flood volcanism in the northern high latitudes of mercury revealed by messenger messenger observations from mercury orbit reveal that a large contiguous expanse of smooth plains covers much of mercury s high northern latitudes and occupies more than 6 of the planet s surface area these plains are smooth embay other landforms are distinct in color show several flow features and partially or completely bury impact craters the sizes of which indicate plains thicknesses of more than 1 kilometer and multiple phases of emplacement these characteristics as well as associated features interpreted to have formed by thermal erosion indicate emplacement in a flood basalt style consistent with x ray spectrometric data indicating surface compositions intermediate between those of basalts and komatiites the plains formed after the caloris impact basin confirming that volcanism was a globally extensive process in mercury s post heavy bombardment era

a morphological evaluation of crater degradation on mercury revisiting crater classification with messenger data abstract observations of impact crater morphology can be used to gain insight into the geological history and evolution of a planet s surface image data from the mariner 10 mission revealed the diversity of impact crater morphologies and degradational states on mercury leading to early studies that sought to establish a stratigraphic column for the planet despite only acquiring image data for 45 of the surface in 2011 the mercury surface space environment geochemistry and ranging messenger spacecraft entered orbit around mercury returning a high resolution global image dataset that enables a robust analysis of crater morphology and degradation to be completed for the entirety of mercury s surface in this study we conducted a visual classification of crater degradation according to initial crater morphology and assigned a degradation state to all craters on mercury 40 km in diameter in our scheme class 1 craters are those that are heavily degraded and class 5 craters are very fresh with bright ray systems we discuss the processes involved in crater degradation and erasure and the challenges associated with applying crater degradation to derive the timing of geological events we found that based on the global spatial density of craters in each class there appears to be a dearth of class 1 craters within the intercrater plains likely due to several ancient basin sized impacts effectively obliterating a considerable portion of craters 40 km in diameter in this region the crater degradation database we present here will serve as a useful tool for future analyses of mercury s geological evolution

new observational evidence of global seismic effects of basin forming impacts on the moon from lunar reconnaissance orbiter lunar orbiter laser altimeter data 1 new maps of kilometer scale topographic roughness and concavity of the moon reveal a very distinctive roughness signature of the proximal ejecta deposits of the orientale basin the hevelius formation no other lunar impact basin even the just preceding imbrium basin is characterized by this type of signature although most have similar types of ejecta units and secondary crater structures the preservation of this distinctive signature and its lack in basins formed prior to orientale is interpreted to be the result of seismically induced smoothing caused by this latest major basin forming event intense seismic waves accompanying the orientale basin forming event preceded the emplacement of its ejecta in time and operated to shake and smooth steep and rough topography associated with earlier basin deposits such as imbrium orientale ejecta emplaced immediately following the passage of the seismic waves formed the distinctive roughness signature that has been preserved for almost 4 billion years

lithification mechanisms for planetary regoliths the glue that binds it is understood how rocks are made on earth however on the moon mercury and to a lesser extent mars and venus there are distinct rock forming processes that we do not fully comprehend the surfaces and crusts of the inner planetary bodies may retain a history of disruption by hypervelocity impact resulting in the generation of disaggregated materials to several kilometers depth the uppermost component of this is called regolith typically 20 m thick on the moon which is part of a more extensive megaregolith that is up to tens of kilometers thick and which in places may pervade the entire crust of a planetary body it is from these pulverized materials that new rocks are reaggregated to form so called breccias this work reviews regolith and megaregolith structure for the inner planetary bodies and investigates how extraterrestrial breccias are produced three principal formation mechanisms are explored thermal sintering shock sintering and the dynamic interaction of impact generated melt w

the global surface roughness of 433 eros from the near laser rangefinder abstract we performed the first global surface roughness assessment of the asteroid 433 eros at baselines horizontal distances of 4 200 m we measured surface roughness using the root mean square rms deviation over a variety of baselines after first detrending the height to remove long wavelength slope effects the global surface roughness of eros is found to be self affine at all baselines investigated the surface roughness is statistically correlated with crater density at baselines of 100 200 m and boulders at a baseline of 5 m no global spatial statistical correlation was found for baselines of 4 200 and mapped tectonic lineaments ponds slope or geopotential elevation the surface roughness of the crater shoemaker charlois regio is controlled by the interplay of a high boulder density producing higher surface roughness values at small baselines and low crater density lowering surface roughness values at long baselines we estimated the mobile regolith thickness regolith that moves around and infills topography to be 0 2 6 2 m from the difference in the surface roughness values at the baseline of 4 m furthermore we find that the change in rms deviation as a function of baseline compares favorably with the moon and differs significantly from existing results for rubble pile asteroid itokawa

lunar topographic roughness maps from lunar orbiter laser altimeter lola data scale dependence and correlation with geologic features and units we present maps of the topographic roughness of the moon at hectometer and kilometer scales the maps are derived from range profiles obtained by the lunar orbiter laser altimeter lola instrument onboard the lunar reconnaissance orbiter lro spacecraft as roughness measures we used the interquartile range of profile curvature at several baselines from 115 m to 1 8 km and plotted these in a global map format the maps provide a synoptic overview of variations of typical topographic textures and utilize the exceptional ranging precision of the lola instrument we found that hectometer scale roughness poorly correlates with kilometer scale roughness because they reflect different sets of processes and time scales hectometer scale roughness is controlled by regolith accumulation and modification processes and affected by the most recent events primarily geologically recent 1 2 ga meteoritic impacts kilometer scale roughness reflects major geological impact volcanic and tectonic events in earlier geological history young large impact craters are rough and their roughness decreases with age the global roughness maps revealed a few unusually dense clusters of hectometer and decameter size impact craters that differ in their morphology and settings from typical secondary crater clusters and chains the origin of these features is enigmatic the maps can assist in the geological mapping of the lunar maria by revealing contacts between volcanic plain units the global roughness maps also clearly reveal cryptomaria old volcanic plains superposed by younger materials primarily crater and basin ejecta

duration of activity on lobate scarp thrust faults on mercury lobate scarps landforms interpreted as the surface manifestation of thrust faults are widely distributed across mercury and preserve a record of its history of crustal deformation their formation is primarily attributed to the accommodation of horizontal shortening of mercury s lithosphere in response to cooling and contraction of the planet s interior analyses of images acquired by the mariner 10 and mercury surface space environment geochemistry and ranging messenger spacecraft during flybys of mercury showed that thrust faults were active at least as far back in time as near the end of emplacement of the largest expanses of smooth plains however the full temporal extent of thrust fault activity on mercury particularly the duration of this activity following smooth plains emplacement remained poorly constrained orbital images from the messenger spacecraft reveal previously unrecognized stratigraphic relations between lobate scarps and impact craters of differing ages and degradation states analysis of these stratigraphic relations indicates that contraction has been a widespread and long lived process on the surface of mercury thrust fault activity had initiated by a time near the end of the late heavy bombardment of the inner solar system and continued through much or all of mercury s subsequent history such deformation likely resulted from the continuing secular cooling of mercury s interior

anisotropy of heterogeneity scale lengths in the lower mantle from pkikp precursors summary the anisotropy of heterogeneity scale lengths in the lower mantle is investigated by modelling its evect on the high frequency precursors of pkikp scattered by the heterogeneities although models having either an isotropic or an anisotropic distribution of scale lengths can fit the observed coda shapes of short period precursors the frequency content of broad band pkikp precursors favours a dominantly isotropic distribution of scale lengths precursor coda shapes are consistent with 1 per cent fluctuations in p velocity in the wavenumber band 0 05 0 5 km 1 extending to 1000 km above the core mantle boundary and with a d region open to circulation throughout the lower mantle the level of excitation of pkikp precursors observed in the frequency band 0 02 2 hz requires a power spectrum of heterogeneity that is nearly white or slowly increasing with wavenumber anisotropy of scale lengths may exist in a d layer having larger horizontal than vertical scale lengths and produce little or no detectable evects on pkikp precursors for p velocity perturbations as high as 3 per cent when averaged over a vertical scale of several kilometres and much higher when averaged over scales of hundreds of metres or less

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

agents of low temperature ocean crust alteration 18o and87sr 86sr isotopic data from smectites calcites and whole rocks together with published isotopic age determinations alkali element concentration data and petrographic observations suggest a sequential model of ocean floor alteration the early stage lasts about 3 m y and is characterized by palagonite and smectite formation and solutions with a large basaltic component increasing with temperature which varies from 15 to 80 c at dsdp site 418a most carbonates are depositedafter this stage from solutions with a negligible basaltic sr component and temperatures of 15 to 40 c water of seawater sr and o isotopic composition is shown to percolate to at least 500 m into the basaltic basement no evidence was found for continuing exchange of strontium or oxygen after 3 m y

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

the surface roughness of mercury from the mercury laser altimeter investigating the effects of volcanism tectonism and impact cratering surface roughness is a statistical measure of change in surface height over a given spatial horizontal scale after the effect of broad scale slope has been removed and can be used to understand how geologic processes produce and modify a planet s topographic character at different scales the statistical measure of surface roughness employed in this study of mercury was the root mean square rms deviation and was calculated from 45 90 n at horizontal baselines of 0 5 250 km with detrended topographic data from individual mercury laser altimeter tracks as seen in previous studies the surface roughness of mercury has a bimodal spatial distribution with the cratered terrain dominated by the intercrater plains possessing higher surface roughness than the smooth plains the measured surface roughness for both geologic units is controlled by a trade off between impact craters generating higher surface roughness values and flood mode volcanism decreasing surface roughness the topography of the two terrain types has self affine like behavior at baselines from 0 5 1 5 km the smooth plains collectively have a hurst exponent of 0 88 0 01 whereas the cratered terrains have a hurst exponent of 0 95 0 01 subtle variations in the surface roughness of the smooth plains can be attributed to differences in regional differences in the spatial density of tectonic landforms the northern rise a 1 000 km wide region of elevated topography centered at 65 n 40 e is not distinguishable in surface roughness measurements over baselines of 0 5 250 km

kilometer scale topographic roughness of mercury correlation with geologic features and units we present maps of the topographic roughness of the northern circumpolar area of 30 mercury at kilometer scales the maps are derived from range profiles obtained by the 31 mercury laser altimeter mla instrument onboard the mercury surface space 32 environment geochemistry and ranging messenger mission as measures of 33 roughness we used the interquartile range of profile curvature at three baselines 0 7 km 34 2 8 km and 11 km the maps provide a synoptic overview of variations of typical 35 topographic textures they show a dichotomy between the smooth northern plains and 36 rougher more heavily cratered terrains analysis of the scale dependence of roughness 37 indicates that the regolith on mercury is thicker than on the moon by approximately a 38 factor of three roughness contrasts within northern volcanic plains of mercury indicate a 39 younger unit inside goethe basin and inside another unnamed stealth basin these new 40 data permit interplanetary comparisons of topographic roughness

mars nature and evolution of young latitude dependent water ice rich mantle 1 high resolution altimetry and imaging have revealed the presence of a meters thick sedimentary layer at middle to high northern and southern latitudes presently covering at least 23 of the planet the layer is interpreted to be water ice rich and to undergone degradation recently its activity very likely coincided with the last major obliquity excursion a few hundred thousand years ago the majority of the layer at higher latitudes however persisted for a much longer time in the late amazonian stratigraphic analysis suggests a complex history of successive episodes of deposition and removal repeated deposition and removal of the mantles are interpreted to be responsible for the unusual statistical properties of kilometer scale topography in the transitional mid latitude zones

locating scatterers in the mantle using array analysis of pkp precursors from an earthquake doublet abstract well separated individual pkp precursors observed at the yellowknife seismic array yk for a high quality doublet of earthquakes provide an opportunity to study the location of the corresponding scatterers and assess the stability of the location estimation based on the comparison of the waveforms of stacked individual precursors and those of pkikp phases we are able to determine that most of these precursors originate from scattering of the pkpbc rather than the pkpab branch above the b caustic on the receiver side this allows a reliable location of the scatterers in the lower mantle their depths range from 2890 km the cmb to 2270 km scattering angles range from 45 8 to 16 0 and surface projections range from southern ontario to northern saskatchewan in canada these locations are associated with transitions from slow to fast velocities in mantle tomographic models and follow the expected general dip direction of fossil slabs under north america this suggests that the subducted slab remnants under north america have retained their compositional signature the fact that we can essentially treat these scatterers as reflections from plane boundaries suggests that the remnant fragments of slab may be spatially extended which should be confirmed using broadband data average differences in measured slowness and back azimuth of the doublet precursors are as small as 0 08 s deg and 1 4 deg respectively our study indicates that it may be possible to locate such scatterers using single earthquakes and small aperture seismic arrays

small scatterers in the lower mantle observed at german broadband arrays seismograms of earthquakes from the south pacific recorded at a german broadband array and network show precursors to pkpdf these precursors mainly originate from off path scattering of pkpab or a nearby pkpbc to p for receiver side scattering or from scattering of p to pkpab or pkpbc on the pkpdf path for source side scattering standard array processing techniques based on plane wave approximations such as vespagram or frequency wavenumber analysis are inadequate for investigating these precursors since scattered waves cannot be approximated as plane waves for arrays and networks larger than 300 300 km for short period waves we therefore develop a migration method to estimate the location of scatterers in the mantle at the core mantle boundary and at the top of the outer core with our method we are able to find isolated scatterers at the source side and the receiver side although the depth of the scatterer is not well constrained however from looking at the first possible arrival time of precursors at different depth and the region where scattering can take place scattering volume we believe that the location of the scatterers is in the lowermost mantle since we have detected scatterers in regions where ultralow velocity zones have been discovered recently we think that the precursor energy possibly originates from scattering at partial melt at the base of the mantle comparing results from broadband and band pass filtered data the detection of small scale structure of the ultralow velocity zones becomes possible

the surface roughness of europa derived from galileo stereo images abstract we assess the surface roughness of europa by reevaluating galileo stereo images roughness provides important information about surface textures which are related to geologic processes operating at a variety of scales roughness also sets important performance requirements for various remote sensing instruments on board the upcoming icy satellite missions to the jovian system in this study roughness is derived for different geologic terrains on europa which have been distinguished through geomorphological mapping for each terrain type we derive the scale dependent root mean square roughness and corresponding hurst exponents we investigate the scale between 30 m and 5 km and show that europa s surface can mostly be described by a monofractal behavior over these baselines i e surface roughness follows power laws with breakpoints located at a few hundred meters hurst exponents are higher below the breakpoints with typical values between 0 4 and 0 8 and lower above typically between 0 2 and 0 6 this behavior is mostly controlled by ridges which are dominating the topography on europa at unity scale 1 m roughness is extrapolated to be around 0 4 0 7 m with strong exceptions on chaos terrain where values over 1 2 m can be achieved assuming that no further breakpoints are present below the resolution of the digital terrain models used in this study

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

an analysis of large scale variations in small scale mantle heterogeneity using global seismographic network recordings of precursors to pkp high frequency precursors to the core phase pkp are caused by scattering off heterogeneities in the lowermost mantle and d regions and they provide a unique window into the small scale structure of the deep earth we study lower mantle scattering by analyzing 412 high quality pkp precursor records at ranges between 120 and 137 5 as obtained from the global seismic networks during the last 10 years to examine regional variations in scattering strength we compare individual records with the globally averaged pkp precursor stack of hedlin et al 1997 we identify strong differences in apparent scattering strength among specific source receiver paths inversion of these data for scattering source regions is complicated by ambiguity between source and receiver side scattering and the sparse and uneven data coverage synthetic tests however suggest that inversions with applied smoothness constraints can resolve large scale differences in scattering strength over significant parts of the lower mantle we use a conjugate gradient method based on an approximation to rayleigh born scattering theory to image differences in the average strength of scattering within the lowermost 1000 km of the mantle our results indicate particularly strong scattering beneath central africa parts of north america and just north of india whereas weaker scattering is seen beneath south and central america eastern europe and indonesia some regions of strong scattering correlate roughly with large scale anomalies revealed by seismic tomography including the african plume and the tethys trench these correlations are tentative rather than definitive because bootstrap resampling tests show that many details in our model are not reliably resolved and the network data alone do not permit complete resolution of the source receiver ambiguity in all areas further progress in this area will require integration of available network recordings with data collected by regional networks and arrays and consideration of the phase velocity of the precursors as well as their temporal variations

size frequency distribution of different secondary crater populations 1 equilibrium caused by secondary impacts accumulation of impact craters is the major reason causing equilibrium of crater populations on airless planetary surfaces besides primary craters the effect of widespread secondaries on the equilibrium of local crater populations is little studied here the different secondary crater populations formed by the hokusai crater on mercury are systematically studied and they are compared with those on the moon to investigate their contribution to the evolution of local crater populations self secondaries cause equilibrium on continuous ejecta deposits in a short time and the equilibrium crater population has a differential sfd slope of about 3 background secondaries are abundant on mercury and equilibrium caused by a combination of primaries and potential background secondaries follows the same pattern on the moon and mercury the spatial dispersion of fragments that form both near field and distant secondaries is the major factor affecting the degree of mutual destruction and thus the final crater sfd some clustered distant secondaries on mercury are likely formed by individual fragments considering their large spatial dispersion and identical morphology with same sized primaries and the sfd rollovers of these secondaries possibly reflect the inherent sfd rollovers of the impact fragments near field secondaries and many other distant secondaries have morphology and spatial distribution that are consistent with being formed by clustered fragments and mutual destruction of secondaries may be the major reason causing the observed sfd rollovers heterogeneous secondary impacts are a potential explanation for both different crater densities within the equilibrium diameter range and different regolith thicknesses on coeval surfaces

topography of the northern hemisphere of mercury from messenger laser altimetry laser altimetry by the messenger spacecraft has yielded a topographic model of the northern hemisphere of mercury the dynamic range of elevations is considerably smaller than those of mars or the moon the most prominent feature is an extensive lowland at high northern latitudes that hosts the volcanic northern plains within this lowland is a broad topographic rise that experienced uplift after plains emplacement the interior of the 1500 km diameter caloris impact basin has been modified so that part of the basin floor now stands higher than the rim the elevated portion of the floor of caloris appears to be part of a quasi linear rise that extends for approximately half the planetary circumference at mid latitudes collectively these features imply that long wavelength changes to mercury s topography occurred after the earliest phases of the planet s geological history

revised constraints on absolute age limits for mercury s kuiperian and mansurian stratigraphic systems following an approach similar to that used for the moon mercury s surface units were subdivided into five time stratigraphic systems based on geologic mapping using mariner 10 images the absolute time scale originally suggested for the time periods associated with these systems was based on the assumption that the lunar impact flux history applied to mercury however we find that the duration and onset of corresponding periods in the stratigraphic sequences on mercury and the moon are not the same using high resolution and multiband image data obtained by the mercury surface space enviroment geochemistry and ranging messenger spacecraft we identify and catalog fresh impact craters interpreted to have formed during mercury s two most recent periods the mansurian and kuiperian we use the densities of the inferred kuiperian and mansurian aged crater populations to estimate new limits for the age boundaries of these time intervals results suggest that both the mansurian and kuiperian periods began more recently and extended for significantly shorter durations of time than previously suggested the kuiperian is estimated to have initiated as recently as 280 60 ma and the mansurian as recently as 1 7 0 2 ga

kilometer scale roughness of mars results from mola data analysis the mars orbiter laser altimeter mola data are used to characterize the kilometer scale surface roughness of mars the median absolute value of the differential slope at a given baseline is proposed as a data derived measure of the surface roughness at this scale study of the scale dependence of roughness for the smoothest terrains gives an independent estimate of 20 cm for mola ranging accuracy the baseline lengths from 0 6 to 20 km are used and kilometer scale roughness is mapped for the entire surface the maps show that different geological units have distinctive roughness characteristics scale dependence of roughness for a number of geological units is presented and discussed the southern polar cap is rougher than the northern at kilometer and subkilometer scale which suggests differences in the sublimation condensation balance the vastitas borealis formation has a distinctive 3 km scale background surface topography which suggests a nonvolcanic origin for its upper layer young volcanic plains in amazonis planitia and the eastern part of elysium planitia are very similar to each other in their roughness characteristics and differ from other volcanic plains on mars which suggests a distinctive eruption style there are systematic latitudinal variations of roughness in both the southern highlands and the northern lowlands terrains at high latitude are smoother at short baselines the characteristic vertical scale related to this difference is several meters processes that could be responsible for formation of this trend include creep of ice rich near surface material at high latitudes treatment of the surface with repetitive deposition and sublimation of seasonal frost climate controlled deposition and or cementation of dust at high latitudes and repetitive sublimation and accumulation of subsurface ice at low latitudes with climate variations all mechanisms of origin could operate more effectively under different climate conditions relevant morphological observations favor mechanisms involving deposition of smooth blankets at high latitudes

an observation of pkjkp inferences on inner core shear properties the seismic phase pkjkp which traverses the inner core as a shear wave and would provide direct evidence for its solidity has been difficult to detect using stacked broadband records from the grafenberg array in germany we documented a high signal to noise phase the arrival time and slowness of which agree with theoretical predictions for pkjkp the back azimuth of this arrival is also consistent with predictions for pkjkp as is the comparison with a pseudoliquid inner core model envelope modeling of the pkjkp waveform implies a shear velocity gradient with depth in the inner core that is slightly larger than that in the preliminary reference earth model

inner core differential motion confirmed by earthquake waveform doublets we analyzed 18 high quality waveform doublets with time separations of up to 35 years in the south sandwich islands region for which the seismic signals have traversed the inner core as pkp df the doublets show a consistent temporal change of travel times at up to 58 stations in and near alaska and they show a dissimilarity of pkp df coda using waveform doublets avoids artifacts of earthquake mislocations and contamination from small scale heterogeneities our results confirm that earth s inner core is rotating faster than the mantle and crust at about 0 3 degrees to 0 5 degrees per year

rates of temperature change of airless landscapes and implications for thermal stress weathering 1 thermal stress weathering may play a role in the evolution of terrestrial planet landscapes particularly those without atmospheres by contributing to rock breakdown regolith production and crater degradation damage occurs in the form of microscopic cracks that result from a thermal cycle or thermal shock terrestrial studies typically evaluate the efficacy of this process by measuring the rate of surface temperature change dt dt using a damage threshold of 2 k min while the extent of this damage is unknown we investigate its relative efficacy by modeling rates of temperature change on various airless surfaces the magnitude of dt dt values is primarily controlled by sunrise set durations on quickly rotating bodies such as vesta and by distance to the sun on slowly rotating bodies such as mercury the strongest temperature shocks are experienced by highly sloped east or west facing surfaces hot thermal shocks dt dt 0 tend to be stronger than cold shocks dt dt 0 and on some bodies daytime shadowing may produce higher dt dt values than those caused by diurnal sunrise set if the topography is optimally oriented we find that high dt dt values are not however always correlated with high temperature gradients within the rock this adds to the ambiguity of the poorly understood damage threshold warranting further research on the topic that goes beyond the simple 2 k min criterion

impact induced thermal effects in the lunar and mercurian regoliths while the moon and mercury are similar objects in a geomorphologic sense they also possess important differences particularly in the context of small scale impact phenomena not only is the surface of mercury notably hotter than that of the moon but the impact flux is also more intense at mercury due to higher impact velocities and a greater spatial density of micrometeoroids by extrapolating the terrestrial micrometeoroid flux to the moon and mercury it is found that the impact rate at mercury is 5 5 times greater and the mean impact velocity more than 60 percent higher than at the moon a model of impact melting and vaporization applied to the lunar and mercurian environments indicates that almost 14 times more impact melt and over 20 times more vapor are generated per unit time in the mercurian regolith although the surface temperature plays a role in determining the melt and vapor volumes the difference in impact velocity appears to be markedly more important average craters formed by identical projectiles under the two velocity distributions on the two planets should be very similar in size because differences in the kinetic energy of the average impactors will be offset by the differences in gravitational acceleration as a result variations in regolith mixing should depend more on the actual impact rates than on disparities in excavated volumes each average impact on mercury however will produce more than twice as much impact melt as its lunar counterpart introducing more glass into the mercurian regolith although substantial quantities of impact vapor are produced on both planets mixing of the regolith should occur quickly enough in each case to spread the deposited vapor over large surface areas the resulting vapor coatings should be negligibly small for most purposes a much larger agglutinate population should exist on mercury and individual agglutinates should be similar to those of the lunar highlands reflectance spectra and visual albedos indicate that the mercurian crust possesses relatively small quantities of fe perhaps similar to those of apollo 16 light and dark matrix breccias laboratory studies have demonstrated that as the magnetic fractions which are correlated with agglutinate abundances of soils derived from these breccias increase the differences in 0 565 mm visible light reflectivity between the soils diminish should the regional variations in fe content of the mercurian crust be similar to those of these apollo 16 samples then the intense agglutination environment could account for the lack of optical contrast across the mercurian surface at the same time the very large fraction of impact glass in the regolith would imply that most pyroxenes have been fused and incorporated into these glasses seriously attenuating the fe2 crystal field absorption near 0 9 mm supporting this picture are the most recently published spectral observations of mercury which provide little or no evidence of such an absorption feature

slabs in the lower mantle and their modulation of plume formation numerical mantle convection models indicate that subducting slabs can reach the core mantle boundary cmb for a wide range of assumed material properties and plate tectonic histories an increase in lower mantle viscosity a phase transition at 660 km depth depth dependent thermal expansivity and depth dependent thermal diffusivity do not preclude model slabs from reaching the cmb we find that ancient slabs could be associated with lateral temperature anomalies 500 c cooler than ambient mantle plausible increases of thermal conductivity with depth will not cause slabs to diffuse away regional spherical models with actual plate evolutionary models show that slabs are unlikely to be continuous from the upper mantle to the cmb even for radially simple mantle structures the observation from tomography showing only a few continuous slab like features from the surface to the cmb may be a result of complex plate kinematics not mantle layering there are important consequences of deeply penetrating slabs our models show that plumes preferentially develop on the edge of slabs in areas on the cmb free of slabs plume formation and eruption are expected to be frequent while the basal thermal boundary layer would be thin however in areas beneath slabs the basal thermal boundary layer would be thicker and plume formation infrequent beneath slabs a substantial amount of hot mantle can be trapped over long periods of time leading to mega plume formation we predict that patches of low seismic velocity may be found beneath large scale high seismic velocity structures at the core mantle boundary we find that the location buoyancy and geochemistry of mega plumes will differ from those plumes forming at the edge of slabs various geophysical and geochemical implications of this finding are discussed

evidence for rapid topographic evolution and crater degradation on mercury from simple crater morphometry examining the topography of impact craters and their evolution with time is useful for assessing how fast planetary surfaces evolve here new measurements of depth diameter d d ratios for 204 craters of 2 5 to 5 km in diameter superposed on mercury s smooth plains are reported the median d d is 0 13 much lower than expected for newly formed simple craters 0 21 in comparison lunar craters that postdate the maria are much less modified and the median crater in the same size range has a d d ratio that is nearly indistinguishable from the fresh value this difference in crater degradation is remarkable given that mercury s smooth plains and the lunar maria likely have ages that are comparable if not identical applying a topographic diffusion model these results imply that crater degradation is faster by a factor of approximately two on mercury than on the moon suggesting more rapid landform evolution on mercury at all scales

global surface slopes and roughness of the moon from the lunar orbiter laser altimeter 1 the acquisition of new global elevation data from the lunar orbiter laser altimeter carried on the lunar reconnaissance orbiter permits quantification of the surface roughness properties of the moon at unprecedented scales and resolution we map lunar surface roughness using a range of parameters median absolute slope both directional along track and bidirectional in two dimensions median differential slope and hurst exponent over baselines ranging from 17 m to 2 7 km we find that the lunar highlands and the mare plains show vastly different roughness properties with subtler variations within mare and highlands most of the surface exhibits fractal like behavior with a single or two different hurst exponents over the given baseline range when a transition exists it typically occurs near the 1 km baseline indicating a significant characteristic spatial scale for competing surface processes the hurst exponent is high within the lunar highlands with a median value of 0 95 and lower in the maria with a median value of 0 76 the median differential slope is a powerful tool for discriminating between roughness units and is useful in characterizing among other things the ejecta surrounding large basins particularly orientale as well as the ray systems surrounding young copernican age craters in addition it allows a quantitative exploration on mare surfaces of the evolution of surface roughness with age

comparison of areas in shadow from imaging and altimetry in the north polar region of mercury and implications for polar ice deposits earth based radar observations and results from the messenger mission have provided strong evidence that permanently shadowed regions near mercury s poles host deposits of water ice messenger s complete orbital image and topographic datasets enable mercury s surface to be observed and modeled under an extensive range of illumination conditions the shadowed regions of mercury s north polar region from 65 n to 90 n were mapped by analyzing mercury dual imaging system mdis images and by modeling illumination with mercury laser altimeter mla topographic data the two independent methods produced strong agreement in identifying shadowed areas all large radar bright deposits those hosted within impact craters 6 km in diameter collocate with regions of shadow identified by both methods however only 46 of the persistently shadowed areas determined from images and 43 of the permanently shadowed areas derived from altimetry host radar bright materials some sizable regions of shadow that do not host radar bright deposits experience thermal conditions similar to those that do the shadowed craters that lack radar bright materials show a relation with longitude that is not related to the thermal environment suggesting that the earth based radar observations of these locations may have been limited by viewing geometry but it is also possible that water ice in these locations is insulated by anomalously thick lag deposits or that these shadowed regions do not host water ice

deformation associated with ghost craters and basins in volcanic smooth plains on mercury strain analysis and implications for plains evolution 1 since its insertion into orbit about mercury in march 2011 the messenger spacecraft has imaged most previously unseen regions of the planet in unprecedented detail revealing extensive regions of contiguous smooth plains at high northern latitudes and surrounding the caloris basin these smooth plains thought to be emplaced by flood volcanism are populated with several hundred ghost craters and basins nearly to completely buried impact features having rims for which the surface expressions are now primarily rings of deformational landforms associated with some ghost craters are interior groups of graben displaying mostly polygonal patterns the origin of these graben is not yet fully understood but comparison with numerical models suggests that the majority of such features are the result of stresses from local thermal contraction in this paper we highlight a previously unreported category of ghost craters quantify extensional strains across graben bearing ghost craters and make use of graben geometries to gain insights into the subsurface geology of smooth plains areas in particular the style and mechanisms of graben development imply that flooding of impact craters and basins led to substantial pooling of lavas to thicknesses of 1 5 km in addition surface strains derived from groups of graben are generally in agreement with theoretically and numerically derived strains for thermal contraction

the global surface roughness of 25143 itokawa abstract surface roughness is an important metric in understanding how the geologic history of an asteroid affects its small scale topography and it provides an additional means to quantitatively compare one asteroid with another in this study we report the first detailed global surface roughness maps of 25143 itokawa at horizontal scales from 8 32 m comparison of the spatial distribution of the surface roughness of itokawa with 433 eros the other asteroid for which this kind of analysis has been possible indicates that the two asteroids are dominated by different geologic processes on itokawa the surface roughness reflects the results of down slope activity that moves fine grained material into geopotential lows and leaves large blocks in geopotential highs on 433 eros the surface roughness is controlled by geologically recent large impact craters in addition large longitudinal spatial variations of surface roughness could impact the role of yorp on itokawa