weathering rates of historic sandstone structures in semiarid environments ebro basin ne spain abstract long term weathering rates of sandstones were obtained for well dated from 400 to 1100 years bp historic structures wall sections of the alberuela castle in a semiarid environment of the central ebro basin ne spain using an exclusive movable contour plotting frame this sandstone has a low durability controlled mainly by textural properties differences observed in weathering rates are clearly controlled by the age of building as well as the orientation of the studied sandstone wall sections weathering processes related to insolation control the asymmetric spatial pattern of weathering rates in surfaces exposed for since 1100 bp which are higher in southeast facing sandstone wall sections 8 4 mm 100 years than in southwestern 4 4 mm 100 years and northern faces 3 9 mm 100 years in addition exponential increase in weathering rates with time from 1 6 2 2 mm 100 years in surfaces exposed for since 800 400 bp to 8 4 mm 100 years in surfaces exposed for since 1100 bp in southeastern faces may be probably related to environmental changes during the last millennium

the effect of rock strength on weathering rates of sandstone used for angkor temples in cambodia abstract this study determined the weathering rates from the depths of hollows formed by the deterioration of sandstone blocks used to build angkor temples in the 7th 12th centuries due to capillary processes these hollows generally develop at the bases of building components such as doorframes false doorframes window frames and pillars in this study 55 locations in 29 temples were selected to measure maximum hollow depth based on the ages of the temples the weathering rate was calculated to be in the range 7 92 mm ka 1 the weathering rate was found to be independent of building component type and aspect the calculated weathering rate corresponds to the physical weathering rate of sandstone caused by the wetting drying cycles that result from the tropical environment along with the dry season an equation that can estimate the maximum depth of a hollow in a sandstone block was proposed based on temple age and the coefficient obtained from two types of rock hardness values the equation coefficient is an effective index that can help predict the deterioration of stone cultural artifacts such as the angkor temples

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

how old are young lunar craters 1 the accurate definition of the lunar cratering chronology is important for deriving absolute model ages across the lunar surface and throughout the solar system images from the lunar reconnaissance orbiter narrow angle cameras and wide angle camera and the selene kaguya terrain camera provide new opportunities to investigate crater size frequency distributions csfds on individual geological units at lunar impact craters we report new csfd measurements for the copernican aged craters north ray tycho and copernicus which are crucial anchor points for the lunar cratering chronology we also discuss possible reasons for an age discrepancy observed between the impact melt and ejecta units our csfds for north ray and tycho crater ejecta deposits are consistent with earlier measurements however for copernicus crater and one of its rays we find significantly lower cumulative crater frequencies than previous studies our new results for copernicus crater fit the existing lunar absolute chronologies significantly better than the previous counts our derived model ages of the ejecta blankets of north ray tycho and copernicus agree well with radiometric and exposure ages of the apollo 16 17 and 12 landing sites respectively and are generally consistent with a constant impact rate over the last 3 ga however small variations of the impact rate cannot be resolved in our data and require further investigations

evidence for young volcanism on mercury from the third messenger flyby during its first two flybys of mercury the messenger spacecraft acquired images confirming that pervasive volcanism occurred early in the planet s history messenger s third mercury flyby revealed a 290 kilometer diameter peak ring impact basin among the youngest basins yet seen having an inner floor filled with spectrally distinct smooth plains these plains are sparsely cratered postdate the formation of the basin apparently formed from material that once flowed across the surface and are therefore interpreted to be volcanic in origin an irregular depression surrounded by a halo of bright deposits northeast of the basin marks a candidate explosive volcanic vent larger than any previously identified on mercury volcanism on the planet thus spanned a considerable duration perhaps extending well into the second half of solar system history

weathering mechanisms and their effects on landsliding in pelitic schist physical chemical and mineralogical analyses of undisturbed drill cores of pelitic schist from a landslide area in japan clarified the mechanisms of chemical weathering of pelitic schist oxidizing surface water percolates downward and reaches an oxidation front where chlorite is altered to al vermiculite graphite and pyrite are oxidized and depleted and goethite precipitates oxidation of pyrite also occurs just below the oxidation front probably by ferric iron pyrite oxidation yields sulphuric acid which penetrates further downward interacting with and weakening the rocks in addition to this chemical weakening stress release and shearing along schistosities form an incipient shear zone which propagates to a sliding zone that forms the rupture surface of a landslide once a sliding zone has developed it inhibits downward groundwater flow across it because of its low permeability slowing the downward propagation of the weathering zone until this filtration barrier is broken by landslide movement copyright 2010 john wiley sons ltd

ages of large lunar impact craters and implications for bombardment during the moon s middle age abstract standard lunar chronologies based on combining lunar sample radiometric ages with impact crater densities of inferred associated units have lately been questioned about the robustness of their interpretations of the temporal dependance of the lunar impact flux in particular there has been increasing focus on the middle age of lunar bombardment from the end of the late heavy bombardment 3 8 ga until comparatively recent times 1 ga to gain a better understanding of impact flux in this time period we determined and analyzed the cratering ages of selected terrains on the moon we required distinct terrains with random locations and areas large enough to achieve good statistics for the small superposed crater size frequency distributions to be compiled therefore we selected 40 lunar craters with diameter 90 km and determined the model ages of their floors by measuring the density of superposed craters using the lunar reconnaissance orbiter wide angle camera mosaic absolute model ages were computed using the model production function of marchi et al marchi s mottola s cremonese g massironi m martellato e 2009 astron j 137 4936 4948 we find that a majority 36 of 40 of our superposed crater size frequency distributions are consistent with the model production function a histogram of the original crater floor model ages indicates the bombardment rate decreased gradually from 3 8 ga until 3 0 ga implying an extended tail to the late heavy bombardment for large craters it also preliminarily suggests that between 3 0 and 1 0 ga bombardment may be characterized by long periods 600 myr of relatively few impacts lulls broken by a short duration 200 myr of relatively more impacts spike while measuring superposed craters we also noted if they were part of a cluster or chain named obvious secondary and analyzed these craters separately interestingly we observe a wide variety of slopes to the differential size frequency power law which demonstrates that there can be considerable variation in individual secondary crater field size frequency distributions finally four of the small superposed crater size frequency distributions are found to be inconsistent with the model production function possible reasons are resurfacing has modified these distributions unrecognized secondary craters and or the model production function has incorrect inputs such as the scaling law for the target terrain the degraded appearance of the superposed craters and indications of resurfacing suggest that the first cause is the most likely

simulated degradation of lunar impact craters and a new method for age dating farside mare deposits with the advent of clementine data it is now possible to determine the lithology and extent of geologic materials on the moon particularly the farside mare deposits however traditional crater counting techniques do not provide reliable age estimates of these materials owing to their small surface areas to support such studies we present a model for estimating their age by analyzing the morphometry of degraded craters 1 3 km in diameter a photoclinometric model was adapted for use with monoscopic 0 750 m ultraviolet visible and high resolution images where we extracted the topography of fresh craters a two dimensional computer model simulating linear diffusional creep was applied to fresh craters at a variety of diameters the resulting profiles were then compared to photoclinometric profiles of degraded craters of known ages for calibration application of the resulting model to degraded craters in the mare deposit of the central apollo basin 36 5 latitude 208 0 longitude indicates that this material was emplaced during the early imbrian period 3 85 ga by calculating the amount of material eroded from each of the degraded craters observed in this unit the average erosion rate is estimated to be 2 0 0 1 10 7 mm yr on the moon since the imbrian the estimated amount of material eroded during any given period suggests that the erosion rate has decreased with time implying that the flux of larger impactors has as well

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

mechanisms of shallow landslides on soil mantled hillslopes with permeable and impermeable bedrocks in the boso peninsula japan abstract rainfall induced shallow landslides play a vital role in hillslope denudation processes in humid temperate regions this study demonstrates the contrasting mechanisms of landslides in adjoining hills with permeable sandstone and impermeable mudstone bedrock in the boso peninsula japan the characteristics of slope hydrology were inferred from pressure head monitoring and rainfall runoff observations an analysis of slope stability provided critical conditions for several previously occurring landslides the results are as follows 1 in slopes with the permeable sandstone infiltrated rainwater percolates through the bedrock as an unsaturated gravitational flow the wetting front migration results in a decrease of soil cohesion and causes landsliding at the steep lower part of the hillslopes 2 in contrast the impermeable mudstone beneath a thin soil layer causes a transient positive pressure head that generates a saturated subsurface storm flow the reduction in effective normal stress triggers shallow soil slipping at the uppermost part of a hollow

mechanism and effect of chemical weathering of sedimentary rocks the mineralogy geochemistry and physical and mechanical properties of rocks from four weathering profiles of miocene to pleistocene mudstones and sandstones in japan showed that chemical weathering of sedimentary rocks is characterized by sequential reaction between percolating groundwater and rock forming minerals pyrite a common mineral contained in sedimentary rocks is especially important in these sequential reactions pyrite is oxidized by oxygen coming from the ground surface and sulfuric acid is generated at the base of the oxidized zone the sulfuric acid in turn dissolves rock forming materials to make a dissolved zone if the fluxes of oxygen and water are in the same direction sulfuric acid generated at the oxidation front migrates farther and forms a dissolved zone if these fluxes are in opposite directions the oxidized and dissolved zones are not differentiated rocks in the dissolved zone are caused to deteriorate by the acid leaching and are acidic if buffering minerals such as calcite and zeolite are absent in the oxidized zone sandstone is strengthened because of cementation by iron oxide or hydroxide while mudstone is weakened because it has greater clay fractions and larger specific surface areas than sandstone

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

spherule layers records of ancient impacts abstract a large extraterrestrial object striking earth at cosmic velocity melts and vaporizes silicate materials which can condense into highly spheroidal sand size particles that get deposited hundreds to thousands of kilometers from the point of impact these particles known as impact spherules have been detected in great abundance in a relatively small number of thin discrete layers ranging in age from less than a million years to 3 47 billion years unaltered impact spherules consist entirely of glass microtektites or a combination of glass and crystals grown in flight microkrystites impact spherule layers form very rapidly and can be very extensive even global in extent e g the cretaceous tertiary k t boundary layer so they form excellent time stratigraphic markers because they are always found in a stratigraphic context spherule layers are probably superior to terrestrial craters and related structures for assessing the environmental and biotic effects of large impacts a reco

the sculptured hills of the taurus highlands implications for the relative age of serenitatis basin chronologies and the cratering history of the moon 1 new images from the lunar reconnaissance orbiter camera show the distribution and geological relations of the sculptured hills a geological unit widespread in the highlands between the serenitatis and crisium basins the sculptured hills shows knobby undulating radially textured and plains like morphologies and in many places is indistinguishable from the similarly knobby alpes formation a facies of ejecta from the imbrium basin the new lroc image data show that the sculptured hills in the taurus highlands is imbrium ejecta and not directly related to the formation of the serenitatis basin this occurrence and the geological relations of this unit suggests that the apollo 17 impact melts may not be not samples of the serenitatis basin forming impact leaving their provenance undetermined and origin unexplained if the apollo 17 melt rocks are serenitatis impact melt up to half of the basin and large crater population of the moon was created within a 30 ma interval around 3 8 ga in a global impact cataclysm either interpretation significantly changes our view of the impact process and history of the earth moon system

weathering mechanism of arenite sandstone with sparse calcite cement content abstract we conducted a multifaceted analysis of the weathering zones of arenite containing a minute amount of calcite cement based on mineralogical chemical and physical properties the survey targeted sandstone with mudstone interbeds belonging to the cretaceous izumi group the survey revealed that the weathering mechanism is a chain reaction the most important process is the pore opening between the grains caused by the dissolution of calcite cement in the initial stage of weathering subsequently the pores between the grains connect and widen separating the sand grains within the sandstone this primary weathering action may be a major difference from wacke during calcite dissolution in the initial stage sulfuric acid generated by pyrite oxidation in sandstone plays an important role chlorite breaks down as calcite dissolves however iron resulting from the breakdown disappears only after the pore water has been replaced by acidic water sandstones with advanced diagenesis are dense and hard and it was previously assumed that thick weathering zones could not develop because they were less susceptible to weathering however we discovered that weathering caused arenite with a minute amount of calcite cement to disintegrate into grains down to a depth of 30 m the surface layer of the weathered arenite lost fine particle fraction and crept to form a fragile soil layer susceptible to landslides

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

lunar impact basins stratigraphy sequence and ages from superposed impact crater populations measured from lunar orbiter laser altimeter lola data 1 impact basin formation is a fundamental process in the evolution of the moon and records the history of impactors in the early solar system in order to assess the stratigraphy sequence and ages of impact basins and the impactor population as a function of time we have used topography from the lunar orbiter laser altimeter lola on the lunar reconnaissance orbiter lro to measure the superposed impact crater size frequency distributions for 30 lunar basins d 300 km these data generally support the widely used wilhelms sequence of lunar basins although we find significantly higher densities of superposed craters on many lunar basins than derived by wilhelms 50 higher densities our data also provide new insight into the timing of the transition between distinct crater populations characteristic of ancient and young lunar terrains the transition from a lunar impact flux dominated by population 1 to population 2 occurred before the mid nectarian this is before the end of the period of rapid cratering and potentially before the end of the hypothesized late heavy bombardment lola derived crater densities also suggest that many pre nectarian basins such as south pole aitken have been cratered to saturation equilibrium finally both crater counts and stratigraphic observations based on lola data are applicable to specific basin stratigraphic problems of interest for example using these data we suggest that serenitatis is older than nectaris and humboldtianum is younger than crisium sample return missions to specific basins can anchor these measurements to a pre imbrian absolute chronology

ages and stratigraphy of lunar mare basalts in mare frigoris and other nearside maria based on crater size frequency distribution measurements 1 we report on ages derived from impact crater counts for exposed mare basalt units in the northern part of the lunar nearside hemisphere mare frigoris the eastern and northeastern part of the nearside hemisphere lacus temporis joliot hubble goddard mare marginis and mare smythii the central part of the nearside hemisphere palus putredinis mare vaporum and sinus medii and the southwestern part of the nearside hemisphere grimaldi cruger rocca a lacus aestatis and schickard in mare frigoris we dated 37 basalt units showing ages from 2 61 to 3 77 gyr with most units being formed in the late imbrian period between 3 4 and 3 8 gyr ago in mare vaporum we dated six spectrally homogeneous units that show model ages of 3 10 to 3 61 gyr our model ages of basalts in mare marginis range from 3 38 to 3 88 gyr and are mostly older than basalts in mare smythii 3 14 3 48 gyr the model ages of four units in sinus medii indicate that the basalts in this region formed 3 63 to 3 79 gyr ago we find an excellent agreement of our crater size frequency model ages of the palus putredinis area which contains the apollo 15 landing site with the radiometric ages of apollo 15 samples according to our crater counts basalts in palus putredinis are 3 34 gyr old and this compares favorably with the radiometric ages of 3 30 3 35 gyr of the olivine normative and quartz normative basalts of the apollo 15 landing site lacus aestatis is a small irregular shaped mare patch in the southwestern nearside and shows an imbrian age of 3 50 gyr basalts in lacus temporis in the northeastern nearside formed between 3 62 and 3 74 gyr ago and are therefore older than the basalts in lacus aestatis we found that basalts in craters of the southwestern nearside schickard grimaldi cruger and rocca a are also mostly younger than basalts in craters of the northeastern nearside hubble joliot and goddard while basalt ages vary between 3 16 and 3 75 gyr in the southwest basalts in the northeast are 3 60 3 79 gyr old these results confirm and extend the general distribution of ages of mare basalt volcanism and further underline the predominance of older mare basalt ages in the eastern and southern nearside and in patches of mare peripheral to the larger maria in contrast to the younger basalt ages on the western nearside oceanus procellarum

the onset of the lunar cataclysm as recorded in its ancient crater populations abstract the earliest bombardment history of the moon potentially provides powerful constraints for solar system evolution models a major uncertainty however is how much of this history is actually recorded in lunar craters for example some argue that most ancient lunar craters and basins were produced by a declining bombardment of leftover planetesimals produced by terrestrial planet formation processes others believe that most lunar craters and large basins were formed in a narrow time interval between 3 8 and 4 0 ga the so called lunar cataclysm in the light of recent improvements in our understanding of early solar system evolution it is possible that the contributions from both scenarios could be represented in the lunar crater record if so when did the declining bombardment end and the lunar cataclysm begin here we show using new counts of 15 150 km diameter craters on the most ancient lunar terrains that the craters found on or near nectaris basin appear to have been created by projectiles hitting twice as fast as those that made the oldest craters on various pre nectarian era terrains this dramatic velocity increase is consistent with the existence of a lunar cataclysm and potentially with a late reconfiguration of giant planet orbits which may have strongly modified the source of lunar impactors this work also suggests that the lunar cataclysm may have started near the formation time of nectaris basin this possibility implies that south pole aitken basin spa the largest lunar basin and one of the oldest by superposition was not created during the cataclysm this view is strengthened by our interpretation that a substantial fraction of ancient craters on spa were made by low velocity impactors finally we believe these results shed new light on the impact history of the primordial earth

from the mid ordovician into the late silurian changes in the micrometeorite flux after the l chondrite parent breakup we present the first reconstruction of the micrometeorite flux to earth in the silurian period we searched 321 kg of condensed marine limestone from the late silurian cellon section southern austria for refractory chrome spinel grains from micrometeorites that fell on the ancient sea floor a total of 155 extraterrestrial spinel grains 10 grains 63 m and 145 in the 32 63 m fraction were recovered for comparison we searched 102 kg of similar limestone from the mid ordovician komstad formation in southern sweden this limestone formed within 1 ma after the breakup of the l chondrite parent body lcpb in the asteroid belt in the sample we found 444 extraterrestrial spinel grains in the 63 m fraction and estimate a content of at least 7000 such grains in the 32 63 m fraction our results show that in the late silurian 40 ma after the lcpb the flux of ordinary equilibrated chondrites has decreased by two orders of magnitude almost down to background levels among the ordinary chondrites the dominance of l chondritic micrometeorites has waned off significantly from 99 in the post lcpb mid ordovician to 60 in the late silurian with 30 h and 10 ll chondritic grains in the late silurian primitive achondrite abundances are similar to today s value contrasting to the much higher abundances observed in pre lcpb mid ordovician sediments

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

soil production on a retreating escarpment in southeastern australia the functional dependence of bedrock conversion to soil on the overlying soil depth the soil production function has been widely recognized as essential to understanding landscape evolution but was quantified only recently here we report soil production rates for the first time at the base of a retreating escarpment on the soil mantled hilly slopes in the upper bega valley southeastern australia concentrations of 10 be and 26 al in bedrock from the base of the soil column show that soil production rates decline exponentially with increasing soil depth these data define a soil production function with a maximum soil production rate of 53 m m y under no soil mantle and a minimum of 7 m m y under 100 cm of soil thus constraining landscape evolution rates subsequent to escarpment retreat the form of this function is supported by an inverse linear relationship between topographic curvature and soil depth that also suggests that simple creep does not adequately characterize the hillslope processes spatial variation of soil production shows a landscape out of dynamic equilibrium possibly in response to the propagation of the escarpment through the field area within the past few million years in addition we present a method that tests the assumption of locally constant soil depth and lowering rates using concentrations of 10 be and 26 al on the surfaces of emergent tors

an archaean heavy bombardment from a destabilized extension of the asteroid belt the late heavy bombardment lasted much longer than previously thought up to 1 7 billion years ago on earth with impacts on the moon and earth coming mostly from the e belt survivor hungaria asteroids the late heavy bombardment was a period of time generally put at about 4 1 billion to 3 8 billion years ago when the inner planets of the solar system were subjected to a high frequency barrage of asteroids this left its mark on the moon but on earth the craters quickly disappeared owing to tectonic processes and erosion in the first of two papers on the bombardment brandon johnson and jay melosh determine the properties of the asteroids by looking at spherule beds layers of debris ejected during the impacts the thickness of spherule layers is expected to vary according to the size of the impactor and the speed at which it hit earth this historical record of impacts indicates that the number of projectiles colliding with earth was substantially higher 3 5 billion years ago than it is today with a gradual decline in the number of strikes after the late heavy bombardment bottke et al modelled the evolution of an asteroid belt that extended farther towards mars than the present one they find that most of the impactors traced by the spherule beds probably originated in this e belt which was disrupted during migrations of some of the giant planets the barrage of comets and asteroids that produced many young lunar basins craters over 300 kilometres in diameter has frequently been called the late heavy bombardment1 lhb many assume the lhb ended about 3 7 to 3 8 billion years gyr ago with the formation of orientale basin2 3 evidence for lhb sized blasts on earth however extend into the archaean and early proterozoic eons in the form of impact spherule beds globally distributed ejecta layers created by chicxulub sized or larger cratering events4 at least seven spherule beds have been found that formed between 3 23 and 3 47 gyr ago four between 2 49 and 2 63 gyr ago and one between 1 7 and 2 1 gyr ago5 6 7 8 9 here we report that the lhb lasted much longer than previously thought with most late impactors coming from the e belt an extended and now largely extinct portion of the asteroid belt between 1 7 and 2 1 astronomical units from earth this region was destabilized by late giant planet migration10 11 12 13 e belt survivors now make up the high inclination hungaria asteroids14 15 scaling from the observed hungaria asteroids we find that e belt projectiles made about ten lunar basins between 3 7 and 4 1 gyr ago they also produced about 15 terrestrial basins between 2 5 and 3 7 gyr ago as well as around 70 and four chicxulub sized or larger craters on the earth and moon respectively between 1 7 and 3 7 gyr ago these rates reproduce impact spherule bed and lunar crater constraints

ages of mare basalts on the lunar nearside the chronology of lunar volcanism is based on radiometric ages determined from apollo and luna landing site samples regional stratigraphic relationships and crater degradation and size frequency distribution data for units largely defined prior to the end of the apollo program here we report on new crater size frequency distribution data for 139 spectrally and morphologically defined basalt units which are exposed in six nearside impact basins australe tranquillitatis humboldtianum humorum serenitatis and imbrium crater size frequency distribution measurements are a statistically robust and accurate method to derive absolute model ages of unsampled regions of the moon compared to crater degradation ages crater size frequency ages performed on spectrally defined units offer significant improvements in accuracy our investigation showed that 1 in the investigated basins lunar volcanism was active for at least 1 5 2 0 b y starting at about 3 9 4 0 b y and ceasing at 2 0 b y 2 most basalts erupted during the late imbrian period at about 3 6 3 8 b y 3 significantly fewer basalts were emplaced during the eratosthenian period 4 basalts of copernican age were not found in any of the investigated basins 5 lunar basin filling volcanism probably started within 100 m y after the formation of the individual basins we also assessed the relationship between impact basin age and the history of mare basalt emplacement in these basins we found that 1 in all pre nectarian basins australe and tranquillitatis as well as in the humboldtianum basin which is of nectarian age the distribution of surface ages is clearly dominated by only a single peak in the number of erupted units at 3 6 3 8 b y 2 in the younger basins humorum serenitatis and imbrium a second peak at 3 3 3 5 b y is observed 3 basalt eruptions younger than 2 6 b y occur only intermittently and 4 in the youngest basins serenitatis and imbrium we see an extended period of active basin filling volcanism 1 5 1 6 b y which is 500 m y longer than in the australe and humorum and even 1 0 b y longer than in tranquillitatis and humboldtianum

color development of iron oxides during rhyolite weathering over 52 000 years abstract to evaluate the behavior of iron during weathering colors of four rhyolites with different degrees of weathering eruption ages 1 1 1 8 26 and 52 ka were studied because the initial chemical compositions of the four lavas were similar to one another it is considered that the four lavas had similar whitish color at the time of eruption however at present more weathered lavas have more yellow brownish color indicating the formation of iron hydr oxides or fe bearing clays in order to characterize those weathering products spectro colorimetry and differential dissolution method were combined the colors of 1 1 and 1 8 ka lavas are close to the color of ferrihydrite whereas those of 26 and 52 ka lavas approach to the color of goethite this indicates that the initial stage of weathering is dominated by the formation of ferrihydrite and with increasing weathering the transformation from ferrihydrite to goethite proceeds it is estimated from the change of rock color that the half life for the transformation reaction of ferrihydrite to goethite in the field is 1100 years which is at least 80 times longer than previously reported values based on laboratory experiments the presence of impurities such as si and al in ferrihydrite can be a possible origin for retarding the transformation rate in the field

the mvacs surface stereo imager on mars polar lander the surface stereo imager ssi a stereoscopic multispectral camera on the mars polar lander is described in terms of its capabilities for studying the martian polar environment the camera s two eyes separated by 15 0 cm provide the camera with range finding ability each eye illuminates half of a single ccd detector with a field of view of 13 8 high by 14 3 wide and has 12 selectable filters between 440 and 1000 nm the 18 optics have a large depth of field and no focusing mechanism is required a mechanical shutter is avoided by using the frame transfer capability of the 528 512 ccd the resolving power of the camera 0 975 mrad pixel is the same as the imager for mars pathfinder camera of which it is nearly an exact copy specially designed targets are positioned on the lander they provide information on the magnetic properties of windblown dust and radiometric standards for calibration several experiments beyond the requisite color panorama are described in detail contour mapping of the local terrain multispectral imaging of interesting features possibly with ice or frost in shaded spots to study local mineralogy and atmospheric imaging to constrain the properties of the haze and clouds eight low transmission filters are included for imaging the sun directly at multiple wavelengths to give ssi the ability to measure dust opacity and potentially the water vapor content this paper is intended to document the functionality and calibration of the ssi as flown on the failed lander

cosmogenic nuclides topography and the spatial variation of soil depth abstract if the rate of bedrock conversion to a mobile layer of soil depends on the local thickness of soil then hillslopes on uniform bedrock in a landscape approaching dynamic equilibrium should be mantled by a uniform thickness of soil conversely if the depth of soil varies across an actively eroding landscape then rates of soil production will also vary and consequently the landscape will not be in morphologic equilibrium the slow evolution of hillslopes relative to the tempo of climatic variations and tectonic adjustments would suggest that local morphologic disequilibrium may be expected in many landscapes here we explore this issue of equilibrium landscapes through a previously developed model that predicts the spatial variation in thickness of soil as a consequence of the local balance between soil production and erosion first we confirm the assumption in the model that soil production varies inversely with the thickness of soil using two independent methods one method uses the theoretical prediction that at local steady state soil production equals removal the depth of soil should vary inversely with hillslope curvature the second method relies on direct measurements of in situ produced concentrations of cosmogenic 10 be and 26 al in bedrock at the base of the soil column for our study site in northern california the two methods agree and yield the expression that the rate of soil production declines exponentially with the thickness of soil from 0 077 mm year with no soil mantle to 0 0077 mm year under 1 m of soil we then use this function of soil production in a coupled production and diffusive model of sediment transport to explore the controls on the spatial variation of the depth of soil on four separate spur ridges noses where we measured the data for the function of soil production model predictions are sensitive to boundary conditions grid scale and run time nonetheless we found good agreement between predicted and observed depths of soil as long as we used the observed function of soil production the four noses each have spatially varying curvature and consequently have varying depths of soil implying morphologic disequilibrium we suggest that our study site has been subjected to a wave of incision and varying intensities of erosion because of tectonic and climatic oscillations that have a frequency shorter than the morphologic response time of the landscape

a sawtooth like timeline for the first billion years of lunar bombardment abstract we revisit the early evolution of the moon s bombardment our work combines modeling based on plausible projectile sources and their dynamical decay rates with constraints from the lunar crater record radiometric ages of the youngest lunar basins and the abundance of highly siderophile elements in the lunar crust and mantle we deduce that the evolution of the impact flux did not decline exponentially over the first billion years of lunar history but also there was no prominent and narrow impact spike 3 9 gy ago unlike that typically envisioned in the lunar cataclysm scenario instead we show the timeline of the lunar bombardment has a sawtooth like profile with an uptick in the impact flux near 4 1 gy ago the impact flux at the beginning of this weaker cataclysm was 5 10 times higher than the immediately preceding period the nectaris basin should have been one of the first basins formed at the sawtooth we predict the bombardment rate since 4 1 gy ago declined slowly and adhered relatively close to classic crater chronology models neukum and ivanov 1994 overall we expect that the sawtooth event accounted for about one fourth of the total bombardment suffered by the moon since its formation consequently considering that 12 14 basins formed during the sawtooth event we expect that the net number of basins formed on the moon was 45 50 from our expected bombardment timeline we derived a new and improved lunar chronology suitable for use on pre nectarian surface units according to this chronology a significant portion of the oldest lunar cratered terrains has an age of 4 38 4 42 gyr moreover the largest lunar basin south pole aitken is older than 4 3 gy and therefore was not produced during the lunar cataclysm

a crater and its ejecta an interpretation of deep impact we apply recently updated scaling laws for impact cratering and ejecta to interpret observations of the deep impact event an important question is whether the cratering event was gravity or strength dominated the answer gives important clues about the properties of the surface material of tempel 1 gravity scaling was assumed in pre event calculations and has been asserted in initial studies of the mission results because the gravity field of tempel 1 is extremely weak a gravity dominated event necessarily implies a surface with essentially zero strength the conclusion of gravity scaling was based mainly on the interpretation that the impact ejecta plume remained attached to the comet during its evolution we address that feature here and conclude that even strength dominated craters would result in a plume that appeared to remain attached to the surface we then calculate the plume characteristics from scaling laws for a variety of material types and for gravity and strength dominated cases we find that no model of cratering alone can match the reported observation of plume mass and brightness history instead comet like acceleration mechanisms such as expanding vapor clouds are required to move the ejected mass to the far field in a few hour time frame with such mechanisms and to within the large uncertainties either gravity or strength craters can provide the levels of estimated observed mass thus the observations are unlikely to answer the questions about the mechanical nature of the tempel 1 surface

weathering rind formation in buried terrace cobbles during periods of up to 300ka abstract weathering rinds formed in mesozoic sandstone and basalt cobbles buried in terrace deposits for up to 300 ka have been investigated the aim was to determine the formation process and elemental mass balances during rind development the ages of terraces distributed in the western part of fukui prefecture central japan have been determined as 50 ka 120 ka and 300 ka based on a tephro stratigraphic method detailed investigations across the weathering rinds consisting of microscopic observations porosity measurements and mineralogical and geochemical analyses using x ray diffractometry xrd x ray fluorescence xrf secondary x ray analytical microscopy sxam scanning electron microanalyser sem and electron probe microanalysis epma have been carried out the results revealed that the fe concentrations in the weathering rind of a basalt cobble slightly decreased from the cobble s surface rim towards the unweathered core in contrast in a sandstone cobble formed under the same environmental conditions over the same period of time there is an fe rich layer at some distance below the cobble s surface elemental mass balances across the rinds were determined by using open system mass balance i j calculations and show that the fe was precipitated as fe oxyhydroxides in the basalt cobbles although fe was slightly removed from the rims in sandstone cobbles on the other hand fe migrated along a fe concentration gradient by diffusion and precipitated as fe oxyhydroxide minerals to form the weathering rinds presumably precipitation was due to the relatively higher ph conditions caused by mineral dissolution within the pores principally involving calcite but probably also silicates including feldspar the detailed characterization of the weathering rinds revealed the influence of lithology on the accumulation and dissolution of fe oxyhydroxides causing weathering rinds with different characteristics to develop in different kinds of buried cobbles under the same conditions relatively large climatic changes in the study area did not cause discernable variations in the mean formation rates of the studied rinds which were in the order of 10 8 m a for both basalt and sandstone cobbles these rates are 1 2 orders of magnitude slower than those reported for tropical areas elsewhere most probably due to the lower rainfall in the studied area

a quantitative evaluation of pyrite weathering namurian sediments at mam tor derbyshire are cut by a major landslide and a geological fault both channel oxygenated waters into fragmented pyritic shales pyrite is rapidly oxidized to sulphuric acid 1 5 g being destroyed by each litre of water passing through the fault crush more than 99 per cent of the acid however is immediately consumed in clay mineral transformation and carbonate dissolution reactions the typical acid sulphate ochre springs thus retain less than 1 per cent of the acid generated in the crush zone these very rapid and large scale chemical transformations probably contribute to the slide s continuing activity

rates of weathering and temporal changes in strength of bedrock of marine terraces in boso peninsula japan the weathering rates of bedrock of the dated erosional marine terraces were examined for two definitions 1 the rates at which the thicknesses of weathered zones with different strengths increase with weathering time and 2 the rates at which the strength of weathered materials at an arbitrary depth decrease with time the bedrock of terraces in the study area consists mainly of tertiary sandstone and mudstone the weathering profiles were observed from drill cores on three levels of terrace surfaces i e 2850 290 and 70 years b p in the emergence age of terraces which is regarded as the weathering time of bedrock the degree of weathering grade was described by the residual strength ratio which is defined as the ratio of the needle penetration hardness of weathered part to that of fresh part of bedrock the weathering rates of the first definition do not decrease monotonically but decelerate logarithmically with time zones with lower weathering grades have faster weathering rates than those with higher weathering grades with respect to the weathering rates of the second definition mudstone starts to weather earlier than sandstone but is exceeded by the latter after a certain time has elapsed because mudstone has a higher slaking susceptibility

stochastic processes of soil production and transport erosion rates topographic variation and cosmogenic nuclides in the oregon coast range landscapes in areas of active uplift and erosion can only remain soil mantled if the local production of soil equals or exceeds the local erosion rate the soil production rate varies with soil depth hence local variation in soil depth may provide clues about spatial variation in erosion rates if uplift and the consequent erosion rates are sufficiently uniform in space and time then there will be tendency toward equilibrium landforms shaped by the erosional processes soil mantle thickness would adjust such that soil production matched the erosion previous work in the oregon coast range suggested that there may be a tendency locally toward equilibrium between hillslope erosion and sediment yield here results from a new methodology based on cosmogenic radionuclide accumulation in bedrock minerals at the base of the soil column are reported we quantify how soil production varies with soil thickness in the southern oregon coast range and explore further the issue of landscape equilibrium apparent soil production is determined to be an inverse exponential function of soil depth with a maximum inferred production rate of 268 m ma 1 occurring under zero soil depth this rate depends however on the degree of weathering of the underlying bedrock the stochastic and large scale nature of soil production by biogenic processes leads to large temporal and spatial variations in soil depth the spatial variation of soil depth neither supports nor rejects equilibrium morphology our observed catchment averaged erosion rate of 117 m ma 1 is however similar to that estimated for the region by others and to soil production rates under thin and intermediate soils typical for the steep ridges we suggest that portions of the oregon coast range may be eroding at roughly the same rate but that local competition between drainage networks and episodic erosional events leads to landforms that are out of equilibrium locally and have a spatially varying soil mantle copyright 2001 john wiley sons ltd

chemical weathering in a tropical watershed luquillo mountains puerto rico i long term versus short term weathering fluxes abstract the pristine rio icacos watershed in the luquillo mountains in eastern puerto rico has the fastest documented weathering rate of silicate rocks on the earth s surface a regolith propagation rate of 58 m ma 1 calculated from iso volumetric saprolite formation from quartz diorite is comparable to the estimated denudation rate 25 50 ma 1 but is an order of magnitude faster than the global average weathering rate 6 ma 1 weathering occurs in two distinct environments plagioclase and hornblende react at the saprock interface and biotite and quartz weather in the overlying thick saprolitic regolith these environments produce distinctly different water chemistries with k mg and si increasing linearly with depth in saprolite porewaters and with stream waters dominated by ca na and si such differences are atypical of less intense weathering in temperate watersheds porewater chemistry in the shallow regolith is controlled by closed system recycling of inorganic nutrients such as k long term elemental fluxes through the regolith e g si 1 7 10 8 moles m 2 s 1 are calculated from mass losses based on changes in porosity and chemistry between the regolith and bedrock and from the age of the regolith surface 200 ma mass losses attributed to solute fluxes are determined using a step wise infiltration model which calculates mineral inputs to the shallow and deep saprolite porewaters and to stream water pressure heads decrease with depth in the shallow regolith 2 03 m h2o m 1 indicating that both increasing capillary tension and graviometric potential control porewater infiltration interpolation of experimental hydraulic conductivities produces an infiltration rate of 1 m yr 1 at average field moisture saturation which is comparable with libr tracer tests and with base discharge from the watershed short term weathering fluxes calculated from solute chemistries and infiltration rates e g si 1 4 10 8 moles m 2 s 1 are compared to watershed flux rates e g si 2 7 10 8 moles m 2 s 1 consistency between three independently determined sets of weathering fluxes imply that possible changes in precipitation temperature and vegetation over the last several hundred thousand years have not significantly impacted weathering rates in the luquillo mountains of puerto rico this has important ramifications for tropical environments and global climate change

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

a late miocene dust shower from the break up of an asteroid in the main belt throughout the history of the solar system earth has been bombarded by interplanetary dust particles idps which are asteroid and comet fragments of diameter approximately 1 1 000 microm the idp flux is believed to be in quasi steady state particles created by episodic main belt collisions or cometary fragmentation replace those removed by comminution dynamical ejection and planetary or solar impact because idps are rich in 3he seafloor sediment 3he concentrations provide a unique means of probing the major events that have affected the idp flux and its source bodies over geological timescales here we report that collisional disruption of the 150 km diameter asteroid that created the veritas family 8 3 0 5 myr ago also produced a transient increase in the flux of interplanetary dust derived 3he the increase began at 8 2 0 1 myr ago reached a maximum of approximately 4 times pre event levels and dissipated over approximately 1 5 myr the terrestrial idp accretion rate was overwhelmingly dominated by veritas family fragments during the late miocene no other event of this magnitude over the past approximately 10 8 yr has been deduced from main belt asteroid orbits one remarkably similar event is present in the 3he record 35 myr ago but its origin by comet shower or asteroid collision remains uncertain

response to comment on earth and moon impact flux increased at the end of the paleozoic hergarten et al interpret our results in terms of erosion and uncertain calibration rather than requiring an increase in impact flux geologic constraints indicate low long term erosion rates on stable cratons where most craters with diameters of 20 kilometers occur we statistically test their proposed recalibration of the lunar crater ages and find that it is disfavored relative to our original calibration

a mechanism of chemical weathering of mudstone in a mountainous area abstract the chemical weathering of marine mudstone in humid mountainous areas of japan is dominated by the oxidation of pyrite a common mineral in marine sedimentary rocks this process occurs at the oxidation front that is the base of the oxidized zone and sulfuric acid is formed chlorite is transformed into smectite due to the oxidizing and acidic conditions as sulfuric acid migrates downward from the oxidation front minerals are dissolved and components are leached out from the underlying dissolved zone whose lower boundary is the dissolution front the oxidation and dissolution fronts migrate downward as weathering progresses in mountainous areas

cratering history and lunar chronology the moon is exceptional and important because it is the only planetary body besides the earth for which we have both a detailed stratigraphic history and datable rock samples that can be related to specific geomorphologic units fig 5 1 the moon has preserved much of its magmatic and impact record of at least the last 4 billion years while its endogenic history is of great interest for the fundamentals of planetary interiors and surfaces the moon has become a calibration plate for the cratering record of the earth moon system and by extrapolation of the entire inner solar system if one assumes a heliocentric origin for impactor populations these populations range from asteroids through long and short period comets to interplanetary dust and cover a size range from hundreds of kilometers to micrometers figure 5 1 telescopic view of the nearside of the earth s moon with landing sites of the apollo and luna missions this chapter reviews the presently available data sets in support of this paradigmatic assumption as follows 1 the phenomenology of lunar impact craters 2 the terrestrial record of the impact cratering process and the interpretation of terrestrial impactites as far as this ground truth is relevant for the interpretation of lunar impact craters and datable lunar impact breccias and melt rocks 3 the theory and numerical simulation of the cratering process and the characteristics of the earth moon crossing population of impactors asteroids and comets 4 the principles of relative age dating of lunar surface units and the general lunar stratigraphy 5 the stratigraphic significance and ages of lunar samples impactites and basalts and based on this data set the absolute ages of lunar surface units 6 the cratering rate of the moon as a function of time and 7 the time calibration of this cratering rate based on the

the illusion of diffusion field evidence for depth dependent sediment transport soil covered upland landscapes are common in much of the habitable world and our understanding of their evolution as a function of different climatic tectonic and geologic regimes is important across a wide range of disciplines erosion laws direct quantitative study of the processes shaping earth s surface and form the basis of landscape evolution modeling but are based on limited field data here we use in situ produced cosmogenic 1 0 be and 2 6 al concentrations from granitic saprolite to quantify an exponential decline in soil production with increasing soil thickness for a new field site in point reyes california results are similar to soil production functions from two different previously studied field sites and are used with extensive measurements of soil thickness to quantify depth integrated sediment transport flux plots of calculated sediment fluxes against the product of soil depth and hillslope gradient provide the first field based evidence that soil transport is a nonlinear depth dependent function data from all sites suggest that the widely used linear diffusion equation is only appropriate for shallow gradient convex up regions while the depth dependent transport law is more broadly applicable quantifying both the mobile soil thickness and landscape morphology is therefore critical to understanding how landscapes evolve

relative and absolute ages of individual craters and the rate of infalls on the moon in the post imbrium period abstract this paper contains a reasonably successful attempt to determine relative ages and then absolute ages of individual craters younger than imbrium and the rate of infalls onto the moon as a function of time after the tail of the massive premare bombardment became depleted before 3 aeons 1 aeon 109 years ago there was a period of minimal numbers of infalls the rate of infalls increased rather steadily from this minimum to the present the rate in the geologically recent past 0 3 aeon was about two times that found for the period immediately after the last of the major lave outpourings 3 2 aeons absolute ages were determined for large craters 8 km from crater counts on the surfaces within and on the rims of the large craters key dates were 0 and 0 3 aeon for terrestrial meteoritic craters 3 2 3 5 3 8 and 3 82 aeons for the various mare surfaces according to the determinations of d e wilhelms 1980 geologic history of the moon u s geol surv prof pap and 3 85 aeons from the formation of imbrium