global distribution of large lunar craters implications for resurfacing and impactor populations by using high resolution altimetric measurements of the moon we produced a catalog of all impact craters 20 kilometers in diameter on the lunar surface and analyzed their distribution and population characteristics the most densely cratered portion of the highlands reached a state of saturation equilibrium large impact events such as orientale basin locally modified the prebasin crater population to 2 basin radii from the basin center basins such as imbrium orientale and nectaris which are important stratigraphic markers in lunar history are temporally distinguishable on the basis of crater statistics the characteristics of pre and postmare crater populations support the hypothesis that there were two populations of impactors in early solar system history and that the transition occurred near the time of the orientale basin event

caloris impact basin exterior geomorphology stratigraphy morphometry radial sculpture and smooth plains deposits the mariner 10 flybys revealed the eastern third of the caloris basin and demonstrated its importance as a chronostratigraphic marker for the geological history of mercury uncertain after that mission were the size of the basin the full distribution of materials deposited or modified during the impact process and the stratigraphic and age relationships between caloris and surrounding terrain images obtained during messenger s january 2008 flyby of mercury revealed that the caloris basin is 15 larger than previously estimated and moderately elliptical 1525 by 1315 km basin related sculpture and secondary craters are dispersed widely in areas surrounding the basin confirming the widespread significance of this event as a stratigraphic marker units mapped around caloris on the basis of mariner 10 data are generally recognized in regions observed for the first time by messenger including most facies of the caloris group the caloris montes odin formation and van eyck formation the only unit without obvious exposures in messenger images is the nervo formation which was originally interpreted as impact melt or fallback ejecta new measurements of the size frequency distribution of impact craters both within the caloris basin and on surrounding units particularly the odin formation support the hypothesis that the vast majority of circum caloris plains postdate the basin and are likely volcanic the messenger observations re emphasize the importance of understanding the exterior stratigraphy of the basin in order to utilize the caloris event as a discrete time horizon in the geological history of mercury

planetary surface dating from crater size frequency distribution measurements partial resurfacing events and statistical age uncertainty abstract we describe the procedure to fit a cumulative production function polynomial to a partial crater size frequency distribution the technique is of particular use in deriving ages for surfaces which have undergone partial resurfacing events namely erosional or depositional events which have affected a limited diameter range of the crater population we demonstrate its use in obtaining times for both the surface formation and the resurfacing event we give a practical outline of the method for making age measurements from crater counts and how to identify resurfacing effects in the results we discuss the conversion of production function polynomials between common presentations and the statistical uncertainty of the determined ages with respect to the non linear chronology function and a minor refinement of data binning

volcanism on mercury evidence from the first messenger flyby for extrusive and explosive activity and the volcanic origin of plains the first messenger flyby of mercury obtained images of 21 of the surface not seen by mariner 10 including the center and western half of the caloris basin and regions near the terminator that show details of the nature of smooth and intercrater plains these new data have helped to address and resolve a series of longstanding questions on the existence and nature of volcanism on mercury and the distribution of volcanic materials data from the mercury dual imaging system mdis on the messenger spacecraft have shown the following 1 numerous volcanic vents in the form of irregularly shaped rimless depressions are concentrated around the interior edge of the caloris basin 2 these vents appear to be sources for effusive volcanism that in one case built a shield in excess of 100 km in diameter and in some cases formed bright haloes around the vents that are interpreted to represent pyroclastic eruptions 3 lobate margins of plains units seen previously in mariner 10 data are documented in messenger images with more clarity and are often distinctive in morphology and color properties supporting the interpretation that these features are the edges of lava flow units 4 the interior of the caloris basin is filled with plains units spectrally distinctive from the rim deposits and comparison with the lunar imbrium basin and superposed impact crater stratigraphy provide evidence that these units are volcanic in origin detailed differences in the mineralogy of lava flow units so prominent in imbrium are not seen in the caloris interior 5 some of the smooth plains surrounding the exterior of the caloris basin show distinct differences in color and morphological properties supporting a volcanic origin 6 some smooth and intercrater plains units distant from the caloris basin show evidence of flooding and embayment relations unrelated to caloris ejecta emplacement local and regional geological and color relationships support a volcanic origin for these plains 7 large impact craters show a sequence of embayment of interior floor and exterior ejecta deposits that supports a volcanic origin for the embayment and filling processes 8 crater embayment and flooding relationships in selected areas suggest volcanic plains thicknesses of many hundreds of meters and local thicknesses inside impact craters of up to several kilometers 9 impact crater size frequency distributions for caloris exterior deposits including the facies of the caloris group and relatively high and low albedo smooth plains show that they are younger than plains interior to caloris and thus must be dominantly the product of post caloris volcanism these new data provide evidence that supports and confirms earlier hypotheses from mariner 10 data that volcanism was important in shaping the surface of mercury the emerging picture of the volcanic style of mercury is similar to that of the moon the other small one plate planetary body there are no major shield volcanoes e g comparable to tharsis montes on mars shallow magma reservoirs are rare and there is little evidence for surface deformation or long lived volcanic sources related to sites of upwelling mantle the close association of volcanic plains and surface deformation features suggests that future observations and analyses can help document the relation between the volcanic flux and the evolving state and magnitude of stress in the lithosphere of mercury

very recent and wide spread basaltic volcanism on mars new spacecraft data provide increasing evidence for a dynamic environment on presenta day mars exogenic processes such as impact cratering mass wasting processes and active dune migration have all been observed to modify the surface no traces of current endogenic activity have been found yet but some studies point to very localized volcanism in the last few millions of years however no systematic study of young volcanic surfaces had been performed so far we present absolute model age determinations of plains volcanism on mars as derived from impact crater sizefrequency distributions extended areas in tharsis the largest volcanoa tectonic region on mars have been resurfaced by lava flows in the last few tens of millions of years we also present results on the rheologic properties of these lava flows inferred from morphometric measurements yield strengths are in the range of 100a 300 pa and viscosities reach values of 10 2 to 10 3 pa s indicating basaltic compositions the results imply that mars retained until recently and probably still retains enough internal heat to produce widea spread plaina style volcanism producing lowa viscosity lava flows throughout large parts of tharsis

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

on the origin of alkali feldspar megacrysts in granitoids the case against textural coarsening alkali feldspar megacrysts up to 20 cm in length are common in granitoids but their origin has been a matter of continued debate in particular the relative role of various magmatic and subsolidus processes has proven difficult to assess given the protracted crystallization history of these megacrysts textural coarsening also known as ostwald ripening has been invoked to explain the extraordinarily large sizes of these feldspar megacrysts while coarsening is a thermodynamically spontaneous process it is important to evaluate the timescales in which it operates during magmatic evolution in this work we consider theories of coarsening of a second phase in the melt in binary systems to evaluate if coarsening can explain the origin of alkali feldspar megacrysts in granitoids the theories explored are all approximate and somewhat incomplete however all of them predict the same basic kinetic equation for the coarsening process calculation of the fundamental rate constant controlling the coarsening processes requires determination of parameters that are not particularly well constrained for minerals in silicate melts nonetheless we demonstrate that diffusion rates are the main control on the rate constant for coarsening using a range of rate constants calculated using suitable parameters for silicate magmatic systems we show that coarsening in silicate magmas cannot lead to crystals larger than tens of m on timescales relevant for igneous processes the timescales required to generate millimeter to centimeter crystals by coarsening are so prohibitively large that the weaknesses of the existing kinetic theories becomes unimportant and the idea of generating megacrysts via textural coarsening can be conclusively abandoned the origin of feldspar megacrysts probably results from the interplay between nucleation and growth rates in magmas a problem that has not been dealt in detail in the literature

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

evidence for basaltic volcanism on the moon within the past 100 million years the majority of basaltic volcanism on the moon occurred more than 3 billion years ago small mounded formations on the lunar nearside may be products of basaltic eruptions less than 100 million years ago suggesting a long decline of magmatic activity

comprehensive analysis of glaciated martian crater greg abstract the 66 km diameter martian crater greg east of hellas hosts various distinctive features including dendritic valleys filled with chevron textured masses south wall and lobate tongues a few kilometers long north wall we analyze these features by various quantitative techniques to illuminate martian geologic and climatic history crater retention model ages indicate that greg is at least 1 3 gy old but surface layers of mantles and glacial features are orders of magnitude younger properties of the dendritic valleys combined with climate models suggest that fluvial activity began under a thicker warmer atmosphere soon after the crater s formation the oldest exposed fluvial systems have surface crater retention ages of a few hundred my indicating runoff in recent geologic time much of greg is covered by ice rich mantle deposits for which we infer gradual accumulation and depths of order 30 85 m they mask pre existing landforms the lobate tongues are interpreted as glaciers with mean slope of 10 2 2 3 and average thickness of 33 19 m our calculations and data suggest that these glaciers were originally ice rich and that their surface layers have been depleted by volatile loss the glaciers probably formed when ice rich mantle deposits reached critical thickness and flowed downhill the top 5 10 m of the mantle and glaciers show crater survival times of order a few my to 15 my which remarkably is the time since the last 1 4 episodes of obliquity 45 global climate models affirm that greg lies in one of two non polar areas with extremes of ice deposition during high obliquity epochs this match with observations supports the use of such models in studies of planetary climate change

martian gases in an antarctic meteorite significant abundances of trapped argon krypton and xenon have been measured in shock altered phases of the achondritic meteorite elephant moraine 79001 from antarctica the relative elemental abundances the high ratios of argon 40 to argon 36 2000 and the high ratios of xenon 129 to xenon 132 2 0 of the trapped gas more closely resemble viking data for the martian atmosphere than data for noble gas components typically found in meteorites these findings support earlier suggestions made on the basis of geochemical evidence that shergottites and related rare meteorites may have originated from the planet mars

present day volcanism on venus evidence from microwave radiometry 1 we present new evidence for volcanic eruptions and lava flow emplacement on venus within the last several decades an integrated study of a radar dark lava flow unit in bereghinia planitia on venus 28 e 39 n based on magellan data obtained in 1993 reveals a significant apparent microwave thermal emission excess consistent with increased subsurface temperature due to very recent lava flow emplacement the flow unit occupies the stratigraphically youngest position in the area and in part is more than 15 years old because it was also observed in the radar map obtained by pioneer venus in 1978 analysis of lava flow cooling rates and geological characteristics point to flow material of mafic composition future missions can employ this microwave radiometry technique to search for and monitor active volcanism on venus

active volcanism on venus in the ganiki chasma rift zone venus is known to have been volcanically resurfaced in the last third of solar system history and to have undergone a significant decrease in volcanic activity a few hundred million years ago however fundamental questions remain is venus still volcanically active today and if so where and in what geological and geodynamic environment here we show evidence from the venus express venus monitoring camera for transient bright spots that are consistent with the extrusion of lava flows that locally cause significantly elevated surface temperatures the very strong spatial correlation of the transient bright spots with the extremely young ganiki chasma their similarity to locations of rift associated volcanism on earth provide strong evidence for their volcanic origin and suggests that venus is currently geodynamically active

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

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

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

the current martian cratering rate abstract the discovery of 248 dated impact sites known to have formed within the last few decades allows us to refine the current cratering rate and slope of the production function at mars we use a subset of 44 of these new craters that were imaged before and after impact by mars reconnaissance orbiter s context camera a thoroughly searched data set that minimizes biases from variable image resolutions we find the current impact rate is 1 65 10 6 craters with an effective diameter 3 9 m km2 yr with a differential slope power law exponent of 2 45 0 36 this results in model ages that are factors of three to five below the hartmann hartmann w k 2005 icarus 174 294 320 and neukum et al neukum g ivanov b a hartmann w k 2001 space sci rev 96 55 86 ivanov ivanov b a 2001 space sci rev 96 87 104 model production functions where they overlap in diameter the best fit production function we measure has a shallower slope than model functions at these sizes but model function slopes are within the statistical errors more than half of the impacts in this size range form clusters which is another reason to use caution when estimating surface ages using craters smaller than 50 m in diameter

crater population and resurfacing of the martian north polar layered deposits present day accumulation in the north polar layered deposits npld is thought to occur via deposition on the north polar residual cap understanding current mass balance in relation to current climate would provide insight into the climatic record of the npld to constrain processes and rates of npld resurfacing a search for craters was conducted using images from the mars reconnaissance orbiter context camera one hundred thirty craters have been identified on the npld 95 of which are located within a region defined to represent recent accumulation high resolution imaging science experiment images of craters in this region reveal a morphological sequence of crater degradation that provides a qualitative understanding of processes involved in crater removal a classification system for these craters was developed based on the amount of apparent degradation and infilling and where possible depth diameter ratios were determined the temporal and spatial distribution of crater degradation is interpreted to be close to uniform through comparison of the size frequency distribution of these craters with the expected production function the craters are interpreted to be an equilibrium population with a crater of diameter d meters having a lifetime of 30 75d 1 14 years accumulation rates within these craters are estimated at 7 2d 0 14 mm yr which corresponds to values of 3 4 mm yr and are much higher than rates thought to apply to the surrounding flat terrain the current crater population is estimated to have accumulated in the last 20 kyr or less

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

large impact basins on mercury global distribution characteristics and modification history from messenger orbital data 1 the formation of large impact basins diameter d 300 km was an important process in the early geological evolution of mercury and influenced the planet s topography stratigraphy and crustal structure we catalog and characterize this basin population on mercury from global observations by the messenger spacecraft and we use the new data to evaluate basins suggested on the basis of the mariner 10 flybys forty six certain or probable impact basins are recognized a few additional basins that may have been degraded to the point of ambiguity are plausible on the basis of new data but are classified as uncertain the spatial density of large basins d 500 km on mercury is lower than that on the moon morphological characteristics of basins on mercury suggest that on average they are more degraded than lunar basins these observations are consistent with more efficient modification degradation and obliteration of the largest basins on mercury than on the moon this distinction may be a result of differences in the basin formation process producing fewer rings relaxation of topography after basin formation subduing relief or rates of volcanism burying basin rings and interiors during the period of heavy bombardment on mercury from those on the moon

volcanism on mercury evidence from the first messenger flyby the origin of plains on mercury whether by volcanic flooding or impact ejecta ponding has been controversial since the mariner 10 flybys 1974 75 high resolution images down to 150 meters per pixel obtained during the first messenger flyby show evidence for volcanic vents around the caloris basin inner margin and demonstrate that plains were emplaced sequentially inside and adjacent to numerous large impact craters to thicknesses in excess of several kilometers radial graben and a floor fractured crater may indicate intrusive activity these observations coupled with additional evidence from color images and impact crater size frequency distributions support a volcanic origin for several regions of plains and substantiate the important role of volcanism in the geological history of mercury

gale crater and impact processes curiosity s first 364 sols on mars abstract impact processes at all scales have been involved in the formation and subsequent evolution of gale crater small impact craters in the vicinity of the curiosity msl landing site and rover traverse during the 364 sols after landing have been studied both from orbit and the surface evidence for the effect of impacts on basement outcrops may include loose blocks of sandstone and conglomerate and disrupted fractured sedimentary layers which are not obviously displaced by erosion impact ejecta blankets are likely to be present but in the absence of distinct glass or impact melt phases are difficult to distinguish from sedimentary volcaniclastic breccia and conglomerate deposits the occurrence of individual blocks with diverse petrological characteristics including igneous textures have been identified across the surface of bradbury rise and some of these blocks may represent distal ejecta from larger craters in the vicinity of gale distal ejecta may also occur in the form of impact spherules identified in the sediments and drift material possible examples of impactites in the form of shatter cones shocked rocks and ropy textured fragments of materials that may have been molten have been observed but cannot be uniquely confirmed modification by aeolian processes of craters smaller than 40 m in diameter observed in this study are indicated by erosion of crater rims and infill of craters with aeolian and airfall dust deposits estimates for resurfacing suggest that craters less than 15 m in diameter may represent steady state between production and destruction the smallest candidate impact crater observed is 0 6 m in diameter the observed crater record and other data are consistent with a resurfacing rate of the order of 10 mm myr considerably greater than the rate from impact cratering alone but remarkably lower than terrestrial erosion rates

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

the distribution and origin of smooth plains on mercury 1 orbital images from the messenger spacecraft show that 27 of mercury s surface is covered by smooth plains the majority 65 of which are interpreted to be volcanic in origin most smooth plains share the spectral characteristics of mercury s northern smooth plains suggesting they also share their magnesian alkali basalt like composition a smaller fraction of smooth plains interpreted to be volcanic in nature have a lower reflectance and shallower spectral slope suggesting more ultramafic compositions an inference that implies high temperatures and high degrees of partial melting in magma source regions persisted through most of the duration of smooth plains formation the knobby and hummocky plains surrounding the caloris basin known as odin type plains occupy an additional 2 of mercury s surface the morphology of these plains and their color and stratigraphic relationships suggest that they formed as caloris ejecta although such an origin is in conflict with a straightforward interpretation of crater size frequency distributions if some fraction is volcanic this added area would substantially increase the abundance of relatively young effusive deposits inferred to have more mafic compositions smooth plains are widespread on mercury but they are more heavily concentrated in the north and in the hemisphere surrounding caloris no simple relationship between plains distribution and crustal thickness or radioactive element distribution is observed a likely volcanic origin for some older terrain on mercury suggests that the uneven distribution of smooth plains may indicate differences in the emplacement age of large scale volcanic deposits rather than differences in crustal formational process

extent age and resurfacing history of the northern smooth plains on mercury from messenger observations abstract messenger orbital images show that the north polar region of mercury contains smooth plains that occupy 7 of the planetary surface area within the northern smooth plains nsp we identify two crater populations those superposed on the nsp post plains and those partially or entirely embayed buried the existence of the second of these populations is clear evidence for volcanic resurfacing the post plains crater population reveals that the nsp do not exhibit statistically distinguishable subunits on the basis of crater size frequency distributions nor do measures of the areal density of impact craters reveal volcanically resurfaced regions within the nsp these results suggest that the most recent outpouring of volcanic material resurfaced the majority of the region and that this volcanic flooding emplaced the nsp over a relatively short interval of geologic time perhaps 100 my or less stratigraphic embayment relationships within the buried crater population including partial crater flooding and the presence of smaller embayed craters within the filled interiors of larger craters and basins indicate that a minimum of two episodes of volcanic resurfacing occurred from the inferred rim heights of embayed craters we estimate the nsp to be regionally 0 7 1 8 km thick with a minimum volume of volcanic material of 4 10 6 to 10 7 km 3 because of the uncertainty in the impact flux at mercury the absolute model age of the post plains volcanism could be either 3 7 or 2 5 ga depending on the chronology applied

mechanisms for incompatible element enrichment on the moon deduced from the lunar basaltic meteorite northwest africa 032 abstract the lunar meteorite northwest africa nwa 032 is a low ti basalt that has incompatible element abundances and th sm ratios characteristic of the involvement of late stage magma ocean crystallization products urkreep in its petrogenesis this sample is very fine grained and contains terrestrial weather products a progressive leaching procedure was therefore developed and applied to magnetic separates and whole rock fractions to obtain rb sr and sm nd ages although many of the leachates as well as the unleached mineral and whole rock fractions contain terrestrial alteration products selected residue fractions yield concordant rb sr and sm nd ages rubidium sr isotopic analyses yield an age of 2947 16 ma with an initial 87 sr 86 sr of 0 700057 17 these characteristics indicate nwa 032 is derived from a source region with an 87 rb 86 sr ratio of 0 044 0 001 this value is higher than all but those determined for kreep basalts and suggests that nwa 032 is derived from a source region that has higher incompatible element abundances than other low ti basalts samarium neodymium isotopic analysis yield a concordant age of 2931 92 ma and an initial e nd of 9 71 0 74 corresponding to a source region with 147 sm 144 nd ratio of 0 246 0 004 the initial nd isotopic composition stands in contrast to the initial sr isotopic composition by requiring nwa 032 to be derived from a source with lower incompatible element abundances than most low ti basalts the source of nwa 032 is therefore unlike those of other lunar basalts modeling of magma ocean cumulate formation demonstrates that unlike other low ti basalt source regions the nwa 032 source is a mixture of olivine pigeonite and clinopyroxene bearing cumulates and only a small amount of urkreep furthermore unlike other mare basalt sources the nwa 032 source does not contain appreciable quantities of plagioclase partial melting models demonstrate that the incompatible element characteristics of the nwa 032 result from formation by smaller degrees of partial melting than other mare basalts thus the incompatible element geochemical signature that is observed in nwa 032 appears to reflect the combined effects of generation from an unusual plagioclase free incompatible element depleted source region by very small degrees of partial melting this study demonstrates that both the presence of urkreep in the source region and small degrees of partial melting generate magmas with similar but not identical incompatible element characteristics in addition it underscores the fact that there is significantly more geochemical diversity on the moon than is represented by samples collected by the american and soviet lunar missions

confident thickness estimates for planetary surface deposits from concealed crater populations abstract an improved technique is presented to determine more accurately deposit thicknesses of any surface units using crater size frequency distributions csfds this new approach enables thickness estimates of deposits that completely cover their underlying unit i e where no flooded craters are observed here the crater populations of the unit of interest and its underlying unit or a representative surface area are measured and their cratering model ages determined the csfd of the younger unit is then treated as if it had the same age as the older unit and the expected cumulative numbers of craters for each bin size are calculated the crater deficiency for each bin size is then determined by subtracting the expected cumulative crater number from the observed cumulative crater number per bin the probability is calculated such that the crater deficiency corresponds to the age of the older underlying unit the crater bin size where we are 95 confident that these craters are there but just covered by the younger unit is chosen to derive the deposit thickness value using known crater diameter rim height relationships we demonstrate the use of the method at two sites of different scales and settings on mars pickering crater and lunae planum and compare our estimates with those obtained by other methods for the lunae planum area a significantly higher minimum thickness of 840 m is predicted

the resurfacing history of venus constraints from buffered crater densities because of atmospheric shielding and endogenic resurfacing the population of impact craters on venus is small about a thousand and consists of large craters this population has been used in numerous studies with the goal of deciphering the geologic and geodynamic history of venus but the nearly spatially random nature of the crater population has complicated efforts to understand this history here we utilize the recent 1 15 m scale global geological map of venus ivanov m a head j w 2011 planet space sci 59 1559 1600 to help address this problem the global geological map provides a stratigraphic sequence of units and known areas where each unit is exposed on the planet for each crater on venus we identify the specific geological units predating and postdating the crater we perform a statistical analysis of this set of observations with a buffered crater density approach which rigorously and consistently takes into account the large size of craters and the fact that many craters are known to predate and or postdate more than one unit in this analysis we consider crater emplacement as random and resurfacing history as determined although unknown we obtain formal confidence intervals for the mean ages of geological units and the mean age differences between the pairs of units at the unit boundaries we find that 1 size frequency distributions of craters superposed on each unit are consistent with each other 2 regional plains and stratigraphically older units have similar crater retention ages 3 stratigraphically younger units have a mean crater retention age significantly younger than the regional plains these findings are readily and consistently explained by global resurfacing scenarios and are difficult to reconcile with equilibrium resurfacing scenarios our analysis also shows that the latest recorded part of intensive resurfacing period lasted on the order of 10 of the mean surface age tens of millions of years the termination of intensive resurfacing may or may not be synchronous over the planet our results also indicate that there are extended deposits associated with large craters that are almost indiscernible in the radar images but obscure radar contrasts between predating lava flows we do not see evidence for any significant and prolonged change of atmospheric pressure following the termination of the intensive resurfacing epoch

candidates source regions of martian meteorites as identified by omega mex abstract the objective of this study is to identify and map spectral analogues of some key martian meteorites basaltic shergottites los angeles shergotty que 94201 lherzolitic shergottite alh a77005 nakhla chassigny and the orthopyroxenite alh 84001 in order to localize terrain candidates for their source regions we develop a best fit procedure to reproduce the near infrared nir spectral properties of the martian surface as seen by the hyperspectral imaging spectrometer omega mex from the nir spectra of the martian meteorites the fitting process is tested and validated and root mean square rms global maps for each meteorite are obtained it is found that basaltic shergottites have nir spectral properties the most representative of the martian surface with the best spectral analogues found in early hesperian volcanic provinces sites with spectral properties similar to those of alh a77005 are scarce they are mainly localized in olivine bearing regions such as nili fossae and small noachian early hesperian terrains the only plausible source region candidate for chassigny is the nili patera caldera dated to 1 6 ga widespread spectral analogues for the alh 84001 meteorite are found northeast of syrtis major and northwest of the hellas basin while this distribution is in agreement with the low calcium pyroxene rich composition and old age 4 1 ga of this meteorite the modal mineralogy of these candidates is not consistent with that of this meteorite no convincing spectral analogue is found for the amazonian aged nakhla meteorite suggesting that its olivine high calcium pyroxene rich composition could be representative of the amazonian terrains buried under dust finally some young rayed craters are proposed as possible candidates for source craters of the studied martian meteorites

crater degradation on the lunar maria topographic diffusion and the rate of erosion on the moon landscape evolution on the moon is dominated by impact cratering in the post maria period in this study we mapped 800 m to 5 km diameter craters on 30 of the lunar maria and extracted their topographic profiles from digital terrain models created using the kaguya terrain camera we then characterized the degradation of these craters using a topographic diffusion model because craters have a well understood initial morphometry these data provide insight into erosion on the moon and the topographic diffusivity of the lunar surface as a function of time the average diffusivity we calculate over the past 3 ga is 5 5 m2 myr with this diffusivity after 3 ga a 1 km diameter crater is reduced to approximately 52 of its initial depth and a 300 m diameter crater is reduced to only 7 of its initial depth and craters smaller than 200 300 m are degraded beyond recognition our results also allow estimation of the age of individual craters on the basis of their degradation state provide a constraint on the age of mare units and enable modeling of how lunar terrain evolves as a function of its topography

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

high velocity collisions from the lunar cataclysm recorded in asteroidal meteorites lunar samples suggest that the inner solar system was bombarded by asteroids about 4 gyr ago radiometric ages of meteorites suggest an unusual number of high velocity asteroids at this time consistent with a dynamical origin of the bombardment in which the asteroids were pushed by outer planet migration onto highly eccentric orbits

martian cratering 10 progress in use of crater counts to interpret geological processes examples from two debris aprons abstract recent controversies about systems of crater count dating have been largely resolved and with continuing refinements crater counts will offer a fundamental geological tool to interpret not only ages but also the nature of geological processes altering the surface of mars as an example of the latter technique we present data on two debris aprons east of hellas the aprons show much shorter survival times of small craters than do the nearby contiguous plains the order of magnitude depths of layers involved in the loss process can be judged from the depths of the affected craters we infer that ice rich layers in the top tens of meters of both aprons have lost crater topography within the last few 10 8 yr probably due to flow or sublimation of ice rich materials mantling by ice rich deposits associated with climate change cycles of obliquity change has probably also affected both the aprons and the plains the crater count tool thus adds chronological and vertical dimensional information to purely morphological studies

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

candidate ice rich material within equatorial craters on mars 1 the floors and walls of many mid latitude 30 60 craters on mars appear to be mantled by relatively young material s with distinct morphology and erosional properties collectively this material fill is often interpreted as ice rich with emplacement and modification related to climatological processes here i document material and associated landforms within 38 craters between 4 13 s in the sinus sabaeus region that appear morphologically similar to material and landforms within mid latitude craters these equatorial mid latitude materials may also share a common composition and emplacement mechanism near surface ice is unstable at equatorial latitudes under present conditions suggesting that emplacement could have occurred under different climate conditions in the past high obliquity 35 45 general circulation model gcm simulations show surface ice accumulation in sinus sabaeus and tharsis where similar material and landforms have been documented these observations are consistent with the hypothesis that past obliquity driven climate change resulted in equatorward volatile migration on mars

secondary crater fields from 24 large primary craters on mars insights into nearby secondary crater production 1 crater statistics are used across a wide variety of applications on planetary surfaces one of the most notable being estimating relative and absolute ages of those surfaces this requires an assumed cratering rate over time and that craters be randomly distributed secondary craters craters that form from the ejecta of an impact event belie this assumption by creating greater crater density in a local area at a single time significantly affecting crater statistics there has been substantial debate over the relative importance of secondary craters and our findings in this mars study indicate that these events can be very significant and cannot be ignored when age dating surfaces we have analyzed secondary crater fields found close to 24 primary craters on mars among other findings such as terrain control over secondary crater field characteristics we conclude that a single large impact event 100 km can significantly affect crater statistics at the 1 5 km diameter level over a non trivial fraction of a planetary surface minimum secondary crater diameters examined were 0 9 km the minimum primary crater diameter was 20 km we also suggest a potential way to avoid significant contamination by the majority of secondary craters that occur close to the primary impact event without the need to manually classify every crater as primary or secondary our findings are specific to mars but further work may show the patterns are applicable to other solid bodies

beginning and end of lunar mare volcanism mare volcanism on the moon is commonly attributed to an important but relatively short lived epoch of internal heating after 3 900 myr bp but before about 2 500 myr bp refs 1 2 although some studies suggested that mare volcanism had started earlier3 5 than times indicated by dated lunar samples only recently have photogeological spectral and geochemical data6 8 documented the importance of a pre 4 000 myr bp epoch similarly early earth based geological mapping revealed a bright rayed crater lichtenberg superposed by mare units9 thereby indicating that mare volcanism must have extended to times significantly more recent than 3 100 myr bp lunar orbital photography confirmed this inferred strati graphical relationship10 but crater statistics indicated that the youngest units were at least twice the age of copernicus or between 1 700 and 2 000 myr bp refs 11 13 we present here the inferred distribution and style of the early phases of mare volcanism based on current evidence and conclude that certain regions of the moon underwent two distinct pulses of igneous activity we then examine crater statistics for the post lichtenberg mare unit and other selected units and conclude that mare volcanism extended to a time comparable with that of the copernicus impact or 1 myr bp these reassessments of the oldest and youngest maria provide new constraints on geophysical models of the internal thermal history of the moon

new crater calibrations for the lunar crater age chronology abstract measuring the spatial density of craters emplaced on geologic units is the primary method used for remotely estimating ages of solar system surfaces the calibration for this method which enables conversion of crater density to absolute age comes from apollo and luna lunar samples for which absolute radiometric ages have been determined researchers throughout the 1970s worked to establish the lunar chronology system based on that calibration correlating crater densities with absolute ages however no uniform crater study has been conducted on all calibration terrains a limitation that was previously unaddressed until this study the latest lunar images from the lunar reconnaissance orbiter were used here to re map the eleven main sampling sites and new crater counts of those surfaces were conducted these show significant differences for many sites crater counts in many cases having more craters than previously identified these results calibrated to the radiometric ages show a revised lunar crater chronology that changes previously established crater based ages by up about 1 billion years surfaces younger than 3 6 gyr and older than 3 9 gyr under the classic chronology are younger in this system and those in between are older in this new system

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

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

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

geomorphic evidence for former lobate debris aprons at low latitudes on mars indicators of the martian paleoclimate 1 circumferential depressions enclosing mesas and plateaus in the northern kasei valles and in the tartarus colles regions of mars are interpreted as indicators of the former extent of lobate debris aprons thought to be mixtures of ice and clastic particles these former lobate debris aprons existed about 1 ga ago and were embayed by lavas or other flow deposits after the lobate debris aprons had been removed by sublimation and deflation topographic depressions with a depth of 50 m and a width of several kilometers were left behind between the mesa or plateau scarp and the solidified flow materials these depressions or moats are located equatorward of 30 at significantly lower latitudes than generally observed for occurrences of modern intact lobate debris aprons this observation provides evidence that the paleoclimate at that time was different than today probably due to a higher averaged obliquity of the planet s rotational axis

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

recent hotspot volcanism on venus from virtis emissivity data the questions of whether venus is geologically active and how the planet has resurfaced over the past billion years have major implications for interior dynamics and climate change nine hotspots areas analogous to hawaii with volcanism broad topographic rises and large positive gravity anomalies suggesting mantle plumes at depth have been identified as possibly active this study used variations in the thermal emissivity of the surface observed by the visible and infrared thermal imaging spectrometer on the european space agency s venus express spacecraft to identify compositional differences in lava flows at three hotspots the anomalies are interpreted as a lack of surface weathering we estimate the flows to be younger than 2 5 million years and probably much younger about 250 000 years or less indicating that venus is actively resurfacing

geologic history of the polar regions of mars based on mars global surveyor data i noachian and hesperian periods during the noachian period the south polar region of mars underwent intense cratering construction of three groups of volcanoes widespread contractional deformation resurfacing of low areas and local dissection of valley networks no evidence for polar deposits ice sheets or glaciation is recognized south polar hesperian geology is broadly characterized by waning impacts volcanism and tectonism emplacement of the polar dorsa argentea formation daf occurred during the hesperian period mars orbiter laser altimeter topographic data and mars orbiter camera images elucidate stratigraphic morphologic and topographic relations permitting the dividing of the daf into eight members which surround and underlie about half of the amazonian south polar layered deposits the lobate fronts and lack of typical volcanic flow morphology of the six plains units indicate that they may be made up of debris flows we think that these flows tens of meters to 200 m thick may have originated by the discharge of huge volumes of slurry fluidized by ground water or liquid co2 perhaps triggered by local impacts igneous activity or basal melting beneath polar deposits the cavi and rugged members include irregular depressions that penetrate the subsurface some of the pits have raised rims the depressions may have formed by collapse due to expulsion of subsurface material in which local explosive activity built up the raised rims further smaller eruptions of volatile rich material may have resulted in narrow sinuous channel deposits within aggrading fine grained unconsolidated material perhaps produced by gaseous discharge of subsurface volatiles preferential erosion of the latter material could have produced the dorsa argentea type sinuous ridges associated mainly with the daf alternatively the ridges may be eskers but the lack of associated glacial and fluvial morphologies casts doubt on this interpretation the knobby degraded materials forming scandia colles may represent the only noachian geologic record exposed in the north polar region most of the north polar region was buried by water or debris ocean sediments during the hesperian period originating from uplands areas and perhaps knobby terrains in the northern plains the sediments either mantle or were deformed by wrinkle ridges radial and concentric to utopia basin and concentric to northern tharsis sources of stress probably included sediment loading in the northern plains and regional magmatic and loading activity at tharsis polar layered deposits began piling up during the early amazonian or later

effect of obliteration on crater count chronologies for martian surfaces 1 the density of impact craters calibrated against lunar data is currently the only quantitative measure of surface age for terrestrial planetary surfaces unlike the moon however mars has been weathered and eroded obliterating some small 10 km diameter craters a phenomenon addressed in the mariner viking days of mars exploration but commonly overlooked in recent studies we present a quantitative model that extends this earlier work to assess the effect of erosion and infilling on surface ages inferred from crater frequency distributions our work affirms that small cratersizedistributionscanbeinterpretedquantitatively in terms of effects of erosion and crater infilling at rates comparable to those reported for mars a reanalysis of prior studies indicates that low to moderate long term rates of erosion and crater infilling can mask an ancient age and result in small crater populations similar to those offered as evidence for young and geologically significant surface activity citation smith m r a r gillespie and d r montgomery 2008 effect of obliteration on crater count chronologies for martian surfaces geophys res lett 35 l10202 doi 10 1029 2008gl033538

crater based dating of geological units on mars methods and application for the new global geological map abstract the new post viking generation of mars orbital imaging and topographical data provide significant higher resolution details of surface morphologies which induced a new effort to photo geologically map the surface of mars at 1 20 000 000 scale although from unit superposition relations a relative stratigraphical framework can be compiled it was the ambition of this mapping project to provide absolute unit age constraints through crater statistics in this study the crater counting method is described in detail starting with the selection of image data type locations both from the mapper s and crater counter s perspectives and the identification of impact craters we describe the criteria used to validate and analyse measured crater populations and to derive and interpret crater model ages we provide examples of how geological information about the unit s resurfacing history can be retrieved from crater size frequency distributions three cases illustrate short intermediate and long term resurfacing histories in addition we introduce an interpretation independent visualisation of the crater resurfacing history that uses the reduction of the crater population in a given size range relative to the expected population given the observed crater density at larger sizes from a set of potential type locations 48 areas from 22 globally mapped units were deemed suitable for crater counting because resurfacing ages were derived from crater statistics these secondary ages were used to define the unit age rather than the base age using the methods described herein we modelled ages that are consistent with the interpreted stratigraphy our derived model ages allow age assignments to be included in unit names we discuss the limitations of using the crater dating technique for global scale geological mapping finally we present recommendations for the documentation and presentation of crater statistics in publications

venus crater distribution and plains resurfacing models detailed analysis of the distribution of craters on venus using mth nearest neighbor analysis coupled with models based upon surface morphology constraints indicates that the hypothesis of complete spatial randomness csr cannot be rejected but is not a unique model of the observed crater distribution based on morphologic mapping the extensive volcanic plains can be divided into four units that have a spread in age of the order of 0 5t the mean surface age of the planet this four unit plains model along with its derivatives produce test statistics that indicate such models also cannot be rejected further the probability of obtaining a result at least as extreme as the observed test statistic given that the null hypothesis model corresponds to venus is true is lowest for the csr model there is no particular reason to pick a csr model along with its implications for catastrophic resurfacing as a constraint on the evolution of venus and there are geological reasons to choose the multiage models we find that we cannot distinguish statistically among models that have two three or four distinct production ages within the plains however the hypothesis that the variation in crater density within all of the plains is due to a single random process can be rejected for two reasons first the binomial probability that such a process could exist within each of the plains units is 0 05 except the smallest and youngest unit pl1 second using a chi squared statistic to test the hypothesis that four plains units have the same age gives a p value of 10 4 indicating confident rejection of the hypothesis thus csr cannot be used as a constraint on models of resurfacing or planetary evolution of venus because of the non uniqueness in matching such a model to the observed crater distribution and the strong indication of distinct ages within the plains with a significant spread in age geological and geophysical constraints provide our best clues for understanding venus

extensive hesperian aged south polar ice sheet on mars evidence for massive melting and retreat and lateral flow and ponding of meltwater local and regional mars orbiter laser altimeter mola topographic data support the presence of an extensive hesperian aged volatile rich south polar deposit the dorsa argentea formation hd and related units underlying the present amazonian aged polar cap api residual ice and apl layered terrain and covering a surface area that could be as large as 2 94 106 km2 about 2 of the surface of mars over twice the area of the present amazonian aged south polar deposits the deposit characteristics indicate that it contained significant quantities of water ice in amounts comparable to present day polar deposits several lines of evidence for melting indicate that the ice sheet deposits underwent melt back and liquid water drainage into surrounding lows including a large valley near the crater schmidt and the argyre basin narrow sinuous ridges lie in a broad linear depression extending from a high near the present polar cap continuously downslope to near the distal portion of hd the new topographic data support the interpretation of these ridges as eskers representing meltwater distribution networks at the base of the receding deposit extensive development of large pits and depressions cavi have previously been interpreted as eolian etching or basal melting of ice rich deposits analysis of mola topography supports the interpretation that they represent basal melting of ice rich deposits and shows that they have links to the esker systems volumetric considerations and topographic lineations suggest that some of the basal melting occurred beneath regions presently occupied by apl and that some of the liquid water formed ponds and lakes in the distal parts of hd the presence of pedestal craters is further evidence of the removal of extensive volatile rich deposits and contributes to the quantitative measure of the former deposit thickness where did the melt products go inspection of the margins of the dorsa argentea formation reveals several large channels that begin there and drain downslope for distances between 900 and 1600 km onto the floor of the argyre basin some 3 5 4 0 km below their origin these channels do not exhibit tributaries their broad lateral distribution supports other evidence that deposit melting was areally very widespread and volumetrically significant and that a large part of the meltwater entered the surface distribution system and was deposited on the floor of the argyre basin over 1000 km away estimates of the present deposit thicknesses together with amounts of the deposit removed by meltback suggest that the original volume could have been as much as 5 9 106 km3 equivalent to a global layer of water 20 m deep if the deposit consisted of 50 volatiles a portion of these volatiles migrated across the surface to pond in adjacent valleys and basins and into the groundwater system a significant portion of the volatiles remain in the deposit representing a net removal from the atmosphere and from the active hydrologic system in early to middle mars history and forming an accessible record of aqueous conditions and possible biological environments dating from that time

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

mercury s geochronology revised by applying model production function to mariner 10 data geological implications 1 model production function mpf chronology uses dynamic models of the main belt asteroids mbas and near earth objects neos to derive the impactor flux to a target body this is converted into the crater size frequency distribution for a specific planetary surface and calibrated using the radiometric ages of different regions of the moon s surface this new approach has been applied to the crater counts on mariner 10 images of the highlands and of several large impact basins on mercury mpf estimates for the plains show younger ages than those of previous chronologies assuming a variable uppermost layering of the hermean crust the age of the caloris interior plains may be as young as 3 59 ga in agreement with messenger results that imply that long term volcanism overcame contractional tectonics the mpf chronology also suggests a variable projectile flux through time coherent with the mbas for ancient periods and then gradually comparable also to the neos

timing and characteristics of the latest mare eruption on the moon unraveling the timing and duration of mare volcanism on the moon is essential for understanding its thermal evolution the end of mare volcanism is poorly constrained because mare basalts are incompletely sampled in this study employing selene kaguya high resolution images we performed new crater size frequency measurements for 49 young mare units 3 0 ga in the procellarum kreep terrane pkt in which the latest magma eruption of the moon occurred mare volcanism in this region continued until 1 5 ga suggesting that volcanic activity in this region ceased 1 0 ga after the magma eruption had globally ceased 2 5 3 0 ga volcanic activity may have peaked 1 8 2 2 ga ago the youngest basalts occur around the aristarchus plateau and the kepler crater which are located in the central region of the pkt it is likely that heating in the crust due to the concentration of heat producing elements in the pkt delayed cooling of a partial melting zone in the underlying mantle in contrast with previous basalt dating in this region our results indicate a higher correlation between ages and spectral types of mare basalts the young mare units in the pkt tend to have spectral types corresponding to high titanium contents while low titanium basalts occur mainly in the early stage the titanium variation in mare basalts may reflect vertical heterogeneity in tio2 content in the upper mantle beneath the pkt

an extended period of episodic northern mid latitude glaciation on mars during the middle to late amazonian implications for long term obliquity history mars is the only planet other than earth in the solar system that has a preserved nonpolar geological record of glaciation on its surface nonpolar ice deposits on mars have been linked to variations in spin axis obliquity that cause mobilization of polar ice and redeposition at lower latitudes forming ice rich and glacial deposits remnant nonpolar glacial deposits are found across the northern mid latitudes where surface ice is not currently stable implying that different climatic conditions existed on mars in the past individual glacial deposits are often too small to date reliably using impact crater size frequency data we describe a novel approach that allows us to derive new information about when glaciation occurred in broad areas of the northern mid latitudes in this region we have classified 1 craters that superpose preexisting glacial deposits and were modified by later accumulation and therefore formed during an epoch when glaciation was occurring and 2 craters that are superposed on glacial deposits but are themselves unmodified by ice accumulation and thus post date significant glaciation the sparse population of post glacial craters reveals that the last period of extensive ice deposition of this type in this latitude band was recent late amazonian the substantial number of craters formed during the recurring glacial periods implies that northern mid latitude glaciation was a long lived recurring process occurring over a period of at least 600 m y on the basis of mars atmospheric general circulation models these results are consistent with higher obliquity being common in the past with recurring periods of obliquity exceeding the 25 axial tilt of mars today these observations support the statistical prediction of j laskar and colleagues that the median obliquity during the amazonian was 35 40

morphological and ecological change on a meander bend the role of hydrological processes and the application of gis environmental change induced by hydrological processes can often be quite small this paper illustrates subtle changes in river planform on a single meander bend of the lower river dee wales and the significance of those changes for the ecology of the present riparian zone geographical information systems gis are shown to provide an excellent framework for integrating historical and contemporary information from different sources and for quantifying possible transcription errors so that true environmental associations and relatively small changes in the context of the spatial scales of the sources can be identified with confidence as a result functional units within the riparian zone of a regulated river can be defined and mapped

planetary surface dating from crater size frequency distribution measurements multiple resurfacing episodes and differential isochron fitting abstract the analysis of crater size frequency distributions and absolute densities forms the basis of current approaches for estimating the absolute and relative ages of planetary surfaces users of the neukum system of crater dating have conventionally used a cumulative presentation of the data but because of the recent proliferation of interest in identifying resurfacing ages it is worth emphasising the utility of the differential presentation of crater data in identifying resurfacing events and particularly in distinguishing the signature of short lived events such as volcanic flows from long acting processes such as aeolian erosion the work describes some additional considerations for making isochron fits to differentially plotted crater populations with respect to the removal of a binning bias for incrementally differentially plotted data the hartmann approach has not typically employed the fitting of isochrons but differential fitting would be a natural choice for this system and is implemented in the craterstats software a revised calculation of the martian epoch boundary times in both chronology systems is provided

the global resurfacing of venus the impact cratering record on venus is unique among the terrestrial planets fully 84 of the craters are in pristine condition and only 12 are fractured remarkably only 2 5 of the craters and crater related features are embayed by lava although intense volcanism and tectonism have affected the entire planet furthermore the spatial and hypsometric distribution of the craters is consistent with a completely random one including stochastic variations monte carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed or unobserved nonrandom crater distributions for resurfacing areas between 0 03 and 100 of the planet s surface these models also are not consistent with the number and nonrandom distribution of volcanoes and the nonrandom distribution of embayed and heavily fractured craters the constraints imposed by the cratering record strongly indicate that venus experienced a global resurfacing event about 300 m y ago followed by a dramatic reduction of volcanism and tectonism this global resurfacing event ended abruptly 10 m y the present crater population has accumulated since then and remains largely intact thermal history models suggest that similar global resurfacing events probably occurred episodically in the past the tesserae statistically have the same average surface age crater retention age as the rest of the planet but they probably represent older rock units deformed by earlier episodes of global resurfacing although they largely survived the latest global resurfacing event their surfaces were severely deformed by it monte carlo simulations indicate that only about 4 6 of the planet has been volcanically resurfaced since the global event and that the lava production rate has been no more than 0 01 0 15 km3 yr during this time this rate is significantly less than the current rate of intraplate volcanism on earth 0 33 0 5 km3 yr most of venus recent volcanism occurs in the beta atla themis region and most of the recent tectonism is associated with the major global scale tectonic disruption zones that lie within and connect the equatorial highlands the approximately 33 of the planet s surface bounded by latitudes 30 n and 30 s longitudes 60 and 300 e contains twice as many heavily fractured craters and 1 4 times more lava embayed craters as the planetary average this region includes most of the major tectonic belts in the equatorial region because the craters are indistinguishable from a statistically random distribution both spatially and hypsometrically this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m y however these craters are simply fractured and or embayed and very few have been subjected to complete tectonic disruption complete burial and subsequent removal from the surface as was the case during the global resurfacing event we show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record the effects of recent geologic activity are much less than those of the earlier global resurfacing event when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity episodic regional resurfacing events that had global effects also occurred on earth e g the mid cretaceous superplume and probably on mars on mars they may have triggered the catastrophic releases of water that formed the outflow channels