buried topography of utopia mars persistence of a giant impact depression knobs partially buried craters ring fractures and some mesas permit a qualitative determination of the topography buried beneath younger northern plains materials these features are widely distributed in the utopia area but are absent in a large roughly circular region centered at about 48 degree n 240 degree w this implies the existence of a circular depression about 3 300 km in diameter buried beneath utopia planitia that is here interpreted to represent the central part of a very large impact basin the presence of buried curved massifs around part of this depression and a roughly coincident mascon lend further support present topography areal geology and paleotopography of buried surfaces all point to the persistence of this major depression for almost the entire history of mars

the simud tiu valles hydrologic system a multidisciplinary study of a possible site for future mars on site exploration abstract when looking for traces of past life on mars we have to look primarily for places where water was present possibly for long time intervals the simud and tiu valles are two large outflow channels connected to the north with the chryse basin oxia palus quadrangle the area carved by water during the noachian early hesperian is characterized by a complex geological evolution the geomorphological analysis shows the presence of fluvial and alluvial structures interpreted as fluvial channels and terraces debris flow fronts and short lasting small water flows coexisting with maar diatremes and mud volcanoes several morphological features indicate a change in water flux direction after the main erosive phase during this period water originated from the masursky crater and flown southwards into the hydraotes chaos this phenomenon caused the studied area to become a depocenter where fine grained material deposition took place possibly in association with ponding water this setting is potentially quite valuable as traces of life may have been preserved the presence of water at various times over a period of about 1 ga in the area is corroborated by mineralogical analyses of different areas that indicate the possible presence of hydrated minerals mixtures such as sulfate bearing deposits given the uniqueness of the evolution of this region the long term interactions between fluvial volcanic and tectonic processes and its extremely favorable landing parameters elevation slope roughness rock distribution thermal inertia albedo etc we decided to propose this location as a possible landing site for the esa exomars 2018 the nasa mars 2020 and future on site missions

late precambrian crustal accretion rates in northeast africa and arabia apparent late precambrian growth rates for continental crust in the arabian nubian craton are implausibly high for simple arcaccretion models if correct these rates imply that some special non uniformitarian process occurred to rapidly produce continental crust assuming that the dominant mechanism for continental growth is by arc magmatism roughly 100 of global island arc production at current rates would have had to accrete in this region from 900 to 600 ma one solution to this paradox is that large amounts of preexisting sialic basement have so far gone undetected and should not be included in the estimate for crustal growth during late precambrian time published radiometric data indicate that older continental material makes up the eastern and western margins of the exposed shield we interpret these and other data to suggest that the larger part of the arabian nubian craton including the platform obscured by phanerozoic cover east and west of the exposed shield is older continental crust in this case a growth rate approximately 20 of the modern global rate is required and nonuniformitarian crustal growth is not necessary to explain late precambrian accretion of the arabian nubian craton

gravity inferred crustal attributes of visible and buried impact basins on mars 1 the impact basins of mars reveal important insights on martian tectonic evolution they involve strongly disrupted depressed regions of crust with likely enhanced porosity and permeability that may locally concentrate water and other crustal fluids hence the crustal properties of impact basins can also be important in the hunt for water and related clues for life on mars we assess the crustal details of impact basins by separating the mars global surveyor free air anomalies into terrain correlated and terrain decorrelated components the separation is based on the correlation spectrum between the free air anomalies and the gravity effects evaluated from the topography mapped by the mars orbital laser altimeter mola for topographically visible multiring basins like isidis striking circular patterns of alternating terrain correlated free air maxima and minima mark the uncompensated components of the central mantle plug and surrounding rings the first vertical derivatives of these anomalies effectively estimate the basin ring locations and a transient cavity depth to diameter ratio of 0 09 that is consistent with the ratio observed for lunar nearside multiring basins for the isidis basin we obtain an excavation depth of roughly 66 km that reflects considerably disrupted crust for concentrating local fluids since the noachian impact furthermore the related crustal terrain decorrelated free air anomalies suggest up to 2 km of high density basin fill may cap the central basin subtle quasi circular depressions in the relatively featureless mola terrain of the northern hemisphere have identified potentially buried impact basins frey et al 2002 an altimetry depression in acidalia planitia and another in utopia are also associated with ringed patterns of terrain decorrelated free air anomalies that may mark the uncompensated mass effects of buried impact basins the gravity derived transient excavation depths for these inferred basins are roughly 41 and 20 km respectively while the related ring diameters d follow the ubiquitous d rule of planetary impact basins the crust of these buried basins is likely to contain water at higher levels than the crust of the equatorial basins that was substantially dewatered with the development of the great northern basin hence analyzing surface fractures faults and other crustal discontinuities that can tap the more saturated crust of these inferred buried basins may facilitate the hunt for water on mars

assessment of global and regional tectonic models for faulting in the ancient terrains of mars structural landforms within parts of the heavily cratered terrain of mars were studied in order to test various models of global fracturing landforms analyzed were scarps grabens simple ridges and channel wall scarps many of these landforms predate graben formation in tharsis very few structural landforms were found to be either radial to tharsis or normal to principal stress trajectories calculated for tharsis isostasy or flexural loading these results suggest either that the structures are locally derived or if they were produced by tharsis activity that the state of stress in tharsis was different from that which produced the classic graben systems association of narrow valley networks with subdued grabens in one portion of margaritifer sinus suggests that one episode of narrow valley formation coincided with flexural loading in tharsis crustal fracturing in oxia palus that predates outflow channel formation does not correlate with tharsis tectonism and may reflect fracturing by the proposed chryse multiring basin to the north in contrast to previous studies no evidence was found for either an orthogonal martian grid or conjugate sets of shear fractures it is concluded that mars probably did not experience significant tidal despinning calculations of thermal stress within a cooling early lithosphere performed for varying lithospheric thicknesses predict large scale tensile near surface failure many of the large ridges in memnonia that predate classical tharsis centered tectonism may reflect normal faulting generated in part by thermal stress

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

polygonal impact craters in the argyre region mars evidence for influence of target structure on the final crater morphology impact craters that in plan view are distinctly polygonal rather than circular or elliptical are common on mars and other planets ohman et al 2005 their actual formation mechanism however is somewhat debatable we studied the polygonal craters of different degradational stages in the region of the argyre impact basin mars the results show that in the same areas heavily degraded moderately degraded and fresh polygonal craters display statistically similar strike distributions of the straight rim segments the fact that the strike distributions are not dependent on lighting conditions was verified by using two data sets viking and moc wa having different illumination geometries but similar resolutions in addition there are no significant differences in the amount of polygonality of craters in different degradational stages these results clearly imply that large scale polygonality is not caused by degradation but originates from the cratering process itself concurring with the findings regarding lunar craters by eppler et al 1983 the straight rims of polygonal craters apparently reflect areal fracture patterns that prevail for a geologically long time

erosional valleys in the thaumasia region of mars hydrothermal and seismic origins analysis of erosional valleys geologic materials and features and topography through time in the thaumasia region of mars using co registered digital spatial data sets reveals significant associations that relate to valley origin valleys tend to originate 1 on noachian to early hesperian stages 1 and 2 large volcanoes 2 within 50 100 km of stages 1 and 2 rift systems and 3 within 100 km of noachian stage 1 impact craters 50 km in diameter these geologic preferences explain observations of higher valley source densities vsds in areas of higher elevations and regional slopes 1 because the volcanoes rifts and craters form high steep topography or occur in terrain of high relief other stage 1 and stage 2 high steep terrains however do not show high vsds the tendency for valleys to concentrate near geologic features and the overall low drainage densities in thaumasia compared to terrestrial surfaces rule out widespread precipitation as a major factor in valley formation as is proposed in wann wet climate scenarios except perhaps during the early noachian for which much of the geologic record has been obliterated instead volcanoes and rifts may indicate the presence of shallow crustal intrusions that could lead to local hydrothermal circulation melting of ground ice and snow and groundwater sapping however impact crater melt would provide a heat source at the surface that might drive away water fonning valleys in the process post stage 1 craters mostly have low nearby vsds which for valleys incised in older rocks suggests burial by ejecta and for younger valleys may indicate desiccation of near surface water and deepening of the cryosphere later hesperian and amazonian stages 3 and 4 valleys originate within 100 200 km of three young large impact craters and near rifts systems at warrego valles and the southern part of coprates rise these valleys likely developed when the cryosphere was a couple kilometers or more thick inhibiting valley development by hydrothermal circulation however eruption of groundwater may have occurred from impact induced fracturing and lateral and perhaps minor upward transport of water due to seismic pumping the two smaller craters formed along the plateau margin where the highest potential hydraulic head would occur in aquifers beneath the plateau in the case of the larger crater lowell 200 km in diameter potential aquifers would likely be at depths of kilometers below the cryosphere seismic energy generated by the lowell impactor would have been much greater pumping both groundwater and perhaps fluidized slurry to the surface from beneath the cryosphere to form the young valleys and flow deposit along the margin of thaumasia tectonic pressurization of groundwater also may have contributed to valley formation dissection of rim materials of the argyre impact may relate to tectonic activity and the unconsolidated state of basin ejecta

geology of central chryse planitia and the viking 1 landing site implications for the mars pathfinder mission 1 500 000 scale geologic mapping in the central chryse planitia region of mars was correlated with ground truth data gathered by the viking 1 lander materials within the chryse basin can be subdivided into plains and channel units that are typically separated from one another by gradational contacts hesperian ridged plains materials unit 1 hr1 are the oldest materials mapped typically these materials contain numerous fresh impact craters and have sharply defined mare like wrinkle ridges similar to those appearing on the lunar maria these materials grade into hesperian ridged plains materials unit 2 hr2 which are characterized by buried and eroded impact craters and subdued wrinkle ridges from analyses of crater age dates and their associated geologic contacts channel materials appear to have formed at the same time as hr2 materials and it is likely both units represent fluvial sediments measurements of buried craters contained in hr2 materials suggest that in places this unit may be 50 m thick but crater size frequency distribution curves suggest that the areal average may be closer to 170 m based on these observations our interpretation is that hr2 materials were deposited into a standing body of water during channel formation this interpretation implies that many of the rocks visible in the viking 1 lander images were emplaced by fluvial processes possibly finer grained sediments remained in suspension and were subsequently transported out of chryse planitia and into the northern plains during draining of the ponded water east west trending surface undulations visible in lander views toward the south may represent aeolian dunes lava flow fronts or sediment waves formed at the bottom of the standing body of water broad physiographic units seen at the surface are not clearly visible in viking orbiter images however they can be projected at the resolution of the orbiter images these units show that concentrations of drift materials are oriented in a northwesterly direction contrary to the strongest prevailing wind direction which is toward the northeast these materials were probably deposited on ridged plains materials unit 2 during a period of time when aeolian processes were more active in the region both earth based radar and viking thermal data suggest that the mars pathfinder landing site will be similar geologically to the viking 1 site if this is true then the mars pathfinder mission provides the opportunity for building directly on results of the viking program some of the outstanding questions that mars pathfinder may be able to address include determining the aeolian modification history of the chryse planitia region the degree and possibly the relative rate of sediment induration the fraction of rocks and boulders emplaced by impact processes the possibility that some materials are the result of in situ weathering and whether materials were emplaced by fluvial processes and the associated depositional environment

structure and evolution of the hengill grensdalur volcanic complex iceland geology geophysics and seismic tomography recent geological and geophysical research indicates the presence of three volcanic systems in the hengill grensdalur area and progressive westerly migration of the accretionary plate boundary the hengill system comprises the active hengill central volcano and a nne trending fissure swarm that is the present locus of crustal accretion in the area extinct volcanic systems are identified with the mountain hromundartindur and the grensdalur area and associated nne trending zones the grensdalur system was formerly the locus of accretion in the area a widespread geothermal area encompasses the whole area and is fueled by at least three distinct heat sources associated with the three volcanic systems a tomographic study of the upper 5 km of crust using local earthquakes imaged three bodies with velocities up to 15 higher than the average background velocities and volumes of several tens of cubic kilometers one of these underlies the grensdalur volcano and a second underlies the olkelduhals area within the hromundartindur system these are interpreted as intrusions that are the solidified magma reservoirs of their respective volcanic systems and the heat sources of those parts of the geothermal area above them the third high velocity body underlies the extinct basalt shield husmuli which is not associated with geothermal resources that body is interpreted as a cold intrusion that is the frozen magma conduit that fed the surface eruptive site a low velocity body with a volume of a few cubic kilometers was imaged in the depth range 2 4 km beneath the northern part of the presently active hengill central volcano this volume may contain partial melt and represent the heat source or part of the heat source fueling the hengill field heat balance calculations show that subsurface magmatism continued in the extinct grensdalur volcano long after volcanic activity and crustal accretion migrated from it to the hengill system

discovery of a 450 km diameter multi ring basin on mars through analysis of mola topographic data mars orbiter laser altimeter mola topographic data have revealed a previously unknown 450 km wide 2 km deep basin centered at 30n 312w near the phison rupes this basin as large as and deeper than the obvious cassini impact basin located 1000 km to the sw is not apparent in the existing but good quality viking imagery gridded mola data show the feature as a closed depression based on analysis of slope breaks readily visible in two mola profiles we suggest this phison rupes basin has three topographic rings with diameters approximately 350 455 and 670 km these rings outline a region of lower impact crater density and smoother inter crater plains similar previously unknown features may exist elsewhere on mars and mola topographic data may be able to locate them

radar altimetry of large martian craters abstract mars topography in the 23 to 22 latitude band has been measured by the goldstone 13 cm radar during the 1971 1982 oppositions a total of 108 scans were produced each scan spans several tens of degrees in longitude the longest scan spans 133 8000 the vertical resolution generally is better than 200 m with some exceptions the areal resolution is 0 16 8 km in longitude and 1 30 80 km in latitude on regional scales elevation differences of as much as 3 km over less than 10 longitude have been observed aeolis memnonia most local relief is provided by structural elements of impact craters one hundred fifty two complex degraded craters some as small as 25 km in diameter have been identified in this data set repeated scans of the same craters yield nearly identical elevation profiles measured crater depths rarely exceed 2 5 km depths of the most degraded craters in the sample are a weak function of crater diameter depths of the largest craters d 125km show no functional relationship to diameters measured rim heights rarely exceed several hundred meters structures with no discernible rim e g ladon have retained a basin shape low exterior relief rather than overall low relief seems to be the distinguishing characteristic of large degraded craters included in the goldstone sample craters or basins that serve as sinks for major channel systems e g gusev ladon appear to be as deep as comparable structures not associated with such systems suggesting that processes other than fluvial sedimentation contributed to crater shallowing

global and regional ridge patterns on mars abstract over 16 000 wrinkle ridges on mars have been classified mapped and digitized to provide a large computer accessible data base for analyzing regions subjected to possible compressive stresses the survey has revealed major compressive structures that occur well beyond the tharsis dominated hemisphere the large variety of affected geologic terrains indicate stresses not simply localized in the intercrater plains one major area of inferred compression occurs in the southern cratered highlands near longitude 180 w where major ridges and scarps extend over 3000 km the occurrence and orientation of many ridges are locally controlled by ancient impact basins the chryse basin in particular has an important effect on ridges in northern lunae planum the removal of all basin concentric ridges reveals however a complex global pattern although such patterns may yet be controlled by heavily degraded impact basins major regional trends also emerge that appear to require broader scale global stresses most ridges in the western hemisphere are shown to be orthogonal to three centers corresponding to tharsis 1 n 122 w syria 12 s 104 w and chryse 19 n 47 w ridges not included in these three sets are generally more random and highly localized most but by no means all ridges in the eastern hemisphere are controlled by the hellas and isidis basins a simple global grid is not yet identifiable and may not be recognizable owing to the large number of regional patterns

a lake in uzboi vallis and implications for late noachian early hesperian climate on mars abstract uzboi vallis centered at 28 s 323 e is 400 km long and comprises the southernmost segment of the northward draining uzboi ladon morava ulm meso scale outflow system that emerges from argyre basin bond and holden craters blocked the valley to the south and north respectively forming a late noachian to hesperian paleolake basin that exceeded 4000 km 3 limited crism data suggest lake deposits in uzboi and underlying basin floor incorporate relatively more mg clays and more fe clays respectively the short lived lake overflowed and breached holden crater s rim at an elevation of 350 m and rapidly drained into the crater fan deltas in holden extend 25 km from the breach and incorporate meter sized blocks and longitudinal grooves along the uzboi basin floor are hundreds of meters long and average 60 m wide suggesting high discharge drainage of the lake precipitation derived runoff rather than regional groundwater or overflow from argyre dominated contributions to the uzboi lake although the failure of most tributaries to respond to a lowering of base level indicates their incision largely ended when the lake drained the uzboi lake may have coincided with alluvial and or lacustrine activity in holden eberswalde and other craters in southern margaritifer terra where fluvial lacustrine activity may have required widespread synoptic precipitation rain or snow perhaps associated with an ephemeral global hydrologic system during the late noachian into the hesperian on mars

the martian hemispheric dichotomy may be due to a giant impact mars is divided into two fundamentally different geological provinces of approximately hemispherical extent1 3 the more southerly province is heavily cratered contains relatively old geological units and superficially resembles the lunar and mercurian highlands the northern province is relatively lightly cratered and contains younger geological units including extensive plains volcanic edifices and volcanic calderas except for the tharsis and elysium regions and other large volcanoes most of the younger northern province consists of lowlands which lie an average of 3 km below the highlands lowlands occupy about one third of mars they are separated from the highlands by a distinct scarp or by a sloping transitional zone as much as 700km wide in which highland materials have been disrupted and partly replaced by lowland deposits fig 1 the transition is characterized by a variety of landforms unknown on other planets the highlands and lowlands are in isostatic equilibrium across the transitional zone4 no generally accepted explanation for the cause of the highland lowland dichotomy has been proposed although thinning of the lithosph re in the northern hemisphere by internal processes has been suggested5 large impacts which were a feature of early solar system history have also been suggested as an important cause of such early planetary surface heterogeneities3 6 7 subcircular lowlands such as argyre chryse hellas and isidis planitia and the plains south of tharsis occupy previously recognized impact basins8 10 we propose here that the largest expanse of lowlands also results from formation of a large impact basin early in mars history that has profoundly shaped the planet s surface

paleohydrology of eberswalde crater mars abstract eberswalde crater mars contains a well preserved fluvial distributary network in a likely deltaic setting the meandering inverted paleochannels and closed drainage basin of this deposit support relatively well constrained estimates of channel forming discharge over an individual event flood timescale runoff production event and annual timescales and longevity of deposition geologic timescale during the late hesperian to early amazonian epochs the width and meander dimensions of two inverted paleochannels reflect the channel forming discharge from event floods 200 to 400 m 3 s the deposit surface indicates the level 1400 to 1350 m and surface area 410 to 810 km 2 of the likely paleolake and the topography and mapped extent of tributaries constrain the watershed area 5000 to 17 000 km 2 based on these results and terrestrial empirical constraints on evaporation and sediment concentration we evaluated three hypothetical water sources meltwater liberated by the nearby holden crater impact continuous deposition over 10 1 10 2 years intermittent rainfall or snowmelt during finite periods controlled by orbital evolution deposition over 10 4 10 6 years and highly infrequent runoff or melting of accumulated snowpacks following distant impacts or secular changes in orbital parameters local impact generated runoff and highly infrequent rainfall or snowmelt require unreasonably high and low rates of evaporation respectively to maintain the paleolake level the local impact hypothesis alternatively depends on one flooding episode with very high concentrations of fluvial sediment that are inconsistent with morphologic considerations multiple primary impact craters in the area postdate holden ejecta but were later dissected indicating fluvial erosion long after the holden impact intermittent rainfall of 1 cm day and seasonal snowmelt are both consistent with our results over a deposition timescale totaling 10 4 10 6 years

evidence for and implications of sedimentary diapirism and mud volcanism in the southern utopia highland lowland boundary plain mars abstract several types of spatially associated landforms in the southern utopia planitia highland lowland boundary hlb plain appear to have resulted from localized geologic activity including 1 fractured rises 2 elliptical mounds 3 pitted cones with emanating lobate materials and 4 isolated and coalesced cavi depressions stratigraphic analysis indicates these features are hesperian or younger and may be associated with resurfacing that preferentially destroyed smaller 8 km diameter impact craters based on landform geomorphologies and spatial distributions the documented features do not appear to be specifically related to igneous or periglacial processes or the back wasting and erosion of the hlb scarp we propose that these features are genetically related to and formed by sedimentary mud diapirs that ascended from zones of regionally confined poorly consolidated and mechanically weak material we note morphologic similarities between the mounds and pitted cones of the southern utopia boundary plain and terrestrial mud volcanoes in the absheron peninsula azerbaijan these analogs provide a context for understanding the geological environments and processes that supported mud diapir related modification of the hlb in southern utopia mud diapirs near the elysium volcanic edifice may have resulted in laccolith like intrusions that produced the fractured rises while in the central boundary plain mud diapirs could have extruded to form pitted cones mounds and lobate flows perhaps related to compressional stresses that account for wrinkle ridges the removal of material a few kilometers deep by diapiric processes may have resulted in subsidence and deformation of surface materials to form widespread cavi collectively these inferences suggest that sedimentary diapirism and mud volcanism as well as related surface deformations could have been the dominant hesperian mechanisms that altered the regional boundary plain we discuss a model in which detritus would have accumulated thickly in the annular spaces between impact generated structural rings of utopia basin we envision that these materials and perhaps buried ejecta of utopia basin contained volatile rich low density material that could provide the source material for the postulated sedimentary diapirs thick water rich low density sediments buried elsewhere along the hlb and within the lowland plains may account for similar landforms and resurfacing histories

evidence for igneous activity and implications for the origin of a fretted channel in southern ismenius lacus mars northern arabia terra mars is characterized by a complex landscape that includes fretted terrain fretted channels completely closed canyons shallow troughs and fracture bounded depressions the importance of ground ice in the initiation and development of these features and of large impact structures in controlling channel courses has long been recognized the development of the fretted channel systems including the integration of closed canyons craters and surface depressions into throughgoing channels has previously been attributed primarily to mass wasting processes the two channel systems in southern ismenius lacus quadrangle mamers valles and ismeniae fossae incorporate craters of probable impact origin but both also include irregular crater like features and scalloped channel reaches that have characteristics not consistent with an impact origin some of these are by analogy with morphologically similar features on earth and elsewhere on mars reasonably interpreted as of volcanic origin the detailed morphologies of a few diagnostic features associated with ismeniae fossae suggest a hydrovolcanic origin an inference supported by the probable presence of abundant ground ice or groundwater at the time of their formation the evidence described here for hydrovolcanism supports the inference that in addition to mass wasting processes explosive excavation intrusion driven sublimation subsurface erosion and erosion by running water may have played important roles in channel formation and evolution as well all of these processes are consistent with magma interacting with groundwater or ground ice

tomographic inversion for the three dimensional seismic velocity structure of the aswan region egypt the thurber iterative simultaneous inversion program is used to determine the three dimensional p wave velocity structure in the aswan seismic region of egypt the tomographic inversion presented in this study is based on 1131 p phase observations at 13 stations from 89 local earthquakes all of which occurred within the kalabsha fault zone the assumed initial velocity model is that deduced from local explosion experiments the results indicate that the aswan region is characterized by a heterogeneous crust consisting of a shallow low velocity zone and a deeper high velocity anomaly seismic velocity structure within the shallow part demonstrates that the inferred change in velocity exists primarily across the east west trending kalabsha fault scarp whereas the high velocity zone is located south of this fault two well resolved low velocity zones appear within the upper 6 km of the crust the first coincides with a graben structure located between the kalabsha and seiyal faults and the second exists between the n s kurkur fault and the main axis of lake aswan both low velocity zones occupy an area of approximately 30 40 km located along the western bank of the lake the most significant result of this study is that the location of the deeper high velocity anomaly coincides with the concentration of seismic activity in the lower crustal layer

are terrestrial plumes from motionless plates analogues to martian plumes feeding the giant shield volcanoes the near one plate planet evolution of mars has led to the edification of long lasting giant shied volcanoes unlike the earth mars would have been a transient convecting planet where plate tectonic would have possibly acted only during the first hundreds of million years of its history on earth where plate tectonic is active most of them are regenerated and recycled through convection however the nubian and antarctic plates could be considered as poorly mobile surfaces of various thicknesses that are acting as conductive lids on top of earth s deeper convective system in these environments volcanoes do not show any linear age progression at least for the last 30 ma but constitute the sites of persistent focused long term magmatic activity rather than a chain of volcanoes as observed in fast moving plate plume environments here the near stationary absolute plate motion probably exerts a primary control on volcanic processes and more specifically on the melting ones the residual depleted mantle that is left behind by the melting processes cannot be swept away from the melting locus over time the thickening of this near stationary depleted layer progressively forces the termination of melting to higher depths reducing the melt production rate such a process gradually leads both to decreasing efficient melt extraction and increasing mantle lithospheric melt interactions the accumulation of this refractory material also causes long term fluctuations of the volcanic activity in generating long periods of quiescence the presence of this residual mantle keel induces over time a lateral flow deflection which translates into a shift of future melting sites around it this process gives rise to the horseshoe like shape of some volcanic islands on slow moving plates e g cape verde crozet finally the pronounced topographic swells bulges observed in this environments may also be supported both by large scale mantle upwelling and their residual mantle roots most of these processes are likely similar to those observed on martian giant shield volcanoes the goal of this presentation will be to describe the essential characteristics of intra oceanic plumes on slow moving plates on the earth and to point out their similarities with those of the large shield volcanoes from the tharsis region

martian mud volcanism terrestrial analogs and implications for formational scenarios the geology of mars and the stratigraphic characteristics of its uppermost crust mega regolith suggest that some of the pervasively occurring pitted cones mounds and flows may have formed through processes akin to terrestrial mud volcanism a comparison of terrestrial mud volcanism suggests that equivalent martian processes likely required discrete sedimentary depocenters volatile enriched strata buried rheological instabilities and a mechanism of destabilization to initiate subsurface flow we outline five formational scenarios whereby martian mud volcanism might have occurred a rapid deposition of sediments b volcano induced destabilization c tectonic shortening d long term load induced subsidence and e seismic shaking we describe locations within and around the martian northern plains that broadly fit the geological context of these scenarios and which contain mud volcano like landforms we compare terrestrial and martian satellite images and examine the geological settings of mud volcano provinces on earth in order to describe potential target areas for piercement structures on mars our comparisons help to evaluate not only the role of water as a functional component of geological processes on mars but also how martian mud volcanoes could provide samples of otherwise inaccessible strata some of which could contain astrobiological evidence

noachian and hesperian modification of the original chryse impact basin topography we propose a new center and ring assignment for the original chryse impact basin based upon photogeologic mapping of noachian outcrops and re examination of the published geology using orthographic projections while others have centered the chryse impact on the topographic low associated with hesperian volcanic and fluvial deposits we suggest that the center of the noachian age excavation cavity was located approximately 800 km to the north and that the basin topography was modified significantly from the noachian into the hesperian evolution of the topographic low included structural modification by a later impact centered in acidalia restricted volcanic deposition and loading localized subsidence and restricted deposition from the circum chryse outflow channels

the origin of fluvial valleys and early geologic history aeolis quadrangle mars in southern aeolis quadrangle in eastern mars parallel slope valleys flat floored branching valleys v shaped branching valleys and flat floored straight canyons dissect the heavily cratered plateau sequence associated knife like ridges are interpreted as fissure eruption vents and thin dark stratiform outcrops are interpreted as exhumed igneous sills or lava flows ridged lava plains are also common but are not themselves modified by fluvial processes i mapped 56 asymmetric scarps or ridges that are probable thrust faults these faults exhibit an orientation vector mean of n63 w 11 95 confidence interval and they transect the lava plains and the older plateau sequence units by comparison the vector mean for the 264 valleys mapped is n48 w 12 with a larger dispersion about the mean the similar orientations displayed by thrust fault and valley axes suggest that valley locations are partly controlled by preexisting thrust faults and related fracture systems most valleys are also arranged orthogonally to and along the perimeter of the ridged plains a possible model for valley development is 1 freshly outgassed water became entombed as frost snow and ice within the cratered terrains during heavy bombardment and the accompanying deposition of impact ejecta volcanic ash and eolian materials 2 effusive volcanism and lava sill emplacement heated subsurface ice in the vicinity of the ridged plains and faults and fractures provided zones of increased permeability for water transport to the surface and 3 headward sapping at thermal springs thermokarst subsidence and limited downvalley fluid flows then carved and modified the valleys

related magma ice interactions possible origins of chasmata chaos and surface materials in xanthe margaritifer and meridiani terrae mars abstract we examine here the close spatial and temporal associations among several unique features of xanthe and margaritifer terrae specifically the valles marineris troughs or chasmata and their interior deposits chaotic terrain the circum chryse outflow channels and the subdued cratered material that covers xanthe margaritifer and meridiani terrae though previous hypotheses have attempted to explain the origin of individual features or subsets of these we suggest that they may all be related all of these features taken together present a consistent scenario that includes the processes of sub ice volcanism and other magma ice interactions results of intrusive events during late noachian to early amazonian times

symmetries of volcanic distributions on mars and earth and their mantle plume dynamics the symmetries in the distribution of major volcanic centers on mars have been analyzed by employing a technique involving the angular self similarity of point fields the distribution of all central vent volcanoes exhibits an axisymmetry along with a reflecting symmetry about a plane whose normal is the axis of axisymmetry the latter axis is skewed by about 30 from the rotational axis and is suggested to represent the early axis of symmetry of the interior dynamics the high level of symmetry of mars volcanic distribution may be the result of a focusing of seismic energy from a major impact in the hellas area that formed the hellas basin and may have triggered the onset of volcanic activity in the greater tharsis area if the highland paterae near the hellas region are not taken into account in the analysis an asymmetry with respect to the reflecting plane appears this is probably representative of the late hesperian and amazonian when the once global volcanic activity had retreated to the tharsis province these angular symmetries suggest a simple pattern of long wavelength axisymmetric martian mantle convection involving a large stationary plume as shown by our three dimensional 3 d spherical shell simulations on the basis of these calculations we venture to propose that the volcanic history of mars reflects the transformation of a bipolar mantle plume structure into a single megaplume by the process of plume plume collision acting in concert with the deep martian phase transitions this transformation might have overcome the original pattern induced by a major impact forming the hellas basin we have repeated the procedure for the earth s hotspot distribution which revealed both axisymmetry and a certain level of reflecting symmetry this similarity between the two planets is striking the symmetries of the earth s hotspot distribution are probably due to the interaction of the lower mantle megaplumes with the endothermic perovskite to spinel phase transition

speculations on the origin and evolution of the utopia elysium lowlands of mars the origin of the overall low and variable topographic character of the northern plains is a fundamental aspect of the martian crustal dichotomy a model for the development of the utopia elysium lowlands in eastern mars in terms of the long term evolution of large overlapping impact basins is presented the effects of relaxation of basin topography volcanic flooding and loading of the basins with subsequent subsidence and downwarping and the growth of a long lived major tectonovolcanic center in the overlap region between the proposed utopia and elysium impact basins are qualitatively described in an early hot thin 106 years the later utopia basin impact drastically altered the structure of the earlier elysium basin uplifting portions of what had been the central depression the heating and fracturing due to the impacts was most intense in the region of overlap which probably provided an easy conduit for volcanic eruptions in this area loading by early hesperian and older basin volcanics caused widespread subsidence which may have extended beyond the basin rim and which was responsible for the generally low character of the northern plains despite the minimum 2 3 km volcanic fill present there prolonged volcanism through the early amazonian would likely keep the overlap region hotter longer opposing later general subsidence of the impact basin floor where it had cooled slower cooling and thickening of the lithosphere in the overlap region is consistent with lower estimates of the early middle amazonian elastic lithosphere thickness 50 km in central elysium by comparison with inferred greater thickness in the early hesperian near the edge of the utopia basin at isidis 120 km prolonged volcanic construction in the overlap region was also probably responsible for the great height of the elysium tectonovolcanic complex by comparison with the other portions of the utopia elysium lowland where most volcanics are hesperian in age we suggest this model may be applicable to the origin of the northern lowlands elsewhere on mars and that overlapping large impacts may be important in the establishment of the fundamental topographic and crustal dichtomy and the origin of major tectonovolcanic complexes on the planet

catastrophic ice lake collapse in aram chaos mars hesperian chaotic terrains have been recognized as the source of outflow channels formed by catastrophic outflows four main scenarios have been proposed for the formation of chaotic terrains that involve different amounts of water and single or multiple outflow events here we test these scenarios with morphological and structural analyses of imagery and elevation data for aram chaos in conjunction with numerical modeling of the morphological evolution of the catastrophic carving of the outflow valley the morphological and geological analyses of aram chaos suggest large scale collapse and subsidence 1500 m of the entire area which is consistent with a massive expulsion of liquid water from the subsurface in one single event the combined observations suggest a complex process starting with the outflow of water from two small channels followed by continuous groundwater sapping and headward erosion and ending with a catastrophic lake rim collapse and carving of the aram valley which is synchronous with the 2 5 ga stage of the ares vallis formation the water volume and formative time scale required to carve the aram channels indicate that a single rapid maximum tens of days and catastrophic flood volume of 9 3 104 km3 event carved the outflow channel we conclude that a sub ice lake collapse model can best explain the features of the aram chaos valley system as well as the time scale required for its formation

origin of the martian global dichotomy by crustal thinning in the late noachian or early hesperian the marked dichotomy in topography surface age and crustal thickness between the northern lowland and southern upland of mars has been explained as due to an initially inhomogeneous crust a single mega impact event several overlapping large basin impacts and first order convective overturn of the martian mantle all of the published hypotheses propose that the dichotomy was formed early in martian history before the end of the primordial heavy bombardment a primordial origin is inherent in the initial crustal inhomogeneity hypothesis and required for both impact hypotheses endogenic hypotheses are not so constrained geological data indicate episodes of fracturing and faulting in the late noachian and the early hesperian this fracturing and faulting occurred primarily within the northern lowland and along the boundary between lowland and highland igneous activity also peaked in the late noachian and early hesperian these data suggest a tectonic event near the noachian hesperian boundary characterized by enhanced heat loss and extensive fracturing including formation of the faults that define much of the highland lowland boundary we argue that the major result of this tectonic event was formation of the dichotomy by thinning of the crust above a large convection cell or plume

the circum chryse region as a possible example of a hydrologic cycle on mars geologic observations and theoretical evaluation the transection and superposition relationships among channels chaos surface materials units and other features in the circum chryse region of mars were used to evaluate relative age relationships and evolution of flood events channels and chaos in contact with one another were treated as single discrete flood carved systems some outflow channel systems form networks and are inferred to have been created by multiple flood events within some outflow channel networks several separate individual channel systems can be traced to a specific chaos which acted as flood source area to that specific flood channel individual flood carved systems were related to widespread materials units or other surface features that served as stratigraphic horizons chryse outflow channels are inferred to have formed over most of the perceivable history of mars outflow channels are inferred to become younger with increasing proximity to the chryse basin in addition outflow channels closer to the basin show a greater diversity in age the relationship of subsequent outflow channel sources to the sources of earlier floods is inferred to disfavor episodic flooding due to the progressive tapping of a juvenile near surface water supply instead we propose the circum chryse region as a candidate site of past hydrological recycling the discharge rates necessary to carve the circum chryse outflow channels would have inevitably formed temporary standing bodies of h2o on the martian surface where the flood waters stagnated and pooled the chryse basin is topographically enclosed these observations and inferences have led us to formulate and evaluate two hypotheses 1 large amounts of the sublimated h2o off the chryse basin flood lakes precipitated snowed onto the flood source highlands and this h2o was incorporated into the near surface recharging the h2o sources making possible subsequent deluges and 2 ponded flood water in chryse basin drained back down an anti basinward dipping subsurface layer accessed long the southern edge of the lake recharging the flood source aquifers h2o not redeposited in the flood source region was largely lost to the hydrologic cycle this loss progressively lowered the vitality of the cycle probably by now killing it our numerical evaluations indicate that of the two hypotheses we formulated the groundwater seep cycle seems by far the more viable optimally approximately 3 4 of the original mass of an ice covered cylindrical lake albedo 0 5 1 km deep 100 km radius draining along its rim for one quarter of its circumference into substrata with a permeability of 3000 darcies can be modeled to have moved underground on timescales of the order of 10 3 years before the competing mechanisms of sublimation and freeze down choked off further water removal once underground this water can travel distances equal to the separation between chryse basin and flood source sites in geologically short approximately 10 6 year scale times conversely we calculate that optimally only approximately 40 of the h2o carried from chryse can condense at the highlands and most of the precipitate would either collect at the base of the highlands lowlands scarp or sublimate at rates greater than it would accumulate over the flood source sites further observations from forthcoming missions may permit the determination of which mechanisms may have operated to recycle the chryse flood waters

paleoecology of late glacial terrestrial deposits with in situ conifers from the submerged continental shelf of western canada extensive portions of the continental shelf off the coast of british columbia were subaerially exposed during late wisconsinan deglaciation due to lowering of relative sea level by as much as 150 m paleoecological analyses were conducted at two sites on the emergent continental shelf where terrestrial surfaces with in situ conifers are preserved the woody plant remains confirm that during the latest period of subaerial exposure terrestrial vegetation was established on the continental shelf microscopic identification of fossil wood and analyses of pollen and plant macrofossils from the associated paleosols and overlying shallow pond sediments indicate that productive pinus contorta dominated communities with abundant alnus crispa and ferns grew on the shelf adjacent to and on the queen charlotte islands around 12 200 14c yr b p dwarf shrubs including salix and empetrum and herbaceous plants such as heracleum lanatum and hippuris vulgaris were also important components of the shelf vegetation near northern vancouver island mixed coniferous forests dominated by pinus contorta with picea tsuga spp alnus spp and ferns occupied the shelf at 10 500 14c yr b p

a new survey of multiring impact basins on mars multiring impact basins have profoundly influenced the geologic evolution of mars we compile and summarize the evidence for martian impact basins and suggest eight new examples multiring basins on mars define three morphologic subclasses with increasing basin size basins having diameters 300 3600 km have extremely shallow topographic profiles and numerous concentric structures expressed as scarps massifs and channels radial and concentric structures analogous to those associated with orientale are not apparent for basins of argyre size or larger these variations in basin morphology and structure may be associated with mechanical interactions between basin forming impacts relatively thin weak lithosphere and for the largest impacts spherical target geometry multiring basins are recognized on all parts of mars including tharsis elysium and the northern lowlands much of the subsequent resurfacing of cratered terrain such as lunae planum ridged plains is associated spatially with multiring basins nucleation of long lived volcanic complexes in tharsis and elysium was probably aided by early impact basins the planetary terrain dichotomy was probably established during the period of heavy meteoritic bombardment and subsequent processes in the northern plains region were not sufficiently vigorous to destroy or completely obscure the underlying multiring basin fabric the revised population of multiring basins is consistent with the size frequency distribution of craters 500 km in diameter on mars basins having diameters 500 d 1850 km are characterized by a cumulative frequency slope of 0 75 0 16 whereas the larger basins define a slope of 1 7 0 45 this change in slope occurs over the same diameter range as the significant change from orientale type basin morphology at smaller diameters to argyre and chryse type basin morphology the population of craters and basins on mars confirms that the relative crater densities on the terrestrial planets due to heavy bombardment are comparable perhaps suggesting a common source for the impacting objects

hydrovolcanic tuff rings and cones as indicators for phreatomagmatic explosive eruptions on mars hydrovolcanism is a common natural phenomenon on earth and should be common on mars too since its surface shows widespread evidence for volcanism and near surface water we investigate fields of pitted cones in the nephentes amenthes region at the southernmargin of the ancient impact basin utopia which were previously interpreted as mud volcanoes the cone fields contain pitted and breached cones with associated outgoing flow like landforms based on stratigraphic relations we determined ahesperian or youngermodel age we test the hypothesis of a hydro volcanic origin based on a detailed morphological and morphometrical analysis and an analysis of the regional context an igneous volcanic origin of these cones as hydrovolcanic edifices produced by phreatomagmatic eruptions is plausible several lines of evidence suggest the existence of subsurface water ice the pitted cones display well developed wide central craters with floor elevations below the preeruptive surface their morphometry and the overall appearance are analogous to terrestrial tuff cones and tuff rings mounds that are also observed in the same region resemble terrestrial lava domes the hydrovolcanic interaction between ascending magma and subsurface water and or water ice may explain the formation of the pitted cones although other scenarios such as mud volcanism cannot be ruled out together with the mounds the cones might represent effusive and explosive edifices of amonogenetic volcanic field composed of lava domes tuff rings tuff cones and possibly maars

impact constraints on the age and origin of the lowlands of mars 1 visible and buried impact basins seen as quasi circular depressions qcds in mola data provide important new constraints on the age of the martian lowlands the buried lowlands are no younger than early noachian at least as old as the oldest exposed visible surface units in the highlands a model absolute age for these buried lowlands is 4 04 4 11 gy or earlier but similar model ages for the largest lowland basins are older yet 4 08 4 18 gy the lowland crust both formed and became low no later than 500 million years after mars formed and likely even earlier this constrains models for the origin of the fundamental crustal topographic dichotomy on mars mechanisms which operated both early and quickly during the earliest history of mars e g large impacts may be more likely than those requiring extended periods of time i e endogenic models

flow patterns of lobate debris aprons and lineated valley fill north of ismeniae fossae mars evidence for extensive mid latitude glaciation in the late amazonian a variety of late amazonian landforms on mars have been attributed to the dynamics of ice related processes evidence for large scale mid latitude glacial episodes existing within the last 100 million to 1 billion years on mars has been presented from analyses of lobate debris aprons lda and lineated valley fill lvf in the northern and southern mid latitudes we test the glacial hypothesis for lda and lvf along the dichotomy boundary in the northern mid latitudes by examining the morphological characteristics of lda and lvf surrounding two large plateaus proximal massifs and the dichotomy boundary escarpment north of ismeniae fossae centered at 45 3n and 39 2e lineations and flow directions within lda and lvf were mapped using images from the context ctx camera the thermal emission imaging spectrometer themis and the high resolution stereo camera hrsc flow directions were then compared to topographic contours derived from the mars orbiter laser altimeter mola to determine the down gradient components of lda and lvf flow observations indicate that flow patterns emerge from numerous alcoves within the plateau walls are integrated over distances of up to tens of kilometers and have down gradient flow directions smaller lobes confined within alcoves and superposed on the main lda and lvf represent a later less extensive glacial phase crater size frequency distributions of lda and lvf suggest a minimum youngest age of 100 ma the presence of ring mold crater morphologies is suggestive that lda and lvf are formed of near surface ice rich bodies from these observations we interpret lda and lvf within our study region to result from formerly active debris covered glacial flow consistent with similar observations in the northern mid latitudes of mars glacial flow was likely initiated from the accumulation and compaction of snow and ice on plateaus and in alcoves within the plateau walls as volatiles were mobilized to the mid latitudes during higher obliquity excursions together with similar analyses elsewhere along the dichotomy boundary these observations suggest that multiple glacial episodes occurred in the late amazonian and that lda and lvf represent significant reservoirs of non polar ice sequestered below a surface lag for hundreds of millions of years

a new global database of mars impact craters 1 km 1 database creation properties and parameters 1 impact craters have been used as a standard metric for a plethora of planetary applications for many decades including age dating geologic mapping and stratigraphic relationships as tracers for surface processes and as locations for sampling lower crust and upper mantle material utilizing craters for these and other investigations is significantly aided by a uniform catalog of craters across the surface of interest consequently catalogs of craters have been developed for decades for the moon and other planets we present a new global catalog of martian craters statistically complete to diameters d 1 km it contains 384 343 craters and for each crater it lists detailed positional interior morphologic ejecta morphologic and morphometric data and modification state information if it could be determined in this paper we detail how the database was created the different fields assigned and statistical uncertainties and checks in our companion paper robbins and hynek 2012 we discuss the first broad science applications and results of this work

origin of the martian crustal dichotomy evaluating hypotheses abstract any hypothesis for the origin of the martian global dichotomy should survive three elementary tests 1 it must account for the observed plan shape and apparent depth of the martian northern lowland 2 it must be physically consistent and 3 it must be compatible with available geological and geophysical data at present there are three contending types of hypotheses for the origin of the dichotomy creation by some endogenic process or processes creation by a single mega impact or creation by several overlapping large impacts none of these hypotheses can survive all three tests without the incorporation of additional processes the endogenic and mega impact hypotheses require the presence of additional impact basins to explain many of the topographic details of the martian northern lowland and of the dichotomy boundary and the mega impact hypothesis probably also requires extensive primordial erosion the multiple impact hypothesis requires an additional process or processes to account for the large portion of the northern lowland that is external to the rims of the basins inferred to be the cause of the lowland while we believe that aspects of the multiple impact hypothesis are required to account for some topographic details of the boundary and of the lowland other processes appear better able to account for the dichotomy as a whole these could include an early mega impact endogenic processes or some combination of these

origin of the valley networks on mars a hydrological perspective abstract the geomorphology of the martian valley networks is examined from a hydrological perspective for the compatibility with an origin by rainfall globally higher heat flow and localized hydrothermal systems comparison of morphology and spatial distribution of valleys on geologic surfaces with terrestrial fluvial valleys suggests that most martian valleys are probably not indicative of a rainfall origin nor are they indicative of formation by an early global uniformly higher heat flow in general valleys are not uniformly distributed within geologic surface materials as are terrestrial fluvial valleys valleys tend to form either as isolated systems or in clusters on a geologic surface unit leaving large expanses of the unit virtually untouched by erosion with the exception of fluvial valleys on some volcanoes most martian valleys exhibit a sapping morphology and do not appear to have formed along with those that exhibit runoff morphology in contrast terrestrial sapping valleys form from and along with runoff valleys the isolated or clustered distribution of valleys suggests localized water sources were important in drainage development persistent groundwater outflow driven by localized but vigorous hydrothermal circulation associated with magmatism volcanism impacts or tectonism is however consistent with valley morphology and distribution snowfall from sublimating ice covered lakes or seas may have provided an atmospheric source of water for the formation of some valleys in regions where the surface is easily eroded and where localized geothermal hydrothermal activity is sufficient to melt accumulated snowpacks

investigation of an argyre basin ring structure using mars reconnaissance orbiter compact reconnaissance imaging spectrometer for mars 1 the argyre basin a large impact basin in the southern highlands of mars excavates early noachian geologic units and thus is an ideal probe of the compositional layering of ancient highland crust a ring structure defined by circumferential graben is believed to represent the outermost topographic scarp of the basin two distinct layers in the fluvially dissected highland plains unit npld have been exposed by the ring graben in northwest argyre a lower unit enriched in high calcium pyroxene and an upper unit showing laterally extensive occurrences of phyllosilicates including chlorite and iron magnesium smectite these phyllosilicates closely follow the unit contact between unit npld and hpl3 in this region the hesperian aged hpl3 caprock directly covers unit npld two nearby knobs also show phyllosilicate bearing materials over a high ca pyroxene enriched unit although the knobs are located in regions mapped as npl1 and nplh from these observations we suggest that an extensive layer of phyllosilicates underlying a layer of high ca pyroxene bearing rock existed in the region prior to the argyre impact these materials were thrown up by the impact event and deposited as an inverted flap this inverted flap of ejecta was then cut by the circumferential graben that formed as the transient argyre crater collapsed exposing the materials

the structure and evolution of ancient impact basins on mars the number of martian impact basins once thought deficient owing to a low number of impactors has been significantly increased through the identification of structural imprints that remain even after nearly complete erasure of basin massifs five ancient multiringed impact basins were selected for detailed study and exhibit a systematic pattern of rejuvenation of the multiring plan through endogenic modification concentric zones of unstable terrain are believed to reflect deep seated impact related fractures that have localized regional igneous processes in a manner analogous with the basin controlled distribution of basalts on the moon most martian outflow channels were found to originate along the unstable ring zones hydrothermal melting and temporary confinement of channel source materials at sites analogous to lunar sinuous rille source regions are proposed to play an important role in the distribution of martian outflow channels ancient basins also can be delineated by the concentric pattern of topographically and structurally controlled narrow valley networks that are particularly abundant along the outer rings where basin ejecta deposits occur remnant massifs and scarps commonly exhibit extensive furrowing by narrow valley networks perhaps due to a different paleoclimate or sapping during a period of geothermal melting of trapped ices the systematic patterns of valley networks stable and unstable annuli scarps and massifs provide bases for interpreting impact structure in general and for recognizing multiringed structures elsewhere the chryse basin in particular is proposed to represent a major multiringed structure that controlled the formation and distribution of eastern valles marineris and the chaotic terrains of margaritifer sinus in response to volcanism tectonism related to tharsis we conclude that 1 mars is probably not deficient in large body impactors 2 the multiring patterns of basins represent deep seated fracture zones that can be reexposed through subsequent volcanism hydrothermal activity sapping and differential erosion 3 many martian terrains and features e g chaotic terrains and outflow channels are controlled by ancient basins and 4 the resurrection and destruction of impact basins reveal an important third dimension depth for interpreting impact basin formation

geologic evolution of the east rim of the hellas basin mars abstract geomorphic analyses and crater statistics indicate the distribution of materials on the east rim 27 5 42 5 s 260 275 w of the hellas basin and constrain the timing nature and interactions of geologic processes which have modified the hellas region the discernible geologic history of the area began with the formation of the impact basin and the emplacement of highlands that became heavily cratered during the noachian period the tectonic setting of the region was established at this time and basin structures promoted and influenced subsequent volcanic tectonic and fluvial activity degradation of the highland units by fluvial mass wasting and aeolian processes began in the noachian and continued into the hesperian and amazonian periods sedimentary plains covered low lying regions of the highlands their pitted surfaces in some areas suggest subsidence and the presence of volatiles emplacement of lobate debris aprons shed from highland peaks and possibly facilitated by ice creep occurred in the amazonian period the volcanic history of the east rim of the hellas basin began with the formation of the highland paterae the low dissected shields of hadriaca and tyrrhena paterae formed in the late noachian or early hesperian epochs and are older than or coeval with the ridged plains of hesperia planum volcanic activity continued throughout the hesperian period with the emplacement of the caldera filling materials at hadriaca patera and the lava flows of the flank flow unit of tyrrhena patera outflow channels and related channeled plains of the hesperian and amazonian periods suggest an extensive and complex history of volatile driven erosion and sedimentary deposition dao and harmahkis valles are interpreted to be collapsed regions of the older plains a combination of surface and subsurface flow has removed materials with deposition on the floor of the hellas basin the geologic evolution of the region determined from the observed surface morphology and stratigraphic relationships and refined by crater statistics indicates that a combination of diverse geologic processes acted in the degradation of the hellas basin the effects of volcanism and volatile driven processes were prominent throughout the hesperian and well into the amazonian periods the proposed transition from explosive to effusive volcanism in the hellas region is supported by the relative ages indicated by crater statistics the heat released by volcanism may have resulted in the melting of ground ice and contributed to the development of the channeled plains and outflow channels the presence of wrinkle ridges on units that are inferred to be relatively young from their crater size frequency distributions suggests deformation in the late hesperian or early amazonian epoch

a new global database of mars impact craters 1 km 2 global crater properties and regional variations of the simple to complex transition diameter 1 we have generated a new 384 343 entry global crater database of mars statistically complete for craters with diameters d 1 km in this release the database contains detailed morphologic and morphometric data for craters d 3 km future releases will extend these to smaller diameters with detailed topographic data for the largest crater database to date we analyzed crater depth to diameter ratios for simple and complex morphologies across various terrains and for the planet as a whole and investigated the simple to complex morphology transition our results are similar to those in the published literature but we found a substantial terrain dependence of the simple to complex transition that occurs at 11 km diameter craters at high latitudes this suggests a model that requires melting of volatiles during high latitude crater formation that fill the crater during the modification phase but will still support the simple morphology to larger diameters we also use this database to reexamine previously observed distributions and patterns to show its fidelity and to further explore other global relationships of fresh craters those with central peaks pits and summit pits we present the global distribution of craters with different types of ejecta and morphometric properties overall this database is shown to be comparable to previous databases where there is overlap and to be useful in extending prior work into new regimes

isidis basin site of ancient volatile rich debris layer abstract the interior plains of the isidis impact basin on mars contain landforms which have previously been interpreted as volcanic features these landforms can be classified into two broad units hillocky terrain and ridged terrain hillocky terrain the most central unit consists of isolated mounds commonly arranged in arcuate chains ridged terrain contains systems of parallel curvilinear ridges and occurs in a narrow band almost completely encircling the hillocky terrain ridges range from approximately 10 to 40 km long 0 5 to 1 0 km wide and 0 15 to 0 3 km high ridge properties and their distribution within isidis appear inconsistent with characteristics of typical volcanic fields on earth and mars alternatively we suggest that the isidis landforms reflect the deposition and subsequent removal of a thick layer of material within the basin crater statistics and the morphology of additional features within isidis provide further evidence that a major resurfacing event occurred within the basin although depositional and erosional processes are currently minimal terrestrial ice cover disintegration results in the development of landforms such as moraines kames and eskers and provides possible analogs for the features which occur in isidis the isidis cover however probably contained a vast amount of debris remained stationary throughout most of its existence and eventually sublimated thus differing from most terretrial glaciers a global survey of viking orbiter images reveals ridges elsewhere on mars with similar characteristics or spatial distribution thereby indicating that the isidis layer may have been part of a more widespread epoch of deposition this epoch possibly coincided with the period of major outflow channel formation when subsurface volatiles were released

impact craters in the northern hemisphere of mars layered ejecta and central pit characteristics mars global surveyor mgs and mars odyssey data are being used to revise the catalog of large martian impact craters analysis of data in the revised catalog provides new details on the distribution and morphologic details of 6795 impact craters in the northern hemisphere of mars this report focuses on the ejecta morphologies and central pit characteristics of these craters the results indicate that single layer ejecta sle morphology is most consistent with impact into an ice rich target double layer ejecta dle and multiple layer ejecta mle craters also likely form in volatile rich materials but the interaction of the ejecta curtain and target produced vapor with the thin martian atmosphere may be responsible for the large runout distances of these ejecta pancake craters appear to be a modified form of double layer craters where the thin outer layer has been destroyed or is unobservable at present resolutions pedestal craters are proposed to form in an ice rich mantle deposited during high obliquity periods from which the ice has subsequently sublimated central pits likely form by the release of vapor produced by impact into ice soil mixed targets therefore results from the present study are consistent with target volatiles playing a dominant role in the formation of crater morphologies found in the martian northern hemisphere

proteus geology shape and catastrophic destruction abstract proteus with a mean radius of 209 8 km is the largest irregular shaped icy body in the solar system it has a primitive heavily cratered surface its most prominent feature is an impact crater 255 km in diameter and 10 to 15 km deep relatively one of the largest craters in the solar system this prominent crater is similar in relative size to giant impacts on several other planets and satellites all of which are larger than a recent theoretical limit for catastrophic disruption consistency is obtained by inclusion of reaccumulation in the disruption analysis based on this reanalysis proteus was not brought to the brink of catastrophic destruction by the giant impact but only to the limit beyond which the surface of the impacted body and the form of the disrupting crater are modified beyond recognition a network of large streaks interpreted as troughs are also discernable this network too extensive to be explained by strains produced in traditional thermal history calculations may represent the onset of significant internal disruption in near catastrophic impacts proteus sphericity is near unity but it has one of the roughest surfaces known thus proteus is a transitional object in the irregular spherical shape spectrum for icy satellites its global figure is relaxed but its surface features are unrelaxed proteus intermediate relaxation state requires relatively high internal temperatures the required temperatures and the observed radius of the shape transition for icy bodies is consistent with radiogenic heating in bodies with deep porous regoliths

geomorphological evidence of water level changes in nepenthes mensae mars in the western sector of nepenthes mensae mars there are some geomorphological features that could be related to a standing water sheet in the area such as fluvial terraces deltas and shorelines a detailed analysis of these features reveals two variations in water level probably related to tectonic processes as suggested by the existence of a fissural volcano at this site

geotraverse from xanthe terra to chryse planitia viking 1 lander region mars high resolution 1 500 000 scale geologic mapping of a 300 km wide and 1500 km long strip or geotraverse across the transition from the xanthe terra cratered highlands to the chryse planitia lowlands encompasses an area on the margin of the chryse basin unaffected by outflow channels it thus characterizes the transition where it has been relatively unmodified by erosion and the stratigraphy of pre outflow units is well preserved two types of highland lowland boundary are identified 1 a topographic boundary and 2 a geologic morphologic boundary the topographic boundary occurs well within the cratered highlands and separates a lower standing surface with characteristics transitional to cratered highlands from the true highland surface of xanthe terra the lower transitional highland surface or transition zone is partially inundated by plains forming materials interpreted to be plains type lava flows of late noachian early hesperian age similar to later basin fill material forming hesperian aged lowland ridged plains the geologic boundary between this transitional surface and ridged lowland plains surface is identified with the traditional highland lowland boundary as mapped in previous studies mapping in this study identifies a moat like region between this geologic boundary and the basin interior is filled with an easily eroded material erosion of the moat filling deposit along the outer margins of the chryse basin accounts for the streamlined tables formed during outflow channeling throughout the margins of chryse planitia the geology of the central basin interior where the viking 1 lander is located is consistent with the presence of fluvial sediments derived from both the adjacent highlands and remobilized from the marginal moat filling unit and deposited near the termini of the maja and kasei valles outflow channels because the plains forming lavas overlie much of the proximal portions of the highlands and moat filling sediments ring the margins of the basin materials eroded from the transition zone and deposited toward the basin interior by later outflow channels may be similar to interior plains lavas and marginal moat filling deposits if the transitional highland surface is typical of the basin elsewhere true highland rocks are likely to be a small component of any outflow channel sediments even in settings near the basin margin rock populations and sediments at the proposed mars pathfinder site located near the geologic boundary further east could contain a significant portion of materials typical of lowlands in addition to some highland materials

geology and mer target site characteristics along the southern rim of isidis planitia mars 1 the southern rim of the isidis basin contains one of the highest densities of valley networks several restricted paleolake basins and the stratigraphically lowest oldest terrain on mars geologic mapping in viking mgs moc and mola data odyssey themis data and other multispectral data products supports the presence of extensive fans of debris and sediments deposited along the inner rim of the isidis basin where large valleys enter the lowlands additional processes subsequent to the period of intense fluvial activity including mass flow analogous to some glacial processes have contributed to the materials accumulated on the margins of the isidis basin these have occurred along preexisting channels and valleys at the termini of major channels where they enter the plains along the highland lowland boundary if the abundant valley networks in highland terrains are the result of runoff accompanied by saturated groundwater flow as has been suggested in previous studies of ancient fluvial highland terrains then the extreme age and abundance of early valley networks in the libya montes highland rocks should have resulted in deposition of materials that record evidence for the long term presence of water in the form of aqueous alteration of polycrystalline constituents the material deposited along the basin margin is likely to consist of ancient altered highland rocks in several physical states weathered rounded and angular exposing both weathered and altered surfaces and exposures of alteration profiles in fractured faces and unweathered material from rock interiors debris fans shed off the southern rim of isidis planitia should contain materials that have experienced possible saturated groundwater flow residence within paleolake basins and derivative materials deposited during the most fluvially intensive part of martian geologic history many of these materials have also been reworked by ice related processes in situ measurements of the ancient crustal materials in the form of rocks within the debris fans and the weathered condition of the rocky material are potential sources for mineralogical evidence of climatic conditions in earliest martian geologic history the absence of alteration within rocks would on the other hand support the hypothesis that fluvial runoff during the earliest history of mars was geologically brief rather than long term and that long term saturated groundwater flow was not present determination of the presence or absence of alteration would have corresponding implications for hypotheses requiring the long term presence of aqueous solutions i e complex organic compounds and life a proposed mer site along the margin addresses realistic field science objectives of the mars exploration rover mission and the current goals of the mars exploration program in situ measurements may be important in deriving estimates of the longevity and intensity of past wetter climates

three dimensional velocity structure in northern california coast ranges from inversion of local earthquake arrival times abstract the three dimensional p wave velocity structure in the upper crust in the northern california coast ranges from santa rosa to ukiah is obtained by simultaneous inversion of local earthquake and refraction travel time data for velocity and hypocentral parameters in this region where there are lateral heterogeneities the three dimensional inversion provides important results not found in two dimensional refraction studies the medium is parameterized with velocities at the intersections gridpoints of a nonuniform three dimensional grid travel times are calculated with approximate ray tracing along arcuate paths which are reasonable for lateral distances less than 45 km inversions with 5 to 10 and 2 to 3 km gridspacing achieved 80 per cent decreases in data variance the inversion with 2 to 3 km gridspacing reduced the rms residual level to approximately the level of timing error anomalously low velocities to at least 6 km depth are found along the maacama and healdsburg rodgers creek hrc fault zones the low velocity region is widest at the en echelon junction of the maacama and hrc fault zones low velocities are associated with the clear lake basin where there is a thick sequence of volcanics interbedded with sedimentary deposits there is a high velocity body located southeast of the geysers between the maacama and collayomi fault zones which may be associated with igneous intrusives or high grade metamorphism related to an underlying inferred partial melt body modeled in previous teleseismic travel time studies there is some indication in the vapor dominated geothermal reservoir of lower velocities relative to surrounding water saturated rock as has been shown in some laboratory experiments hypocenter locations obtained in the inversion show little variation from hypocenters obtained through inversion for one dimensional velocity model and station corrections for the inversion with 2 to 3 km gridspacing however some groups of events become more tightly clustered

tomographic inversion of local earthquake data from the hengill grensdalur central volcano complex iceland parameters is generally low however a quantitative analysis of resolution provides an unambiguous assessment of well resolved volumes within the better resolved regions of the model the averaging volumes are 1 2 km and 2 4 km in vertical and horizontal extent respectively results of tomographic inversion image three distinct bodies of anomalously high velocity two of these extend from near the surface to a depth of about 3 km these high velocity volumes are located directly beneath the surface expressions of the extinct grensdalur volcano and the extinct husmuli basalt shield the third high velocity structure occurs in the depth range of 3 4 km but does not extend to the surface these three high velocity bodies are interpreted to be solidified magmatic intrusions relatively low velocities underlay limited portions of the trace of the present accretionary axis and a low velocity body is imaged in the roots of the active hengill volcano the volume of lower velocities located beneath the surface expression of the hengill volcano is interpreted to be a region of partial melt

evidence of volcanic and glacial activity in chryse and acidalia planitiae mars abstract chryse and acidalia planitiae show numerous examples of enigmatic landforms previously interpreted to have been influenced by a water ice rich geologic history these landforms include giant polygons bounded by kilometer scale arcuate troughs bright pitted mounds and mesa like features to investigate the significance of the last we have analyzed in detail the region between 60 n 290 e and 10 n 360 e utilizing hirise high resolution imaging science experiment images as well as regional scale data for context the mesas may be analogous to terrestrial tuyas emergent sub ice volcanoes although definitive proof has not been identified we also report on a blocky unit and associated landforms drumlins eskers inverted valleys kettle holes consistent with ice emplaced volcanic or volcano sedimentary flows the spatial association between tuya like mesas ice emplaced flows and further possible evidence of volcanism deflated flow fronts volcanic vents columnar jointing rootless cones and an extensive fluid rich substratum giant polygons bright mounds rampart craters allows for the possibility of glaciovolcanic activity in the region landforms indicative of glacial activity on chryse acidalia suggest a paleoclimatic environment remarkably different from today s climate changes on mars driven by orbital obliquity changes or giant outflow channel activity could have resulted in ice sheet related landforms far from the current polar caps

polygonal impact craters in argyre region mars implications for geology and cratering mechanics impact craters are not always circular sometimes their rims are composed of several straight segments such polygonal impact craters pics are controlled by pre existing target structures mainly faults or other similar planes of weakness in the argyre region mars pics comprise 17 of the total impact crater population 7 km in diameter and pics are relatively more common in older geologic units their formation is mainly controlled by radial fractures induced by the argyre and ladon impact basins and to a lesser extent by the basin concentric fractures also basin induced conjugate shear fractures may play a role unlike the pics ridges and graben in the argyre region are mostly controlled by tharsis induced tectonism with the ridges being concentric and graben radial to tharsis therefore the pics primarily reflect an old impact basin centered tectonic pattern whereas tharsis centered tectonism responsible for the graben and the ridges has only minor influence on the pic rim orientations according to current models of pic formation complex pics should form through a different mechanism than simple pics leading to different orientations of straight rim segments however when simple and complex pics from same areas are studied no statistically significant difference can be observed hence in addition to enhanced excavation parallel to the strike of fractures simple craters and slumping along the fracture planes complex craters we propose a third mechanism involving thrusting along the fracture planes this model is applicable to both simple and small complex craters in targets with some dominating orientations of structural weakness

sedimentary history and mass flow structures of chryse and acidalia planitiae mars geologic mapping and crater counting in chryse and acidalia planitiae cap reveal five major sedimentary deposits of hesperian to early amazonian age including 1 a mass flow deposited during the early hesperian near deuteronilus mensae northeast of the map region that may have resulted from the carving of kasei valles 3000 km southwest of the exposed part of the deposit 2 knobby plains material consisting of channel likely from simud and tiu valles and possibly ares and shalbatana valles and mass wasting deposits in central and eastern cap 3 material largely from maja and ares valles emplaced in at least western and southern cap outcrops in southern chryse planitia developed thermokarst 4 a thin mass flow covering much of southern chryse planitia that emanated from simud and tiu valles and 5 a thick extensive perhaps 3500 km across mass flow deposit in central and northern cap derived from accumulation and backflow of the preceding thin mass flow or perhaps melting of polar deposits other possible deposits may not be recognizable owing to burial by younger materials or a lack of morphologic signature various associated landforms appear to be consistent with the mass flow interpretations including lobate and linear scarps along deposit edges fractures related to desiccation of thick sediments troughs and ridges near the edges of the deposit indicative of secondary mass movement and deformation pitted domes and fissure fed flows possibly formed by sedimentary mud eruptions and longitudinal channel grooves perhaps formed by roller vortices no convincing evidence for paleoshorelines or stagnant ice sheets is found in cap these findings suggest that mass flow and hyperconcentrated flooding may have been the predominant processes of outflow channel dissection in cap elsewhere in the northern plains similar landforms are prevalent the mass flow interpretation does not require either multiple episodes of extraordinarily high water discharge rates achieved by freeing huge volumes of water from the crust repetitive recycling of immense volumes of water into highland aquifers at the heads of chryse channels or profound climate change mars pathfinder will most likely land on and inspect the surface of the thin mass flow that originated from the canyons of simud and tiu valles

martian impact craters correlations of ejecta and interior morphologies with diameter latitude and terrain abstract the premise of this study is that global analysis of crater morphologies can discriminate between the effects of impact energy atmosphere and subsurface volatiles based on the correlation between morphology and latitude diameter and terrain this study focuses on seven ejecta and nine interior morphologies associated with 3819 craters 8 km in diameter distributed across the entire martian surface this data set is sufficiently large to provide statistically viable results on a global scale the results of the analyses indicate that changes in ejecta and interior morphology correlate with increases in crater diameter excavation depths corresponding to these diameters are calculated from depth diameter relationships and are compared to the theoretical distribution of subsurface volatiles the different ejecta morphologies can be described by impact into material with varying proportions of volatiles and the latidudinal variations seen for rampart crater morphologies correlate well with the proposed latitude depth relationship for ice and brines across the planet many of the interior structres show distributions indicative of terrain dependent influences for example the concentrations of central pit craters around ancient impact basins and on alba patera suggest that volatiles preferentially collect in these areas however central peak and peak ring interior morphologies show little relationship to planetary properties and probably result from changes in impact energy thus many of the unique morphologies associated with martian impact craters result from the presence of subsurface volatiles

formation of phobos and deimos via a giant impact abstract although the two moons of mars phobos and deimos have long been thought to be captured asteroids recent observations of their compositions and orbits suggest that they may have formed from debris generated by one or more giant impacts of bodies with 0 01 target mass recent studies have both analytically estimated debris produced by giant impacts on mars and numerically examined the evolution of circum mars debris disks we perform a numerical study smoothed particle hydrodynamics simulation of debris retention from giant impacts onto mars particularly in relation to a borealis scale giant impact e 3 10 29 j capable of producing the borealis basin we find that a borealis scale impact is capable of producing a disk of mass 5 10 20 kg 1 4 of the impactor mass sufficient debris to form at least one of the martian moons according to recent numerical studies of martian debris disk evolution while a borealis scale impact may generate sufficient debris to form both phobos and deimos further studies of the debris disk evolution are necessary our results can serve as inputs for future studies of martian debris disk evolution

the origin and timing of fluvial activity at eberswalde crater mars the fan deposit in eberswalde crater has been interpreted as strong evidence for sustained liquid water on early mars with a paleolake formed during the noachian period 3 7 gy this location became a key region for understanding the mars paleo environment eberswalde crater is located 50 km north of the rim of the 150 km diameter crater holden stratigraphic relationships and chronology obtained using recent mars express high resolution stereo camera and mars reconnaissance orbiter context camera images show that eberswalde fluvial activity crosscuts holden ejecta and thus postdates holden crater whose formation age is estimated from crater counts as late hesperian 3 5 gy depending on models fluvial modeling shows that short term activity over several years to hundreds of years involving dense flows with sediment water ratio between 0 01 and 0 3 may be as good an explanation of the fluvial landforms as dilute flow over longer durations modeling of the thermal effect of the holden impact in the eberswalde watershed is used to evaluate its potential role in aqueous activity the relative timing of the holden impact and eberswalde s fan is a constraint for future studies about the origin of these landforms holden ejecta form a weak and porous substrate which may be easy to erode by fluvial incision in a cold climate scenario impact heating could have produced runoff by melting snow or ground ice any attempt to model fluvial activity at eberswalde should take into account that it may have formed as late as in the late hesperian after the great majority of valley network formation and aqueous mineralization on mars this suggests that hypotheses for fan formation at eberswalde by transient and or localized processes i e impact volcanism unusual orbital forcing should be considered on a par with globally warmer climate

large impact basins and the mega impact origin for the crustal dichotomy on mars the origin of the fundamental crustal dichotomy on mars remains the outstanding unsolved problem in martian geologic evolution the hypothesis that this dichotomy is due to a single giant mega impact early in martian history is tested by comparing the observed number of large impact basins on mars with the number expected from a d 2 distribution this test is based on the assumption that the mega impact borealis basin is the largest member of such a distribution the difference between observed and expected basins is the number of missing basins that might have been obscured by subsequent geologic events if the borealis basin were the largest member of a d 2 impact population there would be a very large number of missing basins on mars 1 larger than 5000 km 10 larger than 2000 km and 40 50 larger than about 1000 km diameter the area occupied by these missing basins allowing for overlap is too large to be hidden by the younger surface units a large population of presently unrecognized large impact basins would have to be located in the well preserved cratered highlands by contrast if chryse is the largest member of a d 2 impact population the more modest number of missing basins could in principle be confined to areas of mars that have been resurfaced or reworked by subsequent geologic processes a possible alternative to the mega impact hypothesis is that the crustal dichotomy is due to the cumulative effects of multiple overlapping large but not giant impacts

localization of bedding plane slip and backthrust faults above blind thrust faults keys to wrinkle ridge structure a mechanically based model for wrinkle ridge development is developed that combines wrinkle ridge morphologies regional topographic offsets suggestive of subsurface thrust faults and folding of near surface layers this model provides explicit relationships between observed morphologic elements characteristic of wrinkle ridges and plausible mechanisms in the subsurface a key component that is absent in previous qualitative fault based scenarios for these structures as developed in this paper wrinkle ridges are the surface expression of anticlines that grow above a blind thrust fault as a result of both flexural slip folding of near surface strata and the nucleation and growth of echelon arrays of backthrust faults calculations of displacements both horizontal shortening and vertical uplift and coulomb stress change related to slip along blind thrust faults in the model demonstrate physically important spatial inhomogeneities in these quantities with revealing and useful implications 1 the ratio of shortening due to folding at the surface to the shortening due to faulting at depth is characteristically small for coupled wrinkle ridge blind thrust fault systems and decreases with increasing fault depth the depth of the blind thrust fault s upper tip thus profoundly influences the surface strains 2 folding and uplifted topography forming the topographic ridge are produced above the area of the slipping blind thrust fault plane horizontal and vertical deformation at the surface extend over several ridge widths or a total of at least 50 km for a 10 km wide ridge implying that topographic profiles and geologic studies must extend sufficiently far from the wrinkle ridge to fully characterize the surface deformation 3 calculations of coulomb stress changes suggest that fault slip can localize both bedding plane slip in overlying strata and new backthrust faults that propagate upward to become wrinkles on the trailing side of the ridge initiation of bedding plane slip in association with slip along the blind thrust fault likely determines whether the resulting surface structure becomes a wrinkle ridge or a lobate scarp wrinkle ridge spacing may also be related to stress changes associated with slip along the underlying blind thrust fault

small martian valleys pristine and degraded morphology the equatorial heavily cratered uplands of mars are dissected by two classes of small valleys that are intimately associated in compound networks pristine valleys with steep valley walls preferentially occupy downstream portions of compound basins degraded valleys with eroded walls are laterally more extensive and have higher drainage densities than pristine valleys morphometric and crater counting studies indicate that relatively dense drainage networks were emplaced on mars during the heavy bombardment about 4 0 b y ago over a period of approximately 10 sup 8 years these networks were degraded and subsequently invaded by headwardly extending pristine valleys the pristine valleys locally reactivated the compound networks probably through sapping processes dependent upon high water tables fluvial activity in the heavily cratered uplands generally ceased approximately 3 8 3 9 b y ago coincident with the rapid decline in cratering rates the relict compound valleys on mars are morphometrically distinct from most terrestrial drainage systems the differences might be caused by a martian valley formation episode characterized by hyperaridity by inadequate time for network growth by very permeable rock types or by a combination of factors

sequence and mechanisms of deformation around the hellas and isidis impact basins on mars the distribution and age of early volcanic and tectonic features surrounding the itellas and isidis impact basins are shown to fit a four stage sequence first concentric canyons form outside the basin boundary scarp at or near the time of basin formation followed shortly thereafter by radial troughs extending beyond the boundary scarp after a hiatus concentric graben develop inside the basin within the massif ring finally soon after graben formation widespread volcanic plains are emplaced between the massif ring and the boundary scarp to one side of the basin three models of basin centered deformation are compared io hese observed deformation events elastic flexure due to loading elastic flexure due to uplift and impact fracture a central load of basin fill can account for concentric fractures by elastic flexure around the load formation of the distant concentric canyons however requires lithospheric thicknesses a factor of 5 greater than those indicated by planetary thermal history models and an increase in feature width by an order of magnitude relative to flexural graben identified elsewhere on mars and the moon although inclusion of loading by basin ejecta reduces the required lithospheric thickness to acceptable values flexure still fails to account for the large canyon widths observed impact fracture from a rapid inward flow of asthenosphere during transient cavity collapse however is consistent with the three key observational constraints for the distant canyons their age width and distance from the basin as the basin then undergoes isostatic uplift soon after impact elastic flexure induces a radial pattern of failure consistent with the observed extent and timing of the radial troughs later after a long period of basin infilling elastic failure due to loading of the central basin region can account for the ring grabens in the massif ring the localization of volcanism into a single ridged plains unit on the basin rim indicates a thermal source offset from the basin which is not included in these models and perhaps reflects an additional mechanism unique to mars

evidence for an ancient impact basin in daedalia planum mars geologic mapping has provided observations that some features on mars are morphologically similar to lunar imbrium sculpture materials these grooved features occur in clustered sets of a dozen or more are spaced 1 to 10 km apart and are unlike other linear martian features such as yardangs faults ancient valley networks and mass wasting features we propose that they represent impact materials from an ancient basin the orientations of these features were plotted as lines on a schmidt net and the density of the resulting intersections suggests a likely source area at approximately 26 0 125 0 or roughly the center of daedalia planum numerous features geologic materials and earth based radar data indicate that a basin with a diameter of approximately 4500 km with an early noachian age is centered at this location four interior rings with diameters of 1100 1500 2200 and 3200 km and an exterior ring with a diameter of approximately 6400 km were found the rings are spaced with a ratio of 2 0 1 which is similar to the basin ring spacing on the other terrestrial planets both concentric and infrequently radial basin related faults were recognized including claritas fossae gordii dorsum and the thaumasia graben faults or zones of weakness related to the basin rings may have influenced the position of the mangala valles source area and arsia mons based on the location and morphology of early hesperian to early amazonian volcanics composing daedalia planum the daedalia basin may also have extensive deep seated interior faults capable of extruding mare materials to the surface

hirise imaging of impact megabreccia and sub meter aqueous strata in holden crater mars high resolution imaging science experiment hirise images of holden crater mars resolve impact megabreccia unconformably overlain by sediments deposited during two noachian age phases of aqueous activity a lighter toned lower unit exhibiting phyllosilicates was deposited in a long lived quiescent distal alluvial or lacustrine setting an overlying darker toned and often blocky upper unit drapes the sequence and was emplaced during later high magnitude flooding as an impounded uzboi vallis lake overtopped the crater rim the stratigraphy provides the first geologic context for phyllosilicate deposition during persistent wet and perhaps habitable conditions on early mars

constraints on the depth of magnetized crust on mars from impact craters large diameter greater than 500 km martian impact basins are associated with observed magnetic fields which are statistically distinct from and smaller than fields associated with smaller craters we suggest that this effect arises because impacts cause shock heating and excavation reducing the magnetization of previously magnetized crust for a simple uniformly magnetized model the magnetic field at 100 km altitude is reduced by 50 when a crater shaped demagnetization zone reaches the base of the magnetized layer by analogy with terrestrial data we assume that in martian craters the zone of demagnetization extends to a depth of 0 04 0 15 crater diameters on the basis of this assumption the data suggest that the depth to the base of the magnetized layer on mars if uniform is 35 km with lower and upper bounds of 10 and 100 km respectively these bounds imply magnetizations of 5 40 a m 1 and are consistent with likely mars geotherms at 4 gyr b p

chryse planitia mars topographic configuration outflow channel continuity and sequence and tests for hypothesized ancient bodies of water using mars orbiter laser altimeter mola data many of the largest and most prominent outflow channels on mars debouch into chryse planitia pre mars global surveyor topographic data show chryse to be a closed depression almost 2000 km in diameter new mars orbiter laser altimeter mola data reveal the following 1 chryse is not a locally closed basin but instead opens into the north polar basin 2 the highly distinctive morphology of the six largest predominantly hesperian aged channels kasei maja simud tiu ares and mawrth disappears into the northern lowlands at average elevations that all occur within less than 170 m of a mean elevation of 3742 sd 153 m over a lateral distance in excess of 2500 km 3 the elevations where the distinctive morphology of each channel disappears all fall within 190 m of contact 2 a boundary mapped by parker et al 1993 and interpreted to represent an ancient shoreline and the mean elevation values of contact 2 and circum chryse channel termini fall within 18 m of each other in contrast the termini of seven later amazonian aged channels emerging from elysium into utopia planitia are spread over a vertical range of 1500 m 4 topographic evidence of the continuation of some of the outflow channels can be observed for distances of 250 450 km into the north polar basin but the morphology is subdued and distinctly different 5 the nature of this less distinctive topography and its crosscutting relationships show that simud and tiu are likely to represent the youngest activity specifically crosscutting ares valles 6 the distinctive change in channel morphology is consistent with rapid loss of energy encountered at base level subaerial submarine boundary and emplacement into a shallow submarine environment channel characteristics lack of distinctive deltas or lobes and continuation of subdued channel morphology suggest hyperpychnal flow and the possibility of density turbidity currents estimates of the volumes of individual channel events are wide ranging the minimum volume estimates of carr 1996 suggest that 46 such events would be required to fill the basin to the level of contact 2 and thus that the channels may have emptied into an existing standing body of water volume estimates of baker et al 1991 assume that single individual events may have filled the basin to the level of contact 2 thus requiring significant water loss between events and refilling during subsequent events to essentially the same level in both end member cases these observations are consistent with the presence of large standing bodies of water in the northern lowlands in hesperian early amazonian times