global survey of lunar regolith depths from lroc images abstract we performed the first global survey of lunar regolith depths using lunar reconnaissance orbiter camera lroc data and the crater morphology method for determining regolith depth we find that on both the lunar farside and in the nearside non mare regions the regolith depth is twice as deep as it is within the lunar maria our data compare favorably with previous studies where such data exist we also find that regolith depth correlates well with density of large craters 20 km diameter this result is consistent with the gradual formation of regolith by rock fracture during impact events

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

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

a strain based porosity model for use in hydrocode simulations of impacts and implications for transient crater growth in porous targets numerical modelling of impact cratering has reached a high degree of sophistication however the treatment of porous materials still poses a large problem in hydrocode calculations we present a novel approach for dealing with porous compaction in numerical modelling of impact crater formation in contrast to previous attempts e g p alpha model snowplow model our model accounts for the collapse of pore space by assuming that the compaction function depends upon volumetric strain rather than pressure our new e alpha model requires only four input parameters and each has a physical meaning the model is simple and intuitive and shows good agreement with a wide variety of experimental data ranging from static compaction tests to highly dynamic impact experiments our major objective in developing the model is to investigate the effect of porosity and internal friction on transient crater formation we present preliminary numerical model results that suggest that both porosity and internal friction play an important role in limiting crater growth over a large range in gravity scaled source size

global distribution of lunar impact melt flows abstract in this study we analyzed the distribution and properties of 146 craters with impact melt deposits exterior to their rims many of these craters were only recently discovered due to their unusual radar properties in the near global mini rf data set we find that most craters with exterior deposits of impact melt are small 20 km and that the smallest craters have the longest melt flows relative to their size in addition exterior deposits of impact melt are more common in the highlands than the mare this may be the result of differing target properties in the highlands and mare the difference in titanium content or the greater variation of topography in the highlands we find that 80 of complex craters and 60 of simple craters have melt directions that are coincident or nearly coincident with the lowest point in their rim implying that pre existing topography plays a dominant role in melt emplacement this is likely due to movement during crater modification complex craters or breached crater rims simple craters we also find that impact melt flows have very high circular polarization ratios compared to other features on the moon this suggests that their surfaces are some of the roughest material on the moon at the centimeter to decimeter scale even though they appear smooth at the meter scale

lunar central peak mineralogy and iron content using the kaguya multiband imager reassessment of the compositional structure of the lunar crust ryder and wood 1977 suggested that the lunar crust becomes more mafic with depth because the impact melts associated with the large imbrium and serenitatis basins are more mafic than the surface composition of the moon in this study we reexamine the hypothesis that the crust becomes more mafic with depth we analyze the composition of crater central peaks by using recent remote sensing data and combining the best practices of previous studies we compute the mineralogy for 34 central peaks using 1 nine band visible and near infrared data from the kaguya multiband imager 2 an improved version of hapke s radiative transfer model validated with spectra of lunar soils with well known modal mineralogy and 3 new crustal thickness models from the gravity recovery and interior laboratory data to examine the variation in composition with depth we find that there is no increase in mafic mineral abundances with proximity to the crust mantle boundary or with depth from the current lunar surface and therefore that the crust does not become more mafic with depth we find that anorthosite with very low mafic abundance purest anorthosite or pan is a minority constituent in these peaks and there is no clear evidence of a distinct pan rich layer in the middle crust as previously proposed the composition of most of the central peaks we analyze is more mafic than classically defined anorthosites with an average noritic anorthosite composition similar to that of the lunar surface

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

lunar surface rock abundance and regolith fines temperatures derived from lro diviner radiometer data 1 surface temperatures derived from thermal infrared measurements provide a means of understanding the physical properties of the lunar surface the contrasting thermophysical properties between rocks and regolith fines cause multiple temperatures to be present within the field of view of nighttime multispectral data returned from the lunar reconnaissance orbiter lro diviner radiometer between 60 n s latitudes regolith temperatures are influenced by the presence of rocks in addition to factors such as the thermophysical properties of the regolith fines latitude and local slopes and radiative heating from adjacent crater walls preliminary comparisons of derived rock concentrations with lro camera images show both qualitative and quantitative agreement although comparisons of derived rock concentrations with circular polarization ratio radar data sets display general similarities there are clear differences between the two data sets in the relative magnitude and areal extent of rocky signatures several surface units can be distinguished based on their regolith temperature and rock concentration values and distributions including maria and highlands surfaces rocky impact craters rilles and wrinkle ridges dark mantled deposits and isolated cold surfaces rock concentrations are correlated with crater age and rocks are only preserved on the youngest surfaces or where steep slopes occur and mass wasting prevents mantling with fines the presence of rocky surfaces excavated by young impacts allows for the estimation of minimum regolith thickness from the size of the impact the derived rock concentrations confirm the presence of thicker regolith cover in the highlands and in locations of radar dark haloes

small craters population as a useful geological investigative tool apollo 17 region as a case study abstract small craters below a few decametres in diameter are ubiquitous on the lunar surface and numbered in billions thanks to the expanding availability of global high resolution imagery their size frequency distribution has increasingly been employed as an investigative tool to derive relative or even absolute age estimations of resurfacing events within small regions of geological interest based on a combined survey of tens of thousands of craters this work investigates the 7 50 meter crater distributions across a wide range of lunar terrains centred around the apollo 17 exploration region data analysis using a wide array of approaches reveals distribution patterns consistent with a complex geochronological scenario confirming crater tycho as the source of the main secondary surge which was followed by seismic led modification of the small crater population within the taurus littrow valley and a possible more recent arrival of small secondaries from the fabbroni impact some 70 km to the south this study also confirms that applying standard absolute model ages fits to small craters size frequency distributions to estimate resurfacing ages will only work for surfaces no older than a few tens of ma using an application of monte carlo simulations of crater degradation based on diffusion equations we matched the cumulative sfd of each area to derive size dependent diffusivity rates the results produce equilibrium age estimates that vary among the proposed geological units and crater sizes between 10 and 100 m in diameter this work concludes that the distribution of small craters does not appear to be stochastic in terms of relative size representation and their use for mapping surface variations in terms of physical properties represents a new tool in the remote sensing investigation of planetary surfaces

lunar irregular mare patches classification characteristics geologic settings updated catalog origin and outstanding questions one of the most mysterious lunar features discovered during the apollo era was ina a 2 3 km depression composed of bleb like mounds surrounded by hummocky and blocky terrains subsequent studies identified dozens of similar features in lunar maria describing them as irregular mare patches imps due to the unusual and complex characteristics of imps their specific formation mechanism is debated to improve our understanding of the nature and origin of imps we undertook an updated search and geological characterization of all imps and established a classification approach encompassing the full spectrum of imps we present an updated catalog of 91 imps and survey the detailed characteristics of each imp we find that the majority of imps occur in maria emplaced over three billion years ago contemporaneous with the peak period of global lunar volcanism we utilized geologic context information and characteristics to establish two classification schemes for lunar imps 1 geologic context imps are categorized into a small shield volcano summit pit floor and flank b linear sinuous rille interior and adjacent exterior and c typical maria 2 characteristics imps are classified into a mound floor and b pit only types we showed the range of characteristics of lunar imps was consistent with the waning stage magmatic foam formation and extrusion scenario in different environments our updated catalog and classification raise several outstanding questions concerning the nature and origin of lunar imps assessing these questions will improve our knowledge of lunar thermal and geologic evolution 2020 american geophysical union all rights reserved

space weathering on airless bodies space weathering refers to alteration that occurs in the space environment with time lunar samples and to some extent meteorites have provided a benchmark for understanding the processes and products of space weathering lunar soils are derived principally from local materials but have accumulated a range of optically active opaque particles oaopq that include nanophase metallic iron on in rims formed on individual grains imparting a red slope to visible and near infrared reflectance and larger iron particles which darken across all wavelengths such as are often found within the interior of recycled grains space weathering of other anhydrous silicate bodies such as mercury and some asteroids produce different forms and relative abundance of oaopq particles depending on the particular environment if the development of oaopq particles is minimized such as at vesta contamination by exogenic material and regolith mixing become the dominant space weathering processes volatile rich bodies and those composed of abundant hydrous minerals dwarf planet ceres many dark asteroids outer solar system satellites are affected by space weathering processes differently than the silicate bodies of the inner solar system however the space weathering products of these bodies are currently poorly understood and the physics and chemistry of space weathering processes in different environments are areas of active research

young lunar volcanic features thermophysical properties and formation abstract irregular mare patches imps are small volcanic features on the lunar nearside with young model ages several formation mechanisms have been proposed including caldera collapse explosive outgassing lava flow inflation pyroclastic eruption and regolith drainage we present new observations of the four largest imps sosigenes ina cauchy 5 and maskelyne using the lunar reconnaissance orbiter lro diviner lunar radiometer diviner and evaluate the formation hypotheses in the context of both previous results and the results presented here we find that the imps have a rock abundance slightly higher than their surrounding terrain comparison of the diviner data with thermal models excludes the possibility of extensive competent rocks within 15 cm of the surface at the imps we also derive the thermal inertia at the four largest imps three appear to have thermal inertias slightly higher than typical regolith due to alteration by nearby craters or mass wasting from surrounding steep slopes but ina has a thermal inertia lower than the surrounding terrain in particular the largest smooth mound in ina is the area with the lowest thermal inertia suggesting that the material on the mound is less consolidated than typical regolith and or contains fewer small rocks

role of material properties in the cratering record of young platy ridged lava on mars 1 platy ridged surfaces in the elysium planitia region of mars exhibit different crater densities on rafted plates and polygonally patterned areas between them rather than being indicative of different ages these differences provide insight into the variable strength of different types of lava surface the sizes of small craters and the resulting size frequency distribution sfd depend on the material strength of target surfaces brecciated lava surfaces are likely to have higher crater densities than coherent lava

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

magma supply and storage at kilauea volcano hawaii 1956 1983 abstract shallow crustal magma reservoirs beneath the summit of kilauea volcano and within its rift zones are linked in such a way that the magma supply to each can be estimated from the rate of ground deformation at the volcano s summit our model builds on the well documented pattern of summit inflation as magma accumulates in a shallow summit reservoir followed by deflation as magma is discharged to the surface or into the rift zones magma supply to the summit reservoir is thus proportional to summit uplift and supply to the rift zones is proportional to summit subsidence the average proportionality constant is 0 33 10 6 m 3 rad this model yields minimum supply estimates because it does not account for magma which escapes detection by moving passively through the summit reservoir or directly into the rift zones calculations suggest that magma was supplied to kilauea during july 1956 april 1983 at a minimum average rate of 7 2 10 6 m3 month roughly 35 of the net supply was extruded the rest remains stored within the volcano s east rift zone 55 and southwest rift zone 10 periods of relatively rapid supply were associated with the large kapoho eruption in 1960 and the sustained mauna ulu eruptions in 1969 1971 and 1972 1974 bursts of harmonic tremor from the mantle beneath kilauea were also unusually energetic during 1968 1975 suggesting a close link between kilauea s deep magma supply region and shallow storage reservoirs it remains unclear whether pulses in magma supply from depth give rise to corresponding increases in shallow supply or if instead unloading of a delicately balanced magma transport system during large eruptions or intrusions triggers more rapid ascent from a relatively constant mantle source

the steepest slopes on the moon from lunar orbiter laser altimeter lola data spatial distribution and correlation with geologic features abstract we calculated topographic gradients over the surface of the moon at a 25 m baseline using data obtained by the lunar orbiter laser altimeter lola instrument onboard the lunar reconnaissance orbiter lro spacecraft the relative spatial distribution of steep slopes can be reliably obtained although some technical characteristics of the lola dataset preclude statistical studies of slope orientation the derived slope frequency distribution revealed a steep rollover for slopes close to the angle of repose slopes significantly steeper than the angle of repose are almost absent on the moon due to 1 the general absence of cohesion strength of the fractured and fragmented megaregolith of the lunar highlands and 2 the absence of geological processes producing steep slopes in the recent geological past the majority of slopes steeper than 32 35 are associated with relatively young large impact craters we demonstrate that these impact craters progressively lose their steepest slopes we also found that features of early imbrian and older ages have almost no slopes steeper than 35 we interpret this to be due to removal of all steep slopes by the latest basin forming impact orientale probably by global seismic shaking the global spatial distribution of the steepest slopes correlates moderately well with the predicted spatial distribution of impact rate however a significant paucity of steep slopes in the southern farside remains unexplained

eruption of magmatic foams on the moon formation in the waning stages of dike emplacement events as an explanation of irregular mare patches abstract volcanic eruptions on the moon take place in conditions of low gravity and negligible atmospheric pressure very different from those on earth these differences lead to characteristic lunar versions of hawaiian and strombolian explosive activity and to the production of unusual eruption products neither predicted nor observed on earth in the terminal stages of eruptions these include the unusual mounds and rough hummocky blocky floors of some small shield summit pit crater floors elongate depressions and mare flows similar to those named irregular mare patches imps by braden et al 2014 we examine the ascent and eruption of magma in the waning stages of the eruptive process in small shield summit pit crater floors and show that many imp characteristics can be plausibly explained by basaltic magma behavior as the rise rate of the ascending magma slows to zero volatiles exsolve in the dike and lava lake to form a very vesicular foam and the dike begins to close stresses in the very vesicular and porous lava lake crust produce fractures through which the foam extrudes at a rate determined by its non newtonian rheology waning stage extrusion of viscous magmatic foams to the surface produces convex mounds whose physical properties inhibit typical impact crater formation and regolith development creating an artificially young crater retention age this mechanism for the production and extrusion of very vesicular magmatic foams is also applicable to waning stage dike closure associated with pit craters atop dikes and fissure eruptions in the lunar maria providing an explanation for many irregular mare patches this mechanism implies that imps and associated mare structures small shields pit craters and fissure flows formed synchronously billions of years ago in contrast to very young ages less than 100 million years proposed for imps by some workers

the memin research unit scaling impact cratering experiments in porous sandstones the memin research unit multidisciplinary experimental and modeling impact research network is focused on analyzing experimental impact craters and experimental cratering processes in geological materials memin is interested in understanding how porosity and pore space saturation influence the cratering process here we present results of a series of impact experiments into porous wet and dry sandstone targets steel iron meteorite and aluminum projectiles ranging in size from 2 5 to 12 mm were accelerated to velocities of 2 5 7 8 km s 1 yielding craters with diameters between 3 9 and 40 cm results show that the target s porosity reduces crater volumes and cratering efficiency relative to nonporous rocks saturation of pore space with water to 50 and 90 increasingly counteracts the effects of porosity leading to larger but flatter craters spallation becomes more dominant in larger scale experiments and leads to an increase in cratering efficiency with increasing projectile size for constant impact velocities the volume of spalled material is estimated using parabolic fits to the crater morphology yielding approximations of the transient crater volume for impacts at the same velocity these transient craters show a constant cratering efficiency that is not affected by projectile size

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

geological characterization of the ina shield volcano summit pit crater on the moon evidence for extrusion of waning stage lava lake magmatic foams and anomalously young crater retention ages ina a distinctive 2 3 km d shaped depression is composed of unusual bulbous shaped mounds surrounded by optically immature hummocky blocky floor units the crisp appearance optical immaturity and low number of superposed impact craters combine to strongly suggest a geologically recent formation for ina but the specific formation mechanism remains controversial we reconfirm that ina is a summit pit crater vent on a small shield volcano 3 5 billion years old following detailed characterization we interpret the range of ina characteristics to be consistent with a two component model of origin during the waning stages of summit pit eruption activities the ina pit crater floor is interpreted to be dominated by the products of late stage low rise rate magmatic dike emplacement magma in the dike underwent significant shallow degassing and vesicle formation followed by continued degassing below the solidified and highly microvesicular and macrovesicular lava lake crust resulting in cracking of the crust and extrusion of gas rich magmatic foams onto the lava lake crust to form the mounds these unique substrate characteristics highly porous aerogel like foam mounds and floor terrains with large vesicles and void space exert important effects on subsequent impact crater characteristics and populations influencing 1 optical maturation processes 2 regolith development and 3 landscape evolution by modifying the nature and evolution of superposed impact craters and thus producing anomalously young crater retention ages accounting for these effects results in a shift of crater size frequency distribution model ages from

the effect of long term atmospheric changes on the macroevolution of birds abstract atmospheric conditions are critical for a range of biological functions locomotion among others and long term changes in these conditions have been identified as causal for different macroevolutionary patterns here we examine the influence of variations in atmospheric o2 concentration aoc temperature tair and air density air on the power efficiency as it relates to locomotion during the evolutionary history of birds specifically our study centers on four key evolutionary events 1 the body mass reduction of non avian theropods prior to the rise of birds 2 the emergence of flapping flight in the earliest birds 3 the divergence of basal pygostylians and 4 the diversification of modern birds our results suggest that a marked increase in aoc and air during the middle jurassic coeval with a trend in miniaturization improved the power efficiency of the dinosaurian predecessors of birds likewise an increase in these conditions is hypothesized as having played a major role in the diversification of early pygostylians during the early cretaceous however our analyses do not identify any significant paleoatmospheric effects on either the emergence of flapping flight or the early cladogenesis of modern birds extinct birds flew within the range of atmospheric conditions in which modern birds fly but varying past conditions influenced their flight performance our study thus highlights the importance of considering paleoatmospheric conditions when reconstructing the flight efficiency of the forerunners of modern birds

space weathering from mercury to the asteroid belt the variety of evidence bearing on the nature of space weathering is reviewed the effects of space weathering include spectral darkening reddening and subdued absorption bands and the distinctive magnetic electron spin resonance caused by single domain metallic iron particles ever since the apollo missions two paradigms have dominated the thinking of the planetary science community concerning space weathering 1 the optical effects are caused by impact vitrified glass in agglutinates and 2 the submicroscopic metallic iron results from the reduction of ferrous iron by the impact melting of minerals whose surfaces have been saturated with hydrogen from the solar wind however studies carried out since the apollo program showed that both of these paradigms are invalid a hypothesis first suggested by the author and his colleagues 26 years ago but not generally accepted at that time now appears to be essentially correct both the optical and magnetic effects are caused by metallic iron particles smaller than the wavelength in ubiquitous vapor deposited coatings on soil particle surfaces and inside agglutinates the vapor is generated by both solar wind sputtering and micrometeorite impact vaporization and injected preferentially downward into the porous regolith the iron is reduced by a physical process the selective loss of oxygen that occurs during deposition of the vapor and does not require heating melting or a reducing environment a mathematical theory that describes the optical effects of the submicroscopic iron quantitatively is derived and applied to the regoliths of the moon mercury and an s asteroid

controls on lunar basaltic volcanic eruption structure and morphology gas release patterns in sequential eruption phases assessment of mare basalt gas release patterns during individual eruptions provides the basis for predicting the effect of vesiculation processes on the structure and morphology of associated features we subdivide typical lunar eruptions into four phases phase 1 dike penetrates to the surface transient gas release phase phase 2 dike base still rising high flux hawaiian eruptive phase phase 3 dike equilibration lower flux hawaiian to strombolian transition phase phase 4 dike closing strombolian vesicular flow phase we show how these four phases of mare basalt volatile release together with total dike volumes initial magma volatile content vent configuration and magma discharge rate can help relate the wide range of apparently disparate lunar volcanic features pyroclastic mantles small shield volcanoes compound flow fields sinuous rilles long lava flows pyroclastic cones summit pit craters irregular mare patches imps and ring moat dome structures rmdss to a common set of eruption processes

ina pit crater on the moon extrusion of waning stage lava lake magmatic foam results in extremely young crater retention ages the enigmatic ina feature on the moon was recently interpreted to represent extrusive basaltic volcanic activity within the past 100 m y of lunar history an extremely young age for volcanism on the moon ina is a 2 3 km d shaped depression that consists of a host of unusual bleb like mounds surrounded by a relatively optically fresh hummocky and blocky floor documentation of magmatic volcanic processes from shield volcano summit pit craters in hawai i and new insights into shield building and dike evolution processes on the moon provide important perspectives on the origin of ina we show that the size location morphology topography and optical maturity of ina are consistent with an origin as a subsided summit pit crater lava lake on top of a broad 22 km diameter 3 5 b y old shield volcano new theoretical treatments of lunar shield building magmatic dike events predict that waning stage summit activity was characterized by the production of magmatic foam in the dike and lake the final stages of dike stress relaxation and closure cause the magmatic foam to extrude to the surface through cracks in the lava lake crust to produce the mounds the high porosity of the extruded foams 75 altered the nature of subsequent impact craters the aerogel effect causing them to be significantly smaller in diameter which could bias the crater derived model ages accounting for this effect allows for significantly older model ages to 3 5 b y contemporaneous with the underlying shield volcano thus extremely young volcanic eruptions are not required to explain the unusual nature of ina

a new lunar digital elevation model from the lunar orbiter laser altimeter and selene terrain camera abstract we present an improved lunar digital elevation model dem covering latitudes within 60 at a horizontal resolution of 512 pixels per degree 60 m at the equator and a typical vertical accuracy 3 to 4 m this dem is constructed from 4 5 10 9 geodetically accurate topographic heights from the lunar orbiter laser altimeter lola onboard the lunar reconnaissance orbiter to which we co registered 43 200 stereo derived dems each 1 1 from the selene terrain camera tc 1010 pixels total after co registration approximately 90 of the tc dems show root mean square vertical residuals with the lola data of

lunar mare volcanism stratigraphy eruption conditions and the evolution of secondary crusts abstract lunar mare basalt deposits cover 17 of the lunar surface occur preferentially in topographic lows on the nearside and have a total volume estimated at 1 107 km3 analysis of returned samples and photogeologic and remote sensing studies shows that mare volcanism began prior to the end of heavy bombardment the period of cryptomare formation in pre nectarian times and continued until the copernican period a total duration approaching 3 5 4 ga stratigraphic analyses show that the flux was not constant but peaked in early lunar history during the imbrian period which spans the period 3 85 3 2 ga average volcanic output rate during this peak period was about 10 2 km 3 a very low relative to the present global terrestrial volcanic output rate and comparable to the present local output rates for individual volcanoes such as vesuvius and kilauea hawaii volcanic landforms indicate that many eruptions were of high volume and long duration some eruptions associated with sinuous rules may have lasted of the order of a year and emplaced 103 km3 of lava representing the equivalent in one year of about 70 000 years at the average flux shallow magma chambers were uncommon the nearsidefarside mare deposit asymmetry can be readily explained by differences in crustal thickness magma ascending from the mantle or from a buoyancy trap at the base of the crust should preferentially extrude to the surface on the nearside but should generally stall and cool in dikes in the farside crust extruding only in the deepest basins the occurrence of farside maria within craters whose diameter is generally near to or less than the thickness of the crust may be accounted for by the difference between local and regional compensation dikes that establish pathways to the surface on the nearside should have very high volumes comparable to the volumes associated with many observed flows and sinuous rilles an abundance of dikes should exist in the lower crust of the moon many more than those feeding surface eruptions the upper limit is 37 50 of the crust by volume the presence and abundance of such dike swarms have important implications for the interpretation of the average composition of the lunar crust and the composition of basin and crater ejecta the interplay between thermal contraction and differentiation leads to net cooling and ultimate contraction of the outer portions of the moon resulting in a regional horizontal compressive stress acting on the lunar crust in addition overall cooling and deepening of sources require the production of ever larger volumes of magma in order to reach the surface with time stresses became high enough so that few dikes could open to the surface causing eruptive activity to be severely diminished in the eratosthenian and perhaps to cease in the copernican period lower stress levels are required to terminate eruptive activity on the lunar farside consistent with the imbrian age of the farside maria and the nearside location of the youngest maria lunar mare deposits provide an example of the transition from primary crusts to secondary crusts and illustrate the significance of several factors in the evolution of secondary crusts such as crustal density variations in crustal thickness presence of impact basins state and magnitude of stress in the lithosphere and general thermal evolution these factors are also responsible for the extremely low volcanic flux even during periods of peak extrusion

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

the origin of ina evidence for inflated lava flows on the moon 1 ina is an enigmatic volcanic feature on the moon known for its irregularly shaped mounds the origin of which has been debated since the apollo missions three main units are observed on the floor of the depression 2 9 km across 64 m deep located at the summit of a low shield volcano irregularly shaped mounds up to 20 m tall a lower unit 1 to 5 m in relief that surrounds the mounds and blocky material analyses of lunar reconnaissance orbiter camera images and topography show that features in ina are morphologically similar to terrestrial inflated lava flows comparison of these unusual lunar mounds and possible terrestrial analogs leads us to hypothesize that features in ina were formed through lava flow inflation processes while the source of the lava remains unclear this new model suggests that as the mounds inflated breakouts along their margins served as sources for surface flows that created the lower morphologic unit over time mass wasting of both morphologic units has exposed fresh surfaces observed in the blocky unit ina is different than the terrestrial analogs presented in this study in that the lunar features formed within a depression no vent sources are observed and no cracks are observed on the mounds however lava flow inflation processes explain many of the morphologic relationships observed in ina and are proposed to be analogous with inflated lava flows on earth

newly discovered ring moat dome structures in the lunar maria possible origins and implications we report on a newly discovered morphological feature on the lunar surface here named ring moat dome structure rmds these low domes a few meters to 20 m height with slopes 5 are typically surrounded by narrow annular depressions or moats we mapped about 2 600 rmdss in the lunar maria with diameters ranging from tens to hundreds of meters four candidate hypotheses for their origin involving volcanism are considered we currently favor a mechanism for the formation of the rmds related to modification of the initial lava flows through inflated flow squeeze ups and or extrusion of magmatic foams below a cooling lava flow surface these newly discovered features provide new insights into the nature of emplacement of lunar lava flows suggesting that in the waning stages of a dike emplacement event magmatic foams can be produced extrude to the surface as the dike closes and break through the upper lava flow thermal boundary layer crust to form foam mounds and surrounding moats

imaging of lunar surface maturity the physical evolution of the lunar surface with exposure to the space environment is termed maturation and maturity is the degree to which a particular lunar soil possesses quantitative characteristics consistent with that exposure several quantitative measures or indices of maturity have been proposed and employed including the abundance of solar wind gas abundance of various types of agglutinates various measures of grain size and is feo among the changes attendant with space exposure are striking changes in the optical characteristics of soils mature lunar soils are dark red and exhibit reduced spectral contrast relative to immature soils this paper presents an optical maturity index that quantifies the spectral effects of maturation we show that this optical maturity index correlates with other maturity indices about as well as the accepted maturity indices correlate among themselves and is only weakly coupled to composition the modest correlations among maturity indices suggest important controls on individual maturity indices other than age uncorrelated variations in the rates of accumulation of individual indicators or variations in depths over which indicators are emplaced in addition to mixing these effects conspire to reduce the equivalence of maturity and duration of surface exposure optical maturity illustrates some of these effects showing that ejecta of large and small craters mature at different rates and that the interiors and ejecta of large craters exhibit systematically different optical maturities the same or analogous effects are likely to influence other maturity indices

ring moat dome structures rmdss in the lunar maria statistical compositional and morphological characterization and assessment of theories of origin ring moat dome structures rmdss are positive morphologic features found clustered across many mare regions on the moon of which only a few isolated examples have been previously reported our continuing survey has expanded the known locations of the rmdss from 2 600 to over 8 000 indicating that rmdss are more common geological features than previously thought this work presents a detailed geomorphological analysis of 532 rmdss identified in several mare basins the combination of detailed elemental mapping morphological and morphometric analyses spatial distribution relationships with other geologic structures and comparison with terrestrial analogs lead us to conclude that 1 rmdss represent low circular mounds with diameters of a few hundred meters average about 200 m and a mean height of 3 5 m the mounds are surrounded by moats ranging from tens to over 100 m in width and up to several meters in depth 2 there is a wide variation of titanium abundances although rmdss are more commonly found in mare regions of moderate to high titanium content 3 wt tio2 3 rmdss are found to occur on or around fractures graben and volcanic edifices small shields and cones 4 a spatial association between rmdss and irregular mare patches see braden et al 2014 https doi org 10 1038 ngeo2252 is observed suggesting that both may form from related lava flows 5 comparisons between rmdss and lava inflationary structures on earth support an inflation related extrusive nature and a genetic relationship with host lava flow processes and 6 rmds embayment relationships with craters of different degradation ages superposed on the host mare and regolith development models produces conflicting age relationships and divide theories of rmds origin into two categories 1 synchronous with the emplacement and cooling of the host lava flows 3 4 ga and 2 emplaced substantially after the host mare lava unit in the period 0 3 ga we outline the evidence supporting this age conundrum and implications for the different theories of origin and describe future research avenues to help resolve these outstanding questions 2020 american geophysical union all rights reserved

theory of glacial erosion transport and deposition as a consequence of subglacial sediment deformation a theory of erosion transport and deposition of unlithified sediments by glaciers is presented it predicts the large scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle together with the resultant intensity of large scale lincations drumlins which will be incised in the landscape the theory also predicts the dispersal patterns of subglacial lithologies together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till it predicts major erosional discontinuities within tills and the formation of boulder pavements it suggests that the dominant proportion of the lowland tills produced by pleistocene mid latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine grained the theory is based on an analysis of glacier dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition

mass wasting features on the moon how active is the lunar surface abstract mass wasting features have long been noticed on the moon but there has been no systematic studies for their morphological geometrical or distribution characteristics the effect of mass wasting on topographic evolution and surface erosion on the moon is not fully understood using high resolution image and topography data obtained from the lunar reconnaissance orbiter spacecraft we found that mass wasting has been globally and continuously occurring on the moon meteor impacts and moonquakes are the two major triggering mechanisms in forming lunar mass wasting features we selected over 300 examples of lunar mass wasting features and classified them as falls flows slides slumps and creeps after studying their morphology potential mode of emplacement and sizes of displaced materials the slope and age distributions for their host terrains are studied to estimate the topographic evolutionary sequence of the lunar surface under the effect of mass wasting due to continuous reshaping by mass wasting surface topographies on the moon tend to become smoother with time with fewer mass wasting events occurring on older surfaces pre nectarian surfaces may represent the final stage of surface topographic evolution on slope angles regarding the effect of mass wasting crater densities on lunar surfaces vary with slope angles because the surface erosion rate and regolith production rate on slopes are larger than those on flat areas

model for the origin ascent and eruption of lunar picritic magmas a model for the origin ascent and eruption of the lunar a17 orange glass magma has been constructed using petrological constraints from gas solubility experiments and from analyses of the lunar sample 74220 to better determine the nature and origin of this unique explosive eruption three stages of the eruption have been identified stage 1 of the eruption model extends from 550 km the a17 orange glass magma source region based on phase equilibria studies to 50 km depth in the moon stage 2 extends from 50 km to 500 m where a c o h s gas phase formed and grew in volume based on melt inclusion analyses and measurements the volume of the gas phase at 500 m depth below the surface is calculated to be 7 to 15 vol of the magma closed system using the minimum and maximum estimates of co h2o and s loss from the melt in stage 3 depths shallower than 450 m the rising magma exsolved an additional 800 900 ppm h2o and 300 ppm s increasing the moles in the gas by a factor of 3 to 4 the closed system gas phase is calculated to reach 70 vol at 130 m depth enough to fragment the magma and form pyroclastic beads however fragmentation bead formation is interpreted to have occurred at depths ranging from 600 to 300 m below the lunar surface based on the pressure necessary to explain the c content of the orange glass beads the gas volume 70 required to fragment the ascending magma at this depth is a factor of 5 greater than the volume determined for closed system degassing of an orange glass magma at 500 m strongly implying that the gas was produced by open system degassing as the magma ascended from greater depths formation of the dike carrying the magma up from the 550 km deep source is considered to occur by a crack propagation mechanism wilson and head 2003 2017 the rapid dike propagation process facilitates gas collection by open system degassing in the upper part of the dike this is necessary to achieve the gas volumes required for magma fragmentation at 600 m depths and the magma ascent velocities to explain the wide areal distribution of the bead deposit the explosive nature of the picritic orange glass eruption and the homogeneity of the bead compositions are consistent with this gas assisted eruption scenario as is the evidence of a fe metal forming reduction event during stage 2 followed by a stage 3 oxidation event in the ascending magma

the quantitative relationship between small impact crater morphology and regolith depth abstract small impact craters 10 300 m that encounter a strength transition in the target like a regolith over bedrock have unique morphologies previous studies have used these morphologies as indicators of regolith depth this paper reports on several new analyses that expand our understanding of the quantitative relationship between small crater morphology and target layering i describe three practical situations where the application of the updated method is ambiguous because the specific relationship between the target layering and the crater morphology has never been analyzed in order to resolve the ambiguity i report on new analyses of computer models and lunar data that demonstrate how the dimensions of the crater shape relate to layer depth i also analyze the boundary conditions under which the crater layering relationship will enable determination of layering depth finally in light of the greater understanding of the crater layering relationship i discuss the possible application of this method to mars

lunar gruithuisen and mairan domes rheology and mode of emplacement characteristics of the domes to estimate the yield strengths 10 5 pa plastic viscosities 10 9 pa s and effusion rates 50 m 3 s of the magmas which formed them these values are similar to those of terrestrial rhyolites dacites and basaltic andesites and support the hypothesis that these domes are an unusual variation of typical highlands and mare compositions the dikes which formed them are predicted to have had widths of 50 m and lengths of about 15 km the magma rise speed implied by this geometry is very low 7 10 5 m s and the reynolds number of the motion is 2 10 8 implying a completely laminar flow regime estimates of emplacement duration range from one to several decades these new calculations confirm the unusual nature of these features and support previous qualitative suggestions that they were formed from magmas with significantly higher viscosity than those typical of mare basalts index terms 5480 planetology solid surface planets volcanism 8450 5410 planetology solid surface planets composition 5460 planetology solid surface planets physical properties of materials 8429 volcanology lava rheology and morphology keywords moon volcanism domes

extracting accurate and precise topography from lroc narrow angle camera stereo observations abstract the lunar reconnaissance orbiter camera lroc includes two identical narrow angle cameras nac that each provide 0 5 to 2 0 m scale images of the lunar surface although not designed as a stereo system lroc can acquire nac stereo observations over two or more orbits using at least one off nadir slew digital terrain models dtms are generated from sets of stereo images and registered to profiles from the lunar orbiter laser altimeter lola to improve absolute accuracy with current processing methods dtms have absolute accuracies better than the uncertainties of the lola profiles and relative vertical and horizontal precisions less than the pixel scale of the dtms 2 5 m we computed slope statistics from 81 highland and 31 mare dtms across a range of baselines for a baseline of 15 m the highland mean slope parameters are median 9 1 mean 11 0 standard deviation 7 0 for the mare the mean slope parameters are median 3 5 mean 4 9 standard deviation 4 5 the slope values for the highland terrain are steeper than previously reported likely due to a bias in targeting of the nac dtms toward higher relief features in the highland terrain overlapping dtms of single stereo sets were also combined to form larger area dtm mosaics that enable detailed characterization of large geomorphic features from one dtm mosaic we mapped a large viscous flow related to the orientale basin ejecta and estimated its thickness and volume to exceed 300 m and 500 km 3 respectively despite its 3 8 billion year age the flow still exhibits unconfined margin slopes above 30 in some cases exceeding the angle of repose consistent with deposition of material rich in impact melt we show that the nac stereo pairs and derived dtms represent an invaluable tool for science and exploration purposes at this date about 2 of the lunar surface is imaged in high resolution stereo and continued acquisition of stereo observations will serve to strengthen our knowledge of the moon and geologic processes that occur across all of the terrestrial planets

origin and implications of non radial imbrium sculpture on the moon the widespread rimmed grooves lineations and elongate craters extending from the imbrium impact basin on the moon termed the imbrium sculpture includes a non radial component that is used to infer that the imbrium impactor was the size of a proto planet about half the diameter of vesta the pattern of rimmed grooves lineations and elongate craters around mare imbrium on the moon has been termed the imbrium sculpture rather than radiating from the centre of imbrium the grooves come from different locations reflecting the consequence of a giant oblique collision peter schultz and david crawford use the non radial trends in these patterns to infer that the imbrium impactor was a proto planet with a diameter about half that of the asteroid vesta on the basis of laboratory impact experiments shock physics codes and the observed groove widths the authors conclude that multiple fragments from each oblique basin forming impactor such as the one that formed imbrium should have survived planetary collisions and contributed to the heavy impact bombardment between 4 3 and 3 8 billion years ago rimmed grooves lineations and elongate craters around mare imbrium shape much of the nearside moon this pattern was coined the imbrium sculpture1 and it was originally argued that it must have been formed by a giant oblique 30 impact a conclusion echoed by later studies2 some investigators however noticed that many elements of the imbrium sculpture are not radial to imbrium thereby implicating an endogenic or structural origin3 4 here we use these non radial trends to conclude that the imbrium impactor was a proto planet half the diameter of vesta once part of a population of large proto planets in the asteroid belt such independent constraints on the sizes of the imbrium and other basin forming impactors markedly increase estimates for the mass in the asteroid belt before depletion caused by the orbital migration of jupiter and saturn5 moreover laboratory impact experiments shock physics codes and the groove widths indicate that multiple fragments up to 2 of the initial diameter from each oblique basin forming impactor such as the one that formed imbrium should have survived planetary collisions and contributed to the heavy impact bombardment between 4 3 and 3 8 billion years ago

regolith textures on mercury comparison with the moon abstract surfaces of atmosphereless planetary bodies including the moon and mercury are covered with regolith regolith related processes define surface morphology at scales of meters and tens of meters we systematically surveyed all available images of the surface of mercury with image resolution better than 2 5 m per pixel and compared the observed regolith textures with those on the moon in a manner similar to the moon a typical surface contains many impact craters of different degrees of degradation which indicates that regolith gardening smooths all topographic features textures characteristic of surfaces of large young craters with thin regolith are similar on both bodies there are several types of sharp geologically young textures on mercury that do not have lunar analogs hollows are young sharp irregular depressions on mercury they were discovered in images of lower resolution and attributed to sublimation of unidentified moderately volatile material our survey reveals small decameter size hollows both spatially associated with known large hollow clusters and scattered at great distances from them the existence of small hollows provides new constraints on the nature of these enigmatic features two types of fresh geologically young regolith textures on mercury finely textured slope patches and chevron texture have no analogs on the moon and their formation mechanism is unclear we propose that sintering of regolith particles due to high peak surface temperatures cause formation of a slightly indurated decimeter thick crust at the regolith surface on mercury we hypothesize that such a crust can play a key role in formation of these uniquely regolith textures of mercury

compaction as the origin of the unusual craters on the asteroid mathilde the asteroid mathilde has suffered at least five giant impacts previous studies suggest that mathilde s giant craters should be surrounded by blankets of ejecta that are kilometres deep1 2 yet the craters show no evidence of filling by material excavated during later nearby impacts1 3 computer simulations of impacts have been used to suggest that the absence of ejecta arises because the impact energy is deposited in a small volume due to mathilde s unusually high porosity4 which produces ejecta velocities so high that nearly all of the material escapes mathilde s gravitational field5 here we report laboratory measurements of high velocity impacts into porous material which support an alternative explanation3 the crater is formed mainly by compaction not excavation the small amount of ejecta lofted in our experiments have velocities sufficiently low that nearly all of the material is redeposited within the crater bowl the crater itself results from material being compressed rather than ejected this type of cratering implies that highly porous asteroids are minor contributors of meteorites because essentially no material escapes the asteroids

the effect of target properties on transient crater scaling for simple craters the effects of the coefficient of friction and porosity on impact cratering are not sufficiently considered in scaling laws that predict the crater size from a known impactor size velocity and mass we carried out a systematic numerical study employing more than 1000 two dimensional models of simple crater formation under lunar conditions in targets with varying properties a simple numerical setup is used where targets are approximated as granular or brecciated materials and any compression of porous materials results in permanent compaction the results are found to be consistent with impact laboratory experiments for water low strength and low porosity materials e g wet sand and sands using this assumption we found that both the friction coefficient and porosity are important for estimating transient crater diameters as is the strength term in crater scaling laws i e the effective strength the effects of porosity and friction coefficient on impact cratering were parameterized and incorporated into group scaling laws and predict transient crater diameters within an accuracy of 5 for targets with friction coefficients f 0 4 and porosities 0 30 moreover 90 crater scaling relationships are made available and can be used to estimate transient crater diameters on various terrains and geological units with different coefficient of friction porosity and cohesion the derived relationships are most robust for targets with 10 15 applicable for a lunar environment and could therefore yield significant insights into the influence of target properties on cratering statistics

lunar iron and titanium abundance algorithms based on final processing of clementine ultraviolet visible images the clementine mission to the moon returned global imaging data collected by the ultraviolet visible uvvis camera this data set is now in a final state of calibration and a five band multispectral digital image model dim of the lunar surface will soon be available to the science community we have used observations of the lunar sample return sites and stations extracted from the final dim in conjunction with compositional information for returned lunar soils to revise our previously published algorithms for the spectral determination of the feo and tio2 content of the lunar surface the algorithms successfully normalize the effects of space weathering so that composition may be determined without regard to a surface s state of maturity these algorithms permit anyone with access to the standard archived dim to construct high spatial resolution maps of feo and tio2 abundance such maps will be of great utility in a variety of lunar geologic studies

the cauchy 5 small low volume lunar shield volcano evidence for volatile exsolution eruption patterns and type 1 type 2 hybrid irregular mare patch formation the lunar shield volcano cauchy 5 sitting at the low diameter height volume end of the population is the only known example containing two different types of irregular mare patches imps in very close association 1 the pit crater interior type 1 imp composed of bleb like mounds surrounded by a hummocky and blocky floor unit and 2 type 2 imps small often optically immature pits less than 5 m deep located on the generally block deficient shield flanks a four phase lunar magma ascent eruption model predicts that during a relatively brief eruption low magma rise rates maximize volatile exsolution in lava filling the pit crater bubble rich magmas overtop the pit crater and form extremely vesicular flows on the shield flanks exposure of the flanking flows to vacuum produces a fragmental layer of exploded glassy bubble walls subsequent second boiling upon cooling of the flanking flow interiors releases additional volatiles which migrate and collect forming magmatic foams and gas pockets as magma rise rates slow trapped gas and magmatic foam build up below the cooling pit crater floor magmatic foams are extruded to form type 1 imp deposits type 2 imps on the flanks are interpreted to be due primarily to subsequent impacts causing collapse of the flow surface layer into the extremely vesicle and void rich flow interior anomalously young pit crater floor shield flank crater retention ages compared with surrounding maria ages may be due to effects of cauchy 5 substrate characteristics extreme micro and macroporosity foamy nature and glassy auto regolith on superposed crater formation and retention

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

lunar farside volcanism in and around the south pole aitken basin abstract we identified and mapped 129 mare basalt deposits in and around the south pole aitken spa basin and determined absolute model ages amas for 101 of these units by performing crater size frequency distribution csfd measurements the derived amas range from 2 2 ga to 3 7 ga with the youngest deposits within antoniadi crater and the oldest deposits at jules verne crater our investigations indicate a major peak in volcanic activity between 3 6 ga and 3 2 ga which is a similar time range as the major volcanic activity on the nearside and the rest of the farside however a second peak in volcanic activity 2 2 2 5 ga as observed for the nearside and parts of the farside is not observed for the mare deposits within the spa basin combining all amas derived for farside mare basalts reveals that volcanic activity was more abundant and lasted longer on the nearside than on the farside we propose that the stripping of insulating crust by the large spa forming impact event in combination with lower amounts of heat producing elements like th might be responsible for the reduced volcanic activity in the spa basin in addition we estimated the thicknesses and volumes of the investigated mare deposits with thicknesses between 31 m and 273 m and volumes of 1 km to 2630 km the mare basalt deposits in and around the spa basin show a wide range of dimensions similar to other mare basalts of the near and farsides a trend between the amas and the estimated volumes was not observed but the mare deposits within the large northern craters e g apollo ingenii or leibnitz seem to be generally larger and more voluminous than the mare basalt deposits at the center of the spa basin

the morphology of small fresh craters on mars and the moon the depth diameter ratio for new meter to decameter scale martian craters formed in the last 20 years averages 0 23 only slightly deeper than that expected for simple primary craters on rocky surfaces large variations in depth diameter d d between impact sites indicate that differences between the sites such as target material properties impact velocity angle and physical state of the bolide s are important in determining the depth of small craters in the strength regime on the moon the d d of random fresh small craters with similar diameters averages only 0 10 indicating that either the majority of them are unrecognized secondaries or some proportion are degraded primaries older craters such as these may be shallower due to erosional infilling which is probably not linear over time but more effective over recently disturbed and steeper surfaces processes that are not yet acting on the new martian craters brand new meter to decameter scale craters such as the martian ones studied here are statistically easily distinguishable as primaries but the origins of older craters of the same size such as the lunar ones in this study are ambiguous

the role of substrate characteristics in producing anomalously young crater retention ages in volcanic deposits on the moon morphology topography subresolution roughness and mode of emplacement of the sosigenes lunar irregular mare patch lunar irregular mare patches imps comprise dozens of small distinctive and enigmatic lunar mare features characterized by their irregular shapes well preserved state of relief apparent optical immaturity and few superposed impact craters imps are interpreted to have been formed or modified geologically very recently 100 ma braden et al 2014 however their apparent relatively recent formation modification dates and emplacement mechanisms are debated we focus in detail on one of the major imps sosigenes located in western mare tranquillitatis and dated by braden et al 2014 at 18 ma the sosigenes imp occurs on the floor of an elongate pit crater interpreted to represent the surface manifestation of magmatic dike propagation from the lunar mantle during the mare basalt emplacement era billions of years ago the floor of the pit crater is characterized by three morphologic units typical of several other imps i e 1 bulbous mounds 5 10 m higher than the adjacent floor units with unusually young crater retention ages meters thick regolith and slightly smaller subresolution roughness than typical mature lunar regolith 2 a lower hummocky unit mantled by a very thin regolith and significantly smaller subresolution roughness and 3 a lower blocky unit composed of fresh boulder fields with individual meter scale boulders and rough subresolution surface texture using new volcanological interpretations for the ascent and eruption of magma in dikes and dike degassing and extrusion behavior in the final stages of dike closure we interpret the three units to be related to the late stage behavior of an ancient dike emplacement event following the initial dike emplacement and collapse of the pit crater the floor of the pit crater was flooded by the latest stage magma the low rise rate of the magma in the terminal stages of the dike emplacement event favored flooding of the pit crater floor to form a lava lake and co gas bubble coalescence initiated a strombolian phase disrupting the cooling lava lake surface this phase produced a very rough and highly porous with both vesicularity and macroporosity lava lake surface as the lake surface cooled in the terminal stage of the eruption dike closure with no addition of magma from depth caused the last magma reaching shallow levels to produce viscous magmatic foam due to h2 o gas exsolution this magmatic foam was extruded through cracks in the lava lake crust to produce the bulbous mounds we interpret all of these activities to have taken place in the terminal stages of the dike emplacement event billions of years ago we attribute the unusual physical properties of the mounds and floor units anomalously young ages unusual morphology relative immaturity and blockiness to be due to the unusual physical properties of the substrate produced during the waning stages of a dike emplacement event in a pit crater the unique physical properties of the mounds magmatic foams and hummocky units small vesicles and large void space altered the nature of subsequent impact cratering regolith development and landscape evolution inhibiting the typical formation and evolution of superposed impact craters and maintaining the morphologic crispness and optical immaturity accounting for the effects of the reduced diameter of craters formed in magmatic foams results in a shift of the crater size frequency distribution age from 100 myr to billions of years contemporaneous with the surrounding ancient mare basalts we conclude that extremely young mare basalt eruptions and resulting modification of lunar thermal evolution models to account for the apparent young ages of the imps are not required we suggest that other imp occurrences both those associated with pit craters atop dikes and those linked to fissure eruptions in the lunar maria may have had similar ancient origins

terrestrial analogues for lunar impact melt flows abstract lunar impact melt deposits have unique physical properties they have among the highest observed radar returns at s band 12 6 cm wavelength implying that they are rough at the decimeter scale however they are also observed in high resolution optical imagery to be quite smooth at the meter scale these characteristics distinguish them from well studied terrestrial analogues such as hawaiian p hoehoe and a lava flows the morphology of impact melt deposits can be related to their emplacement conditions so understanding the origin of these unique surface properties will help to inform us as to the circumstances under which they were formed in this work we seek to find a terrestrial analogue for well preserved lunar impact melt flows by examining fresh lava flows on earth we compare the radar return and high resolution topographic variations of impact melt flows to terrestrial lava flows with a range of surface textures the lava flows examined in this work range from smooth hawaiian p hoehoe to transitional basaltic flows at craters of the moon cotm national monument and preserve in idaho to rubbly and spiny p hoehoe like flows at the recent eruption at holuhraun in iceland the physical properties of lunar impact melt flows appear to differ from those of all the terrestrial lava flows studied in this work this may be due to a differences in post emplacement modification processes or b fundamental differences in the surface texture of the melt flows due to the melts unique emplacement and or cooling environment information about the surface properties of lunar impact melt deposits will be critical for future landed missions that wish to sample these materials

size frequency distribution of crater populations in equilibrium on the moon overprinting of craters by subsequent impacts and topographic degradation complicates crater statistics especially for old surfaces and small diameter crater populations a crater population is regarded as in equilibrium at a particular crater size when smaller craters are being produced at the same rate at which they are being destroyed evaluating the equilibrium state of crater populations is challenging and empirical equilibrium densities are frequently inferred by performing careful crater counts and cross comparisons on several lunar surfaces we study the size frequency distributions sfd for the crater populations which have portions in equilibrium the results are one of the few observational constraints on the sfd of crater populations in equilibrium showing that referring to empirical equilibrium densities is not safe for evaluating the equilibrium states of crater populations equilibrium densities are not positively correlated with the ages of crater populations and some populations in equilibrium have crater densities less than those previously believed to represent equilibrium conditions besides the sfd of the production population different crater removal rates at different diameters also affect the sfd of crater populations in equilibrium the equilibrium onset diameter deq of a crater population can be translated to model ages because older populations have larger deq and those for same aged surfaces are comparable we show that the crater populations studied here are in equilibrium at much smaller diameters than those predicted for same aged surfaces by crater degradation models thus indicating lower crater degradation rates on the moon and or younger ages of the counting areas