard classification system for a future Europa orbiter.

The compelling evidence for an ocean beneath the ice shell of Europa makes it a high priority for astrobiological investigations. Future missions to the icy surface of this moon will query the plausibly sulfur-rich materials for potential indications of the presence of life carried to the surface by mobile ice or partial melt. However, the potential for generation and preservation of biosignatures under cold, sulfur-rich conditions has not previously been investigated, as there have not been suitable environments on Earth to study. Here, we describe the characterization of a range of biosignatures within potentially analogous sulfur deposits from the surface of an Arctic glacier at Borup Fiord Pass to evaluate whether evidence for microbial activities is produced and preserved within these deposits. Optical and electron microscopy revealed microorganisms and extracellular materials. Elemental sulfur (S⁰), the dominant mineralogy within field samples, is present as rhombic and needle-shaped mineral grains and spherical mineral aggregates, commonly observed in association with extracellular polymeric substances. Orthorhombic α-sulfur represents the stable form of S⁰, whereas the monoclinic (needle-shaped) γ-sulfur form rosickyite is metastable and has previously been associated with sulfide-oxidizing microbial communities. Scanning transmission electron microscopy showed mineral deposition on cellular and extracellular materials in the form of submicron-sized, needle-shaped crystals. X-ray diffraction measurements supply supporting evidence for the presence of a minor component of rosickyite. Infrared spectroscopy revealed parts-per-million level organics in the Borup sulfur deposits and organic functional groups diagnostic of biomolecules such as proteins and fatty acids. Organic components are below the detection limit for Raman spectra, which were dominated by sulfur peaks. These combined investigations indicate that sulfur mineral deposits may contain identifiable biosignatures that can be stabilized and preserved under low-temperature conditions. Borup Fiord Pass represents a useful testing ground for instruments and techniques relevant to future astrobiological exploration at Europa.

Previous analyses of Galileo images showed the small (≈1 km and smaller) crater population on Callisto to be lower than had been expected (Moore, J.M. et al. [1999]. Icarus 140, 294–312; Bierhaus E.B. et al. [2000]. Lunar Planet. Sci. 31. Abstract #1996). In this paper we examine the small crater population using high-resolution imagery from Callisto flybys during Galileo orbits C3, C10, C21, and C30, including several C30 regions not previously analyzed. Our findings confirm that most small craters are depleted relative to a presumed equilibrium of R = 0.22, and we find that there is significant variability in the small crater counts. While some of the variability in the small crater population on Callisto can be attributed to secondary cratering, some variability also may be explained by resetting of portions of Callisto’s surface by larger impactors. This is expected where the differential size frequency distribution of the crater production population b b represents the exponent of a differential power-law crater-size distribution), such that large impacts affect a greater planetary surface area than smaller craters.

Missions to explore Europa have been imagined ever since the Voyager mission first suggested that Europa was geologically very young. Subsequently, the Galileo spacecraft supplied fascinating new insights into this satellite of Jupiter. Now, an international team is proposing a return to the Jupiter system and Europa with the Europa Jupiter System Mission (EJSM). Currently, NASA and ESA are designing two orbiters that would explore the Jovian system and then each would settle into orbit around one of Jupiter’s icy satellites, Europa and Ganymede. In addition, the Japanese Aerospace eXploration Agency (JAXA) is considering a Jupiter magnetospheric orbiter and the Russian Space Agency is investigating a Europa lander. The Jupiter Europa Orbiter (JEO) would be the NASA-led portion of the EJSM; JEO would address a very important subset of the complete EJSM science objectives and is designed to function alone or in conjunction with ESA’s Jupiter Ganymede Orbiter (JGO). The JEO mission concept uses a single orbiter flight system that would travel to Jupiter by means of a multiple-gravity-assist trajectory and then perform a multi-year study of Europa and the Jupiter system, including 30 months of Jupiter system science and a comprehensive Europa orbit phase of 9 months. The JEO mission would investigate various options for future surface landings. The JEO mission science objectives, as defined by the international EJSM Science Definition Team, include: A. Europa’s ocean: Characterize the extent of the ocean and its relation to the deeper interior. B. Europa’s ice shell: Characterize the ice shell and any subsurface water, including their heterogeneity, and the nature of surface–ice–ocean exchange. C. Europa’s chemistry: Determine global surface compositions and chemistry, especially as related to habitability. D. Europa’s geology: Understand the formation of surface features, including sites of recent or current activity, and identify and characterize candidate sites for future in situ exploration. E. Jupiter system: Understand Europa in the context of the Jupiter system. The JEO orbital mission would provide critical measurements to support the scientific and technical selection of future landed options. The primary challenge of a Europa mission is to perform in Jupiter’s radiation environment, radiation damage being the life-limiting parameter for the flight system. Instilling a system-level radiation-hardened-by-design approach very early in the mission concept would mitigate the pervasive mission and system-level impacts (including trajectory, configuration, fault protection, operational scenarios, and circuit design) that can otherwise result in runaway growth of cost and mass. This paper addresses the JEO mission concept developed by a joint team from JPL and the Applied Physics Laboratory to address the science objectives defined by an international science definition team formed in 2008, while designing for the Jupiter environment.

From 2007 the Russian Academy of Sciences and Roscosmos consider to develop a Europa surface element, in coordination with the Europa Jupiter System Mission (EJSM) international project planned to study the Jupiter system. The main scientific objectives of the Europa Lander are to search for the signatures of possible present and extinct life, in situ studies of the Europa internal structure, the surface and the environment. The mission includes the lander, and the relay orbiter, to be launched by Proton and carried to Jupiter with electric propulsion. The mass of scientific instruments on the lander is ∼50 kg, and its planned lifetime is 60 days. A dedicated international Europa Lander Workshop (ELW) was held in Moscow in February 2009. Following the ELW materials and few recent developments, the paper describes the planned mission, including the science goals, technical design of the mission elements, the ballistic scheme, and the synergy between the Europa Lander and the EJSM.

Elemental sulfur (S(0) ) is deposited each summer onto surface ice at Borup Fiord pass on Ellesmere Island, Canada, when high concentrations of aqueous H(2) S are discharged from a supraglacial spring system. 16S rRNA gene clone libraries generated from sulfur deposits were dominated by β-Proteobacteria, particularly Ralstonia sp. Sulfur-cycling micro-organisms such as Thiomicrospira sp., and e-Proteobacteria such as Sulfuricurvales and Sulfurovumales spp. were also abundant. Concurrent cultivation experiments isolated psychrophilic, sulfide-oxidizing consortia, which produce S(0) in opposing gradients of Na(2) S and oxygen. 16S rRNA gene analyses of sulfur precipitated in gradient tubes show stable sulfur-biomineralizing consortia dominated by Marinobacter sp. in association with Shewanella, Loktanella, Rubrobacter, Flavobacterium, and Sphingomonas spp. Organisms closely related to cultivars appear in environmental 16S rRNA clone libraries; none currently known to oxidize sulfide. Once consortia were simplified to Marinobacter and Flavobacteria spp. through dilution-to-extinction and agar removal, sulfur biomineralization continued. Shewanella, Loktanella, Sphingomonas, and Devosia spp. were also isolated on heterotrophic media, but none produced S(0) alone when reintroduced to Na(2) S gradient tubes. Tubes inoculated with a Marinobacter and Shewanella spp. co-culture did show sulfur biomineralization, suggesting that Marinobacter may be the key sulfide oxidizer in laboratory experiments. Light, florescence and scanning electron microscopy of mineral aggregates produced in Marinobacter experiments revealed abundant cells, with filaments and sheaths variably mineralized with extracellular submicron sulfur grains; similar biomineralization was not observed in abiotic controls. Detailed characterization of mineral products associated with low temperature microbial sulfur-cycling may provide biosignatures relevant to future exploration of Europa and Mars.