possible long term decline in impact rates 1 martian geological data abstract quantin et al quantin c allemand p mangold n delacourt c 2004a icarus 172 555 572 tabulated crater count data for 56 landslides along the walls of valles marineris under the assumption of a constant cratering rate after about 3 gyr ago as used in the 1999 2005 iterations of the crater chronology isochron system of hartmann and in the hartmann and neukum system these data indicate a regularly increasing rate of landslides which would be difficult to explain we suggest that these data may support a decline in inner solar system cratering rates by about a factor of 3 since 3 gyr ago not unlike predictions based on asteroid belt collision models such a decline is also supported by our review of data on lunar impact melts and glass spherules in a companion paper hartmann w k quantin c mangold n 2007 icarus 186 11 23 such models produce not only a more uniform rate of landslides over the last 3 gyr but also a more uniform rate of resurfacing processes which also had an apparent increase under the assumption of a constant cratering rate

do young martian ray craters have ages consistent with the crater count system abstract mcewen et al mcewen a s preblich b s turtle e p artemieva n a golombek m p hurst m kirk r l burr d m christensen p 2005 icarus 176 351 381 developed a useful test for the internal consistency of crater count chronometry systems they argued that certain multi kilometer fresh looking martian craters with prominent rays should be the youngest or near youngest craters in their size range the mcewen et al test is that the ages determined from crater densities of the smallest superimposed craters typically diameter d 5 20 m should thus be comparable to the expected formation intervals of the host primary mcewen et al concluded from moc data that crater chronometry failed this test by factors of 700 2000 we apply hirise and other imagery to eight different young craters in order to re evaluate their arguments we use existing crater chronology systems as well as the reported observed production rate of 16 m craters malin m c edgett k posiolova l mccolley s noe dobrea e 2006 science 314 1573 1557 hartmann w k quantin c mangold n 2007 icarus 186 11 23 kreslavsky 2007 seventh international conference on mars 3325 every case passes the mcewen et al test we conclude that the huge inconsistencies suggested by mcewen et al are spurious many of these craters show evidence of impact into ice rich material and appear to have ice flow features and sublimation pits on their floors as production rate data improve decameter scale craters will provide a valuable way of dating these young martian geological formations and the processes that modify them

distribution of slab derived fluids around the edge of the philippine sea plate from central to northeast japan abstract marginal parts of a plate and subducting slab can play important roles in geodynamics this is because in areas where a plate interacts with other plates or with the mantle thermal geochemical and mechanical interactions are expected the philippine sea phs slab that subducts beneath the japan arcs has such an edge to examine the relationship between arc magmatism and the slab edge in the transition zone from northeast japan to central japan we investigated isotopic systematics of the regional volcanic rocks incorporating data from literature and new data for five isotopic ratios of sr nd and pb the new data included major element compositions of 22 samples from the back arc area and 5 isotopic ratios for 6 samples selected from pleistocene to early quaternary epochs consequently several findings were determined based on the spatial variation of the isotopic ratios and the estimated amount of slab derived fluid 1 the amount of fluid derived from the two subducting slabs i e the pacific slab and the phs slab decreases northward from a significantly high value 5 wt fluid added to the source mantle away from the seismically determined edge of the phs slab 2 the proportion of the phs component in the total slab derived fluid also decays northward and 3 the phs component spreads to the north beyond the seismically determined edge of the phs slab these observations strongly suggest that the existence of an aseismic phs slab beneath southernmost parts of northeast japan delivers the phs component to the arc magmatism as was indicated by previous geodynamical studies subduction of the phs and pac slabs may generate suction force towards the corner of mantle wedge which might account for the large amount of fluid near the seismically determined slab edge as described in 1 above

the geology of the viking lander 2 site revisited abstract reevaluating the geologic history of the prior mars landing sites provides important ground truth for recent and ongoing orbital missions at the viking 2 lander vl2 site topographic measurements of relict landforms indicate that at least 100 m of sedimentary mantle material has been stripped away the observed paucity of impact craters

the case for an imbrium origin of the apollo thorium rich impact melt breccias abstract mafic th rich impact melt breccias most of which are identified with the composition known as low k fra mauro lkfm are the most common rock type in the nonmare regoliths of the apollo lunar landing sites the origin of mafic impact melt breccias bears on many lunar problems the nature of the late meteoroid bombardment cataclysm the spatial distribution of kreep both near the surface and at depth the ages of the major basins and the composition of the early crust of the nearside lunar highlands thus it is crucial that the origin of mafic impact melt breccias be accurately understood because of both intra and intersite differences in compositions of mafic impact melt breccia samples apparent differences in crystallization age and differences in siderophile element ratios previous studies have argued that either 1 most mafic impact melt breccias are the products of several large craters local to the site at which they were found but that some are of basin origin or that 2 they are all from the imbrium apollos 14 and 15 nectaris apollo 16 and serenitatis apollo 17 basins here we reconsider the hypothesis that virtually all of the th rich mafic impact melt breccias from the apollo missions are products of the imbrium impact ejecta deposit modeling based on modern crater scaling indicates that the imbrium event produced ejecta deposits that average hundreds of meters thick or more at all apollo highland sites which is thicker than some previous estimates substantial amounts of imbrium ejecta should have been sampled at every apollo highland site we suggest that the mafic impact melt breccias may be the principal form of those ejecta the imbrium projectile impacted into th rich material that we regard as part of a unique mafic lunar geochemical province we call the high th oval region based on the surface distribution of th only basins within the high th oval region excavated th rich material the th concentrations of the highlands as observed by the apollo orbiting ray experiments are consistent with the estimates from ejecta modeling of the younger basin forming impacts only imbrium was large enough to produce the copious amount of melt required by the ubiquitous presence of mafic impact melt breccias in the apollo sampled regolith the high th oval region still may have been molten or hot at shallow depths 4 ga ago when the imbrium projectile struck we reason that compositional heterogeneity of ejected melt breccia is to be expected under these circumstances we argue that siderophile element fingerprints of mafic impact melt breccias are not inconsistent with production of all common types by a single projectile we suggest that the narrow range of ages of 3 7 4 0 ga for all successfully dated mafic impact melt breccias may reflect a single event whose age is difficult to measure precisely rather than a number of discrete impact events closely spaced in time such that reported age variations among mafic impact melt breccias reflect the ability to measure 40ar 39ar ages with greater precision than the accuracy with which measured portions of mafic impact melt breccias have recorded the time of their formation

the volcanic history of central elysium planitia implications for martian magmatism abstract central elysium planitia cep is located south of elysium mons back to the era of the viking orbiters clues accumulated in favor of recent volcanism in relation with ground water release and the formation of long sub parallel fissures four aqueous flood channel systems emanate from linear fissures recent eruptions of low viscosity lavas originate from these fissures and from low shield volcanoes the objective of this paper is to constrain the volcanic history of this region and to determine the chronological relationships with fluvial erosional processes new observations e g new shield volcanoes and one new fluvial event are summarized on a context map thirty five surfaces have been dated from the count of about 15 000 impact craters ages have been cross checked with relative stratigraphy when possible a probabilistic approach has been introduced to compare similar ages and define periods of volcanic activity our results confirm that some volcanic features are extremely recent 2 my active periods are found at 2 5 3 my 4 3 my 13 5 16 2 my 19 my 21 32 my 58 my 71 my 85 95 my 134 my 173 my and 234 my not excluding the possibility that some of the gaps would be filled with additional crater counts the volcanic activity thus extended for at least the last 250 my the lava volumes have been estimated from the topographic modeling of the floor of depressions filled up by volcanic products including the volumes of several large crater cavities buried under lavas 20 of the total volume our new estimation of the total lava volume is 1 5 0 2 10 5 km 3 this value corresponds to an average thickness of one hundred meters of lavas for the young volcanic plain as a consequence the total eruption rate at cep defined as the total volume of lava divided by the time of emplacement 1 4 10 2 1 8 10 2 m 3 s is lower than values typically estimated for terrestrial hot spots or large igneous provinces suggesting longer inactive periods the concept of mantle plumes responsible for terrestrial flood volcanism may not be applicable to the case of cep and the mechanism proposed in schumacher and dreuer 2007 offers a plausible alternative to explain our observations

support for the lunar cataclysm hypothesis from lunar meteorite impact melt ages lunar meteorites represent a more random sampling of lunar material than the apollo or luna collections and as such lunar meteorite impact melt ages are the most important data in nearly 30 years with which to reexamine the lunar cataclysm hypothesis within the lunar meteorite breccias mac 88105 que 93069 dag 262 and dag 400 seven to nine different impact events are represented with 40ar 39ar ages between 2 76 and 3 92 billion years ago ga the lack of impact melt older than 3 92 ga supports the concept of a short intense period of bombardment in the earth moon system at approximately 3 9 ga this was an anomalous spike of impact activity on the otherwise declining impact frequency curve

martian cratering 9 toward resolution of the controversy about small craters abstract the malin et al malin m c edgett k posiolova l mccolley s noe dobrea e 2006 science 314 1573 1577 report of ongoing production of decameter scale primary craters matches within an order of magnitude the crater production curves of hartmann hartmann w k 2005 icarus 174 294 320 and appears to contradict models that say our curves grossly overpredict primary production nonetheless such models may still predict correct order of magnitude secondary production the new observations if correct require geologically recent martian surface modification in some areas on timescales comparable to the cycles of obliquity changes suggesting episodic martian climatic environmental change driven by the obliquity cycles

constraints on the source of lunar cataclysm impactors multiple impact basins formed on the moon about 3 8 gyr ago in what is known as the lunar cataclysm or late heavy bombardment many workers currently interpret the lunar cataclysm as an impact spike primarily caused by main belt asteroids destabilized by delayed planetary migration we show that morphologically fresh class 1 craters on the lunar highlands were mostly formed during the brief tail of the cataclysm as they have absolute crater number density similar to that of the orientale basin and ejecta blanket the connection between class 1 craters and the cataclysm is supported by the similarity of their size frequency distribution to that of stratigraphically identified imbrian craters majority of lunar craters younger than the imbrium basin including class 1 craters thus record the size frequency distribution of the lunar cataclysm impactors this distribution is much steeper than that of main belt asteroids we argue that the projectiles bombarding the moon at the time of the cataclysm could not have been main belt asteroids ejected by purely gravitational means

geomorphic analysis of lobate debris aprons on mars at mars orbiter camera scale evidence for ice sublimation initiated by fractures 1 lobate debris aprons known to be geomorphic landform indicators of the presence of ground ice are of special interest for future missions devoted to the research of water on mars lobate debris aprons in fretted terrains of deuteronilus and protonilus mensae 35 50 n show typical convex shapes interpreted to be the result of viscous deformation at the scale of mars orbiter camera moc high resolution images the surface of these debris aprons shows complex patterns with small pits and buttes these patterns can be explained by the mantling of dust the accumulation of interstitial ice and the subsequent removal of ice by sublimation the sublimation of the ground ice is especially initiated and accelerated by subsurface heterogeneities like fractures theoretical quantification of sublimation rates therefore minimizes sublimation which is not a homogeneous process at least over the landforms studied crater counts show that the sublimation occurred in the last tens of millions of years up to the recent past in the point of view of future searching of subsurface ice only surface layers are submitted to sublimation favoring the conservation of ground ice in deeper layers since the formation of the landform the geophysical survey of lobate debris aprons would give interesting insights into the subsurface distribution of ice and its seasonal variations especially in order to measure current sublimation of ground ice

geochemistry and 40ar 39ar geochronology of impact melt clasts in feldspathic lunar meteorites implications for lunar bombardment history we studied 42 impact melt clasts from lunar feldspathic regolith breccias macalpine hills mac 88105 queen alexandra range que 93069 dar al gani dag 262 and dag 400 for texture chemical composition and or chronology although the textures are similar to the impactmelt clasts identified in mafic apollo and luna samples the meteorite clasts are chemically distinct from them having lower fe ti k and p thus representing previously unsampled impacts the 40ar 39ar ages on 31 of the impact melts the first ages on impact melt samples from outside the region of the apollo and luna sampling sites range from 4 to 2 5 ga we interpret these samples to have been created in at least six and possibly nine or more different impact events one inferred impact event may be consistent with the apollo impact melt rock age cluster at 3 9 ga but the meteorite impact melt clasts with this age are different in chemistry from the apollo samples suggesting that the mechanism responsible for the 3 9 ga peak in lunar impact melt clast ages is a lunar wide phenomenon no meteorite impact melts have ages more than 1 older than 4 0 ga this observation is consistent with but does not require a lunar cataclysm

recent and episodic volcanic and glacial activity on mars revealed by the high resolution stereo camera the large area coverage at a resolution of 10 20 metres per pixel in colour and three dimensions with the high resolution stereo camera experiment on the european space agency mars express mission has made it possible to study the time stratigraphic relationships of volcanic and glacial structures in unprecedented detail and give insight into the geological evolution of mars here we show that calderas on five major volcanoes on mars have undergone repeated activation and resurfacing during the last 20 per cent of martian history with phases of activity as young as two million years suggesting that the volcanoes are potentially still active today glacial deposits at the base of the olympus mons escarpment show evidence for repeated phases of activity as recently as about four million years ago morphological evidence is found that snow and ice deposition on the olympus construct at elevations of more than 7 000 metres led to episodes of glacial activity at this height even now water ice protected by an insulating layer of dust may be present at high altitudes on olympus mons

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

lithospheric contributions to arc magmatism isotope variations along strike in volcanoes of honshu japan major chemical exchange between the crust and mantle occurs in subduction zone environments profoundly affecting the chemical evolution of earth the relative contributions of the subducting slab mantle wedge and arc lithosphere to the generation of island arc magmas and ultimately new continental crust are controversial isotopic data for lavas from a transect of volcanoes in a single arc segment of northern honshu japan have distinct variations coincident with changes in crustal lithology these data imply that the relatively thin crustal lithosphere is an active geochemical filter for all traversing magmas and is responsible for significant modification of primary mantle melts

mineralogy of recent volcanic plains in the tharsis region mars and implications for platy ridged flow composition volcanism on mars occurred until recently but the mineralogy of recent lava plains is poorly known because few regions display fresh outcrops devoid of dust using visible and near infrared data of the mars express probe two new volcanic plains in noctis labyrinthus have been identified and the existence of a volcanic plain on the floor of echus chasma has been confirmed crater retention ages estimated for these three plains range between 50 and 100 my corresponding to the late amazonian these plains represent an excellent opportunity to constrain the mineralogy of recent volcanic rocks results show that basaltic compositions with plagioclase and high calcium pyroxene are predominant the low olivine proportion suggests that the apparent fluidity of these flat plains is not related to magmas being ultramafic in addition a platy ridged texture is observed in two of the studied regions our study shows for the first time that this texture is associated with volcanic rocks and that these rocks are of typical basaltic mineralogy finally these volcanic plains are located more than 1000 km east of previously known late amazonian volcanic centers of the tharsis region an observation to be taken into account when considering models of recent volcanism on mars c 2009 elsevier bm all rights reserved

evidence for recent volcanism on mars from crater counts impact craters help characterize the age of a planetary surface because they accumulate with time they also provide useful constraints on the importance of surface erosion as such processes will preferentially remove the smaller craters earlier studies of martian crater populations revealed that erosion and dust deposition are important processes on mars they disagreed however on the age of the youngest volcanism these earlier studies were limited by image resolution to craters larger than a few hundred metres in diameter here we report an analysis using new images obtained by the mars global surveyor spacecraft of crater populations that extend the size distribution down to about 16 m our results indicate a wide range of surface ages with one region lava flows within the arsia mons caldera that we estimate to be no older than 40 100 million years we suggest that volcanism is a continuing process on mars

petrology of aoso volcano northeast japan arc temporal variation of the magma feeding system and nature of low k amphibole andesite in the aoso osore volcanic zone the aoso volcano is a member of the newly defined volcanic front of northeast japan characterized by the occurrence of low k and hornblende andesites its activity can be divided into three stages the early caldera forming and late stages while petrographic and geochemical data show all products underwent magma mixing or co mingling sr and nd isotopic ratios indicate that all are consanguineous the end member magmas are basaltic and andesitic in the early stage but basaltic and dacitic in the late stage in the caldera forming stage hornblende free and hornblende rich andesites co mingled which triggered an explosive eruption leading to caldera formation hornblende occurs also in the dacite from the early part of the late stage these hornblende andesites and dacites are lower in magmatic temperature compared to hornblende free andesites the estimated basaltic end member is low k and high in magmatic temperature and can be derived by a high degree of partial melting of mantle under the volcanic front the estimated andesitic and dacitic end members cannot be derived from the basaltic end member magma through fractional crystallization but can be derived from partial re melting of the solidified low k basalt leaving amphibolitic and gabbroic residues respectively

the phanerozoic impact cratering rate evidence from the farside of the moon the relatively recent 1 b y flux of asteroids and comets forming large craters on the earth and moon may be accurately recorded by craters with bright rays on the moon s farside many previously unknown farside rayed craters are clearly distinguished in the low phase angle images returned by the clementine spacecraft some large rayed craters on the lunar nearside are probably significantly older than 1 ga rays remain visible over the maria due to compositional contrasts long after soils have reached optical maturity most of the farside crust has a more homogeneous composition and only immature rays are visible the size frequency distribution of farside rayed craters is similar to that measured for eratosthenian craters up to 3 2 b y at diameters larger than 15 km the areal density of farside rayed craters matches that of a corrected tabulation of nearside copernican craters hence the presence of bright rays due to immature soils around large craters provides a consistent time stratigraphic basis for defining the base of the copernican system the density of large craters less than 3 2 b y old is 3 2 times higher than that of large farside rayed craters alone this observation can be interpreted in two ways 1 the average cratering rate has been constant over the past 3 2 b y and the base of the copernican is 1 ga or 2 the cratering rate has increased in recent geologic time and the base of the copernican is less than 1 ga we favor the latter interpretation because the rays of copernicus 800 850 m y old appear to be very close to optical maturity suggesting that the average copernican cratering rate was 35 higher than the average eratosthenian rate other lines of evidence for an increase in the phanerozoic 545 ga cratering rate are 1 the densities of small craters superimposed on copernicus and apollo landing sites 2 the rates estimated from well dated terrestrial craters 120 m y and from present day astronomical observations and 3 the proterozoic rate suggested by the crater record of australia the hypothesis most consistent with several key observations is that the cratering rate has increased by 2x during the past 300 m y

sediment dispersed extraterrestrial chromite traces a major asteroid disruption event abundant extraterrestrial chromite grains from decomposed meteorites occur in middle ordovician 480 million years ago marine limestone over an area of 250 000 square kilometers in southern sweden the chromite anomaly gives support for an increase of two orders of magnitude in the influx of meteorites to earth during the mid ordovician as previously indicated by fossil meteorites extraterrestrial chromite grains in mid ordovician limestone can be used to constrain in detail the temporal variations in flux of extraterrestrial matter after one of the largest asteroid disruption events in the asteroid belt in late solar system history

utilization of the themis visible and infrared imaging data for crater population studies of the meridiani planum landing site 1 the mars odyssey thermal emission imaging system themis acquires both visible and thermal infrared images of the martian surface this study shows that the daytime infrared images as well as the visible images may be utilized successfully for crater population studies the themis data bridge the resolution gap between higher resolution mars orbiter camera narrow angle images and lower resolution viking orbiter images and themis will provide unprecedented global coverage at 100 m resolution by the end of the nominal mars odyssey mission the crater population data in this work show evidence for the flattening of the mid sized 63 m d 1km crater density curve due to erosion and or deposition and the upturn of the smaller diameter 90 m diameter crater curve signaling the hiatus of major crater obliteration in meridiani planum about 1 10 ma

surface ages and resurfacing rates of the polar layered deposits on mars interpretation of the polar stratigraphy of mars in terms of global climate changes is complicated by the significant difference in surface ages between the north and south polar layered terrains inferred from crater statistics we have reassessed the cratering record in both polar regions using viking orbiter and mariner 9 images no craters have been found in the north polar layered terrain but the surface of most of the south polar layered deposits appears to have been stable for many of the orbital axial cycles that are thought to have induced global climate changes on mars the inferred surface age of the south polar layered deposits about 10 ma is two orders of magnitude greater than the surface age of the north polar layered deposits and residual cap at most 100 ka similarly modeled resurfacing rates are at least 20 times greater in the north than in the south these results are consistent with the hypotheses that polar layered deposit resurfacing rates are highest in areas covered by perennial ice and that the differences in polar resurfacing rates result from the 6 4 km difference in elevation between the polar regions deposition on the portion of the south polar layered deposits that is not covered by the perennial ice cap may have ceased about 5 million years ago when the obliquity of mars no longer exceeded 40

ages of valles marineris mars landslides and implications for canyon history the chronology of landslides of valles marineris the equatorial trough system of mars has been investigated by a crater population study valles marineris landslides have widespread debris aprons which offer a remarkable opportunity to study the crater population with high resolution images from mars orbiter camera moc and from mars odyssey thermal emission imaging system themis sixty six ages were determined within valles marineris including 56 landslide ages and 10 ages of the previous termcanyonnext term floor results reveal that landslides of valles marineris system of previous termcanyonsnext term occurred during a widespread period of time between 3 5 gy and 50 my in some locations the previous termcanyonnext term floor has an apparent age of 3 5 gy suggesting that at least locally within valles marineris no major refreshing processes have occurred for 3 5 gy the temporal repetitivity of landslides implies that the triggering mechanisms of the landslides are reproducible in time landslides have the same features whatever their age the dynamic of these landslides is probably the same either with intervention of water up to recently the last 100 my or without water since 3 5 gy

subduction origin on early earth a hypothesis i propose a hypothesis for the origin of subduction on early earth that is directly coupled with mantle upwelling and the formation of mafi c crust the hypothesis invokes spatial and temporal overlap of both endogenic and exogenic processes broad quasi cylindrical mantle upwelling and large bolide impact respectively leading to subduction and spreading two signature processes of modern plate tectonics the spatial and temporal intersection of these processes could have occurred variably across early earth and thus subduction could have begun at different locations and times globally the hypothesis postulates that the ability of a terrestrial planet to evolve plate tectonics results from a balance between the strength of its lithosphere and the size bolide it can attract and survive both factors to a fi rst order are a function of planet size

probable age of autolycus and calibration of lunar stratigraphy 39 ar 40 ar analyses of three petrographically distinct shocked apollo 15 kreep i e high k rare earth element p and other trace element contents basalt samples demonstrate that a major impact event affected all three samples at about 2 1 ga the copernican system craters aristillus and autolycus are to the north a ray from one of them crosses the landing site and had been suggested prior to the apollo 15 mission as a source of material that could be used to date these craters autolycus the older of the two is in a particularly appropriate terrain and is the most likely source of the 2 1 ga heating and delivery event with this calibration point and if autolycus really is a copernican crater the copernican system lasted twice as long as has previously been suggested furthermore the moon was not subjected to a constant cratering rate over the past 3 b y the average rate in the preceding eratosthenian must have been twice that in the copernican

subduction of a wedge shaped philippine sea plate beneath kanto central japan estimated from converted waves and small repeating earthquakes 1 we estimate the configuration of the philippine sea plate phs subducted beneath kanto to understand the seismotectonics in the tokyo metropolitan area in addition to small repeating earthquakes that delineate both the upper and lower boundaries of the phs previously unrecognized converted waves at its upper boundary from lower boundary repeater sources provide a new constraint on the position of the boundary the results from 794 s to p and 212 p to s converted waves together with 200 repeater locations show that 1 the phs has a wedge like shape decreasing in thickness from 50 km beneath tokyo to zero at its northeastern limit and 2 the phs is relatively flat in the eastern part of kanto and bends upward near its northeastern limit from the fact that the phs in the izu bonin forearc also has a similar wedge like shape we infer that its shape was formed before its subduction on the other hand the relatively flat phs at its northeastern limit and its upward bend are probably a consequence of ongoing deformation from an interaction with the underlying thick pacific plate pac the history of the phs as a forearc before subduction and its contact with the pac after subduction suggests that the temperature of this part of the phs is colder than other regions which probably causes anomalously deep seismicity on the phs in kanto colder phs temperatures imply that the source regions of large earthquakes may extend deeper than expected an important fact when assessing the locations of disastrous earthquakes for tokyo

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

predicted detection rates of regional scale meteorite impacts on mars with the insight short period seismometer abstract in 2016 nasa will launch the insight discovery class mission which aims to study the detailed internal structure of mars for the first time short and long period seismometers form a major component of insight s payload and have the potential to detect seismic waves generated by meteorite impacts large globally detectable impact events producing craters with diameters of 100 m have been investigated previously and are likely to be rare teanby n a wookey j 2011 phys earth planet int 186 70 80 but smaller impacts producing craters in the 0 5 20 m range are more numerous and potentially occur sufficiently often to be detectable on regional scales 1000 km at these distances seismic waves will have significant high frequency content and will be suited to detection with insight s short period seismometer seis sp in this paper i estimate the current martian crater production function from observations of new craters malin m c et al 2006 science 314 1573 1577 daubar i j et al 2013 icarus 225 506 516 model results williams j p pathare a v aharonson o 2014 icarus 235 23 36 and standard isochrons hartmann w k 2005 icarus 174 294 320 these impact rates are combined with an empirical relation between impact energy source receiver distance and peak seismogram amplitude derived from a compilation of seismic recordings of terrestrial and lunar impacts chemical explosions and nuclear tests the resulting peak seismogram amplitude scaling law contains significant uncertainty but can be used to predict impact detection rates i estimate that for a short period instrument with a noise spectral density of 10 8 ms 2 hz 1 2 in the 1 16 hz frequency band approximately 0 1 30 regional impacts per year should be detectable with a nominal value of 1 3 impacts per year therefore small regional impacts are likely to be a viable source of seismic energy for probing mars crustal and upper mantle structure this is particularly appealing as such impacts should be easily located with orbital imagery increasing their scientific value compared to other types of events with unknown origins finally comparison of the empirical results presented here with the modelling study of teanby and wookey teanby n a wookey j 2011 phys earth planet int 186 70 80 provides constraints on the seismic efficiency suggesting that values of 5 10 4 may be appropriate for impact generated seismic waves comparing explosion and impact datasets indicate that buried explosions are 10 times more efficient at generating seismic waves than impacts

40ar 39ar dating of apollo 12 impact spherules 1 we have used the 40ar 39ar isochron technique to determine ages of 81 lunar spherules from apollo 12 soil sample 12023 most spherules are created in meteoroid impacts and their ages correspond to the timing of the impacts that formed them of the 81 impacts we have dated most occurred in the last 500 million years the abundance of spherules from the most recent 10 of the history of the moon is consistent with an increase in the meteoroid bombardment of the inner solar system but does not require this explanation the soil sample from which we extracted our spherules was from the ejecta of a recent impact our spherule age measurements support models of lunar soil mechanics and impact cratering in which ejecta are stratified and inverted relative to the stratigraphy that was present before the impact

geomorphic analysis of small rayed craters on mars examining primary versus secondary impacts 1 twenty confirmed impacts over a 7 year time period on mars were qualitatively and statistically compared to 287 secondary craters believed to originate from zunil an 500 ka 10 km diameter primary crater our goal was to establish criteria to distinguish secondaries from primaries in the general crater population on the basis of their horizontal planforms recent primary impacts have extensive air blast zones distal ray systems 100 crater radii r and ephemeral ejecta recent primaries formed clusters of craters from atmospheric fragmentation of the meteoroid body secondary craters have ejecta blankets with shorter rays that are consistent with emplacement by low impact velocities near 1 km s the mean extent of the continuous ejecta blankets was less distal for secondaries 5 38 1 57r versus primaries 18 07 7 01r though primary ejecta were less fractal fractal dimension index fdi 1 30 and more circular on average circularity ratio cr 0 55 0 25 versus 0 27 0 13 for secondaries crater rims were remarkably circular primaries cr 0 97 0 02 secondaries at 0 94 0 05 though secondaries have the lowest values cr 0 9 secondary crater rims were elongated toward or orthogonal to their primary of origin uprange source directions for most secondaries determined by ejecta planform and crater rim ellipticity point toward zunil although contamination from other primaries is considered in some areas ejecta blanket discrepancies between recent primaries and zunil secondaries are attributable to differences in impact velocity and retention age after removal of the ejecta blanket crater rims are generally not diagnostic for determining crater origin fragmentation of primaries may play some role in steepening the size frequency distribution of crater diameters in the 5 m d 30 m range

martian cratering 8 isochron refinement and the chronology of mars abstract this paper reviews and refines the technique of dating martian surfaces by using impact crater isochrons defined as size distributions of impact craters on undisturbed martian surfaces of specified ages in the 1970s this system identified not only abundant ancient martian volcanic surfaces but also extensive lava plains with ages of a few 108 y old this dating was initially controversial but confirmed in the 1980s and 90s by martian meteorites the present update utilizes updated estimates of the mars moon cratering ratio the most important calibration factor improves treatment of gravity and impact velocity scaling effects combines aspects of the crater size distribution data from earlier work by both neukum and hartmann and for the first time applies a correction for loss of small meteoroids in the martian atmosphere from popova et al 2003 meteorit planet sci 38 905 925 the updated isochrons are not radically different from the previous 2002 iteration but fit observed data better and give somewhat older model ages for features dated from small craters diameter d 100 m crater counts from young lava flows in various areas give good fits to the new isochrons over as much as 3 orders of magnitude in d confirming the general isochron shape and giving crater retention ages in the range of some 106 to some 108 y interpreted as lava flow ages more complex older units are also discussed uncertainties are greatest if only small craters d 100 m are used suggestions by other workers of gross uncertainties due to local secondary craters and deposition exhumation are discussed they do not refute our conclusions of significant volcanic fluvial and other geologic activity in the last few percent of martian geologic time or the importance of cratering as a tool for studying processes such as exhumation indeed crater count data suggest certain very recent episodes of deposition exhumation and ice flow possibly associated with obliquity cycles of 10 7 y timescale evidence from ancient surfaces suggests higher rates of volcanism fluvial activity glaciation and other processes in noachian hesperian time than in amazonian time

the imbrium impact event and the thorium distribution at the lunar highlands surface late in the moon s heavy bombardment period the impact that formed the imbrium basin excavated and melted a tremendous volume of material that was subsequently distributed over most of the moon s surface the apollo orbiting ray experiments metzger et al 1977 show that the imbrium projectile struck in a th rich area called the high th oval region here and regarded as a unique lunar geochemical province imbrium primary ejecta were thus probably rich in th it is shown here using ejecta deposit modeling that the distribution of th in the highlands surface along the ground tracks of the ray spectrometers is consistent with the distribution expected for imbrium ejecta deposits deep basins other than imbrium appear to have excavated regions of lower crust that were mafic and fe bearing but not th rich the possibility that the surface th distribution arises mainly from imbrium ejecta has implications for the nature of the moon s igneous differentiation and the nature of the proposed cataclysm or late bombardment with large meteoroids

accretion rates of meteorites and cosmic dust in the early ordovician abundant fossil meteorites in marine condensed lower ordovician limestones from kinnekulle sweden indicate that accretion rates of meteorites were one to two orders of magnitude higher during an interval of the early ordovician than at present osmium isotope and iridium analyses of whole rock limestone indicate a coeval enhancement of one order of magnitude in the influx rate of cosmic dust enhanced accretion of cosmic matter may be related to the disruption of the l chondrite parent body around 500 million years ago

primary centers and secondary concentrations of tectonic activity through time in the western hemisphere of mars five main stages of radial and concentric structures formed around tharsis from the noachian through the amazonian as determined by geologic mapping of 24 452 structures within the stratigraphic framework of mars and by testing their radial and concentric orientations tectonic activity peaked in the noachian stage 1 around the largest center claritas an elongate center extending more than 20 in latitude and defined by about half of the total grabens which are concentrated in the syria planum thaumasia and tempe terra regions during the late noachian and early hesperian stage 2 extensional structures formed along the length of present day valles marineris and in thaumasia with a secondary concentration near warrego vallis radial to a region just to the south of the central margin of valles marineris early hesperian stage 3 radial grabens in pavonis syria ulysses and tempe terra and somewhat concentric wrinkle ridges in lunae and solis plana and in thaumasia sirenum memnonia and amazonis are centered northwest of syria with secondary centers at thaumasia tempe terra ulysses fossae and western valles marineris late hesperian early amazonian stage 4 structures around alba patera the northeast trending alignment of tharsis montes and olympus mons appears centered on alba patera stage 5 structures middle late amazonian represent the last pulse of tharsis related activity and are found around the large shield volcanoes and are centered near pavonis mons tectonic activity around tharsis began in the noachian and generally decreased through geologic time to the amazonian statistically significant radial distributions of structures formed during each stage centered at different locations within the higher elevations of tharsis secondary centers of radial structures during many of the stages appear related to previously identified local magmatic centers that formed at different times and locations throughout tharsis

megaregolith evolution and cratering cataclysm models lunar cataclysm as a misconception 28 years later the hypothesis of a lunar cataclysmic cratering episode between 3 8 and 3 9 gyr ago lacks proof its strongest form proposes no cratering before about 4 0 gyr followed by catastrophic formation of most lunar craters and basins in 200 myr the premise that zero impact melts implies zero impacts is disproved by data from asteroids on which early collisions clearly occurred but from which early impact melts are scarce plausible cataclysm models imply that any cataclysm should have affected the whole inner solar system but among available lunar and asteroid impact melt and impact age resetting data a narrow strong 3 83 9 gyr spike in ages is seen only in the region sampled by apollo luna reported lunar meteorite data do not show the spike asteroid data show a broader milder peak spreading from about 4 2 to 3 5 gyr these data suggest either that the spike in apollo impact melt ages is associated with unique lunar front side events or that the lunar meteorites data represent different kinds of events than the apollo luna data here we develop an alternate megaregolith evolution hypothesis to explain these data in this hypothesis early impact melts are absent not because there were no impacts but because the high rate of early impacts led to their pulverization the model estimates survival halflives of most lunar impact melts prior to 4 1 gyr at 100 myr after a certain time tcritical 4 0 gyr impact melts began to survive to the present the age distribution differences among impact melts and plutonic rocks are controlled by and hold clues to the history of regolith evolution and the relative depths of sequestration of impact melts versus plutonic rocks both among lunar and asteroidal samples both the zero cratering then cataclysm hypothesis and the megaregolith evolution hypothesis require further testing especially with lunar meteorite impact melt studies

water undersaturated mantle plume volcanism on present day mars based on meteorite evidence the present day martian mantle has a combined abundance of up to a few hundred ppm of h2o cl and f which lowers the solidus and enhances the magma production rate adiabatic decompression melting in upwelling mantle plumes is the best explanation for young last 200 myr volcanism on mars we explore water undersaturated mantle plume volcanism using a finite element mantle convection model coupled to a model of hydrous peridotite melting relative to a dry mantle the reduction in solidus temperature due to water increases the magma production rate by a factor of 1 3 1 7 at 50 ppm water and by a factor of 1 9 3 2 at 200 ppm water mantle water also decreases the viscosity and increases the vigor of convection which indirectly increases the magma production rate by thinning the thermal boundary layer and increasing the flow velocity at conditions relevant to mars these indirect effects can cause an order of magnitude increase in the magma production rate using geologic and geophysical observations of the late amazonian magma production rate and geochemical observations of melt fractions in shergottite meteorites present day mars is constrained to have a core mantle boundary temperature of 1750 to 1800 c and a volume averaged thermal rayleigh number of 2 106 to 107 indicating that moderately vigorous mantle convection has persisted to the present day melting occurs at depths of 2 5 6 gpa and is controlled by the rayleigh number at the low pressure end and by the mantle water concentration at high pressure

evidence of space weathering in regolith breccias ii asteroidal regolith breccias space weathering products such as agglutinates and nanophase iron bearing rims are easily preserved through lithification in lunar regolith breccias thus such products if produced should be preserved in asteroidal regolith breccias as well a study of representative regolith breccia meteorites fayetteville h4 and kapoeta howardite was undertaken to search for physical evidence of space weathering on asteroids amorphous or npfe 0 bearing rims cannot be positively identified in fayetteville although possible glass rims were found extensive friction melt was discovered in the meteorite that is difficult to differentiate from weathered materials several melt products including spherules and agglutinates as well as one irradiated rim and one possible npfe 0 bearing rim were identified in kapoeta the existence of these products suggests that lunar like space weathering processes are or have been active on asteroids

the geologic evolution of mars episodicity of resurfacing events and ages from cratering analysis of image data and correlation with radiometric ages of martian meteorites we have investigated 12 prominent regions on mars in detail with respect to their geologic evolution through time on the basis of detailed geologic mapping exercises and determining age relationships through crater counting techniques using imagery new data in combination with previously obtained data have been analyzed by way of a refined method of cratering age extraction that also gives fine details of periods of resurfacing we have found that there has been volcanic and fluvial glacial geologic activity on the martian surface at all times from 4 ga ago until today this activity shows episodic pulses in intensity of both volcanic and fluvial glacial processes at 3 8 3 3 ga 2 0 1 8 ga 1 6 to 1 2 ga 800 to 300 m y 200 m y and 100 m y and a possible weaker phase around 2 5 2 2 ga ago in between these episodes there was relative quiescence of volcanic and or fluvial glacial activity the episodes we find on the martian surface in the crater frequency analyses of hrsc moc and themis data coincide with some age groups of the martian meteorites 1 3 ga 600 300 m y 170 m y it appears that the surface activity expressions and their episodicity are related to the interior evolution of the planet when convection in the asymptotic stationary state changes from the so called stagnant lid regime to an episodic behavior similarities in episodic behavior are found for the other terrestrial planets venus the earth s moon and the earth itself suggesting a common general relationship in the evolutionary tracks

a prediction of mars seismicity from surface faulting the shallow seismicity of mars has been estimated by measurement of the total slip on faults visible on the surface of the planet throughout geologic time seismicity was calibrated with estimates based on surface structures on the moon and measured lunar seismicity that includes the entire seismogenic lithosphere results indicate that mars is seismically active today with a sufficient number of detectable marsquakes to allow seismic investigations of its interior

the slip distribution of the 1946 nankai earthquake estimated from tsunami inversion using a new plate model abstract a new model for the shape of the philippine sea plate psp subducting beneath southwest japan was constructed by combining marine seismic survey results and seismicity data this resulted in a plate model that is generally 3 5 km shallower than previous estimates and contains new details of plate boundary structure near the nankai trough axis we estimated the slip distribution of the 1946 nankai earthquake by inverting tsunami waveforms using the new plate boundary model and found that the large slip off cape muroto and seismic moment were reduced by about 10 from the previous estimate of tanioka and satake 2001a however we also found reason to suspect the tide gauge closest to the epicenter at shimotsu and this resulted in a much more substantial reduction in the amount of slip in the eastern segment of the rupture area we also find that our new rupture model correlates well with features of the subducting plate boundary such as a large subducting seamount off cape muroto kodaira et al 2000a this suggests that such features have an important effect on either earthquake rupture propagation or the interseismic accumulation of elastic strain energy or both

seismic detection of meteorite impacts on mars abstract meteorite impacts provide a potentially important seismic source for probing mars interior it has recently been shown that new craters can be detected from orbit using high resolution imaging which means the location of any impact related seismic event could be accurately determined thus improving the constraints that could be placed on internal structure using a single seismic station this is not true of other seismic sources on mars such as sub surface faulting which require location using multiple seismic stations this study aims to determine the seismic detectability of meteorite impacts and assess whether they are a viable means of probing deep internal structure first we derive a relation between crater diameter and equivalent seismic moment based on observational data compiled from impact tests controlled explosions and earthquake seismology second this relation was combined with updated cratering rates based on newly observed craters to derive the impact induced seismicity on mars which we estimate to total 10 13 10 14 n m per year finally seismic waveform modelling was used to determine the detectability of these impacts based on reasonable assumptions about likely seismometer performance and background noise levels for our nominal noise instrument case we find that detectable impacts at teleseismic distances source receiver offsets greater than 60 are very rare and occur approximately once every 10 years for our most optimistic noise instrument case approximately one such event occurs each year this suggests that using solely meteorite impacts is not a reliable way of probing the martian interior although local impacts are more frequently detectable and could provide important constraints on near surface seismic properties

martian cratering vi crater count isochrons and evidence for recent volcanism from mars global surveyor this paper develops a methodology to establish absolute martian ages by deriving isochrons on a plot of martian impact crater density vs crater diameter calibrated by lunar crater age data the isochrons illustrated here are based on a mars moon cratering ratio of 1 6 at constant size but there is a factor of 2 to 4 uncertainty in this ratio and the consequent model ages martian crater diameter distributions are determined in several areas down to diameter d 16 45 m the shapes of the curves in young areas are found to be close to that of the predicted isochrons and close to the standard production function found by neukum the youngest areas studied here display the lunar like production function down to d 30 m where saturation equilibrium sets in model crater retention ages of several volcanic units are found to be a few hundred million years or less with estimated uncertainties ranging from a factor of 2 lower to a factor of 4 higher the results are consistent with martian meteorite ages volcanism on mars has probably persisted into the last 10 to 15 of the planet s history and is likely ongoing because surfaces as young as a few hundred million years have reached crater saturation equilibrium at d 60 to 100 m mars is likely to have widespread impact produced regoliths at least a few meters deep and this may contribute to the widespread mobile dust and boulder fields of mars

geomorphic and stratigraphic analysis of crater terby and layered deposits north of hellas basin mars received 20 september 2006 revised 12 february 2007 accepted 17 may 2007 published 24 august 2007 1 the geologic history of crater terby is developed through geomorphic and stratigraphic analyses within the regional context of the hellas basin terby exhibits 2 km thick sequences of layers that consist of repetitive subhorizontal and laterally continuous beds the layers are predominantly fine grained as indicated by their ease of aeolian erosion although a few consolidated layers weather to form rubbly talus the grain size or composition of the deposited materials fluctuated producing layering but the overall properties of the deposits are similar throughout the sequence and are comparable to layered deposits in other crater basins around hellas the original depositional geometry physical and geological characteristics of the layers in terby and the other basins lead us to favor a lacustrine origin but a loess like origin cannot be ruled out the formation of the layers corresponds to a period when the circum hellas region may have been occupied by a lake s up to 3 6 km deep once the lake in hellas decreased the layers in terby were incised by troughs and a moat like depression we attribute this erosion to scour beneath an ice cover due to a lack of integrated fluvial drainage or large aeolian deflation features the presence of viscous flow features in a crater on terby s northwestern rim and lobate features on terby s crater floor are also indicative of ice the lack of depositional features associated with the postulated glacial activity suggests there was a contemporaneous shallow ice covered lake covering the floor of terby that transported material into the greater hellas basin

a catalogue of impact craters larger than 200 m and surface age analysis in the chang e 5 landing area abstract chang e 5 ce 5 is the first sample return mission of china s lunar exploration and will be launched in 2020 the planned landing area is near mons rumker in oceanus procellarum it is important to have a detailed geological analysis of the landing area prior to the lunar sample collection the crater size frequency distribution curve combined with radioisotope dating of components of the returned sample by ce 5 may allow a refinement of the lunar chronology function based on a digital orthophoto map with a pixel size of 1 5 m generated from more than 700 lunar reconnaissance orbiter camera narrow angle camera images of the ce 5 landing area all impact craters larger than 200 m in diameter were extracted using the resultant crater catalogue of 32 277 craters a systematic analysis of the landing area including the crater distribution morphology characteristics and geologic model ages is conducted the results show that craters smaller than 1 000 m are denser in the east and the west parts of the region while those larger than 1 000 m tend to be more concentrated in the central part particularly within the range of 61 5 w 64 5 w within nine geologic units divided using the spectral characteristics the geologic unit of em1 eratosthenian aged mare em near the mons rumker is the youngest with a model age of 2 02 0 16 0 16 ga the three rumker plateau ir units are the oldest parts of the landing area with the largest model age of 3 49 0 12 0 073 ga the crater catalogue and the surface model age analysis results can contribute to the ce 5 mission as well as further scientific research

dating small fresh lunar craters with mini rf radar observations of ejecta blankets 1 the ages of large impact craters on the moon can be estimated by measuring the size frequency distribution of smaller craters superimposed on their ejecta but applying this method to individual craters under 3 km in diameter remains difficult due to their limited areal extent the ejecta blankets of fresh lunar impact craters are expressed as halos of optical and radar bright material that gradually fade with time although compositional differences make inferring crater ages from optical albedo problematic radar is less sensitive to composition and thus may provide a more reliable means of estimating age in this work we use radar data from the miniature radio frequency mini rf instrument on the lunar reconnaissance orbiter lro to characterize the radar backscatter of large numbers of ejecta blankets by analyzing the size frequency distribution of these craters we show that the lifetime of the radar bright discontinuous ejecta blanket varies with crater diameter in a predictable way for craters with diameters between 0 45 and 5 km absolute ages of individual craters can then be estimated by combining this empirically derived model with estimates of the relative degradation state of each individual crater the lifetimes of radar bright discontinuous ejecta blankets are significantly shorter in the highlands than the maria although this is likely due to local topography creating difficulties in applying the method to the highlands the cosmic ray exposure age of south ray crater a fresh crater visited by apollo 16 provides confirmation of these results this method therefore provides a new way to accurately date small fresh craters using the mini rf data set

weak interplate coupling by seamounts and repeating m 7 earthquakes subducting seamounts are thought to increase the normal stress between subducting and overriding plates however recent seismic surveys and laboratory experiments suggest that interplate coupling is weak a seismic survey in the japan trench shows that a large seamount is being subducted near a region of repeating earthquakes of magnitude m 7 both observed seismicity and the pattern of rupture propagation during the 1982 m 7 0 event imply that interplate coupling was weak over the seamount a large rupture area with small slip occurred in front of the seamount its northern bound could be determined by a trace of multiple subducted seamounts whereas a subducted seamount itself may not define the rupture area its width may be influenced by that of the seamount

mars observer camera the mars observer camera moc is a three component system one narrow angle and two wide angle cameras designed to take high spatial resolution pictures of the surface of mars and to obtain lower spatial resolution synoptic coverage of the planet s surface and atmosphere the cameras are based on the push broom technique that is they do not take frames but rather build pictures one line at a time as the spacecraft moves around the planet in its orbit moc is primarily a telescope for taking extremely high resolution pictures of selected locations on mars using the narrow angle camera areas ranging from 2 8 km 2 8 km to 2 8 km 25 2 km depending on available internal digital buffer memory can be photographed at about 1 4 m pixel additionally lower resolution pictures to a lowest resolution of about 11 m pixel can be acquired by pixel averaging these images can be much longer ranging up to 2 8 500 km at 11 m pixel high resolution data will be used to study sediments and sedimentary processes polar processes and deposits volcanism and other geologic geomorphic processes the moc wide angle cameras are capable of viewing mars from horizon to horizon and are designed for low resolution global and intermediate resolution regional studies low resolution observations can be made every orbit so that in a single 24 hour period a complete global picture of the planet can be assembled at a resolution of at least 7 5 km pixel regional areas covering hundreds of kilometers on a side may be photographed at a resolution of better than 250 m pixel at the nadir such images will be particularly useful in studying time variable features such as lee clouds the polar cap edge and wind streaks as well as acquiring stereoscopic coverage of areas of geological interest the limb can be imaged at a vertical and along track resolution of better than 1 5 km different color filters within the two wide angle cameras permit color images of the surface and atmosphere to be made to distinguish between clouds and the ground and between clouds of different composition

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

anomalous low velocity zone and linear alignment of seismicity along it in the subducted pacific slab beneath kanto japan reactivation of subducted fracture zone 1 a detailed investigation of the hypocenter distribution beneath kanto japan reveals a nw se trending linear alignment of seismicity within the subducted pacific slab we estimate the 3d seismic velocity structure in the pacific slab to understand the factors controlling the genesis of such intraslab earthquakes a narrow low velocity zone is imaged within the subducted slab over a length of 150 km which partly penetrates into the mantle portion of the slab the low velocity zone correlates in space with the nw se trending earthquake cluster a reactivation of hydrated fracture zone formed prior to subduction is probably related to the low velocity anomaly dehydration reactions of the hydrated oceanic mantle as well as the oceanic crust might lower the seismic velocity along the fossil fracture zone accompanied by intraslab earthquakes these observations support the hypothesis of dehydration embrittlement as the most viable mechanism for generating intraslab earthquakes

regolith breccia northwest africa 7533 mineralogy and petrology with implications for early mars northwest africa 7533 a polymict martian breccia consists of fine grained clast laden melt particles and microcrystalline matrix while both melt and matrix contain medium grained noritic monzonitic material and crystal clasts the matrix also contains lithic clasts with zoned pigeonite and augite plus two feldspars microbasaltic clasts vitrophyric and microcrystalline spherules and shards the clast laden melt rocks contain clump like aggregates of orthopyroxene surrounded by aureoles of plagioclase some shards of vesicular melt rocks resemble the pyroxene plagioclase clump aureole structures submicron size matrix grains show some triple junctions but most are irregular with high intergranular porosity the noritic monzonitic rocks contain exsolved pyroxenes and perthitic intergrowths and cooled more slowly than rocks with zoned pyroxene or fine grain size noritic material contains orthopyroxene or inverted pigeonite augite calcic to intermediate plagioclase and chromite to cr bearing magnetite monzonitic clasts contain augite sodic plagioclase k feldspar ti bearing magnetite ilmenite chlorapatite and zircon these feldspathic rocks show similarities to some rocks at gale crater like black trout mara and jake m the most magnesian orthopyroxene clasts are close to alh 84001 orthopyroxene in composition all these materials are enriched in siderophile elements indicating impact melting and incorporation of a projectile component except for ni poor pyroxene clasts which are from pristine rocks clast laden melt rocks spherules shards and siderophile element contents indicate formation of nwa 7533 as a regolith breccia the zircons mainly derived from monzonitic melt rocks crystallized at 4 43 0 03 ga humayun et al 2013 and a 147sm 143nd isochron for nwa 7034 yielding 4 42 0 07 ga nyquist et al 2016 defines the crystallization age of all its igneous portions the zircon from the monzonitic rocks has a higher 17o than other martian meteorites explained in part by assimilation of regolith materials enriched during surface alteration nemchin et al 2014 this record of protolith interaction with atmosphere hydrosphere during regolith formation before melting demonstrates a thin atmosphere a wet early surface environment on mars and an evolved crust likely to have contaminated younger extrusive rocks the latest events recorded when the breccia was on mars are resetting of apatite much feldspar and some zircons at 1 35 1 4 ga bellucci et al 2015 and formation of ni bearing pyrite veins during or shortly after this disturbance lorand et al 2015