shatter cones at the keurusselka impact structure and their relation to local jointing shatter cones are the only distinct meso to macroscopic recognition criterion for impact structures yet not all is known about their formation the keurusselka impact structure finland is interesting in that it presents a multitude of well exposed shatter cones in medium to coarse grained granitoids the allegedly 27 km wide keurusselka impact structure was formed about 1150 ma ago in rocks of the central finland granitoid complex special attention was paid in this work to possible relationships between shatter cones and local as well as regionally occurring fracture or joint systems a possible shatter cone find outside the previously suggested edge of the structure could mean that the keurusselka impact structure is larger than previously thought the spacing between joints fractures from regional joint systems was influenced by the impact but impact induced fractures strongly follow the regional joint orientation trends there is a distinct relationship between shatter cones and joints shatter cones occur on and against joint surfaces of varied orientations and belonging to the regional orientation trends planar fractures pf and planar deformation features pdf were found in three shatter cone samples from the central most part of the impact structure whereas other country rock samples from the same level of exposure but further from the assumed center lack shock deformation features pdf occurrence is enhanced within 5 mm of shatter cone surfaces which is interpreted to suggest that shock wave reverberation at preimpact joints could be responsible for this local enhancement of shock deformation some shatter cone surfaces are coated with a quasi opaque material which is also found in conspicuous veinlets that branch off from shatter cone surfaces and resemble pseudotachylitic breccia veins the vein filling is composed of two mineral phases one of which could be identified as a montmorillonitic phyllosilicate the second phase could not be identified yet the original composition of the fill could not be determined further work is required on this material observed joints and fractures were discussed against findings from barringer impact crater they show that impact induced joints in the basement rock do not follow impact specific orientations such as radial conical or concentric

fractal geometry based quantification of shock induced rock fragmentation in and around an impact crater shock induced fragmentation structures of basement rocks and their limestone cover in and around the ries impact crater germany were recorded on outcrop hand sample and thin section scale and quantified mainly by fractal geometry methods quantification was performed by automated procedures and in areas of square centimetres to square decametres with a maximum resolution of micrometre scale in 2d and on all scales the fragmentation structures form complex statistically self similar patterns fractals with specific characteristics i the pattern fractality is scale dependent ii three different power law relationships exist which reflect the effect of three fragmentation processes iii the fracture patterns are anisotropic and inhomogeneous over larger areas iv complexity and anisotropy of the fracture patterns vary systematically such systematic variation appears typical for impact related fragmentation

impact melt and projectile bearing ejecta at barringer crater arizona abstract our understanding of the impact cratering process continues to evolve and even at well known and well studied structures there is still much to be learned here we present the results of a study on impact generated melt phases within ejecta at barringer crater arizona one of the first impact craters on earth to be recognized and arguably the most famous we report on previously unknown impact melt bearing breccias that contain dispersed fragments of the projectile as well as impact glasses that contain a high proportion of projectile material higher than any other glasses previously reported from this site these glasses are distinctly different from so called melt beads that are found as a lag deposit on the present day erosion surface and that we also study it is proposed that the melts in these impact breccias were derived from a more constrained sub region of the melt zone that was very shallow and that also had a larger projectile contribution in addition to low and high fe melt beads documented previously we document ca mg rich glasses and calcite globules within silicate glass that provide definitive evidence that carbonates underwent melting during the formation of barringer crater we propose that the melting of dolomite produces ca mg rich melts from which calcite is the dominant liquidus phase this explains the perhaps surprising finding that despite dolomite being the dominant rock type at many impact sites including barringer crater calcite is the dominant melt product when taken together with our estimate for the amount of impact melt products dispersed on and just below the present day erosional surface it is clear that the amount of melt produced at barringer crater is higher than previously estimated and is more consistent with recent numerical modeling studies this work adds to the growing recognition that sedimentary rocks melt during hypervelocity impact and do not just decompose and or devolatilize as was previously thought this has implications for understanding the processes and products of impacts into sedimentary rocks and for estimating the amount of climatically active gases released by impact events

erosional modification and gully formation at meteor crater arizona insights into crater degradation processes on mars hydrogeological modification of meteor crater produced a spectacular set of gullies throughout the interior wall in response to rainwater precipitation snow melting and possible groundwater discharge the crater wall has an exceptionally well developed centripetal drainage pattern consisting of individual alcoves channels and fans some of the gullies originate from the rim crest and others from the middle crater wall where a lithologic transition occurs broad gullies occur along the crater corner radial faults deeply incised alcoves are well developed on the soft coconino sandstone exposed on the middle crater wall beneath overlying dolomite in general the gully locations are along crater wall radial fractures and faults which are favorable locales of erosion due to preferential rock breakup from faulting and groundwater flow discharge these structural discontinuities are also the locales where the surface runoff from rain precipitation and snow melting can preferentially flow causing erosion and crater degradation channels are well developed on the talus deposits and alluvial fans on the periphery of the crater floor caves exposed on the lower crater level point to percolation of surface runoff and selective discharge through fractures on the crater wall in addition lake sediments on the crater floor provide significant evidence of a past pluvial climate when the water table was higher and groundwater may have seeped from springs on the crater wall although these hydrological processes continue at meteor crater today conditions at the crater are much more arid than they were soon after impact reflecting a climatic shift this climate shift and the hydrological modifications observed at meteor crater provide insights for landscape sculpturing on mars during various parts of its history

cosmic wabi sabi tell tale morphological imperfections in impact crater shapes revealed by numerical analysis abstract geomorphological features result from a combination of factors such as material strength structure and the forces of nature an impact crater is a good example in an ideally simple situation e g a vertical impact of a spherical object into a homogeneous target the resulting crater will obtain a perfectly circular shape any deviation from the perfect circle can reveal important information about the cratering process target properties and even the surficial environment in which the impact occurred e g the presence of an atmosphere however irregularities in the crater shape are not always obvious what appears to be a perfect circle may hide subtle information undetectable to the naked eye here we present a new method of numerical aspect classification that allows even the slightest morphological irregularities to be detected our method is based on a numerical analysis of dip directions of segments within a digital terrain model dtm of the crater rim we show the successful application of our method on a crater with well known polygonal shape meteor crater usa as well as two apparently circular martian impact craters our crater aspect classification cac can be applied on any other micro to mega scale structures for which a dtm can be produced thus allowing applications to fields of science and technology within and beyond the scope of geomorphology

the long lived and recent seismicity at the lunar orientale basin evidence from morphology and formation ages of boulder avalanches tectonics and seismic ground motion

evidence for thermal stress induced rockfalls on mars impact crater slopes abstract here we study rocks falling from exposed outcrops of bedrock which have left tracks on the slope over which they have bounced and or rolled in fresh impact craters 1 10 km in diameter on mars the presence of these tracks shows that these rocks have fallen relatively recently because aeolian processes are known to infill topographic lows over time mapping of rockfall tracks indicate trends in frequency with orientation which in turn depend on the latitudinal position of the crater craters in the equatorial belt between 15 n and 15 s exhibit higher frequencies of rockfall on their north south oriented slopes compared to their east west ones craters 15 n s have notably higher frequencies on their equator facing slopes as opposed to the other orientations we computed solar radiation on the surface of crater slopes to compare insolation patterns with the spatial distribution of rockfalls and found statistically significant correlations between maximum diurnal insolation and rockfall frequency our results indicate that solar induced thermal stress plays a more important role under relatively recent climate conditions in rock breakdown and preconditioning slopes for rockfalls than phase transitions of h2o or co2 at mid and equatorial latitudes thermal stress should thus be considered as an important factor in promoting mass wasting process on impact crater walls and other steep slopes on mars

postglacial rebound observed by lageos and the effective viscosity of the lower mantle sixty four observations of the orbital node made by the lageos satellite over a five year time interval reveal an acceleration of 8 1 or 1 8 x 10 to the 8 power arcseconds day 2 due to a source which is not presently modeled in the geodyn orbit determination computer program this acceleration cannot be explained by the ocean tide with 18 6 year period assuming it to be an equilibrium tide instead it seems to be due to postglacial rebound which changes the j 2 coefficient in the spherical harmonic expansion of the earth s gravitational field at the rate of 8 2 or 18 x 10 to the 19th power s this in turn accelerates the node this rate does not agree with the 32 x 10 to the 19th power s predicted by wu and peltier s 1982 l2 model which has upper and lower mantle effective viscosities of 10 to the 21st and 22nd powers pa s respectively it does agree well with their l1 model which gives about 10 x 10 to the 19th power s since the effective viscosity is 10 to the 21st power pa s throughout the entire mantle in the l1 model the results support the contentions that the efective viscosity is near 10 to the 21st power pa s everyhere in the mantle and this relatively low value for the effective viscosity may have permitted several degrees of polar wander due to glaciation during the quaternary ice age previously announced in star as n84 13705

mantle phase transitions and the free air gravity anomalies over fennoscandia and laurentia the interpretation of free air gravity anomalies over present day centers of postglacial rebound has always been controversial we show that the observed gravity anomalies over both fennoscandia and laurentia and the associated relative sea level variations since deglaciation are explicable with a simple newtonian viscoelastic model of the earth if the earth model includes the density jumps in the mantle associated with the phase transitions at 420 and 670 km depth the result has important implications for mantle convection since the observed free air anomalies are very sensitive to the viscosity of the mantle beneath the deepest phase transition

recent shallow moonquake and impact triggered boulder falls on the moon new insights from the schrodinger basin shallow moonquakes are thought to be of tectonic origin however the geologic structures responsible for these moonquakes are unknown here we report sites where moonquakes possibly occurred along young lobate scarps in the schrodinger basin our analysis of lunar reconnaissance orbiter and chandrayaan 1 images revealed four lobate scarps in different parts of the schrodinger basin the scarps crosscut small fresh impact craters 1500 boulders associated with trails and bouncing marks their origins are largely controlled by recent impact events ejecta rays and secondary crater chains from a 14 km diameter impact crater traversed schrodinger and triggered significant boulder falls about 17 ma therefore a combination of recent shallow moonquakes and impact events triggered the boulder falls in the schrodinger basin

shatter cones branched rapid fractures formed by shock impact 1 shatter cones are rock discontinuities known only from sites of extraterrestrial impacts and they are assumed to be formed by impact induced shock waves here we characterize the structure of shatter cones by field and microanalyses and explain their formation by dynamic fracture mechanics our analyses reveal that shatter cones always occur as multilevel three dimensional networks 0 01 100 m in size with hierarchal branched fractures a typical individual shatter cone is a curved oblate branch that bifurcates from its parent fracture e g a larger shatter cone and expands to form a spoon like surface the unique shatter cone striations are arranged in v shaped pairs whose enclosed angle is constant for a given sample we propose that shatter cones are the natural consequence of tensile rock fracturing at extreme velocities first the structure of shatter cone networks is strikingly similar to the structure of branched networks of experimental dynamic fractures that propagate at high velocities velocities that approach the rayleigh wave speed vr second fracture front waves generated experimentally by the interaction of a rapidly moving tensile fractures and material inclusions create tracks on the fracture surface that correspond to the v shaped striations of shatter cones third applying the front wave concept to our field measurements vredefort impact south africa shows that the shatter cones propagated at velocities of 0 98 0 90vr with a systematic velocity decrease from the impact center these extreme asymptotic velocities require the intense energy flux of impacts our model explains all of the structural features of shatter cones curved surfaces cone directivity unique striations hierarchic multilevel structure and their exclusive occurrence at impact sites

geophysical signature of malingen the minor crater of the lockne malingen doublet impact structure malingen is the 0 7km wide minor crater associated to the 10 times larger lockne crater in the unique lockne malingen doublet the craters formed at 458ma by the impact of a binary asteroid related

glacial isostatic adjustment i the forward problem summary the isostatic adjustment of a radially stratified visco elastic spheroid is treated using space time green functions for the associated surface mass load boundary value problem these impulse response functions are convolved with a heaviside function to give the time dependent deformation of the planet which would be produced by a unit point mass brought up from infinity at t 0 and allowed to remain on the surface the resulting heaviside green functions can be employed to simulate all of the important signatures of glacial isostatic adjustment given a space time dependent surface mass load consisting of ice sheet ablation histories and a model of the simultaneous filling of the ocean basins these source terms are simply convolved with the heaviside green function appropriate to a specific response signature a realistic model of the spatial distribution of the main late pleistocene ice loads and of their temporal disintegration is constructed the response of two visco elastic earth models is computed and compared to a global set of relaxation data relative sea level curves on the basis of this initial comparison of theory and observation the possibility that the lower mantle has a viscosity which is significantly in excess of the viscosity of the upper mantle is excluded in addition clear evidence of the presence of the lithosphere has been found in relaxation data from sites which were near the edge of the laurentide ice sheet such data should therefore prove useful as a basis for analysis of lateral variations in lithosphere thickness further extensions of the calculation are suggested

reassessing rock mass properties and slope instability triggering conditions in valles marineris mars the rock walls of the valles marineris valleys vm in the equatorial area of mars exhibit several gravitational failures which resulted in a series of large landslides up to several hundred cubic kilometers in volume questions arise as to forces at play and rock strength in the stability of the walls of vm in this work we address the stability analysis of the walls of vm by considering the strength of the materials of the walls and the causes of landslides using finite element calculations and the limit analysis upper bound method we explore the range of cohesion and friction angle values associated with realistic failure geometries and compare predictions with the more classical culmann s translational failure model our analysis is based both on synthetic simplified slope profiles and on the real shape of the walls of vm taken from the mola topographic data validation of the calibrated cohesion and friction angle values is performed by comparing the computed unstable cross sectional areas with the observed pre and post failure profiles the estimated failure surface geometry and ridge crest retreat this offers a link between rock mass properties slope geometry and volume of the observed failure represented in dimensionless charts the role of groundwater flow and seismic action on the decrease of slope stability is also estimated pseudo static seismic analyses provide another set of dimensionless charts and show that low seismicity events induced by meteoroid impacts consistent with the size of craters could be a cause for some of the observed landslides if poor rock properties for vm are assumed analyses suggest that rock mass properties are more similar to their earth equivalents with respect to what has been previously supposed

a reappraisal of postglacial decay times from richmond gulf and james bay canada decay times inferred from relative sea level rsl histories of previously glaciated regions provide a potentially important constraint on mantle rheology we present a new compilation of rsl data from richmond gulf and james bay canada this recompilation reveals errors in previous compilations that led to inaccurate estimates for the richmond gulf decay time in a series of recently published articles we derive updated estimates for the decay time at richmond gulf and james bay using a methodology that incorporates errors in both the age and the height of the sea level markers this exercise is guided by a series of synthetic rsl calculations that show that decay time estimates in the region can be significantly biased if the rsl time series are not corrected for global eustatic sea level trends or if the estimates are based on composite rsl histories derived by combining data from both the richmond gulf and the james bay regions our decay time analysis for richmond gulf applies the pioneering approach of walcott 1980 to a large database and we derive a value of 4 0 6 6 kyr where the range is defined by a misfit tolerance 10 per cent higher than the minimum our analysis for james bay is based on the uplift curve derived by hardy 1976 and we estimate a decay time of about 2 0 2 8 kyr the difference between our estimates for richmond gulf and james bay may be due to errors in the observational record from these regions but could also be influenced by lateral variations in lithospheric structure associated with the assembly of laurentia

deglaciation induced vertical motion of the north american continent and transient lower mantle rheology the last deglaciation event of the current ice age which began approximately 18 000 years ago and ended about 7000 years ago continues to exert a profound influence on the vertical motion of the solid earth in and surrounding the regions which were once ice covered observations of such vertical motions may be employed to constrain the radial viscoelastic structure of the planet and therefore to contribute in an important way to the understanding of geodynamic processes in general in this paper both present day rates of vertical motion as recorded on tide gauges and long time scale records of vertical motion as recorded in 14c controlled relative sea level histories are employed to refine previously constructed models of the radial viscoelastic structure an interesting sidelight to this analysis is the demonstration that the lower mantle viscosity inferred from the vertical motion data could represent a transient rather than the steady state value a fact which would help considerably to reconcile the small viscosity stratification required by the glacial rebound data with the large stratification which seems to be required by some recent analyses of the geoid height anomalies associated with internal density heterogeneity of the mantle which has been revealed by application of seismic tomographic techniques a further product of these new analyses is obtained by the application of the glacial isostatic adjustment model to filter this signal from the secular trends of relative sea level observed on tide gauges from both the east and west coasts of the north american continent these results may be important in the context of recently proposed scenarios of climatic change

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

equatorial electrojet responses to intense solar flares under geomagnetic disturbance time electric fields large enhancement in the equatorial electrojet eej current can occur due to sudden increase in the e layer density arising from solar flare associated ionizing radiations as also from background electric fields modified by magnetospheric disturbances when present before or during a solar flare we investigate the eej responses at widely separated longitudes during two x class flares that occurred at different activity phases surrounding the magnetic super storm sequences of 28 29 october 2003 during the 28 october flare we observed intense reverse electrojet under strong westward electric field in the sunrise sector over jicamarca sources of westward disturbance electric fields driving large eej current are identified for the first time model calculations on the e layer density with and without flare and comparison of the results between jicamarca and sao luis suggested enhanced westward electric field due to the flare occurring close to sunrise over jicamarca during the flare on 29 october which occurred during a rapid ae recovery a strong overshielding electric field of westward polarity over jicamarca delayed an expected eej eastward growth due to flare induced ionization enhancement in the afternoon this eej response yielded a measure of the overshielding decay time determined by the storm time region 2 field aligned current this paper will present a detailed analysis of the eej responses during the two flares including a quantitative evaluation of the flare induced electron density enhancements and identification of electric field sources that played dominant roles in the large westward eej at the sunrise sector over jicamarca

impact fragmentation of lonar crater india implications for impact cratering processes in basalt impact fragmentation is an energetic process that has affected all planetary bodies to understand its effects in basalt we studied lonar crater which is a rare terrestrial simple impact crater in basalt and analogues to kilometer scale simple craters on mars the lonar ejecta consists of basaltic fragments with sizes ranging from silt to boulder the cumulative size and mass frequency distributions of these fragments show variation of power index for different size ranges suggesting simple and complex fragmentation the general shape of the fragments is compact platy bladed and elongated with an average edge angle of 100 the size distribution of cobble to boulder sized fragments is similar to the fracture spacing distribution in the upper crater wall indicating the provenance of those large fragments its consistency with a theoretical spallation model suggests that the large fragments were ejected from near surface of the target whereas the small fragments from deeper level the petrophysical properties of the ejecta fragments reflect the geophysical heterogeneity in the target basalt that significantly reduced the original size of spall fragments the presence of fe mg phyllosilicates smectites both in the ejecta and wall indicates the role of impact in excavating the phyllosilicates from the interior of basaltic target affected by aqueous alteration the seismic images reveal a thickness variation in the ejecta blanket segregation of boulders fractures and faults in the bedrock beneath the crater rim the fracturing fragmentation and fluvial degradation of lonar crater have important implications for mars

ionospheric electron content within the equatorial f 2 layer anomaly belt this paper describes some new results on the day to day behavior of the location of the equatorial anomaly in the columnar electron content the measurements were made from a unique network of stations covering dip latitudes from 0 n to 25 n during the period in 1975 1976 near sunspot minimum when the ats 6 satellite was visible from india the latitudinal distribution of the total electron content on a particular day was found to depend only on the strength of the electrojet current as determined by the difference of the horizontal magnetic field at stations on the magnetic equator and another outside the equatorial electrojet rather than on the horizontal magnetic field measured only at the equatorial station during magnetically disturbed periods the presence or absence of the anomaly was also found to depend only upon the strength of the electrojet current the varying electrojet current that is the equatorial electric field is very effective in moving the ionization to the anomaly latitudes of 15 20 but the columnar electron content over the magnetic equator remains relatively constant

measurement requirements for glacial uplift detection of nonadiabatic density gradients in the mantle recent isotope studies have rekindled interest in the possibility that the mantle may be chemically stratified and therefore nonadiabatic previous calculations of the last ice age are compatible only with a 1022p mantle calculations of the isostatic response of an adiabatic partially nonadiabatic and fully nonadiabatic 1022p mantle to the load cycle of the last ice age show that the uplift responses of an adiabatic and even partially nonadiabatic mantle are very different however geoid perturbations mask these differences and the emergence curves of the two mantle types are nearly identical except in the regions near and under the ice loads at the early stages of isostatic readjustment the calculations suggest that we must seek shoreline emergence data around glaciated areas more than 6000 years before present if nonadiabatic density gradients in the mantle are to be detected from emergence data

emplacement of sharp walled sulfide veins during the formation and reactivation of impact related structures at the broken hammer mine sudbury ontario broken hammer is a hybrid cu ni platinum group element pge footwall deposit located in archean basement rocks below the impact induced sudbury igneous complex sic canada the deposit consists

structural record of an oblique impact abstract most impactors strike their target at an oblique angle the common criterion for identifying craters formed by an oblique impact is the pattern of the ejecta blanket on earth however ejecta blankets are rarely preserved and other morphological structural or geophysical criteria are needed here we present structural details from the central uplift of the upheaval dome impact structure that are diagnostic of the kinematics during crater collapse and central uplift formation a characteristic imbrication of thrust slices towards the se the pattern of strata orientation within the central uplift dominant radial faults that accommodated nw se shortening and an elliptical bedding outline indicate that the displacement field during crater collapse has not been axial symmetric instead an additional lateral component roughly towards the se is preserved in the internal structure of the central uplift the structural asymmetries are largest in the core of the central uplift and disappear outwards thereby preserving the large scale circular shape of the main structural elements rim monocline ring syncline comparison with numerical models of oblique impacts suggests that the additional lateral displacement component reflects a downrange material transport during the initial stages of central uplift formation similar patterns are identified in other impact structures and may serve as general criteria for identifying the impact direction of deeply eroded impact structures in sedimentary targets

the formation of a giant collapse caprock sinkhole on the barda negra plateau basalts argentina magnetic mineralogical and morphostructural evidences the 1 5 km wide 40 m deep crater like structure in the 10 ma old barda negra basaltic plateau in central argentina was discovered in the early 2000s based on remote sensing surveys and on its morphological characteristics similar to those of the barringer crater in arizona the structure is described to be originated by an impact in this study we ran several field work campaigns and collected and analysed samples in order to find more evidences to endorse or reject this hypothesis we observe a circular depression not generally surrounded by raised rims in contrast to craters produced either by a meteorite impact or phreatomagmatic eruption maars mineralogical investigations of rocks and sediments do not show high pressure and temperature minerals such as coesite or stishovite or any remnants of an impactite or impact melt glass likewise no textural evidences for impact related fracturing or stress are observed a detailed geomorphological mapping indicates a successive crater development which invokes local stepwise subsidence magnetic mapping performed with the eu funded newton multisensor novel instrument shows a 2000 nt field anomaly associated to the edges of the crater and susceptibility measurements cast an important contrast between the basaltic rims and plateau and the crater interior therefore we propose a sink hole origin for the crater with a former collapse of the plateau basalts and a latter infill with sedimentary material this hypothesis is supported by the fact that the 40 to 85 m thick and 14 to 15 ma old carbonate bearing collon cura formation underneath the 100 to 150 m thick basaltic plateau lava sheet represents ideal rocks for dissolution and karst formation remote sensing data show other nearby sinkholes 20 km westward with extensions of 3 6 km and 100 m depth which are emplaced within a cogenetic neighboring basaltic plateau with a similar underlying lithology and the consistence of the magnetic model computed with this scheme and on measured rock remanence and susceptibilities of the structure and surroundings these giant collapse sinkholes up to 6 km in diameter within caprocks of very thick plateau basalts represent unique examples for planetary surface shaping processes which also occur on mars and comets in areas with ba salts or rigid caprocks

response of the equatorial and low latitude ionosphere to an intense x class solar flare x7 2b as observed on 09 august 2011 1 in this paper we present response of equatorial and low latitude ionosphere to an intense solar flare of class x7 2b that peaked at 08 05 ut on 09 august 2011 in the solar cycle 24 global positioning system total electron content tec observations in the sunlit hemisphere show enhancement of 3 tec units while geomagnetic h component observations indicate sudden decrease and increase in their strength at equatorial and low latitude stations respectively at several stations in the sunlit hemisphere in addition equatorial electrojet strength over indian region reveals commencement of counter electrojet simultaneous canadian advanced digital ionosonde observations at tirunelveli an equatorial station in india show the disappearance of ionogram echoes during the flare event indicating absorption of radio signals in the d region strong equatorial blanketing type es layer was observed in the ionogram records at tirunelveli prior to the occurrence of the solar flare that continued for several hours though it became weak absent during the flare event ionogram records on the control day show regular f layer movement without any blanketing type es layer very low frequency vlf observations at allahabad an indian low latitude station show enhanced vlf amplitude signal during the same time revealing the sudden enhancement of d region ionization using the observations presented here an attempt has been made to study the impact of the solar flares on the electrodynamics of the equatorial and low latitude ionosphere

new constraints on transient lower mantle rheology and internal mantle buoyancy from glacial rebound data a model of mantle rheology which has an explicit transient component of its relaxation spectrum is shown to reconcile two apparently conflicting inferences of lower mantle viscosity if recent analyses of isostatic geoid anomalies are correct in requiring a large increase of viscosity with depth then the weak viscosity stratification demanded by postglacial rebound data implies the importance of such rheological behaviour at least in the earth s lower mantle

the subsurface character of meteor crater arizona as determined by ground probing radar ground probing radar has provided new two dimensional stratigraphic and structural information on meteor crater arizona four 50 mhz transects inside the crater and one 100 mhz transect on a northeast radial across the exterior ejecta blanket were carried out the frequencies used were in relation to the desired depth of investigation greater than 50 m at 50 mhz and 30 m for 100 mhz the results obtained inside the crater show that the water table can be clearly detected at depths of 65 m as can the contact between the crater wall and the infilling allochthonous breccia lens in addition we were able to detect 1 point reflectors from relatively coherent blocks 1 5m within the breccia lens 2 a zone 7 m thick relatively free of point reflectors from the immediate fall back unit 3 a series of subhorizontal reflectors a few meters apart due to bedding in the post impact crater sediments and 4 five separate subhorizontal reflectors from what we interpret to be a series of postimpact paleo water table levels in the breccia lens the ejecta blanket transect indicates a 4 10 m thick ejecta layer and the underlying moenkopi formation sandstone as well as the moenkopi kaibab contact and bedding within the kaibab formation in addition reverse faults with displacements up to 4 m were detected in the bedrock as well as changes from subhorizontal to 30 in the dip of the bedrock as the crater rim is approached from the northeast beneath the ejecta blanket the total uplift observed at the crater rim is around 55 m the radar data indicate uplift effects out to a radial distance of 400 m from the rim farther than that determined from drilling 300 m the results demonstrate the utility of continuous ground probing radar cross sections in studying the near subsurface 50 m character at meteor crater this relatively rapid low cost technique can be extended to other structures but confident interpretation requires supplemental ground truth constraints

origin of the structure and planform of small impact craters in fractured targets endurance crater at meridiani planum mars we present observations and models that together explain many hallmarks of the structure and growth of small impact craters forming in targets with aligned fractures endurance crater at meridiani planum on mars diameter 150 m formed in horizontally layered aeolian sandstones with a prominent set of wide orthogonal joints a structural model of endurance crater is assembled and used to estimate the transient crater planform the model is based on observations from the mars exploration rover opportunity a bedding plane orientations and layer thicknesses measured from stereo image pairs b a digital elevation model of the whole crater at 0 3 m resolution and c color image panoramas of the upper crater walls this model implies that the crater s current shape was mostly determined by highly asymmetric excavation rather than long term wind mediated erosion we show that modal azimuths of conjugate fractures in the surrounding rocks are aligned with the square component of the present day crater planform suggesting excavation was carried farther in the direction of fracture alignments this was previously observed at barringer crater in arizona and we show the same relationship also holds for tswaing crater in south africa we present models of crater growth in which excavation creates a stellate transient cavity that is concave cuspate in planform these models reproduce the lenticular crescentic layering pattern in the walls of some polygonal impact craters such as endurance and barringer craters and suggest a common origin for tear faults and some crater rays we also demonstrate a method for detailed error analysis of stereogrammetric measurements of bedding plane orientations

dynamic fracturing field and experimental observations abstract we analyzed a system of complex joints in thick dolomite layers that are exposed within the western margins of the dead sea basin these joints display two dominant features tree like branching and a gradual increase of density that leads to local fragmentation the development of this joint system is investigated in laboratory experiments with samples of brittle ductile layered composites the samples were subjected to layer parallel extension and displayed three styles of fracturing planar fractures known from previous tests branching fractures and clustering fractures observed here for the first time in layered composites based on fracture morphology we deduced that the branching and clustering fractures in the experiments and the tree like closely spaced joints in the field propagated at dynamic high velocity growth rates it is proposed that the morphological features described here could be used as field criteria to recognize dynamic rates of rock fracturing

glacial isostatic adjustment and the free air gravity anomaly as a constraint on deep mantle viscosity summary a gravitationally self consistent theory for relative sea level variations forced by pleistocene deglaciation events is employed to explore the extent to which rsl and free air gravity observations together constrain the viscosity of the mantle beneath the seismic discontinuity at 670 km depth a trade off is revealed between errors in the assumed deglaciation history and errors in the inferred value of the viscosity of the lower mantle taking full account of such uncertainty plausible values of the viscosity beneath the transition region are bounded above by 1023 poise cgs units the preferred value is at least a factor of 2 lower than this and is strongly constrained by the observed free air gravity anomaly over hudson bay the calculations described in detail here show for the first time that the relatively large gravity anomalies observed over sites of wurm wisconsin deglaciation do not require any pronounced increase of mantle viscosity with depth in order to explain them

a deep groundwater origin for recurring slope lineae on mars the recurring slope lineae on mars have been hypothesized to originate from snow melting deliquescence dry flow or shallow groundwater except for the dry flow origin these hypotheses imply the presence of surficial or near surface volatiles placing the exploration and characterization of potential habitable environments within the reach of existing technology here we present observations from the high resolution imaging science experiment heat flow modelling and terrestrial analogues which indicate that the source of recurring slope lineae could be natural discharge along geological structures from briny aquifers within the cryosphere at depths of approximately 750 m spatial correlation between recurring slope lineae source regions and multi scale fractures such as joints and faults in the southern mid latitudes and in valles marineris suggests that recurring slope lineae preferably emanate from tectonic and impact related fractures we suggest that deep groundwater occasionally surfaces on mars in present day conditions observations and heat flow modelling suggest that briny groundwater surfacing from fractures forms recurring slope lineae on mars

shock pressure estimates in target basalts of a pristine crater a case study in the lonar crater india the study of shock pressure indicators can provide important clues for understanding the cratering process though the estimation of shock pressures in weakly shocked rocks is commonly difficult in this study we selected a very young and well preserved impact structure the lonar crater in india the crater devoid of any tectonic overprint can be assumed as pristine we used a combination of rock magnetic and microfracture studies to estimate shock pressure in the crater rim on the basis of present results the magnetic fabrics are interpreted to be of magmatic origin related to the deccan basalt emplacement the high coercivity component of the natural remnant magnetization in the crater rim basalt is similar to that in the unshocked basalt the lack of any shock related magnetic overprint on the crater rim basalt is therefore evident in the lonar crater in contrast radial and concentric microfractures observed in basalts at the crater rim and farther away show symmetric distribution with respect to the crater the concentric microfractures consistently overprint the radial microfractures we infer that the radial and concentric microfractures were developed during propagation of the early compressional and the late decompressional shock wave components respectively the results of our rock magnetic and microfracture studies when interpreted in light of published experimental and numerical simulation studies on the lonar basalt reveal that the shock pressure in the lonar crater rim was less than 0 5 gpa but greater than 0 2 gpa this shock pressure was high enough to produce fractures but too low to affect the magnetic fabrics these results give new information on the relationship between shock pressure and resulting microfractures

impact spallation processes on the moon a case study from the size and shape analysis of ejecta boulders and secondary craters of censorinus crater abstract impact spallation is a fundamental process responsible for formation of ejecta boulders from impact craters although theoretical spallation models were developed about three decades ago only limited geological observations have been made so far to test these models the 3 8 km censorinus crater on the moon provides an excellent opportunity for studying the impact spallation processes associated with a fresh simple crater formed by oblique impact using the lunar reconnaissance orbiter narrow angle camera images we prepared the ejecta boulder distribution map of censorinus crater and measured the boulder sizes and shapes mapping of about 242 000 ejecta boulders enabled us to document the size distribution of boulders both radial and concentric to the impact crater larger size boulders dominate the crater rim areas while they become smaller away from the crater the boulder distribution exhibits a radial asymmetry suggesting censorinus is a oblique impact in which the uprange ejecta have smaller ranges with larger concentration of boulders near the southwestern crater rim while the downrange ejecta are in general characterized by smaller boulders with high spatial dispersion the cumulative size frequency distribution csfd of boulders shows a highly variable fragmentation history in which the uprange boulders suffered more complex fragmentation the ejecta boulders also exhibit a variety of shapes that are gleaned from their axial ratios and edge angle characteristics there is a general decrease of axial ratios away from the crater rim rectangular boulders dominate the crater rim and they become more equant away from the crater in addition to the boulder sizes the boulder shape distribution also exhibits a mild asymmetry in response to the oblique impact small size fresh impact craters 84 000 craters are abundant on the censorinus ejecta and post date censorinus these craters are found in two morphologic types in which a large majority of craters have subdued ejecta rayless craters while some possess bright rayed ejecta bright rayed craters the csfd of rayless craters show a steep power law slope with a b value of 4 0 similar to the secondary craters produced by the impact of ejecta from primary craters we therefore interpret the rayless craters as the secondary craters of censorinus on the other hand the csfd of bright rayed craters have smaller power law slope b value 2 7 which is a characteristic of primary craters and thus provide 3 ma age for censorinus crater when the characteristics of censorinus boulders are compared with the theoretical spallation models that are sensitive to the petrophysical properties of the target lunar highland the models generally agree with the censorinus boulders however the observed shape and size characteristics of the censorinus boulders are found to be more complex than the theoretical spallation models the ejecta boulders suffered more complex fragmentation and asymmetric distribution in response to the oblique impact the spallation models accounting oblique impacts have not yet been developed therefore our censorinus boulder observations can be used to develop and validate the new theoretical spallation models for the effects of oblique impacts

structural effects of meteorite impact on basalt evidence from lonar crater india received 31 january 2005 revised 17 august 2005 accepted 26 august 2005 published 8 december 2005 1 lonar crater is a simple bowl shaped near circular impact crater in the 65 myr old deccan volcanic province in india as lonar crater is a rare terrestrial crater formed entirely in basalt it provides an excellent opportunity to study the impact deformation in target basalt which is common on the surfaces of other terrestrial planets and their satellites the present study aims at documenting the impact deformational structures in the massive basalt well exposed on the upper crater wall where the basalt shows upward turning of the flow sequence resulting in a circular deformation pattern three fracture systems radial concentric and conical fractures are exposed on the inner crater wall on the fracture planes plumose structures are common uplift and tilting of the basalt sequence and formation of the fractures inside the crater are clearly related to the impact event and are different from the preimpact structures such as cooling related columnar joints and fractures of possible tectonic origin which are observed outside the crater slumping is common throughout the inner wall and listric faulting displaces the flows in the northeastern inner wall the impact structures of lonar crater are broadly similar to those at other simple terrestrial craters in granites and clastic sedimentary rocks and even small scale experimental craters formed in gabbro targets as lonar crater is similar to the strength controlled laboratory craters impact parameters could be modeled for this crater provided maximum depth of fracture formation would be known citation kumar p s 2005 structural effects of meteorite impact on basalt evidence from lonar crater india j geophys res 110 b12402 doi 10 1029 2005jb003662

origin and significance of decameter scale polygons in the lower peace vallis fan of gale crater mars abstract decameter scale polygons are extensively developed in the bedded fractured bf unit of the lower peace vallis fan the polygons occur in a likely extension of the gillespie lake member north of yellowknife bay the section first drilled by the mars science laboratory msl mission we examine hypotheses for the origin of these polygons to provide insight into the history of gale crater the polygons are 4 30 m across square to rectangular and defined by 0 5 4 m wide generally straight troughs with orthogonal intersections polygon networks are typically oriented orthogonal systems with occasional nearly circular patterns hundreds of meters across potential origins include cooling of lava and for sedimentary units syneresis unloading weathering desiccation impact processes and cold climate thermal contraction cold climate thermal contraction is the hypothesis most consistent with the sedimentary nature of the bf unit and the polygon morphology geometry networks and apparent restriction to the coarse grained gillespie lake member a periglacial setting further provides the best analogs for the circular networks and is consistent with geologic context and msl data most of the decametric polygons appear to be ancient they are confined to the hesperian bf unit and only a few of their bounding fractures extend into younger or recently exposed units in this regard they differ from the majority of proposed thermal contraction polygons on mars as those are generally thought to be young features and accordingly the history of formation preservation and reactivation of the decametric polygons is likely to be more complex than that of any proposed young polygons on mars the decametric polygons in the bf unit may represent landforms developed in a cold but still comparatively wet interlude between a clement early mars and the much drier and colder planet of today

geology of lonar crater india lonar crater india is one of the youngest and best preserved impact structures on earth the 1 88 km diameter simple crater formed entirely within the deccan traps making it a useful analogue for small craters on the basaltic surfaces of the other terrestrial planets and the moon in this study we present a meter scale resolution digital elevation model geological map of lonar crater and the surrounding area and radiocarbon ages for histosols beneath the distal ejecta impact related deformation of the target rock consists of upturned basalt fl ows in the upper crater walls and recumbent folding around rim concentric subhorizontal noncylindrical fold axes at the crater rim the rim fold hinge is preserved around 10 15 of the crater although tearing in the rim fold is inferred from fi eld and paleomagnetic observations no tear faults are identifi ed indicating that large displacements in the crater walls are not characteristic of small craters in basalt one signifi cant normal fault structure is observed in the crater wall that offsets slightly older layer parallel slip faults there is little fl uvial erosion of the continuous ejecta blanket portions of the ejecta blanket are overlain by aerodynamically and rotationally sculpted glassy impact spherules in particular in the eastern and western rim as well as in the depression north of the crater known as little lonar the emplacement of the continuous ejecta blanket can be likened to a radial groundhugging debris fl ow based on the preserved thickness distribution of the ejecta the effi cient exchange of clasts between the ejecta fl ow and the underlying histosol and the lack of sorting and stratifi cation in the bulk of the ejecta the ejecta profi le is thickened at the distal edge and similar to fl ejecta structures observed on mars

a paleoclimate model of northern hemisphere ice sheets abstract a model for predicting the growth and decay of ice sheets based on the astronomical theory of climate change is presented the purpose of the study in part is to isolate the role of the ice sheet physics and earth response under varying ice load by simplifying to the extreme the role of the hydrosphere atmosphere ice sheet physics and the response of the lithosphere asthenosphere under the ice load are modeled explicitly insolation anomalies taken at a fixed latitude directly force latitudinal displacement of the snow line accumulation rate a and ablation rate a evaluated at mean sea level are specificed as external constants a a decrease linearly with ice sheet elevation rough tuning of the model to the general shape of the ice volume record of the last two major glacials determines the external constants model predictions of the ages of several events in the last major glaciation compare well with the radiological ages the six glacial terminatios i vi over the last 600 000 yr are identified and the predicted ages compare reasonably well with the 18 o record for two deep sea cores a direct comparison of model power spectra of ice volume as a function of period with spectra of the 18 o record shows apparent underprediction of power near 100 000 yr when a quantitative but heuristic method for taking into account the red noise spectrum evident in the geological records is used a much more favorable comparison is possible the model prediction lends support to the hypothesis that the nonlinearity of the ice sheet physics is responsible for the 100 000 yr periodicity in the geological record of the late pleistocene

fluids mobilization in arabia terra mars depth of pressurized reservoir from mounds self similar clustering abstract arabia terra is a region of mars where signs of past water occurrence are recorded in several landforms broad and local scale geomorphological compositional and hydrological analyses point towards pervasive fluid circulation through time in this work we focus on mound fields located in the interior of three casters larger than 40 km firsoff kotido and unnamed crater 20 km to the east and showing strong morphological and textural resemblance to terrestrial mud volcanoes and spring related features we infer that these landforms likely testify the presence of a pressurized fluid reservoir at depth and past fluid upwelling we have performed morphometric analyses to characterize the mound morphologies and consequently retrieve an accurate automated mapping of the mounds within the craters for spatial distribution and fractal clustering analysis the outcome of the fractal clustering yields information about the possible extent of the percolating fracture network at depth below the craters we have been able to constrain the depth of the pressurized fluid reservoir between 2 5 and 3 2 km of depth and hence we propose that mounds and mounds alignments are most likely associated to the presence of fissure ridges and fluid outflow their process of formation is genetically linked to the formation of large intra crater bulges previously interpreted as large scale spring deposits the overburden removal caused by the impact crater formation is the inferred triggering mechanism for fluid pressurization and upwelling that through time led to the formation of the intra crater bulges and after compaction and sealing to the widespread mound fields in their surroundings

influence of glacial isostatic adjustment on tide gauge measurements of secular sea level change modem tide gage records of secular sea level change are strongly contaminated by the ongoing influence of glacial isostatic adjustment in this paper we employ an accurate high resolution model of the glacial isostatic adjustment process to filter this signal from time series consisting of annual mean heights observed on over 500 tide gages from which records of duration longer than 10 years are available these records are all contained in the archive of the permanent service for mean sea level at bidston in the united kingdom prior to removal of the isostatic adjustment contamination from these records the secular sea level change signal is seen to exhibit both rising and falling levels depending upon geographic location with regions of falling level corresponding to those that were ice covered during the last glaciation event of the present ice age after filtering the geographic heterogeneity in rates is considerably reduced so much so that a globally coherent component consisting of a rise of sea level at a rate of 2 4 0 9 mm yr 1 is revealed this could conceivably be driven by the greenhouse effect although no particular mechanism has been unambiguously identified by our analyses we investigate the impact of a number of different variants of the analysis procedure upon the inferred global signal

rim uplift and crater shape in meteor crater effects of target heterogeneities and trajectory obliquity 1 we have analyzed the rim structure of meteor crater arizona in order to understand the mechanism of rim uplift in simple craters and the causes of the shape of polygonal impact craters for this purpose we systematically determined bedding orientation of the autochthonous crater wall and overturned flap and analyzed the kinematics of major radial faults we found that rim uplift correlates with the crater shape and increases in the corners of the crater the two main mechanisms of differential uplift are the formation of horizontal interthrust wedges leading to the doubling of strata in the rim and radial corner faults or tear faults that vertically displace bedrock the development of meteor crater s quadrangular shape is caused by more effective crater excavation flow parallel to major joint sets additionally we infer the impact direction with a newly developed technique the two corners model and review the arguments in favor of an oblique trajectory while the data set is ambiguous several indicators suggest an impact direction from the nnw we conclude that oblique impacts should have an effect on early cratering and excavation flow whereas target heterogeneities like joints start to play a prominent role in later stages when the stresses induced by the excavation flow are in the same order of strength as the material involved

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

continental lithospheric thickness and deglaciation induced true polar wander the secular drift of the earth s rotation pole which is evident in ils data is a memory of the planet of the last deglaciation event of the current ice age in order to fit the observed rate and direction of drift with an earth model whose viscosity profile is capable of reconciling the free air gravity and relative sea level data of postglacial rebound requires a lithospheric thickness which is substantially in excess of that which might be considered an appropriate average thickness for the entire planet this required thickness which is greater than 250 km is nevertheless in accord with independent estimates of the thickness of the continental lithosphere

seismic constraints on the morphology of deep slabs residual sphere images from deep earthquakes not only detect the presence of slab associated velocity anomalies but also lend insight into the flow and deformation of lithosphere subducted into the lower mantle we have compared travel times from deep events in the kuril and mariana arcs with the seismic velocity anomalies implied by kinematical models that thicken the slab perpendicular to its plane by reducing the vertical velocity of the flow with depth we assume that the details of the deformation whether the slab buckles imbricates fragments etc are averaged out along the ray paths and hence our models constrain the scale not the mode of slab thickening the deep event travel times are best fit by undeformed models but the ability of the residual sphere method to resolve slab thickness is limited by ray bending effects although the mariana times are consistent with advective thickening factors of 5 or more factors larger than 3 are ruled out by the kuril data for all models examined the data require that slab material extends to depths of 900 1000 km global tomographic models and regional studies which delineate high velocity anomalies in the lower mantle beneath zones of cenozoic subduction are consistent with our results as is recent work on pulse distortion by slab gradients comparison of observed and predicted rates of seismic moment release suggests that if substantial advective thickening does occur it is largely aseismic

dynamic tensile strength of terrestrial rocks and application to impact cratering dynamic tensile strengths and fracture strengths of 3 terrestrial rocks san marcos gabbro coconino sandstone and sesia eclogite were determined by carrying out flat plate pmma and aluminum impact experiments on disc shaped samples in the 5 to 60 m sec range tensile stresses of 125 to 300 mpa and 245 to 580 mpa were induced for gabbro and eclogite respectively with duration time of 1 s for sandstone porosity 25 tensile stresses normal to bedding of 13 to 55 mpa were induced with duration times of 2 4 and 1 4 s tensile crack failure was detected by the onset of shock induced damage p and s wave velocity reduction the dynamic tensile strength of gabbro determined from p and s wave velocity deficits agrees closely with the value of previously determined values by post impact microscopic examination 150 mpa tensile strength of coconino sandstone is 20 mpa for a 14 s duration time and 17 mpa for a 2 4 s duration time for sesia eclogite the dynamic tensile strength is 240 mpa the fracture strength for gabbro is 250 mpa 500 mpa for eclogite and 40 mpa for sandstone relative crack induced reduction of s wave velocities is less than that of post impact p wave velocity reductions for both gabbro and eclogite indicating that the cracks were predominantly spall cracks impacts upon planetary surfaces induce tensile failure within shock processed rocks beneath the resulting craters the depth of cracking beneath impact craters can be determined both by seismic refraction methods for rocks of varying water saturation and for dry conditions e g the moon from gravity anomalies in principle depth of cracking is related to the equations of state of projectile and target projectile dimension and impact velocity we constructed a crack depth model applicable to meteor crater for the observed 850 m depth of cracking our preferred strength scaling model yields an impact velocity of 33 km s and impactor radius of 9 m for an iron projectile

study of ionospheric response to the 4b flare on 28 october 2003 using international gps service network data 1 using the gps data from as many as 114 gps stations of the international gps service for geodynamics igs the morphological features of the ionospheric total electron content tec variations on the sunlit hemisphere during the 4b solar flare on 28 october 2003 is studied it is found that the strongest sudden increase of tec sitec happened during the flare and the magnitudes of sitec vary at regions with different local solar zenith angle sza in the northern hemisphere the tec enhancement is approximately symmetrical to the local noon and its value is usually greater than 14 tecu 1 tecu 1016 m2 if the sza is less than 60 on the whole as the sza increases the value of tec enhancement in the northern hemisphere decreases it is worth mentioning that even in the regions of sza between 90 and 100 the sitec was still seen from the temporal tec curves using a photochemical model the electron production rate over the sunlit boundary region is calculated and some obvious features of sitec over this region are analyzed in the polar region the effect of this flare on the ionosphere exceeds the effect of the ionospheric scintillations and it seems that the ionosphere in the northern polar region responses more sensitively to this flare in the end superimposed on the curves of the rate of tec change there are some small disturbances spikes synchronously appearing on all curves and thus indicating an existence of similar structures in the euv band of the flare

viscous gravitational relaxation summary this paper is concerned with a detailed examination of the response of maxwell models of the planet to surface mass loads particular attention is devoted to an examination of the factors which determine the isostatic response since the understanding of this response is crucial in a number of different geodynamic problems one particular example which we discuss in detail is concerned with the prediction of free air gravity anomalies produced by large scale deglaciation events using the methods developed here we are able to provide the first direct assessment of the importance of initial isostatic disequilibrium on the observed relative sea level variations and free air gravity anomalies forced by the melting of the laurentide ice sheet we are therefore able to estimate the extent to which such initial disequilibrium might influence the inference of mantle viscosity from isostatic adjustment data our calculations establish that free air gravity data although they are sensitive to the degree of initial disequilibrium provide an extremely high quality constraint upon the viscosity of the lower mantle

pleistocene deglaciation and the global gravity field the present day global geoid and free air gravity anomaly signals induced by pleistocene deglaciation have been computed along with their secular variations a revised green function has been derived for the free air gravity anomaly and we find that an earth model with a lower mantle viscosity moderately larger than the upper mantle value satisfies observations of the anomaly over both hudson s bay and fennoscandia using this preferred earth model the predicted global geoid anomaly map is characterized by peak negative values of 36 7 9 4 and 21 9 m over hudson s bay northern europe and antarctica respectively and by a smooth small amplitude 2 5 m upwarping over the major ocean basins although the field has an order of magnitude less power than the observed there is a significant spatial correlation between the two inferences of mantle viscosity based upon comparisons between the observed geoid and the computed deglaciation induced anomaly are hampered by the large contribution from the convective circulation in the earth s mantle we have subtracted a predicted convection signal based on seismic tomography from the observed field however the residual geoid thus obtained is still too inaccurate to allow a useful comparison in contrast a filtered version of the observed geoid over canada which retains harmonic coefficients in the degree range 10 22 provides a relatively uncontaminated by convection datum that is indeed satisfied by the preferred earth model secular variations in the earth s gravitational field are naturally dominated by the signal from pleistocene deglaciation in this respect we compute zonal harmonics in the expansion of the present day rate of change of the geopotential and in particular we predict amplitudes comparable to that for the previously analysed j2 for j4 j5 j7 and j9 all of which reflect in large part the response due to the melting of the massive laurentide ice sheet this analysis suggests that measurement of these higher order secular variations of the zonal harmonics based upon analysis of long time series of lageos ranging data could provide very useful additional constraints on the radial variation of mantle viscosity these constraints along with all others described in this paper are shown to be relatively insensitive to assumptions concerning the deflection during the glacial isostatic adjustment process of the 400 and 670 km density discontinuities within the mantle

pleistocene deglaciation and the earth s rotation a new analysis summary this paper is concerned with an analysis of the effect of the pleistocene glacial cycle upon the earth s rotation we demonstrate that two important geophysical observables may be explained as aspects of the rotational response to surface mass loading by ice sheets these are the astronomically observed non tidal component of the acceleration of planetary rotation and the secular drift of the rotation pole relative to the surface geography which is evident in the ils pole path the former observation is shown to provide an unambiguous constraint upon the viscosity of the planetary mantle and requires a depth dependence of this parameter which is the same as that which has previously been inferred in studies of the relative sea level variations and free air gravity anomalies associated with post glacial rebound the observed secular drift of the pole cannot provide an independent estimate of the viscosity of the mantle because it depends jointly upon mantle viscosity and lithospheric thickness with the viscosity profile fixed by the rebound data the observed speed of polar wander suggests a continental lithospheric thickness which is in excess of that appropriate for old ocean basins but nevertheless in accord with independent constraints

relative depths of simple craters and the nature of the lunar regolith abstract we assessed the morphologies of more than 930 simple impact craters diameters 40 m 10 km on the moon using digital terrain models dtms of a variety of terrains in order to characterize the variability of fresh crater morphology as a function of crater diameter from lunar reconnaissance orbiter camera lroc narrow angle camera nac dtms we determined depth to diameter d d ratios for an extremely fresh set of these craters with diameters less than 400 m and found that their d d ratios range from 0 11 to 0 17 using both nac and kaguya terrain camera dtms we also determined the d d ratios for the set of fresh simple craters larger than 400 m in diameter the d d ratios of these larger craters are typically near 0 21 as expected of gravity dominated crater excavation fresh craters less than 400 m in diameter on the other hand exhibit significantly lower d d ratios various possible factors affect the morphologies and relative depths d d ratios of small strength dominated craters including impactor and target properties e g effective strength strength contrasts porosity pre existing weaknesses impact angle and velocity and degradation state while impact conditions resulting from secondary impacts can also affect crater morphologies we found that d d ratio alone was not a unique discriminator of small secondary craters to investigate the relative influences of degradation and target properties on the d d ratios of small strength dominated craters we examined a subset of fresh craters located on the geologically young rim deposits of tycho crater these craters are deeper and steeper than other craters of similar diameter and degradation state consistent with their relative freshness and formation in the relatively coherent melt rich deposits in this region the d d ratios of globally distributed small craters of similar degradation state and size range on the other hand are relatively shallow with lower average wall slopes consistent with crater excavation in a weak or poorly cohesive layer the widespread predominance of these small shallow craters is consistent with the pervasive poorly cohesive upper regolith

boulder abundances and size frequency distributions on oxia planum mars scientific implications for the 2020 esa exomars rover abstract this paper presents the abundances and the size frequency distributions sfd of boulders identified on a sector of the prime landing site for the exomars 2020 rover in oxia planum region by means of a hirise image boulders 1 75 m across have been identified and subdivided according to the two main oxia planum geological units the noachian clay rich formation nc and the amazonian volcanic deposit av the spatial density of boulders 1 75 m over the entire study area is 6 75 10 4 m2 with a size frequency that is best fit both with power law and exponential law curves with indices of 4 9 0 1 0 2 and 1 29 0 04 0 06 respectively significant differences were found by analysing separately the av and nc geological units the data collected in the av unit are well fitted with a power law curve with an index equal to 4 8 0 2 and with an exponential law curve with an index of 1 24 0 05 0 06 whilst in the nc unit such indices are 5 5 0 3 0 4 and 1 70 0 09 0 12 power law and exponential law curve respectively the spatial density of boulders in the av unit is 7 0 times larger than in the nc one this may be due primarily to the distinct mechanical properties of the two units that may result in a different production rate or preservation of the boulders secondly the av unit overlies the nc unit possibly resulting in more impacts and or different weathering processes throughout the ages this study provides a quantitative evaluation of the abundances of boulders 1 75 m across on oxia planum it is therefore a reference for the exomars 2020 mission both during the landing phase and the rover traverse to specific areas of interest the landing ellipse presents much higher abundances of boulders 1 75 m than all previous martian rover landing areas this is particularly evident when the rougher av unit is taken into account contrarily the nc unit shows a much more comparable value but still slightly higher to the mars pathfinder landing site we provide illustrative navigating scenarios for both the nc and the av units as well

physical properties of basalt ejecta boulders at lonar crater india insights into the target heterogeneity and impact spallation processes in basalt with application to mars

gullies and landslides on the moon evidence for dry granular flows 1 high resolution images from chandrayaan 1 terrain mapping camera and lunar reconnaissance orbiter camera reveal landslides and gully formation on the interior wall of a 7 km diameter simple crater emplaced in schrodinger basin on the farside of the moon these features occur on the steep upper crater wall where the slope is 35 the gullies show a typical alcove channel fan morphology some gullies incise bedrock where impact related faults are present slope failure along the concentric faults also led to formation of landslides dark slope streaks are abundant at the bright gully regions especially near the fan and channel deposits spectral characteristics inferred from data obtained by hyperspectral imager and moon mineralogy mapper on board chandrayaan 1 show that the gullies and landslides are characterized by high optical immaturity and devoid of prominent spectral absorption features related to water or hydroxyl molecules suggesting youthful dry granular flows mass movements on the crater wall led to the formation of arcuate ridges and ponding of fine grained sediments on the crater floor runout flows from small impact craters on the slopes indicate that impact induced seismic shaking was responsible for the downslope mass movements crater size frequency distributions suggest a minimum age of 18 2 ma for the gullies and 2 ma for the landslides while age of the host crater ejecta was inferred to be about 175 ma the gullies and landslides also occur on the interior wall of other impact craters elsewhere on the moon and probably formed by similar processes

outcrops on lunar crater rims implications for rim construction mechanisms ejecta volumes and excavation depths the raised rims of impact craters consist of ejecta deposited onto structurally uplifted target rocks although it is commonly accepted that ejecta makes up 50 75 of the total rim height no previous measurements on well preserved naturally occurring impact craters exist relying on data from the lunar reconnaissance orbiter i report the first direct measurements of the rim forming constituents from 21 lunar craters ranging in diameter from 2 2 km to 45 km results show that the ejecta deposit accounts for no more than 20 of rim relief and structural uplift is the primary mechanism of rim development in both simple and complex craters thick quasi horizontal outcrops of coherent bedrock indicate that this uplift is the result of subsurface injection of debris or melt into the surrounding target rocks crater growth at least during the latest portion of the excavation stage therefore proceeds mainly through injection rather than ejection of material these results suggest that ejecta volumes and excavation depths may be factors of 3 to 4 less than previously considered

impact fracturing and structural modification of sedimentary rocks at meteor crater arizona 1 meteor crater provides a rare opportunity to study impact deformation of sedimentary target rocks and isolate those features from preexisting tectonic deformation and impact generated reactivation of preexisting tectonic features this study reports over 2500 new measurements of orientations of bedding faults and fractures in crater walls and in surrounding bedrock target rocks are characterized by horizontal bedding planes that are cut by at least three prominent sets of preimpact tectonic fracture systems the crater rim is also cut by three distinct groups of fractures radial concentric and conical fractures when the crater rim is restored to preimpact condition the radial and concentric fractures resemble preimpact fracture populations indicating that crater wall deformation and rim uplift were partly accommodated by activation of preexisting fractures in contrast the conical fractures are dissimilar to the preimpact fractures and apparently formed as a direct result of impact deformation some of the preimpact fractures were transformed into tear faults during the impact event and motion along those faults appears to have controlled the geometry of the impact deformational features the crater rim is thus square in plan view rather than circular faults occurring in the crater diagonals are prominent ones allowing greater vertical displacement the deformation pattern of meteor crater is different from that at lonar crater which was excavated in basalt with fewer preimpact fractures the differences between deformation at meteor crater and lonar crater may reflect the same disparities seen in simple craters produced in different target lithologies on mars and other planetary surfaces