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

the propagation of internal solitary waves over variable topography in a horizontally two dimensional framework abstractthis paper presents a horizontally two dimensional theory based on a variable coefficient kadomtsev petviashvili equation which is developed to investigate oceanic internal solitary waves propagating over variable bathymetry for general background density stratification and current shear to illustrate the theory a typical monthly averaged density stratification is used for the propagation of an internal solitary wave over either a submarine canyon or a submarine plateau the evolution is essentially determined by two components nonlinear effects in the main propagation direction and the diffraction modulation effects in the transverse direction when the initial solitary wave is located in a narrow area the consequent spreading effects are dominant resulting in a wave field largely manifested by a significant diminution of the leading waves together with some trailing shelves of the opposite polarity on the other hand if the initial solitary wave is uniform in the transverse direction t

mercury s global contraction much greater than earlier estimates observations of compressional structures on mercury have fallen short of accommodating the global contraction that is required owing to cooling of the planet s interior mapping of folds and faults across mercury s surface using messenger spacecraft images reveals deformation consistent with a planet that has contracted radially as much as seven kilometres over its history

floor subsidence and rebound of large venus craters the topography and geology of large craters on venus reveal no evidence for floor rebound or relaxation rather the floors have subsided depressions of the floor centers relative to the margins are evident in topography and floor faulting is interpreted as contractional failure of the likely processes responsible only subsolidus thermal contraction applies to craters that both have and have not been infilled by lavas assuming volcanism occurred within a few tens of millions of years after the impact thermal subsidence satisfies the measured floor depressions for reasonable scaling of impact energies and temperature distributions in the lithosphere further the predicted stresses are generally consistent with observed floor fracturing we constrain the impact heat deposited in the lithosphere to be less than roughly 5 1023 j for diameters of 100 km the absence of perceptible floor subsidence at craters this size on the moon and icy satellites is readily explained by the scaling dependence of impact energy not only on transient crater diameter but also on gravity and target density the unfractured melt sheets of three large young bright floored craters imply sufficient lithospheric rigidity to support the crater cavities an elastic flexural rebound model restricts the elastic plate thickness to at least 10 15 km for the three craters corresponding to maximum geotherms of 20 30 k km 1 structural evidence for early rebound in older dark floored craters may have been buried by subsequent volcanism

role of crustal flexure in initiation of low angle normal faults and implications for structural evolution of the basin and range province the mechanics of low angle normal faulting has proved difficult to establish despite clear geological evidence for the existence of such faults gently dipping shear stress trajectories are necessary for nucleation and propagation of low angle faults in a structurally isotropic medium such trajectories are not produced by simple horizontal extension but do arise in the presence of flexural stresses we model stress conditions in a cross section through the strong upper crust using airy stress functions for a two dimensional elastic medium numerical modeling indicates that high ratios of flexural stress to extensional stress produce conditions favorable for low angle normal fault initiation at intermediate depths in an elastic medium we propose that the gentle dip of cenozoic low angle normal faults in the basin and range province resulted from flexural stresses that were produced by isostatically uncompensated surface and especially moho relief resurgent mid cenozoic magmatism and the consequent reduction of the thermally dependent flexural strength of the mantle lithosphere transferred flexural stresses to the upper crust causing or promoting initiation of low angle normal faults temporal changes in structural style from dominantly low angle to dominantly high angle normal faulting may reflect reduction of the magnitude of flexural stresses in the upper crust due to flattening of the moho and cooling of the mantle lithosphere

timing of the faulting on the wispy terrain of dione based on stratigraphic relationships with impact craters the trailing hemisphere of dione is characterized by the wispy terrain where it exhibits a hemispheric scale network of extensional tectonic faults superposed on the moon s cratered surface the faults likely reflect past endogenic activity and dione s interior thermal history although fresh exposures of pristine scarps indicate that the timing of the faulting is relatively recent the absolute age of the faulting remains uncertain to estimate the timing of the faulting we investigated stratigraphic relationships between impact craters and faults using high resolution images obtained by iss cameras onboard the cassini spacecraft we investigated craters with diameters exceeding or equal to 10 km that coincide spatially with the faults and classified the craters as crosscut craters or superposed craters as a result at least 82 of the craters were interpreted as clear examples of crosscut craters and 12 of the craters were interpreted to be candidates of superposed craters although stratigraphic relationships are often ambiguous the paucity of superposed craters and a predicted cratering rate indicate that the faulting of the wispy terrain is 0 30 0 79 ga if 12 18 of the craters are assumed to be superposed the timing of the faulting could be in the range 0 30 0 79 ga however it is possible that the faulting of the wispy terrain is still ongoing

duration of activity on lobate scarp thrust faults on mercury lobate scarps landforms interpreted as the surface manifestation of thrust faults are widely distributed across mercury and preserve a record of its history of crustal deformation their formation is primarily attributed to the accommodation of horizontal shortening of mercury s lithosphere in response to cooling and contraction of the planet s interior analyses of images acquired by the mariner 10 and mercury surface space environment geochemistry and ranging messenger spacecraft during flybys of mercury showed that thrust faults were active at least as far back in time as near the end of emplacement of the largest expanses of smooth plains however the full temporal extent of thrust fault activity on mercury particularly the duration of this activity following smooth plains emplacement remained poorly constrained orbital images from the messenger spacecraft reveal previously unrecognized stratigraphic relations between lobate scarps and impact craters of differing ages and degradation states analysis of these stratigraphic relations indicates that contraction has been a widespread and long lived process on the surface of mercury thrust fault activity had initiated by a time near the end of the late heavy bombardment of the inner solar system and continued through much or all of mercury s subsequent history such deformation likely resulted from the continuing secular cooling of mercury s interior

crater fault interactions a metric for dating fault zones on planetary surfaces abstract constructing a tectonic history of a planetary surface requires determining precise fault ages a task not always possible with current analytical methods here we introduce a new method to constrain the ages of faults improving upon earlier methods that used cross cutting relationships with crater dated host surfaces and apply it to faulted terrains by counting all craters and categorizing them into faulted unfaulted and unclear types to date deformation directly additionally we construct a simple probabilistic model to account for regions of low fault density this new technique is applied to the tectonically complex thaumasia plateau mars to assess the timing of regional faulting and demonstrate its usefulness and ease of application

age determination of linear surface features using the buffered crater counting approach case studies of the sirenum and fortuna fossae graben systems on mars buffered crater counting bcc offers a possibility to determine ages of linear curvilinear surface features that provide no or only very limited surface areas for the conventional crater counting approach in this study we applied the bcc analysis to two tectonic fault systems fortuna fossae and a subsection of sirenum fossae we compared bcc results with age estimates derived from conventional crater counting on the surrounding geologic units and investigated to what extent crater ejecta blankets can be used for determining the stratigraphic placement of craters pre or post dating the formation of linear features furthermore we introduce a new functionality of the cratertools software for arcgis which allows for a user friendly semi automatic application of the otherwise time consuming procedure of bcc analysis the software provides the resulting crater size frequency data in a standard format which can be read and analyzed in the craterstats analysis software our case studies showed that the bcc approach provides equivalent or even more precise age results compared to the conventional stratigraphic approach here we found that the investigated section of sirenum fossae is younger than previously thought the derived formation age from the bcc analysis is 3 44 0 25 0 1ga which corresponds to late instead of early hesperian fortuna fossae formed shortly after the emplacement of its now fractured geologic host unit late hesperian ages derived from bcc analysis vary between 3 53 0 11 0 06ga and 3 50 0 11 0 07ga furthermore we recommend the use of crater ejecta blankets to position them in the stratigraphic sequence in order to improve crater statistics however the accuracy of the results depends on the extent and preservation state of the continuous ejecta blankets in the region of interest thus the applied buffer width has to be chosen carefully according to investigated crater sizes and local observations

internal waves in the strait of messina studied by a numerical model and synthetic aperture radar images from the ers 1 2 satellites a new numerical two layer model is presented which describes the generation of internal tidal bores and their disintegration into internal solitary waves in the strait of messina this model is used to explain observations made by the synthetic aperture radar sar from the european remote sensing satellites ers 1 and ers 2 the analysis of available ers 1 2 sar data of the strait of messina and adjacent sea areas show that 1 northward as well as southward propagating internal waves are generated in the strait of messina 2 southward propagating internal waves are observed more frequently than northward propagating internal waves 3 sea surface manifestations of southward as well as northward propagating internal waves are stronger during periods where a strong seasonal thermocline is known to be present 4 southward propagating internal bores are released from the sill between 1 and 5 hours after maximum northward tidal flow and northward propagating internal bores are released between 2 and 6 hours after maximum southward tidal flow and 5 the spatial separation between the first two internal solitary waves of southward propagating wave trains is smaller in the period from july to september than in the period from october to june the numerical two layer model is a composite of two models consisting of 1 a hydrostatic generation model which describes the dynamics of the water masses in the region close to the strait s sill where internal bores are generated and 2 a weakly nonhydrostatic propagation model which describes the dynamics of the water masses outside of the sill region where internal bores may disintegrate into internal solitary waves due to a technique for movable lateral boundaries the generation model is capable of simulating the dynamics of a lower layer that may intersect the bottom topography the proposed generation propagation model depends on one space variable only but it retains several features of a fully three dimensional model by including a realistic channel depth and a realistic channel width it is driven by semidiurnal tidal oscillations of the sea level at the two open boundaries of the model domain numerical simulations elucidate several observed characteristics of the internal wave field in the strait of messina such as north south asymmetry times of release of the internal bores from the strait s sill propagation speeds and spatial separations between the first two solitary waves of internal wave trains

avalanche slope angles in low gravity environments from active martian sand dunes 1 the properties of granular material have an important effect on surface landforms and processes recently it has been suggested that material properties called dynamic and static angle of repose vary with gravitational acceleration which would have a significant effect on many planetary surface processes such as crater collapse and gully formation in order to test that hypothesis we measured lee slopes of active aeolian sand dunes on mars using the high resolution imaging experiment hirise dtms digital terrain model we examined dune fields in nili patera herschel crater and gale crater our measurements showed that the dynamic angles of repose for the sands in these areas are 33 34 in the first region and 30 31 in the other two these results fall within the 30 to 35 window for the dynamic angles of repose for terrestrial dunes with similar flow depths and grain properties and thus show that this angle does not significantly vary with decreasing gravity

friction of ice on ice 1 new measurements have been made of the friction coefficient of freshwater polycrystalline ice sliding slowly 5 10 8 to 1 10 3 m s 1 upon itself at temperatures from 98 to 263 k under low normal stresses 98 kpa sliding obeys coulomb s law the shear stress is directly proportional to the normal stress across the interface while cohesion offers little contribution to frictional resistance the coefficient of kinetic friction of smooth surfaces varies from k 0 15 to 0 76 and at elevated temperatures 223 k exhibits both velocity strengthening at lower velocities 10 5 to 10 4 m s 1 and velocity weakening at higher velocities strengthening and weakening are attributed to creep deformation of asperities and localized melting respectively at intermediate temperatures of 173 and 133 k the kinetic coefficient appears to not exhibit significant dependence upon velocity however at the low temperature of 98 k the coefficient of kinetic friction exhibits moderate velocity strengthening at both the lowest and the highest velocities but velocity independence over the range of intermediate velocities no effect was detected of either grain size or texture over the range of roughness 0 4 10 6 m ra 12 10 6 m a moderate effect was detected where k ra0 08 slide hold slide experiments revealed that the coefficient of static friction increases by an amount that scales logarithmically with holding time implications of the results are discussed in relation to shearing across tiger stripe faults within the icy crust of saturn s enceladus sliding of the arctic sea ice cover and brittle compressive failure of cold ice

a nonlinear model of the shoaling and refraction of interfacial solitary waves in the ocean part i development of the model and investigations of the shoaling effect abstract oceanic internal waves are frequently observed to be of large amplitude and to exhibit significant wave front curvature as seen in synthetic aperture radar images the aim of this paper is to present a refraction model in two horizontal dimensions including moderate amplitude effects to model these common properties of internal waves the model incorporates second order nonlinearity and uses a ray method to allow arbitrary refraction for these preliminary investigations with the model a two layer ocean is assumed with an upper layer thickness of 50 m and a density difference across the layers of 0 5 kg m 3 an important aspect affecting the refraction of nonlinear waves is shoaling as the phase speed of a nonlinear wave depends on its amplitude in part i of this paper the development of the model is described and the shoaling phenomenon is investigated in detail the shoaling of interfacial internal solitary waves over both slowly varying and rapidly varying bathymetry is studied a wedg

mass wasting and ground collapse in terrains of volatile rich deposits as a solar system wide geological process the pre galileo view the polar terrains of mars are covered in many places with irregular pits and retreating scarps as are some of the surfaces of the outer planet satellites these features are interpreted by us as diagnostic of exogenic degradation due to the loss of a volatile rock forming matrix or cement in this study we propose that sublimation degradation is a plausible solar system wide geological process candidate examples have been identified on mars io and triton and possibly europa and ganymede we envision this process as having two end member expressions pits and scarps for which we hypothesize two end member mechanisms massive localized lenses and areally extensive basal layers in this study we focus on the role this process may play on the surfaces of the galilean satellites our principle modeling results are that for these satellites h2s co2 and nh3are the only viable candidate volatiles for sublimation degradation of landforms in light of galilean satellite cosmochemistry for io s polar regions only h2s and then only from slopes that face the sun and have thin lags is volatile enough to cause the observed sublimation induced erosion at those latitudes so2is not a viable candidate as an agent of erosion especially for these polar landforms in the case of europa only co2and h2s are viable candidates given surface age constraints both species could be efficient eroders in nonpolar regions h2s could generate erosion within the polar regions if the deposition and erosion conditions were essentially identical as those we invoked for io s polar regions for ganymede and callisto nh3might be an agent of erosion in equatorial terrains of great age the sublimation of co2and h2s is much more robust than nh3 the much slower rate of sublimation degradation from nh3might be detectable by galileo and used as a compositional indicator

a model for the alboran sea internal solitary waves abstract the propagation into the alboran sea of the interface depression generated at the strait of gibraltar by the interaction of the semidiurnal tidal current with the main camarinal sill is studied numerically by using a unidirectional model with two horizontal space dimensions an initial waveform within the strait is determined whose evolution corresponds with the train of internal solitary waves detected in the alboran sea by means of current measurements at different depths as described by kinder the agreement is good for two points located at about 45 km east of the strait the first aligned with the strait axis and the second 20 km to the north moreover the shape and amplitude of this initial condition are in agreement with a typical interface depression generated in the strait by the interaction of the tidal current with the sill a series of numerical experiments with different initial conditions is also performed to study the sensitivity of the model results to changes in the initial i

dione s spectral and geological properties abstract we present a detailed analysis of the variations in spectral properties across the surface of saturn s satellite dione using cassini vims data and their relationships to geological and or morphological characteristics as seen in the cassini iss images this analysis focuses on a local region on dione s anti saturnian hemisphere that was observed by vims with high spatial resolution during orbit 16 in october 2005 the results are incorporated into a global context provided by vims data acquired within cassini s first 50 orbits our results show that dione s surface is dominated by at least one global process bombardment by magnetospheric particles is consistent with the concentration of dark material and enhanced co 2 absorption on the trailing hemisphere of dione independent of the geology local regions within this terrain indicate a special kind of resurfacing that probably is related to large scale impact process in contrast the enhanced ice signature on the leading side is associated with the extended ejecta of the fresh impact crater creusa 49 n 76 w although no geologically active regions could be identified dione s tectonized regions observed with high spatial resolution partly show some clean h 2 o ice implying that tectonic processes could have continued into more recent times

numerical modeling of the semidiurnal tidal exchange through the strait of gibraltar 1 a three dimensional sigma coordinate free surface model is used to investigate the semidiurnal tidal exchange through the strait of gibraltar the model makes use of a coastal following curvilinear orthogonal grid that includes the gulf of cadiz and the alboran sea with very high resolution in the strait 500 m a lock exchange initial condition is used the western part of the model domain is filled with atlantic water whereas the eastern part is filled with mediterranean water the model is forced at the open boundaries through the specification of the semidiurnal m2 and s2 tidal surface elevation the model is run over a spring neap cycle fortnightly period and the results are compared with available observed data simulated cotidal maps of the m2 and s2 tidal elevation components are in quantitative and qualitative good agreement with observed data as well as with the simulated major and minor axis of tidal ellipse the model reproduces the generation and the subsequent propagation of internal bores both eastward and westward showing that they are always generated during the fortnightly period however the principal aim of this work is to quantify the effects of tidal forcing on mean quantities entrainment and transport of atlantic and mediterranean water along the strait model results reveal that the contribution of the semidiurnal tidal component m2 to the transport is relevant over camarinal sill whereas it is negligible at the eastern end of the strait model results indicate also that the effect of the semidiurnal tide is to increment the mean transport by about 30 both for the inflow and the outflow

a statistical analysis of extreme events in current variations due to internal waves from the australian north west shelf measurements of current fluctuations due to internal waves obtained from two fixed point moorings on the australian north west shelf have been analyzed with the aim of calculating the probability of short scale large amplitude internal waves exceeding particular values such waves are generated as a result of breaking or hydrodynamical instability of the semidiurnal internal tide the statistical stationarity of each record has been checked and it is shown that the records can be used for the calculation of the frequency by which currents exceed a particular value for wave heights double amplitudes of less than 60 cm s 1 the theoretical forms for exceedance frequency from the theory of extreme statistics have been checked and it is shown that for one location a power asymptotic provides a slightly better descriptor of the statistics than an exponential asymptotic and for the other location an exponential asymptotic is found to provide the better fit the exceedance frequency for internal waves near the australian north west shelf is found to be less than 1 3 waves per hour the exceedance probability for each location has been calculated using poisson statistics

a link between geomagnetic reversals and events and glaciations abstract the apparent duration of geomagnetic polarity events in arctic ocean sediments is much longer than in sediments from lower latitudes in fact while the remanence of brunhes age sediment cores from the yermak plateau at 82 n is fully reversed for 30 of their lengths 1 the events often evade detection in many other continuously deposited sediments for example the laschamp event is absent in an otherwise high resolution record of secular variation from lac du bouchet 2 which is located near the laschamp volcanics where the event was first detected very short event durations of a few hundred years at the most have been suggested before 2 3 because sedimentation rates in the arctic ocean were increased during glaciations the exaggerated proportion of reverse polarities in sediments from high latitudes suggests a link between glaciation and field reversals this suggestion is supported by magnetostratigraphic results obtained from thick loess paleosol sequences in china 4 these demonstrate that all polarity boundaries separating chrons and subchrons since the gauss matuyama field reversal have been recorded in loess and thus during periods of cold climate although conflicting evidence exists for some boundaries furthermore the ages of 22 events and chron boundaries have been compared with the oxygen isotope record 5 thought to represent global ice volume all events and reversals younger than 2 6 ma may have occurred during periods of global cooling or during cold stages however some ages are still too poorly dated for a definite correlation climatic signals also exist in the two longest relative paleointensity records 6 7 but these are suspected to be caused by climatically driven variations in the rock magnetic parameters a mechanism for field reversals may be the acceleration of the earth s rotation caused by lowering of the sea level during glaciations the short duration of events also implies that the geomagnetic field can reverse an order of magnitude faster than commonly assumed

a propagation model for the internal solitary waves in the northern south china sea a two dimensional regularized long wave equation model is developed to study the dynamic mechanisms of the propagation and evolution of the internal solitary waves isws in the northern south china sea scs it is shown that the bottom topography would cause the polarity reversal of isws the change of the local wave crestline shape and some diminution in wave amplitude even if the isws are induced at the small sill channel along the luzon strait they could propagate westward with their crestlines covering a large area in the latitudinal direction in the northern scs when there are two trains of isws propagating from the same source site with a time lag but different amplitudes of initial solitons the latter train of isws with a larger amplitude may catch then swallow the former one with a smaller amplitude and the wave amplitude of the merged isw train decreases while the wave number increases when there are two trains of isws propagating from the different source sites at the same time with the same amplitude of initial solitons the crestlines of the two isw trains may meet and a new leading soliton is induced at the connection point once the isw trains collide with the island before the island a weak isw train is reflected behind the island the former crestlines of the isw train are torn by the island into two new trains which may reconnect after passing around the island the propagation direction the wave amplitude and the reconnection point of the new merged isw train behind the island depend on the relative orientation of the original soliton source site to the island

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

voyager 2 in the uranian system imaging science results voyager 2 images of the southern hemisphere of uranus indicate that submicrometersize haze particles and particles of a methane condensation cloud produce faint patterns in the atmosphere the alignment of the cloud bands is similar to that of bands on jupiter and saturn but the zonal winds are nearly opposite at mid latitudes 70 degrees to 27 degrees where winds were measured the atmosphere rotates faster than the magnetic field however the rotation rate of the atmosphere decreases toward the equator so that the two probably corotate at about 20 degrees voyager images confirm the extremely low albedo of the ring particles high phase angle images reveal on the order of 10 2 new ringlike features of very low optical depth and relatively high dust abundance interspersed within the main rings as well as a broad diffuse low optical depth ring just inside the main rings system nine of the newly discovered small satellites 40 to 165 kilometers in diameter orbit between the rings and miranda the tenth is within the ring system two of these small objects may gravitationally confine the e ring oberon and umbriel have heavily cratered surfaces resembling the ancient cratered highlands of earth s moon although umbriel is almost completely covered with uniform dark material which perhaps indicates some ongoing process titania and ariel show crater populations different from those on oberon and umbriel these were probably generated by collisions with debris confined to their orbits titania and ariel also show many extensional fault systems ariel shows strong evidence for the presence of extrusive material about halfof miranda s surface is relatively bland old cratered terrain the remainder comprises three large regions of younger terrain each rectangular to ovoid in plan that display complex sets of parallel and intersecting scarps and ridges as well as numerous outcrops of bright and dark materials perhaps suggesting some exotic composition

internal hydraulics solitons and associated mixing in a stratified sound observations of tidally forced flow in a constricted and sill like region of a highly stratified sound are examined as a problem of two layer hydraulic exchange based on the current and salinity data time and space dependent hydraulic conditions along the sound are discussed it will be shown that the sound is subject to internal hydraulic controls at several locations which account for the generation of strong internal hydraulic bores and drops in the upper layer internal solitary waves with 3 6 rain period were found in the upstream propagating bore using results from different models of the solitary wave based on the korteweg de vfies kdv equation it is shown that the second order nonlinear term must be included in the two layer model results from a first order continuously stratified model solved using the lowest mode eigenfunction gave similar results as the second order two layer model this implies that two layer models may ignore some properties of the real fluid and that internal soiltons are also sensitive to the stratification characteristics of the water column the vertical velocity shear hydraulic characteristics of the flow and the presence of solitary waves were major contributors to vertical mixing in the sound

shapes of the saturnian icy satellites and their significance the sizes and shapes of six icy saturnian satellites have been measured from cassini imaging science subsystem iss data employing limb coordinates and stereogrammetric control points mimas enceladus tethys dione and rhea are well described by triaxial ellipsoids iapetus is best represented by an oblate spheroid all satellites appear to have approached relaxed equilibrium shapes at some point in their evolution but all support at least 300 m of global wavelength topography the shape of enceladus is most consistent with a homogeneous interior if enceladus is differentiated its shape and apparent relaxation require either lateral inhomogeneities in an icy mantle and or an irregularly shaped core iapetus supports a fossil bulge of over 30 km and provides a benchmark for impact modification of shapes after global relaxation satellites such as mimas that have smoother limbs than iapetus and are expected to have higher impact rates must have relaxed after the shape of iapetus was frozen

ad hoc committee on global climate issues annual report the aapg ad hoc committee on global climate issues has studied the supposition of human induced climate change since the committee s inception in january 1998 this paper details the progress and findings of the committee through june 1999 at that time there had been essentially no geologic input into the global climate change debate the following statements reflect the current state of climate knowledge from the geologic perspective as interpreted by the majority of the committee membership the committee recognizes that new data could change its conclusions the earth s climate is constantly changing owing to natural variability in earth processes natural climate variability over recent geological time is greater than reasonable estimates of potential human induced greenhouse gas changes because no tool is available to test the supposition of human induced climate change and the range of natural variability is so great there is no discernible human influence on global climate at this time

resurfacing saturn s satellites models of partial differentiation and expansion abstract the faulting seen on the surfaces of saturn s icy moons may have been caused either by external events such as large impacts or internal stresses caused by the expansion of the moons as long lived radionuclides produced internal heating and phase changes we estimate the stress as a function of radius expansion is 44 r r kbar the extensional stress needed for fracture is probably something less than 40 bar so extensional fracture is likely to occur when r r is greater than one part in a thousand the radius change for these moons can be calculated analytically given suitable assumptions in addition detailed time dependent computer models of the thermal and physical evolution of tethys dione rhea and iapetus were carried out from these calculations we conclude that the most reasonable cause for rifting on dione and rhea is the refreezing of an ammonia water eutectic melt inside these moons roughly two billion years after their formation while the rift on tethys was caused by a large impact and little rifting should be expected on iapetus

polygonal impact craters on dione evidence for tectonic structures outside the wispy terrain abstract plan view impact crater geometries can be indicative of pre impact structures within the target material impact events that occur on a pre fractured surface generate craters exhibiting one or more straight rim segments termed polygonal impact craters pics impact craters that appear to be pics are abundant on the surface of saturn s icy satellite dione both within the wispy terrain a region with large visible fractures and also outside the wispy terrain the non wispy terrain where less evidence for fracturing has been observed in the non wispy terrain subtle lineaments are hypothesized to be ne sw nw se and e w trending fractures suggesting that tectonism may have been an important process in this terrain results of previous studies have shown that pic straight rim segment azimuths pic azimuths commonly parallel pre impact fracture azimuths although disagreements about this relationship exist in the literature we investigated the hypothesis that fractures either subtle or nonvisible with available spacecraft images are present within dione s non wispy terrain our first step was to assess the relationship between pics and pre existing fracture azimuths in the wispy terrain our results from this initial assessment show a parallel relationship between pic azimuths and fracture azimuths based on this correlation in the wispy terrain we find it likely that this relationship would hold true in the non wispy terrain if pics are present we tested for pics using crater rim azimuth data collected from randomly distributed study locations throughout the non wispy terrain from these data we identified widespread pics in this terrain which supports the hypothesis that subtle fractures are also present analysis of the pic azimuth data yield a pattern for these inferred fractures across dione s surface that is consistent with the hypothesized global deformation that would result from a combination of satellite despinning and volume expansion our results provide evidence for these previously hypothesized events in dione s history and demonstrate that mapping pics and their azimuths is a useful tool for investigating subtle fractures on solar system bodies

recent extensional tectonics on the moon revealed by the lunar reconnaissance orbiter camera on the moon extensional tectonic features have only been observed close to the influence of the mare basalt filled basins and floor fractured craters analysis of lunar reconnaissance orbiter camera images reveals several potentially very young extensional tectonic features in the farside highlands implying that extensional stresses may locally exceed compressional ones

study of the generation and propagation of internal waves in the strait of gibraltar using a numerical model and synthetic aperture radar images of the european ers 1 satellite a weakly nonhydrostatic two layer numerical model based on the boussinesq equations is presented which is capable of describing among others the generation and propagation of nonlinear weakly dispersive internal waves in the strait of gibraltar the model depends on one space coordinate only but it retains several features of a fully three dimensional model by including a realistic bottom profile a variable channel width and a trapezoidal channel cross section the nonlinear primitive boussinesq equations include horizontal diffusion bottom friction and friction between the two water layers the model is driven by a height difference of the mean interface depth between the atlantic and the mediterranean boundaries and by semidiurnal tidal oscillations of the barotropic transport the model presented in this paper describes 1 the mean and tidal flow in the strait of gibraltar 2 the variation of the depth of the interface during a tidal cycle 3 the generation of strong depressions of the interface at the western sides of the spartel sill and the camarinal sill 4 the generation of strong eastward propagating internal bores and 5 their disintegration into trains of internal solitary waves the surface convergence patterns associated with depressions of the interface at the camarinal sill internal bores and internal solitary waves are calculated and compared with roughness patterns visible on synthetic aperture radar sar images of the first european remote sensing satellite ers 1 in total 155 ers 1 sar scenes from 94 satellite overflights over the strait of gibraltar which were acquired in the period from january 1992 to march 1995 have been analyzed it is shown that the proposed model is capable of explaining the observed temporal and spatial evolution of surface roughness patterns associated with eastward propagating internal waves inside the strait of gibraltar as well as the observed east west asymmetry of the internal wave field

glacial cycles and astronomical forcing narrow spectral features in ocean sediment records offer strong evidence that the cycles of glaciation were driven by astronomical forces two million years ago the cycles match the 41 000 year period of earth s obliquity this supports the croll milankovitch theory which attributes the cycles to variations in insolation but for the past million years the spectrum is dominated by a single 100 000 year feature and is a poor match to the predictions of insolation models the spectrum can be accounted for by a theory that derives the cycles of glaciation from variations in the inclination of earth s orbital plane

the geology of dione abstract dione is one of the more geologically complex of saturn s satellites several geologic units have been identified including ancient heavily cratered terrain two plains units cratered plains and lightly cratered plains lobate deposits crater rim deposits and bright wispy material the only structural features observed are a series of troughs which cross portions of the surface and subtle northeast and northwest trending lineaments the troughs are associated with volcanic deposits and are interpreted to be the vents through which material was erupted correlations exist between telescopically observed albedo patterns and the distribution of geologic units

topographic effect on oblique internal wave wave interactions based on a variable coefficient kadomtsev petviashvili kp equation the topographic effect on the wave interactions between two oblique internal solitary waves is investigated in the absence of rotation and background shear the model set up featuring idealised shoaling topography and continuous stratification is motivated by the large expanse of continental shelf in the south china sea when the bottom is flat the evolution of an initial wave consisting of two branches of internal solitary waves can be categorised into six patterns depending on the respective amplitudes and the oblique angles measured counterclockwise from the transverse axis using theoretical multi soliton solutions of the constant coefficient kp equation we select three observed patterns and examine each of them in detail both analytically and numerically the effect of shoaling topography leads to a complicated structure of the leading waves and the emergence of two types of trailing wave trains further the case when the along crest width is short compared with the transverse domain of interest is examined and it is found that although the topographic effect can still modulate the wave field the spreading effect in the transverse direction is dominant

sequence and mechanisms of deformation around the hellas and isidis impact basins on mars the distribution and age of early volcanic and tectonic features surrounding the itellas and isidis impact basins are shown to fit a four stage sequence first concentric canyons form outside the basin boundary scarp at or near the time of basin formation followed shortly thereafter by radial troughs extending beyond the boundary scarp after a hiatus concentric graben develop inside the basin within the massif ring finally soon after graben formation widespread volcanic plains are emplaced between the massif ring and the boundary scarp to one side of the basin three models of basin centered deformation are compared io hese observed deformation events elastic flexure due to loading elastic flexure due to uplift and impact fracture a central load of basin fill can account for concentric fractures by elastic flexure around the load formation of the distant concentric canyons however requires lithospheric thicknesses a factor of 5 greater than those indicated by planetary thermal history models and an increase in feature width by an order of magnitude relative to flexural graben identified elsewhere on mars and the moon although inclusion of loading by basin ejecta reduces the required lithospheric thickness to acceptable values flexure still fails to account for the large canyon widths observed impact fracture from a rapid inward flow of asthenosphere during transient cavity collapse however is consistent with the three key observational constraints for the distant canyons their age width and distance from the basin as the basin then undergoes isostatic uplift soon after impact elastic flexure induces a radial pattern of failure consistent with the observed extent and timing of the radial troughs later after a long period of basin infilling elastic failure due to loading of the central basin region can account for the ring grabens in the massif ring the localization of volcanism into a single ridged plains unit on the basin rim indicates a thermal source offset from the basin which is not included in these models and perhaps reflects an additional mechanism unique to mars

shallow normal fault slopes on saturnian icy satellites fault dips are a function of the coefficient of internal friction i of the lithospheric material laboratory deformation experiments of h2o ice at conditions applicable to icy bodies yield 0 i 0 55 such that normal faults dip between 45 and 59 we tested the hypothesis that normal faults on icy bodies reflect these values by using digital elevation models to examine geometries of large extensional systems on three saturnian satellites analyzed faults within ithaca chasma on tethys and avaiki chasmata on rhea all exhibit shallower than predicted topographic slopes across the fault scarp which we term fault slopes a scarp of padua chasmata within dione s wispy terrain also has a shallow fault slope although three others that make up palatine chasmata exhibit steeper slopes as predicted we infer that viscous relaxation is the most viable explanation for these shallow fault slopes and we model the potential role of viscous relaxation in creating shallow fault slopes our modeling results support formation of these normal faults with steep dips consistent with deformation experiments followed by their relaxation due to lithospheric heating events related to radionuclide decay the steepest fault slopes in this terrain yield 0 i 0 73 for dione s lithospheric ice which overlaps the dip range predicted from experiments results of this work suggest that viscous relaxation substantially affected fault slopes on tethys rhea and dione by implication these processes may have also affected fault geometries on other icy satellites

climate change conflict of observational science theory and politics discussion gerhard 2004 argues that our reasoning on the causes of climate change should be guided by observational science instead of by results derived from computer models given that the nature and balance of the controls of the earth s climate remain uncertain this appears to be a sound suggestion on the strength of his own reading of recent observational science gerhard adheres to the view set out by the aapg ad hoc committee on global climate issues in its 1999 annual report because no tool is available to test the supposition of human induced climate change and the range of natural variability is so great there is no discernible human influence on global climate at this time gerhard and hanson 2000 p 466 gerhard 2004 p 1219 now asks why is it that we spend so much time and resources trying to prove a theory of greenhouse gas climate control instead of testing the hypothesis a contrary view is expressed in a position paper of the stratigraphy commission of the geological society london zalasiewicz et al 2004 p 1 we find that the evidence for human induced climate change is now persuasive and the need for direct action compelling oreskes 2004 p 1686 cites evidence that such a view is representative of a notable consensus in the refereed scientific literature and that all major scientific bodies in the united states whose members expertise bears directly on the matter have issued similar statements where does that leave gerhard and the aapg that question can finally be answered when this particular scientific dispute is agreed by all rational parties to have been settled by the weight of observational evidence instead of by majority voting or social pressures this particular debate on climate change is of major economic and political significance as well as

simulations of internal solitary wave interactions with mesoscale eddies in the northeastern south china sea abstractwith the combined analysis of synthetic aperture radar image and satellite altimeter data collected in the northeastern south china sea scs this study found one type of distorted phenomenon of internal solitary wave isw with the long front caused by the oceanic mesoscale eddy motivated by these satellite observations the authors carried out numerical experiments using the fully nonhydrostatic and nonlinear mitgcm to investigate the perturbation of isws by an isolated cyclonic or anticyclonic eddy the results show that the isw front is distorted by these oceanic eddies due to the retardation and acceleration effects at their two sides the isw energy along the front is focused onto scattered from the wave fragment where a concave convex pattern is formed and the previously accumulated energy in the focusing region is gradually released after the isw propagates away from the eddies the isw amplitude is modulated greatly by the eddies due to the energy redistribution along the front se

the tectonic and volcanic history of dione abstract the tectonic and volcanic modifications of dione are described and interpreted it is proposed that after the formation of a brittle outer shell but before the end of heavy meteoritic bombardment global expansion due to radionuclide heating and perhaps a loss of oblateness due to tidal despinning and orbital recession produced a global system of lineaments an nh 3 h 2 o melt was produced and erupted on the surface to form plains units cooling of the interior or a phase change led to horizontal compression in the surface compression of a thick deposit of plains material possibly overlying a decollement surface is proposed to explain cratered plains ridges developed as thrust or high angle reverse faults following formation of ridges and smooth plains the surface experienced light cratering

thermal stress tectonics on the satellites of saturn and uranus most of the major saturnian and uranian satellites have undergone some form of extensional tectonism and or resurfacing early in their histories we examine the stresses created by the early thermal evolution of these satellites and consider the geologic implications of these stresses starting from an initial accretional temperature profile we calculate the satellites temperature profiles as they evolve by conduction solid state convection when it occurs is handled using a simple parameterized convection scheme the effects of radiogenic heating and release of heat of phase change are included the thermal stresses built up in the satellites are then calculated assuming the satellites to have a maxwell viscoelastic rheology the stress calculation takes into account both the effects of thermal stresses and of shifts in the position of the ice i ice ii phase boundary in some satellites if accretion takes place in a cold gas free environment the stresses that build up in the smaller satellites mimas miranda and enceladus are minor in the larger satellites however stresses are large enough to cause fracturing and extensional tectonism under these circumstances the stresses result largely from rapid cooling contraction of a thin outer shell the ice ii ice i phase transition also contributes significantly for the largest satellites if accretion occurs in a warm gaseous nebula that dissipates rapidly stresses large enough to cause failure are attained in all of the satellites in all cases the material immediately underlying the zone of failure is the warmest in the satellite and hence may be buoyant and mobile enough to be extruded to the surface through fractures created by the extension we conclude that thermal stresses following accretion led to a substantial early episode of extensional tectonism and perhaps also associated resurfacing on many of the saturnian and uranian satellites if evidence of this early episode of extensional tectonism has survived subsequent impacts then the process may have been responsible for much of the early tectonism and resurfacing observed on these satellites if not then it may only be responsible for the very early global resurfacing that some of the satellites are inferred to have undergone