sedimentary resurfacing and fretted terrain development along the crustal dichotomy boundary aeolis mensae mars 1 the evolution of the martian crustal dichotomy boundary which separates the southern cratered highlands from the northern lowland plains by 1 3 km of elevation remains among the fundamental outstanding issues in mars research for a study area at aeolis mensae we show that fretted terrain formed exclusively in a 2 km thick late noachian 3 7 ga sedimentary deposit that overlies the base of an older cratered dichotomy boundary slope in this equatorial study area fretted terrain does not exhibit the debris aprons or lineated valley fills that are attributed to ground ice in otherwise similar midlatitude fretted terrain in arabia terra the massive deposit of fine sand or loess was probably transported from the north by wind and trapped against the precursor dichotomy slope producing a similar initial form to the younger medusae fossae layered materials that occur east of aeolis mensae contemporary with the latest noachian to early hesperian decline in fluvial erosion the fretting process likely initiated as the massive layer s indurated surface was compromised by fracture cratering or collapse into possible voids in these depressions grain impact or contact with water disaggregated the fine sedimentary materials which were then largely deflated by wind the fretting process largely ended when liquid water was no longer widely available for weathering during the early hesperian period although some degradation of the region by aeolian and slope processes has continued to the present

planetary science into thin martian air a dense early atmosphere has been invoked to explain the strong greenhouse effect inferred for early mars yet an analysis of the smallest impact craters suggests that the atmospheric pressure on mars 3 6 billion years ago was surprisingly low

the age of the medusae fossae formation evidence of hesperian emplacement from crater morphology stratigraphy and ancient lava contacts abstract the medusae fossae formation mff covering about 2 1 10 6 km 2 with an estimated volume of 1 4 10 6 km 3 and straddling the equatorial region of mars east of tharsis has historically been mapped and dated as amazonian in age analysis of the mff using a range of new observations from recent mission data at multiple resolutions reveals evidence that the formation is older than previously hypothesized with parts of the mff having formed in the hesperian and parts having been reworked and reformed throughout the amazonian up to the present ancient outcroppings of the mff edged with jagged yardangs became a mold for embaying hesperian aged lavas the erosion of the mff left solidified lava casts in the embaying lava unit this lava edge morphology permits the identification of ancient contacts between the mff and hesperian aged lava terrain additionally the flanking fan of the hesperian aged apollinaris patera volcano embays the formation at its foot indicating that parts of the mff were formed in the hesperian erosion has erased and inverted many of the superposed craters in the region showing that very young amazonian ages derived from impact crater size frequency distributions are resurfacing ages and not emplacement ages we find abundant evidence that the formation is extremely mobile and continuously reworked we conclude that a significant part of the mff may have originally been emplaced in the hesperian these observations place new constraints on the mode of origin of the mff

interpreting avulsion process from ancient alluvial sequences guadalope matarranya system northern spain and wasatch formation western colorado alluvial deposits of the guadalope matarranya system oligocene ebro basin spain and the wasatch formation eocene western colorado provide time integrated records of the process of river channel avulsion these sequences consist of isolated channel belt sandstones incised into and abruptly overlain by flood plain siltstones indicating deposition by avulsive river systems the geometry and distribution of channel incisions suggest that avulsion was not controlled by tectonics climate or base level changes but formed by autocyclic processes measurements from 221 channel fills in the guadalope matarranya system and 38 from the wasatch formation allow us to statistically characterize channel geometries we infer to be associated with establishment and abandonment of individual river avulsions paleoflow depths in both systems average 1 4 to 1 6 m aggradation height superelevation of channel margin levees are on average 0 6 and 1 1 times paleoflow depth in the guadalope matarranya and wasatch systems respectively these results are consistent with values from recently avulsed modern rivers and suggest that 1 flow depth is the appropriate parameter against which to scale the critical superelevation necessary for channel avulsion and 2 the increase in potential energy due to channel perching drives the lateral instability that is needed for avulsion to be successful numerous stacked channel fills indicate repeated reoccupation of the same site by avulsing channels these reoccupation channels indicate that inherited flood plain topography here abandoned channel forms was an important control on the arrival site of newly avulsed channels comparison of our results to others suggests two end member types of avulsion can take place incisional avulsion seen here is characterized by an early incision phase followed by infilling by migrating bar forms aggradational avulsion begins with aggradation followed in time by stream integration into a single downcutting channel we suggest that the type of avulsion is strongly influenced by whether or not the adjacent flood plain is well or poorly drained in both cases subsequent aggradation and channel perching increase the chances that some triggering event will lead to avulsion

loess and floods high resolution multi proxy data of last glacial maximum lgm slackwater deposition in the flinders ranges semi arid south australia abstract terrace remnants of late pleistocene fine grained valley fill deposits silts deeply entrenched by ephemeral traction load streams in arid areas remain a puzzle they have been attributed to a variety of origins ranging from lacustrine to alluvial floodplains we here report a centimetre scale multi proxy study of a 7 m section of silts in the semi arid flinders ranges of south australia which span the lead up to and peak of the last glacial maximum lgm the results of detailed lithostratigraphic mapping high resolution parametric particle size analysis automated mineralogy induced magnetic susceptibility carbon stable isotope geochemistry and a chronostratigraphy based on 27 ams radiocarbon and 6 luminescence ages are discussed in terms of sediment provenance depositional environment and weathering history with the aim of reconstructing the regional palaeo environment the data are consistent with a fluctuating aeolian fluvial interplay dominating the extended lgm environment with a greater impact on the landscape than all combined geomorphic processes since then accordingly weathered slope mantles and loess accessions were eroded and entrained by numerous small and at least a dozen large scale flood events and trapped in an intra montane floodplain extending into brachina gorge upstream of this narrow constriction recurrent backflooding is discussed resulting in a thick sequence of layered to laminated slackwater couplets aggradation and degradation of valley fills appear to be largely controlled by fine sediment supply from the valley slopes replenished by wind blown dust from upwind playa lakes and source bordering dunefields in conclusion this study demonstrates how dust storms and flooding rains can account for pluvial features previously explained by the opposing effects of reduced precipitation and evaporation in the colder more arid glacial landscape of southern australia

modeling the thermal and physical evolution of mount sharp s sedimentary rocks gale crater mars implications for diagenesis on the msl curiosity rover traverse gale crater the mars science laboratory msl landing site contains a central mound named aeolis mons informally mount sharp that preserves 5 km of sedimentary stratigraphy formation scenarios include 1 complete filling of gale crater followed by partial sediment removal or 2 building of a central deposit with morphology controlled by slope winds and only incomplete sedimentary fill here we model temperature time paths for both scenarios compare results with analyses provided by msl curiosity and provide scenario dependent predictions of temperatures of diagenesis along curiosity s future traverse the effects of variable sediment thermal conductivity and historical heat flows are also discussed modeled erosion and deposition rates are 5 37 m yr consistent with previously published estimates from other mars locations the occurrence and spatial patterns of diagenesis depend on sedimentation scenario and surface paleotemperature for 1 temperatures experienced by sediments decrease monotonically along the traverse and up mount sharp stratigraphy whereas for 2 temperatures increase along the traverse reaching maximum temperatures higher up in mount sharp s lower units if early mars surface temperatures were similar to modern mars mean 50 c only select locations under select scenarios permit diagenetic fluids in contrast if early mars surface temperatures averaged 0 c or brines had lowered freezing points diagenesis is predicted in most locations with temperatures 225 c comparing our predictions with future msl results on diagenetic textures secondary mineral assemblages and their spatial variability will constrain past heat flow mount sharp s formation processes the availability of liquid water on early mars and sediment organic preservation potential

cosmogenic 3 he in hematite and goethite from brazilian canga duricrust demonstrates the extreme stability of these surfaces helium isotopes were measured in hematite and goethite samples from several lateritiric duricrusts canga developed on banded iron formations these samples uniformly have high 3 he concentrations which must arise from long periods of cosmic ray exposure from coexisting phases from the quadrilatero ferrifero in east central brazil we determined the ratio of cosmogenic 3 he in hematite to that of 21 ne in quartz to be 3 96 0 19 combined with best current estimates of the 21 ne production rate in quartz this ratio implies a sea level high latitude slhl 3 he production rate in hematite of 68 1 8 1 atoms g yr from the chemical composition we estimate the 3 he production rate in goethite to be 5 higher we use these production rate estimates to interpret 3 he concentrations measured in goethite and hematite from a 10 m depth profile collected from a surface canga in carajas in the amazon basin of brazil we find that the carajas canga has experienced a very low rate of surface erosion 0 16 0 54 m myr over at least the last few millions of years this iron rich canga surface is remarkably resistant to erosion despite its location in a wet tropical environment details of the depth profile suggest that despite its stability the canga has also been internally dynamic translocation of material solution and reprecipitation over million year timescales

transience and persistence in the depositional record of continental margins 1 continental shelves and coastal plains are large persistent depositional landforms which are stationary nonmigrating at their proximal ends and characterized by relatively steady long term growth in detail however their surface form and stratigraphic record is built of transient freely migrating landscape elements we derive the timescales of crossover from transient to persistent topographic forms using empirical scaling relations for mean sediment accumulation as a function of averaging time based upon tens of thousands of empirical measurements a stochastic noisy diffusion model with drift predicts all the gross features of the empirical data it satisfies first order goals of describing both the surface morphology and stratigraphic completeness of depositional systems the model crossover from noise dominated to drift dominated behavior corresponds to the empirical crossover from transport dominated autogenic transient behavior to accommodation dominated subsidence persistent behavior which begins at timescales of 102 103 years and is complete by scales of 104 105 years because the same long term scaling behavior emerges for off shelf environments it is not entirely explicable by steady subsidence fluctuations in sediment supply and routing probably have significant influence at short term transient scales the exponents of the scaling relations vary with environment particularly the prevalence of channeled sediment transport at very small scales modeling sediment transport as a diffusive process is inappropriate our results indicate that some of the timescales of interest for climate interpretation may fall within the transitional interval where neither accommodation nor transport processes are negligible and deconvolution is most challenging

veneers rinds and fracture fills relatively late alteration of sedimentary rocks at meridiani planum mars veneers and thicker rinds that coat outcrop surfaces and partially cemented fracture fills formed perpendicular to bedding document relatively late stage alteration of ancient sedimentary rocks at meridiani planum mars the chemistry of submillimeter thick buff colored veneers reflects multiple processes at work since the establishment of the current plains surface veneer composition is dominated by the mixing of silicate rich dust and sulfate rich outcrop surface but it has also been influenced by mineral precipitation including nacl and possibly by limited physical or chemical weathering of sulfate minerals competing processes of chemical alteration perhaps mediated by thin films of water or water vapor beneath blanketing soils and sandblasting of exposed outcrop surfaces determine the current distribution of veneers dark toned rinds several millimeters thick reflect more extensive surface alteration but also indicate combined dust admixture halite precipitation and possible minor sulfate removal cemented fracture fills that are differentially resistant to erosion occur along the margins of linear fracture systems possibly related to impact these appear to reflect limited groundwater activity along the margins of fractures cementing mechanically introduced fill derived principally from outcrop rocks the limited thickness and spatial distribution of these three features suggest that aqueous activity has been rare and transient or has operated at exceedingly low rates during the protracted interval since outcropping meridiani strata were exposed on the plains surface

on the structure and dynamics of indian monsoon depressions abstractera interim reanalysis data from the past 35 years have been used with a newly developed feature tracking algorithm to identify indian monsoon depressions originating in or near the bay of bengal these were then rotated centralized and combined to give a fully three dimensional 106 depression composite structure a considerably larger sample than any previous detailed study on monsoon depressions and their structure many known features of depression structure are confirmed particularly the existence of a maximum to the southwest of the center in rainfall and other fields and a westward axial tilt in others additionally the depressions are found to have significant asymmetry owing to the presence of the himalayas a bimodal midtropospheric potential vorticity core a separation into thermally cold 1 5 k and neutral 0 k cores near the surface with distinct properties and the center has very large cape and very small cin variability as a function of background state has also been explo

degradation of the organic molecules in the shallow subsurface of mars due to irradiation by cosmic rays 1 detection of the organic matter on mars is one of the main goals of the future martian landing missions yet the degradation of organic molecules by cosmic ray irradiation on mars is often ignored we calculate the radiation dose accumulation rates from solar and galactic cosmic rays at various depths in the shallow martian subsurface we demonstrate that a 1 billion year outcrop on mars accumulates the dosage of 500 mgy in the top 0 2 cm and 50 mgy at 5 10 cm depths we show that the preservation of ancient complex organic molecules in the shallow 10 cm depth subsurface of rocks could be highly problematic if the exposure age of a geologic outcrop would exceed 300 myr we demonstrate that more simple organic molecules with masses 100 amu should have a good chance to survive in the shallow subsurface of rocks implications to the sampling strategy for the oncoming martian missions are discussed

the timing of martian valley network activity constraints from buffered crater counting abstract valley networks concentrations of dendritic channels that often suggest widespread pluvial and fluvial activity have been cited as indicators that the climate of mars differed significantly in the past from the present hyperarid cold desert conditions some researchers suggest that the change in climate was abrupt while others favor a much more gradual transition thus the precise timing of valley network formation is critical to understanding the climate history on mars we examine thirty valley network incised regions on mars including both cratered upland valley networks and those outside the uplands and apply a buffered crater counting technique to directly constrain when valley network formation occurred the crater populations that we derive using this approach allow assessment of the timing of the last activity in a valley network independent of the mapping of specific geological units from these measurements we find that valley networks cluster into two subdivisions in terms characteristics and age 1 valley network activity in the cratered highlands has an average cessation age at the noachian hesperian boundary and all valleys that we crater counted are early hesperian or older no evidence is found for valley networks in the cratered uplands of late hesperian or amazonian age the timing of the cessation of cratered upland valley network activity at the noachian hesperian boundary also corresponds to a decline in the intensity of large crater formation and degradation and to the apparent end of phyllosilicate type weathering 2 a few valley network incised regions formed outside of the cratered uplands on volcanic edifices in association with younger impact craters and on the rim of valles marineris we applied our buffered crater counting technique to four such valleys on the volcanoes ceraunius tholus hecates tholus and alba patera and on the rim of echus chasma and find that each has distinctive and different late hesperian or early amazonian ages indicating that valley networks formed from time to time in the post noachian period unlike the cratered upland valley networks these isolated occurrences are very local and have been interpreted to represent local conditions e g snowpack melted during periods of intrusive volcanic activity in contrast to a gradual cessation in the formation of valley networks proposed by some workers our new buffered crater counting results indicate a relatively abrupt cessation in the formation of the widespread cratered upland valley networks at approximately the end of the noachian followed only by episodic and very localized valley network formation in later mars history very likely due to specific conditions e g local magmatic heating these valley network ages and correlations are thus consistent with a major change in the near surface aqueous environment on mars at approximately the noachian hesperian boundary the noachian environment supported surface running water and fluvial erosion across mars in the cratered uplands enhanced crater degradation and a weathering environment favoring the formation of phyllosilicates the hesperian amazonian environment was more similar to the hyperarid cold desert of today with valley networks forming only extremely rarely and confined to localized special conditions sources of water for these latter occurrences are likely to be related to periodic mobilization and equatorward migration of polar volatiles due to variations in spin axis orbital parameters and to periodic catastrophic emergence of groundwater

extraterrestrial iridium sediment accumulation and the habitability of the early earth s surface we have measured the concentrations of iridium and platinum in ancient metasediments from akilia island southern west greenland to obtain the first direct constraints on the extraterrestrial mass flux at the earth s surface before 3 8 gyr the lunar impact record indicates this was a time of heavy bombardment the average lunar cratering rate between 4 15 and 3 8 gyr was elevated 150 times the modern value hence heightened ir and pt abundances are possible in sediments from this time however the observed concentrations of ir and pt in the akilia metasediments are extremely low in most samples ir 3 ppt and pt 20 ppt in contrast the average modern crustal abundances of ir and pt are 50 ppt and 2 ppb respectively the very low concentrations observed in the akilia metasediments cherts and banded iron formations can be reconciled with the lunar record by quantifying the effects of sedimentation rate on the efficiency with which the extraterrestrial mass flux is sampled the observations are explained if rapidly accumulating sediments sampled stochastic bombardment by an impactor population governed by a power law mass distribution such that exogenous ir and pt are concentrated in rare horizons the required sedimentation rates are geologically plausible this explanation has two consequences first evidence of massive impact events may be rare in sediments older than 3 8 gyr despite heavy bombardment second conditions at the earth s surface may have been conducive for extensive accumulation of sedimentary sequences and for microbial life during most of the period 4 15 3 8 gyr it is possible that the earth s surface hosted microbial habitats during most of this time if microbes living in deep sea or deep crustal environments survived photic zone vaporizing impacts that would have occurred every 30 myr hence evidence of life is expected even in the earliest sedimentary sequences

surficial properties in gale crater mars from mars odyssey themis data abstract we examine the nature of the surface layer in gale crater as determined from high resolution thermal and visible mars odyssey thermal emission imaging system themis data as well as how our conclusions compare to past analyses at themis resolution the thermal surface structure is dominated by local control thus providing us with detailed images that contain thermophysical information as well using these data sets we have created a map of the area defining units based primarily on their geomorphology as determined from the daytime thermal and visible images and then using the nighttime thermal data to interpret the nature of the surface layer within each unit seven units have been defined i partially blanketed knobby plateaus ii crater walls with terrain similar to that on the plateaus on the upper half and exposed rocky surfaces on the lower half iii v three floor units with varying combinations of bedrock and indurated and or particulate deposits vi sand sheets and vii a central mound consisting of indurated and or rocky material forming layers terraces and slides covered by particulate material that tapers in thickness downslope additionally dozens of channels have been observed on the crater walls and central mound the results indicate that aeolian processes have played a major role in shaping much of the present surface layer within gale and may still be active today because of the dramatic size and structure of gale the winds are most likely controlled by the local topography additionally the presence and frequency of channels within gale bolster hypotheses involving aqueous episodes in the history of the crater

real and apparent changes in sediment deposition rates through time 1 field measurements show that estimated sediment deposition rate decreases as a power law function of the measurement interval this apparent decrease in sediment deposition has been attributed to completeness of the sedimentary record the effect arises because of incorporation of longer hiatuses in deposition as averaging time is increased we demonstrate that a heavy tailed distribution of periods of nondeposition hiatuses produces this phenomenon and that observed accumulation rate decreases as t 1 over multiple orders of magnitude where 0 1 is the parameter describing the tail of the distribution of quiescent period length by using continuous time random walks and limit theory we can estimate the actual average deposition rate from observations of the surface location over time if geologic and geometric constraints place an upper limit on the length of hiatuses then average accumulation rates approach a constant value at very long times our model suggests an alternative explanation for the apparent increase in global sediment accumulation rates over the last 5 million years

the dynamics of heat lows a numerical model is used to investigate dynamical aspects of the structure and evolution of a heat low in an idealized flow configuration with an area of land surrounded by sea of particular interest is the evolution of the distributions of relative vorticity and potential vorticity while the heat low has a minimum surface pressure in the late afternoon following strong solar heating of the land the relative vorticity is strongest in the early morning hours following a prolonged period of low level convergence thus the heat low is not approximately in quasi geostrophic balance the low level convergence is associated with the sea breeze and later with the nocturnal low level jet the effects of differing sea area land area and coriolis parameter on various aspects of the heat low are investigated although a cyclonic vortex the heat low is characterized by an anticyclonic potential vorticity anomaly relative to its environment throughout much of the lower troposphere on account of the greatly reduced static stability in the convectively well mixed boundary layer however the surface temperature maximum over land corresponds with a cyclonic potential vorticity anomaly at the surface the reduced static stability in the mixed layer has the further consequence that the horizontal components of relative vorticity and horizontal potential temperature gradient make a non negligible contribution and of opposite sign to the potential vorticity in certain flow regions two processes associated with the flow evolution in the model appear to be fundamental to understanding a range of low level atmospheric phenomena over the arid interior of australia these are the deep convective mixing over land during the daytime and the development of a nocturnal low level jet which leads to convergence in the trough such phenomena include the diurnal behaviour of dry cold fronts and the generation of nocturnal wind surges and bores it is reasonable to assume that similar processes operate in other arid regions of the world where deep convective mixing over land produces local maxima of diabatic heating in the lower atmosphere

stratigraphy of aeolis dorsa mars stratigraphic context of the great river deposits unraveling the stratigraphic record is the key to understanding ancient climate and past climate changes on mars grotzinger j et al 2011 astrobiology 11 77 87 stratigraphic records of river deposits hold particular promise because rain or snowmelt must exceed infiltration plus evaporation to allow sediment transport by rivers therefore river deposits when placed in stratigraphic order could constrain the number magnitudes and durations of the wettest and presumably most habitable climates in mars history we use crosscutting relationships to establish the stratigraphic context of river and alluvial fan deposits in the aeolis dorsa sedimentary basin 10 e of gale crater at aeolis dorsa wind erosion has exhumed a stratigraphic section of sedimentary rocks consisting of at least four unconformity bounded rock packages recording three or more distinct episodes of surface runoff early deposits 700 m thick are embayed by river deposits 400 m thick which are in turn unconformably draped by fan shaped deposits 900 m thick unconformably drape all previous deposits river deposits embay a dissected landscape formed of sedimentary rock the river deposits are eroding out of at least two distinguishable units there is evidence for pulses of erosion during the interval of river deposition the total interval spanned by river deposits is 1 10 6 2 10 7 yr and this is extended if we include alluvial fan deposits alluvial fan deposits unconformably postdate thrust faults which crosscut the river deposits this relationship suggests a relatively dry interval of 4 10 7 yr after the river deposits formed and before the fan shaped deposits formed based on probability arguments yardang forming layered deposits unconformably postdate all of the earlier deposits they contain rhythmite and their induration suggests a damp or wet near surface environment the time gap between the end of river deposition and the onset of yardang forming layered deposits is constrained to 1 10 8 yr by the high density of impact craters embedded at the unconformity the time gap between the end of alluvial fan deposition and the onset of yardang forming layered deposits was at least long enough for wind induced saltation abrasion to erode 20 30 m into the alluvial fan deposits we correlate the yardang forming layered deposits to the upper layers of gale crater s mound mt sharp aeolis mons and the fan shaped deposits to peace vallis fan in gale crater alternations between periods of low mean obliquity and periods of high mean obliquity may have modulated erosion deposition cycling in aeolis this is consistent with the results from an ensemble of simulations of solar system orbital evolution and the resulting history of the obliquity of mars 57 of our 61 simulations produce one or more intervals of continuously low mean mars obliquity that are long enough to match our aeolis dorsa unconformity data

timescales of fluvial activity and intermittency in milna crater mars milna crater mars 23 4s 12 3w exhibits signs of fluvial modification early in mars history including a large multi lobed fan deposit cut by several sinuous valleys we describe the past hydrologic conditions in milna and the surrounding area including a potential lake with a volume of 50 km 3 we also introduce new methods i to calculate the timescale of sediment deposition by considering fluvial sediment input into the entire crater while accounting for non fluvial input and ii to place improved constraints on the channel dimensions through which sediment was delivered to milna by comparing to the dimensions of inner channels found in valleys in the region surrounding milna by calculating the flux of fluid and sediment into the crater we find that the crater cavity was flooded for at least months and that the time of active fluvial sediment transport without hiatus is on the order of decades to centuries with a preferred timescale of centuries we also calculate the total amount of water required to transport the volume of sediment we measure in milna and conclude that impacts alone are likely insufficient to deliver enough water to milna to allow the sedimentary fill we see combining the timescales of fluvial activity in the adjacent parana valles with estimates for global noachian erosion rates we calculate an intermittency factor for fluvial activity of 0 01 0 1 during 10 5 10 6 yr near the noachian hesperian boundary in the parana valles region these values are comparable to arid climates on earth where the majority of fluvial sedimentary transport takes place during floods with multi year to decadal recurrence intervals our calculations of intermittency help to quantitatively reconcile the divergent estimates of the short and long timescales of fluvial activity on mars reported in the literature

coherent synoptic disturbances in the australian monsoon abstractcoherent synoptic scale weather systems within the australian monsoon are identified and tracked in the isentropic potential vorticity pv field from the ecmwf interim reanalysis era interim dataset during the southern hemisphere summer the resulting dataset is then used to compile statistics and synoptic composites of australian monsoon disturbances on average a synoptic system is found in the region every 2 5 days however the time interval between consecutive events is highly variable meaning that the synoptic activity in the australian monsoon is not well represented by commonly employed spectral techniques the analysis reveals that most synoptic systems originate within the australian monsoon but at the 315 k level approximately 700 hpa a significant proportion of systems are first detected near the east coast of the continent where extratropical rossby waves are observed to frequently break the average australian monsoon weather system propagates from east to west at approximatel

mars sedimentary rock erosion rates constrained using crater counts with applications to organic matter preservation and to the global dust cycle abstract small crater counts on mars light toned sedimentary rock are often inconsistent with any isochron these data are usually plotted then ignored we show using an 18 hirise image 104 crater dataset that these non isochron crater counts are often well fit by a model where crater production is balanced by crater obliteration via steady exhumation for these regions we fit erosion rates we infer that mars light toned sedimentary rocks typically erode at 102 nm yr when averaged over 10 km2 scales and 107 108 yr timescales crater based erosion rate determination is consistent with independent techniques but can be applied to nearly all light toned sedimentary rocks on mars erosion is swift enough that radiolysis cannot destroy complex organic matter at some locations e g paleolake deposits at sw melas but radiolysis is a severe problem at other locations e g oxia planum the data suggest that the relief of the valles marineris mounds is currently being reduced by wind erosion and that dust production on mars

the formation of martian river valleys by impacts we explore the role of large impacts in creating the martian valley networks recent dating shows that some large impact basins are contemporaneous with the valley networks the mass deposited and volatiles released by impacts is large and comparable with the mass from the tharsis volcanic construct steam atmospheres formed after large impacts can produce more than 600 m of rainfall followed by rainfall from water vapor greenhouse atmospheres and snowmelt the erosion rates from impacts that created the currently visible craters are somewhat less than the erosion rates suggested for the noachian 4 2 to 3 82 gya there are several possible explanations for this difference and it is possible that erosion rates are overestimated because the burial of small craters by global debris layers from impacts has not been considered rainfall after the noachian was low because the impact rate and co2 pressure declined we suggest tests of the hypothesis that impacts caused the river valleys

role of ocean mesoscale structures in shaping the angola low pressure system and the southern africa rainfall southern african climate is under the influence of both tropical and subtropical systems which result in a complex region where important interactions co exist over a large spectrum of spatiotemporal scales the angola low al situated on boundary between tropical and subtropical southern africa has been diagnosed as a key driver of moisture distribution in the region on daily to seasonal time scales it has been demonstrated that the al pressure system is sensitive to the dynamics of the neighbouring oceans but to date no study has considered the model resolution of air sea interactions required to simulate this sensitivity using sensitivity experiments with a regional atmospheric model which differ only in the mesoscale sea surface temperature sst forcing characteristics either the full spectrum of sst variability or only its large scale components are included we first quantify the importance of sst gradients on the al strength and variability the results suggest that the mesoscale sst variability of the angola benguela frontal zone abfz plays a key role in al activity particularly during the late summer synoptic scale tropical lows which form the al are automatically detected and the results suggest more extreme events occur when the model is forced by mesoscale ssts everywhere and in the abfz area only the rainfall resulting from those events suggests that tropical low episodes are associated with nearly 15 of the total rain in angola and namibia the link between al dynamics and wet spells is also discussed with the former showing a different spatial pattern as well as frequency when the ocean is fully resolved

the dynamics of heat lows in simple background flows a three dimensional numerical model is used to study the evolution of an idealized heat low that develops over an isolated level land mass surrounded by sea calculations are carried out without a background flow and for simple background flows in all cases the flow shows a significant diurnal variation in which the sea breeze circulations are the prominent feature during the daytime and the nocturnal low level jet is the prominent feature at night if the land area is large enough so that the sea breezes do not cover it entirely during the diurnal cycle strong convergence associated with the low level jet leads to the formation of intense but shallow cold fronts along the inland boundaries of sea breeze air these fronts decay rapidly after sunrise when surface heating leads to renewed vertical mixing which destroys the low level stable layer the presence of a uniform easterly flow leads to the formation of a west coast trough similar to the situation commonly observed over western australia the trough broadens during the day due to the heating of the land while at night it sharpens and again frontogenesis occurs at low levels near the leading edge of the sea breezes to form shallow cold fronts in these calculations there is an east west asymmetry in the sea breeze circulations the cool air behind the west coast sea breeze is shallower than that behind the east coast sea breeze but the vertical circulation associated with the west coast sea breeze is deeper the east coast sea breeze front penetrates further inland but is more diffuse and is recognizable more by its signature in the relative vorticity than by that in the horizontal temperature gradient the presence of a horizontal shear flow leads to the deformation of the heat trough a process that appears to play an important role in the formation of cold fronts over central australia again the calculations show the formation of shallow fronts over the land during the night which frontolyse rapidly after sunrise despite the idealized nature of the calculations the associated patterns of low level vorticity divergence and horizontal temperature gradient that develop overnight show remarkable similarity to those observed over central australia indeed the calculations help to interpret the observations of nocturnal frontogenesis over this region copyright 2005 royal meteorological society

formation timescales of large martian valley networks abstract assessing timescales for formation of the ancient martian valley networks is key to interpreting the early climate on mars we determined the likely formation times for seven of the largest ancient valley networks on mars using the darcy weisbach equation for average flow velocity three different sediment transport models and a range of input parameters to encompass possible formation conditions with runoff rates similar to intense storms in arid regions on earth the minimum formation timescales of these martian valley networks range from 10 5 to 10 7 yr depending on the specific valley network shorter formation timescales require hurricane scale flows that if minimized with assumptions of continuous formation unlike even terrestrial rates could complete large valley network formation in as little as 200 to 5000 yr though this is not the preferred interpretation while these results do not support impact induced climate change as the sole mechanism for creating precipitation sufficient to incise the large valley networks neither do they extend the amount of time required to form the networks beyond their range in age consistent with hypotheses that valley incision was constrained to a relatively short period of martian history near the noachian hesperian boundary approximately 3 6 to 3 8 ga

fluvial sinuous ridges of the morrison formation usa meandering scarp retreat and implications for mars sinuous ridges have been interpreted as evidence for ancient rivers on mars but relating ridge geometry to paleo hydraulics remains uncertain three analog ridge systems from the morrison formation utah are composed of sandstone caprocks up to 50 m wide and 8 m thick atop mudstone flanks ridge caprocks have narrowed significantly compared to sandstone bodies preserved in outcrop consistent with a new ridge erosion model that can be used to estimate original sandstone body extent ridge networks represent caprocks intersecting at distinct stratigraphic levels rather than a preserved channel network caprocks are interpreted as amalgamated channel belts rather than inverted channels with dune and bar cross stratification that was used to reconstruct paleo channel dimensions curvilinear features on ridge tops are outcropping lateral accretion sets las from point bars and indicate meandering we found that caprock thickness scales with paleo channel depth and las curvature scales with paleo channel width application of these relations to a ridge in aeolis dorsa mars yielded consistent water discharge estimates 310 1800 m s in contrast using ridge width or ridge radius of curvature as paleo channel proxies overestimated discharge by a factor of 30 500 in addition the ridge erosion model suggests that scarp retreat may be less efficient on mars resulting in taller and wider ridges with more intact caprocks altogether our results support the hypothesis that ridges are exhumed channel belts and floodplain deposits implying long lived fluvial activity recorded within a depositional basin

the eocene biostratigraphy of new mexico the san jose galisteo and baca formations are continental deposits in new mexico containing eocene vertebrate fossils the almagre and largo local faunas of the san jose formation in the san juan basin are lysitean middle wasatchian the cerrillos local fauna of the galisteo formation also is wasatchian possibly lysitean strata of the baca formation in south central new mexico contain the titanotheres palaeosyops sp and cf manteoceras sp and are the only bridgerian strata known in new mexico the tonque local fauna of the galisteo formation and fossil vertebrates from the baca formation in west central new mexico are correlatives of the lapoint utah pearson ranch california and porvenir texas local faunas and are duchesnean precise placement of the paleocene eocene and eocene oligocene boundaries in new mexico using vertebrate biostratigraphy is impossible because of a lack of fossils at critical horizons the near absence of bridgerian and total absence of uintan fossils in new mexico may result from a lack of collecting particularly in parts of the baca formation

late alluvial fan formation in southern margaritifer terra mars 1 crater statistics indicate alluvial fans crater floor and fill mantling deposits within impact craters 50 km in diameter in southern margaritifer terra were likely emplaced during multiple epochs fans formed during the amazonian or near the amazonian hesperian boundary crater floor deposits are likely hesperian in age and most fill mantling deposits are amazonian the regional distribution of fans points to a late period of widespread water driven degradation two of the final candidate landing sites for the 2011 mars science laboratory mission would land on or near some of these fans which appear younger than previously considered

hemispheres apart the crustal dichotomy on mars the hemispheric dichotomy is a fundamental feature of mars expressed by a physiographic and geologic divide between the heavily cratered southern highlands and the relatively smooth plains of the northern lowlands the origin of the dichotomy which may have set the course for most of the subsequent geologic evolution of mars remains unclear internally driven models for the dichotomy form the lowlands by mantle convection plate tectonics or early mantle overturn externally driven models invoke one giant impact or multiple impacts areal densities of buried basins expressed by quasicircular depressions and subsurface echoes in radar sounding data suggest that the dichotomy formed early in the geologic evolution of mars tectonic features along the dichotomy boundary suggest late stage modification by flexure or relaxation of the highlands after volcanic resurfacing of the northern lowlands subsequent deposition and erosion by fluvial aeolian and glacial processes shaped the present day dichotomy boundary

study of ice related flow features around tanaica montes mars implications for late amazonian debris covered glaciation abstract lobate debris aprons lda and lineated valley fill lvf have been broadly recognized in the mid latitudes of mars and their subsequent analyses using data from the shallow radar sharad instrument has suggested evidence for contemporary ice preserved beneath these features in this study we conduct detailed characterization of newly identified lda flow units within the tanaica montes region 39 55 n 269 17 e of mars to assess and understand the similarities in their emplacement with respect to lda flow units mapped in other regions of mars we utilize the mars reconnaissance orbiter mro context camera ctx images and shallow radar sharad datasets for geomorphic and subsurface analysis and mars global surveyor mgs mars orbiter laser altimeter mola point tracks for topographic analysis geomorphic observation of lda flow units surrounding the montes flanks and massif walls reveal integrated pattern of convergence and divergence and evidence of bending and deflection within the flow lines that resulted in concentric loop like flow patterns in the downslope brain terrain texture and craters with varying morphological characteristics ring mold type is suggestive that ldas may be similar to ice rich debris covered glaciers mola point track based convex up topographic profiles of ldas suggest that their thickness vary in the range of 100 200 m in both the northwestern and southeastern portions of study region further the slope values of mapped lda surfaces within the study region are within 0 1 4 the extent of mapped ldas within the study region is such that some of the low elevation 0 8 1 3 km portions of montes flanks are surrounded by relatively less extent up to 0 5 0 8 km of lda flow units geomorphic and topographic evidence for flow units that appear to be superposed on the main lda body collectively suggest the possibility of episodic glacial activity in the region furthermore based on the alignment of subsurface reflectors with the surrounding plains when a permittivity of ice 3 2 is assumed and the radargram is depth corrected we infer that some of the portions of lda flow units have preserved ice in their subsurface up to 300 500 m depth crater size frequency distribution of craters counted on lda surface indicates that the best fit age is 110 ma in addition the lda surfaces exhibit different best fit ages for different types of crater morphologies bowl shaped ring mold and infilled craters included in the crater count statistics together these observations and the interpretations suggest that most if not all of the ldas in the study region are like classical ldas mapped in other regions of mars e g along the mid latitude dichotomy boundary and eastern hellas region these results indicate that a widespread accumulation and preservation of ice has occurred during the late amazonian as suggested in previous studies

characterizing the synoptic expression of the angola low the angola low is a key feature of the southern africa wet season atmosphere that influences precipitation across the continent this paper uses era interim to show that the synoptic expression of the angola low is a combination of dry heat lows and moist tropical low pressure systems the angola heat low and angola tropical low composites are contrasted against similar lows observed in other continental tropical regions and found to be broadly comparable the implications that the distinction between dry and moist events has for the interannual relationship among the angola low precipitation and enso are examined the tropical lows exhibit unusual semistationary behavior by lingering in the angola region rather than traveling offshore this behavior is proposed to be caused by an integrated sea breeze anabatic wind that enhances inhibits cyclonic vorticity stretching and convection inland near the coast the combined effect of the heat lows and the anchored tropical lows creates the angola low in the climatological average by elucidating the mechanisms of the angola low this research improves the foundation of process based evaluation of southern africa present and future climate in cmip and amip models

circulation anomalies associated with tropical temperate troughs in southern africa and the south west indian ocean daily rainfall variability over southern africa sa and the southwest indian ocean swio during the austral summer months has recently been described objectively for the first time using newly derived satellite products the principle mode of variability in all months is a dipole structure with bands of rainfall orientated northwest to southeast across the region these represent the location of cloud bands associated with tropical temperate troughs ttt this study objectively identifies major ttt events during november to february and on the basis of composites off ncep reanalysis data describes the associated atmospheric structure the two phases of the rainfall dipole are associated with markedly contrasting circulation patterns there are also pronounced intra seasonal variations in early summer the position of the temperate trough and ttt cloud band alternates between the swio and southwest atlantic in late summer the major ttt axis lies preferentially over the swio associated with an eastward displacement in the indian ocean high in all months positive events in which the ttt cloud band lies primarily over the swio are associated with large scale moisture flux anomalies in which convergent fluxes form a pronounced poleward flux along the cloud band this suggests that ttt events are a major mechanism of poleward transfer of energy and momentum moisture transport occurs along three principle paths 1 the northern or central indian ocean where anomalous fluxes extend eastward to the maritime continent 2 south equatorial africa and the equatorial atlantic 3 from the south within a cyclonic flow around the tropical temperate trough the relative importance of 2 is greatest in late summer thus synoptic scale ttt events over sa swio often result from large scale planetary circulation patterns hovmoeller plots show that ttt development coincides with enhanced tropical convection between 10 30 e itself exhibiting periodicity of around 5 days and often with convergence of eastward and westward propagating convection around 40 e harmonic analysis of 200 hpa geopotential anomalies show that ttt features are forced by a specific zonally asymmetric wave pattern with wave 5 dominant or significant in all months except february when quasi stationary waves 1 2 and 3 dominate these findings illustrate the importance of tropical and extratropical dynamics in understanding ttt events finally it is suggested that in november januar ttt rainfall over sa swio may be in phase with similar rainfall dipole structures observed in the south pacific and south atlantic convergence zones

pacing early mars river activity embedded craters in the aeolis dorsa region imply river activity spanned 1 20 myr we find net sedimentation rate 13 200 m yr in the aeolis dorsa region mars using the frequency of crater river interactions this sets a lower bound of 1 20 myr on the total interval spanned by fluvial activity which we correlate to the noachian hesperian transition the main uncertainty is the impact flux at the time of deposition this result rules out basin filling by a single catastrophic episode such as a single impact induced water vapor greenhouse

decline of crater obliteration rates during early martian history abstract a change in the global climate on early mars is suspected from the geologic evidence of warmer wetter conditions that prevailed during the noachian but stands in contrast to the dry and cold conditions observed currently however the timing and evolution of this climatic change represents a gap in our understanding of the history of mars here we document the time dependence of the crater obliteration rates on mars that decreased continuously between 3 8 ga and 3 ga our results indicate that the erosion responsible for crater modification and obliteration declined gradually through the noachian and into the hesperian periods these results suggest that the climatic conditions also changed gradually and the amount of water available on the surface of mars slowly decreased over time

redefinition of the crater density and absolute age boundaries for the chronostratigraphic system of mars abstract for the boundaries of each chronostratigraphic epoch on mars we present systematically derived crater size frequencies based on crater counts of geologic referent surfaces and three proposed standard crater size frequency production distributions as defined by a a simple 2 power law b neukum and ivanov c hartmann in turn these crater count values are converted to model absolute ages based on the inferred cratering rate histories we present a new boundary definition for the late hesperian early amazonian transition our fitting of crater size frequency distributions to the chronostratigraphic record of mars permits the assignment of cumulative counts of craters down to 100 m 1 km 2 km 5 km and 16 km diameters to martian epochs due to differences in the standard crater size frequency production distributions a generalized crater density based definition to the chronostratigraphic system cannot be provided for the diameter range used for the boundary definitions the resulting model absolute age fits vary within 1 5 for a given set of production function and chronology model ages crater distributions translated to absolute ages utilizing different curve descriptions can result in absolute age differences exceeding 10

the dynamics of heat lows over flat terrain the numerical model for a heat low developed by racz and smith is extended to include a representation of radiative heating and cooling the model is run with a higher horizontal resolution than the original version and is used to investigate additional dynamical aspects of the structure and evolution of a heat low over a subcontinental or continental scale circular island surrounded by sea of particular interest is the diurnal and day to day evolution of the upper and lower level circulations and the degree of balance that exists in these the heat low is surmounted by an anticyclone the development of which is closely tied to the outflow branch of the sea breeze the anticyclone has a much smaller diurnal variation than the heat low and unlike the heat low is largely in balance except in the region affected by the upward propagating gravity wave induced by the inland penetrating sea breeze there is a strong analogy to certain aspects of tropical cyclones which have a warm core a shallow unbalanced boundary layer and which are surmounted also by an anticyclone principles governing the absolute angular momentum budget are the same as those relating to the tropical cyclones and to the zonal mean flow over antarctica implications of these principles for obtaining a realistic steady state in long term integrations of axisymmetric models are discussed copyright 2008 royal meteorological society

drylines in southern africa rediscovering the congo air boundary abstractprojected rainfall decline in southern africa is likely to be highly sensitive to subtleties in the local atmospheric circulation in an effort to understand the regional circulation comple

african easterly jet south control maintenance mechanisms and link with southern subtropical waves the african easterly jet south aej s is an important feature of the central african mid tropospheric circulation and has been identified as a key contributor to convection over the region this study uses 21 year 1983 2003 reanalysis data sets of era interim ncep2 and merra2 to establish mechanisms related to aej s dynamics results demonstrate that aej s is dominated by rotational circulation and is maintained by a mid level high that forms over the kalahari region during september to november because of the role played by this high pressure system in the aej s dynamics an effort is made to understand how this anticyclone develops we show that this mid tropospheric high over the kalahari region is established in response to lower tropospheric dry convection over the region from september to november at the core of this high anticyclonic circulation is induced and maintains aej s located at the northern flank of the high a link between aej s dynamics and southern subtropical westerly waves is also revealed it is shown that when waves amplify over the southern subtropics they modify lower tropospheric heating depending on the phase of the wave this modifies the cross latitude temperature gradient throughout equatorial regions therefore modulating the intensity of the jet by clarifying the mechanisms that govern the aej s this research work contributes to insight into central africa climate mechanisms and also suggests a link between central africa and southern africa climate systems this research work also improves the foundation of new mechanisms that help to identify suitable metrics for the evaluation of global models over the central africa region

hesperian age for western medusae fossae formation mars the medusae fossae formation mff on mars is an intensely eroded deposit north of the cratered highlands it is widely thought that mff materials were emplaced through ignimbrite eruptions recent geologic mapping of western mff identified outliers of mff materials well beyond the previously mapped western extent for the deposit including outliers close to gale crater we report counts of impact craters on the mff units that have implications for our understanding of the general history of mff and the uppermost layered materials on the gale crater mound

localized precipitation and runoff on mars 1 we use the mars regional atmospheric modeling system mrams to simulate lake storms on mars finding that intense localized precipitation will occur for lake size 103 km2 mars has a low density atmosphere so deep convection can be triggered by small amounts of latent heat release in our reference simulation the buoyant plume lifts vapor above condensation level forming a 20 km high optically thick cloud ice grains grow to 200 m radius and fall near or in the lake at mean rates up to 1 5 mm h 1 water equivalent maximum rates up to 6 mm h 1 water equivalent because atmospheric temperatures outside the surface layer are always well below 273 k supersaturation and condensation begin at low altitudes above lakes on mars in contrast to earth lake effect storms lake storms on mars involve continuous precipitation and their vertical velocities and plume heights exceed those of tropical thunderstorms on earth for lake sizes 102 5 to 103 5 km plume vertical velocity scales linearly with lake area convection does not reach above the planetary boundary layer for lakes 103 km2 or for atmospheric pressure o 102 mbar instead vapor is advected downwind with little cloud formation precipitation occurs as snow and the daytime radiative forcing at the land surface due to plume vapor and storm clouds is too small to melt snow directly 10 w m 2 however if orbital conditions are favorable then the snow may be seasonally unstable to melting and produce runoff to form channels we calculate the probability of melting by running thermal models over all possible orbital conditions and weighting their outcomes by probabilities given by long term integrations of the chaotic diffusion of solar system orbital elements with this approach we determine that for an equatorial vapor source sunlight 15 fainter than at present and snowpack with albedo 0 28 0 35 melting may occur with 4 0 1 probability this rises to 56 12 if the ancient greenhouse effect was modestly 6 k greater than today

deltaic deposits indicative of a paleo coastline at aeolis dorsa mars abstract recent work at aeolis dorsa mars has identified exposure of fluvial sedimentary outcrop deposited early in martian history likely during the late hesperian or earlier here we examine a 1200 km2 exposure of sedimentary outcrop in southeast aeolis dorsa total thickness of the stratigraphic section exceeds 100 m we identify eight discrete complexes each of which is primarily composed of branching channel filling fluvial deposits associated with a primary feeder or trunk channel deposit relative ages for the branching complexes are derived from observed stacking relationships and there is a pattern of eastward younging using digital elevation models derived from context camera ctx stereo pair images we measure the strike and dip of stratal surfaces mapped throughout the study area there is a trend of increasing dip magnitude from north to south the branching complexes are interpreted as exhumed remnants of the lobes of an aggrading river delta that shifted its depo center from west to east over time the increasing dip magnitude of stratal surfaces towards the south records topset foreset transitions an architectural feature associated with river deltas and indicates paleo transport was locally southward the interpreted deltaic lobes indicate that aeolis dorsa was a coastal region not presently bound by a modern crater basin and the studied outcrop is part of a larger set of coastal sedimentary deposits identified in the region

global drainage patterns and the origins of topographic relief on earth mars and titan rivers have eroded the topography of mars titan and earth creating diverse landscapes however the dominant processes that generated topography on titan and to some extent on early mars are not well known we analyzed drainage patterns on all three bodies and found that large drainages which record interactions between deformation and erosional modification conform much better to long wavelength topography on titan and mars than on earth we use a numerical landscape evolution model to demonstrate that short wavelength deformation causes drainage directions to diverge from long wavelength topography as observed on earth we attribute the observed differences to ancient long wavelength topography on mars recent or ongoing generation of long wavelength relief on titan and the creation of short wavelength relief by plate tectonics on earth

effect of obliteration on crater count chronologies for martian surfaces 1 the density of impact craters calibrated against lunar data is currently the only quantitative measure of surface age for terrestrial planetary surfaces unlike the moon however mars has been weathered and eroded obliterating some small 10 km diameter craters a phenomenon addressed in the mariner viking days of mars exploration but commonly overlooked in recent studies we present a quantitative model that extends this earlier work to assess the effect of erosion and infilling on surface ages inferred from crater frequency distributions our work affirms that small cratersizedistributionscanbeinterpretedquantitatively in terms of effects of erosion and crater infilling at rates comparable to those reported for mars a reanalysis of prior studies indicates that low to moderate long term rates of erosion and crater infilling can mask an ancient age and result in small crater populations similar to those offered as evidence for young and geologically significant surface activity citation smith m r a r gillespie and d r montgomery 2008 effect of obliteration on crater count chronologies for martian surfaces geophys res lett 35 l10202 doi 10 1029 2008gl033538

a global climatology of monsoon low pressure systems the first global climatology of monsoon low pressure systems is presented here based on the era interim reanalysis low pressure systems are classified into three intensity categories and particular focus is given to systems in the category corresponding to a traditional definition of monsoon depressions vortex tracks are identified using an automated algorithm applied to the distributions of 850 hpa relative vorticity sea level pressure and surface wind speed for 1979 2012 roughly two to three times as many monsoon low pressure systems form in the northern hemisphere as in the southern hemisphere during local summer the frequency of genesis typically peaks in local summer but low pressure systems form throughout the year in every monsoon region interannual variability is weak with standard deviations of summer counts typically being below 10 of the long term summer mean regional composites reveal that monsoon depressions in india the western pacific and northern australia share a common structure consisting of a warm over cold core and a top heavy column of potential vorticity that extends from the surface to the upper troposphere a separate class of monsoon low pressure systems develops over dry regions of west africa and western australia with a shallow composite structure having a warm core in the lower troposphere and cyclonic potential vorticity confined to a thin near surface layer low pressure systems in nearly all monsoon regions are estimated to account for a large fraction from about 40 to more than 80 of summer precipitation on the poleward edge of the climatological mean precipitation maxima

martian cratering 8 isochron refinement and the chronology of mars abstract this paper reviews and refines the technique of dating martian surfaces by using impact crater isochrons defined as size distributions of impact craters on undisturbed martian surfaces of specified ages in the 1970s this system identified not only abundant ancient martian volcanic surfaces but also extensive lava plains with ages of a few 108 y old this dating was initially controversial but confirmed in the 1980s and 90s by martian meteorites the present update utilizes updated estimates of the mars moon cratering ratio the most important calibration factor improves treatment of gravity and impact velocity scaling effects combines aspects of the crater size distribution data from earlier work by both neukum and hartmann and for the first time applies a correction for loss of small meteoroids in the martian atmosphere from popova et al 2003 meteorit planet sci 38 905 925 the updated isochrons are not radically different from the previous 2002 iteration but fit observed data better and give somewhat older model ages for features dated from small craters diameter d 100 m crater counts from young lava flows in various areas give good fits to the new isochrons over as much as 3 orders of magnitude in d confirming the general isochron shape and giving crater retention ages in the range of some 106 to some 108 y interpreted as lava flow ages more complex older units are also discussed uncertainties are greatest if only small craters d 100 m are used suggestions by other workers of gross uncertainties due to local secondary craters and deposition exhumation are discussed they do not refute our conclusions of significant volcanic fluvial and other geologic activity in the last few percent of martian geologic time or the importance of cratering as a tool for studying processes such as exhumation indeed crater count data suggest certain very recent episodes of deposition exhumation and ice flow possibly associated with obliquity cycles of 10 7 y timescale evidence from ancient surfaces suggests higher rates of volcanism fluvial activity glaciation and other processes in noachian hesperian time than in amazonian time

the south indian convergence zone and interannual rainfall variability over southern africa abstract the south indian convergence zone sicz is identified in this paper as a region of enhanced precipitation extending off the southeast coast of southern africa during austral summer unlike the south pacific convergence zone the sicz is a land based convergence zone lbcz with position and intensity at least partially determined by surface conditions over southern africa an idealized gcm simulation is used to explore the basic dynamics of lbczs in the tropics and subtropics output from a realistic gcm simulation and the national centers for environmental prediction national center for atmospheric research 40 year reanalysis are analyzed to apply this basic dynamical framework to the case of the sicz in contrast to the intertropical convergence zone where column moisture convergence is primarily due to meridional wind convergence in the moist environment precipitation within the sicz and the lbczs in general is also supported by zonal wind convergence moisture convergence by transient eddy

pervasive aqueous paleoflow features in the aeolis zephyria plana region mars abstract a survey of themis visible wavelength images in the aeolis zephyria plana region over the two western lobes of the equatorial medusae fossae formation mff shows 150 sinuous ridges having a variety of morphologies and contexts to systematize investigation we use a classification scheme including both individual ridge and ridge network types as well as associations with impact craters and fan shaped features the morphology of the ridges their location downslope from higher topography e g crater rims and scarps and their association with fan shaped forms indicate that most sinuous ridges formed through overland aqueous flow analysis of observations by individual ridge type leads to interpretation of most of these sinuous ridges as inverted fluvial channels or floodplains and a few as possible eskers with the origin of the remaining ridges under continuing investigation about 15 of the sinuous ridges are associated with impact craters but data analysis does not support a genetic relationship between the craters and the sinuous ridges instead analysis of one sinuous ridge network associated with a crater indicates that the water source for the network was atmospheric in origin namely precipitation runoff the broad areal distribution of these 150 ridges and the network morphologies in particular the branched and subparallel types suggest that an atmospheric water source is generally applicable to the population of sinuous ridges as a whole this concentration of sinuous ridges is the largest single population of such landforms on mars and among the youngest these ridges are situated at a paleoscarp between cerberus palus and the aeolis highlands suggesting that the precipitation that formed them was orographic in origin the ages of the equatorial mff units in which this population of sinuous ridges is found imply that this orographic rain and or snow fell during some period from the late hesperian through the middle amazonian

channel slope reversal near the martian dichotomy boundary testing tectonic hypotheses abstract faults along the martian dichotomy boundary are evidence of tectonic activity which by analogy with terrestrial tectonism may cause changes in or even reversal of fluvial longitudinal profiles in the eastern hemisphere between 30 e and 150 e this tectonic activity has been hypothesized to result from lower crustal flow or from lithospheric flexure for which loading e g by material deposition of the northern lowlands is a possible cause watters 2003a nimmo 2005 the topographic slope changes resulting from these two different mechanisms are distinct and can provide a means for distinguishing between them although other processes may complicate interpretations of reversed longitudinal profiles two fan shaped networks of inverted channels are located near 150 e in the aeolis dorsa region just north of the dichotomy boundary their original flow directions inferred from planform morphology suggest flow to the northeast in contrast to their current longitudinal profiles sloping down to the southwest this contrast indicates slope reversal we investigate the lower crustal flow and flexure mechanisms for slope reversal by testing three different hypotheses 1 lower crustal flow 2 flexure caused by material erosion from the highlands and deposition in the lowlands and 3 flexure caused by highland erosion and deposition in the lowlands plus deposition of the medusae fossae formation in the lowlands we test these three hypotheses by comparing the inferred magnitudes of the slope reversals with predicted slope changes from geophysical models for these processes taking the possibility of non tectonic i e collapse processes into account our results suggest that among these three models the slope reversal is most consistent with the predicted tectonic response to erosion and deposition of highland material in conjunction with deposition of the medusae fossae formation contrary to previous findings our results do not support the mechanism of lateral crustal flow although they do not rule it out because lower crustal flow and erosion may both have been operating but on different time scales the result of this hypothesis testing provides insight into the evolution of the dichotomy boundary and martian crust in this location

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

inverted fluvial features in the aeolis zephyria plana region mars formation mechanism and initial paleodischarge estimates 1 a subset of the sinuous ridges srs in the aeolis zephyria plana azp region of mars has been previously hypothesized to be inverted fluvial features although the precise induration and erosion mechanisms were not specified morphological observations and thermal inertia data presented here support this hypothesis a variety of mechanisms can cause inversion and identification of the specific events that lead to fluvial sr formation can provide insights into the sedimentological geochemical and climatic processes of the region reconnaissance of two terrestrial lava capped ridges suggests some criteria that may be used to identify inverted fluvial features formed by lava infill on mars but these criteria are not satisfied by the majority of the azp fluvial srs armoring also appears inconsistent with terrestrial analogs layering and surface textures of fluvial srs indicate that the most likely induration mechanism was geochemical cementation of fluvial sediments and that the primary erosional mechanism that exposed the fluvial srs was aeolian abrasion this analysis of formation mechanism provides a foundation for estimating paleodischarge using an empirical form discharge approach to which we have applied scaling for martian gravity for those fluvial srs meeting a set of criteria for accurate paleodischarge estimates paleodischarge values generally range between 101 and 103 m3 s 1 the largest of these initial estimates are comparable to paleodischarge estimates for some late stage noachian fluvial channels on mars and provide a constraint on the atmospheric conditions at this equatorial location during the late hesperian to early amazonian time frame

subsurface structure of planum boreum from mars reconnaissance orbiter shallow radar soundings abstract we map the subsurface structure of planum boreum using sounding data from the shallow radar sharad instrument onboard the mars reconnaissance orbiter radar coverage throughout the 1 000 000 km2 area reveals widespread reflections from basal and internal interfaces of the north polar layered deposits npld a dome shaped zone of diffuse reflectivity up to 12 s 1 km thick underlies two thirds of the npld predominantly in the main lobe but also extending into the gemina lingula lobe across chasma boreale we equate this zone with a basal unit identified in image data as amazonian sand rich layered deposits byrne s murray b c 2002 j geophys res 107 5044 12 pp doi 10 1029 2001je001615 fishbaugh k e head j w 2005 icarus 174 444 474 tanaka k l rodriguez j a p skinner j a bourke m c fortezzo c m herkenhoff k e kolb e j okubo c h 2008 icarus 196 318 358 elsewhere the npld base is remarkably flat lying and co planar with the exposed surface of the surrounding vastitas borealis materials within the npld we delineate and map four units based on the radar layer packets of phillips et al phillips r j and 26 colleagues 2008 science 320 1182 1185 that extend throughout the deposits and a fifth unit confined to eastern gemina lingula we estimate the volume of each internal unit and of the entire npld stack 821 000 km3 exclusive of the basal unit correlation of these units to models of insolation cycles and polar deposition laskar j levrard b mustard j f 2002 nature 419 375 377 levrard b forget f montmessin f laskar j 2007 j geophys res 112 e06012 18 pp doi 10 1029 2006je002772 is consistent with the 4 2 ma age of the oldest preserved npld obtained by levrard et al levrard b forget f montmessin f laskar j 2007 j geophys res 112 e06012 18 pp doi 10 1029 2006je002772 we suggest a dominant layering mechanism of dust content variation during accumulation rather than one of lag production during periods of sublimation

tropical extratropical interactions over southern africa three cases of heavy summer season rainfall abstract the synoptic evolution of three tropical extratropical te interactions each responsible for extreme rainfall events over southern africa is discussed in detail along with the consideration of previously studied events common features of these heavy rainfall producing tropical temperate troughs ttts over southern africa are discussed it is found that 2 days prior to an event northeasterly moisture transports across botswana set up by the angola low are diverted farther south into the semiarid region of subtropical southern africa the ttts reach full maturity as a te cloud band rooted in the central subcontinent which is triggered by upper level divergence along the leading edge of an upper tropospheric westerly wave trough convection and rainfall within the cloud band is supported by poleward moisture transports with subtropical air rising as it leaves the continent and joins the midlatitude westerly flow it is shown that these systems fit within a theoretical framework describing

low palaeopressure of the martian atmosphere estimated from the size distribution of ancient craters the decay of the martian atmosphere which is dominated by carbon dioxide is a component of the long term environmental change on mars from a climate that once allowed rivers to flow to the cold and dry conditions of today the minimum size of craters serves as a proxy for palaeopressure of planetary atmospheres because thinner atmospheres permit smaller objects to reach the surface at high velocities and form craters the aeolis dorsa region near gale crater on mars contains a high density of preserved ancient craters interbedded with river deposits and thus can provide constraints on atmospheric density at the time of fluvial activity here we use high resolution images and digital terrain models from the mars reconnaissance orbiter to identify ancient craters in deposits in aeolis dorsa that date to about 3 6 gyr ago and compare their size distribution with models of atmospheric filtering of impactors we obtain an upper limit of 0 9 0 1 bar for the martian atmospheric palaeopressure rising to 1 9 0 2 bar if rimmed circular mesas interpreted to be erosionally resistant fills or floors of impact craters are excluded we assume target properties appropriate for desert alluvium if sediment had rock mass strength similar to bedrock at the time of impact the paleopressure upper limit increases by a factor of up to two if mars did not have a stable multibar atmosphere at the time that the rivers were flowing as suggested by our results then a warm and wet co 2 h 2o greenhouse is ruled out and long term average temperatures were most likely below freezing

inverted fluvial features in the aeolis zephyria plana western medusae fossae formation mars evidence for post formation modification 1 the aeolis and zephyria plana contain the western most portion of the medusae fossae formation mff an enigmatic and extensive light toned deposit located in the martian equatorial region and dated from the hesperian to amazonian epochs this area hosts a large population of sinuous ridges srs interpreted as inverted fluvial features formed by precipitation indurated by chemical cementation buried by subsequent deposition and finally exhumed this interpretation of srs as uniformly fluvial represents a modification to an earlier hypothesis for one particular sr of possible glaciofluvial i e esker formation these srs provide a tool to investigate the degree and character of post fluvial modification processes in this region we combined digital terrain models made from context camera ctx and high resolution imaging science experiment hirise stereo image pairs with individual data points from the mars orbiter laser altimeter mola to estimate relief cross sectional profiles longitudinal profiles and slope directions of selected srs longitudinal profiles of several srs display undulations with amplitudes of up to order 100 m while some of the lower amplitude undulations may be due to differential erosion undulations having amplitudes in excess of sr relief require alternative explanations our combined morphologic and topographic analysis suggests that multiple post flow processes including compaction of the deposits and tectonic displacements have modified the original sr profiles specification of the type s and magnitudes of these modification processes will contribute to understanding both the potential of post flow modification of fluvial profiles elsewhere on mars as well as the nature and properties of the mff