a 1 d evolutionary model for icy satellites applied to enceladus abstract we develop a long term 1 d evolution model for icy satellites that couples multiple processes water migration and differentiation geochemical reactions and silicate phase transitions compaction by self gravity and ablation the model further considers the following energy sources and sinks tidal heating radiogenic heating geochemical energy released by serpentinization or absorbed by mineral dehydration gravitational energy and insolation and heat transport by conduction convection and advection we apply the model to enceladus by guessing the initial conditions that would render a structure compatible with present day observations assuming the initial structure to have been homogeneous assuming the satellite has been losing water continually along its evolution we postulate that it was formed as a more massive more icy and more porous satellite and gradually transformed into its present day state due to sustained long term tidal heating we consider several initial compositions and evolution scenarios and follow the evolution for the age of the solar system testing the present day model results against the available observational constraints our model shows the present configuration to be differentiated into a pure icy mantle several tens of km thick overlying a rocky core composed of dehydrated rock at the center and hydrated rock in the outer part for enceladus it predicts a higher rock ice mass ratio than previously assumed and a thinner ice mantle compatible with recent estimates based on gravity field measurements although obviously the model cannot be used to explain local phenomena it sheds light on the internal structure invoked in explanations of localized features and activities

cassini imaging science initial results on phoebe and iapetus the cassini imaging science subsystem acquired high resolution imaging data on the outer saturnian moon phoebe during cassini s close flyby on 11 june 2004 and on iapetus during a flyby on 31 december 2004 phoebe has a heavily cratered and ancient surface shows evidence of ice near the surface has distinct layering of different materials and has a mean density that is indicative of an ice rock mixture iapetus s dark leading side cassini regio is ancient heavily cratered terrain bisected by an equatorial ridge system that reaches 20 kilometers relief local albedo variations within and bordering cassini regio suggest mass wasting of ballistically deposited material the origin of which remains unknown

ferrovolcanism iron volcanism on metallic asteroids metallic asteroids the exposed cores of disrupted planetesimals are expected to have been exposed while still molten some would have cooled from the outside in crystallizing a surface crust which would then grow inward because the growing crust is expected to be more dense than the underlying melt this melt will tend to migrate toward the surface whenever it is able compressional stresses produced in the crust while it cools will be relieved locally by thrust faulting which will also provide potential conduits for melt to reach the surface we predict iron volcanism to have occurred on metallic asteroids as they cooled and discuss the implications of this process for both the evolution and the modern appearance of these bodies

on the origin of south polar folds on enceladus recent high resolution cassini images of the south polar terrain of enceladus reveal regions of short wavelength deformation inferred to be compressional folds between the baghdad and damascus tiger stripes spencer j r barr a c esposito l w helfenstein p ingersoll a p jaumann r mckay c p nimmo f waite j h 2009a enceladus an active cryovolcanic satellite in saturn after cassini huygens springer new york pp 683 722 here we use fourier analysis of the bright dark variations to show that the folds have a dominant wavelength of 1 1 0 4 km we use the simple model of lava flow folding from fink fink j 1980 geology 8 250 254 to show that the folds could form in an ice shell with an upper high viscosity boundary layer of thickness 400 m with a driving stress of 40 80 kpa and strain rate between 10 14 s 1 and 10 12 s 1 such deformation rates imply resurfacing of the spt in 0 05 5 myr consistent with its estimated surface age measurements of fold topography and more sophisticated numerical modeling can narrow down the conditions of fold formation and provide valuable constraints on the thermal structure of the ice shell on enceladus

the temperature and width of an active fissure on enceladus measured with cassini vims during the 14 april 2012 south pole flyover abstract the width and temperature of the active fissures on saturn s satellite enceladus provide key observable constraints on physical models of these geyser like eruptions we analyze a sequence of high spatial resolution near infrared spectra acquired with vims at 0 025 s intervals during a 74 km altitude flyover of the south pole of enceladus by the cassini spacecraft on 14 april 2012 utc a thermal emission spectrum covering 3 to 5 m wavelengths was detected as the field of view crossed one of the four major fissures baghdad sulcus within 1 km of 82 36s latitude and 28 24w longitude we interpret this spectrum as thermal emission from a linear fissure with temperature 197 20 k and width 9 m at the above wavelengths the spectrum is dominated by the warmest temperature component looking downward into the fissure at only 13 from the vertical we conclude that our results measure the temperature of the interior fissure walls and the h 2 o vapor at depths within 40 m of the surface

timing of water plume eruptions on enceladus explained by interior viscosity structure water plume eruptions on saturn s icy moon enceladus are delayed relative to the peak tidal stresses simulations suggest the delay can be explained by the moon s interior structure and the presence of a subsurface ocean at the south pole of saturn s icy moon enceladus eruptions of water vapour and ice emanate from warm tectonic ridges1 2 3 4 observations in the infrared5 and visible6 spectra have shown an orbital modulation of the plume brightness which suggests that the eruption activity is influenced by tidal forces however the observed activity seems to be delayed by several hours with respect to predictions based on simple tidal models6 7 here we simulate the viscoelastic tidal response of enceladus with a full three dimensional numerical model8 9 and show that the delay in eruption activity may be a natural consequence of the viscosity structure in the south polar region and the size of the putative subsurface ocean by systematically comparing simulations of variations in normal stress along faults with plume brightness data we show that the observed activity is reproduced for two classes of interior models with contrasting thermal histories a low viscosity convective region above a polar sea extending about 45 60 from the south pole at a depth below the surface as small as 30 km or a convecting ice shell of 60 70 km in thickness above a global ocean our analysis further shows that the eruption activity is controlled by the average normal stress applied across the cracks thus providing a constraint on the eruption mechanism

cryoclastic origin of particles on the surface of enceladus analogous to volcanic deposits on earth we can infer eruption characteristics on enceladus from the relationship between particle size and distance from the vent we develop a model in which ice particles feeding plumes are accelerated by the gas we consider two cases drag limited and collision limited acceleration which link particle size to exit velocity after being ejected at the vent particles follow ballistic trajectories we fit the model to observations of particle size on the surface inferred from modeled vims data collected by the cassini spacecraft we obtain a relationship between gas temperature and characteristic acceleration length whereby lower gas temperatures require longer acceleration lengths the model shows that the large size of particles on the surface is consistent with the size of particles observed with the cda and vims instruments at heights of cassini flybys and the size of particles that reach escape velocity and are found in saturn s e ring

fracture penetration in planetary ice shells abstract the proposed past eruption of liquid water on europa and ongoing eruption of water vapor and ice on enceladus have led to discussion about the feasibility of cracking a planetary ice shell we use a boundary element method to model crack penetration in an ice shell subjected to tension and hydrostatic compression we consider the presence of a region at the base of the ice shell in which the far field extensional stresses vanish due to viscoelastic relaxation impeding the penetration of fractures towards a subsurface ocean the maximum extent of fracture penetration can be limited by hydrostatic pressure or by the presence of the unstressed basal layer depending on its thickness our results indicate that europa s ice shell is likely to be cracked under 1 3 mpa tension only if it is 2 5 km thick enceladus ice shell may be completely cracked if it is capable of supporting 1 3 mpa tension and is less than 25 km thick

the implications of tides on the mimas ocean hypothesis we investigate whether a present day global ocean within mimas is compatible with the lack of tectonic activity on its surface by computing tidal stresses for ocean bearing interior structure models derived from observed librations we find that for the suite of compatible rheological models peak surface tidal stresses caused by mimas high eccentricity would range from a factor of two smaller to an order of magnitude larger than those on tidally active europa thermal stresses from a freezing ocean or a past higher eccentricity would enhance present day tidal stresses exceeding the magnitudes associated with europa s ubiquitous tidally driven fractures and in some cases the failure strength of ice in laboratory studies therefore in order for mimas to have an ocean its ice shell cannot fail at the stress values implied for europa furthermore if mimas ocean is freezing out the ice shell must also be able to withstand thermal stresses that could be an order of magnitude higher than the failure strength of laboratory ice samples in light of these challenges we consider an ocean free mimas to be the most straightforward model best supported by our tidal stress analysis

prerequisites for explosive cryovolcanism on dwarf planet class kuiper belt objects explosive extrusion of cold material from the interior of icy bodies or cryovolcanism has been observed on enceladus and perhaps europa triton and ceres it may explain the observed evidence for a young surface on charon pluto s surface is masked by frosts here we evaluate prerequisites for cryovolcanism on dwarf planet class kuiper belt objects kbos we first review the likely spatial and temporal extent of subsurface liquid proposed mechanisms to overcome the negative buoyancy of liquid water in ice and the volatile inventory of kbos we then present a new geochemical equilibrium model for volatile exsolution and its ability to drive upward crack propagation this novel approach bridges geophysics and geochemistry and extends geochemical modeling to the seldom explored realm of liquid water at subzero temperatures we show that carbon monoxide co is a key volatile for gas driven fluid ascent whereas co 2 and sulfur gases only play a minor role n 2 ch 4 and h 2 exsolution may also drive explosive cryovolcanism if hydrothermal activity produces these species in large amounts a few percent with respect to water another important control on crack propagation is the internal structure a hydrated core makes explosive cryovolcanism easier but an undifferentiated crust does not we briefly discuss other controls on ascent such as fluid freezing on crack walls and outline theoretical advances necessary to better understand cryovolcanic processes finally we make testable predictions for the 2015 new horizons flyby of the pluto charon system

a combined spectrophotometric and morphometric study of the lunar mare dome fields near cauchy arago hortensius and milichius in this study we examine the spectral and morphometric properties of the four important lunar mare dome fields near cauchy arago hortensius and milichius we utilize clementine uv vis mulfispectral data to examine the soil composition of the mare domes while employing telescopic ccd imagery to compute digital elevation maps in order to determine their morphometric properties especially flank slope height and edifice volume after reviewing previous attempts to determine topographic data for lunar domes we propose an image based 3d reconstruction approach which is based on a combination of photoclinometry and shape from shading accordingly we devise a classification scheme for lunar marc domes which is based on a principal component analysis of the determined spectral and morphometric features for the effusive mare domes of the examined fields we establish four classes two of which are further divided into two subclasses respectively where each class represents distinct combinations of spectral and morphometric dome properties as a general trend shallow and steep domes formed out of low tio2 basalts are observed in the hortensius and milichius dome fields while the domes near cauchy and arago that consist of high tio2 basalts are all very shallow the intrusive domes of our data set cover a wide continuous range of spectral and morphometric quantities generally characterized by larger diameters and shallower flank slopes than effusive domes a comparison to effusive and intrusive mare domes in other lunar regions highland domes and lunar cones has shown that the examined four mare dome fields display such a richness in spectral properties and 3d dome shape that the established representation remains valid in a more global context furthermore we estimate the physical parameters of dome formation for the examined domes based on a rheologic model each class of effusive domes defined in terms of spectral and morphometric properties is characterized by its specific range of values for lava viscosity effusion rate and duration of the effusion process for our data set we report lava viscosities between about 10 2 and 10 8 pas effusion rates between 25 and 600 m 3 s 1 and durations of the effusion process between three weeks and 18 years lava viscosity decreases with increasing r 415 r 750 spectral ratio and thus tio2 content however the correlation is not strong implying an important influence of further parameters like effusion temperature on lava viscosity

resolved spectrophotometric properties of the ceres surface from dawn framing camera images we present a global spectrophotometric characterization of the ceres surface using dawn framing camera fc images we identify the photometric model that yields the best results for photometrically correcting images corrected fc images acquired on approach to ceres were assembled into global maps of albedo and color generally albedo and color variations on ceres are muted the albedo map is dominated by a large circular feature in vendimia planitia known from hst images li et al 2006 and dotted by smaller bright features mostly associated with fresh looking craters the dominant color variation over the surface is represented by the presence of blue material in and around such craters which has a negative spectral slope over the visible wavelength range when compared to average terrain we also mapped variations of the phase curve by employing an exponential photometric model a technique previously applied to asteroid vesta schroder et al 2013b the surface of ceres scatters light differently from vesta in the sense that the ejecta of several fresh looking craters may be physically smooth rather than rough high albedo blue color and physical smoothness all appear to be indicators of youth the blue color may result from the desiccation of ejected material that is similar to the phyllosilicates water ice mixtures in the experiments of poch et al 2016 the physical smoothness of some blue terrains would be consistent with an initially liquid condition perhaps as a consequence of impact melting of subsurface water ice we find red terrain positive spectral slope near ernutet crater where de sanctis et al 2017 detected organic material the spectrophotometric properties of the large vendimia planitia feature suggest it is a palimpsest consistent with the marchi et al 2016 impact basin hypothesis the central bright area in occator crater cerealia facula is the brightest on ceres with an average visual normal albedo of about 0 6 at a resolution of 1 3 km per pixel six times ceres average the albedo of fresh bright material seen inside this area in the highest resolution images 35 m per pixel is probably around unity cerealia facula has an unusually steep phase function which may be due to unresolved topography high surface roughness or large average particle size it has a strongly red spectrum whereas the neighboring less bright vinalia faculae are neutral in color we find no evidence for a diurnal ground fog type haze in occator as described by nathues et al 2015 we can neither reproduce their findings using the same images nor confirm them using higher resolution images fc images have not yet offered direct evidence for present sublimation in occator

unstable extension of enceladus lithosphere abstract regions near enceladus equator sarandib and diyar planitia contain extensive sets of parallel ridges and troughs that may be diagnostic of the region s formation conditions we present photoclinometry profiles across these ridges and troughs which indicate that they are periodic low slope features with dominant wavelengths of 3 to 4 km and amplitudes between 100 and 400 m the morphology of these terrains is consistent with formation via unstable extension of the lithosphere our numerical modeling demonstrates that unstable extension can generate large scale topography under enceladus like conditions comparison of our photoclinometry profiles with the dominant wavelengths produced by our numerical model permits estimation of the background heat flow at the time the sarandib diyar province formed we estimate heat flows of 110 to 220 mw m 2 suggesting that resurfacing of the planitiae was accompanied by strong localized heating the extension necessary to produce the ridges and troughs may have been caused by now inactive diapirs internal phase changes or other mechanisms our heat flux estimates imply elastic thickness at the time of resurfacing of 0 4 to 1 4 km which are sufficient to have allowed satellite reorientation if the province was underlain by a low density region it is therefore plausible that enceladus has experienced multiple heating events each leading to localized resurfacing and global reorientation

researching the earth and a few of its neighbors during my career our knowledge of erupting geysers and volcanoes in the solar system has exploded in this prefatory i tell how i became fascinated with high speed processes through studying meteorite impact dynamics and then how my initial idea of studying old faithful geyser as a volcanic analog led me to work not only on the dynamics of eruption of mount st helens in 1980 but also on geysers erupting on io a fiery satellite of jupiter triton a frigid satellite of neptune and enceladus an active satellite of saturn unforeseeably the study of these events also led to work on mineral thermodynamics and the hydraulics and geomorphic evolution of rapids in the grand canyon this is a narrative not a formal review article but the reader can find references in the related resources section to explore topics in more detail

implications of nonsynchronous rotation on the deformational history and ice shell properties in the south polar terrain of enceladus abstract numerous studies of the south polar region of saturn s moon enceladus have focused on the four main fractures the tiger stripes their associated water plume and the effects of diurnal tidal stresses on these fractures however due to their small magnitude 10 100 kpa diurnal stresses alone are unable to completely explain the initial formation of the tiger stripes and the many additional fractures in the region we suggest nonsynchronous rotation nsr stresses induced by a freely rotating ice shell over a global ocean are large enough to overcome the tensile strength of the ice shell and initiate fracturing on enceladus using the tidal stress calculation program satstressgui we demonstrate the dependence of the magnitude of the nsr stress on the thickness of the ice shell particularly the brittle outer layer of the ice shell we suggest the brittle layer in the south polar region is 2 4 km thick whereas in the more northern regions it is 6 8 km the difference is most likely due to the elevated energy flux at the south pole compared to the regions to the north resulting in a warmer and thinner ice shell in the south polar region the thinner ice around the south pole permits nsr tensile stresses of 5 mpa enough to fracture the ice however the thicker ice to the north reduces the nsr stress to less than 2 mpa making fracturing less likely the difference in ice shell thickness and resultant nsr stress can help explain the activity in the south but relative quiescence in the north our modeled nsr stress magnitudes and orientations suggests a nsr period on the order of 1 myr and advocate that the tiger stripes have rotated 45 clockwise about the south pole since their initial formation in response to nsr of the ice shell if the ice shell is rotating at a constant rate the tiger stripes would thus be up to 100 000 years old an apparent decreasing angular separation between consecutive fracture sets that formed in the south polar region suggests an ice shell that has weakened in strength over the course of its recent geologic history this could be a result of increased heating thinning of the ice shell or a combination of the two as the ice shell thinned the nsr stresses were enhanced making fracturing more likely after a smaller amount of nsr compared to the previous fracturing episode

eruptions at lone star geyser yellowstone national park usa 1 energetics and eruption dynamics 1 geysers provide a natural laboratory to study multiphase eruptive processes we present results from a 4 day experiment at lone star geyser in yellowstone national park usa we simultaneously measured water discharge acoustic emissions infrared intensity and visible and infrared video to quantify the energetics and dynamics of eruptions occurring approximately every 3 h we define four phases in the eruption cycle 1 a 28 3 min phase with liquid and steam fountaining with maximum jet velocities of 16 28 m s 1 steam mass fraction of less than 0 01 intermittently choked flow and flow oscillations with periods increasing from 20 to 40 s are coincident with a decrease in jet velocity and an increase of steam fraction 2 a 26 8 min posteruption relaxation phase with no discharge from the vent infrared ir and acoustic power oscillations gliding between 30 and 40 s 3 a 59 13 min recharge period during which the geyser is quiescent and progressively refills and 4 a 69 14 min preplay period characterized by a series of 5 10 min long pulses of steam small volumes of liquid water discharge and 50

structural mapping of enceladus and implications for formation of tectonized regions global structural mapping of high resolution cassini images of enceladus reveals a richly varied surface most notable are three main regions of deformation each containing multiple structural units in addition to the well known south polar terrain spt there are two other large regions of deformation that we term leading hemisphere terrain lht and trailing hemisphere terrain tht each of these three terrains includes a circumferential belt that encloses one or more other structurally deformed units areal extents range from about 80 000 km2 spt to 195 000 km2 lht or 160 to 250 km equivalent circular radius based on relative crater densities the tht is inferred to be older than the lht the geologically active spt is the youngest the overall similarities in shape and dimension of the three tectonized terrains suggest similar formational processes plausibly related to broad loading of a thin elastic shell a viable scenario is that each tectonized terrain formed above a large scale region of warm upwelling ice with subsequent downwarping triggered by cooling and or subsurface melting however differences in morphological detail suggest that the specific evolution of each tectonized terrain has been different

patterns of fracture and tidal stresses on europa abstract the hypothesis that lineaments on europa are fractures produced by tidal distortion and planetary volume change is examined by comparing the orientations of dark bands triple bands and cuspate ridges to fracture patterns predicted for tidal distortion due to orbital recession and orbital eccentricity if short reticulate dark band nnear the anti jove point are tension cracks which formed in response to tidal distortion they could only have been produced by orbital eccentricity long arcuate dark band and triple bands peripheral to the anti jove point orientations which suggest that they are strike slip faults which formed in response to orbital recession if cuspate ridges are compressional features their orientations and distribution suggest that they formed in response to combined orbital recession and a decrease in planetary volume stresses due to orbital eccentricit could have produced tension cracks near the anti jove point only if tensile failure occurred either prior to the accumulation of orbital recession stresses or after they had relaxed surface fracturing if a consequence of tidal deformation places important constraints on the orbital evolution of europa

characteristics of ice grains in the enceladus plume from cassini observations ice grains in the enceladus plume have been observed by several cassini instruments during many enceladus encounters in this paper we study the ice grains ranging from less than one nanometer to micrometers in size based on multiple instrument observations we have analyzed the nanograin data from the e17 and e18 cassini plasma spectrometer caps energy spectra using the approach of hill et al 2012 and studied the charging of the grains using the e3 e6 radio and plasma wave spectrometer rpws langmuir probe lp data presented by morooka et al 2011 to bridge the gap between caps nanograin observations and cosmic dust analyzer and rpws micrograin observations we propose a composite size distribution and fit it to the e3 e5 e17 and e18 caps and rpws data the resulting size distribution peaks at 2 nm and provides a total grain mass density 20 that of the water vapor measured by ion and neutral mass spectrometer at the densest part of the plume and a total grain charge density much smaller than that inferred from the rpws lp plasma data charge balance with the rpws lp plasma data would require many more grains than provided by the best fit composite distribution to the caps and rpws data and would provide a grain mass density comparable to that of the water vapor on the basis of these results we study the subsequent motion of the ice grains and estimate the resulting grain current 104 to 105a and grain mass production rate 15 65 kg s

fault geometries on uranus satellite miranda implications for internal structure and heat flow abstract miranda a 470 km diameter uranian icy satellite has a surface that exhibits evidence of a complex tectonic history tectonic structures are mostly localized in three regions termed coronae but also form a rift system inferred to be global in extent ridges within the boundary of arden corona and those that make up the 340 chasma part of the global rift system have been interpreted as normal fault blocks using voyager data we test the hypothesis that these arden corona faults as well as those at the northern edge of the 340 chasma are listric in geometry for this testing we use four geometric criteria for listric faults 1 progressive down dip decrease in fault scarp dip 2 progressive down dip increase in back tilted face slope 3 concavity of the exposed scarp surface and 4 presence of a rollover structure results of this analysis support the hypothesis that the faults within the arden corona boundary are listric in geometry but do not strongly support the hypothesis for the faults within the 340 chasma by analogy with terrestrial structures the listric character of faults within the arden corona boundary suggests the presence of a subsurface detachment this detachment likely occurred at miranda s brittle ductile transition zone at the time of faulting measurements of the arden corona fault system geometry are used to estimate depths to the proposed brittle ductile transition zone at the time of faulting resulting in values of 6 7 9 0 km those depths in turn are used to estimate a thermal gradient of 6 25 k km 1 and a surface heat flux of 31 112 mw m 2 the weaker evidence of a listric geometry for the faults of the 340 chasma suggests that those faults did not interact with a brittle ductile transition at the time of their formation our estimated thermal gradient of the arden corona region is consistent with a previous heating event on miranda that was as significant as europa s current resonance induced tidal heating this heating event may be associated with a hypothesized previous tidal resonance of miranda with umbriel and or ariel

nature distribution and origin of co2 on enceladus abstract we present the first map of co2 at the surface of enceladus using data obtained by the cassini visible infrared mapping spectrometer vims in order to measure the weak and narrow co2 absorption band depths around 4 26 m we improved 1 the calibration of vims spectra 2 the calculation of geographic coordinates and 3 the projection techniques we averaged multiple observations of a given area to obtain a signal to noise ratio high enough to map the co2 abundances co2 is reliably detected mostly in the south polar region this region includes active faults tiger stripes the highest observed surface temperatures and locations of active plume eruptions the occurrence here of co2 is consistent with an endogenic origin controlled by tectonics both pure co2 ice and complexed co2 are detected from the positions of absorption bands between 4 27 and 4 24 m the highest concentrations of co2 are between the main active faults of the south polar region where the surface temperature is low pure co2 ice deposits at the surface of enceladus are best modeled by the formation of gas pockets below the icy crust and above the surface of the internal ocean these pockets eventually release cold co2 gas 70 to 119 k at low velocity seeping between the tiger stripes matson et al 2018 icarus 302 18 26 co2 clathrate hydrates may form in the ocean and may be subsequently released when a co2 gas pocket blows out and erupts other mechanisms may contribute to reinforcing the anti correlation of the co2 distribution of any type with respect to the location of the tiger stripes such as successive sublimation of co2 and condensation on colder areas and partial frost cover by h2o releases from plume eruptions

a unified nomenclature for tectonic structures on the surface of enceladus abstract enceladus has experienced widespread and diverse tectonic deformation the diversity and nonuniform distribution of structures visible on the surface attests to a complicated and perhaps long lived tectonic history currently no fundamental classification scheme based on morphology exists for the tectonic structures on enceladus which limits analysis and discussion of tectonic structures and regional and global tectonic histories characterization and classification of structures on enceladus is an important first step in understanding its tectonic history here we propose a global morphologic i e nongenetic classification scheme for tectonic structures on enceladus five classes of tectonic structures on enceladus have been identified based on images and limited topographic data troughs scarps chasmata ridges and bands morphological variation of structures exists within these classes reinforcing the complicated tectonic history of this small body

photometrically corrected global infrared mosaics of enceladus new implications for its spectral diversity and geological activity between 2004 and 2017 spectral observations have been gathered by the visual and infrared mapping spectrometer vims on board cassini brown et al 2004 during 23 enceladus close encounters in addition to more distant surveys the objective of the present study is to produce a global hyperspectral mosaic of the complete vims data set of enceladus in order to highlight spectral variations among the different geological units this requires the selection of the best observations in terms of spatial resolution and illumination conditions we have carried out a detailed investigation of the photometric behavior at several key wavelengths 1 35 1 5 1 65 1 8 2 0 2 25 2 55 and 3 6 m characteristics of the infrared spectra of water ice we propose a new photometric function based on the model of shkuratov et al 2011 when combined corrected mosaics at different wavelengths reveal heterogeneous areas in particular in the terrains surrounding the tiger stripes on the south pole and in the northern hemisphere around 30 n 90 w those areas appear mainly correlated to tectonized units indicating an endogenous origin potentially driven by seafloor hotspots

deciphering sub micron ice particles on enceladus surface the surface of saturn s moon enceladus is composed primarily by pure water ice the cassini spacecraft has observed present day geologic activity at the moon s south polar region related with the formation and feeding of saturn s e ring plumes of micron sized particles composed of water ice and other non ice contaminants e g co2 nh3 ch4 erupt from four terrain s fractures named tiger stripes some of this material falls back on enceladus surface to form deposits that extend to the north at 40 w and 220 w with the highest concentration found at the south pole in this work we analyzed vims ir data to identify plumes deposits across enceladus surface through the variation in band depth of the main water ice spectral features to characterize the global variation of water ice band depths across enceladus the entire surface was sampled with an angular resolution of 1 in both latitude and longitude and for each angular bin we averaged the value of all spectral indices as retrieved by vims the position of the plumes deposits predicted by theoretical models display a good match with water ice band depths maps on the trailing hemisphere whereas they diverge significantly on the leading side space weathering processes acting on enceladus surface ionize and break up water ice molecules resulting in the formation of particles smaller than one micron we also mapped the spectral indices for sub micron particles and we compared the results with the plumes deposits models again a satisfactory match is observed on the trailing hemisphere only finally we investigated the variation of the depth of the water ice absorption bands as a function of the phase angle in the visible range some terrains surrounding the tiger stripes show a decrease in albedo when the phase angle is smaller than 10 this unusual effect cannot be confirmed by near infrared data since observations with a phase angle lower than 10 are not available for phase angle values greater than 10 the depth of the water ice features remains quite constant within a broad range of phase angle values

enceladus an active ice world in the saturn system enceladus one of the mid sized icy moons of saturn has an importance to planetary science far greater than its modest 504 km diameter would suggest intensive exploration of enceladus by the cassini saturn orbiter has revealed that it is the only known icy world in the solar system with ongoing deep seated geological activity active tectonic fractures at enceladus s south pole dubbed tiger stripes warmed by internal tidally generated heat spew supersonic jets of water vapor other gases and ice particles into circum saturnian space a subsurface saltwater sea probably exists under the south pole between the ice shell and the silicate core because of evidence that liquid water is probably present at the jet sources enceladus is also of great astrobiological interest as a potential habitat for life

a clathrate reservoir hypothesis for enceladus south polar plume we hypothesize that active tectonic processes in the south polar terrain of enceladus the 500 kilometer diameter moon of saturn are creating fractures that cause degassing of a clathrate reservoir to produce the plume documented by the instruments on the cassini spacecraft advection of gas and ice transports energy supplied at depth as latent heat of clathrate decomposition to shallower levels where it reappears as latent heat of condensation of ice the plume itself which has a discharge rate comparable to old faithful geyser in yellowstone national park probably represents small leaks from this massive advective system

joule heating of the south polar terrain on enceladus 1 we report that joule heating in enceladus resulting from the interaction of enceladus with saturn s magnetic field may account for 150 kw to 52 mw of power through enceladus electric currents passing through subsurface channels of low salinity and just a few kilometers in depth could supply a source of power to the south polar terrain providing a small but previously unaccounted for contribution to the observed heat flux and plume activity studies of the electrical heating of jupiter s moon europa have concluded that electricity is a negligible heating source since no connection between the conductive subsurface and alfven currents has been observed here we show that contrary to results for the jupiter system electrical heating may be a source of internal energy for enceladus contributing to localized heating production of water vapor and the persistence of the tiger stripes this contribution is of order 0 001 0 25 of the total observed heat flux and thus joule heating cannot explain the total south polar terrain heat anomaly the exclusion of salt ions during refreezing serves to enhance volumetric joule heating and could extend the lifetime of liquid water fractures in the south polar terrain

the formation of enceladus tiger stripe fractures from eccentricity tides abstract enceladus has a young tectonically active south polar region which is erupting material from a prominent set of fractures called tiger stripes no comparable activity is observed at the north pole which is heavily cratered with limited tectonism given the many lines of evidence supporting a global ocean under enceladus icy shell the reason for the dichotomy in geologic activity is unclear we model the formation of the tiger stripes as tidally driven fractures and examine the magnitudes of tidal stresses with different ice shell structures in order to explore whether and how tidal stress might explain enceladus distribution of tectonic activity we find that eccentricity driven tidal stresses would produce fractures of nearly identical orientations to the observed tiger stripe fractures and that a 10 km difference in ice shell thickness between the north and south poles can result in substantially different tidal stress magnitudes providing a natural explanation for the hemispheric dichotomy in tectonic activity on enceladus finally we synthesize these results with enceladus global geologic record to offer insight into the evolution of this enigmatic moon

modeling cryogenic mud volcanism on pluto abstract subsurface ice cryofluids and ice interactions with ammonia for cryovolcanic materials give the potential ingredients for the formation of mud volcanoes on pluto in order to constrain the possible morphology from images acquired by the new horizons spacecraft we have modeled the formation of mud volcanoes isostatic layers of ice overlain by denser material from a series of possible solid liquid mud compositions methane and ammonia as well as accounting for the full range of eruption variables e g mud viscosity conduit diameter and eruption duration we categorize them into nine specific scenarios based on the composition of the top ice layer and mud composition from these scenarios the maximum height of the potential mud volcano can be calculated we find that ammonia mud sources produce smaller edifice heights and wider flow profiles compared to methane dominant mud cryovolcanoes assuming certain preliminary parameters we also propose the evolution and development of the edifice of wright mons in as little as 3 8 million years given a 10 wt ammonia subsurface mud composition if characterized on pluto s surface mud volcanoes would provide an interesting insight on the subsurface dynamics of solids and cryofluids on pluto

enceladus s and dione s floating ice shells supported by minimum stress isostasy enceladus gravity and shape have been explained in terms of a thick isostatic ice shell floating on a global ocean in contradiction of the thin shell implied by librations here we propose a new isostatic model minimizing crustal deviatoric stress and demonstrate that gravity and shape data predict a 38 4km thick ocean beneath a 23 4km thick shell agreeing with but independent from libration data isostatic and tidal stresses are comparable in magnitude south polar crust is only 7 4km thick facilitating the opening of water conduits and enhancing tidal dissipation through stress concentration enceladus resonant companion dione is in a similar state of minimum stress isostasy its gravity and shape can be explained in terms of a 99 23km thick isostatic shell overlying a 65 30km thick global ocean thus providing the first clear evidence for a present day ocean within dione

modeling europa s dust plumes the discovery of jupiter s moon europa maintaining a probably sporadic water vapor plume constitutes a huge scientific opportunity for nasa s upcoming mission to this galilean moon measuring properties of material emerging from interior sources offers a unique chance to understand conditions at europa s subsurface ocean exploiting results obtained for the enceladus plume we simulate possible europa plume configurations analyze particle number density and surface deposition results and estimate the expected flux of ice grains on a spacecraft due to europa s high escape speed observing an active plume will require low altitude flybys preferably at altitudes of 5 100 km at higher altitudes a plume may escape detection our simulations provide an extensive library documenting the possible structure of europa dust plumes which can be quickly refined as more data on europa dust plumes are collected

eruptions arising from tidally controlled periodic openings of rifts on enceladus the cassini flyby of 14 july 2005 revealed plumes of water vapour and ice associated with the tiger stripe features on the surface of saturn s icy moon enceladus since then the challenge has been to explain the nature of the plumes and the forces driving them two papers this week offer an explanation that accounts for both the plume characteristics and the presence of hot spots without the need to assume the existence of near surface liquid water a requirement of some previous models nimmo et al identify tidally driven lateral fault motions near the tiger stripes as the most likely drivers of heat and vapour production and hurford et al show that as enceladus orbits saturn the parent planet s tides make the satellite s ice flex this may cause the tiger stripes to open and close periodically exposing volatile gases and allowing them to be released plumes of gas have been seen near the south polar region of enceladus a small icy satellite of saturn and observations revealed large rifts in the crust informally called tiger stripes hurford et al report that during each orbit every portion of each tiger stripe rift spends about half the time in tension which allows the rift to open exposing volatiles and allowing eruptions plume activity is expected to vary periodically in 2005 plumes were detected near the south polar region of enceladus1 a small icy satellite of saturn observations of the south pole revealed large rifts in the crust informally called tiger stripes which exhibit higher temperatures than the surrounding terrain and are probably sources of the observed eruptions2 models of the ultimate interior source for the eruptions are under consideration1 3 4 5 other models of an expanding plume6 require eruptions from discrete sources as well as less voluminous eruptions from a more extended source to match the observations no physical mechanism that matches the observations has been identified to control these eruptions here we report a mechanism in which temporal variations in tidal stress open and close the tiger stripe rifts governing the timing of eruptions during each orbit every portion of each tiger stripe rift spends about half the time in tension which allows the rift to open exposing volatiles and allowing eruptions in a complementary process periodic shear stress along the rifts also generates heat along their lengths7 8 9 which has the capacity to enhance eruptions plume activity is expected to vary periodically affecting the injection of material into saturn s e ring10 and its formation evolution and structure moreover the stresses controlling eruptions imply that enceladus icy shell behaves as a thin elastic layer perhaps only a few tens of kilometres thick

planetary science tectonic overturn on enceladus the south pole of saturn s icy moon enceladus is anomalously warm geologically youthful and cryovolcanically active episodic convective overturn explains how the moon s modest sources of internal heat can be channelled into intense geological activity

the volcanic and tectonic history of enceladus abstract enceladus has a protracted history of impact cratering cryovolcanism and extensional compressional and probable strike slip faulting it is unique in having some of the outer solar system s least and most heavily cratered surfaces enceladus cratering record tectonic features and relief elements have been analyzed more comprehensively than done previously like few other icy satellites enceladus seems to have experienced major lateral lithospheric motions it may be the only icy satellite with global features indicating probable lithospheric convergence and folding ridged plains 500 km across consist of a central labyrinthine ridge complex atop a broad dome surrounded by smooth plains and peripheral sinuous ridge belts the ridged plains have few if any signs of extension almost no craters and an average age of just 107to 108years ridge belts have local relief ranging from 500 to 2000 m and tend to occur near the bottoms of broad regional troughs between swells our reanalysis of peter thomas dermott s f and p c thomas 1994 the determination of the mass and mean density of enceladus from its observed shape icarus 109 241 257 limb profiles indicates that high peaks probably ridge belts also occur in unmapped areas sinuous ridges appear foldlike and are similar to terrestrial fold belts such as the appalachians if they are indeed folds it may require that the ridged plains are mechanically perhaps volcanically layered regional topography suggests that folding may have occurred along zones of convective downwelling the cratered plains in contrast to the ridged plains are heavily cratered and exhibit extensional structures but no obvious signs of compression cratered plains contain a possible strike slip fault isbanir fossa along which two pairs of fractures seem to have 15 km of right lateral offset the oldest cratered plains might date from shortly after the formation of the saturnian system or the impact disruption and reaccretion of enceladus another area of cratered plains has modified craters e g ali baba and aladdin which some workers have explained by anomalous heat flow and viscous relaxation lateral shear and shield building volcanism also may have been important a young rift like structure northern samarkand sulci has few craters and a concentration of cracks or grabens and flattened flooded and rifted craters pit chains and cratered domes suggest explosive volcanism smooth plains may have formed by cryovolcanic equivalents of flood basalt volcanism pure h2o would be difficult to extrude through an icy crust and is cosmochemically improbable as a cryovolcanic agent density relations rule out eutectic brine lavas on enceladus but nh3 h2o volcanism is possible current steady state tidal dissipation may cause melting of ammonia hydrate at a depth of 25 km if the crust is made of ammonia hydrate or 100 km if it is made of water ice

characterizing deposits emplaced by cryovolcanic plumes on europa abstract in the absence of direct observations of europa s particle plumes deposits left behind during eruptive events would provide the best evidence for recent geological activity and would serve as indicators of the best places to search for ongoing activity on the icy moon here we model the morphological and spectral signatures of europan plume deposits utilizing constraints from recent hubble space telescope observations as model inputs we consider deposits emplaced by plumes that are 1 km to 300 km tall and find that in the time between the galileo mission and the arrival of the europa clipper spacecraft plumes that are

an oceanic composition on early and today s enceladus 1 the oceanic composition on saturn s moon enceladus is evaluated through calculations of thermochemical equilibria at hydrothermal and freezing settings conditions of rock alteration are constrained from assumptions and models for the moon s interior composition and thermal evolution and from the composition of enceladus plume results show that an early ocean was an alkaline na cl hco3 solution underlying altered rocks consisted of mg phyllosilicates magnetite fe and ni sulfides and carbonates subsequent freezing of oceanic water caused the deposition of a nacl hydrate na k and ca carbonates and the formation of a salt free ice shell if an aqueous phase exists on today s enceladus it could consist of eutectic na cl hco3 brine that at least locally decouples the ice shell and facilitates tidal heating a lack of firm detection of na and cl at enceladus is consistent with the accumulation of salts at the ice rock boundary and implies the plume formation via sublimation in the ice shell

memory of past random wave conditions in submarine groundwater discharge submarine groundwater discharge sgd is an integral part of the hydrological cycle and represents an important aspect of land ocean interactions we used a numerical model to simulate flow and salt transport in a nearshore groundwater aquifer under varying wave conditions based on yearlong random wave data sets including storm surge events the results showed significant flow asymmetry with rapid response of influxes and retarded response of effluxes across the seabed to the irregular wave conditions while a storm surge immediately intensified seawater influx to the aquifer the subsequent return of intruded seawater to the sea as part of an increased sgd was gradual using functional data analysis we revealed and quantified retarded cumulative effects of past wave conditions on sgd including the fresh groundwater and recirculating seawater discharge components the retardation was characterized well by a gamma distribution function regardless of wave conditions the relationships between discharge rates and wave parameters were quantifiable by a regression model in a functional form independent of the actual irregular wave conditions this statistical model provides a useful method for analyzing and predicting sgd from nearshore unconfined aquifers affected by random waves key points cumulative effects of antecedent wave conditions on sgd long memory of past wave conditions back to over a hundred days found in the sgd wave effects characterized well by a gamma distribution function

cassini observes the active south pole of enceladus cassini has identified a geologically active province at the south pole of saturn s moon enceladus in images acquired by the imaging science subsystem iss this region is circumscribed by a chain of folded ridges and troughs at 55 s latitude the terrain southward of this boundary is distinguished by its albedo and color contrasts elevated temperatures extreme geologic youth and narrow tectonic rifts that exhibit coarse grained ice and coincide with the hottest temperatures measured in the region jets of fine icy particles that supply saturn s e ring emanate from this province carried aloft by water vapor probably venting from subsurface reservoirs of liquid water the shape of enceladus suggests a possible intense heating epoch in the past by capture into a 1 4 secondary spin orbit resonance

viscoelastic models of tidal heating in enceladus abstract tidal heating in enceladus is investigated using multilayered viscoelastic models of the satellite dissipation in a homogeneous maxwell model of enceladus at the current eccentricity can be as large as 920 gw nearly a factor of 2000 greater than previously thought reflecting the fact that the dynamic love number can approach unity the elastic love number 10 3 the theoretical tidal heating maximum violates an upper bound based on saturn s q and models with this much heating are unreasonable plausible multilayered models are shown to be thermally stable at the current eccentricity with 10 km thick lithosphere and surface heat flow 5 mw m 2 the models require a combination of near surface insulation subsurface solar heating or anomalously low lithosphere conductivity to simultaneously fit dynamic and geologic constraints a two layer model with a conductive lithosphere over a maxwell mantle has mantle viscosity of about 10 13 pa sec and mantle temperature about 210 k a three layer model with an internal ocean of accumulated nh 3 h 2 o eutectic melt that decouples the lithosphere from a maxwell core has slightly lower internal viscosity and slightly higher temperature than the two layer model dissipation in the ocean layer may be significant if the tidally forced flow is turbulent the models suggest that enceladus thermal and dynamical evolution may have been relatively simple involving only dione in a 2 1 resonance the current eccentricity could have been maintained since resonance onset with recurring surface activity due to a thermally active interior

a fracture history on enceladus provides evidence for a global ocean 1 the region surrounding the south pole of saturn s moon enceladus shows a young pervasively fractured surface that emanates enough heat to be detected by the cassini spacecraft to explain the elevated heat and eruptive icy plumes originating from large cracks informally called tiger stripes in the surface many models implicitly assume a global liquid ocean beneath the surface here we show that the fracture patterns in the south polar terrain spt of enceladus are inconsistent with contemporary stress fields but instead formed in a temporally varying global stress field related to nonsynchronous rotation of a floating ice shell above a global liquid ocean this finding increase to at least three the number of outer planet satellites likely to possess a subsurface liquid water layer

coupling mantle convection and tidal dissipation applications to enceladus and earth like planets 1 anelastic dissipation of tidal forces likely contributes to the thermal budget of several satellites of giant planets and earth like planets closely orbiting other stars in order to address how tidal heating influences the thermal evolution of such bodies we describe here a new numerical tool that solves simultaneously mantle convection and tidal dissipation in a three dimensional spherical geometry since the two processes occur at different timescales tidal dissipation averaged over a forcing period is included as a volumetric heat source for mantle dynamics for the long term flow a purely viscous material is considered whereas a maxwell like formalism is employed for the tidal viscoelastic problem due to the strongly temperature dependent rheological properties of both mechanisms the coupling is achieved via the temperature field the model is applied to two examples enceladus and an earth like planet for enceladus our new 3 d method shows that the tidal strain rates are strongly enhanced in hot upwellings when compared with classical methods moreover the heat flux at the base of enceladus ice shell is strongly reduced at the poles thus favoring the preservation of a liquid reservoir at depth for earth like planets tidal dissipation patterns are predicted for different orbital configuration thermal runaway is observed for orbital periods smaller than a critical value e g 30 days for an eccentricity of 0 2 and 3 2 resonance this is likely to promote large scale melting of the mantle and io like volcanism

tidal dissipation in enceladus uneven fractured ice shell abstract analysis of enceladus gravity topography and libration data suggests that the thickness of the ice crust significantly varies which may have important consequences for the heat transfer across and the tidal dissipation within the ice shell understanding these processes is a critical prerequisite for analyzing enceladus thermal evolution and assessing the long term stability of its subsurface ocean here we investigate the impact of ice shell thickness variations on the tidal deformation of the moon and the associated heat production using a finite element model that includes faults tiger stripes in the south polar region spr since the tidal deformation and the thermal structure of the ice shell are coupled through temperature and deviatoric stress we simultaneously solve the equations governing the anelastic deformation of ice and also equations which model conductive heat transfer in the ice shell we find that tidal heating is concentrated in a narrow low viscosity zone near the base of the ice shell and along faults outside the spr the thickness of this zone is about 1 10 of the local ice thickness and the associated volumetric heating is 1 0 6 w m 3 corresponding to less than 1 gw of dissipated power in the spr the tidal effects are enhanced by the combined action of faults and ice shell thinning although the volumetric heating in this relatively small region may be larger than 10 4 w m 3 the total heat production in this region does not exceed 1 1 gw our computations show that tidal heating in the ice shell can explain only a small fraction of enceladus heat production derived from astrometric observations implying that enceladus heat engine is powered by dissipation in the core or in the ocean

modification of the plasma in the near vicinity of enceladus by the enveloping dust 1 the plasma near saturn s equator is quasi corotating but those fluid elements entering the near vicinity of the moon enceladus become uniquely modified besides the solid body the moon has a surrounding dust envelop that we show herein to be detected 20 enceladus radii 1 re 252 km both north and south of the body previous reports indicate that corotating plasma slows down substantially in the near vicinity of enceladus we show herein that the commencement of this plasma slow down matches closely with cassini s entry into the dense portions of the enveloping dust in the northern hemisphere above the moon we also examine in detail the source of the dust about 400 km above the south polar fissures we find that a large positive potential must exist between the south pole of the moon and the spacecraft to account for ions streaming away from the pole on connecting magnetic field lines

brittle ice shell thickness of enceladus from fracture distribution analysis abstract we determine the depth of fracture penetration in multiple regions of enceladus by performing self similar clustering and length distribution analysis of fractures the statistical characterization of fault population attribute such as length and clustering provide a productive avenue for exploring deformation rate stress transmission mode rheology of the medium and mechanical stratification of the ice satellite through this analysis we estimate the depth of the mechanical discontinuity of enceladus ice shell that is the depth to which fractures penetrate the brittle ice layer above the ductile one in this work we find that for the south polar terrain spt the brittle ice shell interested by fracture penetration is about 30 km and corresponds to the total depth of the ice shell because the spt has a very high thermal gradient and hence fractures likely reach the ocean ice interface in the other regions analyzed the depth of fracture penetration increases from 31 to 70 km from the south pole to northern regions up to 75

enceladus near surface co 2 gas pockets and surface frost deposits abstract solid co2 surface deposits were reported in enceladus south polar region by brown et al 2006 they noted that such volatile deposits are temporary and posited ongoing replenishment we present a model for this replenishment by expanding on the matson et al 2012 model of subsurface heat and chemical transport in enceladus our model explains the distributions of both co2 frost and complexed co2 clathrate hydrate as seen in the cassini visual and infrared mapping spectrometer vims data we trace the journey of co2 from a subsurface ocean the ocean water circulation model of matson et al 2012 brings water up to near the surface where gas exsolves to form bubbles some of the co2 bubbles are trapped and form pockets of gas in recesses at the bottom of the uppermost ice layer when fissures break open these pockets the co2 gas is vented gas pocket venting is episodic compared to the more or less continuous eruptive plumes emanating from the tiger stripes that are supported by plume chambers two styles of gas pocket venting are considered 1 seeps and 2 blowouts the presence of co2 frost patches suggests that the pocket gas slowly seeped through fractured cold ice and when some of the gas reached the surface it was cold enough to condense i e t 70 to 119 k if the fissure opening is large a blowout occurs the rapid escape of gas and drop in pocket pressure causes water in the pocket to boil and create many small aerosol droplets of seawater these may be carried along by the erupting gas electrically charged droplets can couple to the magnetosphere and be dragged away from enceladus most of the co2 blowout gas escapes from enceladus and the remainder is distributed globally however co2 trapped in a clathrate structure does not escape it is much heavier and slower moving than the co2 gas its motion is ballistic and has an average range of about 17 km thus it contributes to deposits in the vicinity of the vent local heat flow indicates that gas pockets can be located as deep as several tens of meters below the surface gas pockets can be reused and we explore their life cycle

true polar wander of enceladus from topographic data many objects in the solar system are suspected to have experienced reorientation of their spin axes as their rotation rates are slow and their shapes are nearly spherical the formation of mass anomalies by either endogenic or exogenic processes can change objects moments of inertia therefore the objects reorient to align their largest moment of inertia with their spin axis such a phenomenon is called true polar wander tpw here we report the discovery of a global series of topographic lows on saturn s satellite enceladus that we interpret to show that this synchronously locked moon has undergone tpw by 55 about the tidal axis we use improved topographic data from the spherical harmonic expansion of cassini limb and stereogrammetric measurements to characterize regional topography over the surface of enceladus we identify a group of nearly antipodal basins orthogonal to a basin chain tracing a non equatorial circumglobal belt across enceladus surface we argue that the belt and the antipodal regions are fossil remnants of earlier equator and poles respectively we argue that these lows arise from isostasic compensation and that their pattern reflects spatial variations in internal dynamics of the ice shell our hypothesis is consistent with a variety of geological features visible in cassini images

enceladus an estimate of heat flux and lithospheric thickness from flexurally supported topography fluxes of 200 270 mw m 2 at the time of formation the heat fluxes are comparable to average heat flux values measured in enceladus active south polar region and they are consistent with estimates derived via models of unstable extension of the lithosphere at this location surface porosity can reduce the obtained heat fluxes to an estimated minimum of 45 60 mw m 2 c rater size frequency counts fix the time of formation from presentday of the rift zone at either 3 5 0 4 0 1 ga assuming an

formation of ridges on europa above crystallizing water bodies inside the ice shell abstract jupiter s second galilean satellite europa is a moon sized body with an icy shell and global ocean approximately 100 km thick surrounding a rocky interior its surface displays extensive tectonic activity in a geologically recent past europa s most ubiquitous surface features double ridges have a central trough flanked by two raised edifices double ridges can extend hundreds of kilometers and appear genetically related to cracks formed in the europan ice shell the origin of the raised flanks has been the center of much debate and many models have been proposed there are also ridges without a central trough single ridges these ridges are far less common than their double ridge counterparts however there are locations where along strike changes in ridge type appear to occur we explore an elastic model in which the ridges form in response to crystallization of a liquid water intrusion in our model liquid water fills tension cracks that open in the europan crust in response to tidal stress or perhaps overpressure of a subsurface ocean the crack would be long and essentially continuous similar to dikes on earth explaining the remarkable continuity and lack of segmentation of europan ridges the freezing of the water would cause a volume expansion compressing and buckling the adjacent crust we find that the geometry of the intruding water body controls the shape of the resulting ridges with single ridges forming above sill like intrusions and double ridges above dike like intrusions in order to match the ridge heights observed for double ridges we would need approximately 1 5 km2 of water intruded at a shallow depth in the ice shell potentially over the course of multiple events deeper intrusions result in a broader lower amplitude ridge than shallow intrusions

tectonics and water on europa the tectonics of europa one of jupiter s moons are complex this satellite probably hosts a subsurface water ocean but the thickness of the outer ice crust is poorly constrained and the episodic presence of liquid water at the surface is debated we argue that some surface features of europa are formed by soft ice that is heated by viscous dissipation of tidal motion along faults and do not depend on a shallow ocean our model suggests that transient pockets of liquid water or brine could form at shallow depths in the crust

pressurized oceans and the eruption of liquid water on europa and enceladus 1 it is difficult for liquid water to erupt onto the surface of icy satellites such as europa and enceladus because liquid water is more dense than ice if the ice shell thickens the volume expansion of ice upon freezing increases pressure in the subsurface ocean the excess pressure is determined by a balance between compression of ocean water and elastic expansion of the ice shell we show that on europa the freezing of 1 10 km of ice generates tangential stresses that exceed the tensile strength of ice excess pressure however is insufficient for liquid water to erupt to the surface within smaller icy satellites such as enceladus ocean pressure can become large enough to cause an eruption of large amounts of liquid water

europan double ridge morphometry as a test of formation models abstract double ridges on the jovian satellite europa consist of two parallel ridges with a central trough although these features are nearly ubiquitous on europa their formation mechanism s is are not yet well understood previous hypotheses for their formation can be divided into two groups based on 1 the expected interior slope angles and 2 the magnitude of interior exterior slope symmetry the published hypotheses in the first fracture group entail brittle deformation of the crust either by diapirism shear heating or buckling due to compression because these mechanisms imply uplift of near vertical fractures their predicted interior slopes are steeper than the angle of repose aor with shallower exterior slopes the second flow group includes cryosedimentary and cryovolcanic processes explosive or effusive cryovolcanism and tidal squeezing which are predicted to form ridge slopes at or below the aor explosive cryovolcanism would form self symmetric ridges whereas effusive cryolavas and cryo sediments deposited during tidal squeezing would likely not exhibit slope symmetry to distinguish between these two groups of hypothesized formation mechanisms we derived measurements of interior slope angle and interior exterior slope symmetry at multiple locations on europa through analysis of data from the galileo solid state imaging ssi camera two types of data were used i elevation data from five stereo pair digital elevation models dems covering four ridges 580 individual measurements and ii ridge shadow length measurements taken on individual images over 40 ridges 200 individual measurements our results shows that slopes measured on our dems located in the cilix and banded plains regions typically fall below the aor and slope symmetry is dominant two different shadow measurement techniques implemented to calculate interior slopes yielded slope angles that also fall below the aor the shallow interior slopes derived from both techniques weigh against brittle deformation mechanisms although shallow slopes could result from degradation interior exterior ridge symmetry weighs against ridge degradation as the sole reason for shallow interior slopes thus our results suggest that for the double ridges analyzed in this work cryovolcanic or cryosedimentary formation is more likely than brittle deformation and of those formation mechanisms explosive cryovolcanism is the double ridge formation mechanism best supported on the basis of interior exterior slope symmetry

enceladus south polar sea recent observations of the south pole of saturn s moon enceladus by the cassini spacecraft have revealed an active world powered by internal heat in this paper we propose that localized subsurface melting on enceladus has produced an internal south polar sea evidence for this localized sea comes from the shape of enceladus which does not match a differentiated body at its current orbital position we show that melting induced by the observed heat flow at the south pole produces a large enough pit to match the shape of enceladus with a differentiated rock and ice interior numerical modeling of melting and ice flow shows that the sea produced beneath the south pole is stable against inflow of ductile ice from its surroundings for the duration of the heating the shape modification due to melting also produces a negative degree two gravity anomaly which can reorient the spin axis of enceladus in order to place the sea at the pole

the evolution of enceladus saturn s small icy moon enceladus shows evidence of several episodes of geologic resurfacing and extensional tectonism spread over much of its history freezing of liquid in the interior caused global expansion and fracturing of the crust resurfacing took place by eruption of fresh material perhaps containing nh3 to the surface solid state convection could take place in enceladus for a crustal thickness greater than about 30 km assuming thermal properties like those of pure h2o ice melting in the interior may have been made more likely by the presence of nh3 as the h2o nh3 system has a eutectic point at 173 k tidal dissipation seems to be the only heating mechanism capable of melting enceladus for the thermal properties of pure h2o the orbital eccentricity would have to be higher by a factor of 5 7 than the present value of 0 0044 to maintain a molten interior and may have to be greater by as much as a factor of 20 to cause melting in an initially frozen body if the thermal conductivity is diminished by inclusion of clathrate hydrates a significant enhancement over the present eccentricity would still be required to initiate melting but it might be possible to maintain a molten interior and allow geologic activity with the present eccentricity removal of eccentricity forcing would result in rapid eccentricity damping freezing and cessation of tectonic activity

linking europa s plume activity to tides tectonics and liquid water abstract much of the geologic activity preserved on europa s icy surface has been attributed to tidal deformation mainly due to europa s eccentric orbit although the surface is geologically young 30 80 myr there is little information as to whether tidally driven surface processes are ongoing however a recent detection of water vapor near europa s south pole suggests that it may be geologically active initial observations indicated that europa s plume eruptions are time variable and may be linked to its tidal cycle saturn s moon enceladus which shares many similar traits with europa displays tidally modulated plume eruptions which bolstered this interpretation however additional observations of europa at the same time in its orbit failed to yield a plume detection casting doubt on the tidal control hypothesis the purpose of this study is to analyze the timing of plume eruptions within the context of europa s tidal cycle to determine whether such a link exists and examine the inferred similarities and differences between plume activity on europa and enceladus to do this we determine the locations and orientations of hypothetical tidally driven fractures that best match the temporal variability of the plumes observed at europa specifically we identify model faults that are in tension at the time in europa s orbit when a plume was detected and in compression at times when the plume was not detected we find that tidal stress driven solely by eccentricity is incompatible with the observations unless additional mechanisms are controlling the eruption timing or restricting the longevity of the plumes the addition of obliquity tides and corresponding precession of the spin pole can generate a number of model faults that are consistent with the pattern of plume detections the locations and orientations of these hypothetical source fractures are robust across a broad range of precession rates and spin pole directions analysis of the stress variations across the fractures suggests that the plumes would be best observed earlier in the orbit true anomaly 120 our results indicate that europa s plumes if confirmed differ in many respects from the enceladean plumes and that either active fractures or volatile sources are rare

spatial distribution of ice blocks on enceladus and implications for their origin and emplacement we have mapped the locations of over 100 000 ice blocks across the south polar region of saturn s moon enceladus thus generating the first quantitative estimates of ice block number density distribution in relation to major geological features ice blocks were manually identified and mapped from twenty of the highest resolution 4 25 m per pixel cassini imaging science subsystem iss narrow angle images using arcgis software the 10 100 m diameter positive relief features are marginally visible at the resolution of the images making ice block identifications difficult but not impossible our preliminary results reveal that ice blocks in the southern hemisphere are systematically most concentrated within the geologically active south polar terrain spt and exhibit peak concentrations within 20 km of the tiger stripe fractures as well as close to the south pole we find that ice blocks are concentrated just as heavily between tiger stripe fractures as on the directly adjacent margins although significant local fluctuations in ice block number density do occur we observe no clear pattern with respect to the tiger stripes or jet sources we examine possible roles of several mechanisms for ice block origin emplacement and evolution impact cratering ejection from fissures during cryovolcanic eruptions tectonic disruption of lithospheric ice mass wasting seismic disturbance and vapor condensation around icy fumeroles we conclude that impact cratering as well as mass wasting perhaps triggered by seismic events cannot account for a majority of ice block features within the inner spt the pervasiveness of fracturing at many size scales the ubiquity of ice blocks in the inner spt as well as the occurrence of linear block arrangements that parallel through cutting crack networks along the flanks of tiger stripes indicate that tectonic deformation is an important source of blocky ice features in the spt ejection during catastrophic cryovolcanic eruptions and condensation around surface vents however cannot be ruled out further sublimation processes likely erode and disaggregate ice blocks from solid exposures of ice especially near the warm tiger stripe fractures the relative paucity of blocks beyond the bounds of the spt particularly on stratigraphically old cratered terrains may be explained in part by mantling of the surface by fine particulate ice grains that accumulate over time

antarctic analog for dilational bands on europa europa s surface shows signs of extension which is revealed as lithospheric dilation expressed along ridges dilational bands and ridged bands ridges the most common tectonic feature on europa comprise a central crack flanked by two raised banks a few hundred meters high on each side together these three classes may represent a continuum of formation in tufts dilational model ridge formation is dominated by daily tidal cycling of a crack which can be superimposed with regional secular dilation the two sources of dilation can combine to form the various band morphologies observed new gps data along a rift on the ross ice shelf antarctica is a suitable earth analog to test the framework of tufts dilational model as predicted by tufts dilational model tensile failures in the ross ice shelf exhibit secular dilation upon which a tidal signal can be seen from this analog we conclude that tufts dilational model for europan ridges and bands may be credible and that the secular dilation is most likely from a regional source and not tidally driven