the spatial distribution of impact craters on ryugu abstract asteroid 162173 ryugu has numerous craters the initial measurement of impact craters on ryugu by sugita et al 2019 is based on hayabusa2 onc images obtained during the first month after the arrival of hayabusa2 in june 2018 utilizing new images taken until february 2019 we constructed a global impact crater catalogue of ryugu which includes all craters larger than 20 m in diameter on the surface of ryugu as a result we identified 77 craters on the surface of ryugu ryugu shows variation in crater density which cannot be explained by the randomness of cratering there are more craters at lower latitudes and fewer at higher latitudes and fewer craters in the western bulge 160 e 290 e than in the region around the meridian 300 e 30 e this variation implies a complicated geologic history for ryugu it seems that the variation in crater density indicates that the equatorial ridge located in the western hemisphere is relatively young while that located in the eastern hemisphere is a fossil structure formed during the short rotational period in the distant past

ejecta from impact craters an important feature of impacts into solar system bodies is the fate of crater ejecta the near surface material launched during the highly dynamic crater formation process laboratory measurements of impact crater ejecta from 18 studies are summarized the data are examined and used to assess our understanding of how the ejecta velocity and mass distributions depend on the conditions of an impact event the effects of impact speed on the ejecta are reasonably well understood but the dependences on target properties such as strength and porosity are only poorly constrained a point source scaling model for the ejecta mass and velocity distributions is developed and fit to the data for several classes of materials distinguished by porosity

impact craters and venus resurfacing history venusian impact crater size frequency distributions locations and preservation states were analyzed to reconstruct the history of resurfacing by tectonism and volcanism an atmospheric transit model for meteoroids demonstrates that for craters larger than about 30 km the size frequency distribution is close to the atmosphere free case with this result and assuming that the surface records a crater production population a catastrophic resurfacing model crm an age of cessation of rapid resurfacing of 500 ma is obtained crater locations are widely dispersed across venus and the hypothesis that they are completely spatially random csr cannot be rejected however craters that show embayment by plains materials or modification by throughgoing faults i e tectonized are preferentially found in areas with relatively few craters overall the primary region where these modified craters are found is the aphrodite volcanotectonic zone extending from ovda regio on the west to the region east of atla regio these results together with the appearance of plains material on most crater floors and evidence for complex volcanic stratigraphy imply that a range of surface ages are recorded by the impact crater population e g the aphrodite zone is relatively young an end member model equilibrium resurfacing model erm was developed to quantify resurfacing scenarios in the erm venus has been resurfacing at an average rate of approximately 1 km2 yr 1 however the crm and erm are idealized end member representations of possible resurfacing histories for both models the resurfacing rate can be expressed as the product of resurfacing patch area a normalized by planetary surface area and the frequency of resurfacing events numerical simulations of resurfacing showed that there are two solution branches that satisfy the csr constraint a 0 1 74 diameter circle the former range corresponds to resurfacing diameters smaller than the average intercrater distance whereas the latter is associated with large infrequent events resurfacing 10 of the planet every 50 ma to 100 every 500 ma the observed fraction of embayed and tectonized craters further constrains values of a and only values near 0 0003 are admissible the resurfacing model that best fits all of the statistical and geological constraints has resurfacing with small patches that occurs in any given geological episode in only a limited number of regions on the planet

mass fluxes in the canary basin abstract ocean studies in the 1970s provided an improved knowledge of the coastal upwelling region off nw africa while in the 1980s and 1990s they led to a good description of the open ocean flow patterns in the canary basin it was not until the late 1990s that major research addressed the open coastal ocean coupled response here we examine the mean and seasonal circulation patterns in the canary basin with data from four hydrographic cruises carried out in the region between cape ghir madeira island and the canary islands we apply an inverse box model to an ocean divided into 14 layers with several layers representing each water mass or stratum to obtain mass fluxes consistent with the thermal wind equation an optimum flow description is obtained using conservation of mass salt and heat anomaly biologically corrected oxygen and silicate and allowing for ekman transport in the surface layer and dianeutral mixing between adjacent layers the deep waters show no predominant flow direction while the intermediate waters display localized southward flowing mediterranean water far from shore and northward flowing antarctic intermediate water near the continental slope specially in the passage between the eastern canary islands and the african slope the mean upper thermocline canary current composed of north atlantic central water flows south with an open ocean branch transporting about 3 1 sv 1 sv 10 6 m 3 s 1 10 9 kg s 1 and an upwelling related branch near the continental slope carrying 1 0 3 sv the seasonal transport by the open ocean branch intensifies and moves offshore from spring to fall 2 8 1 2 sv in spring 2 9 1 1 sv in summer and 4 5 1 2 sv in fall while it carries its lowest southward mass flux in winter 1 7 1 0 sv possibly as a result of a migration offshore the sampled region upwelling related southward flow is present in spring and summer 1 9 0 1 sv and 2 4 0 1 sv respectively while in fall and winter it merges with the offshore southward branch this westward migration allows a northward mass flux between the canary islands and the african coast 1 8 0 1 sv that by winter reaches cape ghir 0 5 0 2 sv seasonal air sea heat fluxes fit well with the climatological values the net phosphate transport in the surface layer indicates that primary production was negligible in this region during january 1997 and april 1998 though in this last month production was probably starting as a result of significant nutrient supply

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

residual currents in a semi enclosed bay of the seto inland sea japan 1 on the basis of the princeton ocean model a robust diagnostic model is developed to calculate the residual currents in the hiuchi nada a semi enclosed bay of the seto inland sea the input water temperature and salinity data are from six hydrographic surveys in 2002 and the model results in the summer show a clockwise eddy in the western part of the bay and an anticlockwise eddy in the eastern part the magnitude of two eddies is 10 cm s the modeled flow pattern is consistent with early observations derived from moored arrays the two eddies are persistent throughout the summer because winds are usually weak and cannot influence their existence the tide induced residual currents are appreciable only in the close vicinity of the kurushima strait and bisan strait the two straits connecting the bay to the other bays and have little affect on the two eddies that are inside the bay thus the two density driven eddies in the hiuchi nada are the basic pattern of circulation in the summer using the calculated residual currents we examined the intrusion route of waters from the kurushima strait and bisan strait with passive tracer experiments the results show that the water in the kurushima strait intrudes into the bay mainly through the middle and bottom layers while the water in the bisan strait intrudes mainly through the surface layer furthermore the tracer cannot reach the center of the anticlockwise eddy in the eastern part of the bay this indicates that the eddy prevents water exchange between its center and outer edges and provides favorable conditions for the formation of oxygen deficient waters in the central part of the eddy

gravity tides and topography on small satellites and asteroids application to surface features of the martian satellites abstract the influence of gravitational potential on the disposition of loose material on the surfaces of small satellites is investigated using the martian satellites as examples gravity on small satellites can depend strongly on tidal and rotational effects especially for objects of low mean density such as phobos numerical shape models of phobos and deimos allow the mapping of the relative gravitational positions of surface features dynamic height derived from measures of potential energy is a convenient approximation to heights above a reference equipotential surface for these irregular bodies deimos shows a strong correlation of indicators of global downslope movement with dynamic height for phobos the distribution of dynamic heights across its surface has changed substantially over time due to its tidally evolving orbit that has induced changes in the rotational and tidal components of surface accelerations areas of minimum and maximum regolith thickness correlate better with high and low regions of dynamic height under conditions of rapid but synchronous rotation than with dynamic heights that occurred at more distant orbits less influenced by tides and rotation however some model dependent measures of regolith depth such as pit spacing in grooves show little correlation with dynamic height at any orbit radius the rougher surface of phobos does not provide satellitewide pathways for near surface downslope transport of material as does the surface of deimos although the total amount of regolith on phobos greatly exceeds that which can be attributed to the visible cratering record the smoother surface of deimos probably results from an even greater covering by debris from nearly catastrophic impacts the phobos and deimos examples suggest that asteroids have global redistribution of ejecta only if debris from nearly catastrophic impacts provides smooth surfaces for long distance downslope motion driven by small impact gardening seismic effects of larger impacts and thermal cycling

integral measurements of mass transport and heat content in the strait of gibraltar from acoustic transmissions three view the mathml sourceacoustic transceivers were deployed on either side of the eastern entrance of the strait of gibraltar during april may 1996 to determine the feasibility of using acoustic methods to make routine rapidly repeated horizontally integrated measurements of flow and temperature in straits reciprocal transmissions between the transceivers were used to test the feasibility of using traditional ray differential travel times to monitor the component of flow along the acoustic paths transmissions directly across the strait were used to test the feasibility of using horizontal arrival angle fluctuations and acoustic intensity scintillations to monitor the flow perpendicular to the acoustic path the geometry was selected to provide ray paths that only sample the lower layer mediterranean water so that the feasibility of monitoring the mediterranean outflow using the various methods could be evaluated the acoustic scintillation method did not yield useful current estimates but the experimental parameters were not optimized for this approach since the low frequency variability in log amplitude was found to be highly correlated at receivers view the mathml source apart it is possible that acoustic scintillation measurements using different receiver spacings and more rapid sampling might yield better results the horizontal deflection method gave encouraging results at the time of neap tides but less so during spring tides for this approach both theoretical estimates and measured phase differences between the horizontally separated receivers suggest that internal wave induced horizontal arrival angle fluctuations may fundamentally limit the precision with which arrival angles can be measured further work is needed to determine if a smaller horizontal spacing and higher signal to noise ratios would yield better results reciprocal travel time measurements diagonally across the strait performed the best of the three methods giving absolute flow estimates consistent with those derived from current meter data the fractional uncertainty variance for the lower layer tidal transport from a single tomographic path was estimated to be 0 017 i e 98 of the a priori tidal transport variance was resolved the spatial scales of the sub tidal flow are thought to be significantly shorter than those of the tidal flow however which means that a more elaborate monitoring network is required to achieve the same performance for sub tidal variability finally sum travel times from the reciprocal transmissions were found to provide good measurements of the temperature and heatcontent in the lower layer

does crater saturation equilibrium occur in the solar system abstract similarity is found in crater densities on the most heavily cratered surfaces throughout the solar system this is hypothesized to result from a steady state saturation equilibrium being approached or achieved by cratering processes this hypothesis conflicts with some recent interpretations however it accounts for 1 a similarity in maximum relative crater density below certain theoretically predicted values on all heavily cratered surfaces 2 a leveling off at this same relative density among 100 m scale secondary craters in populations on lunar maria and other sparsely cratered lunar surfaces 3 the approximate uniformity of maximum relative densities on saturn satellites in spite of dramatic variations predicted from nonsaturation models assuming heliocentric impactors the lunar frontside upland crater population sometimes described as a well preserved production function useful for interpreting other planetary surfaces is found not to be a production function it was modified by intercrater plains formed at least partly by early upland basaltic lava flooding recently confirmed spectrophotometrically consistent with this counts in pure uplands those lacking intercrater plains match the proposed saturation equilibrium density variations among large d 64 km crater populations are found but these may involve several hypothesized mechanisms that rapidly obliterate large craters especially on icy surfaces recent models in which different populations of interplanetary bodies hit different planets need further appraisal

seasonal climatologies of nutrients and hydrographic properties on quasi neutral surfaces for four coastal upwelling systems seasonal climatologies of upper ocean hydrographic properties and nutrients are presented for four different major coastal upwelling centers the california peru humboldt canary and benguela regions the climatologies are built from historical hydrographic data primarily to serve as a tool for comparative studies of productivity variations and nutrient fluxes in the near coastal zone care was taken to ensure that quantities were interpolated vertically to and averaged horizontally on approximately neutral surfaces to constrain averaging among only those water masses that would naturally mix easily rather than rely on potential density surfaces that are not coincident with neutral surfaces in regions with large vertical excursions of isopycnals the neutral surfaces were properly approximated through the use of specific volume anomaly surfaces and were spaced closely in the vertical approximately 0 1 unit apart horizontal averaging to a 1 6 spatial grid was accomplished via the application of bicubic splines under tension and provided sufficient spatial resolution for resolving upwelling patterns near the coastline seasonal mean nutrient to nutrient relationships also were constructed as another diagnostic tool for inter regional comparisons and are consistent with expected redfield ratios a consistent high resolution climatology is now publicly available as a tool for local and comparative studies of four different upwelling zones

variations of current path velocity and volume transport of the kuroshio in relation with the large meander abstract the variations of the kuroshio current path and their relations with current velocity volume transport and upstream position of the kuroshio are examined using sea level data after the long time non large meander nlm path during 1963 75 the kuroshio takes a large meander lm path during most of 1975 91 1975 80 1981 84 1986 88 and 1989 91 the study of these meanders confirms the description of the path of the kuroshio based on the three typical paths and the transitions between them with an exception being the 1981 84 lm path the formation and decay of the large meander are associated with velocity and position of the kuroshio in the tokara strait south of kyushu a small meander precursor of the large meander seems to be formed southeast of kyushu in relation with a temporary increase of kuroshio velocity and a northward shift of the kuroshio in the tokara strait an eastward propagation of the small meander leading to the lm formation is associated with remaining in the northe

regolith migration and sorting on asteroid itokawa high resolution images of the surface of asteroid itokawa from the hayabusa mission reveal it to be covered with unconsolidated millimeter sized and larger gravels locations and morphologic characteristics of this gravel indicate that itokawa has experienced considerable vibrations which have triggered global scale granular processes in its dry vacuum microgravity environment these processes likely include granular convection landslide like granular migrations and particle sorting resulting in the segregation of the fine gravels into areas of potential lows granular processes become major resurfacing processes because of itokawa s small size implying that they can occur on other small asteroids should those have regolith

assimilation of surface velocity data into a primitive equation coastal ocean model summer of 1998 are 0 42 and 0 78 respectively demonstrating the effectiveness of the das our estimates of the error covariances are shown to be appropriate through a series of objective statistical tests analysis of the term balances of the model equations show that the dominant modeled dynamical balances are preserved by the das and that uncertainties in the spatial variability of the wind forcing are likely to be one source of model error by varying the relative magnitudes of the estimated forecast and observation error covariances the das is shown to be most effective when approximately 80 of the analysis is made up of the model solution index terms 4263 oceanography general ocean prediction 4219 oceanography general continental shelf processes 4255 oceanography general numerical modeling 3337 meteorology and atmospheric dynamics numerical modeling and data assimilation 4516 oceanography physical eastern boundary currents 4594 oceanography physical instruments and techniques keywords practical data assimilation numerical modeling upwelling highfrequency radar data

spatial distribution of impact craters on deimos abstract deimos one of the martian moons has numerous impact craters however it is unclear whether crater saturation has been reached on this satellite to address this issue we apply a statistical test known as nearest neighbor analysis to analyze the crater distribution of deimos when a planetary surface such as the moon is saturated with impact craters the spatial distribution of craters is generally changed from random to more ordered we measured impact craters on deimos from viking and hirise images and found 1 that the power law of the size frequency distribution of the craters is approximately 1 7 which is significantly shallower than those of potential impactors and 2 that the spatial distribution of craters over 30 m in diameter cannot be statistically distinguished from completely random distribution which indicates that the surface of deimos is inconsistent with a surface saturated with impact craters although a crater size frequency distribution curve with a slope of 2 is generally interpreted as indicating saturation equilibrium it is here proposed that two competing mechanisms seismic shaking and ejecta emplacement have played a major role in erasing craters on deimos and are therefore responsible for the shallow slope of this curve the observed crater density may have reached steady state owing to the obliterations induced by the two competing mechanisms such an occurrence indicates that the surface is saturated with impact craters despite the random distribution of craters on deimos therefore this work proposes that the age determined by the current craters on deimos reflects neither the age of deimos itself nor that of the formation of the large concavity centered at its south pole because craters should be removed by later impacts however a few of the largest craters on deimos may be indicative of the age of the south pole event

planetary surface dating from crater size frequency distribution measurements spatial randomness and clustering we describe a quantitative procedure to measure the degree of clustering in an observed crater population relative to a series of randomly distributed populations we split the population by according to crater size to be able to identify clustering at different scales and find that a clustering analysis based on the mean 2nd closest neighbour distance measure more closely corresponds to visual interpretations of the spatial configuration than the mean closest neighbour distance standard deviation of adjacent area is found in certain cases to be an even more sensitive measure the technique is demonstrated for two sites on mars we were able to make use of a case where the spatial distribution was less clustered than random to reveal the transition between a crater population superimposed on a lava flow and that belonging to an underlying unit in general this type of analysis may give a better insight into the post formation modification of studied units enabling a more precise classification of which sizes of craters derive from the original accumulation population and which from areas resurfaced by later modification events consequentially improving the accuracy of dating resurfacing events clustering analysis is thus an additional tool for understanding the structure of a crater population and the effects causing an observed population to differ from an ideal one

the climatology of the north atlantic abstract data from approximately 144 000 hydrographic stations in the north atlantic have been retrieved from the national oceanic data centre and analysed to produce maps of mean pressure temperature salinity and oxygen on selected potential density surfaces for the domain bounded by 0 85 w and 0 65 n the data span the period from 1904 to 1990 with the majority of the data from the last four decades the data set for this region is 60 larger than that used in the production of levitus climatological world atlas this increase in stations coupled with smoothing scales specific to the north atlantic rather than the global ocean considerably improves the resolution of the basin s features the mean property fields and their associated standard deviations are resolved on a one degree grid with little smoothing contrasting with the levitus atlas where properties although presented on a one degree grid have been smoothed on the order of 1000km another important feature of this database is the process by which irregularly spaced data are averaged onto a regular grid in a significant departure from the levitus analysis which averaged on depth surfaces these data were averaged on potential density surfaces thus eliminating an artificial mixing of water mass properties the database is used to describe the baroclinicity of well known features such as the gulf stream the north atlantic current and the deep western boundary current and to elucidate the recirculations associated with these currents it additionally resolves several new features in the intermediate and deep north atlantic these features include the signature of a large scale deep recirculation that extends southwestward from the eastward extension of the north atlantic current to the separation point of the gulf stream near cape hatteras this recirculation which spans approximately 2000m of the water column encompasses more local recirculations and potentially mixes subpolar and subtropical waters furthermore in the upper thermocline this database reveals a coherent azores current that stretches from the gulf stream system south of the tail of the grand banks to madeira this flow is marked by divergences to the south and convergences from the north such that its downstream transport is not much changed

accurate imaging and prediction of kanmon strait tidal current structures by the coastal acoustic tomography data 1 the kanmon strait acoustic tomography data acquired at a 5 min interval are assimilated sequentially into a 2d ocean model on the basis of the ensemble kalman filter scheme to image strong tidal current structures occurring in the strait when the accurate range averaged currents obtained for the sound transmission lines connecting eight tomography stations are used as assimilation data the complicated vortex imbedded currents are imaged with horizontal resolution and accuracy much better than the result of tomographic inversion the assimilated currents are well compared to the shipboard adcp data with a rms difference of about 24 cm s 1 for both the horizontal velocity components the assimilated volume transport across the strait also shows a good agreement with the transport estimated from the range averaged current on a pair of transmission lines crossing the strait making a rms difference of 3700 m3 s 1

assimilation of coastal acoustic tomography data into a barotropic ocean model a data assimilation technique was applied to analyze the first coastal acoustic tomography data obtained in the neko seto channel of the seto inland sea japan on march 2 3 1999 the ensemble kaiman filter technique accompanied by the forecast of error covariances by means of the monte carlo method is found to be best fitted to the present problem because currents in the tomography region are dominated by the strongly nonlinear tidal vortices furthermore the analyzed results are in better agreement with the shipboard adcp results than those obtained by the inverse analysis of the travel time difference data for the pairs of acoustic stations

seasonal climatology of hydrographic conditions in the upwelling region off northern chile over 30 years of hydrographic data from the northern chile 18os 24os upwelling region are used to calculate the surface and subsurface seasonal climatology extending 400 km offshore the data are interpolated to a grid with sufficient spatial resolution to preserve cross shelf gradients and then presented as means within four seasons austral winter july september spring october december summer january march and fall april june climatological monthly wind forcing surface temperature and sea level from three coastal stations indicate equatorward upwelling favorable winds throughout the year weakest in the north seasonal maximum alongshore wind stress is in late spring and summer december march major water masses of the region are identified in climatological t s plots and their sources and implied circulation discussed surface fields and vertical transects of temperature and salinity confirm that upwelling occurs year round strongest in summer and weakest in winter bringing relatively fresh water to the surface nearshore surface geostrophic flow nearshore is equatorward throughout the year during summer an anticyclonic circulation feature in the north which extends to at least 200 rn depth is evident in geopotential anomaly and in both temperature and geopotential variance fields subsurface fields indicate generally poleward flow throughout the year strongest in an undercurrent near the coast this undercurrent is strongest in summer and most persistent and organized in the south south of 21os a subsurface oxygen minimum centered at 250 m is strongest at lower latitudes low salinity subsurface water intrudes into the study area near 100 m predominantly in offshore regions strongest during summer and fall and in the southernmost portion of the region the climatological fields are compared to features off baja within the somewhat analogous california current and to measurements from higher latitudes within the chile peru current system

chlorophyll variability in eastern boundary currents the first three years of seawifs data 199 2000 provide the most complete quantification to date of chlorophyll seasonal variability along the full latitudinal ex tent of the four major eastern boundary currents ebcs comparisons to previously published chlorophyll seasonal climatologies deduced from the relatively sparse coverage provided by the coastal zone color scanner czcs show significant differences in both southern hemisphere ebcs while northern hemisphere regions are qualitatively simi lar comparisons between chlorophyll and cross shelf ek man transport seasonal cycles calculated from coincident satellite scatterometer data show seasonal maxima have similar phases over most of the california current at higher 32os latitudes in the peru chile and benguela currents 30os and at lowest latitudes 20on in the canary current latitudinal zones within which phases diverge are indicative of alternate and or more distant forcing

observed fluxes of water salt and suspended sediment in a partly mixed estuary abstract observations of the residual fluxes of water salt and suspended sediment are presented for seven stations along the tamar estuary the data include measurements over single spring and neap tidal cycles and are generally applicable to medium or high run off conditions surface to bed differences in salinity are typically of the order of several parts per thousand gravitational circulation is an important component of residual flow in the deep lower reaches of the estuary here stokes drift is insignificant in the shallow upper reaches the major residual currents are generated by stokes drift and freshwater inputs data are compared with predictions from hansen and rattray s 1966 model of estuarine circulation salt fluxes due to tidal pumping and vertical shear are directed up estuary at spring tides tidal pumping being dominant tidal pumping of salt is also directed up estuary at neap tides although it is insignificant in the lower reaches where vertical shear dominates tidal pumping of suspended sediment is directed up estuary near the head at spring tides and probably contributes to the formation of the turbidity maximum the existence of the turbidity maximum is predicted using a simplified model of the transport of water and sediment the model shows that an additional mechanism for the existence of the turbidity maximum is an up estuary maximum in the tidal current speeds and thus resuspension in the lower reaches transport of suspended sediment is directed down estuary at both spring and neap tides and sediment is essentially flushed to sea with the fresh water

the shapes of small satellites abstract improved measurement techniques allow quantitative testing of the physical significance of the shapes of small satellites such as the possible occurrence of equilibrium ellipsoid forms shapes of the small satellites of mars jupiter saturn and uranus have been measured using limb coordinates from spacecraft images limb derived ellipsoidal models can measure volumes of irregular objects accurately ellipsoidal models of phobos and deimos derived from limb coordinates are nearly identical to those obtained independently from stereogrammetry duxbury and callahan 1988 these ellipsoidal approximations however are incomplete descriptions of the shapes of small satellites in that average residuals are much larger than measurement errors ellipsoidal models of eight small satellites compared with equilibrium ellipsoids show that shapes of even moderately irregular satellites are poor predictors of mean densities icy satellites smaller than mean radius r m 150 km are irregularly shaped and have limb roughnesses of several percent of mean radius rms residuals 03 08 r m there is no trend in roughness with size below r m 150 km larger icy satellites are ellipsoidal and have limb roughness below 0 01 r m rocky satellites of r m 6 110 km are also irregularly shaped the only larger rocky satellites are the moon and io r m 1740 and 1820 km which have roughnesses well below 0 01 r m data on rocky satellites combined with published data for ceres and juno millis et al 1981 1987 suggest a gradual transition from irregular to ellipsoidal rocky objects limb topography and images suggest that the small satellites retain impact scars of diameter of up to 1 7 times the satellite mean radius these large indentations are the primary difference in shape between small and large satellites

the global effects of impact induced seismic activity on fractured asteroid surface morphology abstract impact induced seismic vibrations have long been suspected of being an important surface modification process on small satellites and asteroids in this study we use a series of linked seismic and geomorphic models to investigate the process in detail we begin by developing a basic theory for the propagation of seismic energy in a highly fractured asteroid and we use this theory to model the global vibrations experienced on the surface of an asteroid following an impact these synthetic seismograms are then applied to a model of regolith resting on a slope and the resulting downslope motion is computed for a full range of impactor sizes next this computed downslope regolith flow is used in a morphological model of impact crater degradation and erasure showing how topographic erosion accumulates as a function of time and the number of impacts finally these results are applied in a stochastic cratering model for the surface of an eros like body same volume and surface area as the asteroid with craters formed by impacts and then erased by the effects of superposing craters ejecta coverage and seismic shakedown this simulation shows good agreement with the observed 433 eros cratering record at a main belt exposure age of 400 200 myr including the observed paucity of small craters the lowered equilibrium numbers loss rate production rate for craters less than 100 m in diameter is a direct result of seismic erasure which requires less than a meter of mobilized regolith to reproduce the near observations this study also points to an upper limit on asteroid size for experiencing global surface modifying seismic effects from individual impacts of about 70 100 km depending upon asteroid seismic properties larger asteroids will experience only localized regional seismic effects from individual impacts

fortnightly variation in the bottom thermal front and associated circulation in a semienclosed sea abstractsummer hydrographic data from 1971 to 2000 demonstrate the presence of bottom cold water bcw and a bottom thermal front around the bcw in a semienclosed sea in the seto inland sea of japan the horizontal gradient of water temperature across the bottom front was larger in neap tide than in spring tide which is the opposite the pattern observed in the fortnightly variations in other areas e g the irish sea a numerical model for the inland sea reproduced the presence of bcw and a bottom front as well as the same fortnightly variation in the horizontal gradient of water temperature across the bottom front as in observational data being the same bcw as those in other areas the presence of bcw in the inland sea is also caused by spatial variation in tidal mixing the intensification of the bottom front in neap tide in the inland sea results from a combination of tidal mixing and horizontal advection of warm water to the bcw the presence of a bottom residual current in the direction across th

a survey of possible impact structures on 25143 itokawa abstract we determined the morphologies and dimensions of possible impact craters on the surface of asteroid 25143 itokawa from images taken by the hayabusa spacecraft circular depressions circular features with flat floors or convex floors and circular features with smooth surfaces were identified as possible craters the survey identified 38 candidates with widely varying morphologies including rough smooth and saddle shaped floors a lack of raised rims and fresh material exposures the average depth diameter ratio was 0 08 0 03 these craters are very shallow relative to craters observed on other asteroids these shallow craters are a result of 1 target curvature influencing the cratering process 2 raised rim not being generated by this process and 3 fines infilling the craters as many of the crater candidates have an unusual appearance we used a classification scheme that reflects the likelihood of an observed candidate s formation by a hypervelocity impact we considered a variety of alternative interpretations while developing this scheme including inherited features from a proto itokawa spall scars created by the disruption of the proto itokawa spall scars following the formation of a large crater on itokawa itself and apparent depressions due to random arrangements of boulders the size frequency distribution of the crater candidates was close to the empirical saturation line at the largest diameter and then decline with decreasing diameter

annual cycles of nutrients and oxygen in the upper layers of the north atlantic ocean abstract monthly climatologies of nutrients and oxygen in the upper north atlantic ocean are analyzed and used to estimate the rates of spring summer new production and remineralization the annual cycle of surface nutrients is characterized by a spring summer biological drawdown and a fall winter increase due to vertical mixing the drawdown is accompanied by maximum oxygen supersaturations in the subsurface layer 100 200 m the decrease of oxygen concentrations during the spring summer period reflects the remineralization of organic matter the spring summer drawdown of surface nutrients implies new production rates that are high in the high latitudes around 40 g c m2 on average and low in the low and temperate latitudes about 15 g c m2 on average in the subsurface layer respiration rates are computed from the oxygen concentration decrease during spring summer period results show that most of the new production in the low latitudes and half or less of it in the high latitudes is remineralized above 200 m the spring summer new production in the north atlantic ocean is 1 34 pg c considering that the spring summer production that is not mineralized in the top 200 m of the ocean is mainly composed of particulate organic matter would lead to an export lying between 0 4 and 0 6 gt c period for the whole north atlantic ocean

mapping of a typhoon driven coastal upwelling by assimilating coastal acoustic tomography data a typhoon driven upwelling event was observed with coastal acoustic tomography in hiroshima bay during september 2013 the tomography data were used to obtain state estimates from an ocean model employing the ensemble kalman filter enkf for data assimilation hiroshima bay was represented as a two layer system with a fresh near surface layer overlaying a more saline lower layer wind from the typhoon forced the surface layer southward thus drawing the lower layer northward and causing upwelling after the upwelling these currents reversed for a period during the upwelling the total volume transports for the upper and lower layers were well balanced showing continuity of the two layers however the total upper layer volume transported northward during the reverse flow period was significantly smaller than the total upper layer volume transported southward during the upwelling and significantly larger than the total lower layer volume transported southward during the reverse flow period the downstream reductions of transported volume indicated mixing fractions of 24 30 with water mixing at the interface of the upper and lower layers errors in state estimates were less than the changes in current and salinity associated with the upwelling

historical position of the north atlantic current the path of the north atlantic current nac within the newfoundland basin is determined from the historical hydrographic database 1910 1988 horizontal probability distributions for dynamical and property front locations on specific volume anomaly surfaces are derived from an analysis of all available hydrographic stations in the basin these probability distributions indicate where the fronts were most likely to have been observed in the past they are observed to exhibit a pattern of stationary meanders with troughs located over the southcast newfoundland rise over the newfoundland seamounts and to the east of the flemish cap the width of the nac s envelope varies from 300 km over the southeast newfoundland rise temporal subsamples of the historical data indicate that the path and meander patterns are persistent features for all seasons and for the four decades between 1950 and 1990

hydrographic variability in the st helena bay region of the southern benguela ecosystem cross shelf distributions of temperature salinity water masses and dissolved oxygen in st helena bay revealed substantial vertical and seasonal variations in the surface layers nearshore and offshore temperature and salinity patterns differed with bay scale variability linked to upwelling dynamics and coastal processes while the offshore region was influenced by solar insolation spectral analysis revealed that an annual signal prevailed at most stations and corroborated contrasting patterns between the offshore and nearshore regions with phase differences suggesting shoreward propagation of the offshore temperature signal the shelf was dominated by modified upwelled water muw and subantarctic mode water samw which comprised the primary source of upwelled water clear zonation of muw was evident across the shelf resulting from seasonal variations in locations of the oceanic and bifurcated shelf break fronts dynamics within st helena bay consistently differed from those further offshore due to the influences of the shelf break front cape columbine upwelling plume and cyclonic recirculation which appeared to be associated with an intraannual signal with a periodicity of 3 4 months persistent hypoxia in the bottom waters suggested the occurrence of a permanent reservoir of low oxygen water low seasonal shoreward and offshore expansion of low occurred throughout the upwelling season with maximum extent reached during summer and autumn due to the coupled effects of advection and local phytoplankton decay while wind mixing ventilated the water column at nearshore stations in winter and the onset of upwelling during spring introduced oxygen richer water from further offshore hypoxia persisted in the center of the bay

impact induced seismic activity on asteroid 433 eros a surface modification process high resolution images of the surface of asteroid 433 eros revealed evidence of downslope movement of a loose regolith layer as well as the degradation and erasure of small impact craters less than 100 meters in diameter one hypothesis to explain these observations is seismic reverberation after impact events we used a combination of seismic and geomorphic modeling to analyze the response of regolith covered topography particularly craters to impact induced seismic shaking applying these results to a stochastic cratering model for the surface of eros produced good agreement with the observed size frequency distribution of craters including the paucity of small craters