local geoelectrical models of the martian subsurface for shallow groundwater detection using sounding radars received 15 february 2002 revised 20 july 2002 accepted 28 august 2002 published 6 march 2003 1 low frequency sounding radars should be able to probe the martian subsurface layers down to varying depths depending on the geoelectrical properties of the sounded sites we present in this work four frequency dependent geoelectrical models of the martian subsurface in the 1 20 mhz frequency band based on laboratory electromagnetic characterization of martian soil analogues those models correspond to local martian sites that we considered to be of particular interest in the search for water using mainly the ground penetrating radar gpr instrument of the netlander mission results and discussion are also valid for both sounding experiments marsis and sharad the four models of the martian subsurface are designed to represent terrains where recent fluviallike features suggest the presence of near subsurface ground ice and probably liquid water we performed measurements on volcanic and sedimentary materials that may be present on these sites under the appropriate geophysical conditions that may exist in those terrains we then simulated the backscattered radar echo arising from each site in the 2 mhz frequency band using the finite difference time domain fdtd algorithm in order to evaluate the instrument performances to probe the subsurface stratigraphy of each site our results confirm that the near subsurface rich iron oxide mineralogy controls the instrument performances in terms of penetration depth and signal to noise ratio in the 2 mhz frequency band we finally discuss the geophysical and geoelectrical sounding conditions that could lead to an ambiguous detection of shallow subsurface water on mars for the netlander gpr index terms 3210 mathematical geophysics modeling 1794 history of geophysics instruments and techniques 5144 physical properties of rocks wave attenuation 5109 physical properties of rocks magnetic and electrical properties keywords mars hydrology gpr sounding simulation fdtd

tectonic history of the tharsis region mars the faults of the tharsis region of mars have been mapped and delineated on the basis of age the orientation of each group of faults was analyzed to determine if they were radial to a point the results indicate that there are at least four discrete centers of faulting within the tharsis area which are of significantly different age the four centers proposed are from oldest to youngest 1 in the thaumasia highlands 2 in northern syria planum and finally 3 and 4 near pavonis mons while episodes 3 and 4 occupy the same geographic position they are separated in time by the development of the tharsis lava plains and shields each episode of tectonism was separated from the next by a period of major basaltic volcanism the tectonic features of the tharsis area are thus viewed as being the cumulative result of the superposition of several discrete episodes of faulting rather than the result of one episode about a single point

mars outflow channels a reappraisal of the estimation of water flow velocities from water depths regional slopes and channel floor properties methods used so far to assess the flow velocities of the water commonly assumed to be responsible for forming the major outflow channel systems on mars have relied widely on various versions of the manning equation this has led to problems in allowing for the difference between the accelerations due to gravity on mars and earth and for the differences of scale between martian floods and most river systems on earth we reanalyze the problem of estimating water flow velocities in martian outflow channels using equations based on the darcy weisbach friction factor instead of the manning n factor we give simplified formulae appropriate to mars for the darcy weisbach friction coefficient as a function of bedrock size distribution for a given channel floor slope and water flood depth similar mean flow velocities are implied for a wide range of values of the ratio of bed roughness to water depth relevant to martian outflow channels using a recent rederivation of manning s equation based on turbulence theory we obtain a new value of 0 0545 s m 1 3 for the manning n coefficient appropriate to martian channels and show that previous analyses have generally overestimated though in some cases underestimated water flow velocities on mars by a factor of order two combining the consequences of this flow velocity overestimate with likely overestimates of flow depth from assuming bank full flow we show that discharges may have been overestimated by a factor of up to 25 leading to corresponding overestimates of subsurface aquifer permeabilities rates of filling of depressions with water and grain sizes of sediments on channel floors despite the availability of an improved value for the manning n coefficient for mars we strongly recommend that modified forms of the original version of the manning equation should be replaced by the modern form or preferably by the darcy weisbach equation in future work

the relation of catastrophic flooding of mangala valles mars to faulting of memnonia fossae and tharsis volcanism detailed stratigraphic relations indicate two coeval periods of catastrophic flooding and tharsis centered faulting producing memnonia fossae in the mangala valles region of mars major sequences of lava flows of the tharsis montes formation and local lobate plains flows were erupted during and between these channeling and faulting episodes first late hesperian channel development overlapped in time the tharsis centered faulting that trends north 75 to 90 e next late hesperian early amazonian flooding was coeval with faulting that trends north 55 to 70 e in some reaches resistant lava flows filled the early channels resulting in inverted channel topography after the later flooding swept through both floods likely originated from the same graben which probably was activated during each episode of faulting faulting broke through groundwater barriers and tapped confined aquifers in higher regions west and east of the point of discharge the minimum volume of water required to erode mangala valles about 5 1012 m3 may have been released through two floods that drained a few percent pore volume from a relatively permeable aquifer the peak discharges of the floods may have lasted from days to weeks the perched water discharged from the aquifer may have been produced by hydrothermal groundwater circulation induced by tharsis magmatism tectonic uplift centered at tharsis montes and compaction of saturated crater ejecta due to loading by lava flows

landform hierarchy and evolution in gorgonum and atlantis basins mars abstract this paper describes the evolution of landforms in atlantis and gorgonum basins using a geomorphologic approach which interprets landform distribution and hierarchy rather than looking at the distribution of large area 10 6 km 2 geologic sequences this study focuses on interpreting the local scale 3 km 2 cratered terrains tectono structural basins and local manifestation of exogenic processes specifically the evolution of fluvio lacustrine landforms is interpreted as being functionally subordinated to the evolution of the cratered terrains and to the tectono structural modifications of the landscape results show that three major phases of landscape evolution in atlantis and gorgonum basins can be identified a major impact cratering during the heavy bombardment period b tectonic displacements in response to volcano emplacement in the tharsis region and simultaneous landform creation by fluvial and lacustrine processes and c exogenic modification of the older landforms through weathering and eolian processes our results show that the smaller morphological features which form on the older geological units are not necessarily old themselves and can in fact be relatively recent e g amazonian the main implication of these results is that martian morphology did not form only during a period immediately following the heavy bombardment as commonly postulated but rather that landform evolution continued throughout the entire martian history

outflow channel sources reactivation and chaos formation xanthe terra mars abstract the undulating warped and densely fractured surfaces of highland regions east of valles marineris located north of the eastern aureum chaos east of the hydraotes chaos and south of the hydaspis chaos resulted from extensional surface warping related to ground subsidence caused when pressurized water confined in subterranean caverns was released to the surface water emanations formed crater lakes and resulted in channeling episodes involved in the excavation of ares tiu and simud valles of the eastern part of the circum chryse outflow channel system progressive surface subsidence and associated reduction of the subsurface cavernous volume and or episodes of magmatic driven activity led to increases of the hydrostatic pressure resulting in reactivation of both catastrophic and non catastrophic outflow activity ancient cratered highland and basin materials that underwent large scale subsidence grade into densely fractured terrains collapse of rock materials in these regions resulted in the formation of chaotic terrains which occur in and near the headwaters of the eastern circum chryse outflow channels the deepest chaotic terrain in the hydaspis chaos region resulted from the collapse of pre existing outflow channel floors the release of volatiles and related collapse may have included water emanations not necessarily linked to catastrophic outflow basal warming related to dike intrusions thermokarst activity involving wet sediments and or dissected ice enriched country rock permafrost exposed to the atmosphere by extensional tectonism and channel incision and or the injection of water into porous floor material may have enhanced outflow channel floor instability and subsequent collapse in addition to the possible genetic linkage to outflow channel development dating back to at least the late noachian clear disruption of impact craters with pristine ejecta blankets and rims as well as preservation of fine tectonic fabrics suggest that plateau subsidence and chaos formation may have continued well into the amazonian period the geologic and paleohydrologic histories presented here have important implications as new mechanisms for outflow channel formation and other fluvial activity are described and new reactivation mechanisms are proposed for the origin of chaotic terrain as contributors to flooding detailed geomorphic analysis indicates that subterranean caverns may have been exposed during chaos formation and thus chaotic terrains mark prime locations for future geologic hydrologic and possible astrobiologic exploration

an igneous origin for features of a candidate crater lake system in western memnonia mars the association of channels inner terraces and delta like features with martian impact craters has previously been interpreted as evidence in favor of the past existence of crater lakes on mars however examination of a candidate crater lake system in western memnonia suggests instead that its features may have formed through igneous processes involving the flow and ponding of lava accumulations of material in craters and other topographic lows throughout much of the study region have characteristics consistent with those of volcanic deposits and terraces found along the inner flanks of some of these craters are interpreted as having formed through drainage or subsidence of volcanic materials channels previously identified as inlets and outlets of the crater lake system are interpreted instead as volcanic rilles these results challenge previous interpretations of terrace and channel features in the study region and suggest that candidate crater lakes located elsewhere should be reexamined

new martian paradigms several interrelated paradigms concerning the nature and the evolution of mars are discussed these include an impact origin of the northern lowland mars as a water rich planet former oceans in the northern lowlands a warm wet climate on early mars and the possibility of ancient life on mars locations on mars suggested for the search of possible ancient life include the heavily cratered terrain dissected by valley networks possible lacustrine deposits in valles marineris the oxidizing layer below the surface sites along the fringes of the hypothetical northern circumpolar ocean and in evaporite deposits

mangala valles mars a reassessment of formation processes based on a new geomorphological and stratigraphic analysis of the geological units abstract mangala valles has always been viewed as the typical outflow channel formed by catastrophic floods of water a new analysis has shown that the geomorphological traces of fluvial or lacustrine processes within mangala valles can be better explained by fluid lava flooding the channels and filling pre existing impact craters as for the circum chryse outflow channels where no clear source of water or mechanism able to replenish water at its hydraulic head is observed there is no geologic trace of a sudden removal of a volume of water ice necessary to carve mangala valles neither maars nor rootless cones typical volcanic features indicative of interaction between lava and ground ice were found past works suggested that the formation of mangala valles occurred in late amazonian age when the climate of mars was similar to that seen today that is absolutely not liquid water friendly the present work shows how the origin of mangala valles may go back to noachian or even pre noachian when other studies have concluded that the climate was not liquid water friendly even assuming limited periods of obliquity favourable to liquid water in the history of mars which is at odds with the widespread presence of unaltered olivine and jarosite it is very difficult to find plausible mechanisms of aquifer recharge or signs of catastrophic water release at the notch of mangala valles that could feed the multiple episodes or even a single episode of fluvial flooding suggested in the literature this evidence and other analysis will show that the presence of water and eventually ground ice is not incontrovertible in the equatorial regions and should not be given for granted as commonly done so far in the literature the geomorphological analysis of the mars reconnaissance orbiter mro images provided in this paper combined with themis and mola data show how mangala fossa from which mangala valles originated as a breakout is an erosional channel formed by the flow of lava in a original tube coming from daedalia planum rather than a tectonic graben or the sign of a dike rupturing to the surface

elysium utopia flows as mega lahars a model of dike intrusion cryosphere cracking and water sediment release 1 high resolution altimetry from the mars orbiter laser altimeter mola provides new data to test the interpretation that unusual flow like deposits in elysium utopia were emplaced as lahars mass flows fluidized by water that are induced by volcanism using several data products derived from mola altimetry we confirm that two major types of terrain dominate the region 1 lava flows and 2 deposits consistent with emplacement by debris flows along medial channels supporting the lahar hypothesis of previous workers associated terrain types are interpreted to be the result of modification of lavas by water and or ice associated with the debris outflows and by later climatically controlled deposition and removal of a volatile rich dust layer at high latitudes both lava flows and debris flows originate from northwest trending radial fossae on the flanks of the elysium rise the fossae are interpreted here to have been formed by laterally propagating dikes from elysium mons that have intersected or approached the surface fossae that are sources for only lava flows lie at higher elevations 3400 m and above than those that are the sources of lahars and channels 3100 m to 4300 m this discrepancy in source location suggests that the availability of groundwater responsible for lahar formation is elevation dependent and reflects a hydrostatic equilibrium distribution of water in the subsurface these observations at elysium allow a geologic test of the prevailing hypothesis on the configuration of the subsurface martian hydrosphere that an interconnected groundwater system at hydrostatic equilibrium may exist below a confining cryosphere if water at a certain elevation is under hydrostatic pressure and this cryosphere is disrupted the potential arises for groundwater to flow to the surface we propose the following flow emplacement model in which geologic activity in northwest elysium is consistent with the preceding hydrological model laterally propagating dikes from elysium approached or intersected the surface on the flanks of the rise resulting in effusion of lava flows and possibly pyroclastics disruption of the cryosphere by the dikes allowed groundwater to escape to the surface and form lahars by interaction with erupted and existing material at elevations at and below the maximum level of subsurface saturated ground this model provides a consistent explanation for the major structures and units both lava flows and lahars and their elevation and spatial relationships in the elysium utopia region this type of mega lahar with groundwater as its primary water source is rare on earth and occurs on mars due to martian climatic and hydrologic conditions and the presence of a global confining cryosphere this study supports the idea that groundwater outflow resulting from disruption of a confining cryosphere may be an important phenomenon in martian geology in particular general association of major outflow with major volcanic areas on mars is highly consistent with a model of disruption of a groundwater confining cryosphere by dikes expected to be intruded in these areas further geologic analyses of features and processes related to groundwater outflow will help constrain the degree and causes of variability within the martian hydrologic system

ancient groundwater flow in the valles marineris on mars inferred from fault trace ridges long linear ridges in the valles marineris region of mars most likely represent fault zones that have been cemented by water deposited minerals this implies that water in the martian crust could have traversed long distances via faults and fractures

amazonis planitia the role of geologically recent volcanism and sedimentation in the formation of the smoothest plains on mars 1 amazonis planitia located between the two main volcanic provinces on mars tharsis and elysium is characterized by extremely smooth topography at several scale lengths as smooth as oceanic abyssal plains topography on earth we use mars global surveyor mgs data primarily very high resolution mars orbiter laser altimeter mola topography and derivative slope maps gradient maps and detrended maps to examine the surface morphology of amazonis planitia and the stratigraphic relationships among previously mapped and newly defined units these new data reveal the presence of a 1300 km diameter noachian impact basin in northwest amazonis planitia and an extensive late hesperian lava flow unit that appears to have originated from the olympus mons source area prior to aureole formation the presence of this previously unrecognized flow unit strongly suggests that olympus mons activity dates back to at least the hesperian as did activity on the tharsis montes emplacement of this 100 meter thick flow unit formed a barrier along the northern margin of amazonis planitia which had a profound influence on the subsequent geologic history of the region formation of olympus mons aureole deposits created an eastern topographic barrier and subsequent tharsis montes lava flows entered the basin from the south flowing around the aureole these three barriers degraded noachian crater rim protoolympus mons flow unit and olympus mons aureole caused subsequent lava flows and outflow channel effluents primarily from the elysium region to the west to pond on the floor of amazonis planitia preferentially smoothing the terrain there mars orbiter camera moc images substantiate that at least two very fluid lava flows alternated with fluvial episodes from elysium planitia flowing through marte valles onto the floor of the amazonis planitia basin within amazonis planitia moc images show flow like textures heavily mantled by sediments and radar data reveal the presence of rough lava flow surfaces underlying the sedimentary debris these data thus suggest that the unique smoothness of amazonis planitia is the result of deposition of thin fluid lava flows and fluvial sediments in an enclosed basin crater counts suggest that the most recent resurfacing may have occurred in the latest amazonian period in the last 1 of the history of mars in light of its unique history it is somewhat ironic to note that amazonis planitia was originally thought to be a typical young martian surface and therefore used to name the amazonian era index terms 6225 planetology solar system objects mars 5415 planetology solid surface planets erosion and weathering 5470 planetology solid surface planets surface materials and properties 5480 planetology solid surface planets volcanism 8450 4267 oceanography general paleoceanography keywords mars water amazonis planitia marte vallis volcanism amazonian citation fuller e r and j w head iii amazonis planitia the role of geologically recent volcanism and sedimentation in the formation of the smoothest plains on mars j geophys res 107 e10 5081 doi 10 1029 2002je001842 2002

thick shell tectonics on one plate planets applications to mars a theory for stress distributions in thick lithospheric shells on one plate planets is developed based on the zero frequency equations of a self gravitating elastic spherical shell overlying a strengthless fluid stress distributions in lithospheres are reviewed for both the compensated and flexural modes in the former case surface stresses only depend on surface topography while for the latter case it is shown for long wavelengths that stress trajectories are mainly dependent on the lithospheric lateral density distribution and not on elastic properties computational analyses are carried out for mars and it is found that isostatically compensated models correctly predict the graben structure in the immediate tharsis region and a flexural loading model is satisfactory in explaining the graben in the regions surrounding tharsis a three stage model is hypothesized for the evolution of tharsis isostasy with north south graben formation on tharsis followed by flexural loading and radial graben formation on the perimeter of tharsis followed by a last stage of loading with little or no regional deformation this model is consistent with the martian lithosphere monotonically thickening over geologic time

formation and evolution of the chaotic terrains by subsidence and magmatism hydraotes chaos mars the origin of the martian chaotic terrains is still uncertain and a variety of geologic scenarios have been proposed we provide topographic profiles of different chaos landscapes notably aureum and hydraotes chaos showing that an initial shallow ground subsidence occurred at the first step of the chaos formation we infer that the subsidence was caused by intrusion of a volcanic sill which could have produced consequent melting as well as release of ground water from disrupted aquifer signs of a volcanic activity are observed on the floor of hydraotes chaos a complex and deep depression located at the junction of three channels the volcanic activity is represented by small 0 5 to 1 5 km diameter rounded cones with summit pits the cone s size and morphology as well as the presence of possible surrounding lava flows suggest that they are primary volcanic cones similar to terrestrial cinder cones the identification of volcanic activity on the deepest chaos where the lower crustal thickness and the faults fractures system contributed to the magma rising reveals that magmatic activity proved by the cones and possibly help by structural activity has been a major factor in the formation of chaotic terrains

mars paleostratigraphic restoration of buried surfaces in tharsis montes volcanism in the tharsis province of mars occurred in several different areas and was generally continuous without large time intervals between eruptive episodes major lava flow units are numerous and extensive but relatively thin in many places impact craters on buried surfaces project above younger flows that overlie them a new application of crater dating methods has been developed to aid in the identification of these buried surfaces and to determine their lateral extent the technique is especially adaptable to the tharsis region where the stratigraphic succession of major flow units has been established by detailed geologic mapping knowledge of the overall stratigraphy allows correlations to be made between known and unknown surfaces by comparing their crater frequencies at diameters large enough to insure their recognition on the buried unit the method has been applied to aid in the restoration of buried rock units and to construct a series of paleostratigraphic maps showing the sequence of major eruptive events in the tharsis region

evidence for remnants of ancient ice rich deposits mangala valles outflow channel mars high resolution spacecraft data reveal the presence of a distinctive unit on the upper reaches of the floor of mangala valles an ancient outflow channel on mars in contrast to abundant evidence for scour intense erosion and hydrodynamic shaping typical of the floors and margins of mangala and other outflow channels this unit is smooth surfaced has cuspate margins is superposed on the scoured valley floor and is extensively pitted we interpret this unit to be the sublimation lag deposit derived from an ice thermal boundary layer on the aqueous flood that formed the outflow channel system and the remnant of floodwaters trapped in channel lows this interpretation further supports the likelihood that climate conditions at the time of emplacement were similar to the hyper arid cold desert conditions of today frozen floodwaters from the ancient martian subsurface could still be preserved in these deposits representing attractive and accessible exobiological exploration objectives

high outflow channels on mars indicate hesperian recharge at low latitudes and the presence of canyon lakes abstract a number of martian outflow channels were carved by discharges from large dilational fault zones these channels were sourced by groundwater not surface water and when observed on high standing plateaus they provide indicators of elevated paleo groundwater levels we identify three outflow channels of hesperian age that issued from a 750 km long fault zone extending from candor chasma to ganges chasma two of these channels allegheny vallis and walla walla vallis have sources 2500 m above the topographic datum too high to be explained by discharge from a global aquifer that was recharged solely in the south polar region the indicated groundwater levels likely required regional sources of recharge at low latitudes the floodwaters that erupted from ophir cavus to form allegheny vallis encountered two ridges that restricted the flow forming temporary lakes the flow probably breached or overtopped these obstructions quickly catastrophically draining the lakes and carving several scablands after the last obstacle had been breached a single main channel formed that captured all subsequent flow we performed hydrologic analyses of this intermediate phase of the flooding prior to incision of the channel to its present depth using floodwater depths of 30 60 m we calculated flow velocities of 6 15 m s 1 and discharges in the range of 0 7 3 10 6 m 3 s 1 locally higher flow velocities and discharges likely occurred when the transient lakes were drained variable erosion at the channel and scabland crossing of mola pass 10644 suggests that the upper 25 30 m may consist of poorly consolidated surface materials underlain by more cohesive bedrock we infer that an ice covered lake with a surface elevation 2500 m probably existed in eastern candor chasma because this canyon is intersected by the ophir catenae fault system from which allegheny vallis and walla walla vallis originated we introduce a new hydrology concept for mars in which the groundwater system was augmented by recharge from canyon lakes that were formed when water was released by catastrophic melting of former ice sheets in tharsis by effusions of flood basalts this model could help to reconcile the expected presence of a thick cryosphere during the hesperian with the abundant evidence for groundwater as a source for some of the circum chryse outflow channels

the evolution of lacustrine environments on mars is mars only hydrologically dormant abstract this study surveys the evolution of martian impact crater paleolakes through time by establishing their age comparing their locations and describing the formation of lacustrine sedimentary structures and features over time we explore their meaning in terms of global climate changes but also on local and regional scales we observe a co location between recent impact crater paleolakes and regions where gcm modeling shows that mars is currently experiencing days above triple point and below boiling point of water some source areas show morphologies that could be consistent with ancient glacier features in the argyre basin darwin crater region in locations that are also consistent with gcm models of past snow precipitation on mars we propose that at least for some paleolakes our results are best explained by recent and abundant aqueous activity on mars with some meteorological contribution these results are supported by other recent findings on martian meteorite ages by the analysis of new mgs data showing recent volcanic and fluvial activity and by climate modeling they point toward the possibility that mars currently is only hydrologically dormant and that there is a potential for hydrological activity to be generated again in the future following changes in the planet s orbital parameters

the geologic evolution of mars episodicity of resurfacing events and ages from cratering analysis of image data and correlation with radiometric ages of martian meteorites we have investigated 12 prominent regions on mars in detail with respect to their geologic evolution through time on the basis of detailed geologic mapping exercises and determining age relationships through crater counting techniques using imagery new data in combination with previously obtained data have been analyzed by way of a refined method of cratering age extraction that also gives fine details of periods of resurfacing we have found that there has been volcanic and fluvial glacial geologic activity on the martian surface at all times from 4 ga ago until today this activity shows episodic pulses in intensity of both volcanic and fluvial glacial processes at 3 8 3 3 ga 2 0 1 8 ga 1 6 to 1 2 ga 800 to 300 m y 200 m y and 100 m y and a possible weaker phase around 2 5 2 2 ga ago in between these episodes there was relative quiescence of volcanic and or fluvial glacial activity the episodes we find on the martian surface in the crater frequency analyses of hrsc moc and themis data coincide with some age groups of the martian meteorites 1 3 ga 600 300 m y 170 m y it appears that the surface activity expressions and their episodicity are related to the interior evolution of the planet when convection in the asymptotic stationary state changes from the so called stagnant lid regime to an episodic behavior similarities in episodic behavior are found for the other terrestrial planets venus the earth s moon and the earth itself suggesting a common general relationship in the evolutionary tracks

amazonian geologic history of the echus chasma and kasei valles system on mars new data and interpretations abstract new high resolution datasets have prompted a mapping based study of the 2500 km long echus chasma and kasei valles system that utilizes geomorphic details stratigraphic relations and cratering statistics derived from the new datasets our results suggest that between the hesperian and amazonian epochs on mars 3 7 ga to recent the study area was affected by at least 4 episodes of widespread volcanic activity and 4 periods of episodic fluvio glacial activity this paper discusses the amazonian

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

a record of temperature and monsoon intensity over the past 40 kyr from groundwater in the north china plain abstract the well known temperature dependence of noble gas solubilities in water enables the determination of absolute paleotemperature records from radiocarbon dated groundwater an advantage of the noble gas paleothermometer is that noble gas temperatures ngts can directly be related to stable isotope ratios from the same groundwater archive in order to calibrate the stable isotope thermometer over long time scales our record from heavily used aquifers of the north china plain constitutes a unique combined ngt and stable isotope data set from the region affected by the east asian monsoon in addition to providing a reliable estimate for the glacial interglacial temperature change the data enable the separation of the effects of temperature and precipitation monsoon strength on the stable isotopes although the record extends from modern waters to near the limit of the radiocarbon dating range at about 40 kyr bp the last glacial maximum is not represented probably because arid conditions limited groundwater recharge during that period the coldest observed period at about 30 kyr bp was 4 6 1 2 c cooler than the holocene whereas ngts show no systematic variation during the holocene the 18 o values increased by about 1 between the early and late holocene a similar increase of 18 o has been observed before in speleothems and ascribed to decreasing monsoon strength in response to decreasing insolation during the holocene our results confirm that the holocene trend in stable isotopes is not related to temperature in contrast 18 o and ngt are correlated within the pleistocene group of samples indicating a 18 o ngt slope of 0 18 0 05 c 1 similar to the temperature effect in modern precipitation of the region thus of the overall increase of 18 o by about 2 between 30 kyr bp and the late holocene about half is due to a temperature increase whereas the other half is due to a decrease of monsoon strength