global multispectral mosaics of the icy galilean satellites multispectral mosaics of the three icy galilean satellites have been produced from low to moderate resolution voyager images the digital image data have been calibrated using preflight and in flight calibration data and reduced to normal albedos for four color band passes centered at 0 35 0 41 0 48 and 0 59 microns the individual images have been geometrically corrected mosaiked and presented in a normal cylindrical map projection at a uniform scale of 8 km pixel preliminary analysis of the normal albedo data shows that 1 the darkest terrains on the satellites are the dark regions on callisto 2 the brightest regions are bright plains regions on europa and bright fresh craters on ganymede 3 bright craters on ganymede and its dark cratered regions are both significantly brighter than comparable terrains on callisto and 4 europa exhibits several distinct spectral units with regions on the trailing hemisphere having lower relative ultraviolet spectral reflectance than areas on the leading hemisphere this is consistent with proposals that io related plasma impact on europa s trailing side has altered its optical properties

the galileo imaging team plan for observing the satellites of jupiter the galileo spacecraft carries a 1500 nm focal length camera with a 800 800 ccd detector that will provide images with a spatial resolution of 10 rad pixel the spacecraft will fly by io at the time of jupiter orbit insertion joi and subsequently while in jupiter s orbit will have a total of 10 close passes by europa ganymede and callisto these close passes together with more distant encounters will be used by the imaging experiment primarily to obtain high resolution coverage of selected targets to fill gaps left in the voyager coverage to extend global color coverage of each satellite and to follow changes in the volcanic activity of io the roughly 390 mbit allocated for imaging during the tour will be distributed among several hundred frames compressed by factors that range from 1 to possibly as high as 50 after obtaining high resolution samples during the initial io encounter at joi roughly 10 of imaging resources are devoted to near terminator mapping of io s topography at 2 to 10 km resolution monitoring color and albedo changes of the ionian surface and monitoring plume activity observations of europa range in resolution from several kilometers per pixel to 10 m pixel the objectives of europa are 1 to determine the nature origin and age of the tectonic features 2 to determine the nature rates and sequence of resurfacing events 3 to assess the satellite s cratering history and 4 to map variations in spectral and photometric properties europa was poorly imaged by voyager so the plan includes a mix of high and low resolution sequences to provide context the imaging objectives at ganymede are 1 to characterize any volcanism 2 to determine the nature and timing of any tectonic activity 3 to determine the history of formation and degradation of impact craters and 4 to determine the nature of the surface materials because ganymede was well imaged by voyager most of the resources at ganymede are devoted to high resolution observations the callisto observations will be directed mostly toward 1 filling voyager gaps 2 acquiring high resolution samples of typical cratered terrain and components of the valhalla and asgaard basins 3 acquiring global color and 4 determining the photometric properties of the surface a small number of frames will be used to better characterize the small inner satellites of jupiter thebe amalthea metis and adrastea

rheological properties of water ice applications to satellites of the outer planets 1 abstract the icy moons of the outer solar system have not been quiescent bodies in part because many have a substantial water component and have experienced significant internal heating we can begin to understand the thermal evolution of the moons and the rate of viscous relaxation of surface topography because we now have good constraints on how ice in several of its polymorphic forms flows under deviatoric stress at planetary conditions details of laboratory derived flow laws for pure polycrystalline ice are reviewed in detail one of the more important questions at hand is the role of ice grain size grain size may be a dynamic quantity within the icy moons and it may or may not significantly affect rheology one recent beneficiary of revelations about grain size sensitive flow is the calculation of the rheological structure of europa s outer ice shell which may be no thicker than 20 km

haughton impact structure structural and morphological aspects haughton is a complex meteorite impact structure of tertiary age on devon island in the canadian arctic the 20 5 km diameter site was formed in essentially flat lying paleozoic carbonates roughly 1700 m in thickness overlying precambrian gneisses the structure comprises three concentric rings surrounding a central basin with interior peaks the outer ring diameter 20 5 km is an annulus of peaks separating the allen bay dolomites of the regional plateau from the same unit that has been downdropped towards the crater interior along a series of circumferential faults the middle ring of peaks diameter 14 15 km comprises both faulted allen bay dolomites and blocks of the underlying bay fiord formation uplifted 250 m hills of the inner elliptical ring 3 5 5 5 km in diameter are also formed from bay fiord formation and the underlying eleanor river formation brought up 600 m an interconnected series of normal faults occupies most of the annulus between the inner and outer rings within the central b

titan radar mapper observations from cassini s t3 fly by cassini s titan radar mapper imaged the surface of saturn s moon titan on its february 2005 fly by denoted t3 collecting high resolution synthetic aperture radar and larger scale radiometry and scatterometry data these data provide the first definitive identification of impact craters on the surface of titan networks of fluvial channels and surficial dark streaks that may be longitudinal dunes here we describe this great diversity of landforms we conclude that much of the surface thus far imaged by radar of the haze shrouded titan is very young with persistent geologic activity

effects of dispersed particulates on the rheology of water ice at planetary conditions we have investigated the effects of initial grain size and hard particulate impurities on the transient and steady state flow of water ice i at laboratory conditions selected to provide more quantitative constraints on the thermomechanical evolution of the giant icy moons of the outer solar system our samples were molded with particulate volume fractions of 0 001 to 0 56 and particle sizes of 1 to 150 m deformation experiments were conducted at constant shortening rates of 4 4 10 7 to 4 9 10 4 s 1 at pressures of 50 and 100 mpa and temperatures 77 to 223 k for the pure ice samples initial grain sizes were 0 2 0 6 mm 0 75 1 75 mm and 1 25 2 5 mm stress strain curves of pure ice i under these conditions display a strength maximum u at plastic strains e 0 01 after initial yield followed by strain softening and achievement of steady state levels of stress ss at e 0 1 to 0 2 finer starting grain size in pure ice generally raises the level of u petrography indicates that the initial transient flow behavior is associated with the nucleation and growth of recrystallized ice grains and the approach to ss evidently corresponds to the development of a steady state grain texture effects of particulate concentrations 0 1 are slight at these concentrations a small but significant reduction in u with respect to that for pure water ice occurs mixed phase ice with 0 1 is significantly stronger than pure ice the strength of samples with 0 56 approaches that of dry confined sand the magnitude of the strengthening effect is far greater than expected from homogeneous strain rate enhancement in the ice fraction or from pinning of dislocations orowan hardening this result suggests viscous drag occurs in the ice as it flows around the hard particulates mixed phase ice is also tougher than pure ice extending the range of bulk plastic deformation versus faulting to lower temperatures and higher strain rates the high pressure phase ice ii formed in 0 56 mixed phase ice during deformation at high stresses bulk planetary compositions of ice rock 0 4 0 5 are roughly 2 orders of magnitude more viscous than pure ice promoting the likelihood of thermal instability inside giant icy moons and possibly explaining the retention of crater topography on icy planetary surfaces

volumetric analysis of complex lunar craters implications for basin ring formation the transition in hypervelocity impact structures from complex craters to basins is defined by a transition in uplifted central features from peaks to peak rings volumetric and morphologic analyses of the central peaks in complex lunar craters are combined with subsurface data from terrestrial complex impact structures in an effort to characterize the crater to basin transition central peak volumes in lunar craters with rim diameters drc 80 km central peak volume is relatively reduced and a second power law relationship is defined the relative reduction in peak volume in craters with drc 80 km is associated with a relative decrease in peak height and is accompanied by the development of a concentric zone of enhanced floor roughening studies of terrestrial complex impact structures suggest that the amount of uplifted material as judged from its depth of origin continues to increase with increasing rim diameter this suggests that a redistribution of uplifted material away from a centralized peak may occur in the larger craters the volumetric and morphologic changes described occur over a rim diameter range of 51 80 km far below the previously proposed range for the crater to basin transition drc 140 175 km evidence is presented to support a crater to basin transition beginning at drc 51 80 km characterized by a relative reduction in central peak volume and the development of rings of floor roughening which are suggested to be precursors of peak ring development

fluid volcanism on miranda and ariel flow morphology and composition several types of volcanic units have been recognized on the icy uranian satellites miranda and ariel on miranda ridges characterized by crest grooves are up to 10 km wide and 500 m high a continuous flat topped flow band also 10 km wide and 500 m high forms the outer southern margin of elsinore corona elsinore coronae at least appears to comprise coalesced flow bands and ridges on ariel in addition to at least one ridge unit similar to these on miranda flood plains material has covered the floors of deep chasmata grabens and an irregular depression flows on both satellites are characterized by linear vent geometries and distinct topographic margins which indicate extrusion of a relatively viscous material the topography of the flows can be used to estimate flow viscosity or yield strength using a bingham plastic model extrusion viscosity estimates incorporating plausible volcanologically based emplacement time scales and a rigid crust correction range from 107 to 109 p 1013 p in the unlikely absence of a chilled crust viscosity estimates are dependent on the assumed emplacement time scale however and could be as high as 1016 p if a solid stale based time scale is assumed bingham models indicate that it is yield strength that controls the final flow width and thickness that we see yield strength estimates are probably more reliable because of uncertainties in the viscous flow time scale yield strength estimates range from 10 1 to 10 2 bars consistent with yield strengths of partially crystallized silicate melts on other planets nevertheless both the viscosities and yield strengths are well within the range of and in some cases lower than silicate flows silicate compositions are not possible for these flows because of the ice rich composition of these satellites measured properties of ammonia water liquid crystal mixtures are consistent with the observed and estimated flow rheologic properties however and extrusion of such material s is a viable alternative to solid state extrusion for the origin of these flow features

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

elastoviscoplastic relaxation of impact crater topography with application to ganymede and callisto 1 we use an elastoviscoplastic rheological model to study the long term relaxation of impact crater topography on ganymede and callisto we employ a complete set of rheological parameters for ductile creep in ice ih and implement a new shallower initial shape model under high heat flow conditions stresses are relieved in deeper less viscous material and the remainder concentrates as flexural bending stresses in a viscously stiffer near surface layer essentially a mechanical lithosphere forms that focuses stresses but with uplifts and stress levels that are always greater than those obtained for a purely viscous half space we discuss how classic elastic flexure and purely viscous relaxation are special cases of this more generalized rheological model stress concentration in the flexing i e bending layer can exceed the plastic yield strength of ice for high enough heat flows weakening the layer somewhat and increasing the total amount of relaxation these plastic strains tend to be limited to small magnitudes 1 mm permits both retention and relaxation of topography at the lower and higher ends respectively of the anticipated heat flow range consideration of relaxed craters in marius regio and of the palimpsests and penepalimpsests suggests ancient heat flows in excess of 10 mw m 2 consistent with other estimates

the geology of the occator quadrangle of dwarf planet ceres floor fractured craters and other geomorphic evidence of cryomagmatism abstract the dawn science team is conducting a geological mapping campaign at ceres during the nominal mission including iterative mapping using data obtained during each orbital phase we are using geological mapping as a method to identify the geologic processes that have modified the surface of dwarf planet ceres we here present the geology of the ac 9 occator quadrangle located between 22 s 22 n and 216 288 e the ac 9 map area is within the topographically high region named hanami planum features of note within the profile include impact craters occator azacca lociyo nepen and kirnis four of these craters have fractured shallow floors morphometrically comparable to floor fractured craters ffcs on the moon similar to models for the formation of the lunar ffcs we suggest that these ceres ffcs are a result of cryomagmatic uplift under the crater floors a set of regional linear structures called the samhain catenae do not have any obvious relationship to impact craters many of the catenae are comprised of smaller structures that have linked together suggestive of en echelon fractures it has been suggested that these fractures formed due to the uplift of hanami planum due to cryromagmatic plumes

ganymede and callisto complex crater formation and planetary crusts new depth diameter d d rim and peak height and width measurements of fresh craters on ganymede and callisto are utilized to explore not only crater mechanics on icy satellites but intersatellite crater scaling and crustal properties and composition significant systematic differences in crater morphology on icy and rocky planets are confirmed simple to complex transition diameters on icy satellites including ganymede and callisto are much lower than those on rocky planets also complex crater depths and rim heights on ganymede and callisto are inherently 60 70 shallower than lunar complex craters despite similar surface gravity hence viscous relaxation is not as important as is generally assumed central peaks on most icy satellites are 1 2 km higher and considerably wider with respect to diameter than for similar sized lunar craters terracing and rim slumping is also rare in these craters central peaks in craters on ganymede and callisto larger than 15 km however are relatively small in both width and height however correlating with the occurrence of rim slumping thus central peak dimensions are an indirect indicator of the degree of rim slumping transition diameters for the occurrence of central peaks and rim slumps and for d d curve inflections for icy satellites scale approximately with the inverse of surface gravity except possibly at very low g the transition diameters for central peaks and for rim slumping differ however by 5 15 km on ganymede and callisto but by 50 or more km on the smaller icy satellites these large differences suggest that d d curves on icy satellites are comprised of three segments of decreasing slope one for smaller simple craters a second for central peak craters and a third for larger true complex craters with both central peaks and rim slumping the shallow slopes of d d curves on ganymede and callisto resemble that of the moon consistent with the occurrence of both central peaks and rim failure the high slopes of d d curves on the middle sized icy satellites indicate that they are middle segments consistent with the dominance of central peaks in those craters complex crater depths on icy satellites in general scale with the inverse of surface gravity the depths of all the observed large basin scale craters on these small satellites when scaled to ganymede gravity are very similar to those of complex craters on ganymede and callisto these large craters including odysseus 425 km diameter on tethys are probably true complex craters that have not undergone significant viscous relaxation ithaca chasma on tethys probably formed during the prompt collapse of the transient odysseus crater crater morphology is clearly controlled by both gravity and by large variations in material properties i e composition and should be useful in constraining bulk crustal composition i e ice rock ratios the similarity of various morphological transition diameters and complex crater depths on ganymede and callisto two geophysically very similar but geologically divergent large icy satellites indicates that the crusts of both bodies are dominated by water ice with only a minor rocky component

radii shapes and topography of the satellites of uranus from limb coordinates abstract limb coordinates are used to find the radii shapes and local topography of the five large satellites of uranus the technique provides a direct measure of ellipsoidal shapes of satellites and fixes radii to subpixel accuracy umbriel titania and oberon are best fit by spheres miranda and ariel are ellipsoids whose equatorial bulges are consistent with published mean densities limb topography on miranda shows substantial deformation of both old cratered terrain and the younger coronae and complex faulting and uplift at the margins of the coronae the maximum deformation is about 10 km umbriel s limb shows a basin of undetermined origin about 500 km across and 6 km deep

investigating target versus impactor influences on martian crater morphology at the simple complex transition comparing craters of identical diameter on a planet is an empirical method of studying the effects of different target and impactor properties while holding total impact energy nearly constant we have analyzed the martian crater population within a narrow diameter range 7 km crater diameter 9 km at the simple complex crater transition using three approaches we looked for correlations of morphology with surface geology using a global crater database and global geologic map we examined selected regions in detail with high resolution images to further understand the relationship between crater morphology and bulk target properties finally we examined craters in close proximity to each other in order to hold target properties constant so that we could isolate impactor effects on crater morphology we found a strong correlation between target properties and interior crater morphology and we found little evidence that impactor properties other than impact angle affect crater appearance central uplift and wall slumping are enhanced for less consolidated targets layered targets affected both the excavation and modification stages of complex crater formation the resulting craters have pseudoterraces flat floors and central pits

three layered models of ganymede and callisto compositions structures and aspects of evolution abstract three layered structural models are determined for ganymede and callisto each consists of a rock core a mixed ice rock lower mantle and a pure ice upper mantle this structure results from differentiation subsequent to accretional melting attention is given to evaluating various candidates for the rock component and three alternatives representing various degrees of silicate hydration and oxidation are modeled and incorporated structures are calculated on the basis of a 250 k isotherm which is a reasonablev approximation to the gentle adiabats expected to occur in icy satellites differentiation of an ice rock satellite generally involves an increase in radius and the three layered approach allows this process to be examined in some detail it is determined that satellite expansion is most significant early in the process and much less so as differentiation proceeds to completion if tectonics are due to global expansion distinguishing on this basis between a completely differentiated satellite and one that is only partially differentiated is difficult the postaccretional global expansion of ganymede which may have left a tectonic record was probably limited to 1 in radius in agreement with observed limits useful quantities such as silicate mass and volume fraction uncompressed density j2 c22 binding energy and surface heat flow are also determined nonhydrostatic contributions to j2 and c22 are estimated and shown to be nonnegligible encounters with jupiter orbiting spacecraft are unlikely to determine callisto s degree of central condensation we conclude by calculating the relative likelihood of postaccretional melting caused by radiogenic heating three layered satellites have generally hotter interiors because additional thermal boundary layers divide the separately convecting upper and lower mantles inhibiting heat transport ganymedes and callistos that are less than about 1 3 differentiated by mass should experience a second episode of melting as these boundary layers are either above the level of the water ice minimum melting temperature or intersect the melting curve at deeper levels runaway differentiation to at least a depth corresponding to a pressure in the ice v stability field is likely the main point here is that if satellite tectonics are tied to differentiation by melting or its aftermath as in the instability following ocean closure of kirk and stevenson moderate or small amounts of accretional differentiation are unlikely to explain an absence of tectonics as in callisto because extensive differentiation ultimately occurs

friction of ice the frictional strength of ice seems to be well below that for all other rocks triaxial testing of cylinders of pure water ice containing a 45 inclined sawcut at temperatures of 77 t 115 k and confining pressures 0 1 p 250 mpa reveals the frictional laws 0 20 n 8 3 mpa for p 10 mpa and 0 55 n 1 0 mpa for p 5 mpa where and n are the shear and normal stresses on the sawcut surface friction is independent of t and over the one order of magnitude tested of average sliding velocity the sliding behavior is invariably stick slip examination of sliding surfaces shows only minor gouge development a curious arrangement of densely packed short vertical fractures occurs primarily in samples with anomalously low strength these results apply to models of near surface tectonic activity on jupiter s three large icy moons europa callisto and ganymede they indicate for example that a global expansion on ganymede of 3 linear percent will cause extensional movement on preexisting faults at depths to 7 3 km

the central pit and dome at cerealia facula bright deposit and floor deposits in occator crater ceres morphology comparisons and formation abstract the prominent bright deposit cerealia facula ceres coincides with the central depression or central pit of the recently formed 92 km wide complex crater occator the central pit is 9 10 km wide and up to 1 km deep and is partially filled with a 700 m high 2 km wide dome the upper surface of the central dome is densely fractured but the flanks are not indicating that uplift of the dome surface occurred after the bright deposit was emplaced and primarily through uplift of the surface from below by laccolithic intrusion or volume expansion the pit is rimless except for two prominent massifs that flank it to the east and west this pit dome morphology bears a strong resemblance in morphology topography and dimensions to central pit craters observed on ganymede and callisto but is essentially absent on the similar sized ice rich moons of uranus saturn and pluto the lack of pit craters on midsize icy moons and the possible lack of central pit craters at the cold poles of ceres suggest that temperature is a significant controlling factor in central pit formation beyond the canonical inverse gravity scaling of complex crater transitions while renewed modeling will benefit from these new constraints the mapping of central pits on ceres including occator suggests that either the rheologically layered target model or more likely the melted central uplift model may be the most probable explanation for central pit formation on ceres lack of similar high resolution imaging on the galilean icy satellites precludes definitive evaluation of models there large volumes of lobate floor fill material at least 600 m thick in some places cover large areas of the southern and eastern floor of occator and small outcrops of this unit are found perched high on closed topographic depressions on the occator terrace zone and other higher areas of the crater floor the swirling and knobby surface textures of these lobate deposits are essentially indistinguishable from those on impact melt sheets on large lunar craters such as tycho these observations indicate that this unit is most likely an impact melt sheet formed from melted water ice admixed with unmelted particulate or dissolved phyllosilicates salts and or carbonates and large quantities of fragmented debris melt of ice phases within ceres crust is also supported by independent numerical models of impact at occator and would provide a ready source of fluids to promote hydrothermal deposition of the observed bright na carbonates on the surface of the central pit at occator if the deposit formed within a million years or so of impact the central dome finds analogs in salt domes on earth and pingos frost mounds on earth and mars which form by water and solute migration and freeze expansion later freezing of a central reservoir of volatiles beneath the central pit at occator or mobilization of ice or other phases during the cooling and refreezing process could explain the uplift of the central dome finally no changes were detected in the cerealia and vinalia facula bright deposits during the course of the prime dawn mapping mission placing additional constraints on their formation time and limits on ongoing activity

geology of the selk crater region on titan from cassini vims observations abstract observations of titan obtained by the cassini visual and infrared mapping spectrometer vims have revealed selk crater a geologically young bright rimmed impact crater located 800 km north northwest of the huygens landing site the crater rim crest diameter is 90 km its floor diameter is 60 km a central pit peak 20 30 km in diameter is seen the ratio of the size of this feature to the crater diameter is consistent with similarly sized craters on ganymede and callisto all of which are dome craters the vims data unfortunately are not of sufficient resolution to detect such a dome the inner rim of selk crater is fluted probably by eolian erosion while the outer flank and presumed ejecta blanket appear dissected by drainages particularly to the east likely the result of fluvial erosion terracing is observed on the northern and western walls of selk crater within a 10 15 km wide terrace zone identified in vims data the terrace zone is bright in sar data consistent with it being a rough surface the terrace zone is slightly wider than those observed on ganymede and callisto and may reflect differences in thermal structure and or composition of the lithosphere the polygonal appearance of the crater likely results from two preexisting planes of weakness oriented at azimuths of 21 and 122 east of north a unit of generally bright terrain that exhibits similar infrared color variation and contrast to selk crater extends east southeast from the crater several hundred kilometers we informally refer to this terrain as the selk bench both selk and the bench are surrounded by the infrared dark belet dune field hypotheses for the genesis of the optically bright terrain of the bench include wind shadowing in the lee of selk crater preventing the encroachment of dunes impact induced cryovolcanism flow of a fluidized ejecta blanket similar to the bright crater outflows observed on venus and erosion of a streamlined upland formed in the lee of selk crater by fluid flow vestigial circular outlines in this feature just east of selk s ejecta blanket suggest that this might be a remnant of an ancient cratered crust evidently the southern margin of the feature has sufficient relief to prevent the encroachment of dunes from the belet dune field we conclude that this feature either represents a relatively high viscosity fluidized ejecta flow a class intermediate to ejecta blankets and long venusian style ejecta flows or a streamlined upland remnant that formed downstream from the crater by erosive fluid flow from the west northwest

europa ganymede and callisto new radar results from arecibo and goldstone observations of the icy galilean satellites conducted during 1987 1991 with the arecibo 13 cm system and the goldstone 3 5 cm system yield significant improvements in our knowledge of the satellites radar properties hardly any wavelength dependence is seen for either the total power radar albedo or the circular polarization ratio c for europa ganymede and callisto our 13 cm estimates of mean values and rms dispersions are 1 39 0 14 and 0 69 0 06 and c 1 53 0 03 1 43 0 06 and 1 17 0 04 radar albedo features are seen on each satellite evidence for c features is lacking except for indications of a weak hemispheric asymmetry for callisto that intersatellite and intrasatellite fractional variations in albedo greatly exceed variations in c is consistent with predictions of coherent backscatter theory and implies that albedo might be a crude indicator of relative silicate abundance the satellites albedo distributions overlap the most prominent radar features are tentatively identified with galileo regio and the valhalla basin estimates of echo doppler frequencies show callisto to be lagging its ephemeris by 200 50 km

formation of crater palimpsests on ganymede crater palimpsests on ganymede are circular features of high albedo apparently formed by impacts very early in the satellite s history some palimpsests have distinct outward facing slopes at their outer margins suggesting that they are distinct deposits of significant thickness this view is supported by observations of superimposed craters that excavate through palimpsest deposits into underlying materials of contrasting albedo we suggest that crater palimpsests may result from volcanic extrusions triggered by large impact events early in ganymede s history impact excavation that occurred to a sufficient depth would penetrate the satellite s thin primordial lithosphere allowing mobile buoyant material from a vigorously convecting mantle to rise to the surface upon reaching the surface the material would spread radially to produce a circular deposit of relatively high albedo as the lithosphere cooled and thickened palimpsest formation would have ceased consistent with the observed concentration of palimpsest formation in the earliest part of ganymede s recorded geologic history constraints on the nature of the extruded material may be derived from a simple model of emplacement we use an analytic model for axisymmetric viscous flow to constrain the flow viscosity in terms of the aspect height radius ratio of the palimpsest a measured value of 0 02 would imply a viscosity marginally consistent with glacial ice viscosities smaller values of which are likely would imply still lower viscosities more appropriate to partially liquid flow if such flows were described by a bingham rheology their yield strengths were of the order of 1 kpa this is lower than is typical for even very mobile silicate lavas but may be reasonable for a water ice slush ice extrusion could have been a slow process taking thousands of years while slush extension would have been essentially contemporaneous with the impact event for the case of either solid ice and slush extrusion our conclusions for the thermal structure of ganymede at the time of palimpsest formation are similar there was a thin 10 km conducting lithosphere underlain by a vigorously convecting mantle that was very close to the solidus temperature

a proposed origin for palimpsests and anomalous pit craters on ganymede and callisto palimpsests and anomalous pit craters are two classes of craters found on the icy satellites ganymede and callisto that have no obvious counterparts among the crater populations found on rocky planets the distinct characteristics of palimpsests and anomalous pit craters in morphology morphometry and population statistics which differentiate them from the more numerous normal craters and basins on the icy satellites also preclude their origin as normal craters that have nearly vanished through viscous relaxation an alternate origin is proposed palimpsests are formed when the flow of material during the modification stage of crater formation is dominated by wet fluid flow as opposed to dry granular flow which forms normal craters conditions of wet modification flow occur when the volume of melt remaining in the crater attains a volume comparable to the volume of the transient crater by use of an impact model that includes 1 plausible impact velocity distributions 2 crater scaling in ice 3 shock melting in ice 4 thermal profiles as a function of time in ganymede and 5 cratering flow field constraints conditions for wet modification flow are found to occur for sufficiently large impacts or high velocity impactors the range of normal crater palimpsest transition diameters and the morphologic characteristics inferred from the impact model are consistent with the observed characteristics of palimpsests thus normal craters are inferred to result from the majority of impactors which have impact velocities near the average of 15 km s anomalous pit craters inferred to be transitional forms from intermediate velocity impactors at about 30 km s and palimpsests by impactors travelling in excess of about 40 km s the thermal state of the crust at the time of impact is a second order effect but sufficient to account for the difference in palimpsest population between ganymede and callisto if ganymede once had a liquid mantle while callisto remained unmelted and undifferentiated or solidified prior to the formation of the presently observed crater population

interdecadal water mass changes in the southern ocean between 30 e and 160 e interdecadal water mass changes in the indian western pacific sectors of the southern ocean were investigated using the japanese antarctic research expeditions and historical hydrographic observations from the 1950s to 1990s freshening and cooling occurred on the neutral density surfaces of 27 0 kgm3 equatorward of sub antarctic front results for the area south of the polar front show warm and saline anomalies and oxygen decreases on the surfaces around 27 9 kgm3 which correspond to the upper circumpolar deep water these latter anomalies are most simply explained by the mixing of these shallow waters with warmer and fresher surface waters steric sea level has also increased with an average change of 1mmyr1 from the 1970s to 1990s the changes are larger north of the sub antarctic front implying a strengthening of the antarctic circumpolar current it appears that the observed changes are consistent with the results from coupled climate model results for a similar period

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

a global inventory of central pit craters on the moon distribution morphology and geometry abstract the origin of central pit craters on the moon has long been an enigma and a primary reason is that their geographic distribution and morphometric characteristics were unknown we investigated a global inventory of lunar central pit craters using high resolution image and topography data obtained from the lunar reconnaissance orbiter 56 certain and 35 probable central pit craters are found on both the lunar maria and highlands the certain pit craters are 9 57 km in diameter the average diameter ratio between the central pits and their parent craters is 0 12 and the average depth diameter ratio for the central pits is 0 072 with irregular shaped rims the central pits have conical profiles and some have flat floors the central pits occur on both crater floors and central peaks the floor pits are generally larger deeper and with more irregular shape compared with summit pits both the summit and floor pit craters have formed in every lunar stratigraphic epoch from nectarian to copernican target properties of background terrains affect the morphology and size of central pits but they do not determine whether or not a central pit forms during a cratering event the lunar central pits may have formed by deformation of central peaks caused by some mechanical processes during or soon after the cratering process of their parent craters

valleys in pit craters on mars characteristics distribution and formation mechanisms abstract new observations of central pit craters on mars reveal that some craters contain prominent interior valley networks that drain into their central pit in this study we catalog the distribution of pit craters with associated valleys and characterize in detail five diverse well preserved examples in some instances sedimentary deposits we interpret as either alluvial fans or deltas are found in the central pits and evidence suggests that at least some of these pits once contained lakes fluvial activity within the five pit craters we detail appears to have occurred relatively late in mars history during the hesperian or amazonian periods for this reason further understanding the formation mechanisms of these valleys has important implications for the history of water on mars

crater formation and modification on the icy satellites of uranus and saturn depth diameter and central peak occurrence depth diameters d d for fresh craters on the intermediate sized icy satellites of uranus and saturn have been determined using photoclinometry and shadow lengths and are compared with similar measurements on the terrestrial planets simple bowl shaped craters on icy satellites including those on miranda for which the highest resolution data are available are systematically 20 40 shallower than on the terrestrial planets this pronounced difference between crater depths on icy and rocky surfaces indicates that differences in impact velocity or surface gravity are not as important as the differences in mechanical properties between ice and rock in controlling simple crater morphology experimental impact studies indicate that differences in material properties such as angle of internal friction can control crater depth alternatively breccia lenses may be thicker in icy satellite simple craters complex craters on the icy satellites become significantly deeper with increasing crater diameters unlike complex craters on terrestrial planets which are nearly constant in depth central peaks and hence floor rebound are also volumetrically and morphologically more prominent than wall slumping rim collapse on the icy satellites the magnitude of viscous relaxation of very large craters e g herschel and odysseus on the icy satellites can be estimated from extrapolation of the d d fits at smaller crater diameters the transition diameter from simple to complex morphology is inversely correlated with gravity as it is on the terrestrial planets but at significantly smaller diameters than would be expected from a simple extrapolation of the terrestrial trend thus crater modification especially floor rebound is easier to initiate on icy satellites estimated effective cohesions for shocked icy material are lower by a factor of 10 20 relative to rock although estimated effective viscosities are similar the distinct systematic differences between crater morphology on icy and rocky worlds indicate that gross material property differences between rock and ices play a key role in crater formation and modification but gravity is still the primary driving force

the geomorphology of ceres introduction observations of ceres the largest object in the asteroid belt have suggested that the dwarf planet is a geologically differentiated body with a silicate core and an ice rich mantle data acquired by the dawn spacecraft were used to perform a three dimensional characterization of the surface to determine if the geomorphology of ceres is consistent with the models of an icy interior rationale instruments on dawn have collected data at a variety of resolutions including both clear filter and color images digital terrain models have been derived from stereo images a preliminary 1 10 m scale geologic map of ceres was constructed using images obtained during the approach and survey orbital phases of the mission we used the map along with higher resolution imagery to assess the geology of ceres at the global scale to identify geomorphic and structural features and to determine the geologic processes that have affected ceres globally results impact craters are the most prevalent geomorphic feature on ceres and several of the craters have fractured floors geomorphic analysis of the fracture patterns shows that they are similar to lunar floor fractured craters ffcs and an analysis of the depth to diameter ratios shows that they are anomalously shallow compared with average ceres craters both of these factors are consistent with ffc floors being uplifted due to an intrusion of cryomagma kilometer scale linear structures cross much of ceres some of these structures are oriented radially to large craters and most likely formed due to impact processes however a set of linear structures present only on a topographically high region do not have any obvious relationship to impact craters geomorphic analysis suggests that they represent subsurface faults and might have formed due to crustal uplift by cryomagmatic intrusion domes identified across the ceres surface present a wide range of sizes 100 km basal shapes and profiles whether a single formation mechanism is responsible for their formation is still an open question cryovolcanic extrusion is one plausible process for the larger domes although most small mounds 10 km diameter are more likely to be impact debris differences in lobate flow morphology suggest that multiple emplacement processes have operated on ceres where three types of flows have been identified type 1 flows are morphologically similar to ice cored flows on earth and mars type 2 flows are comparable to long runout landslides type 3 flows morphologically resemble the fluidized ejecta blankets of rampart craters which are hypothesized to form by impact into ice rich ground conclusion the global trend of lobate flows suggests that differences in their geomorphology could be explained by variations in ice content and temperature at the near surface geomorphic and topographic analyses of the ffcs suggest that cryomagmatism is active on ceres whereas the large domes are possibly formed by extrusions of cryolava although spectroscopic analysis to date has identified water ice in only one location on ceres the identification of these potentially ice related features suggests that there may be more ice within localized regions of ceres crust figure 1 dawn high altitude mapping orbit imagery 140 meters per pixel of example morphologic features a occator crater arrows point to floor fractures b linear structures denoted by arrows c a large dome at 42 n 10 e visible in the elevation map d a small mound at 45 5 s 295 7 e e type 1 lobate flow arrows point to the flow front analysis of dawn spacecraft framing camera image data allows evaluation of the topography and geomorphology of features on the surface of ceres the dwarf planet is dominated by numerous craters but other features are also common linear structures include both those associated with impact craters and those that do not appear to have any correlation to an impact event abundant lobate flows are identified and numerous domical features are found at a range of scales features suggestive of near surface ice cryomagmatism and cryovolcanism have been identified although spectroscopic analysis has currently detected surface water ice at only one location on ceres the identification of these potentially ice related features suggests that there may be at least some ice in localized regions in the crust 1 pending yes

radioelement concentrations in the deep profile through precambrian basement of the vredefort structure the vredefort basement granite in south africa forms the core of an undomed and overturned sequence of precambrian strata slawson 1976 proposed that the granite core has also been overturned exposing to view a thick section through the archean crust incrasing in depth toward the center of the dome profiles across the vredefort basement provide an opportunity for studying the distribution of u th and k with depth in the upper and middle crust detailed radioelement concentration profiles across the granitic basement showed that 1 the central part of the basement core is depleted in th and u relative to the perimeter and the concentration of th and u falls off regularly from the basement margin inward 2 the distribution of the radioelements and the distribution of heat production with depth over the outer 8 km are consistent with an exponential depth function 3 the felsic rocks of the central basement are characterized by high k u and th u ratios and the ratios k u k th and th u increase from the margin inward and 4 mafic granulites which occur in the central regions of the dome have normal k u k th and th u ratios high grade metamorphic environment resulted in the depletion of bothmore u and th relative to k in the greater proportion of the felsic rocks from the central regions of the vredefort basement depletion of u was far greater than that of th the normal th u k u and k th ratios in mafic rocks in close proximity to the felsic rocks suggest that mineral assemblages have a strong control on the behavior of the radioelements during regional metamorphism in the vredefort basement less

multiringed impact craters on venus an overview from arecibo and venera images and initial magellan data abstract examination of earth based arecibo and venera 15 16 radar images of venus reveals the presence of unequivocal peak ring and larger ringed impact craters we identify 15 peak ring craters defined by a distinct radar bright outer rim and an inner ring of concentrically arranged peaks ejecta blankets are generally radar bright rough well defined and exhibit both symmetrical and asymmetrical forms in plan the ratio of crater diameter to peak ring diameter ranges from 3 5 to 2 with increasing crater diameter and magellan data indicate the existence of even larger ratios at the central peak to peak ring transition the overall morphology of venusian peak ring craters is similar to that of peak ring craters on other terrestrial planets effective viscosities of the target rock estimated from melosh s model of crater collapse are proportional to crater diameter indicating that the scale dependence of viscosity may dominate gravity in the formation of peak ring craters two larger ringed craters klenova and lise meitner are different in morphology from the peak ring forms they are each characterized by two well defined radar bright rings that we interpret to represent scarp mountains similar to the cordillera and outer rook of orientale on the moon also klenova exhibits an inner ring of peaks that is similar to orientale s inner rook and meitner may display an additional partial scarp like outer ring diameter ratios between the major rings of these two large craters are 1 6 close to the 2 often suggested for unequivocal lunar multiringed basins we interpret klenova and meitner to be true multiranged impact basins we argue that a third large multiringed structure mead is seen in magellan images it is characterized by two well developed rings that may also represent fault scarps similar to the cordillera and outer rook rings the ring diameter ratio for mead is variable but 1 4 ejecta blankets for meitner and mead are distinct but their appearance in radar images is suppressed relative to that of peak ring crater ejecta although multiringed basins on venus occur at much smaller diameters than on the other terrestrial planets except perhaps the earth effective viscosities are low enough at depth to allow inward asthenospheric flow which is the model of melosh and mckinnon can cause circumferential faulting block rotation and ring formation

terrace width variations in complex lunar craters the widths of terrace structures in complex craters on the moon are compared to existing theoretical models of their origin terrace widths in an individual crater increase monotonically outward toward the crater rim similarly the width w of the terraces lying closest to the rim of a crater of diameter d increases monotonically obeying a least squares power law relation w km 0 09d0 87 km a simple model of slumping that ignores inertial forces and assumes a constant bedrock yield strength is in good agreement with the observations

viscosity of rock ice mixtures and applications to the evolution of icy satellites theory and experiments are used to establish lower and upper bounds on the ratio of actual viscosity to pure ice viscosity for a suspension of rock particles in a water ice matrix for typical conditions encountered in icy satellites this ratio is of order ten or possibly larger depending on unknown factors such as the particle size distribution it is shown that even this modest increase in viscosity may be enough to have caused a failure of solid state convective self regulation early in the evolution of a homogeneous rock water ice satellite provided the satellite is large enough and sufficiently silicate rich the criteria for this failure are satisfied by ganymede and are marginal for callisto if the silicates are hydrated failure of self regulation means that the viscosity is too high for the interior to remain completely solid and eliminate the heat production of long lived radioisotopes by solid state convection partial melting of the ice then occurs it is further shown that satellites of this size may then undergo runaway differentiation into a rock core and almost pure ice mantle because the gravitational energy release is sufficient to melt nearly all the ice and the rayleigh taylor instability time scale is short although the high pressure phases of ice melt the resulting water quickly refreezes at a higher level it is conjectured that these results explain the striking surface dissimilarity of ganymede and callisto if these satellites accreted cold and undifferentiated ganymede may have gone supercritical melted and differentiated because of a failure of self regulation whereas callisto remained undifferentiated to the present day like all proposed explanations for the ganymede callisto dichotomy this conjecture cannot be quantified with confidence because of inadequate or incomplete observations theory and experimental data

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

galileo s first images of jupiter and the galilean satellites the first images of jupiter io europa and ganymede from the galileo spacecraft reveal new information about jupiter s great red spot grs and the surfaces of the galilean satellites features similar to clusters of thunderstorms were found in the grs nearby wave structures suggest that the grs may be a shallow atmospheric feature changes in surface color and plume distribution indicate differences in resurfacing processes near hot spots on io patchy emissions were seen while io was in eclipse by jupiter the outer margins of prominent linear markings triple bands on europa are diffuse suggesting that material has been vented from fractures numerous small circular craters indicate localized areas of relatively old surface pervasive brittle deformation of an ice layer appears to have formed grooves on ganymede dark terrain unexpectedly shows distinctive albedo variations to the limit of resolution

central pit and dome craters exposing the interiors of ganymede and callisto central pit craters on the large icy satellites ganymede and callisto are an unusual crater class perhaps related to the unusual properties of water ice the detailed morphology and dimensions of pit crater features differ from those of smaller normal complex craters on these two satellites pit crater depths appear to be constant regardless of diameter rim collapse is also restricted relative to that in lunar craters or smaller ganymede craters the floors of pits in craters larger than 60 km are occupied by smooth bright domes pit to crater and dome to crater diameter ratios increase linearly with crater diameter and are indistinguishable on ganymede and callisto and with terrain type or surface longitude the floors and central structures of young bright rayed pit craters such as osiris are covered by a thin uniformly bright impact melt or frost deposit thus domes and pits form rapidly on the time scale of the impact itself rather than by long term postimpact intrusion or extrusion the bright domes in pit craters are most simply explained as the uplift and exposure of relatively ice rich material from depths of 3 5 to 5 km during impact this process is almost directly analogous to the uplift and exposure of anamolous deep seated material observed in terrestrial and lunar craters the unusual pit morphology on icy satellites may be the result of impact into crust that is mechanically much weaker at shallow depth than on rocky bodies such as the moon because crater morphology is strongly dependent on ice rock composition the similarity of pit and dome dimensions on ganymede and callisto indicates that the structure and rheology of the crusts of these bodies down to depths of 10 km are very similar and have been for several b y pit crater morphology indicates that the crusts of both satellites are probably ice rich and differentiated

morphology and origin of palimpsests on ganymede based on galileo observations palimpsests are large circular low relief impact scars on ganymede and callisto these structures were poorly understood based on voyager era analysis but high resolution galileo images allow more detailed inspection we analyze images of four ganymedean palimpsests targeted by galileo memphis and buto faculae epigeus and zakar ganymedean craters and europan ring structures are used as tools to help better understand palimpsests based on morphologic similarities from analysis of galileo images palimpsests consist of four surface units central plains an unoriented massif facies a concentric massif facies and outer deposits using as a tie point the location in these structures where secondary craters begin to appear outer deposits of palimpsests are analogous to the outer ejecta facies of craters the concentric massif facies of palimpsests are analogous to the pedestal facies of craters and the unoriented massif facies and central plains are analogous to crater interiors these analogies are supported by the presence of buried preexisting structure beneath the outer two and absence of buried structure beneath the inner two units our observations indicate that palimpsest deposits represent fluidized impact ejecta rather than cryovolcanic deposits or ancient crater interiors

newly discovered ring moat dome structures in the lunar maria possible origins and implications we report on a newly discovered morphological feature on the lunar surface here named ring moat dome structure rmds these low domes a few meters to 20 m height with slopes 5 are typically surrounded by narrow annular depressions or moats we mapped about 2 600 rmdss in the lunar maria with diameters ranging from tens to hundreds of meters four candidate hypotheses for their origin involving volcanism are considered we currently favor a mechanism for the formation of the rmds related to modification of the initial lava flows through inflated flow squeeze ups and or extrusion of magmatic foams below a cooling lava flow surface these newly discovered features provide new insights into the nature of emplacement of lunar lava flows suggesting that in the waning stages of a dike emplacement event magmatic foams can be produced extrude to the surface as the dike closes and break through the upper lava flow thermal boundary layer crust to form foam mounds and surrounding moats

what we know about mars from its impact craters martian impact craters provide insights into formation ages of terrain units nearsurface properties and the planet s degradational history crater size frequency distribution analyses allow determination of terrain unit relative ages and extrapolation of the lunar crater chronology can provide estimates of absolute ages for these units such analyses indicate that mars has been geologically active throughout its history but it also retains extensive regions dating back to ca 4 ga higher cratering rates during the late heavy bombardment lhb helped to create the martian regolith and formation of large impact basins helped to erode the initially denser martian atmosphere and may have created the hemispheric dichotomy low global degradation rates since the end of lhb ca 3 8 ga resulted in well preserved impact crater morphologies the presence of layered ejecta blankets and central pits associated with fresh to moderately eroded craters suggests a volatile rich crust impact crater depth diameter relationships allow estimation of the vertical extent of crustal volatiles to depths greater than those probed by current spacecraft instrumentation temporal changes in subsurface ice reservoir depths are revealed by differences in layered ejecta onset diameter as a function of crater age an understanding of the morphology and morphometry of pristine impact craters facilitates efforts to determine the amount and types of degradation and the periods over which they operated impact craters therefore play a major role in unraveling the climatic and geologic history of mars

tectonic analysis of fracturing associated with occator crater abstract occator crater a ceres crater that hosts multiple bright spots on its floor has several sets of fractures associated with it a tectonic analysis of each of these fracture sets suggests that the concentric and radial fractures around the central pit and the concentric fractures high on the crater wall near the rim most likely formed due to well known impact cratering processes uplift of the crater floor due to magmatic injection is suggested to have resulted in the concentric floor fractures at the base of the crater wall as well as the cross cutting fractures in the lower part of the southwestern wall a mathematical analysis shows that a cryomagmatic plume diapir could have both reached the height necessary to cause fracturing within occator and that the stresses required are reasonable expansion a lobate flow from beneath is proposed to have formed those linear fractures associated with the vinalia faculae circumferential fractures surrounding occator appear to have formed due to the volumetric compaction of the ejecta blanket over the buried pre existing topography

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

central pit craters on ganymede central pit craters are common on mars ganymede and callisto and thus are generally believed to require target volatiles in their formation the purpose of this study is to identify the environmental conditions under which central pit craters form on ganymede we have conducted a study of 471 central pit craters with diameters between 5 and 150 km on ganymede and compared the results to 1604 central pit craters on mars diameter range 5 160 km both floor and summit pits occur on mars whereas floor pits dominate on ganymede central peak craters are found in similar locations and diameter ranges as central pit craters on mars and overlap in location and at diameters 60 km on ganymede central pit craters show no regional variations on either ganymede or mars and are not concentrated on specific geologic units central pit craters show a range of preservation states indicating that conditions favoring central pit formation have existed since crater retaining surfaces have existed on ganymede and mars central pit craters on ganymede are generally about three times larger than those on mars probably due to gravity scaling although target characteristics and resolution also may play a role central pits tend to be larger relative to their parent crater on ganymede than on mars probably because of ganymede s purer ice crust a transition to different characteristics occurs in ganymede s icy crust at depths of 4 7 km based on the larger pit to crater diameter relationship for craters in the 70 130 km diameter range and lack of central peaks in craters larger than 60 km diameter we use our results to constrain the proposed formation models for central pits on these two bodies our results are most consistent with the melt drainage model for central pit formation

implications of using broadband photometry for compositional remote sensing of icy objects water ice has such a low absorption coefficient at visual wavelengths 0 01 cm 1 that a very small amount of particulate material can significantly darken an icy surface a variety of ice plus particle mixtures were studied to show that particulate contaminations of 1 by weight even 0 1 or less in some cases in ice or frosts result in reflectance levels close to that of the contaminants in a very clear ice no bubbles it is plausible to have a reflectance 0 05 for particulate contaminations 10 7 by weight for submicron dark particles such as carbon lampblack scattering conditions compete for domination with contaminants for control of visual reflectance implying that the apparent reflectivity level and color of a surface is a poor indicator of ice content a dark surface e g albedo 0 05 does not necessarily imply that there us very little water ice present infrared jhk colors of water ice and other minerals including ice mineral mixtures show that some orthopyroxenes can have jhk colors very similar to fine grained water frosts in general it is possible that the jhk colors of an ice plus particulate mixture can fall anywhere in the classical j h versus h k diagram thus the diagram cannot be used to distinguish a predominately rock surface from one which is predominantely ice for one specific case an important exception is the case where both the j h and h k colors are 0 2 it appears that such colors indicate a relatively pure icy surface in some cases the diagram might be used as a statistical tool to distinguish between the compositions of surfaces within a class of objects but the validity of such comparisons decreases for different classes such as the main belt asteroids when compared to outer solar system satellites where water ice is more stable

morphology and scaling of ejecta deposits on icy satellites 1 continuous ejecta deposits on ganymede consist of two major units or facies a thick inner hummocky pedestal facies and a relatively thin outer radially scoured facies defined also by the inner limit of the secondary crater field both ejecta facies have a well defined power law relationship to crater diameter for craters ranging from 15 to 600 km across this relationship can be used to estimate the nominal crater diameter for impact features on icy satellites such as palimpsests and multiring basins for which the crater rim is no longer recognizable ejecta deposits have also been mapped on 4 other icy satellites although morphologically similar to eject deposits on the moon ejecta deposits for smaller craters are generally significantly broader in extent on the icy satellites in apparent defiance of predictions of self similarity a greater degree of rim collapse and enlargement on the moon may explain the observed difference

modeling the morphological diversity of impact craters on icy satellites abstract impact craters on icy satellites display a wide range of morphologies some of which have no counterpart on rocky bodies numerical simulation studies have struggled to reproduce the diversity of features such as central pits and transitions in crater depth with increasing diameter observed on the icy galilean satellites the transitions in crater depth at diameters of about 26 and 150 km on ganymede and callisto have been interpreted as reflecting subsurface structure using the cth shock physics code we model the formation of craters with diameters between 400 m and about 200 km on ganymede using different subsurface temperature profiles our calculations include recent improvements in the model equation of state for h 2 o and quasi static strength parameters for ice we find that the shock induced formation of dense high pressure polymorphs ices vi and vii creates a gap in the crater excavation flow which we call discontinuous excavation for craters larger than about 20 km discontinuous excavation concentrates a hot plug of material 270 k and mostly on the melting curve in the center of the crater floor the size and occurrence of the hot plug are in good agreement with the observed characteristics of central pit craters and we propose that a genetic link exists between them we also derive depth versus diameter curves for different internal temperature profiles in a 120 k isothermal crust calculated craters larger than about 30 km diameter are deeper than observed and do not reproduce the transition at about 26 km diameter calculated crater depths are shallower and in good agreement with observations between about 30 and 150 km diameter using a warm thermal gradient representing a convective interior hence the depth to diameter transition at about 26 km reflects thermal weakening of ice finally simulation results generally support the hypothesis that the anomalous interior morphologies for craters larger than 100 km are related to the presence of a subsurface ocean

dome craters on ganymede voyager observations of ganymede show broad high albedo topographic domes situated within the central pits of some impact craters referred to in this study as dome craters of 56 dome craters identified on ganymede all but two can be placed into one of two classes based on the ratio of dome diameter to crater rim diameter two new hypotheses for the origin of the domes involving diapirism as an agent of dome formation are offered implicit in both hypotheses are possible regional heat flux variations under these scenarios relaxation of crater relief may not be homogeneous in space and or time and crater morphology may not be a consistent indicator of crater age plutonic intrusions within the upper lithospheres of ganymede and callisto may have played a far more important role in heat transport on these satellites than previously noted

volcanic constructs on ganymede and enceladus topographic evidence from stereo images and photoclinometry paul m schenklunar and planetary institute houston texasjeffrey m moorespace sciences division and the seti institute nasa ames research center moffett field californiaabstract the morphology of volcanic features on ganymede differs significantly from that onthe terrestrial planets few if any major volcanic landforms such as thick flows or shieldvolcanoes have been identified to date using new stereo voyager images we have searchedganymede for relief generating volcanic constructs we observed seven major types of volcanicstructures including several not previously recognized the oldest are broad flat topped domespartially filling many older craters in dark terrain similar domes occur on enceladus togetherwith smooth dark deposits these domes indicate that the volcanic history of the dark terrain iscomplex bright terrain covers vast areas although the style of emplacement remains unclear smooth bright materials embay and flood older terrains and may have been emplaced as low viscosity fluids associated with smooth bright material are a number of scalloped shaped semi enclosed scarps that cut into preexisting terrain in planform these structures resemble terrestrialcalderas the youngest volcanic materials identified are a series of small flows that may haveflooded the floor of the multiring impact structure gilgamesh forming a broad dome theidentification of volcanic constructs up to 1 km thick is the first evidence for extrusion ofmoderate to high viscosity material on ganymede viscosity and yield strength estimates for thesematerials span several orders of magnitude indicating that volcanic materials on ganymede have arange of compositions and or were extruded under a wide range of conditions and or eruptive styles introductionthe voyager survey of the outer solar system has revealedan astonishing variety of endogenic landforms on the surfacesof icy satellites despite extensive endogenic resurfacing ofsomc satellites for example ganymede enceladus andtriton volcanism on these surfaces has little topographicexpression we do not see massive volcanic edifices similarto olympus mons or mauna kea even the best putativeexamples of volcanic landforms seen on ariel miranda triton and enceladus exhibit rarely more than 1 2 km ofrelief jankowski and squyres 1988 pozio and kargel 1989 schenk 1991a croft et al 1995 this may berelated to the soft theology of ice see poirier 1982 durham et al 1983 and kirby et al 1987 or torheology and volcanic styles peculiar to icy petrologicsystems e g kargel 1991 kargel et al 1991 well over half of the surface of ganymede radius 2634 kin has been covered by endogenic effusions it is now consideredlikely that ganymede experienced one or more episodes ofchaotic orbital behavior malhotra 1991 possiblyresulting in tidal heating volcanic resurfacing constitutes arecord of ganymede s compositional and thermal evolution andmay record some of these putative tidal heating events yetthis satellite has been the subject of morphological study for15 years without the unambiguous detection of relief copyright 1995 by the american geophysical union paper number 95je01854 0148 0227 95 95je 01854505 00generating volcanic constructions on its landscape the styleof emplacement and composition of those volcanic depositsthat have been identified remains unclear on any planetary surface volcanic and other nonimpactconstructs can be difficult to recognize in spacecraft images particularly those obtained under high sun conditions e g malin 1978 topography is especially low on the galileansatellites and imaging resolution is at best 500 m pixel major local albedo variations on ganymede further complicategeologic and topographic mapping with these problems inmind we have undertaken the production of stereo pairs ofvoyager images where coverage permits to aid in the searchfor and characterization of volcanic landforms on ganymede ssurface in addition to an examination of previously identifiedvolcanic materials we report the discovery of severalpreviously unrecognized volcanic landforms and at least twotypes with substantial relief and describe their morphologyand geologic setting morphology of volcanic depositsindicates that ganymede extrusions probably had a range oftheologies and may be compositionally variable we concludewith speculations and predictions regarding their mode ofemplacement and the implications of volcanic landforms forthe geologic evolution of ganymede stereo imaging with voyagerthis study was motivated by the recognition that asignificant fraction of the surfaces of most of the outer planetsatellites were imaged by voyager in stereo e g croft andsoderblom 1991 schenk and moore 1993 we can usestereo imaging to detect subtle topographic features that were19 009

the galilean satellites nasa s galileo mission to jupiter and improved earth based observing capabilities have allowed major advances in our understanding of jupiter s moons io europa ganymede and callisto over the past few years particularly exciting findings include the evidence for internal liquid water oceans in callisto and europa detection of a strong intrinsic magnetic field within ganymede discovery of high temperature silicate volcanism on io discovery of tenuous oxygen atmospheres at europa and ganymede and a tenuous carbon dioxide atmosphere at callisto and detection of condensed oxygen on ganymede modeling of landforms seen at resolutions up to 100 times as high as those of voyager supports the suggestion that tidal heating has played an important role for io and europa

on lava dome growth with application to the 1979 lava extrusion of the soufriere of st vincent abstract a theoretical analysis is presented for the spread of a viscous liquid flowing under its own hydrostatic pressure on a horizontal surface in order to model lava dome formation two situations are considered in detail the spreading of a constant volume of liquid and the case where the amount of liquid is continually increased experiments with silicone liquids show close agreement with theory the formation of a basaltic andesite lava extrusion in 1979 on the crater floor of the soufriere of st vincent west indies provided the motivation for and an application of the model the extrusion reached a diameter of 868 m and a height of 133 m over a period of 150 days over the first 90 days the growth relationships were consistent with those predicted by theory application of the theory to the soufriere dome suggests an effective viscosity of 2 x 10 12 poise for the basaltic andesite lava the large effective viscosity calculated for the lava may be attributed to the dominant influence of a high viscosity skin which forms at the margins of the flow as it cools after 70 days the rate of growth of the extrusion markedly decreased because a substantial collar of rubble accumulated at the flow front due to this collar the growth of the extrusion ceased after 150 days from approximately two weeks after the initiation of the extrusion the discharge rate of lava decreased approximately linearly with increasing dome height this observation suggests that the lava ascended under a decreasing hydrostatic driving pressure and that extrusion ceased when the lava column reached hydrostatic equilibrium

the origin of lunar concentric craters abstract lunar concentric craters are a unique class of impact craters because the interior of the craters contains a concentric ridge but their formation mechanism is unknown in order to determine the origin of concentric craters we examined multiple working hypotheses which include eight impact related and endogenic processes we analyzed data sets that originated from instruments onboard clementine kaguya and the lunar reconnaissance orbiter to characterize the morphology spatial distribution composition and absolute model ages of 114 concentric craters concentric craters contain five key properties 1 a concentric ridge 2 anomalously shallow floors 3 their occurrence is concentrated near mare margins and in mare pond regions 4 the concentric ridge composition is similar to the surrounding area and 5 concentric crater ages are eratosthenian and older these five key properties served as constraints for testing impact related and endogenic mechanisms of formation we find that most impact related hypotheses cannot explain the spatial and age distribution of concentric craters as for endogenic hypotheses we deduce that igneous intrusions are the likely mechanism that formed concentric craters because of the close relationship between concentric craters and floor fractured craters and the concentration of both features near mare highland boundaries and in mare ponds furthermore we observe that floor fractured craters are common at crater diameters 15 km whereas concentric craters are common at crater diameters 15 km produce floor fractured craters

shortwave infrared spectral analysis of hydrothermal alteration associated with the pebble porphyry copper gold molybdenum deposit iliamna alaska the pebble cu au mo porphyry deposit is located approximately 320 km southwest of anchorage alaska shortwave infrared swir spectroscopy on drill core from the deposit has been used to document the distribution of alteration assemblages characterized by subtle variations in phyllosilicate minerals that cannot be confidently distinguished by visual criteria alone at pebble these phyllosilicate alteration types have different histories of metal introduction and or redistribution delineation of the distribution of these assemblages is critical to the geologic and genetic interpretations of the deposit spectral absorption features between 1 300 and 2 500 nm in particular small shifts in the position of the absorption wavelength related to aloh bonds around 2 200 nm allows distinction among illite sericite kaolinite and pyrophyllite bearing alteration assemblages electron microprobe and x ray diffraction analyses were used to validate the chemical composition and crystallinity of the phyllosilicate minerals identified using spectral data the results confirm the use of swir spectroscopy to confidently identify and spatially delineate phyllosilicate alteration assemblages at pebble these alteration types include potassic illite kaolinite quartz illite pyrite sericite pyrophyllite quartz sericite pyrite and sodic potassic assemblages the highest gold and copper concentrations within the deposit are in the eastern pluton and are coincident with low aloh values associated with pyrophyllite and sericite alteration an additional zone of low aloh values not associated with high metal grades occurs in the northeast along the margins of the deposit coincident with quartz sericite pyrite alteration the approach described here has significantly improved three dimensional alteration mapping and shows that short wave infrared spectroscopy may successfully distinguish variations in phyllosiclicate species this has implications for exploration because clay speciation is genetically related to the distribution of metals in the pebble deposit the recognition and utilization of these relationships has produced a robust three dimensional alteration model which can be applied to optimizing mine planning comminution and mineral process design

composition of the lunar highland crust from near infrared spectroscopy near infrared reflectance spectra of the lunar highlands are synthesized to obtain global information about the composition of the lunar near side crust immature exposed surfaces from small fresh craters mountains and massifs and large craters with central peaks are included in a survey of rock types across the lunar near side compositions recognized in these data include anorthosite noritic gabbric and troctolitic mineral assemblages three quarters of the areas studied that represent samples from the upper 1 2 km of lunar crust exhibit noritic compositions with different amounts of pyroxene and or brecciation alteration norites are ubiquitous across the near side and show no spatial clustering associated with any of the major basins in contrast to this noritic megaregolith of the upper crust material representing stratigraphically deeper zones 5 15 km of the lunar crust is dominated by gabbros anorthosites and troctolites with less than a quarter of the areas studied exhibiting a noritic composition the origin of the noritic megaregolith and its relation to deeper crustal material is thus somewhat of an enigma the current limited statistics for areas identified as anorthosites and troctolites are insufficient to derive global spatial distributions gabbroic areas however are notably concentrated in the western hemisphere although incomplete for a global assessment of crustal materials these data demonstrate that the lunar near side crust is clearly mineralogically heterogeneous both laterally and vertically and not well mixed below 1 2 km

the theoretical plausibility of central pit crater formation via melt drainage central pit craters are seen in large craters on some icy satellites and on mars we investigate the hypothesis that central pits form when impact melt drains into fractures beneath the impact crater for this process to occur the volume of melt generated during the impact the volume of void space in fractures beneath the impact crater and the volume of melt able to drain before the fractures freeze shut all must exceed the volume of the observed central pits we estimate the volume of melt generated using results from previous numerical modeling studies the fracture volume is estimated using gravity anomalies over terrestrial craters to estimate the amount of melt able to drain before freezing we consider flow through plane parallel fractures these calculations all suggest that enough liquid water could drain into fractured ice beneath a crater on ganymede to form a central pit on earth and the moon silicate impact melt will freeze before a large volume is able to drain so we do not expect to see central pits in impact craters in targets with no ice in summary we find our calculations are consistent with observed central pits in craters on ganymede and the lack of central pits in craters on earth and the moon

impact craters in the northern hemisphere of mars layered ejecta and central pit characteristics mars global surveyor mgs and mars odyssey data are being used to revise the catalog of large martian impact craters analysis of data in the revised catalog provides new details on the distribution and morphologic details of 6795 impact craters in the northern hemisphere of mars this report focuses on the ejecta morphologies and central pit characteristics of these craters the results indicate that single layer ejecta sle morphology is most consistent with impact into an ice rich target double layer ejecta dle and multiple layer ejecta mle craters also likely form in volatile rich materials but the interaction of the ejecta curtain and target produced vapor with the thin martian atmosphere may be responsible for the large runout distances of these ejecta pancake craters appear to be a modified form of double layer craters where the thin outer layer has been destroyed or is unobservable at present resolutions pedestal craters are proposed to form in an ice rich mantle deposited during high obliquity periods from which the ice has subsequently sublimated central pits likely form by the release of vapor produced by impact into ice soil mixed targets therefore results from the present study are consistent with target volatiles playing a dominant role in the formation of crater morphologies found in the martian northern hemisphere

the geology of callisto the geology of callisto is not boring although cratered terrain dominates callisto a key end member of the jovian satellite system a number of more interesting features are apparent cratered terrain is broken into irregular mappable bright and dark subunits that vary in albedo by a factor of 2 and several relatively smooth units are depleted of small craters some of these areas may have been volcanically resurfaced lineaments including parallel and radial sets may be evidence for early global tectonism frost deposition occurs in cold traps and impact scars have formed from tidally disrupted comets geologic evidence suggests that callisto does have a chemically differentiated crust central pit and central dome craters and palimpsests are common the preferred interpretation is that a relatively ice rich material at depths of 5 km or more has been mobilized during impact and exposed as domes or palimpsests the close similarity in crater morphologies and dimensions indicates that the outermost 10 km or so of callisto may be as differentiated as on ganymede the geology of cratered terrain on callisto is simpler than that of cratered terrain on ganymede however orbital evolution and tidal heating may provide the answer to the riddle of why callisto and ganymede are so different malhotra 1991 we should expect a few surprises and begin to answer some fundamental questions when callisto is observed by galileo in late 1996

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

pristine impact crater morphology on pluto expectations for new horizons abstract this paper combines previous cratering studies and numerical modeling of the impact process at different impact velocities to predict crater morphology on pluto as an icy body pluto s craters are expected to be similar in morphology to those on the icy satellites lesser depth diameter ratios d d shallower wall slopes and the development of central uplifts in craters of smaller rim to rim diameter than craters on rocky bodies of similar gravity the low impact velocity of the pluto system 2 km s 1 might cause deviation from this generalization as the simulations presented in this work suggest that decreasing impact velocity from 10 km s 1 to 2 km s 1 results in deeper craters larger d d and a simple to complex transition diameter at larger crater sizes than predicted based on gravity scaling alone d 6 km conversely decreasing impact velocity from 2 km s 1 to 300 m s 1 produced smaller d d akin to the lower d d noted for secondary craters this complex relationship between impact velocity and d d suggests that there might be a larger range of pristine simple crater depths on pluto than on bodies with higher mean impact velocity the low impact velocities and correspondingly low volumes of impact melt generated at pluto might prevent the occurrence or limit the size of floor pits if their formation involves impact melt water the presence or not of central floor pit craters on pluto will thus provide a valuable test of floor pit formation theories the presence of summit pits or concentric craters on pluto is plausible and would indicate the presence of layering in the near sub surface palimpsests multi ring basins and other crater morphologies associated with high heat flow are not expected and would have important implications for pluto s thermal history if observed by new horizons