differential cratering of synchronously rotating satellites by ecliptic comets abstract we use monte carlo methods to simulate impacts of ecliptic comets on the synchronously rotating satellites of giant planets we reconfirm the long standing prediction that the cratering rate should be much higher on the leading hemispheres than on the trailing hemisphere indeed we find that previously published analytical formulations modestly underestimate the degree of apex antapex asymmetry to be expected we then compare our results to new mapping of impact craters on ganymede callisto and triton ganymede reveals a pronounced apex antapex asymmetry that is nonetheless much less than predicted all of triton s confirmed impact craters are clustered toward the apex of motion far exceeding the predicted asymmetry no asymmetry is observed on callisto in each case at least one of our basic assumptions must be wrong likely candidates include the following i the surfaces of all but the most sparsely cratered satellites are saturated or nearly saturated with impact craters ii these satellites have rotated nonsynchronously over geological time iii most of the craters are made not by heliocentric sun orbiting comets and asteroids but rather by planetocentric planet orbiting debris of indeterminate origin or iv pathological endogenic resurfacing has created illusions of structure callisto s surface is readily classified as nearly saturated ganymede s bright terrains although less heavily cratered than those of callisto can also be explained by crater densities approaching saturation on a world where endogenic processes were active the leading alternative is nonsynchronous rotation an explanation supported by the distribution of catenae crater chains produced by impact of tidally disrupted comets triton s craters can be explained by planetocentric debris or by capricious resurfacing but both hypotheses are inherently improbable

a new look at the saturn system the voyager 2 images voyager 2 photography has complemented that of voyager i in revealing many additional characteristics of saturn and its satellites and rings saturn s atmosphere contains persistent oval cloud features reminiscent of features on jupiter smaller irregular features track out a pattern of zonal winds that is symmetric about saturn s equator and appears to extend to great depth winds are predominantly eastward and reach 500 meters per second at the equator titan has several haze layers with significantly varying optical properties and a northern polar collar that is dark at short wavelengths several satellites have been photographed at substantially improved resolution enceladus surface ranges from old densely cratered terrain to relatively young uncratered plains crossed by grooves and faults tethys has a crater 400 kilometers in diameter whose floor has domed to match tethys surface curvature and a deep trench that extends at least 270 around tethys circumference hyperion is cratered and irregular in shape iapetus bright trailing hemisphere includes several dark floored craters and phoebe has a very low albedo and rotates in the direction opposite to that of its orbital revolution with a period of 9 hours within saturn s rings the birth of a spoke has been observed and surprising azimuthal and time variability is found in the ringlet structure of the outer b ring these observations lead to speculations about saturn s internal structure and about the collisional and thermal history of the rings and satellites

a geophone subarray beam steering process the outputs of geophone array elements are conventionally summed into a single output trace this summation attenuates incoherent noise horizontally propagating surface waves and obliquely incident events the geophone subarray beam steering process is a plane wave stacking technique which removes the differential moveout and improves the resolution of seismic data by directing the subarray peak gain to the incident angle of the seismic wavefront the plane wave stacking process transforms the data from the offset domain to the ray parameter p domain and restricts the range of p as a function of time studies of synthetic and marine field data show that the beam steering process improves the signal to noise ratio of obliquely incident events as compared to conventional subarray summing operations the beam steering process compressing high density data while preserving the high frequency content of the seismic signal is a cost effective technique to process large quantities of closely spaced seismi

fates of satellite ejecta in the saturn system abstract we assess the fates of ejecta from the large craters aeneas on dione and ali baba on enceladus 161 and 39 km in diameter respectively as well as that from herschel 130 km in diameter on mimas the ejecta are treated either as spalls launched from hard surfaces or as rubble launched from a weak rubble pile regolith once in orbit we consider the ejecta as massless test particles subject to the gravity of saturn and its classical satellites the great majority of escaped ejecta get swept up by the source moons the best fit to the ejecta population decay is a stretched exponential with exponent near 1 2 dobrovolskis et al icarus 188 481 505 2007 we bracket the characteristic ejecta sizes corresponding to grady kipp fragments and spalls based on this and computed impact velocities and incidence angles the resulting sesquinary craters if they exist should have diameters on the order of a few meters to a few km the observed longitude distribution of small craters on mimas along with the findings of bierhaus et al that small moons should not have a secondary crater population icarus 218 602 621 2012 suggest that the most likely place to find sesquinary craters in the saturn system is the antapex of mimas

split comets and the origin of crater chains on ganymede and callisto when the voyager 1 spacecraft flew through the jovian system in january 1979 it returned images of several prominent chains of impact craters on the surface of the moon callisto fig 1 these impressively straight chains or catenae are composed of between 4 and 25 craters and are up to 620 km long they were initially thought to be secondary craters produced by debris from a larger primary impact1 but detailed searches for source craters have been largely unsuccessful a satisfactory explanation for the crater chains has yet to be found inspired by the recent observations of comet shoemaker levy 9 which split into a line of about 20 fragments as it swept past jupiter2 we suggest that the impact of previous split comets might be responsible for at least some of the catenae on callisto in support of this hypothesis we find that nearly all of callisto s crater chains are on the jupiter facing hemisphere as are an additional three catenae that we have found on ganymede we present a simple model of tidal breakup which both reproduces the range of observed chain lengths and indicates that the parent comets responsible for the callisto catenae were typically no more than about 10 km in diameter

nonuniform cratering of the terrestrial planets article history we estimate the impact flux and cratering rate as a function of latitude on the terrestrial planets using a model distribution of planet crossing asteroids and comets bottke w f morbidelli a jedicke r petit j m levison h f michel p metcalfe t s 2002 icarus 156 399 433 after determining the planetary impact probabilities as a function of the relative encounter velocity and encounter inclination the impact positions are calculated analytically assuming the projectiles follow hyperbolic paths during the encounter phase as the source of projectiles is not isotropic latitudinal variations of the impact flux are predicted the calculated ratio between the pole and equator is 1 05 for mercury 1 00 for venus 0 96 for the earth 0 90 for the moon and 1 14 for mars over its long term obliquity variation history by taking into account the latitudinal dependence of the impact velocity and impact angle and by using a crater scaling law that depends on the vertical component of the impact velocity the latitudinal variations of the cratering rate the number of craters with a given size formed per unit time and unit area is in general enhanced with respect to the equator the polar cratering rate is about 30 larger on mars and 10 on mercury whereas it is 10 less on the earth and 20 less on the moon the cratering rate is found to be uniform on venus the relative global impact fluxes on mercury venus the earth and mars are calculated with respect to the moon and we find values of 1 9 1 8 1 6 and 2 8 respectively our results show that the relative shape of the crater size frequency distribution does not noticeably depend upon latitude for any of the terrestrial bodies in this study nevertheless by neglecting the expected latitudinal variations of the cratering rate systematic errors of 20 30 in the age of planetary surfaces could exist between equatorial and polar regions when using the crater chronology method

investigation of crater saturation using spatial statistics abstract conventional techniques for investigation of crater populations make use of size frequency distributions but typically disregard the statistical properties of the spatial distribution of craters cratering of a surface is generally a spatially random process so the distributions of craters on lightly cratered surfaces can be expected to be random as crater obliteration becomes important however the situation changes every random distribution has regions of sparseness and regions of clustering craters that form in areas of sparseness obliterate few older craters so the sparseness is filled in craters that form in areas of clustering obliterate more older craters relatively reducing the clustering as sparseness is filled in and clustering is reduced the spatial distribution should progress from one that is random to one that is more uniform than random we show that the spatial distribution of craters on a lightly cratered lunar maria surface is consistent with randomness but that the distributions on heavily cratered portions of rhea and callisto are markedly more uniform than random we develop a simple model of crater formation and obliteration and use it to investigate the statistical behavior of crater spatial distributions as crater saturation is approached we find that the use of spatial distributions to investigate saturation is more robust to model uncertainties than is the use of crater density alone using a simple statistical parameter the z statistic we find that 25 of the craters formed to date on our study areas on rhea and callisto have been obliterated by subsequent cratering adjusting the z statistic for crater floor area effects we find evidence that crater saturation has been reached on rhea

impact cratering on titan ii global melt escaping ejecta and aqueous alteration of surface organics abstract new three dimensional hydrodynamic simulations of hypervelocity impacts into the crust of titan were undertaken to determine the fraction of liquid water generated on the surface of saturn s largest moon over its history and hence the potential for surface modification of hydrocarbons and nitriles by exposure to liquid water we model in detail an individual impact event in terms of ejecta produced and melt generated and use this to estimate melt production over titan s history taking into account the total flux of the impactors and its decay over time our estimates show that a global melt layer at any time after the very beginning of titan s history is improbable but transient melting local to newly formed craters has occurred over large parts of the surface local maxima of the melt are connected with the largest impact events we also calculate the amount of volatiles delivered at the impact with various impact velocities from 3 km s for possible hyperion fragments to 11 km s for jupiter family comets and their retention as a possible source of titan s atmosphere we find the probability of impact ejecta escaping titan with its modern dense and thick atmosphere is rather low and dispersal of titan organics throughout the rest of the solar system requires impactors tens of kilometers in diameter water ice melting and exposure of organics to liquid water has been widespread because of impacts but burial or obscuration of craters by organic deposits or cryovolcanism is aided by viscous relaxation the largest impactors may breach an ammonia water mantle layer creating a circular albedo contrast rather than a crater

nonuniform cratering of the moon and a revised crater chronology of the inner solar system abstract we model the cratering of the moon and terrestrial planets from the present knowledge of the orbital and size distribution of asteroids and comets in the inner solar system in order to refine the crater chronology method impact occurrences locations velocities and incidence angles are calculated semi analytically and scaling laws are used to convert impactor sizes into crater sizes our approach is generalizable to other moons or planets the lunar cratering rate varies with both latitude and longitude with respect to the global average it is about 25 lower at 65 n 90 e and larger by the same amount at the apex of motion 0 n 90 w for the present earth moon separation the measured size frequency distributions of lunar craters are reconciled with the observed population of near earth objects under the assumption that craters smaller than a few kilometers in diameter form in a porous megaregolith varying depths of this megaregolith between the mare and highlands is a plausible partial explanation for differences in previously reported measured size frequency distributions we give a revised analytical relationship between the number of craters and the age of a lunar surface for the inner planets expected size frequency crater distributions are calculated that account for differences in impact conditions and the age of a few key geologic units is given we estimate the orientale and caloris basins to be 3 73 ga old and the surface of venus to be 240 ma old the terrestrial cratering record is consistent with the revised chronology and a constant impact rate over the last 400 ma better knowledge of the orbital dynamics crater scaling laws and megaregolith properties are needed to confidently assess the net uncertainty of the model ages that result from the combination of numerous steps from the observation of asteroids to the formation of craters our model may be inaccurate for periods prior to 3 5 ga because of a different impactor population or for craters smaller than a few kilometers on mars and mercury due to the presence of subsurface ice and to the abundance of large secondaries respectively standard parameter values allow for the first time to naturally reproduce both the size distribution and absolute number of lunar craters up to 3 5 ga ago and give self consistent estimates of the planetary cratering rates relative to the moon

moon meteoritic seismic hum steady state prediction 1 we use three different statistical models describing the frequency of meteoroid impacts on earth to estimate the seismic background noise due to impacts on the lunar surface because of diffraction seismic events on the moon are typically characterized by long codas lasting 1 h or more we find that the small but frequent impacts generate seismic signals whose codas overlap in time resulting in a permanent seismic noise that we term the lunar hum by analogy with the earth s continuous seismic background seismic hum we find that the apollo era impact detection rates and amplitudes are well explained by a model that parameterizes 1 the net seismic impulse due to the impactor and resulting ejecta and 2 the effects of diffraction and attenuation the formulation permits the calculation of a composite waveform at any point on the moon due to simulated impacts at any epicentral distance the root mean square amplitude of this waveform yields a background noise level that is about 100 times lower than the resolution of the apollo long period seismometers at 2 s periods this noise level is more than 1000 times lower than the low noise model prediction for earth s microseismic noise sufficiently sensitive seismometers will allow the future detection of several impacts per day at body wave frequencies

impact excavation on asteroid 4 vesta hubble space telescope results hubble space telescope images of asteroid 4 vesta obtained during the favorable 1996 apparition show an impact crater 460 kilometers in diameter near the south pole color measurements within the 13 kilometer deep crater are consistent with excavation deep into a high calcium pyroxene rich crust or olivine upper mantle about 1 percent of vesta was excavated by the crater formation event a volume sufficient to account for the family of small vesta like asteroids that extends to dynamical source regions for meteorites this crater may be the site of origin for the howardite eucrite and diogenite classes of basaltic achondrite meteorites

did a large impact reorient the moon abstract the moon is currently locked in a spin orbit resonance of synchronous rotation of which one consequence is that more impacts should occur near the moon s apex of motion 0 n 90 w than near its antapex of motion 0 n 90 e several of the largest lunar impact basins could have temporarily unlocked the moon from synchronous rotation and after the re establishment of this state the moon would have been left in either its initial orientation or one that was rotated 180 about its spin axis we show that there is less than a 2 probability that the oldest lunar impact basins are randomly distributed across the lunar surface furthermore these basins are preferentially located near the moon s antapex of motion and this configuration has less than a 0 3 probability of occurring by chance we postulate that the current near side of the moon was in fact its far side when the oldest basins formed one basin with the required size and temporal characteristics to account for a 180 reorientation is the smythii basin

the lunar rayed crater population characteristics of the spatial distribution and ray retention abstract the global statistics for young impact craters on the moon is used to unravel potential spatial asymmetries that may have been introduced by the particular orbital configuration of a synchronously rotating satellite only craters that exhibit bright ejecta rays extending for several crater radii were considered in this study this crater population is younger than about 750 ma the shape of the crater size frequency distribution does not show strong dependence on the target properties mare vs highlands however slightly lower frequencies indicate a shorter retention of the visibility of rays in mare units when their visibility is purely due to immaturity and not due to composition rays of small craters fade away much faster large old rayed craters sustain their visibility longer than the average crater population because of the compositional contrast between rays and mare material and thus obscure the cratering record when investigated for spatial variations using the existence of rays purely based on optical maturity instead of visibility as marker horizon for the copernican eratosthenian boundary suggests a shift from 1 1 ga to 750 ma the spatial distribution of lunar rayed craters namely the latitudinal and longitudinal frequency variations does not agree with previous analytical and numerical studies although there is an apparent hemispherical asymmetry centred close to the apex the density distribution is patchy and no predicted spatial pattern could be confirmed spatial distribution corrections accounting for the lower frequencies in the mare areas did not result in a better agreement with the analytical estimates density variations are less than 15 over vast parts of the lunar surface and the uncertainties for absolute surface ages are similar however variations of up to 50 are found even for the more numerous small craters these extreme values are located at high latitudes a combination of crater forming projectile flux distribution and micrometeorite bombardment which acts on maturation of the ray systems could prove as an explanation for the contradicting observed rayed crater distribution an analysis of the older craters is more challenging on the moon because earlier geological processes complicate the setting and the orbital configuration of the moon earth sun projectile system altered with time

reorientation of icy satellites by impact basins 1 large impact basins are present on many of the icy satellites of the outer solar system assuming that their present day topography is uncompensated such basins can cause significant poleward reorientations for slow rotating satellites this reorientation may have been accompanied by transient large amplitude wobble the largest basins on tethys rhea and titania are predicted to have caused reorientations of roughly 4 7 and 12 respectively resulting in global tectonic stresses up to 0 5 mpa the potential anomalies associated with the basins can be up to one third of those expected for a hydrostatic tidally and rotationally deformed body and may complicate interpretation of the satellite interior structure pluto and charon because of their slow rotation are also likely to have undergone reorientation of 10 20 if they possess impact basins of comparable sizes to those of the saturnian satellites

global atmospheric loading and gravity abstract the purpose of this paper is to model atmospheric pressure effects on surface gravity pressure corrections are usually done with the help of an empirical trans fer function called barometric admittance between pressure and gravity both measured locally we show the validity of this local approach with the help of spectral coherency between air pressure and gravity residuals after subtraction from the superconducting gravimeter observations in strasbourg france of luni solar tides polar motion and instrumental drift contributions we compute the atmospheric surface loading effects using a green s function formalism and global surface pressure data provided by the european centre for medium range weather forecasts ecmwf we model a specific oceanic response to atmospheric loading which incorporates interactions between ocean atmosphere and solid earth and differs from the classical inverted or non inverted barometer hypotheses we investigate the contribution of this global pressure correction in a 3000 day 1987 1996 tidal analysis of the french superconducting gravimeter

cometary nuclei and tidal disruption the geologic record of crater chains on callisto and ganymede prominent crater chains on ganymede and callisto are most likely the impact scars of comets tidally disrupted by jupiter and are not secondary crater chains we have examined the morphology of these chains in detail in order to place constraints on the properties of the comets that formed them and the disruption process in these chains intercrater spacing varies by no more than a factor of 2 and the craters within a given chain show almost no deviation from linearity although the chains themselves are on gently curved small circles all of these crater chains occur on or very near the jupiter facing hemisphere for a given chain the estimated masses of the fragments that formed each crater vary by no more than an order of magnitude the mean fragment masses for all the chains vary by over four orders of magnitude w b mckinnon and p m schenk 1995 geophys res lett 13 1829 1832 however the mass of the parent comet for each crater chain is not correlated with the number of fragments produced during disruption but is correlated with the mean mass of the fragments produced in a given disruption event also the larger fragments are located near the center of each chain all of these characteristics are consistent with those predicted by disruption simulations based on the rubble pile cometary nucleus model in which nuclei are composed on numerous small fragments weakly bound by self gravity and with those observed in comet d shoemaker levy 9 similar crater chains have not been found on the other icy satellites but the impact record of disrupted comets on callisto and ganymede indicates that disruption events occur within the jupiter system roughly once every 200 to 400 years

asymmetrical distribution of rayed craters on the moon the synchronous rotation of the satellite ought to cause a spatial variation in the cratering rate over its surface the crater density is expected to be maximum at the apex of the orbital motion and decrease with the increase of the angular distance from the apex the ratio of the density at the apex maximum to that of the antapex minimum depends on the average encounter velocity of impactors to the satellite although the moon is also in a state of the synchronous rotation it has been supposed that the asymmetry in the crater density on the moon can be hardly observed we report here a spatial variation in the density of rayed craters on the moon which may be associated with the synchronous rotation since the lifetime of a ray is relatively short 0 8 billion years the results provide information on recent impacts rayed craters are identified on clementine 750 nm mosaic images we investigate craters in a lower latitude zone from 42 n to 42 s to avoid an effect of material difference on the ray preservation we analyze craters on the highland from 70 e to 290 e in east longitude a total of 222 rayed craters larger than 5 km in diameter are identified in the study area of about 1 4 107 km2 the average density of rayed craters on the leading side is substantially higher than that on the trailing side the crater density decreases as a sinusoidal function of the angular distance from the apex the observed ratio of the density at the apex to that at the antapex is about 1 5 the ratio suggests that recent craters on the moon are formed mainly by near earth asteroids rather than comets with higher encounter velocities

low frequency seismic deghosting we evaluated a solution to seismic deghosting that deghosts the low frequency components of the seismic pressure data in an approximation that neglected the dependence on wavenumbers the low frequency deghosted pressure field was computed trace by trace as the sum of the pressure field and its scaled temporally integrated and temporally differentiated fields we gave simple numerical examples that demonstrated the concept the method was found to deghost data up to a frequency that is typically half of the second notch frequency on the low frequency side the deghosting method was limited by the signal to noise ratio the lowfrequency deghosting technique can be appropriate to apply to the part of seismic data that have penetrated and reflected beneath complex and attenuating overburdens such as basalt salt and chalk

distribution of icy particles across enceladus surface as derived from cassini vims measurements abstract the surface of enceladus consists almost completely of water ice as the band depths of water ice absorptions are sensitive to the size of particles absorptions can be used to map variations of icy particles across the surface the visual and infrared mapping spectrometer vims observed enceladus with a high spatial resolution during three cassini flybys in 2005 orbits en 003 en 004 and en 011 based on these data we measured the band depths of water ice absorptions at 1 04 1 25 1 5 and 2 m these band depths were compared to water ice models that represent theoretically calculated reflectance spectra for a range of particle diameters between 2 m and 1 mm the agreement between the experimental vims and model values supports the assumption that pure water ice characterizes the surface of enceladus and therefore that variations in band depth correspond to variations in water ice particle diameters our measurements show that the particle diameter of water ice increases toward younger tectonically altered surface units with the largest particles exposed in relatively fresh surface material the smallest particles were generally found in old densely cratered terrains the largest particles 0 2 mm are concentrated in the so called tiger stripes at the south pole in general the particle diameters are strongly correlated with geologic features and surface ages indicating a stratigraphic evolution of the surface that is caused by cryovolcanic resurfacing and impact gardening

broadband seismic over under sources and their designature deghosting abstractthe source depth influences the frequency band of seismic data due to the source ghost effect it is advantageous to deploy sources deep to enhance the low frequency content of seismic data but for a given source volume the bubble period decreases with the source depth thereby degrading the low frequency content at the same time deep sources reduce the seismic bandwidth deploying sources at shallower depths has the opposite effects a shallow source provides improved high frequency content at the cost of degraded low frequency content due to the ghosting effect whereas the bubble period increases with a lesser source depth thereby slightly improving the low frequency content a solution to the challenge of extending the bandwidth on the low and high frequency side is to deploy over under sources in which sources are towed at two depths we have developed a mathematical ghost model for over under point sources fired in sequential and simultaneous modes and we have found an inverse mode

europa s small impactor flux and seismic detection predictions abstract europa is an attractive target for future lander missions due to its dynamic surface and potentially habitable sub surface environment seismology has the potential to provide powerful new constraints on the internal structure using natural sources such as faults or meteorite impacts here we predict how many meteorite impacts are likely to be detected using a single seismic station on europa to inform future mission planning efforts to this end we derive 1 the current small impactor flux on europa from jupiter impact rate observations and models 2 a crater diameter versus impactor energy scaling relation for icy moons by merging previous experiments and simulations and 3 scaling relations for seismic signal amplitudes as a function of distance from the impact site for a given crater size based on analogue explosive data obtained on earth s ice sheets finally seismic amplitudes are compared to predicted noise levels and seismometer performance to determine detection rates we predict detection of 0 002 20 small local impacts per year based on p waves travelling directly through the ice crust larger regional and global scale impact events detected through mantle refracted waves are predicted to be extremely rare 10 8 1 detections per year so are unlikely to be detected by a short duration mission estimated ranges include uncertainties from internal seismic attenuation impactor flux and seismic amplitude scaling internal attenuation is the most significant unknown and produces extreme uncertainties in the mantle refracted p wave amplitudes our nominal best guess attenuation model predicts 0 002 5 local direct p detections and 6 10 6 0 2 mantle refracted detections per year given that a plausible europa landed mission will only last around 30 days we conclude that impacts should not be relied upon for a seismic exploration of europa for future seismic exploration faulting due to stresses in the rigid outer ice shell is likely to be a much more viable mechanism for probing europa s interior

differential impact cratering of saturn s satellites by heliocentric impactors saturnian satellites are thought to have been struck by two different types of impactors those with heliocentric origins and those with planetocentric origins many of the impacts are suggested to come from planetocentric debris while many crater count studies assume an ecliptic comet origin when determining the ages of the surfaces to assess the contribution of planetocentric impactors this study examines the global distribution and apex antapex asymmetry of impact craters on rhea and iapetus the results demonstrate that the craters of rhea more than 20 km in diameter and iapetus more than 30 km in diameter show an apex antapex asymmetry this suggests that most of the large craters are formed from heliocentric impacts in contrast the craters less than 20 km in diameter seem to show no asymmetry possible explanations for this are either planetocentric impactor origins or saturation with impact craters

rough sea deghosting using a single streamer and a pressure gradient approximation we present a method for receiver ghost correction of towed streamer data that accounts for the rough sea surface the method explicitly uses the fact that the pressure is zero at the free sea surface to estimate the vertical pressure gradient continuous elevation measurements of the wave height directly above the hydrophones are required a measurement which is currently unavailable the new deghosting method is fundamentally limited to frequencies below the first ghost notch the lowest order implementation requires that the streamer is towed no deeper than approximately 6 m and a receiver spatial sampling interval of about 3 m or less using the lowest order and simplest implementation of the new method the rough sea error is reduced from 1 5 2 5 db to about 1 1 5 db in amplitude and from 20 to 10 in phase at 50 hz in a 4 m significant wave height sea higher order terms in the approximation promise to further reduce the error

variable source depth acquisition for improved marine broadband seismic data in marine seismic data acquisition varying the source depth along a sail line gives diversity in sequential shot gather frequency spectra undesired alterations of the frequency spectra are created by the source ghost and by air gun bubble oscillations by deliberately varying the source depth along a sail line it is possible to obtain a seismic data set that will have energy more evenly distributed within the main frequency band of the source output this is obtained when data acquired with different source depths are stacked in imaging we formulated a simple inverse problem that seeks to find the optimal distribution of source depths over a sequential series of shots that shape the amplitude spectrum of the final image into a desired shape we assumed that the data are receiver side deghosted that designature could be applied to each shot gather and that the shot gathers could be redatumed to a common datum prior to imaging

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

ocean worlds in the outer solar system many outer solar system bodies are thought to harbor liquid water oceans beneath their ice shells this article first reviews how such oceans are detected we then discuss how they are maintained when they formed and what the oceans likely characteristics are we focus in particular on europa ganymede callisto titan and enceladus bodies for which there is direct evidence of subsurface oceans we also consider candidate ocean worlds such as pluto and triton

the impact cratering record on triton impact craters on triton are scarce owing to the relatively recent resurfacing by icy melts the most heavily cratered surface has a crater density about the same as the lunar maria the transition diameter from simple to complex craters occurs at a diameter of about 11 kilometers and the depth diameter relationship is similar to that of other icy satellites when gravity is taken into account the crater size frequency distribution has a differential 3 slope cumulative 2 slope and is the same as that for the fresh crater population on miranda the most heavily cratered region is on the leading hemisphere in triton s orbit triton may have a leading trailing asymmetry in its crater population based primarily on the similarity of size distributions on triton and miranda and the relatively young surface on triton the source of triton s craters is probably comets the very peculiar size distribution of sharp craters on the cantaloupe terrain and other evidence suggests they are volcanic explosion craters

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

elevation distribution of titan s craters suggests extensive wetlands abstract using a new global topographic map of titan we find that craters on titan preferentially lie at higher than average elevations we explore several explanations for this observed behavior and judge the most reasonable explanation to be the presence of widespread wetlands of liquid hydrocarbons at low elevations over much of geologic time impacts into a shallow marine environment or a saturated layer of sediments more than several hundred meters thick would produce crater morphologies similar to terrestrial submarine impacts these are known to lack significant topographic expression and would thus be difficult to observe with the cassini spacecraft since titan s near surface methane inventory likely fluctuated over geologic time with episodic delivery and continuous depletion a few craters at low elevations can nonetheless be expected

secondary and sesquinary craters on europa abstract we address impact cratering on io and europa with the emphasis on the origin of small craters on europa as secondary to the primary impacts of comets on io europa and ganymede in passing we also address the origin of secondary craters generated by zunil a well studied impact crater on mars that is a plausible analog to impact craters on io at nominal impact rates and taking volcanic resurfacing into account we find that there should be 1 3 impact craters on io equally likely to be of any diameter between 100 m and 20 km the corresponding model age of europa s surface is between 60 and 100 ma this range of ages does not include a factor three uncertainty stemming from the uncertain sizes and numbers of comets the mass of basaltic impact ejecta from io to reach europa is found to meet or exceed the micrometeoroid flux as a source of rock forming elements to europa s ice crust to describe impact ejecta in more detail we adapt models for impact generated spalls and grady kipp fragments originally developed by melosh our model successfully reproduces the observed size number distributions of small craters on both mars and europa however the model predicts that a significant fraction of the 200 500 m diameter craters on europa are not traditional secondary craters but are instead sesquinary craters caused by impact ejecta from io that had gone into orbit about jupiter this prediction is not supported by observation which implies that high speed spalls usually break up into smaller fragments that make smaller sesquinary craters iogenic basalts are also interesting because they provide stratigraphic horizons on europa that in principle could be used to track historic motions of the ice and they provide materials suitable to radiometric dating of europa s surface

influence of the asymmetrical cratering rate on the lunar cratering chronology the lunar cratering rate studied over the past 1 1 gyr which is a foundation of the lunar cratering chronology is a decreasing function of the angular distance from the apex of the orbital motion due to the synchronous rotation of the moon we here evaluate an influence of the asymmetrical rate upon the age determination

the orbital thermal evolution and global expansion of ganymede article history the tectonically and cryovolcanically resurfaced terrains of ganymede attest to the satellite s turbulent geologic history yet the ultimate cause of its geologic violence remains unknown one plausible scenario suggests that the galilean satellites passed through one or more laplace like resonances before evolving into the current laplace resonance passage through such a resonance can excite ganymede s eccentricity leading to tidal dissipation within the ice shell to evaluate the effects of resonance passage on ganymede s thermal history we model the coupled orbital thermal evolution of ganymede both with and without passage through a laplace like resonance in the absence of tidal dissipation radiogenic heating alone is capable of creating large internal oceans within ganymede if the ice grain size is 1 mm or greater for larger grain sizes oceans will exist into the present epoch the inclusion of tidal dissipation significantly alters ganymede s thermal history and for some parameters e g ice grain size tidal q of jupiter a thin ice shell 5 to 20 km can be maintained throughout the period of resonance passage the pulse of tidal heating that accompanies laplace like resonance capture can cause up to 2 5 volumetric expansion of the satellite and contemporaneous formation of near surface partial melt the presence of a thin ice shell and high satellite orbital eccentricity would generate moderate diurnal tidal stresses in ganymede s ice shell larger stresses result if the ice shell rotates non synchronously the combined effects of satellite expansion its associated tensile stress rapid formation of near surface partial melt and tidal stress due to an eccentric orbit may be responsible for creating ganymede s unique surface features

timing and characteristics of the latest mare eruption on the moon unraveling the timing and duration of mare volcanism on the moon is essential for understanding its thermal evolution the end of mare volcanism is poorly constrained because mare basalts are incompletely sampled in this study employing selene kaguya high resolution images we performed new crater size frequency measurements for 49 young mare units 3 0 ga in the procellarum kreep terrane pkt in which the latest magma eruption of the moon occurred mare volcanism in this region continued until 1 5 ga suggesting that volcanic activity in this region ceased 1 0 ga after the magma eruption had globally ceased 2 5 3 0 ga volcanic activity may have peaked 1 8 2 2 ga ago the youngest basalts occur around the aristarchus plateau and the kepler crater which are located in the central region of the pkt it is likely that heating in the crust due to the concentration of heat producing elements in the pkt delayed cooling of a partial melting zone in the underlying mantle in contrast with previous basalt dating in this region our results indicate a higher correlation between ages and spectral types of mare basalts the young mare units in the pkt tend to have spectral types corresponding to high titanium contents while low titanium basalts occur mainly in the early stage the titanium variation in mare basalts may reflect vertical heterogeneity in tio2 content in the upper mantle beneath the pkt

is it optimal to tow air guns shallow to enhance low frequencies abstractin marine seismic acquisition it used to be commonly accepted that it is optimal to tow the source deep to enhance the low frequency content in the seismic data however mayne and quay found in 1971 that the low frequency response of air guns actually improves as the source depth decreases we evaluated a simple ad hoc theory that demonstrates that two effects are counteracting each other the free surface effect favors deep towed sources whereas the bubble time period increasing with decreasing source depth favors shallower tow depths from a fjord test we found that combining several source depths in an air gun array might flatten and improve the low frequency part of the source spectrum the experiment confirms that various source depths result in local characteristic maxima in the low frequency spectrum corresponding to the bubble time period of the air gun

current bombardment of the earth moon system emphasis on cratering asymmetries abstract we calculate the current spatial distribution of projectile delivery to the earth and moon using numerical orbital dynamics simulations of candidate impactors drawn from a debiased near earth object neo model we examine the latitude distribution of impactor sites and find that for both the earth and moon there is a small deficiency of time averaged impact rates at the poles the ratio between deliveries within 30 of the pole to that of a 30 band centered on the equator is small for earth 0 958 0 001 and somewhat greater for the moon 10 0 903 0 005 the terrestrial arrival results are examined to determine the degree of am pm asymmetry to compare with the pm excess shown in meteorite fall times we find that the average lunar impact velocity is 20 km s which has ramifications in converting observed crater densities to impactor size distributions we determine that current crater production on the leading hemisphere of the moon is 1 28 0 01 that of the trailing when considering the ratio of craters within 30 of the apex to those within 30 of the antapex and that there is virtually no nearside farside asymmetry in agreement with observations of rayed craters as expected the degree of leading trailing asymmetry increases when the moon s orbital distance is decreased

cratering asymmetry on the moon new insight from the apollo passive seismic experiment 1 the synchronous rotation of the moon can lead to an asymmetry in the cratering rate between the leading and trailing sides of lunar orbital motion the observation of the apollo passive seismic experiment can be viewed as a continuous seismic observation of lunar impacts it provides information of the current bombardments and small impact events of meter sized craters to evaluate the current leading trailing cratering asymmetry on the moon we investigated the spatial distribution of impact sites determined by apollo seismic observation we determined that the number density of impacts on the leading side is substantially higher than that on the trailing side in addition we discovered a possible enhancement of the number density of 1 4 0 4 to 1 9 0 5 on the leading side relative to that of the trailing side this indicated that the cratering asymmetry predicted from historical cratering records also exists in the current low magnitude impacts

crater modification and geologic activity in enceladus heavily cratered plains evidence from the impact crater distribution abstract although we can observe current activity on saturn s satellite enceladus with cassini insight into past activity is best achieved for now through studying the impact crater distributions furthermore approximation of terrain ages can only be attained through calculations using crater densities and estimations of impact rates in the saturnian system here we focus on what the impact crater distribution in enceladus heavily cratered plains can tell us about enceladus geologic history we use cassini iss images to count craters in the heavily cratered plains on enceladus along with rhea dione tethys and mimas as references to develop and compare their size frequency distributions comparisons of our counts show that enceladus cratered plains distribution is unique in that it appears to have a relative deficiency of craters for diameters 2 km and 6 km compared to the other satellites heavily cratered plains our data also indicates that the impact crater density within the cratered plains changes with latitude specifically both the north and south mid latitude regions have approximately three times higher density than the equatorial region we hypothesize that the missing small and large craters in enceladus cratered plains is due to a combination of viscous relaxation of the larger craters and burial of the relaxed large craters and small craters by south polar plume and possibly e ring material we also conclude that the spatial density distribution is not consistent with recent polar wander