the importance of secondary cratering to age constraints on planetary surfaces small craters less than one kilometer diameter can be primary craters produced by impact of interplanetary debris or they can be secondary craters produced by fallback of high velocity ejecta blocks from much larger but infrequent primary impacts the prevalent assumption over recent decades has been that primaries are most abundant so most small craters are independent random events and can be used for dating however recent results from europa and mars support the early theory that distant secondaries globally dominate the number of small lunar craters this would invalidate part of production functions that have been widely used for age dating crater excavation results in higher mean ejection velocities for smaller fragments resulting in a steeper size frequency distribution for secondary craters than is produced by the same size frequency distribution of interplanetary debris this review also discusses how small craters can sometimes be used to derive meaningful age constraints

a sensitivity analysis of meteoric infrasound 1 we perform a multicomponent sensitivity analysis of how the physical and dynamical parameters that characterize a meteor in fall affect the ground overpressure and period of a plausible emitted n wave signal the nonlinear propagation model used throughout is based upon whitham s nonlinearization method which is modified to take into account a stratified atmosphere we use sensitivity indices derived using a fourier amplitude sensitivity test to measure how the meteor parameters uncertainties affect the uncertainty in the overpressure and period of an emitted n wave the investigated parameters include the azimuth entry angle diameter drag coefficient density and initial velocity of the meteor as well as the atmosphere the method is used to re examine the crater forming meteorite fall near carancas peru 2007 we obtain good agreement between the simulated signals and observed waveforms it is shown that ground overpressure uncertainty depends on the atmospheric uncertainties that are strongly correlated with the unknown trajectory whereas the period is governed by the diameter uncertainties finally we consider new waveform parameters that help characterize the meteor

hydrothermal alteration of tertiary igneous rocks from the isle of skye northwest scotland hydrothermal alteration of tertiary granites from skye involed the reaction of igneous amphibole biotite ferroaugite fayalite alkali feldspar plagioclase feldspar magnetite and ilmenite with aqueous fluid primarily to combinations of secondary alkali feldspar chlorite and montmorillonite lesser amounts of muscovite calcite epidote prehnite zeolite and pyrite also developed during mineralogical alteration of the granites there was a net addition to rock of h and a net loss of ca fe and probably na there is no positive evidence for a significant change in their al si mg or k contents hydrothermal alteration of skye granite involved not only large scale migration of 18o 16o d and h but also of ca fe and probably na mineral thermometry combined with textural observations suggests that chemical isotopic and most mineralogical alteration were synchronous and occurred at 350 450 c textural observations further suggest that the mechanism of isotopic exchange between granite and hydrothermal fluid primarily was solution reprecipitation

theoretical analysis of secondary cratering on mars and an image based study on the cerberus plains the issue of crater retention age estimates on planetary surfaces is discussed with an attempt to quantify the effect of overlapping primary and secondary impact crater populations in restricted crater diameter ranges the approach to this problem is illustrated with a simple model production function where the secondary crater input is artificially enhanced extrapolation of such a secondary crater model distribution to a global record results in extraordinarily high crater frequencies that do not exist on mars and implies the need of detailed studies of the size frequency distribution for remote secondary craters to date poorly known a key case the martian crater zunil and its secondary crater field illustrate that reasonable predictions for the secondary crater size frequency distribution at small 100 m crater diameters affected the standard model crater retention age for the cerberus plains less than the statistical uncertainty these observations show that age determination based on appropriate crater counting statistics is valid in a wide primary crater diameter range

the 2013 russian fireball largest ever detected by ctbto infrasound sensors 1 on 15 february 2013 a large earth impacting fireball disintegrated over the ural mountains this extraordinary event is together with the 1908 tunguska fireball among the most energetic events ever instrumentally recorded it generated infrasound returns after circling the globe at distances up to 85 000 km and was detected at 20 infrasonic stations of the global international monitoring system ims for the first time since the establishment of the ims infrasound network multiple arrivals involving waves that traveled twice round the globe have been clearly identified a preliminary estimate of the explosive energy using empirical period yield scaling relations gives a value of 460 kt of tnt equivalent in the context of the future verification of the comprehensive nuclear test ban treaty this event provides a prominent milestone for studying in detail infrasound propagation around the globe for almost 3 days as well as for calibrating the performance of the ims network

the flux of small near earth objects colliding with the earth asteroids with diameters smaller than approximately 50 100 m that collide with the earth usually do not hit the ground as a single body rather they detonate in the atmosphere these small objects can still cause considerable damage such as occurred near tunguska siberia in 1908 the flux of small bodies is poorly constrained however in part because ground based observational searches pursue strategies that lead them preferentially to find larger objects a tunguska class event the energy of which we take to be equivalent to 10 megatons of tnt was previously estimated to occur every 200 300 years with the largest annual airburst calculated to be approximately 20 kilotons kton tnt equivalent ref 4 here we report satellite records of bolide detonations in the atmosphere over the past 8 5 years we find that the flux of objects in the 1 10 m size range has the same power law distribution as bodies with diameters 50 m from this we estimate that the earth is hit on average annually by an object with approximately 5 kton equivalent energy and that tunguska like events occur about once every 1 000 years

the mixing of lunar regolith vital updates to a canonical model abstract in this work we update the regolith mixing model presented by gault et al 1974 including new input values and reworking key parameters much as gault et al did we present a way to calculate the rate at which lunar regolith is overturned at depth the model describes a mixing front that proceeds downward from the surface following a power law function of time our most important update is the inclusion of secondary impacts our calculations show that secondaries are necessary to produce the reworking rate inferred from the depth distribution of surface correlated material in apollo cores fruchter et al 1977 morris 1978 blanford 1980 from the rate at which splotches rework the top 3 cm of regolith speyerer et al 2016 and from the rate at which diviner cold spots bandfield et al 2014 and crater rays pieters et al 1985 hawke et al 2004 werner and medvedev 2010 are reworked into background regolith overturn calculations that only consider the impact of primaries fail to describe observed reworking rates at all depths and timescales we conclude that secondary impacts dominate mixing in the top meter of lunar regolith

fractal dimensions of diamictic particle size distributions simulations and evaluation it has been proposed that deformation tills have fractal particle size distributions reflecting scale invariant debris comminution processes numerical simulations however show that fractal dimensions m typical of deformation tills can be produced by the mixing of gaussian parent populations analogous to the mechanical mixing of coarse medium and fine grained sorted sediments the simulated distributions have high r2 values 0 98 and narrow 95 confidence limits indicating that fractal analysis cannot distinguish diamictons that have been created by shearing from those formed by other mechanisms such as the mixing of preexisting sediments the fractal dimensions of diamictons reflect the relative amounts of coarse and fine material in the distribution but are not uniquely diagnostic of particular genetic processes

critical crater diameter and asteroid impact seismology if impact stress reverberation is the primary gradational process on an asteroid at global scales then the largest undegraded crater records an asteroid s seismological response the critical crater diameter dcrit is defined as the smallest crater whose formation disrupts all previous craters globally up to its size it is solved for by combining relationships for crater growth and for stress attenuation the computation for dcrit gives a simple explanation for the curious observation that small asteroids have only modest undegraded craters in comparison to their size whereas large asteroids have giant undegraded craters dcrit can even exceed the asteroid diameter in which case all craters are local and the asteroid becomes crowded with giant craters dcrit is the most recent crater to have formed on a blank slate when it is equated to the measured diameter of the largest undegraded crater on known asteroids peak particle velocities are found to attenuate with the 1 2 1 3 power of distance less attenuative than strong shocks and more characteristic of powerful seismic disturbances this is to be expected since global degradation can result from seismic cm s 1 particle velocities on small asteroids attenuation as modeled appears to be higher on asteroids known to be porous although these are also bodies for which different crater scaling rules might apply

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

compositionally zoned cl rich amphiboles from north dabie shan china monitor of high pressure metamorphic fluid rock interaction processes abstract amphiboles containing up to 4 2 wt cl are found in felsic granulites from yanzihe within the north dabie area of the dabie sulu ultrahigh and high pressure metamorphic belt in eastern china most amphibole grains show considerable zonations with cl contents ranging from 0 to 4 2 wt based on their textural features amphiboles can be divided into four generations 1 amphibole occurring as inclusions in orthopyroxene am in with cl contents around 3 5 wt 2 amphibole forming cores of grains in the matrix am i with cl contents between 3 0 and 4 2 wt 3 amphibole with cl contents of 0 2 to 2 5 wt am ii occurring as hydrothermally altered parts of the original amphibole 4 cl free amphibole am iii usually developed at the outermost rim of the grain major and rare earth elements show significant variations for am i am ii and am iii different generations of amphiboles are related to different metamorphic stages of the granulite in yanzihe and provide a monitor for fluid rock interactions and p t evolution during the high pressure metamorphism of dabie shan pressure and temperature estimates suggest that am in was formed during prograde metamorphism of 10 kbar and 700 800 c am i was formed under peak metamorphic conditions 20 kbar 800 960 c whereas am ii and am iii were formed during retrograde metamorphic stages 560 770 c and 5 7 kbar and 520 670 c and

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

growth and evolution of asteroids asteroids are what is left of the precursors to the terrestrial planets they are stunning in their diversity ranging from charcoal black worlds the size of a hilltop spinning like a carnival ride to dog bone shaped metallic remnants of some cataclysmically disrupted planetary core to worlds as stately as ceres and vesta and fragments thereof to garden variety fractured and blocky nuggets that dominate near earth space asteroid belts are common around sun like stars when properly seen as unaccreted residues as scraps on the floor of the planetary bakery the diversity of asteroids can be fully appreciated for to paraphrase tolstoy accreted planets are all alike every unaccreted planet is unaccreted in its own way

chelyabinsk meteorite explains unusual spectral properties of baptistina asteroid family we investigated the spectral and compositional properties of chelyabinsk meteorite to identify its possible parent body in the main asteroid belt our analysis shows that the meteorite contains two spectrally distinct but compositionally indistinguishable components of ll5 chondrite and shock blackened impact melt material our x ray diffraction analysis confirms that the two lithologies of the chelyabinsk meteorite are extremely similar in modal mineralogy the meteorite is compositionally similar to ll chondrite and its most probable parent asteroid in the main belt is a member of the flora family our work confirms previous studies e g vernazza et al 2008 nature 454 858 860 de leon j licandro j serra ricart m pinilla alonso n campins h 2010 astron astrophys 517 a23 dunn t l burbine t h bottke w f clark j p 2013 icarus 222 273 282 linking ll chondrites to the flora family intimate mixture of ll5 chondrite and shock blackened impact melt material from chelyabinsk provides a spectral match with 8 flora the largest asteroid in the flora family the baptistina family and flora family overlap each other in dynamical space mineralogical analysis of 298 baptistina and 11 small family members shows that their surface compositions are similar to ll chondrites although their absorption bands are subdued and albedos lower when compared to typical s type asteroids a range of intimate mixtures of ll5 chondrite and shock blackened impact melt material from chelyabinsk provides spectral matches for all these baf members we suggest that the presence of a significant shock impact melt component in the surface regolith of baf members could be the cause of lower albedo and subdued absorption bands the conceptual problem with part of this scenario is that impact melts are very rare within ordinary chondrites of the 42 000 ordinary chondrites less than 0 5 203 of them contain impact melts a major reason that impact melts are rare in meteorites is that high impact velocities v 10 km s are needed to generate the necessary shock pressures and temperatures e g pierazzo e melosh h j 1998 hydrocode modeling of oblique impacts the fate of the projectile in origin of the earth and moon proceedings of the conference lpi contribution no 957 unless the target material is highly porous nearly all asteroid impacts within the main belt are at 5 km s bottke w f nolan m c greenberg r kolvoord r a 1994 collisional lifetimes and impact statistics of near earth asteroids in tucson gehrels t ed hazards due to comets and asteroids the university of arizona press arizona pp 337 357 which prevents them from producing much impact melt unless they are highly porous however shock darkening is an equally efficient process that takes place at much lower impact velocities 2 km s and can cause the observed spectral effects spectral effects of shock darkening and impact melt are identical the parent asteroid of baf was either a member of the flora family or had the same basic composition as the floras ll chondrite the shock pressures produced during the impact event generated enough impact melt or shock blackening to alter the spectral properties of baf but keep the baf composition largely unchanged collisional mixing of shock blackened impact melt and ll5 chondritic material could have created the baptistina asteroid family with composition identical to those of the floras but with subdued absorption

meteoric smoke fallout over the holocene epoch revealed by iridium and platinum in greenland ice an iridium anomaly at the cretaceous tertiary boundary layer has been attributed to an extraterrestrial body that struck the earth some 65 million years ago1 it has been suggested that during this event the carrier of iridium was probably a micrometre sized silicate enclosed aggregate2 or the nanophase material of the vaporized impactor3 but the fate of platinum group elements such as iridium that regularly enter the atmosphere via ablating meteoroids remains largely unknown here we report a record of iridium and platinum fluxes on a climatic cycle timescale back to 128 000 years ago from a greenland ice core4 we find that unexpectedly constant fallout of extraterrestrial matter to greenland occurred during the holocene whereas a greatly enhanced input of terrestrial iridium and platinum masked the cosmic flux in the dust laden atmosphere of the last glacial age we suggest that nanometre sized meteoric smoke particles5 6 formed from the recondensation of ablated meteoroids in the atmosphere at altitudes 70 kilometres are transported into the winter polar vortices by the mesospheric meridional circulation7 and are preferentially deposited in the polar ice caps this implies an average global fallout of 14 5 kilotons per year of meteoric smoke during the holocene

physical characteristics of near earth asteroids from thermal infrared spectrophotometry thermal infrared spectrophotometric observations of the near earth asteroids 433 eros 1980 tezcatlipoca and 3671 dionysus are presented and their implications for the physical characteristics of the objects discussed sizes and albedos are derived on the basis of published thermal models and comparison of the infrared data with optical observations the results for 433 eros are in very good agreement with radar and other earlier findings and demonstrate the accuracy achievable with one of the thermal models used the new data together with earlier results from the literature suggest the surface thermal inertia of 433 eros is no larger than three times that of the moon the derived effective diameter and optical albedo of 1980 tezcatlipoca around lightcurve maximum are d 6 7 1 0 km and pv 0 15 0 05 and for 3671 dionysus d 1 0 0 2 km and pv 0 40 0 15 these diameters are significantly different from previous estimates the thermal continua of tezcatlipoca and dionysus are indicative of high surface thermal inertias a near infrared spectrum of dionysus coupled with the high albedo suggests an m or e classification such taxonomic types are relatively rare amongst near earth asteroids

a semi empirical model of the contribution from sporadic meteoroid sources on the meteor input function in the mlt observed at arecibo 1 in this paper we present a modeling and observational study of the micrometeor input function with a focus on understanding how each of the extraterrestrial sporadic meteoroid sources contributes to the observed meteoric flux in the mesosphere and lower thermosphere mlt atmospheric region for this purpose we expand the model presented by janches et al 2006 using a monte carlo technique and incorporating 1 a widely accepted global mass flux which is divided into different proportions among the known sporadic meteoroid sources as the initial input above earth s atmosphere 2 contemporary knowledge on the source s velocity and radiant distributions and 3 the full integration of the canonical meteor equations that describe the meteoroid entry and ablation physics in addition we constrain the initial input through a comparison of our modeled results with meteor observations obtained with the 430 mhz high power and large aperture hpla arecibo radar in puerto rico that covers all seasons the predicted meteor rates and velocity distributions are in excellent agreement with the observed ones without the need for any additional normalization factor our results indicate that although the earth s apex centered radiant source which is characterized by high geocentric speeds 55 km s appears to be 33 of the meteoroids in the solar system at 1 au it accounts for 60 of the meteors observed by the arecibo hpla radar in the atmosphere the remaining 40 of observed meteors originate mostly from the helion and anti helion sources with a very small but constant during the day contribution of the south and north toroidal sources these results also suggest that particles smaller than 10 3 g with slow velocities 30 km s will not significantly ablate and never become observable meteors the motivation of this effort is to construct a new and more precise mif model needed for the subsequent modeling of the atmospheric phenomena related to the meteoric flux

the carancas meteorite impact crater peru geologic surveying and modeling of crater formation and atmospheric passage the recent carancas meteorite impact event caused a worldwide sensation an h4 5 chondrite struck the earth south of lake titicaca in peru on september 15 2007 and formed a crater 14 2 m across it is the smallest youngest and one of two eye witnessed impact crater events on earth the impact violated the hitherto existing view that stony meteorites below a size of 100 m undergo major disruption and deceleration during their passage through the atmosphere and are not capable of producing craters fragmentation occurs if the strength of the meteoroid is less than the aerodynamic stresses that occur in flight the small fragments that result from a breakup rain down at terminal velocity and are not capable of producing impact craters the carancas cratering event however demonstrates that meter sized stony meteoroids indeed can survive the atmospheric passage under specific circumstances we present results of a detailed geologic survey of the crater and its ejecta to constrain the possible range of impact parameters we carried out numerical models of crater formation with the isale hydrocode in two and three dimensions depending on the strength properties of the target the impact energies range between approximately 100 1000 mj 0 024 0 24 t tnt by modeling the atmospheric traverse we demonstrate that low cosmic velocities 12 1 4 kms 1 and shallow entry angles 20 are prerequisites to keep aerodynamic stresses low 10 mpa and thus to prevent fragmentation of stony meteoroids with standard strength properties this scenario results in a strong meteoroid deceleration a deflection of the trajectory to a steeper impact angle 40 60 and an impact velocity of 350 600 ms 1 which is insufficient to produce a shock wave and significant shock effects in target minerals aerodynamic and crater modeling are consistent with field data and our microscopic inspection however these data are in conflict with trajectories inferred from the analysis of infrasound signals

experimental modelling of corona textures formation of corona textures along olivine plagioclase and orthopyroxene plagioclase interfaces has been experimentally reproduced at 670 and 700 c and 5 kbar with either a pure h2o fluid phase or 0 1 and 37 m nacl h2o solution fluid in these experiments we investigate the interaction of primary olivine and or orthopyroxene and plagioclase in powders and polished crystals and in small samples of a natural gabbro the experiments result in the formation of corona textures with several layers of different assemblages according to the experimental conditions consisting of garnet grossular clinopyroxene orthopyroxene amphibole chlorite and phlogopite the experiments show major differences in the number of layers the mineral assemblages and mineral composition and in the trends of composition of plagioclase in coronas around olivine and orthopyroxene the fluid phase composition influences the corona assemblages and the composition of the minerals in the experimental coronas for example garnet appears in the coronas in the second experiment where the nacl h2o ratio is low experimental modelling of corona textures confirms a model of simultaneous growth of layers by the mechanism of diffusion metasomatism with participation of a fluid phase through which mass is transferred zoning in the experimental coronas shows opposing diffusion of al and ca from plagioclase and mg and fe from olivine orthopyroxene difference in the mobility of the components is inferred from observations in the coronas the experimental corona textures are compared with natural coronas from the belomorian belt baltic shield developed at 670 690 c and 7 8 kbar and the marun keu complex polar urals developed at 670 700 c and 14 16 kbar where the corona textures correspond to a transitional stage of the gabbro to eclogite transformation

origin of discrepancies between crater size frequency distributions of coeval lunar geologic units via target property contrasts abstract recent work on dating copernican aged craters using lunar reconnaissance orbiter lro camera data re encountered a curious discrepancy in crater size frequency distribution csfd measurements that was observed but not understood during the apollo era for example at tycho copernicus and aristarchus craters csfds of impact melt deposits give significantly younger relative and absolute model ages amas than impact ejecta blankets although these two units formed during one impact event and would ideally yield coeval ages at the resolution of the csfd technique we investigated the effects of contrasting target properties on csfds and their resultant relative and absolute model ages for coeval lunar impact melt and ejecta units we counted craters with diameters through the transition from strength to gravity scaling on two large impact melt deposits at tycho and king craters and we used pi group scaling calculations to model the effects of differing target properties on final crater diameters for five different theoretical lunar targets the new csfd for the large king crater melt pond bridges the gap between the discrepant csfds within a single geologic unit thus the observed trends in the impact melt csfds support the occurrence of target property effects rather than self secondary and or field secondary contamination the csfds generated from the pi group scaling calculations show that targets with higher density and effective strength yield smaller crater diameters than weaker targets such that the relative ages of the former are lower relative to the latter consequently coeval impact melt and ejecta units will have discrepant apparent ages target property differences also affect the resulting slope of the csfd with stronger targets exhibiting shallower slopes so that the final crater diameters may differ more greatly at smaller diameters besides their application to age dating the csfds may provide additional information about the characteristics of the target for example the transition diameter from strength to gravity scaling could provide a tool for investigating the relative strengths of different geologic units the magnitude of the offset between the impact melt and ejecta isochrons may also provide information about the relative target properties and or exposure degradation ages of the two units robotic or human sampling of coeval units on the moon could provide a direct test of the importance and magnitude of target property effects on csfds

dating very young planetary surfaces from crater statistics a review of issues and challenges determining the ages of young planetary surfaces relies on using populations of small often sub km diameter impact craters due to the higher frequency at which they form smaller craters however can be less reliable for estimating ages as their size frequency distribution is more susceptible to alteration with debate as to whether they should be used at all with the current plethora of meter scale resolution images acquired of the lunar and martian surfaces small craters have been widely used to derive model ages to establish the temporal relation of recent geologic events in this review paper we discuss the many factors that make smaller craters particularly challenging to use and should be taken into consideration when crater counts are confined to small crater diameters establishing confidence in a model age ultimately requires an understanding of the geologic context of the surface being dated as reliability can vary considerably and limitations of the dating technique should be considered in applying ages to any geologic interpretation

diffusion controlled olivine corona textures in granitic rocks from lofoten norway calculation of onsager diffusion coefficients thermodynamic modelling and petrological implications reactions occurring during cooling of charnockitic intrusives on the lofoten islands produce characteristic diffusion controlled textures around fayalite and fe ti oxides thermobarometry indicates the corona textures formed at 780 840 c and pressures of 4 10 kbar whereas the magmatic assemblage of the charnockite clinopyroxene olivine quartz crystallized at about 850 870 c and 4 kbar the succession olivine orthopyroxene magnetite orthopyroxene garnet and olivine orthopyroxene magnetite amphibole developed where olivine reacted with adjacent plagioclase or k feldspar but the modes and the thicknesses of the corona textures vary according to the feldspar type indicating that the primary magmatic ternary feldspar was already exsolved into albitic plagioclase and alkali feldspar when the corona formation began simultaneously in other parts of the rock primary magmatic clinopyroxene reacted to amphibole and fe ti oxides reacted to orthopyroxene garnet coronas or to amphibole textures demonstrate significant al diffusion in the rocks under granulite facies conditions and they suggest that no pervasive fluid influx occurred and that amphibole formation was dependant on a local source of h2o probably related to water release during the last stages of magmatism calculation of the net reaction by accounting for all observed reactions at different sites in the rock indicates that the system can be regarded as balanced on a hand specimen scale with respect to all elements except for na and h2o the larger variety of textures developed in rocks of granitic bulk composition provide more constraints than textures from gabbroic compositions and permitted calculation of a set of relative diffusion coefficients which also reproduce textures in the gabbroic and anorthositic rocks from the lofoten islands the following set of relative diffusion coefficients li lfe reproduces the observed textures in the lofoten rocks si 0 82 mg 0 59 mn 0 05 na 0 38 k 0 39 al 0 05 and ca 0 07

evidence for a meteoritic origin of the september 15 2007 carancas crater on september 15th 2007 around 11 45 local time in peru near the bolivian border the atmospheric entry of a meteoroid produced bright lights in the sky and intense detonations soon after a crater was discovered south of lake titicaca these events have been detected by the bolivian seismic network and two infrasound arrays operating for the comprehensive nuclear test ban treaty organization situated at about 80 and 1620 km from the crater the localization and origin time computed with the seismic records are consistent with the reported impact the entry elevation and azimuthal angles of the trajectory are estimated from the observed signal time sequences and backazimuths from the crater diameter and the airwave amplitudes the kinetic energy mass and explosive energy are calculated using the estimated velocity of the meteoroid and similarity criteria between orbital elements an association with possible parent asteroids is attempted the favorable setting of this event provides a unique opportunity to evaluate physical and kinematic parameters of the object that generated the first actual terrestrial meteorite impact seismically recorded

an entry model for the tagish lake fireball using seismic satellite and infrasound records we present instrumental observations of the tagish lake fireball and interpret the observed characteristics in the context of two different models of ablation from these models we estimate the pre atmospheric mass of the tagish lake meteoroid to be 56 tonnes and its porosity to be between 37 and 58 with the lowest part of this range most probable these models further suggest that some 1300 kg of gram sized or larger tagish lake material survived ablation to reach the earth s surface representing an ablation loss of 97 for the fireball satellite recordings of the tagish lake fireball indicate that 1 1 1012 j of optical energy were emitted by the fireball during the last 4 s of its flight the fraction of the total kinetic energy converted to light in the satellite pass band is found to be 16 infrasonic observations of the airwave associated with the fireball establish a total energy for the event of 1 66 0 70 kt tnt equivalent energy the fraction of this total energy converted to acoustic signal energy is found to be between 0 10 and 0 23 examination of the seismic recordings of the airwave from tagish lake have established that the acoustic energy near the sub terminal point is converted to seismic body waves in the upper most portion of the earth s crust the acoustic energy to seismic energy coupling efficiency is found to be near 10 6 for the tagish lake fireball the resulting energy estimate is near 1 7 kt corresponding to a meteoroid 4 m in diameter the seismic record indicates extensive nearly continuous fragmentation of the body over the height intervals from 50 to 32 km seismic and infrasound energy estimates are in close agreement with the pre atmospheric mass of 56 tonnes established from the modeling the observed flight characteristics of the tagish lake fireball indicate that the bulk compressive strength of the pre atmospheric tagish lake meteoroid was near 0 25 mpa while the material compressive strength most appropriate to the recovered meteorites was closer to 0 7 mpa these are much lower than values found for fireballs of ordinary chondritic composition the behavior of the tagish lake fireball suggests that it represents the lowest end of the strength spectrum of carbonaceous chondrites or the high end of cometary meteoroids the bulk density and porosity results for the tagish lake meteoroid suggest that the low bulk densities measured for some small primitive bodies in the solar system may reflect physical structure dominated by microporosity rather than macroporosity and rubble pile assemblages

the influence of nacl h2o fluids on reactions between olivine and plagioclase an experimental study at 0 8 gpa and 800 900 c abstract the lower to middle continental and oceanic crust is dominated by mafic gabbroic rocks which are variably metamorphosed at amphibolite granulite facies conditions forming reaction rims or veins containing secondary pyroxenes amphibole and phlogopite metamorphic fluids in the mid crust are typically nacl rich brines in this study olivine and plagioclase were reacted with varying amounts of fluid 0 2 wt added that had compositions ranging from pure h2o to nacl solutions 0 2 mol in a series of experiments using the piston cylinder apparatus at 0 8 gpa and temperatures from 800 to 900 c coronas of clinopyroxene clinopyroxene amphibole and clinopyroxene phlogopite were formed the thicknesses of reaction rims i e the relative amount of reaction rim products increase with the relative amount of fluid present in the system and are therefore linked to the level of fluid saturation along grain boundaries and interphase boundaries samples with no added h2o show little to no signs of reaction indicating the necessity for fluids in initiating a reaction between olivine and plagioclase the concentration of nacl in the fluid also increases the rates of rim growth at 900 c and 2 wt added fluid cl rich hydrous dacite trachyte melt compositions formed and are similar to low degree partial melts of oceanic crust the cl contents of secondary amphiboles are positively correlated with the feo and k2o contents of the amphibole consumption of fluid during the formation of hydrous rim phases may lead to high cl within transient fluid filled pores along grain boundaries which may lead to locally cl rich amphiboles

infrasound detection of the chelyabinsk meteor at the usarray abstract on february 15 2013 a small asteroid entered earth s atmosphere near chelyabinsk russia this extremely rare event was recorded by the 400 station usarray deployed in the continental united states and alaska these stations recorded infrasound signals from the event at distances from 6000 to 10 000 km across a sector spanning 55 that encompassed the north pole this dense extensive network permitted a detailed study of long range infrasound propagation and source characteristics we observe long wavetrains at all stations ranging to over 100 min but clear variations in the character of the wavetrains across the network ray tracing through a spatially and temporally varying atmospheric model indicates the source excited resonance in the thermospheric duct to all stations resonance was also excited in a persistent stratospheric duct between the source and stations in alaska and along the west coast of the united states due to favorable winds at those azimuths leading to higher group velocities and frequency content at these stations than those to the east an attenuation formula derived from parabolic equation simulations is used to estimate infrasound transmission losses at all stations using simplified models of the effective sound speed along each source receiver path observed variations in signal energies from higher than expected at stations in the thermospheric duct in the eastern united states to lower than expected in alaska at azimuths nearly orthogonal to the asteroid s mach cone lead us to conclude that 1 the source was dominantly isotropic and 2 the model overestimates attenuation in the thermospheric duct

the moravka meteorite fall 3 meteoroid initial size history structure and composition the properties and history of the parent meteoroid of the mora vka h5 6 ordinary chondrites have been studied by a combination of various methods the pre atmospheric mass of the meteoroid was computed from fireball radiation infrasound seismic signal and the content of noble gases in the meteorites all methods gave consistent results the best estimate of the pre atmospheric mass is 1500 500 kg the fireball integral bolometric luminous efficiency was 9 and the acoustic efficiency was 0 14 the meteoroid cosmic ray exposure age was determined to be 6 7 1 0 10 6 yr the meteorite shows a clear deficit of helium both 3 he and 4 he this deficit can be explained by solar heating numerical backward integration of the meteoroid orbit determined in a previous paper from video records of the fireball shows that the perihelion distance was probably lower than 0 5 au and possibly as low as 0 1 au 5 ma ago the collision which excavated mora vka probably occurred while the parent body was on a near earth orbit as opposed to being confined entirely to the main asteroid belt an overview of meteorite macroscopic properties petrology mineralogy and chemical composition is given the meteorites show all mineralogical features of h chondrites the shock level is s2 minor deviations from other h chondrites in abundances of trace elements la ce cs and rb were found the ablation crust is enriched with siderophile elements

in situ fragmentation of lunar blocks and implications for impacts and solar induced thermal stresses abstract this study deals with an aspect of blocks observed on many rocky planetary surfaces in situ fragmentation using lroc nac images we characterized the morphology morphometry and abundance of in situ fractured blocks observed on the rim of six large impact craters of known emplacement age on the moon the relative number of disrupted blocks increases with crater retention age of surfaces on which blocks are hosted consistent with fragmentation post emplacement due to impacts of small meteoroids the type of break up morphologies we observe appears to be independent of surface exposure age of the blocks the inferred flux and size frequency distribution of projectiles responsible for disrupting blocks is consistent with expected lunar impact fluxes block fragmentation due to insolation driven thermal stresses is subordinate to impacts the possible effects of thermal stresses are evident as meridional cracks which have preferred orientations in a young block population 4 ma and as loose material fillet developing on top of surviving blocks in old populations 800 ma

metasomatism of gabbro mineral replacement and element mobilization during the sveconorwegian metamorphic event widespread metasomatism affected the 100 km long and 25 km wide proterozoic bamble and modum kongsberg sectors south norway resulting in the chemical and mineralogical transformation of wide segments of continental crust scapolitization was associated with veining and was followed by albitization transforming metagabbros pervasively over large areas fluids played an active role in these reactions forming h2o co2 and cl bearing phases at the expense of the primary volatile free minerals causing depletion in fe and infiltration of k mg na b and p the transformation of gabbro to scapolite metagabbro is observed as a fluid front replacing the primary magmatic mineral assemblage in three stages during an incipient amphibolitization stage the primary mafic minerals were replaced by anthophyllite or hastingsite followed by pargasitic and edenitic ca amphibole magnetite was dissolved while rutile formed by the breakdown of ilmenite plagioclase was replaced by cl rich scapolite me19 42 reflecting cl saturation while k and mg saturation produced phlogopite enstatite sapphirine and rare corundum the high modal contents of chlorapatite and tourmaline in the scapolite metagabbro imply infiltration of b and p the albitites consist dominantly of albite ab95 98 with varying generally small amounts of chlorite calcite rutile epidote and pumpellyite they formed from a h2o co2 fluid rich in na the gabbro yields a zircon u pb age of 1149 7 ma and tonalite 1294 38 ma whereas rutile from scapolite metagabbro and albitite has u pb ages of 1090 1084 ma and phlogopite produced during scapolitization rb sr ages of 1070 1040 ma temperature conditions for the scapolitization are inferred to have been 600 700 c the reported ages combined with mineralogical and petrographic observations and inferred p t conditions indicate that the metasomatism was a part of the regional sveconorwegian amphibolite facies metamorphic phase initial 87sr 86sr of the scapolite ranges from 0 704 to 0 709 the sr signature the cl and b rich environment and regional distribution of lithologies suggest that the fluid may have originated from evaporites that were mobilized during the regional metamorphism

a near earth asteroid population estimate from the linear survey i estimate the size and shape of the near earth asteroid nea population using survey data from the lincoln near earth asteroid research linear project covering 375 000 square degrees of sky and including more than 1300 nea detections a simulation of detection probabilities for different values of orbital parameters and sizes combined with the detection statistics in a bayesian framework provides a correction for observational bias and yields the nea population distribution as a function of absolute magnitude semi major axis eccentricity and inclination the nea population is more highly inclined than previously estimated and the total number of kilometer sized neas is 1227 90 170 1

tracing a meteoric trajectory with infrasound 1 a bright bolide appeared over the netherlands on the evening of 2003 february 19 infrasound generated by this bolide was detected at two nearby infrasound arrays as two distinct events the origins of the infrasonic signals were located in the stratosphere at heights of 30 and 57 km the results were obtained independently of visual observations of the bolide but confirm the reported time of occurrence the ability to distinguish between a moving and static source is crucial for the application of infrasound as a monitoring technique this study also illustrates the dependence of the infrasound technique on signal coherency inter array distance and atmospheric knowledge

diffusion controlled growth of albite and pyroxene reaction rims the growth rates of albite and pyroxene enstatite diopside spinel reaction rims were measured at 1000 c and 700 mpa and found to be parabolic indicating diffusion controlled growth the parabolic rate constants for the pyroxene spinel rims in samples with 0 5 wt h2o added or initially vacuum dried at 25 c and 250 c are 1 68 0 09 0 54 0 05 and 0 25 0 06 m2 h respectively the values for albite rim growth in samples initially dried at 60 c and with 0 1 wt h2o added are 0 25 0 04 and 0 33 0 03 m2 h respectively the latter values were used to derive the product of the grain boundary diffusion coefficient d a where a sio2 naalo2 or naalsi 1 and the grain boundary thickness in albite the calculated d sio2 in the albite aggregate for the situations of two different water contents are about 9 9 10 23 and 1 4 10 22 m3 s 1 respectively both the rate constants and the calculated d a demonstrate that the effect of water content on the grain boundary diffusion rate in monomineralic albite and polymineralic pyroxene spinel aggregates is small consistent with recent studies of monomineralic enstatite and forsterite rims

composition of fluids in the lower crust inferred from metamorphic salt in lower crustal rocks knowledge of the rheological properties of the lower crust and the metamorphic processes that operate there is important for our understanding of orogenic processes and granite genesis the rheological properties critically depend on whether fluids are present in the lower crust1 2 and if present on their composition3 4 5 6 fluid inclusion7 8 9 and phase equilibria4 10 studies of lower crustal granulites have shown that fluids with low water activities due to the presence of dissolved components such as ch4 n2 co co2 and chlorides 11 are present at least episodically in the lower crust here we report the occurrence of a solid salt solution nacl kcl found together with chlorine rich amphibole and biotite in lower crustal granulites a desiccation mechanism explains how salt and chlorine rich minerals formed from an originally water rich fluid through a short lived series of hydration reactions in the granulites during which chlorine was progressively enriched in the fluid consequently it would appear that fluid was present in the lower crust in only small amounts and was not stable over geologically long periods of time leading to the conclusion that the lower crust is devoid of a free fluid phase during most of its history

seismic observations of meteors coupling theory and observations to date coupling of acoustic waves directly with the earth s surface represents the bulk of the observed meteor related seismic signals while precursory and impact related seismic waves remain an observational rarity with proliferation of infrasound and seismic monitoring systems new opportunities exist to explore the relationship between earth s atmosphere andsurface continued studyofmeteorseismologywillleadto new methods to constrain energies sizes and fluxes for moderately cm to m sized meteoroids on earth and potentially on mars

the hazard of near earth asteroid impacts on earth near earth asteroids neas have struck the earth throughout its existence during epochs when life was gaining a foothold f4 ga the impact rate was thousands of times what it is today even during the phanerozoic the numbers of neas guarantee that there were other impacts possibly larger than the chicxulub event which was responsible for the cretaceous tertiary extinctions astronomers have found over 2500 neas of all sizes including well over half of the estimated 1100 neas 1 km diameter neas are mostly collisional fragments from the inner half of the asteroid belt and range in composition from porous carbonaceous chondrite like to metallic nearly one fifth of them have satellites or are double bodies when the international telescopic spaceguard survey which has a goal of discovering 90 of neas 1 km diameter is completed perhaps as early as 2008 nearly half of the remaining impact hazard will be from land or ocean impacts by bodies 70 600 m diameter comets are expected to contribute only about 1 of the total risk the consequences of impacts for civilization are potentially enormous but impacts are so rare that worldwide mortality from impacts will have dropped to only about 150 per year averaged over very long durations after the spaceguard goal has presumably ruled out near term impacts by 90 of the most dangerous ones that is in the mid range between very serious causes of death disease auto accidents and minor but frightening ones like shark attacks differences in perception concerning this rather newly recognized hazard dominate evaluation of its significance the most likely type of impact events we face are hyped or misinterpreted predicted impacts or near misses involving small neas d 2004 elsevier b v all rights reserved

the dakhleh glass product of an impact airburst or cratering event in the western desert of egypt impact cratering is a ubiquitous geological process on the terrestrial planets meteorite impact craters are the most visible product of impact events but there is a growing recognition that large aerial bursts or airbursts should occur relatively frequently throughout geological time in this contribution we report on an unusual impact glass the dakhleh glass dg which is distributed over an area of 400 km 2 of the dakhleh oasis egypt this region preserves a rich history of habitation stretching back to over 400 000 years before the emergence of homo sapiens we report on observations made during recent fieldwork and subsequent analytical analyses that strengthen previous suggestions that the dg formed during an impact event the wide distribution and large size of dg specimens up to 50 cm across the chemistry e g cao and al2o3 contents up to 25 and 18 wt respectively the presence of lechatelierite and burnt sediments and the inclusion of clasts and spherules in the dg is inconsistent with known terrestrial processes of glass formation the age and other textural characteristics rule out a human origin instead we draw upon recent numerical modeling of airbursts to suggest that the properties of dg coupled with the absence of a confirmed crater can best be explained by melting of surficial sediments as a result of a large airburst event we suggest that glass produced by such events should therefore be more common in the rock record than impact craters assuming that the glass formed in a suitable preserving environment

the orbit atmospheric dynamics and initial mass of the park forest meteorite the fireball accompanying the park forest meteorite fall l5 was recorded by ground based videographers satellite systems infrasound seismic and acoustic instruments this meteorite shower produced at least 18 kg of recovered fragments on the ground simon et al 2004 by combining the satellite trajectory solution with precise ground based video recording from a single site we have measured the original entry velocity for the meteoroid to be 19 5 0 3 km s the earliest video recording of the fireball was made near the altitude of 82 km the slope of the trajectory was 29 from the vertical with a radiant azimuth astronomical of 21 and a terminal height measured by infrared satellite systems of 18 km the meteoroid s orbit has a relatively large semi major axis of 2 53 0 19 au large aphelion of 4 26 0 38 au and low inclination the fireball reached a peak absolute visual magnitude of 22 with three major framentation episodes at the altitudes of 37 29 and 22 km acoustic recordings of the fireball airwave suggest that fragmentation was a dominant process in production of sound and that some major fragments from the fireball remained supersonic to heights as low as 10 km seismic and acoustic recordings show evidence of fragmentation at 42 36 29 and 17 km examination of implied energies initial masses from all techniques satellite optical infrasound seismic modeling leads us to conclude that the most probable initial mass was 11 3 10 3 kg corresponding to an original energy of 0 5 kt tnt 2 1 x 10 12 j and a diameter of 1 8 m these values correspond to an integral bolometric efficiency of 7 2 early fragmentation ram pressures of 30 seconds

amphibolitized sheared gabbros from ophiolites as indicators of the evolution of the oceanic crust bay of islands newfoundland abstract in the bay of islands ophiolite complex both the basal part of the cumulate gabbro sequence and the underlying peridotites have been plastically deformed higher in the series the cumulate gabbros are undeformed except in localized shear zones slightly to strongly oblique to the layering the resulting shear bands are narrow and composed either of mylonitic amphibole bearing gabbros or of highly foliated amphibolites the amphibole composition and nature and the chemistry of coexisting minerals combined with structural evidence imply that a first episode of shearing subhorizontal in the crust occurred at decreasing temperatures ranging from 750 to 450 c this shearing probably represent the response of the lithosphere to stresses also responsible of asthenospheric flow in the upper mantle a second episode of shearing oblique to the first one and probably related to cooling of the crust away from the ridge occurred at lower temperatures ranging from 500 to 300 c the surrounding gabbros have undergone static hydrothermal metamorphism at temperatures decreasing from 650 to 450 c all metamorphic events occurred in hydrous conditions in the vicinity of the ridge axis

analysis of impact crater populations and the geochronology of planetary surfaces in the inner solar system analyzing the density of impact craters on planetary surfaces is the only known technique for learning their ages remotely as a result crater statistics have been widely analyzed on the terrestrial planets since the timing and rates of activity are critical to understanding geologic process and history on the moon the samples obtained by the apollo and luna missions provide critical calibration points for cratering chronology on mercury venus and mars there are no similarly firm anchors for cratering rates but chronology models have been established by extrapolating from the lunar record or by estimating their impactor fluxes in other ways this review provides a current perspective on crater population measurements and their chronological interpretation emphasis is placed on how ages derived from crater statistics may be contingent on assumptions that need to be considered critically in addition ages estimated from crater populations are somewhat different than ages from more familiar geochronology tools e g radiometric dating resurfacing processes that remove craters from the observed population are particularly challenging to account for since they can introduce geologic uncertainty into results or destroy information about the formation age of a surface regardless of these challenges crater statistics measurements have resulted in successful predictions later verified by other techniques including the age of the lunar maria the existence of a period of heavy bombardment in the moon s first billion years and young volcanism on mars

systematic ranging and late warning asteroid impacts we describe systematic ranging an orbit determination technique suitable to assess the near term earth impact hazard posed by newly discovered asteroids for these late warning cases the time interval covered by the observations is generally short perhaps a few hours or even less which leads to severe degeneracies in the orbit estimation process the systematic ranging approach gets around these degeneracies by performing a raster scan in the poorly constrained space of topocentric range and range rate while the plane of sky position and motion are directly tied to the recorded observations this scan allows us to identify regions corresponding to collision solutions as well as potential impact times and locations from the probability distribution of the observation errors we obtain a probability distribution in the orbital space and then estimate the probability of an earth impact we show how this technique is effective for a number of examples including 2008 tc 3 and 2014 aa the only two asteroids to date discovered prior to impact

very low strengths of interplanetary meteoroids and small asteroids we have assembled data on thirteen cases of meteorite falls with accurate tracking data on atmospheric passage in all cases we estimate the bulk strength of the object corresponding to its earliest observed or inferred fragmentation in the high atmosphere and can compare these values with measured strengths of meteorites in the taxonomic class for that fall in all thirteen cases the strength corresponding to earliest observed or inferred fragmentation is much less than the compressive or tensile strength reported for that class of stony meteorites we find a more random relation between bulk sample strength and sample mass than is suggested by a commonly used empirical power law we estimate bulk strengths on entry being characteristically of the order of 10 1 to 10 2 times the tensile strengths of recovered samples

petrogenesis of oceanic plagiogranites by partial melting of gabbros an experimental study we performed hydrous partial melting experiments at shallow pressures 0 2 gpa under slightly oxidizing conditions nno oxygen buffer on oceanic cumulate gabbros drilled by odp ocean drilling program cruises to evaluate whether the partial melting of oceanic gabbro can generate sio2 rich melts with compositions typical of oceanic plagiogranites the experimental melts of the low temperature runs broadly overlap those of natural plagiogranites at 940 c the normalized sio2 contents of the experimental melts of all systems range between 60 and 61 wt and at 900 c between 63 and 68 wt these liquids are characterized by low tio2 and feotot contents similar to those of natural plagiogranites from the plutonic section of the oceanic crust but in contrast to fe and ti rich low temperature experimental melts obtained in morb systems at 950 c the 1 500 m long drilled gabbroic section of odp hole 735b legs 118 and 176 at the southwest indian ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high temperature shearing the compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins while the compositions of the crystals of low temperature runs correspond to those of minerals from high temperature microscopic veins occurring in the gabbroic section of the hole 735b this suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water rich fluid phase if the temperature estimations for high temperature shear zones are correct up to 1 000 c and a water rich fluid phase is present the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust

the production of small primary craters on mars and the moon abstract we model the primary crater production of small d d 10 20 cm well below the downturn observed in the distribution of fresh craters detected by the mars global surveyor mgs mars orbiter camera moc or the mars reconnaissance orbiter mro context camera ctx crater counts are conducted on the ejecta blanket of zunil crater and the interior of pangboche crater on mars and north ray and cone craters on the moon our model isochrons produce a similar slope and age estimate for the formation of zunil crater as the hartmann production function 1 ma we derive an age of 35 1 ma for pangboche when accounting for the higher elevation 20 km higher than zunil a factor 2 younger than estimated using the hartmann production function which assumes 6 mbar surface pressure we estimate ages of 52 3 ma and 23 9 ma for north ray and cone crater respectively consistent with cosmic ray exposure ages from apollo samples our results indicate that the average cratering rate has been constant on these bodies over these time periods since our monte carlo simulations demonstrate that the existing crater chronology systems can be applied to date young surfaces using small craters on the moon and mars we conclude that the signal from secondary craters in the isochrons must be relatively small at these locations as our monte carlo model only generates primary craters

anatomy of a young impact event in central alberta canada prospects for the missing holocene impact record small impact events recorded on the surface of earth are signifi cantly underrepresented based on expected magnitude frequency relations we report the discovery of a 36 m diameter late holocene impact crater located in a forested area near the town of whitecourt alberta canada although undetectable using visible imagery the presence of the crater is revealed using a bare earth digital elevation model obtained through airborne light detection and ranging lidar the target material comprises deglacial quaternary sediments with impact ejecta burying a late holocene soil dated to ca 1100 14 c yr b p most of the 74 iron meteorites 0 1 1196 g recovered have an angular exterior morphology these meteorites were buried at depths 25 cm and are interpreted to result from fragmentation of the original projectile mass either at low altitude or during the impact event impact of the main mass formed the simple bowl shaped impact structure associated with an ejecta blanket and crater fi ll the increasing availability of lidar data for many terrestrial surfaces will serve as a useful tool in the discovery of additional small impact features

metagabbros in the modum complex southern norway an important heat source for sveconorwegian metamorphism abstract the precambrian modum complex in southern norway is characterized by supracrustal rocks penetrated by numerous gabbro intrusions occurring as elongated bodies subparallel with the regional foliation these gabbroic bodies show a partial alteration to amphibolites the best preserved parts of the metagabbros show a subophitic texture with olivine and plagioclase as the major cumulus phases and clinopyroxene orthopyroxene and ilmenite as the major intercumulus phases subsequent alteration of these igneous phases has led to formation of coronas around olivine in contact with plagioclase and recrystallization of clinopyroxene the igneous phases indicate intrusion temperatures in the range of 1100 1000 c and pressures below 5 8 kbar formation of coronas and recrystallization of clinopyroxene occurred in the pressure temperature range of 7 10 kbar and 800 600 c a 1224 15 ma sm nd mineral isochron obtained from igneous phases indicates emplacement at the start of the sveconorwegian metamorphism a complex evolution is suggested for the sveconorwegian metamorphic event involving both magmatism and high grade metamorphism

the population of near earth asteroids abstract we describe a methodology of estimating the size frequency distribution sfd of near earth asteroids neas we estimate the completion versus size of present surveys based on the re detection ratio that is the fraction of all detections over a recent period that are re detections of already discovered objects rather than new discoveries the re detection ratio is a robust measure of completion but must be corrected for the obvious bias caused by differences in ease of discovery due to specific orbital geometries we do this with a computer survey simulation using a large set of synthetic orbital elements matching as best possible the distribution of the real nea population once suitably calibrated to match re detections of the real survey the completion estimate versus size derived from the simulation can be extended both to large size where few if any new detections are recorded and to small sizes beyond where re detection numbers are statistically significant thereby providing an estimate of the population and survey completion over the entire range from the largest neas down to the smallest sizes detected 3 m diameter here we update our previous population estimates and survey progress using discoveries by surveys from august 2012 through july 2014 we estimate that there are 990 20 neas larger than 1 km in diameter absolute magnitude h 17 75 of which about 90 have been discovered as of august 2014 we confirm a dip in the sfd in the range from a few tens to a few hundreds of meters diameter which may be due to the transition from larger rubble pile bodies to smaller monolithic bodies we compare our population estimate at the smallest sizes with recent ones based on bolide frequency and find excellent agreement within estimated errors the same survey simulation methodology can be used to investigate population and survey completion of various subset populations for example earth crossing asteroids ecas with orbits crossing 1 au heliocentric distance potentially hazardous asteroids phas with orbits passing within 0 05 au of the earth s orbit or interior to earth asteroids ieos with orbits entirely interior to the earth s orbit lastly we have investigated the population and completion of so called arm target asteroids of size 10 m diameter in orbits passing within 0 03 au of the earth s orbit with very low earth encounter velocity

the fall of the st robert meteorite the st robert quebec canada meteorite shower occurred on 1994 june 15 at 0h02m ut accompanied by detonations audible for 200 km from the fireball endpoint the fireball was recorded by visual observers in vermont new york state new hampshire quebec and ontario as well as by optical and infrared sensors in earth orbit penetration to an altitude of 36 km occurred 60 km to the northeast of montreal where the bolide experienced several episodes of fragmentation a total of 20 fragments of this h5 chondrite comprising a total mass of 25 4 kg were recovered in an ellipse measuring 8 3 5 km one fragment of the shower partially penetrated the aluminum roof of a shed interpretation of the visual and satellite data suggests that the fireball traveled from south southwest to north northeast with a slope from the horizontal of 55 61 a statistical evaluation of the likely heliocentric orbits for the body prior to collision with the earth coupled with theoretical modeling of the entry suggests an entry velocity in the range of 12 7 13 3 km s the meteoroid had moved in a low inclination orbit with orbital perihelion located extremely close to the earth s orbit from satellite optical data it is found that the photometric mass consumed during the largest detonation is 1200 kg estimation of the amplitude of the acoustic signal detected by the most distant observer yields a source energy near 0 5 kt tnt equivalent energy which corresponds to a mass of order 10 metric tonnes this measure is uncertain to approximately one order of magnitude theoretical modeling of the entry of the object suggests a mass near 1600 kg cosmogenic radionuclide activities constrain the lower initial mass to be 700 kg with an upper limit near 4000 kg seismic data possibly associated with the fireball suggest extremely poor coupling between the airwave and the ground the total mass estimated to have reached the ground is 100 kg in material comprising 55 g fragments while the preatmospheric mass is found to be most probably in the range of 1200 2000 kg

efficient disruption of small asteroids by earth s atmosphere accurate modelling of the interaction between the atmosphere and an incoming bolide is a complex task but crucial to determining the fraction of small asteroids that actually hit the earth s surface most semi analytical approaches have simplified the problem by considering the impactor as a strengthless liquid like object pancake models1 2 but recently a more realistic model has been developed that calculates motion aerodynamic loading and ablation for each separate particle or fragment in a disrupted impactor3 4 here we report the results of a large number of simulations in which we use both models to develop a statistical picture of atmosphere bolide interaction for iron and stony objects with initial diameters up to 1 km we show that the separated fragments model predicts the total atmospheric disruption of much larger stony bodies than previously thought in addition our data set of 1 000 simulated impacts combined with the known pre atmospheric flux of asteroids with diameters less than 1 km5 6 7 8 9 10 11 12 elucidates the flux of small bolides at the earth s surface we estimate that bodies 220 m in diameter will impact every 170 000 years

large impacts detected by the apollo seismometers impactor mass and source cutoff frequency estimations abstract meteoroid impacts are important seismic sources on the moon as they continuously impact the moon they are a significant contribution to the lunar micro seismic background noise they also were associated with the most powerful seismic sources recorded by the apollo seismic network we study in this paper the largest impacts we show that their masses can be estimated with a rather simple modeling technique and that high frequency seismic signals have reduced amplitudes due to a relatively low about 1 s corner frequency resulting from the duration of the impact process and the crater formation if synthetic seismograms computed for a spherical model of the moon are unable to match the waveforms of the observations they nevertheless provide an approximate measure of the energy of seismic waves in the coda the latter can then be used for an estimation of the mass of the impactors when the velocity of the impactor is known this method for the artificial impacts of the lm and sivb apollo upper stages allows us to retrieve the mass within 20 of relative error the estimated mass of the largest impacts observed during the 7 years of activity of the apollo seismic network provides an explanation for the non detection of surface waves on the seismograms the specifications of future moon seismometers in order to provide the detection of surface waves are given in conclusion

a survey of debris trails from short period comets abstract we observed 34 comets using the 24 m camera on the spitzer space telescope each image contains the nucleus and covers at least 10 6 km of each comet s orbit debris trails due to mm sized or larger particles were found along the orbits of 27 comets 4 comets had small particle dust tails and a viewing geometry that made debris trails impossible to distinguish and only 3 had no debris trail despite favorable observing conditions there are now 30 jupiter family comets with known debris trails of which 22 are reported in this paper for the first time the detection rate is 80 indicating that debris trails are a generic feature of short period comets by comparison to orbital calculations for particles of a range of sizes ejected over 2 yr prior to observation we find that particles comprising 4 debris trails are typically mm sized while the remainder of the debris trails require particles larger than this the lower limit masses of the debris trails are typically 10 11 g and the median mass loss rate is 2 kg s the mass loss rate in trail particles is comparable to that inferred from oh production rates and larger than that inferred from visible light scattering in comae

the record of miocene impacts in the argentine pampas argentine pampean sediments represent a nearly continuous record of deposition since the late miocene 10 ma previous studies described five localized concentrations of vesicular impact glasses from the holocene to late pliocene two more occurrences from the late miocene are reported here one near chasico ch with an 40ar 39ar age of 9 24 0 09 ma and the other near baha blanca bb with an age of 5 28 0 04 ma in contrast with andesitic and dacitic impact glasses from other localities in the pampas the ch and bb glasses are more mafic they also exhibit higher degrees of melting with relatively few xenoycrysts but extensive quench crystals in addition to evidence for extreme heating 1700 c shock features are observed e g planar deformation features pdfs and diaplectic quartz and feldspar in impact glasses from both deposits geochemical analyses reveal unusually high levels of ba 7700 ppm in some samples which is consistent with an interpretation that these impacts excavated marine sequences known to be at depth these two new impact glass occurrences raise to seven the number of late cenozoic impacts for which there is evidence preserved in the pampean sediments this seemingly high number of significant impacts over a 10 6 km 2 area in a time span of 10 myr is consistent with the number of bolides larger than 100 m expected to enter the atmosphere but is contrary to calculated survival rates following atmospheric disruption the pampean record suggests therefore that either atmospheric entry models need to be reconsidered or that the earth has received an enhanced flux of impactors during portions of the late cenozoic evidence for the resulting collisions may be best preserved and revealed in rare dissected regions of continuous low energy deposition such as the pampas additionally the rare earth element ree concentrations of the target sediments and impact melts associated with the chasic event resemble the hna k australites of similar age this suggests the possibility that those enigmatic tektites could have originated as high angle distal ejecta from an impact in argentina thereby accounting for their rarity and notable chemical and physical differences from other australasian impact glasses

estimates of yet to find impact crater population on earth abstract one hundred and seventy six craters on earth are currently accepted as resulting from bolide impact this list is growing at a rate of approximately two craters per year because of new discoveries and revised interpretations here we take the subset of craters most relevant to sedimentary basin exploration and utilization phanerozoic kilometre scale craters and tackle the question of how many remain to be found in phanerozoic strata on earth a surprising range in predictions of crater numbers is obtained from published crater population functions reaching two orders of magnitude variation for craters in the range 1 5 km diameter this range in predictions is largely due to variations in treatment of impactor breakup in the atmosphere also required is quantification of the total area and timespan of phanerozoic sediment currently preserved on earth an estimate of 8 87 10 15 km 2 a is derived here together with a relatively conservative crater population function this yields a prediction of 845 craters subtracting the 131 known phanerozoic craters larger than 1 km leaves a predicted population yet to find of 714 of which 228 are larger than 2 5 km uncertainties subsets of this population and spatial distribution are also considered

the kamil crater in egypt we report on the detection in southern egypt of an impact crater 45 meters in diameter with a pristine rayed structure such pristine structures are typically observed on atmosphereless rocky or icy planetary bodies in the solar system this feature and the association with an iron meteorite impactor and shock metamorphism provides a unique picture of small scale hypervelocity impacts on earth s crust contrary to current geophysical models ground data indicate that iron meteorites with masses of the order of tens of tons can penetrate the atmosphere without substantial fragmentation

a collision in 2009 as the origin of the debris trail of asteroid p 2010 a2 the peculiar object p 2010 a2 was discovered 1 in january 2010 and given a cometary designation because of the presence of a trail of material although there was no central condensation or coma the appearance of this object in an asteroidal orbit small eccentricity and inclination in the inner main asteroid belt attracted attention as a potential new member of the recently recognized 2 class of main belt comets if confirmed this new object would expand the range in heliocentric distance over which main belt comets are found here we report observations of p 2010 a2 by the rosetta spacecraft we conclude that the trail arose from a single event rather than a period of cometary activity in agreement with independent results 3 the trail is made up of relatively large particles of millimetre to centimetre size that remain close to the parent asteroid the shape of the trail can be explained by an initial impact ejecting large clumps of debris that disintegrated and dispersed almost immediately we determine that this was an asteroid collision that occurred around 10 february 2009

the villalbeto de la pena meteorite fall i fireball energy meteorite recovery strewn field and petrography an impressive daylight fireball was observed from spain portugal and the south of france at 16h46m45s utc on january 4 2004 the meteoroid penetrated into the atmosphere generating shock waves that reached the ground and produced audible booms the associated airwave was recorded at a seismic station located 90 km north of the fireball trajectory in spain and at an infrasound station in france located 750 km north east of the fireball the absolute magnitude of the bolide has been determined to be 18 1 from a casual video record the energy released in the atmosphere determined from photometric seismic and infrasound data was about 0 02 kilotons kt a massive fragmentation occurred at a height of 28 0 2 km resulting in a meteorite strewn field of 20 6 km the first meteorite specimen was found on january 11 2004 near the village of villalbeto de la pena in northern palencia spain to date about 4 6 kg of meteorite mass have been recovered during several recovery campaigns the meteorite is a moderately shocked s4 l6 ordinary chondrite with a cosmic ray exposure age of 48 5 ma radioisotope analysis shows that the original body had a mass of 760 150 kg which is in agreement with the estimated mass obtained from photometric and seismic measurements

the origin of olivine plagioclase coronas in metagabbros from the adirondack mountains new york olivine plagioclase coronas in metagabbros from the adirondack mountains new york usa are spatially well organized reaction textures consisting most commonly of sequential layers of orthopyroxene clinopyroxene plagioclase and garnet the textures are characteristic of diffusion controlled reaction kinetics although similar coronas have been interpreted by previous workers in terms of an isochemical steady state diffusion model petrographical relations and material balance calculations establish that coronas in the adirondack metagabbros cannot be treated as isochemical and do not form in a single stage steady state process instead they evolve through time in a complex open system reaction in this study the isochemical diffusion model is modified to account for elemental fluxes across the outer boundaries of the coronal reaction band thereby approximating the open system behaviour of the coronas the sequence and relative proportions of product minerals calculated by the open system steady state model correspond closely to those observed in coronas of the adirondacks over a wide range of values for the relative diffusivities of chemical components involved in the reaction regardless of the particular method used to determine material balance in the reaction texture despite this correspondence petrographical evidence for successive replacement of coronal product layers reveals that the adirondack coronas evolved through one or more transient states rather than forming in a single stage steady state process there is no evidence that the successive replacement of coronal product layers resulted from changes in pressure or temperature but there is petrographical evidence that these changes resulted from modification of the composition of reactant plagioclase as the corona forming reaction proceeded this is confirmed by the fact that the evolution of the coronas over time can be replicated with the open system diffusion model by simulating the effect of the gradual exhaustion of plagioclase as a source of the ca and si components required for reaction these simulations suggest that successive stages in the evolution of the coronas are characterized by these product sequences i orthopyroxene clinopyroxene plagioclase garnet ii orthopyroxene clinopyroxene garnet and iii orthopyroxene garnet all of these stages and the transitions between them are observed petrographically coronas in adirondack metagabbros appear therefore to have originated in a complex open system diffusion controlled reaction in which the product assemblages changed as the reaction progressed

dependence of reaction kinetics on h2o activity as inferred from rates of intergranular diffusion of aluminium quantitative constraints on the accelerative effects of h2o on the kinetics of metamorphic reactions arise from a comparison of rates of intergranular diffusion of al in natural systems that are fluid saturated hydrous but fluid undersaturated and nearly anhydrous widths of symplectitic reaction coronas around partially resorbed garnet crystals in the contact aureole of the makhavinekh lake pluton northern labrador combined with time temperature histories from conductive thermal models yield intergranular diffusivities for al from 700 900 c under nearly anhydrous conditions those rates when extrapolated down temperature are approximately three orders of magnitude slower than rates derived from re analysis of garnet resorption coronas formed under hydrous but fluid undersaturated conditions near 575 c in rocks of the llano uplift of central texas which are in turn approximately four orders of magnitude slower than rates at comparable temperatures derived from numerical simulations of prograde garnet growth in fluid saturated conditions in rocks from the picuris range of north central new mexico thus even at constant temperature rates of intergranular diffusion of al and corresponding length scales and timescales of metamorphic reaction and equilibration may vary by as much as seven orders of magnitude across the range of h2o activities found in nature