Deciphering the origin of the regular satellites of gaseous giants - Iapetus: The Rosetta ice-moon

Ever since their discovery the regular satellites of Jupiter and Saturn have held out the promise of providing an independent set of observations with which to test theories of planet formation. Yet elucidating their origins has proven elusive. Here we show that Iapetus can serve to discriminate between satellite formation models. Its accretion history can be understood in terms of a two-component gaseous subnebula, with a relatively dense inner region, and an extended tail out to the location of the irregular satellites, as in the SEMM model of Mosqueira and Estrada (2003a,b) (Mosqueira, I., Estrada, P.R. [2003a]. Icarus 163, 198-231; Mosqueira, I., Estrada, P.R. [2003b]. Icarus 163, 232-255). Following giant planet formation, planetesimals in the feeding zone of Jupiter and Saturn become dynamically excited, and undergo a collisional cascade. Ablation and capture of planetesimal fragments crossing the gaseous circumplanetary disks delivers enough collisional rubble to account for the mass budgets of the regular satellites of Jupiter and Saturn. This process can result in rock/ice fractionation as long as the make up of the population of disk crossers is non-homogeneous, thus offering a natural explanation for the marked compositional differences between outer solar nebula objects and those that accreted in the subnebulae of the giant planets. For a given size, icy objects are easier to capture and to ablate, likely resulting in an overall enrichment of ice in the subnebula. Furthermore, capture and ablation of rocky fragments become inefficient far from the planet for two reasons: the gas surface density of the subnebula is taken to drop outside the centrifugal radius, and the velocity of interlopers decreases with distance from the planet. Thus, rocky objects crossing the outer disks of Jupiter and Saturn never reach a temperature high enough to ablate either due to melting or vaporization, and capture is also greatly diminished there. In contrast, icy objects crossing the outer disks of each planet ablate due to the melting and vaporization of water-ice. Consequently, our model leads to an enhancement of the ice content of Iapetus, and to a lesser degree those of Titan, Callisto and Ganymede, and accounts for the (non-stochastic) compositions of these large, low-porosity outer regular satellites of Jupiter and Saturn. For this to work, the primordial population of planetesimals in the Jupiter-Saturn region must be partially differentiated, so that the ensuing collisional cascade produces an icy population of ?1 m size fragments to be ablated during subnebula crossing. We argue this is likely because the first generation of solar nebula ∼10 km planetesimals in the Jupiter-Saturn region incorporated significant quantities of ²⁶Al. This is the first study successfully to provide a direct connection between nebula planetesimals and subnebulae mixtures with quantifiable and observable consequences for the bulk properties of the regular satellites of Jupiter and Saturn, and the only explanation presently available for Iapetus’ low density and ice-rich composition.     

2.7- to 4.1-μm spectrophotometry of icy satellites of Saturn and Jupiter

We present spectrophotometry in the 27–41 μm spectral region for icy satellites of Saturn (Tethys, Dione, Rhea, Iapetus, and Hyperion) and Jupiter (Europa, Ganymede, and Callisto). The 3.6-μm reflectance peak characteristic of fine-grained water ice is observed prominently on the satellites of Saturn, faintly on the leading side of Europa, and not all on Ganymede, Callisto, or the dark side of Iapetus. The spectral reflectances of these icy satellites may be affected by their equilibrium surface temperatures and magnetospheric effects.     

Thermal convection with a water ice I rheology: Implications for icy satellite evolution

We model stagnant-lid convection for water ice I using a multicomponent rheology, combining grain boundary sliding, dislocation and diffusion creep mechanisms. For the superplastic flow-dislocation creep rheology, dislocation creep (n=4) dominates the deformation within the actively convecting sublayer whilst superplastic flow (n=1.8) is the dominant process within the stagnant-lid whilst for the superplastic flow-diffusion creep rheology, superplastic flow is the dominant deformation mechanism within the convecting sublayer while diffusion creep (n=1) is the dominant deformation process in the stagnant-lid. These results suggest deformation in the actively convecting sublayer is likely to be dominated by the mechanism with the largest stress exponent. We also provide heat flux scaling relationships for the superplastic flow, basal slip, dislocation creep-superplastic flow and superplastic flow-diffusion creep rheologies and provide a simple parameterized convection model of an icy satellite thermal evolution.     

Irregular satellite capture during planetary resonance passage

The passage of Jupiter and Saturn through mutual 1:2 mean-motion resonance has recently been put forward as explanation for their relatively high eccentricities [Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F., 2005. Nature 435, 459-461] and the origin of Jupiter's Trojans [Morbidelli, A., Levison, H.F., Tsiganis, K., Gomes, R., 2005. Nature 435, 462-465]. Additional constraints on this event based on other small-body populations would be highly desirable. Since some outer satellite orbits are known to be strongly affected by the near-resonance of Jupiter and Saturn (“the Great Inequality”; ?uk, M., Burns, J.A., 2004b. Astron. J. 128, 2518-2541), the irregular satellites are natural candidates for such a connection. In order to explore this scenario, we have integrated 9200 test particles around both Jupiter and Saturn while they went through a resonance-crossing event similar to that described by Tsiganis et al. [Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F., 2005. Nature 435, 459-461]. The test particles were positioned on a grid in semimajor axes and inclinations, while their initial pericenters were put at just 0.01 AU from their parent planets. The goal of the experiment was to find out if short-lived bodies, spiraling into the planet due to gas drag (or alternatively on orbits crossing those of the regular satellites), could have their pericenters raised by the resonant perturbations. We found that about 3% of the particles had their pericenters raised above 0.03 AU (i.e. beyond Iapetus) at Saturn, but the same happened for only 0.1% of the particles at Jupiter. The distribution of surviving particles at Saturn has strong similarities to that of the known irregular satellites. If saturnian irregular satellites had their origin during the 1:2 resonance crossing, they present an excellent probe into the early Solar System's evolution. We also explore the applicability of this mechanism for Uranus, and find that only some of the uranian irregular satellites have orbits consistent with resonant pericenter lifting. In particular, the more distant and eccentric satellites like Sycorax could be stabilized by this process, while closer-in moons with lower eccentricity orbits like Caliban probably did not evolve by this process alone.     

Topography on satellite surfaces and the shape of asteroids

Calculations of the topography and shape of planetary bodies are presented for two sets of models. One set of models deals with the effects of static loading on bodies, taking into account strengths of materials, density, and size. The other set considers the effects of creep deformation on model bodies of differing composition, size and temperature. Application of these models to asteroids and satellites of the major planets indicates that model, even the largest asteroids could retain highly nonspherical shapes, and the four large satellites of Jupiter could sustain statically loaded topography on the order of 10 km. (2) If silicate asteroids have not been heated to near the melting temperature of silicates, initial topography should survive for at least 109 yr under creep deformation. Topography on an insulated icy asteroid will be rapidly reduced if it is of larger scale than the insulating layer, no matter what the thermal history. (3) Of the Galilean satellites of Jupiter, J1 and J2 should retain topography created on silicate surfaces since their formation (or since the surfaces were near the silicate melting temperature. If ice layers of any significant thickness exist, topography on a scale smaller than the layer's thickness will be reduced rapidly. (4) J4 and J3 probably fit an icy model throughout and topography of all scales may be reduced with relaxation times < 10⁶yr. These satellites are thus likely to preserve only very recent features on their surfaces, in contrast to the other Galilean satellites. If melting has taken place since formation, these conclusions become even stronger. (5) Of the satellites of the other planets, only Titan appears likely to have undergone topographic reduction by creep, under the models presented. However, if ices other than water are present in large proportion on these satellites relaxation times for topography may be shorter than calculated from the water ice models.     

Constraining the surface properties of Saturn's icy moons, using Cassini/CIRS emissivity spectra

We have conducted a search for emissivity features in the thermal infrared spectrum of the icy satellites of Saturn, Phoebe, Iapetus, Enceladus, Tethys, and Hyperion, observed by the Composite Infrared Spectrometer (CIRS) on board the Cassini spacecraft. Despite the heterogeneity of the composition of these bodies depicted by Earth-based and Cassini/VIMS observations, the CIRS spectra of all satellites are undistinguishable from black-body spectra, with no detectable emissivity feature. However, several materials, which have been detected on the surface of the same bodies, present emissivity features in the analyzed spectral range. In particular, water ice presents features with sufficient contrast to be detected by CIRS. Here we study the physical causes of the absence of features by simulating the effects of intimate mixtures using models of directional emissivity for optically thick surfaces for different particle sizes and abundances, and porosities. The simulations include a set of materials detected on the Phoebe's surface, like water ice, hydrated silicates, and organics. We find that featureless spectra can be produced in three scenarios: (1) ice particles with large sizes, (2) mixtures of ices dominated by dark contaminants, and (3) small particles with large porosity. Constraints imposed by the NIR spectra of the satellites favors the latter scenario as the more likely explanation to the absence of emissivity features on the icy satellites of Saturn.     

Cassini spectra and photometry 0.25-5.1 μm of the small inner satellites of Saturn

The nominal tour of the Cassini mission enabled the first spectra and solar phase curves of the small inner satellites of Saturn. We present spectra from the Visual Infrared Mapping Spectrometer (VIMS) and the Imaging Science Subsystem (ISS) that span the 0.25-5.1 μm spectral range. The composition of Atlas, Pandora, Janus, Epimetheus, Calypso, and Telesto is primarily water ice, with a small amount (∼5%) of contaminant, which most likely consists of hydrocarbons. The optical properties of the “shepherd” satellites and the coorbitals are tied to the A-ring, while those of the Tethys Lagrangians are tied to the E-ring of Saturn. The color of the satellites becomes progressively bluer with distance from Saturn, presumably from the increased influence of the E-ring; Telesto is as blue as Enceladus. Janus and Epimetheus have very similar spectra, although the latter appears to have a thicker coating of ring material. For at least four of the satellites, we find evidence for the spectral line at 0.68 μm that Vilas et al. [Vilas, F., Larsen, S.M., Stockstill, K.R., Gaffley, M.J., 1996. Icarus 124, 262-267] attributed to hydrated iron minerals on Iapetus and Hyperion. However, it is difficult to produce a spectral mixing model that includes this component. We find no evidence for CO₂ on any of the small satellites. There was a sufficient excursion in solar phase angle to create solar phase curves for Janus and Telesto. They bear a close similarity to the solar phase curves of the medium-sized inner icy satellites. Preliminary spectral modeling suggests that the contaminant on these bodies is not the same as the exogenously placed low-albedo material on Iapetus, but is rather a native material. The lack of CO₂ on the small inner satellites also suggests that their low-albedo material is distinct from that on Iapetus, Phoebe, and Hyperion.     

Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects

The detection of induced magnetic fields in the vicinity of the jovian satellites Europa, Ganymede, and Callisto is one of the most surprising findings of the Galileo mission to Jupiter. The observed magnetic signature cannot be generated in solid ice or in silicate rock. It rather suggests the existence of electrically conducting reservoirs of liquid water beneath the satellites' outermost icy shells that may contain even more water than all terrestrial oceans combined. The maintenance of liquid water layers is closely related to the internal structure, composition, and thermal state of the corresponding satellite interior. In this study we investigate the possibility of subsurface oceans in the medium-sized icy satellites and the largest trans-neptunian objects (TNO's). Controlling parameters for subsurface ocean formation are the radiogenic heating rate of the silicate component and the effectiveness of the heat transfer to the surface. Furthermore, the melting temperature of ice will be significantly reduced by small amounts of salts and/or incorporated volatiles such as methane and ammonia that are highly abundant in the outer Solar System. Based on the assumption that the satellites are differentiated and using an equilibrium condition between the heat production rate in the rocky cores and the heat loss through the ice shell, we find that subsurface oceans are possible on Rhea, Titania, Oberon, Triton, and Pluto and on the largest TNO's 2003 UB₃₁₃, Sedna, and 2004 DW. Subsurface oceans can even exist if only small amounts of ammonia are available. The liquid subsurface reservoirs are located deeply underneath an ice-I shell of more than 100 km thickness. However, they may be indirectly detectable by their interaction with the surrounding magnetic fields and charged particles and by the magnitude of a satellite's response to tides exerted by the primary. The latter is strongly dependent on the occurrence of a subsurface ocean which provides greater flexibility to a satellite's rigid outer ice shell.     

Physisorption of CO2 on non-ice materials relevant to icy satellites

CO₂ is known to adsorb onto clay and other minerals when a significant atmospheric pressure is present. We have found that CO₂ can also adsorb onto some clays when the CO₂ partial pressure is effectively zero under ultra-high vacuum (UHV) if cooled to the surface temperatures of the icy satellites of Jupiter and Saturn. The strength of adsorption and the spectral characteristics of the adsorbed CO₂ infrared (IR) ν₃ absorption band near 4.25 μm depend on the composition and temperature of the adsorbent. CO₂ remains adsorbed onto the clay mineral montmorillonite for >10 s of min when exposed to a vacuum of ∼1×¹⁰⁻⁸ Torr at ∼125 K. CO₂ does not adsorb onto serpentine, goethite, or palagonite under these conditions. A small amount may adsorb onto kaolinite. When heated above 150 K under vacuum, the CO₂ desorbs from the montmorillonite within a few minutes. The ν₃ absorption band of CO₂ adsorbed onto montmorillonite at 125 K is similar to that of the CO₂ detected on the saturnian and Galilean satellites and is markedly different from CO₂ adsorbed onto montmorillonite at room temperature. We infer the adsorption process is physisorption and postulate that this mechanism may explain the presence and spectral characteristics of the CO₂ detected in the surfaces of these outer satellites.     

The roughness of the dark side of Iapetus from the 2004 to 2005 flyby

The roughness of a planetary surface offers clues to its past geologic history. We apply a surface roughness model developed by Buratti and Veverka (Buratti, B.J., Veverka, J. [1985]. Icarus 64, 320-328) to Cassini ISS data from the January 1st, 2005 flyby of Iapetus. This model uses the observed scattering behavior to provide a depth to radius factor q quantifying the size of idealized craters on the surface. Our findings indicate that the surface on the dark side is significantly smoother than the surfaces of other icy low-albedo saturnian satellites. We have found that the average depth to radius on the leading (dark) side is 0.084, corresponding to a Hapke mean slope angle of 6°. As compared to the 13-33° Hapke mean slope angle of other icy satellites (Buratti, B.J., and 10 colleagues [2008]. Icarus 193, 309-322), our results present a clearly different picture for the leading surface of Iapetus, suggesting that the dark deposit contributes to the decrease in macroscopic surface roughness of the leading side. Attempts were made to obtain an average depth to radius value for the trailing (bright) side; however the scans of the bright side from this flyby exhibited large variations in albedo, resulting in results that were physically unrealistic.     

Arecibo radar observations of Rhea, Dione, Tethys, and Enceladus

We have measured the bulk radar reflectance properties of the mid-size saturnian satellites Rhea, Dione, Tethys, and Enceladus with the Arecibo Observatory's 13 cm wavelength radar system during the 2004 through 2007 oppositions of the Saturn system. Comparing to the better studied icy Galilean satellites, we find that the total reflectivities of Rhea and Tethys are most similar to Ganymede while Dione is most similar to Callisto. Enceladus' reflectivity falls between those of Ganymede and Europa. The mean circular polarization ratios of the saturnian satellites range from ∼0.8 to 1.2, and are on average lower than those of the icy Galilean satellites at this wavelength although still larger than expected for single reflections off the surface. The ratio for the trailing hemisphere of Enceladus may be the exception with a value ?0.56. The 13 cm wavelength radar albedos and polarization ratios may be systematically lower than similar results from the Cassini orbiter's RADAR instrument at 2.2 cm wavelength [Ostro, S.J., and 19 colleagues, 2006. Icarus 183, 479-490]. Overall, these reflectivities and polarization properties, together with the shapes of the echo spectra, suggest subsurface multiple scattering to be the dominant reflection mechanism although operating less efficiently than on the large icy moons of Jupiter. All these saturnian moons and icy jovian moons are atmosphere-less, low temperature water ice surfaces, and any differences in radar properties may be indicative of differences in composition or the effects of various processes that modify the regolith structure. The degree of variation in radar properties with wavelength on each satellite may constrain the thickness and efficiency of the scattering layer.     

The topography of Iapetus' leading side

We have used Cassini stereo images to study the topography of Iapetus' leading side. A terrain model derived at resolutions of 4-8 km reveals that Iapetus has substantial topography with heights in the range of −10 km to +13 km, much more than observed on the other middle-sized satellites of Saturn so far. Most of the topography is older than 4 Ga [Neukum, G., Wagner, R., Denk, T., Porco, C.C., 2005. Lunar Planet. Sci. XXXVI. Abstract 2034] which implies that Iapetus must have had a thick lithosphere early in its history to support this topography. Models of lithospheric deflection by topographic loads provide an estimate of the required elastic thickness in the range of 50-100 km. Iapetus' prominent equatorial ridge [Porco, C.C., and 34 colleagues, 2005. Science 307, 1237-1242] reaches widths of 70 km and heights of up to 13 km from their base within the modeled area. The morphology of the ridge suggests an endogenous origin rather than a formation by collisional accretion of a ring remnant [Ip, W.-H., 2006. Geophys. Res. Lett. 33, doi:10.1029/2005GL025386. L16203]. The transition from simple to complex central peak craters on Iapetus occurs at diameters of 11±3 km. The central peaks have pronounced conical shapes with flanking slopes of typically 11° and heights that can rise above the surrounding plains. Crater depths seem to be systematically lower on Iapetus than on similarly sized Rhea, which if true, may be related to more pronounced crater-wall slumping (which widens the craters) on Iapetus than on Rhea. There are seven large impact basins with complex morphologies including central peak massifs and terraced walls, the largest one reaches 800 km in diameter and has rim topography of up to 10 km. Generally, no rings are observed with the basins consistent with a thick lithosphere but still thin enough to allow for viscous relaxation of the basin floors, which is inferred from crater depth-to-diameter measurements. In particular, a 400-km basin shows up-domed floor topography which is suggestive of viscous relaxation. A model of complex crater formation with a viscoplastic (Bingham) rheology [Melosh, H.J., 1989. Impact Cratering. Oxford Univ. Press, New York] of the impact-shocked icy material provides an estimate of the effective cohesion/viscosity at . The local distribution of bright and dark material on the surface of Iapetus is largely controlled by topography and consistent with the dark material being a sublimation lag deposit originating from a bright icy substrate mixed with the dark components, but frost deposits are possible as well.     

The high-pressure phase relation of the MgSO4-H2O system and its implication for the internal structure of Ganymede

We have determined the phase relation of the MgSO₄-H₂O binary system using an externally heated diamond anvil cell in the compositional range of 0-30 wt.% MgSO₄, and under temperature and pressure conditions from 298 to 500 K and up to 4.5 GPa. Using our experimental results, we were able to estimate the composition of the ice mantle of the large icy satellites of Jupiter, such as Ganymede.In our experiments, we identified the following phases in the MgSO₄-H₂O system up to 4 GPa at 298 K: Ices VI and VII, magnesium heptahydrate, MgSO₄·7H₂O, and a liquid phase. The present phase relations suggest that there may be a deep internal ocean down to a depth about 800 km in the interior of Ganymede.     

Micrometeorite impact annealing of ice in the outer Solar System

The spectra of water ice on the surfaces of icy satellites and Kuiper Belt Objects (KBOs) indicate that the surface ice on these bodies is in a crystalline state. This conflicts with theoretical models, which predict that radiation (galactic cosmic rays and solar ultraviolet) should damage the crystalline structure of ice on geologically short timescales. Temperatures are too low in the outer Solar System for the ice to anneal, and reflectance spectra of these bodies should match those of amorphous solid water (ASW). We assess whether the kinetic energy deposited as heat by micrometeorite impacts on outer Solar System bodies is sufficient to anneal their surface ice down to a near-infrared optical depth . We calculate the kinetic energy flux from interplanetary micrometeorite impacts, including gravitational focusing. We also calculate the thermal diffusion of impact heat in various surfaces and the rate of annealing of ice. We conclude that the rate of annealing from micrometeorite impacts is sufficient to explain the crystallinity of ice on nearly all the surfaces of the saturnian, uranian and neptunian satellites. We discuss how the model can be used in conjunction with spectra of KBOs to probe dust fluxes in the Kuiper Belt.     

Aegaeon (Saturn LIII), a G-ring object

Aegaeon (Saturn LIII, S/2008 S1) is a small satellite of Saturn that orbits within a bright arc of material near the inner edge of Saturn’s G-ring. This object was observed in 21 images with Cassini’s Narrow-Angle Camera between June 15 (DOY 166), 2007 and February 20 (DOY 051), 2009. If Aegaeon has similar surface scattering properties as other nearby small saturnian satellites (Pallene, Methone and Anthe), then its diameter is approximately 500 m. Orbit models based on numerical integrations of the full equations of motion show that Aegaeon’s orbital motion is strongly influenced by multiple resonances with Mimas. In particular, like the G-ring arc it inhabits, Aegaeon is trapped in the 7:6 corotation eccentricity resonance with Mimas. Aegaeon, Anthe and Methone therefore form a distinctive class of objects in the Saturn system: small moons in corotation eccentricity resonances with Mimas associated with arcs of debris. Comparisons among these different ring-arc systems reveal that Aegaeon’s orbit is closer to the exact resonance than Anthe’s and Methone’s orbits are. This could indicate that Aegaeon has undergone significant orbital evolution via its interactions with the other objects in its arc, which would be consistent with the evidence that Aegaeon’s mass is much smaller relative to the total mass in its arc than Anthe’s and Methone’s masses are.     

Rate-dependent strength of porous ice–silica mixtures and its implications for the shape of small to middle-sized icy satellites

Porosity is one of the most important physical properties in the rheology of small icy satellites composed of ice–silicate mixtures. Deformation experiments involving ice and 1 μm silica bead mixtures were conducted to clarify the effect of porosity on the flow law of ice–silica mixtures. Mixtures with silica mass contents of 0, 30, and 50 wt.% were used for the experiments, and the porosity was changed from 0% to 25% in each mixture. The temperature ranged from −10 to −20 °C, and the strain rate was changed from 1.2 × 10⁻⁶ to 4.2 × 10⁻⁴ s⁻¹. As a result, it was found that the ice–silica mixtures deformed plastically, and that the relationship between the maximum stress, σ_max, on the stress–strain curve and the applied strain rate, , could be described by the following flow law: . The mixture became softer as the porosity or silica mass content increased, and the stress exponent n and activation energy Q were independent of porosity, depending only on the silica mass content. Furthermore, the parameter A₀ could be written as A₀ = B(1 − ?)^−α, where ? is the porosity. The constants B and α also depended only on the silica mass content, and they increased with the increase in this content. The Maxwell relaxation time was calculated in order to estimate the conditions for topographic relaxation of icy satellites, and it was found that topographic relaxation occurred at temperatures higher than 160 K in the case of icy satellites with mean radii of 200 km.     

The thermal evolution of Mars as constrained by paleo-heat flows

Lithospheric strength can be used to estimate the heat flow at the time when a given region was deformed, allowing us to constrain the thermal evolution of a planetary body. In this sense, the high (>300 km) effective elastic thickness of the lithosphere deduced from the very limited deflection caused by the north polar cap of Mars indicates a low surface heat flow for this region at the present time, a finding difficult to reconcile with thermal history models. This has started a debate on the current heat flow of Mars and the implications for the thermal evolution of the planet. Here we perform refined estimates of paleo-heat flow for 22 martian regions of different periods and geological context, derived from the effective elastic thickness of the lithosphere or from faulting depth beneath large thrust faults, by considering regional radioactive element abundances and realistic thermal conductivities for the crust and mantle lithosphere. For the calculations based on the effective elastic thickness of the lithosphere we also consider the respective contributions of crust and mantle lithosphere to the total lithospheric strength. The obtained surface heat flows are in general lower than the equivalent radioactive heat production of Mars at the corresponding times, suggesting a limited contribution from secular cooling to the heat flow during the majority of the history of Mars. This is contrary to the predictions from the majority of thermal history models, but is consistent with evidence suggesting a currently fluid core, limited secular contraction for Mars, and recent extensive volcanism. Moreover, the interior of Mars could even have been heating up during part of the thermal history of the planet.     

Tectonics on Iapetus: Despinning, respinning, or something completely different?

Saturn’s moon Iapetus is unique in that it has apparently despun while retaining a substantial equatorial bulge. Stresses arising from such a non-hydrostatic shape should in principle cause surface deformation (tectonics). As part of a search for such a tectonic signature, lineaments (linear surface features) on Iapetus were mapped on both its bright and dark hemispheres. Lineament orientations were then compared to model stress patterns predicted for spin-down from a rotation period of 16.5 h (or less) to its present synchronous period, and for a range of lithospheric thicknesses. Many lineaments are straight segments of crater rimwalls, which may be faults or joints reactivated during complex crater collapse. Most striking are several large troughs on the bright, trailing hemisphere. These troughs appear to be extensional and are distinctive on that hemisphere, because the interior floors and walls of the troughs contain dark material. Globally, no specific evidence of strike slip or thrust offsets are seen, but this could be due to the age and degraded nature of any such features. We find that observed lineament orientations do not correlate with predicted patterns due to despinning on either hemisphere (the equatorial ridge was specifically excluded from this analysis, and is considered separately). Modest evidence for preferred orientations ±40° from north could be construed as consistent with respinning, which is not necessarily far-fetched. Assuming the rigidity of unfractured ice, predicted maximum lithospheric differential stresses from despinning range from ∼1 MPa to ∼160 MPa for the elastic spheroid and thin lithosphere limits, respectively (although it is only for thicker elastic lithospheres that we expect a nonhydrostatic state to be maintained over geologic time against lithospheric failure). The tectonic signature of despinning may have been obscured over time because the surface of Iapetus is very ancient, Iapetus’ thick lithosphere may have inhibited the full tectonic expression of despinning, or both. Several prominent lineaments strike E–W, and are thus parallel to the equatorial ridge (though not physically close to it), but a tectonic or volcanic origin for the ridge is highly problematic.     

The heat flow of Europa

The heat flow from Europa has profound implications for ice shell thickness and structure, as well as for the existence of an internal ocean, which is strongly suggested by magnetic data. The brittle-ductile transition depth and the effective elastic thickness of the lithosphere are here used to perform heat flow estimations for Europa. Results give preferred heat flow values (for a typical geological strain rate of ¹⁰⁻¹⁵ s⁻¹) of 70-110 mW m⁻² for a brittle-ductile transition 2 km deep (the usually accepted upper limit for the brittle-ductile transition depth in the ice shell of Europa), 24-35 mW m⁻² for an effective elastic thickness of 2.9 km supporting a plateau near the Cilix impact crater, and >130 mW m⁻² for effective elastic thicknesses of ?0.4 km proposed for the lithosphere loaded by ridges and domes. These values are clearly higher than those produced by radiogenic heating, thus implying an important role for tidal heating. The ?19-25 km thick ice shell proposed from the analysis of size and depth of impact structures suggests a heat flow of ?30-45 mW m⁻² reaching the ice shell base, which in turn would imply an important contribution to the heat flow from tidal heating within the ice shell. Tidally heated convection in the ice shell could be capable to supply ∼100 mW m⁻² for superplastic flow, and, at the Cilix crater region, ∼35-50 mW m⁻² for dislocation creep, which suggests local variations in the dominant flow mechanism for convection. The very high heat flows maybe related to ridges and domes could be originated by preferential heating at special settings.