numerical simulations of marine hydrothermal plumes for europa and other icy worlds abstract the liquid water interiors of europa and other icy moons of the outer solar system are likely to be driven by geothermal heating from the sea floor leading to the development of buoyant hydrothermal plumes these plumes potentially control icy surface geomorphology and are of interest to astrobiologists we have performed a series of simulations of these plumes using the mit gcm ocean circulation model we assume here that europa s ocean is deep of order 100 km and unstratified and that plume buoyancy is controlled by temperature not composition our experiments explore a limited region of parameter space with ocean depth h ranging from 50 to 100 km deep source heat flux q between 0 1 and 10 gw and coriolis parameter f corresponding to europa latitudes between 9 and 47 as predicted by earlier work the plumes in our simulations form narrow cylindrical chimneys a few km across under the influence of the coriolis effect these plumes broaden over time until they become baroclinically unstable breaking up into cone shaped eddies when they become 10 35 km in diameter the shed eddies are of a similar size large scale currents in the region of the plume range between 1 and 5 cm s temperature anomalies in the plume far from the seafloor are tiny varying between 10 and 180 k variations in plume size shape speed and temperature are in excellent agreement with previous laboratory tank experiments and in rough agreement with theoretical predictions plume dynamics and geometry are controlled by a natural rossby number which depends strongly on depth h and coriolis parameter f but only weakly on source heat flux q however some specific theoretical predictions are not borne out by these simulations this may occur because the plumes are reingesting their own emissions a process not considered in our earlier theory

numerical modeling of asteroid survivability and possible scenarios for the morokweng crater forming impact the fate of the impactor is an important aspect of the impact cratering process defining impactor material as surviving if it remains solid i e does not melt or vaporize during crater formation previous numerical modeling and experiments have shown that survivability decreases with increasing impact velocity impact angle with respect to the horizontal and target density here we show that in addition to these impactor survivability depends on the porosity and shape of the impactor increasing impactor porosity decreases impactor survivability while prolate shaped polar axis equatorial axis impactors survive impact more so than spherical and oblate shaped polar axis equatorial axis impactors these results are used to produce a relatively simple equation which can be used to estimate the impactor fraction shocked to a given pressure as a function of these parameters by applying our findings to the morokweng crater forming impact we suggest impact scenarios that explain the high meteoritic content and presence of unmolten fossil meteorites within the morokweng crater in addition to previous suggestions of a low velocity and or high angled impact this work suggests that an elongated and or low porosity impactor may also help explain the anomalously high survivability of the morokweng impactor

patterns of fracture and tidal stresses due to nonsynchronous rotation implications for fracturing on europa this study considers the global patterns of fracture that would result from nonsynchronous rotation of a tidally distorted planetary body the incremental horizontal stresses in a thin elastic or viscous shell due to a small displacement of the axis of maximum tidal elongation are derived and the resulting stress distributions are applied to interpret the observed pattern of fracture lineaments on europa the observed pattern of lineaments can be explained by nonsynchronous rotation if these features formed by tension fracturing and dike emplacement tension fracturing can occur for a small displacement of the tidal axis so that the resulting lineaments may be consistent with other evidence suggesting a young age for the surface

holocene paleoseismicity temporal clustering and probabilities of future large m 7 earthquakes on the wasatch fault zone utah the chronology of m 7 paleoearthquakes on the central five segments of the wasatch fault zone wfz is one of the best dated in the world and contains 16 earthquakes in the past 5600 years with an average repeat time of 350 years repeat times for individual segments vary by a factor of 2 and range from about 1200 to 2600 years four of the central five segments ruptured between 620 30 and 1230 60 calendar years b p the remaining segment brigham city segment has not ruptured in the past 2120 100 years comparison of the wfz space time diagram of paleoearthquakes with synthetic paleoseismic histories indicates that the observed temporal clusters and gaps have about an equal probability depending on model assumptions of reflecting random coincidence as opposed to intersegment contagion regional seismicity suggests that for exposure times of 50 and 100 years the probability for an earthquake of m 7 anywhere within the wasatch front region based on a poisson model is 0 16 and 0 30 respectively a fault specific wfz model predicts 50 and 100 year probabilities for an m 7 earthquake on the wfz itself based on a poisson model as 0 13 and 0 25 respectively in contrast segment specific earthquake probabilities that assume quasi periodic recurrence behavior on the weber provo and nephi segments are less 0 01 0 07 in 100 years than the regional or fault specific estimates 0 25 0 30 in 100 years due to the short elapsed times compared to average recurrence intervals on those segments the brigham city and salt lake city segments however have time dependent probabilities that approach or exceed the regional and fault specific probabilities for the salt lake city segment these elevated probabilities are due to the elapsed time being approximately equal to the average late holocene recurrence time for the brigham city segment the elapsed time is significantly longer than the segment specific late holocene recurrence time

liquid water and active resurfacing on europa voyager images of the jovian moon europa show a bright icy surface transected by a complex pattern of long linear fracture like markings with a slightly lower albedo1 very few impact craters are observed with sizes generally hi the range 10 20 km indicators of surface composition and texture include deep ir h2o absorption features2 uv absorption features characteristic of sulphur in a water matrix on the trailing hemisphere3 and a photometric function indicating much more homogeneous scattering than areas of equal albedo on ganymede and callisto4 europe s mean density is 3 03 g cm 3 indicating a primarily silicate composition however if the density of the silicates is the same as for io the h2o mass fraction would be 6 enough to form an outer layer of h2o 100 km thick if completely differentiated several models have been suggested for the evolution and present state of europa5 8 we present here arguments for recent resurfacing by h2o from a liquid layer based on new interpretations of recent spacecraft and earth based observations and revised theoretical calculations

cratering on titan impact melt ejecta and the fate of surface organics abstract we conduct three dimensional hydrodynamical simulations of hypervelocity impacts into the crust of titan to determine the fraction of liquid water generated under the reasonable assumption that the crust is largely water ice and to track the fate of the organic rich layer that is thought to overlie the ice over much of the surface impactors larger than a kilometer in diameter are only slightly affected by the atmosphere while those well under that size are strongly decelerated and broken up before reaching the surface impact of a 2 km diameter icy projectile into the crust at velocities of 7 km per second or higher and angles of impact between 30 and 45 generate about 2 5 melt by volume within the crater our results for the amount of aqueous melt generated in impacts on titan are broadly consistent with the analytic model developed by thompson and sagan 1992 although our numerical model allows us to more precisely quantify the fraction of melt and fate of the organics as a function of the impact parameters while much of the organic surface layer is heavily shocked and ejected from the immediate region of the crater a significant fraction located behind the oblique impact trajectory is only lightly shocked and is deposited in the liquid water at the crater base simple calculations suggest that the resulting aqueous organic phase may remain liquid for hundreds of years or longer enough time for the synthesis of simple precursor molecules to the origin of life

viscoelastic coupling model of the san andreas fault along the big bend southern california the big bend segment of the san andreas fault is the 300 km long segment in southern california that strikes about n65 w roughly 25 counterclockwise from the local tangent to the small circle about the pacific north america pole of rotation the broad distribution of deformation of trilateration networks along this segment implies a locking depth of at least 25 km as interpreted by the conventional model of strain accumulation continuous slip on the fault below the locking depth at the rate of relative plate motion whereas the observed seismicity and laboratory data on fault strength suggest that the locking depth should be no greater than 10 to 15 km the discrepancy is explained by the viscoelastic coupling model which accounts for the viscoelastic response of the lower crust thus the broad distribution of deformation observed across the big bend segment can be largely associated with the san andreas fault itself not subsidiary faults distributed throughout the region the working group on california earthquake probabilities 1995 in using geodetic data to estimate the seismic risk in southern california has assumed that strain accumulated off the san andreas fault is released by earthquakes located off the san andreas fault thus they count the san andreas contribution to total seismic moment accumulation more than once leading to an overestimate of the seismicity for magnitude 6 and greater earthquakes in their type c zones

impact features on europa results of the galileo europa mission gem during the galileo europa mission gem impact features on europa were observed with improved resolution and coverage was compared with voyager or the galileo nominal mission we surveyed all primary impact features 4 km in diameter seen on europa through orbit e19 the transition from simple to complex crater morphology occurs at a diameter of about 5 km we calculated the transient crater dimensions and excavation depths of all craters surveyed the largest impact feature tyre probably had a transient crater depth between 5 and 10 km and transported material to the surface from a depth of not greater than 4 km craters 30 km in diameter such as manannan and pwyll formed within targets whose immediate subcrater materials exhibited nonfluid behavior on time scales of the impact event and that are capable especially in the case of pwyll of supporting significant local topographic loads such as a central peak these craters are nevertheless quite shallow with very subdued floors and we suspect that manannan and pwyll s small depth to diameter ratios are due to the isostatic adjustment of large scale topography facilitated by warm plastically deformable ice at depth morphological similarities between callanish and tyre strongly imply that conclusions reached regarding callanish in j moore et al 1998 icarus135 127 145 also apply to tyre which was that callanish is the consequence of impact into target materials that are mechanically very weak at depth new evidence that callanish s circumferential rings formed before the proximal ejecta became immobile implies a low viscosity substrate at the time of impact we also report additional evidence that a component of the proximal ejecta of callanish was emplaced as a fluid our observations of pwyll secondaries support the conclusions stated in alpert and melosh 1999 that impacts on icy bodies eject smaller fragments and that fragment size decreases more gradually as velocity increases than observed for impacts on silicate bodies at equivalent ejection velocities examination of pwyll s secondary craters reveals azimuthal variations with ejecta fragment sizes being larger near the center of a ray than off the ray our initial analysis of the characteristic size distribution of pwyll s secondary craters shows that they form a differential slope slightly shallower than 4 similar steep slopes for small craters on ganymede imply that small craters there are mostly formed by secondary impact and the jovian system may thus be deficient in small impacts relative to the environment of the terrestrial planets

rotation of europa constraints from terminator and limb positions abstract tidal torques on europa due to jupiter would tend to drive europa s rotation to a rate slightly faster than synchronous unless synchroneity is maintained by a permanent asymmetry in europa s mass distribution moreover global fracture patterns on europa s surface as well as the complex fine scale tectonics revealed by galileo high resolution imagery may be evidence of stress due to nonsynchronous rotation a direct measurement of europa s rotation rate is made from the positions of surface features relative to the terminator in a galileo image and comparing the results with similar measurements of the positions of the same features relative to the terminator in a voyager 2 image taken 17 years earlier any nonsynchronous rotation must be slower than a few tenths of a degree in 17 years a complete rotation in 10 4 years relative to the direction of jupiter

forming europa s folds strain requirements for the production of large amplitude deformation abstract the surface of jupiter s iconic icy satellite europa is dominated by tectonic features that have formed in extension in contrast features that have formed in contraction appear to be rare thus there exists an apparent imbalance between the observed surface expression and relative magnitudes of extensional and contractional strain in this paper we reexamine the amount of strain that can be accommodated during fold formation on europa using a two dimensional finite element model that allows the examination of fold formation at large strains and under a broad range of initial conditions we find that in contrast to previous assessments relatively large strains can be accommodated by the development of relatively low amplitude folds under our nominal conditions creating peak to trough fold amplitudes of 200 m consistent with europa s astypalaea folds requires 7 contraction of the lithosphere decreasing either the strain rate or the ice grain size allows the formation of larger amplitude folds at smaller strains but we find no cases where 200 m amplitude folds form at less than 3 strain only if both of these conditions are met simultaneously do large amplitude folds form at contractional strains 3 the requirement of larger strains relative to previous analytical models results from the combined effects of an initial period of fold nucleation generally 1 2 contractional strain in which limited amplification occurs and smaller amplitude growth rates overall the requirement of several percent strain to produce even moderate amplitude folds is consistent with previous numerical models of single layer and lithospheric scale viscoelastic folding under terrestrial conditions because the formation of large amplitude folds requires relatively large strain small but significant amounts of contraction can be accommodated predominantly through uniform thickening of the lithosphere during the nucleation phase of fold growth without the formation of obvious surface deformation the accommodation of contractional strains via such pre folding lithospheric thickening provides at least a partial solution to europa s apparent strain imbalance

large impact features on europa results of the galileo nominal mission the galileo orbiter examined several impact features on europa at considerably better resolution than was possible from voyager the new data allow us to describe the morphology and infer the geology of the largest impact features on europa which are probes into the crust we observe two basic types of large impact features 1 classic impact craters that grossly resemble well preserved lunar craters of similar size but are more topographically subdued e g pwyll and 2 very flat circular features that lack the basic topographic structures of impact craters such as raised rims a central depression or central peaks and which largely owe their identification as impact features to the field of secondary craters radially sprayed about them e g callanish our interpretation is that the classic craters all 30 km diameter formed entirely within a solid target at least 5 to 10 km thick that exhibited brittle behavior on time scales of the impact events some of the classic craters have a more subdued topography than fresh craters of similar size on other icy bodies such as ganymede and callisto probably due to the enhanced viscous relaxation produced by a steeper thermal gradient on europa pedestal ejecta facies on europa and ganymede may be produced by the relief flattening movement of plastically deforming but otherwise solid ice that was warm at the time of emplacement callanish and tyre do not appear to be larger and even more viscously relaxed versions of the classic craters rather they display totally different morphologies such as distinctive textures and a series of large concentric structural rings cutting impact feature related materials impact simulations suggest that the distinctive morphologies would not be produced by impact into a solid ice target but may be explained by impact into an ice layer 10 to 15 km thick overlying a low viscosity material such as water the very wide near antipodal separation of callanish and tyre imply that 10 15 km may have been the global average thickness of the rigid crust of europa when these impacts occurred the absence of detectable craters superposed on the interior deposits of callanish suggests that it is geologically young 108years hence it seems likely that our preliminary conclusions about the subsurface structure of europa apply to the current day

tidal heating in an internal ocean model of europa considerable evidence suggests that europe s internal structure might consist of a liquid water layer that decouples a thin 30 km overlying ice lithosphere from an underlying silicate core1 2 a lack of impact features extremely subdued topography and positive spectroscopic identification of h2o all imply recent resurfacing by water in addition curvilinear features resembling cracks are ubiquitous over the surface their orientations are broadly consistent with tidally controlled tectonic activity3 compositional models of europea indicate that the h2o layer could be over 100 km thick1 cassen et al 4 first showed how tidal heating if sufficiently intense might stabilize a thin 30 km ice lithosphere over the internal ocean and prevent the water from freezing other models of europa do not include an internal ocean and instead have an ice layer perhaps as thick as 100 km or as thin as a few kilometres resting directly on the silicate interior5 6 however based on studies of crater relaxation thomas and schubert7 have shown the version of this model with a thin ice layer to be unlikely the plausibility of the internal ocean model vis a vis the thick ice model depends critically on the level of tidal heating in europa here we present new calculations of tidal heating based on a more realistic three layer model of europa the tidal distortion of a decoupled ice lithosphere is only half that previously thought at the current value of orbital eccentricity tidal heating is only marginally able to prevent the internal ocean from freezing

the canyon diablo impact event 2 projectile fate and target melting upon impact abstract despite its centennial exploration history there are still unresolved questions about meteor crater the first recognized impact crater on earth this theoretical study addresses some of these questions by comparing model results with field and laboratory studies of meteor crater our results indicate that meteor crater was formed by a high velocity impact of a fragmented projectile ruling out a highly dispersed swarm as well as a very low impact velocity projectile fragmentation caused many fragments to fall separately from the main body of the impactor making up the bulk of the canyon diablo meteorites most of these fragments were engulfed in the expansion plume as they approached the surface without suffering high shock compression and were redistributed randomly around the crater thus the distribution of canyon diablo meteorites is not representative of projectile trajectory as is usual for impactor fragments in smaller strewn fields at least 50 of the main impactor was ejected from the crater during crater excavation and was dispersed mostly downrange of the crater as molten particles spheroids and highly shocked solid fragments shrapnel when compared with the known distribution model results suggest an impactor from the sw overall every model case produced much higher amounts of pure projectile material than observed the projectile target mixing was not considered in the models however this process could be the main sink of projectile melt as all analyzed melt particles have high concentrations of projectile material the fate of the solid projectile fragments is still not completely resolved model results suggest that the depth of melting in the target can reach the coconino sandstone formation however most of the ejected melt originates from 30 40 m depth and thus is limited to moenkopi and upper kaibab material some melt remains in the target based on the estimated volume of the breccia lens at meteor crater our models suggest at most a 2 content of melt in the breccia finally a high water table at the time of impact could have aided strong dispersion of target and projectile melt

fault offsets and lateral crustal movement on europa evidence for a mobile ice shell abstract right lateral structural offsets of 25 km have been identified on the icy galilean satellite europa these occur along dark lineaments oriented orthogonally to wedge shaped bands which are also 25 km wide wedge shaped bands are interpreted as dilated tension fractures which formed as crustal blocks or plates 50 100 km across separated and slipped past each other along flanking strike slip faults this style of deformation does not appear to be characteristic of other lineament types and with the exception of earth appears to be unique to europa together the subparallel wedge shaped b bands form a broad nw se trending belt 1500 km long and less than 500 km across near the anti jovian point this belt is interpreted as a major crustal fracture or rift zone with a pole of rotation determined by the strike slip faults near 47 s 144 w and an approximate ne sw direction of maximum tensile stress extension may have been areally compensated at agenor linea a bright band of possible compressional origin global expansion tidal distortion and nonsynchronous rotation do not explain the inferred minimum principal i e least compressive stress directions alternatively fracturing near the anti jovian point may be a result of i solid state convection in the lower ice crust possibly triggered by uneven heat flow from the silicate interior ii rotation of the icy shell about the sub and anti jovian points induced by latitudinal lithospheric thickness variations or iii preferential strain accumulation from the rest of the icy shell no significant distortion of the crustal blocks occurred during fracturing and rotation indicating that the icy crust was probably mechanically decoupled from the silicate interior in this region over the time scale of fracturing decoupling on a global scale is also likely the simple geometry of lineaments argues for formation in an icy lithosphere not a silicate one and other evidence for fracturing caused by nonsynchronous rotation stress is not compatible with a tidally locked silicate interior which we show is likely unless the ice shell rotates independently decoupling could have been due to either warm ice or liquid water near the base of the icy crust mechanical bounds on lithospheric thickness a few to 10 km lead to heat flow estimates that admit both possibilities but favor decoupling by liquid water

tidal reorientation and the fracturing of jupiter s moon europa the most striking characteristic of europa is the network of long linear albedo markings over the surface suggestive of global scale tectonic processes various explanations for the fractures have been proposed freezing and expansion of an early liquid water ocean1 planetary expansion due to dehydration of hydrated silicates2 localization by weak points in the crust generated by impacts3 and a combination of stresses due to planetary volume change and tidal distortions from orbital recession and orbital eccentricity4 5 calculations by yoder6 and greenberg and weidenschilling7 have shown that europa may rotate slightly more rapidly than the synchronous rate with a rotation period reorientation through 360 ranging from 20 to 103 yr if a liquid mantle is present or up to 1010 yr if the satellite is essentially solid7 helfen stein and parmentier8 modelled the stresses due to nonsynchronous rotation and concluded that this could explain the long fractures in part of the anti jovian hemisphere in this note i present a global map of lineaments with long arc lengths 20 or 550 km and compare the lineament orientations to the tensile stress trajectories due to tidal distortions changes in the lengths of three principal semiaxes and to nonsynchronous rotation longitudinal reorientation of two of the principal semiaxes an excellent orthogonal fit to the lineaments is achieved by the stresses due to nonsynchronous rotation with the axis radial to jupiter located 25 east of its present position this fit suggests that nonsynchronous rotation occurred at some time in europa s history

ganymede crater dimensions implications for central peak and central pit formation and development the morphology of impact craters on the icy galilean satellites differs from craters on rocky bodies the differences are thought due to the relative weakness of ice and the possible presence of sub surface water layers digital elevation models constructed from galileo images were used to measure a range of dimensions of craters on the dark and bright terrains of ganymede measurements were made from multiple profiles across each crater so that natural variation in crater dimensions could be assessed and averaged scaling trends constructed the additional depth slope and volume information reported in this work has enabled study of central peak formation and development and allowed a quantitative assessment of the various theories for central pit formation we note a possible difference in the size morphology progression between small craters on icy and silicate bodies where central peaks occur in small craters before there is any slumping of the crater rim which is the opposite to the observed sequence on the moon conversely our crater dimension analyses suggest that the size morphology progression of large lunar craters from central peak to peak ring is mirrored on ganymede but that the peak ring is subsequently modified to a central pit morphology pit formation may occur via the collapse of surface material into a void left by the gradual release of impact induced volatiles or the drainage of impact melt into sub crater fractures

planetary science europa awakening brines percolating in the icy crust of jupiter s moon europa may be responsible for the satellite s enigmatic chaotic terrains a new model predicts that one such terrain is currently forming over shallow subsurface water see letter p 502

ice diapirs on europa implications for liquid water early examinations of galileo images of europa revealed features that look like low topographic domes these small 5 10 km in radius domes have been interpreted as surface manifestations of diapirs as a way to probe the subsurface structure of europa we investigate the possibility that thermally driven ice diapirism created these surface features we use a previously developed analytic model for rising diapirs to estimate the initial radii of the diapirs their initial depth the temperature of the medium through which they rise and their formation timescales we assume that the diapirs originate at the boundary layer between solid ice and whatever underlies it whether it be liquid water or solid rock our results show that the diapirs originate at a depth of no more than a few tens of kilometers since the h2o layer on europa is substantially thicker than this our results support the view that liquid water recently existed beneath the ice on europa at this location at a depth of no more than a few tens of kilometers further the ice must have been warm 250 270 k and was therefore likely to have been convecting

europa s ice tectonics new insights from physical wax experiments with implications for subduction initiation and global resurfacing processes abstract jupiter s moon europa has one of the youngest geological surfaces in our solar system with an age of 40 90 ma implying an intense history of resurfacing the surface of europa indeed shows abundant evidence of tectonic deformation related to extension strike slip and shortening however observed features related to shortening are scarce compared with pervasive extensive extensional features such as dilational bands and do not suffice as the sole mechanism for recycling aging terranes recently evidence for potential plate tectonics associated with subduction zones has been discovered on europa this could be responsible for recycling most of europa s surface however basic physical parameters needed to initiate subduction on europa such as thickness of the brittle layer deformation rates and orientation of pre existing zones of weakness at which subduction could start are not well understood here we aim to better understand the process and the conditions that could lead to initiation of subduction on europa through physical experiments using wax to simulate europa s two layered i e convective icy crust by deforming the wax strain on europa s surface possibly caused by diurnal tides or its nonsynchronous rotation is simulated our results indicate that subduction could initiate over a broad range of surface thicknesses and deformation rates above a minimum conductive layer thickness but is strongly dependent on the orientation of the pre existing zones of weakness very thin conductive layer experiments however result in a previously undescribed process that we term ductile rolldown which creates surface features similar to double ridges observed on europa thus subduction and ductile rolldown represent physically plausible mechanisms that could play a critical role in resurfacing europa throughout its geologic history these results could yield significant implications for europa s thermal history and evolution habitability and future spacecraft missions

thickness of a europan ice shell from impact crater simulations several impact craters on jupiter s satellite europa exhibit central peaks on the terrestrial planets central peaks consist of fractured but competent rock uplifted during cratering therefore the observation of central peaks on europa indicates that an ice layer must be sufficiently thick that the impact events did not completely penetrate it we conducted numerical simulations of vapor and melt production during cratering of water ice layers overlying liquid water to estimate the thickness of europa s icy crust because impacts disrupt material well beyond the zone of partial melting our simulations put a lower limit on ice thickness at the locations and times of impact we conclude that the ice must be more than 3 to 4 kilometers thick

a schematic model of crater modification by gravity models are proposed to account for the formation of slump terraces central peaks peak rings and concentric rings in large impact structures crater slumping and central peak formation is attributed to the process of acoustic fluidization which endows a volume of the target surrounding the crater with a bingham plastic rheology for a short time after the impact the parameters of this bingham plastic model are remarkably constant throughout the solar system the bingham yield stress ranges from a maximum of 80 bars on mercury to a minimum of 7 bars on callisto the bingham viscosity is between 109 and 1010 p at the onset of central peak formation for all bodies studied although it appears to rise to 1011 p as crater diameter grows to a few hundred kilometers asymmetric or scarp bounded concentric rings are due to a fundamentally different process they form when the target planet s lithosphere fractures under stresses imposed by the flow of the asthenosphere toward the transient crater cavity lithosphere yield strengths of less than a few hundred bars are implied by observed ring development on the moon and callisto

convective instability in europa s floating ice shell models of the tidally heated floating ice shell proposed for the jovian satellite europa generally find shell thicknesses less than 30 km past parameterized convection models indicated that such shells are stable against convective overturn which otherwise ostensibly leads to freezing of the ocean underneath here i apply the temperature dependent viscosity convection scaling developed by solomatov and coworkers to the europan ice shell the temperature dependent properties of ice are linearized about 260 k as any convective interior should be close to this temperature with the colder ice forming an essentially passive stagnant lid ice shells 10 km thick are found to be unstable to convection at their base for melting point viscosities of 1013 pa s as linearized by tidal stresses if the ice deforms by superplastic creep but such low viscosities require small grain sizes 1 mm this requirement may be met if grain sizes observed in terrestrial polar glaciers can be strain rate scaled to europa regardless convection at the base of the ice shell if initiated may not freeze the ocean because of tidal heating a stagnant lid regime ice shell is much more dissipative than a conductive shell of the same thickness such a shell should thin not thicken and the potential exists for further thermal instabilities and runaways

creep of water ices at planetary conditions a compilation many constitutive laws for the flow of ice have been published since the advent of the voyager explorations of the outer solar system conflicting data have occasionally come from different laboratories and refinement of experimental techniques has led to the publication of laws that supersede earlier ones in addition there are unpublished data from ongoing research that also amend the constitutive laws here we compile the most current laboratory derived flow laws for water ice phases i ii iii v and vi and ice i mixtures with hard particulates the rheology of interest is mainly that of steady state and the conditions reviewed are the pressures and temperatures applicable to the surfaces and interiors of icy moons of the outer solar system advances in grain size dependent creep in ices i and ii as well as in phase transformations and metastability under differential stress are also included in this compilation at laboratory strain rates the several ice polymorphs are rheologically distinct in terms of their stress temperature and pressure dependencies but with the exception of ice iii have fairly similar strengths hard particulates strengthen ice i significantly only at high particulate volume fractions ice iii has the potential for significantly affecting mantle dynamics because it is much weaker than the other polymorphs and its region of stability which may extend metastably well into what is nominally the ice ii field is located near likely geotherms of large icy moons

habitability of europa s crust the role of tidal tectonic processes jupiter s satellite europa has been identified as one of the most likely sites for life in the solar system the tidal tectonic processes that appear to have governed europa s geology seem to require interaction with an ocean under only a very thin crust providing a variety of evolving environmental niches the mutually dependent relationship between orbital evolution and tidal processes in turn controls europa s rotation heating and stress surface lineaments are correlated with global stress patterns demonstrating that they form by crustal cracking but only if a substantial ocean is present to give adequate tidal amplitude tidal driving of strike slip faulting indicates that cracks penetrate to a fluid layer which is possible only with a very thin ice crust the characteristic ridge sets that cover tectonic terrain are likely built by tidal pumping of fluid and slush to the surface on a daily basis widespread tectonic dilation creates new surface as material rises from below chaotic terrain has morphology and other characteristics indicative of melt through from below surface colorants correlate with locations such as along large scale ridge systems and around chaotic terrain where ocean water reached the surface this model implies that as a result of tides liquid water regularly bathed crustal cracks and surfaces with heat and whatever nutrients are included in the oceanic chemistry creating a variety of habitable environments the processes were recent and thus most likely continue today longer term evolution of environmental conditions provided the need for adaptation and opportunity for evolution

fossil earthquakes recorded by pseudotachylytes in mantle peridotite from the alpine subduction complex of corsica paleo earthquakes recorded by pseudotachylytes have recently been discovered in the blueschist facies subduction complex of alpine corsica pseudotachylytes occur in ophiolite gabbro and mantle peridotite belonging to the schistes lustres of cape corse ultramafic pseudotachylyte fault and injection veins are found within well preserved peridotite lenses and are progressively hydrated together with the host rock along the margins of the lenses numerous pseudotachylytes ranging in thickness from less than 1 to 380 mm have been identified veins thicker than 3 mm may show flow banded chilled glassy margins and cores with dendritic to spherulitic quench textures the newly formed minerals are zoned olivine fo93 89 clino and ortho pyroxene with compositions indicative of high crystallization temperatures 1300 1400 8c zoned cr spinel and a glassy to micro vesicular hydrous matrix showing that frictional melts contained up to 4 water frictional heating on co seismic faults raised the temperature from ambient blueschist facies conditions 450 8c and 1 1 5 gpa to more than 1700 8c which is required for 75 disequilibrium melting of spinel peridotite at 1 5 gpa the characteristic fault vein thicknesses observed are 1 to 3 cm but several fault veins are thicker than 10 cm co seismic displacement of 1 m a stress of 300 mpa and seismic efficiency of 5 may melt ca 60 kg peridotite pr m 2 fault surface corresponding to 20 mm thick layer of ultramafic pseudotachylyte the ultramafic pseudotachylytes described here formed by disequilibrium melting of peridotite in the upper part of the alpine subduction zone if the interpretations of typical displacements of approximately 1 m are correct the most common pseudotachylyte fault veins are related to magnitude ca 7 or larger subduction earthquakes d 2005 elsevier b v all rights reserved

impact penetration of europa s ice crust as a mechanism for formation of chaos terrain ice thickness estimates and impactor dynamics indicate that some impacts must breach europas ice crust and outcomes of impact experiments using ice over water targets range from simple craters to chaos like destroyed zones depending on impact energy and ice competence firstorder impacts into thick ice or at low impact energy produce craters second order impacts punch through the ice making holes that resemble raft free chaos areas third order impacts into thinnest ice or at highest energy produce large irregular raft filled zones similar to platy chaos other evidence for an impact origin for chaos areas comes from the size frequency distribution of chaos craters on europa which matches the impact production functions of ganymede and callisto and from small craters around the large chaos area thera macula which decrease in average size and density per unit area as a function of distance from theras center there are no tiny chaos areas and no craters 50 km diameter this suggests that small impactors never penetrate whereas large ones berpenetrators 2 5 km diameter at average impact velocity always do existence of both craters and chaos areas in the size range 2 40 km diameter points to spatial temporal variation in crust thickness but in this size range craters are progressively outnumbered by chaos areas at larger diameters suggesting that probability of penetration increases with increasing scale of impact if chaos areas do represent impact sites then europas surface is older than previously thought the recalculated resurfacing age is 480 302 960 ma greater than prior estimates but still very young by solar system standards

folds on europa implications for crustal cycling and accommodation of extension regional scale undulations with associated small scale secondary structures are inferred to be folds on jupiter s moon europa formation is consistent with stresses from tidal deformation potentially triggering compressional instability of a region of locally high thermal gradient folds may compensate for extension elsewhere on europa and then relax away over time

ocean worlds in the outer solar system 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

layering and double diffusion style convection in europa s ocean abstract we assess the effects of fluid composition and depth dependent hydrostatic pressure on dynamics in a europan ocean primarily composed of aqueous na 2 so 4 and mgso 4 for both a saturated and a dilute ocean we observe a salinity and pressure dependent check on buoyancy in putative upwellings which could act as a mechanism for storing heat in the ocean s base uptake of salt to a warmed parcel of water from the seafloor environment causes upwellings to lose buoyancy before reaching the base of the overlying ice implying a tendency toward ocean stratification we make an analogy to double diffusive convecting systems observed in the red sea and elsewhere in earth s waters using currently understood parameterizations for onset and stability we estimate a range of temperatures and salinities for which double diffusive convection may occur in europa s ocean in the red sea boundary layers separating convecting zones have been observed to move upward as the lower layer acquires heat and salt we examine the possibility of stratification and double diffusive convection as mechanisms for heat storage in europa s ocean and possible correlation with recently inferred changes in ice surface alteration style over the last 30 80 myr

subsurface oceans on europa and callisto constraints from galileo magnetometer observations magnetic field perturbations measured during galileo flybys of europa and callisto are consistent with dipole fields induced by the temporal variations of the ambient jovian magnetospheric field these fields are close to those expected for perfectly conducting moons we investigate the implications of these observations for the electrical structure of the moon s interiors using a simple shell model it is found that europa and callisto must possess regions where the conductivity exceeds 0 06 and 0 02 s m at a depth of less than 200 and 300 km below the surface respectively this conductivity is unattainable in ice or silicates unless the ice layer is at least partially molten or very large temperature gradients can be maintained below the ice an ionosphere or a cloud of pick up ions are probably also insufficiently conductive global earth like oceans under the surface of both moons could account for the observations provided they are at least a few kilometers thick c 2000 academic press

hydrocode simulation of ganymede and europa cratering trends how thick is europa s crust abstract one of the continuing debates of outer solar system research centers on the thickness of europa s ice crust as it affects both the habitability and accessibility of its sub surface ocean here we use hydrocode modeling of the impact process in layered ice and water targets and comparison to europan cratering trends and galileo derived topographic profiles to investigate the crustal thickness full or partial penetration of the ice crust by an impactor occurred in simulations in which the ice thickness was less than 14 times the projectile radius craters produced in these thin shell simulations were consistently smaller than for larger ice thicknesses which will complicate inference of large impactor population sizes simulations in which the resultant crater was 3 times the ice layer thickness resulted in summit pit morphology this work supports that summit pit craters noted on both rocky and icy bodies can be created by the presence of a weaker layer at depth we suggest that floor pits seen only on ice rich bodies require a different formation mechanism to summit pits pristine craters formed in a target with high heat flow were shallower than for the same impact into a target of lesser heat flow suggesting that the starting crater morphology for viscous relaxation isostatic readjustments and erosion rate studies is different for craters formed in times of different heat flow we find that the crater depth diameter trend of europa can only be recreated when simulating impact into an upper brittle ice layer of 7 km depth with a corresponding geothermal gradient of 0 025 k m as this ice thickness estimate is below 10 km results from this work suggest that convective overturn of the surface ice may occur or have occurred on europa making the development of indigenous life a possibility

foreshocks aftershocks and earthquake probabilities accounting for the landers earthquake the equation to determine the probability that an earthquake occurring near a major fault will be a foreshock to a mainshock on that fault is modified to include the case of aftershocks to a previous earthquake occurring near the fault the addition of aftershocks to the background seismicity makes its less probable that an earthquake will be a foreshock because nonforeshocks have become more common as the aftershocks decay with time the probability that an earthquake will be a foreshock increases however fault interactions between the first mainshock and the major fault can increase the long term probability of a characteristic earthquake on that fault which will in turn increase the probability that an event is a foreshock compensating for the decrease caused by the aftershocks

ozone perturbation from medium size asteroid impacts in the ocean abstract we present results of an investigation aimed at characterizing the effects of oceanic impacts of 500 m and 1 km diameter asteroids on the lower and middle atmosphere estimating ozone loss and potential danger from uv radiation at the earth s surface little work has been done so far to assess the atmospheric perturbation from the impact of objects in this size range even though their sizes are close to the threshold for causing global environmental effects in particular at the earth s surface oceanic impacts are twice more likely to occur than land impacts this work represents the first attempt at combining impact simulations with a three dimensional shock physics code sova and atmospheric simulations using the general circulation model with interactive chemistry waccm sova simulations provided an estimate of the amount and state of material ejected into the atmosphere by the impacts estimated water vapor in the upper atmosphere was then introduced in the initial conditions for the waccm simulations that modeled the subsequent perturbation of atmospheric chemistry final estimates of the change over time in uv flux at the surface due to the impact induced ozone change are then carried out using the tuv radiative transfer model the results suggest that mid latitude oceanic impacts of 1 km asteroids can produce a significant global perturbation of upper atmospheric chemistry including multi year global ozone depletion comparable to ozone hole records registered in the mid 1990 s asteroids 500 m in diameter cause limited perturbations of upper atmospheric chemistry with significant ozone depletion confined to the hemisphere in which the impact occurred impact induced ozone depletion affects uv irradiance at the earth s surface resulting in levels of uv b irradiance that can be dangerous for living organisms and in the tropics and initial midlatitude summers far exceed levels currently experienced anywhere on earth

habitats and taphonomy of europa abstract jupiter s moon europa possesses an icy shell kilometers thick that may overlie a briny ocean the inferred presence of water tidal and volcanic energy and nutrients suggests that europa is potentially inhabited by some kind of life indeed europa is a primary target in the search for life in the solar system although no evidence yet exists for any kind of life the thickness of the icy crust would impose limits on life but at least 15 broad kinds of habitats seem possible for europa they include several on the sea floor at least 3 in the water column and many in the ice itself all of these habitats are in or could be transported to the icy shell where they could be exposed by geologic activity or impacts so they might be explored from the surface or orbit by future planetary missions taphonomic processes that transport preserve and expose habitats include buoyant ice removing bottom habitats and sediment to the underside of the ice water currents depositing components of water column habitats on the ice bottom cryovolcanoes depositing water on the surface tidal pumping bringing water column and ice habitats to the near surface ice and subice freezing and diapiric action incorporating water column and bottom ice habitats into the lower parts of the icy shell the preserved habitats could be exposed at or near the surface of europa chiefly in newly formed ice tilted or rotated ice blocks ridge debris surface deposits fault scarps the sides of domes and pits and impact craters and ejecta future exploration of europa for life must consider careful targeting of sites where habitats are most likely preserved or exist close to the surface

probability of one or more m 7 earthquakes in southern california in 30 years eight earthquakes of magnitude greater than or equal to seven have occurred in southern california in the past 200 years if one assumes that such events are the product of a poisson process the probability of one or more earthquakes of magnitude seven or larger in southern california within any 30 year interval is 67 23 95 confidence interval because five of the eight m 7 earthquakes in southern california in the last 200 years occurred off of the san andreas fault system the probability of one or more m 7 earthquakes in southern california but not on the san andreas fault system occurring within 30 years is 52 27 95 confidence interval

tidal tectonic processes and their implications for the character of europa s icy crust 1 jupiter s satellite europa is covered by an icy crust likely overlying a deep global liquid water ocean the mutually dependent relationship between the orbit and tidal processes controls europa s rotation heating and stress observations of the surface and investigation of tidal tectonic processes indicate that the dominant types of surface terrain have been created by frequent repeated exposure of an underlying ocean to the surface surface lineaments including ubiquitous cycloidal chains of arcs features are correlated with tidal stress patterns demonstrating that they form by crustal cracking but only if a substantial ocean is present to give adequate tidal amplitude stratigraphy of tectonic features combined with models of how and where they formed demonstrates a moderate rate of nonsynchronous rotation tidal driving of strike slip displacement in which surface areas shear passed one another by daily 85 hours on europa tidal stresses suggests that cracks penetrate to a liquid layer the characteristic ridge sets that cover tectonic terrain may be built by tidal pumping of fluid and slush to the surface on a daily basis widespread tectonic dilation creates new surface as material rises from below in addition to those tectonic terrains nearly half of the surface is chaotic terrain with morphology and other characteristics indicative of melt through from below consistent with the tectonic indications of thin ice and plausible tidal heating rates formation of both chaotic and tectonic terrains has continually resurfaced the satellite while connecting the ocean to the surface surface colorants correlate with locations where ocean water reached the surface such as along large scale ridge systems and around chaotic terrain as a result of tides liquid water may have bathed crustal cracks and surfaces with heat transporting and mixing substances vertically such daily transported materials would include substances from the oceanic reservoir oxidants and fuels created at the surface as well as any organisms and their chemical products thus a variety of habitable environments likely exist in the crust deactivation of individual cracks after thousands of years due to the tidally driven nonsynchronous rotation constrains processes like ridge building and strike slip displacement and would require any living organisms to adapt to such change

does europa have a subsurface ocean evaluation of the geological evidence it has been proposed that jupiter s satellite europa currently possesses a global subsurface ocean of liquid water galileo gravity data verify that the satellite is differentiated into an outer h2o layer about 100 km thick but cannot determine the current physical state of this layer liquid or solid here we summarize the geological evidence regarding an extant subsurface ocean concentrating on galileo imaging data we describe and assess nine pertinent lines of geological evidence impact morphologies lenticulae cryovolcanic features pull apart bands chaos ridges surface frosts topography and global tectonics an internal ocean would be a simple and comprehensive explanation for a broad range of observations however we cannot rule out the possibility that all of the surface morphologies could be due to processes in warm soft ice with only localized or partial melting two different models of impact flux imply very different surface ages for europa the model favored here indicates an average age of 50 myr searches for evidence of current geological activity on europa such as plumes or surface changes have yielded negative results to date the current existence of a global subsurface ocean while attractive in explaining the observations remains inconclusive future geophysical measurements are essential to determine conclusively whether or not there is a liquid water ocean within europa today

galileo magnetometer measurements a stronger case for a subsurface ocean at europa on 3 january 2000 the galileo spacecraft passed close to europa when it was located far south of jupiter s magnetic equator in a region where the radial component of the magnetospheric magnetic field points inward toward jupiter this pass with a previously unexamined orientation of the external forcing field distinguished between an induced and a permanent magnetic dipole moment model of europa s internal field the galileo magnetometer measured changes in the magnetic field predicted if a current carrying outer shell such as a planet scale liquid ocean is present beneath the icy surface the evidence that europa s field varies temporally strengthens the argument that a liquid ocean exists beneath the present day surface

formation of cycloidal features on europa cycloidal patterns are widely distributed on the surface of jupiter s moon europa tensile cracks may have developed such a pattern in response to diurnal variations in tidal stress in europa s outer ice shell when the tensile strength of the ice is reached a crack may occur propagating cracks would move across an ever changing stress field following a curving path to a place and time where the tensile stress was insufficient to continue the propagation a few hours later when the stress at the end of the crack again exceeded the strength propagation would continue in a new direction thus one arcuate segment of the cycloidal chain would be produced during each day on europa for this model to work the tensile strength of europa s ice crust must be less than 40 kilopascals and there must be a thick fluid layer below the ice to allow sufficient tidal amplitude

europa s differentiated internal structure inferences from four galileo encounters radio doppler data from four encounters of the galileo spacecraft with the jovian moon europa have been used to refine models of europa s interior europa is most likely differentiated into a metallic core surrounded by a rock mantle and a water ice liquid outer shell but the data cannot eliminate the possibility of a uniform mixture of dense silicate and metal beneath the water ice liquid shell the size of a metallic core is uncertain because of its unknown composition but it could be as large as about 50 percent of europa s radius the thickness of europa s outer shell of water ice liquid must lie in the range of about 80 to 170 kilometers

an endogenic origin of cerean organics abstract localized deposits of aliphatic organic rich material have been found on the surface of dwarf planet ceres and based on mineralogical context it has been suggested that these organics formed within ceres de sanctis et al 2017 however no obvious source and exposure mechanism can be inferred from the geological settings of the organic rich terrains the alternative hypothesis is that the organics were delivered from an exogenic source via impact pieters et al 2018 we numerically simulate the thermal degradation of aliphatic organics delivered to ceres during hypervelocity impacts as well as the thermal degradation and excavation of organics from the subsurface our models show that exogenic delivery of aliphatic organics is inefficient as most of the spectral signature of the organic species would be thermally degraded and diluted by mixing with target material in the ejecta blanket of a given crater instead our models suggest that the formation of ernutet crater excavated a subsurface reservoir of aliphatic organics from a depth of 3 6 km bringing it close to the surface where it could be subsequently exhumed by small craters and landslides compatible with observations

copernicus crater central peak lunar mountain of unique composition olivine is identified as the major mafic mineral in a central peak of copernicus crater information on the mineral assemblages of such unsampled lunar surface material is provided by near infrared reflectance spectra 0 7 to 2 5 micrometers obtained with earth based telescopes the composition of the deep seated material comprising the copernicus central peak is unique among measured areas other lunar terra areas and the wall of copernicus exhibit spectral characteristics of mineral assemblages comparable to the feldspathic breccias returned by the apollo missions with low calcium orthopyroxene being the major mafic mineral