investigating the origin of candidate lava channels on mercury with messenger data theory and observations received 12 april 2012 revised 31 august 2012 accepted 1 november 2012 published 31 march 2013 1 volcanic plains identified on mercury are morphologically similar to lunar mare plains but lack constructional and erosional features that are prevalent on other terrestrial planetary bodies we analyzed images acquired by the messenger spacecraft to identify features on mercury that may have formed by lava erosion we used analytical models to estimate eruption flux erosion rate and eruption duration to characterize the formation of candidate erosional features and we compared results with analyses of similar features observed on earth the moon and mars results suggest that lava erupting at high effusion rates similar to those required to form the teepee butte member of the columbia river flood basalts 0 1 1 2 10 6 m 3 s 1 would have been necessary to form wide valleys 15km wide observed in mercury s northern hemisphere first by mechanical erosion to remove an upper regolith layer then by thermal erosion once a lower rigid layer was encountered alternatively results suggest that lava erupting at lower effusion rates similar to those predicted to have formed rima prinz on the moon 4400 m 3 s 1 would have been required to form via thermal erosion narrower channels 7km wide observed on mercury although these results indicate how erosion might have occurred on mercury the observed features may have formed by other processes including lava flooding terrain sculpted during the formation of the caloris basin in the case of the wide valleys or impact melt carving channels into impact ejecta in the case of the narrower channels citation hurwitz d m j w head p k byrne z xiao s c solomon m t zuber d e smith and g a neumann 2013 investigating the origin of candidate lava channels on mercury with messenger data theory and observations j geophys res planets 118 471 486 doi 10 1029 2012je004103

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

geologic mapping of ejecta deposits in oppia quadrangle asteroid 4 vesta abstract oppia quadrangle av 10 288 360 e 22 is a junction of key geologic features that preserve a rough history of asteroid 4 vesta and serves as a case study of using geologic mapping to define a relative geologic timescale clear filter images stereo derived topography slope maps and multispectral color ratio images from the framing camera on nasa s dawn spacecraft served as basemaps to create a geologic map and investigate the spatial and temporal relationships of the local stratigraphy geologic mapping reveals the oldest map unit within av 10 is the cratered highlands terrain which possibly represents original crustal material on vesta that was then excavated by one or more impacts to form the basin feralia planitia saturnalia fossae and divalia fossae ridge and trough terrains intersect the wall of feralia planitia indicating that this impact basin is older than both the veneneia and rheasilvia impact structures representing pre veneneian crustal material two of the youngest geologic features in av 10 are lepida 45 km diameter and oppia 40 km diameter impact craters that formed on the northern and southern wall of feralia planitia and each cross cuts a trough terrain the ejecta blanket of oppia is mapped as dark mantle material because it appears dark orange in the framing camera clementine type color ratio image and has a diffuse gradational contact distributed to the south across the rim of rheasilvia mapping of surface material that appears light orange in color in the framing camera clementine type color ratio image as light mantle material supports previous interpretations of an impact ejecta origin some light mantle deposits are easily traced to nearby source craters but other deposits may represent distal ejecta deposits emplaced 5 crater radii away in a microgravity environment

cooling fractures in impact melt deposits on the moon and mercury implications for cooling solely by thermal radiation we study the distribution morphology and geometrical properties of fractures in several young impact melt deposits on the moon and mercury and the ways that these fractures may form from cooling by thermal radiation in each impact melt complex the topography of the underlying terrain determines the orientation of cooling fractures such that interior fractures that formed in the relatively thick interior areas of the melt unit are wider and have a larger spacing than marginal fractures that formed in the relatively thin areas near the unit s margins solid debris entrained in molten deposits provides prefracture flaws that can seed cooling fractures but too much solid debris prevents cooling fractures from growing to macroscopic sizes the appearance of subparallel fractures is mainly caused by subsidence of the deposits during the process of cooling and solidification tensile stresses caused by thermal radiation are large enough to initiate cooling fractures on both the moon and mercury which may represent the initial stage of columnar joints formation but the cooling rate caused solely by thermal radiation is not large enough to form well organized columnar joints that feature polygonal colonnades we therefore propose that thermal conduction and convection are the major contributors in the formation of columnar joints on planetary bodies

long term monitoring of martian gully formation and evolution with mro hirise abstract gully landforms are commonly taken as evidence for surface liquid water in the recent geological history of mars repeat observations with the high resolution imaging science experiment hirise instrument on the mars reconnaissance orbiter demonstrate widespread activity in gullies in the southern hemisphere particularly in those with the freshest morphologies this activity includes substantial channel incision and large scale mass movements and constitutes ongoing gully formation rather than degradation of older landforms new apron deposits that are bright dark and neutrally toned have all been observed the timing of gully activity is seasonally controlled and occurs during the period when seasonal frost is present and defrosting these observations support a model in which currently active gully formation is driven mainly by seasonal co 2 frost gullies in the northern hemisphere are far less active than those in the south this may be due to the current timing of perihelion near the northern winter solstice integrated over time activity like that observed within the past few years appears capable of forming all of the martian gully landforms on timescales of millions of years additionally the current style and rate of activity is able to erase meter to decameter scale surface features that might have been uniquely produced by other processes during the last obliquity high 0 4 ma although it is impossible to rule out a past role for water in the formation of martian gullies a model in which gullies form only through currently active processes with little or no liquid water is consistent with our observations

scaling impact melting and crater dimensions implications for the lunar cratering record abstract the dimensions of large craters formed by impact are controlled to a large extent by gravity whereas the volume of impact melt created during the same event is essentially independent of gravity this differential scaling fosters size dependent changes in the dynamics of impact crater and basin formation as well as in the final morphologies of the resulting structures a variety of such effects can be observed in the lunar cratering record and some predictions can be made on the basis of calculations of impact melting and crater dimensions among them are the following 1 as event magnitude increases the volume of melt created relative to that of the crater will grow and more will be retained inside the rim of the crater or basin 2 the depth of melting will exceed the depth of excavation at diameters that essentially coincide with both the inflection in the depth diameter trend and the simple to complex transition 3 the volume of melt will exceed that of the transient cavity at a cavity diameter on the order of the diameter of the moon this would arguably correspond to a moon melting event 4 small lunar craters only rarely display exterior flows of impact melt because the relatively small volumes of melt created can become choked with clasts increasing the melt s viscosity and chilling it rapidly larger craters and basins should suffer little from such a process 5 deep melting near the projectile s axis of penetration during larger events will yield a progression in cen tral structure morphology with growing event magnitude this sequence should range from single peaks through multiple peaks to peak rings 6 the minimum depth of origin of central peak material should co incide with the maximum depth of melting the main central peak in a crater the size of tycho should have had a preimpact depth of close to 15 km

the gao guenie impact melt breccia sampling a rapidly cooled impact melt dike on an h chondrite asteroid the gao guenie h5 chondrite that fell on burkina faso march 1960 has portions that were impact melted on an h chondrite asteroid at 300 ma and through later impact events in space sent into an earth crossing orbit this article presents a petrographic and electron microprobe analysis of a representative sample of the gao guenie impact melt breccia consisting of a chondritic clast domain quenched melt in contact with chondritic clasts and an igneous textured impact melt domain olivine is predominantly fo80 82 the clast domain contains low ca pyroxene impact melt grown pyroxene is commonly zoned from low ca pyroxene in cores to pigeonite and augite in rims metal troilite orbs in the impact melt domain measure up to 2 mm across the cores of metal orbs in the impact melt domain contain 7 9 wt of ni and are typically surrounded by taenite and ni rich troilite the metallography of metal troilite droplets suggest a stage i cooling rate of order 10 c s 1 for the superheated impact melt the subsolidus stage ii cooling rate for the impact melt breccia could not be determined directly but was presumably fast an analogy between the ni rim gradients in metal of the gao guenie impact melt breccia and the impact melted h6 chondrite orvinio suggests similar cooling rates probably on the order of 5000 40 000 c yr 1 a simple model of conductive heat transfer shows that the gao guenie impact melt breccia may have formed in a melt injection dike 0 5 5 m in width generated during a sizeable impact event on the h chondrite parent asteroid

siwalik mammals of the jammu sub himalaya india an appraisal of their diversity and habitats abstract the red bed sequence of the lower siwaliks around ramnagar and the nagrota formation of the upper siwaliks in the uttarbeni parmandal section are the two highly fossiliferous intervals of the siwaliks of jammu sub himalaya india the lower siwalik mammalian fauna of ramnagar is best represented in its upper 350 m interval small and medium sized forms mostly belonging to browsing species dominate the mammalian assemblage the ramnagar fauna represents the chinji mammalian fauna middle miocene of the potwar plateau pakistan the assemblage indicates a woodland type ecosystem having substantial forest component and subordinately developed grassy subareas the mammalian fauna of the nagrota formation in the uttarbeni parmandal section in contrast exhibits relatively low species diversity but considerable increase in the mean body size and progressive hypsodonty of the cheek teeth in the herbivores this becomes more pronounced above the 2 5 ma old volcanic tuff layers occurring at the middle of the formation broadly a savanna type ecosystem is indicated by the mammalian communities of nagrota formation which appears to have been dominated by grassland communities above the tuff zone the above tuff faunal zone of the nagrota formation can be correlated with the pinjor faunal zone plio pleistocene of the siwalik hills punjab

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

cratering on mars with almost no atmosphere or volatiles pangboche crater pangboche crater 17 2 n 226 7 e 10 4 km dia lies close to the summit of olympus mons volcano mars at an elevation of 20 9 km above the datum given a scale height of 11 1 km for the atmosphere this relatively large fresh crater most likely formed at an atmospheric pressure 1 mbar in essentially volatile free young lava flows detailed analysis of pangboche crater from high resolution imaging science experiment hirise and context camera ctx images reveals that volatile related features e g fluidized ejecta layers and pitted floor material are absent in contrast abundant impact melt occurs on the floor inner walls and rim of the crater and there is an extensive field of secondary craters that extend up to approximately 45 km from the rim crest all of these attributes argue that it was the absence of volatiles in the target rocks at the time of crater formation rather than the thin atmosphere which had a controlling influence on crater morphology digital elevation data derived from the ctx images reveal that pangboche crater has a depth of about 954 m depth diameter approximately 0 092 and that uplifted target rocks comprise about 58 of the relief of the 180 m high north rim as the target material comprised a sequence of layered lava flows pangboche crater may well represent the best crater on mars for direct comparison with craters formed on the moon permitting variations in gravitational effects to be investigated or on mercury allowing the role of the atmosphere to be studied

revisiting the field geology of taurus littrow abstract integration of apollo 17 field observations and photographs sample investigations lunar reconnaissance orbiter camera images chandrayaan 1 moon mineralogy mapper m3 spectra and miniature radio frequency mini rf s band radar images provides new insights into the geology of the valley of taurus littrow on the moon connecting the various remote observations to sample data enables a set of new conclusions to be drawn regarding the geological evolution of the valley structural considerations and published and recalculated 40ar 39ar analyses of samples from the north massif and the sculptured hills indicate that the crisium basin formed about 3 93 ga the serenitatis basin about 3 82 ga and the imbrium basin no earlier than 3 82 ga and no later than the average of 3 72 ga for 33 age dates from samples of the valley s mare basalts strong evidence continues to support the conclusion of others lucchitta 1972 spudis et al 2011 fassett et al 2012 that the sculptured hills physiographic unit consists of imbrium ejecta interpretation of m3 spectral data and apollo 17 samples indicate that rock units of the sculptured hills consist of a largely coherent mg suite pluton lroc nac stereo images and mini rf data indicate the presence of several exposed pyroclastic fissures across the sculptured hills rim boulders at camelot crater constitute nearly in situ wall rocks of that crater rather than ejecta and provide an opportunity for investigations of remanent magnetic field orientation at the time of the eruption of late mare basalt lavas in the valley paleomagnetic field orientation information also may be obtained relative to melt breccia contacts in north massif boulders that suggest original horizontal orientations lroc images indicate the existence of two temporally separate light mantle avalanche deposits the origin potential flow mechanisms and geology of the youngest avalanche from the south massif have been clarified the existence of two distinct light mantle avalanches raises doubt about the association of either light mantle avalanche with secondary impacts related to the tycho impact event alternatively the lee lincoln thrust fault appears to have triggered the second light mantle avalanche between 70 and 110 ma a simple structural analysis shows that this thrust fault dips 20 25 to the southwest where it crosses the north massif and to the west where it crosses the valley floor mini rf data reveal a line of reduced reflections roughly perpendicular to contours on the north massif about 3 km to the east of the lee lincoln fault although this line is possibly an older ancillary fault lroc nac stereo images indicate that it may be best explained as a pyroclastic fissure a debris flow of dark apparent pyroclastic ash lies below the southeast end of the potential fissure finally young lunar impact glass sample 70019 has been precisely located within lroc nac images and oriented for the first time using 60 mm f l sample documentation photographs sample 70019 can now be employed in lunar paleomagnetic field orientation studies

geology of the king crater region new insights into impact melt dynamics on the moon 1 new geologic sketch maps and digital elevation models for king crater and the surrounding lunar farside highlands were created from lunar reconnaissance orbiter camera lroc wide angle camera wac and narrow angle camera nac images nac images reveal that high volume impact melt accumulations exhibit evidence of dynamic processes during and following emplacement that resulted in downwarped zones and other morphologic anomalies visible at the 50 cm pixel scale among the most significant of these forms are negative relief features some of which may represent evidence for near surface caverns offering points of access to subsurface environments and possible shelter from surface hazards other negative relief features may represent regions of extension and separation in response to possible subsurface drainage together with isostatic readjustments contraction and or compaction in the cooling impact melt crater counts on the continuous ejecta blanket suggest a late eratosthenian to copernican age for king crater which is older than the estimate of young 1977 but consistent with those of others

distal ejecta from lunar impacts extensive regions of rocky deposits abstract lunar reconnaissance orbiter lro diviner radiometer mini rf and lro camera data were used to identify and characterize rocky lunar deposits that appear well separated from any potential source crater two regions are described 1 a 18 000 km 2 area with elevated rock abundance and extensive melt ponds and veneers near the antipode of tycho crater 167 5 e 42 5 n this region has been identified previously using radar and imaging data 2 a much larger and more diffuse region covering 730 000 km 2 centered near 310 e 35 s containing elevated rock abundance and numerous granular flow deposits on crater walls the rock distributions in both regions favor certain slope azimuths over others indicating a directional component to the formation of these deposits the spatial distribution of rocks is consistent with the arrival of ejecta from the west and northwest at low angles 10 30 above the horizon in both regions the derived age and slope orientations of the deposits indicate that the deposits likely originated as ejecta from the tycho impact event despite their similar origin the deposits in the two regions show significant differences in the datasets the tycho crater antipode deposit covers a smaller area but the deposits are pervasive and appear to be dominated by impact melts by contrast the nearside deposits cover a much larger area and numerous granular flows were triggered however the features in this region are less prominent with no evidence for the presence of impact melts the two regions appear to be surface expressions of a distant impact event that can modify surfaces across wide regions resulting in a variety of surface morphologies the tycho impact event may only be the most recent manifestation of these processes which likely have played a role in the development of the regolith throughout lunar history

geologic map of the northern hemisphere of vesta based on dawn framing camera fc images abstract the dawn framing camera fc has imaged the northern hemisphere of the asteroid 4 vesta at high spatial resolution and coverage this study represents the first investigation of the overall geology of the northern hemisphere 22 90 n quadrangles av 1 2 3 4 and 5 using these unique dawn mission observations we have compiled a morphologic map and performed crater size frequency distribution csfd measurements to date the geologic units the hemisphere is characterized by a heavily cratered surface with a few highly subdued basins up to 200 km in diameter the most widespread unit is a plateau cratered highland unit similar to although of lower elevation than the equatorial vestalia terra plateau large scale troughs and ridges have regionally affected the surface between 180 e and 270 e these tectonic features are well developed and related to the south pole veneneia impact saturnalia fossae trough unit elsewhere on the hemisphere they are rare and subdued saturnalia fossae cratered unit in these pre rheasilvia units we observed an unexpectedly high frequency of impact craters up to 10 km in diameter whose formation could in part be related to the rheasilvia basin forming event the rheasilvia impact has potentially affected the northern hemisphere also with s n small scale lineations but without covering it with an ejecta blanket post rheasilvia impact craters are small

widespread crater related pitted materials on mars further evidence for the role of target volatiles during the impact process abstract recently acquired high resolution images of martian impact craters provide further evidence for the interaction between subsurface volatiles and the impact cratering process a densely pitted crater related unit has been identified in images of 204 craters from the mars reconnaissance orbiter this sample of craters are nearly equally distributed between the two hemispheres spanning from 53 s to 62 n latitude they range in diameter from 1 to 150 km and are found at elevations between 5 5 to 5 2 km relative to the martian datum the pits are polygonal to quasi circular depressions that often occur in dense clusters and range in size from 10 m to as large as 3 km pit sizes scale with both the host crater s diameter and the host deposit size these pits have subtle raised rims and unlike primary and secondary impact craters they lack well defined ejecta deposits and overlapping stratigraphic relationships they also lack any sign of any preferential alignment expected of volcanic or tectonic collapse features morphologic and stratigraphic evidence in support of an impact origin includes the observation that pitted materials primarily occur as ponded and flow like deposits on crater floors behind terraces and infilling the lowest local topographic depressions atop the ejecta blanket similar to the distribution of impact melt bearing bodies on the moon based on the observations and comparisons to terrestrial and lunar analogs we conclude that the pit bearing materials are impactite deposits the presence of these deposits in older craters where preserved suggests that they have formed on mars throughout most of its geologic history thus understanding their origin may help to constrain the hydrological and climate history of mars

new insight into lunar impact melt mobility from the lro camera 1 the lunar reconnaissance orbiter camera lroc is systematically imaging impact melt deposits in and around lunar craters at meter and sub meter scales these images reveal that lunar impact melts although morphologically similar to terrestrial lava flows of similar size exhibit distinctive features e g erosional channels although generated in a single rapid event the post impact mobility and morphology of lunar impact melts is surprisingly complex we present evidence for multi stage influx of impact melt into flow lobes and crater floor ponds our volume and cooling time estimates for the post emplacement melt movements noted in lroc images suggest that new flows can emerge from melt ponds an extended time period after the impact event

the formation of infilled craters on mars evidence for widespread impact induced decompression of the early martian mantle flat floored craters have long been recognized on mars with early work hypothesizing a sedimentary origin more recently high resolution thermal inertia measurements show that these craters contain some of the rockiest materials on the planet inconsistent with poorly consolidated sedimentary materials in this study the distribution physical properties morphology and thermal inertia and composition of these craters are thoroughly investigated over the entire planet the majority of the 2800 rocky crater floors identified are concentrated in the low albedo 0 1 0 17 cratered southern highlands these craters were infilled at 3 5 ga and are associated with the highest thermal inertia values and some of the most mafic materials identified on the planet although several processes may have led to the formation of the crater floors the most likely scenario is volcanic infilling through fractures created by the impact event the primitive magma source directly results from decompression melting of the martian mantle by the removal of the crustal material excavated by the impactor volcanic infilling of craters by decompression melting appears to only have occurred in early martian history when the lithosphere was still relatively thin and the thermal gradient was high this process was widespread and responsible for the eruption of significant volumes of primitive material inside and likely outside of craters impact induced decompression melting of the martian mantle accounts for the unusual infilling of martian craters and is a widespread planetary process that has gone previously undocumented

emplacement and inflation of pahoehoe sheet flows observations and measurements of active lava flows on kilauea volcano hawaii inflated pahoehoe sheet flows have a distinctive horizontal upper surface which can be several hundred meters across and are bounded by steep monoclinal uplifts the inflated sheet flows we studied ranged from 1 to 5 m in thickness but initially propagated as thin sheets of fluid pahoehoe lava generally 20 30 cm thick individual lobes originated at outbreaks from the inflated front of a prior sheet flow lobe and initially moved rapidly away from their source velocities slowed greatly within hours due to radial spreading and to depletion of lava stored within the source flow as the outward flow velocity decreases cooling promotes rapid crustal growth at first the crust behaves plastically as pahoehoe toes form after the crust attains a thickness of 2 5 cm it behaves more rigidly and develops enough strength to retain incoming lava thus increasing the hydrostatic head at the flow front the increased hydrostatic pressure is distributed evenly through the liquid lava core of the flow resulting in uniform uplift of the entire sheet flow lobe initial uplift rates are rapid flows thicken to 1 m in 1 2 hours but rates decline sharply as crustal thickness increases and as outbreaks occur from the margins of the inflating lobe one flow reached a final thickness of nearly 4 m after 350 hr inflation data define power law curves whereas crustal cooling follows square root of time relationships the combination of data can be used to construct simple models of inflated sheet flows as the flow advances preferred pathways develop in the older portions of the liquid cored flow these pathways can evolve into lavatube systems within a few weeks formation of lava tubes results in highly efficient delivery of lava at velocities of several kilometers per hour to a flow front that may be moving 1 2 orders of magnitude slower if advance of the sheet flow is terminated the tube remains filled with lava that crystallizes in situ rather than draining to form the cave like lava tubes commonly associated with pahoehoe flows inflated sheet flows from kilauea and mauna loa are morphologically similar to some thick icelandic and submarine sheet flows suggesting a similar mechanism of emplacement the planar sheet like geometry of flood basalt flows may also result from inflation of sequentially emplaced flow lobes rather than nearly instantaneous emplacement as literal floods of lava

the initial cooling of pahoehoe flow lobes in this paper we describe a new thermal model for the initial cooling of pahoehoe lava flows the accurate modeling of this initial cooling is important for understanding the formation of the distinctive surface textures on pahoehoe lava flows as well as being the first step in modeling such key pahoehoe emplacement processes as lava flow inflation and lava tube formation this model is constructed from the physical phenomena observed to control the initial cooling of pahoehoe flows and is not an empirical fit to field data we find that the only significant processes are a heat loss by thermal radiation b heat loss by atmospheric convection c heat transport within the flow by conduction with temperature and porosity dependent thermal properties and d the release of latent heat during crystallization the numerical model is better able to reproduce field measurements made in hawai i between 1989 and 1993 than other published thermal models by adjusting one parameter at a time the effect of each of the input parameters on the cooling rate was determined we show that a the surfaces of porous flows cool more quickly than the surfaces of dense flows b the surface cooling is very sensitive to the efficiency of atmospheric convective cooling and c changes in the glass forming tendency of the lava may have observable petrographic and thermal signatures these model results provide a quantitative explanation for the recently observed relationship between the surface cooling rate of pahoehoe lobes and the porosity of those lobes jones 1992 1993 the predicted sensitivity of cooling to atmospheric convection suggests a simple field experiment for verification and the model provides a tool to begin studies of the dynamic crystallization of real lavas future versions of the model can also be made applicable to extraterrestrial submarine silicic and pyroclastic flows

comparison of small lunar landslides and martian gullies some lunar crater wall landslides strongly resemble martian gullies despite the lack of geologically active water on the moon today or in the past the lunar features indicate that alcove channel apron morphology attributed on mars to seepage of liquid water can also form via a dry landslide mechanism therefore a more stringent test than just an alcove channel apron morphology is necessary to differentiate dry landslides from water carved gullies

the lunar crater giordano bruno as seen with optical roughness imagery abstract images acquired by the lro nac allow 3d phase ratio imagery of several areas in the crater giordano bruno this is a new optical remote sensing technique that allows a determination of the optical roughness of the lunar surface our study confirms complicated impact melt movement on the flanks and the floor of the crater in many cases however flow structures seen on the inner wall can be attributed to regolith debris taluses rather than impact melt flows it was found that the whirlpool like formation seen near the western side of the crater giordano bruno has a small central depression that can be interpreted as either a vortex cavity or a feature which resembles such a vortex formed by viscous flows coming from the crater flanks we discuss several features that confirm the young age of the crater concluding however that it was not formed within human history its age is likely on the order of 1 my

structure and origin by injection of lava under surface crust of tumuli lava rises lava rise pits and lava inflation clefts in hawaii tumuli are positive topographic features that are common on hawaiian pahoehoe lava flow fields particularly on shallow slopes and 75 measured examples are presented here to document the size range tumuli form by up tilting of crustal plates without any crustal shortening and are thus distinguished from pressure ridges which are up buckled by laterally directed pressure the axial or star like systems of deep clefts that characterize tumuli are defined here as lava inflation clefts their tips advanced into red hot lava and they widened as uplift proceeded and while the lava crust was thickening flat surfaced uplifts formed like tumuli by injection of lava under a surface crust were previously called pressure plateaus but lava rise is proposed instead the pits that abound among lava rises previously attributed to collapse or subsidence are generally formed because the lava around them rose and the name lava rise pit is proposed unique examples of tumuli and lava rises from which lava drained out under a surface crust 1 5 to 2 5 m thick are described from kilauea caldera these examples show that in tumuli and lava rises the crust floats on considerable bodies of fluid lava and is able to do so because of its higher vesicle content the fluid lava loses many of its gas bubbles during residence beneath the crust the bulk densities of samples from tumuli show a general downward increase the form of the density profile is consistent with the relationship that for any given crustal thickness the density of fluid lava closely matched the average density of that crust suggesting that the lava was stably density stratified it is inferred that stable stratification was regulated by out flows of the more vesicular lava fractions loss of bubbles through the lava inflation clefts and entry of injected lava at its level of neutral buoyancy below the uppermost meter the downward decrease in vesicularity closely conforms with that expected by compression of a uniform mass of gas per unit mass of lava

origin of small pits in martian impact craters we propose a numerical model for the formation of the closely spaced pits found in the thin ejecta related deposits superposed on the floors interior terrace blocks and near rim ejecta blankets of well preserved martian impact craters our model predicts the explosive degassing of water from this pitted material which is assumed to originally be water bearing impact melt rich breccia at the time of deposition this process is analogous to what occurred in the fall out suevite deposits at the ries impact structure in germany at ries impact heating of water bearing target material resulted in the rapid degassing of its water and other volatiles the martian environment plays an important role in enhancing the effects of this degassing by increasing the flow speed of the escaping gas the high flow rate of gas through particulate materials such as suevite tends to quickly form segregation channels or vent pipes similar to those found in the ries deposits these pipes act as conduits for the efficient high speed escape of the gas and small clasts that it entrains escaping gas and entrained clasts abraded and eroded the conduit walls flaring them to form pits above a network of pipes

distribution formation mechanisms and significance of lunar pits abstract lunar reconnaissance orbiter camera images reveal the presence of steep walled pits in mare basalt n 8 impact melt deposits n 221 and highland terrain n 2 pits represent evidence of subsurface voids of unknown extents by analogy with terrestrial counterparts the voids associated with mare pits may extend for hundreds of meters to kilometers in length thereby providing extensive potential habitats and access to subsurface geology because of their small sizes relative to the local equilibrium crater diameters the mare pits are likely to be post flow features rather than volcanic skylights the impact melt pits are indirect evidence both of extensive subsurface movement of impact melt and of exploitable sublunarean voids due to the small sizes of pits mare highland and impact melt and the absolute ages of their host materials it is likely that most pits formed as secondary features

the lunar reconnaissance orbiter mission six years of science and exploration at the moon abstract since entering lunar orbit on june 23 2009 the lunar reconnaissance orbiter lro has made comprehensive measurements of the moon and its environment the seven lro instruments use a variety of primarily remote sensing techniques to obtain a unique set of observations these measurements provide new information regarding the physical properties of the lunar surface the lunar environment and the location of volatiles and other resources scientific interpretation of these observations improves our understanding of the geologic history of the moon its current state and what its history can tell us about the evolution of the solar system scientific results from lro observations overturned existing paradigms and deepened our appreciation of the complex nature of our nearest neighbor this paper summarizes the capabilities measurements and some of the science and exploration results of the first six years of the lro mission

the theoretical plausibility of central pit crater formation via melt drainage central pit craters are seen in large craters on some icy satellites and on mars we investigate the hypothesis that central pits form when impact melt drains into fractures beneath the impact crater for this process to occur the volume of melt generated during the impact the volume of void space in fractures beneath the impact crater and the volume of melt able to drain before the fractures freeze shut all must exceed the volume of the observed central pits we estimate the volume of melt generated using results from previous numerical modeling studies the fracture volume is estimated using gravity anomalies over terrestrial craters to estimate the amount of melt able to drain before freezing we consider flow through plane parallel fractures these calculations all suggest that enough liquid water could drain into fractured ice beneath a crater on ganymede to form a central pit on earth and the moon silicate impact melt will freeze before a large volume is able to drain so we do not expect to see central pits in impact craters in targets with no ice in summary we find our calculations are consistent with observed central pits in craters on ganymede and the lack of central pits in craters on earth and the moon

the emplacement of pahoehoe toes field observations and comparison to laboratory simulations we observed active pahoehoe lobes erupted on kilauea during may june 1996 and found a range of emplacement styles associated with variations in local effusion rate flow velocity and strain rate these emplacement styles were documented and quantified for comparison with earlier laboratory experiments at the lowest effusion rates velocities and strain rates smooth surfaced lobes were emplaced via swelling where new crust formed along an incandescent lip at the front of the lobe and the rest of the lobe was covered with a dark crust at higher effusion rates strain rates and velocities lobes were emplaced through tearing or cracking tearing was characterized by ripping of the ductile crust near the initial breakout point and most of the lobe surface was incandescent during its emplacement this mechanism was observed to generate both smooth surfaced lobes and when the lava encountered an obstacle folded lobes cracking lobes were similar to those emplaced via tearing but involved breaking of a thicker brittle crust at the initial breakout of the lobe and therefore required somewhat higher flow rates than did tearing cracking lobes typically formed ropy folds in the center of the lobe and smooth margins at the highest effusion rates strain rates and flow velocities the lava formed open channels with distinct levees the final lobe morphologies were compared to results from laboratory simulations which were designed to infer effusion rate from final flow morphology to quantitatively test the laboratory results on the scale of individual natural pahoehoe lobes there is general agreement between results from laboratory simulations and natural lavas on the scale of individual pahoehoe lobes but there are disparities between laboratory flows and lava flows on the scale of an entire pahoehoe lava flow field