active strike slip faulting in el salvador central america several major earthquakes have affected el salvador central america during the past 100 yr as a consequence of oblique subduction of the cocos plate under the caribbean plate which is partitioned between trench orthogonal compression and strike slip deformation parallel to the volcanic arc focal mechanisms and the distribution of the most destructive earthquakes together with geomorphologic evidence suggest that this transcurrent component of motion may be accommodated by a major strike slip fault el salvador fault zone we present field geological structural and geomorphological data collected in central el salvador that allow the constraint of the kinematics and the quaternary activity of this major seismogenic strike slip fault system data suggest that the el salvador fault zone consists of at least two main e w fault segments san vicente and berlin segments with associated secondary synthetic wnw ese and antithetic nnw sse riedel shears and nw se tensional structures the two main fault segments overlap in a dextral en echelon style with the formation of an intervening pull apart basin our original geological and geomorphologic data suggest a late pleistocene holocene slip rate of 11 mm yr along the berlin segment in contrast with low historical seismicity the kinematics and rates of deformation suggested by our new data are consistent with models involving slip partitioning during oblique subduction and support the notion that a trench parallel component of motion between the caribbean and cocos plates is concentrated along e w dextral strike slip faults parallel to the volcanic arc

the atmosphere of titan an analysis of the voyager 1 radio occultation measurements abstract two coherently related radio signals transmitted from voyager 1 at wavelengths of 13 cm s band and 3 6 cm x band were used to probe the equatorial atmosphere of titan the measurements were conducted during the occultation of the spacecraft by the satellite on november 12 1980 an analysis of the differential dispersive frequency measurements did not reveal any ionization layers in the upper atmosphere of titan the resolution was approximately 3 10 3 and 5 10 3 electrons cm 3 near the evening and morning terminators respectively abrupt signal changes observed at ingress and egress indicated a surface radius of 2575 0 0 5 km leading to a mean density of 1 881 0 002 g cm 3 for the satellite the nondispersive data were used to derive profiles in height of the gas refractivity and microwave absorption in titan s troposphere and stratosphere no absorption was detected the resolution was about 0 01 db km at the 13 cm wavelength the gas refractivity data which extend from the surface to about 200 km altitude were interpreted in two different ways in the first it is assumed that n 2 makes up essentially all of the atmosphere but with very small amounts of ch 4 and other hydrocarbons also present this approach yielded a temperature and pressure at the surface of 94 0 0 7 k and 1496 20 mbar respectively the tropopause which was detected near 42 km altitude had a temperature of 71 4 0 5 k and a pressure of about 130 mbar above the tropopause the temperature increased with height reaching 170 15 k near the 200 km level the maximum temperature lapse rate observed near the surface 1 38 0 10 k km corresponds to the adiabatic value expected for a dry n 2 atmosphere indicating that methane saturation did not occur in tbis region above the 3 5 km altitude level the lapse rate dropped abruptly to 0 9 0 1 k km and then decreased slowly with increasing altitude crossing zero at the tropopause for the n 2 atmospheric model the lapse rate transition at the 3 5 km level appears to mark the boundary between a convective region near the surface having the dry adiabatic lapse rate and a higher stable region in radiative equilibrium in the second interpretation of the refractivity data it is assumed instead that the 3 5 km altitude level corresponds to the bottom of a ch 4 cloud layer and that n 2 and ch 4 are perfectly mixed below this level these assumptions lead to an atmospheric model which below the clouds contains about 10 ch 4 by number density the temperature near the surface is about 95 k arguments concerning the temperature lapse rates computed from the radio measurements appear to favor models in which methane forms at most a limited haze layer high in the troposphere

the importance of secondary cratering to age constraints on planetary surfaces small craters less than one kilometer diameter can be primary craters produced by impact of interplanetary debris or they can be secondary craters produced by fallback of high velocity ejecta blocks from much larger but infrequent primary impacts the prevalent assumption over recent decades has been that primaries are most abundant so most small craters are independent random events and can be used for dating however recent results from europa and mars support the early theory that distant secondaries globally dominate the number of small lunar craters this would invalidate part of production functions that have been widely used for age dating crater excavation results in higher mean ejection velocities for smaller fragments resulting in a steeper size frequency distribution for secondary craters than is produced by the same size frequency distribution of interplanetary debris this review also discusses how small craters can sometimes be used to derive meaningful age constraints

polar lightning and decadal scale cloud variability on jupiter although lightning has been seen on other planets including jupiter polar lightning has been known only on earth optical observations from the new horizons spacecraft have identified lightning at high latitudes above jupiter up to 80 n and 74 s lightning rates and optical powers were similar at each pole and the mean optical flux is comparable to that at nonpolar latitudes which is consistent with the notion that internal heat is the main driver of convection both near infrared and ground based 5 micrometer thermal imagery reveal that cloud cover has thinned substantially since the 2000 cassini flyby particularly in the turbulent wake of the great red spot and in the southern half of the equatorial region demonstrating that vertical dynamical processes are time varying on seasonal scales at mid and low latitudes on jupiter

a new crater near insight implications for seismic impact detectability on mars a new 1 5 m diameter impact crater was discovered on mars only 40 km from the insight lander context camera images constrained its formation between 21 february and 6 april 2019 follow up high resolution imaging science experiment images resolved the crater during this time period three seismic events were identified in insight data we derive expected seismic signal characteristics and use them to evaluate each of the seismic events however none of them can definitively be associated with this source atmospheric perturbations are generally expected to be generated during impacts however in this case no signal could be identified as related to the known impact using scaling relationships based on the terrestrial and lunar analogs and numerical modeling we predict the amplitude peak frequency and duration of the seismic signal that would have emanated from this impact the predicted amplitude falls near the lowest levels of the measured seismometer noise for the predicted frequency hence it is not surprising this impact event was not positively identified in the seismic data finding this crater was a lucky event as its formation this close to insight has a probability of only 0 2 and the odds of capturing it in before and after images are extremely low we revisit impact seismic discriminators in light of real experience with a seismometer on the martian surface using measured noise of the instrument we revise our previous prediction of seismic impact detections downward from a few to tens to just 2 per earth year still with an order of magnitude uncertainty

saturn s visible lightning its radio emissions and the structure of the 2009 2011 lightning storms visible lightning on saturn was first detected by the cassini camera in 2009 at 35 south latitude we report more lightning observations at 35 south later in 2009 and lightning in the 2010 2011 giant lightning storm at 35 north the 2009 lightning is detected on the night side of saturn in a broadband clear filter the 2011 lightning is detected on the day side in blue wavelengths only in other wavelengths the 2011 images lacked sensitivity to detect lightning which leaves the lightning spectrum unknown the prominent clouds at the west edge or the head of the 2010 2011 storm periodically spawn large anticyclones which drift off to the east with a longitude spacing of 10 15 10 000 km the wavy boundary of the storm s envelope drifts with the anticyclones the relative vorticity of the anticyclones ranges up to f 3 where f is the planetary vorticity the lightning occurs in the diagonal gaps between the large anticyclones the vorticity of the gaps is cyclonic and the atmosphere there is clear down to level of the deep clouds in these respects the diagonal gaps resemble the jovian belts which are the principal sites of jovian lightning the size of the flash illuminated cloud tops is similar to previous detections with diameter 200 km this suggests that all lightning on saturn is generated at similar depths 125 250 km below the cloud tops probably in the water clouds optical energies of individual flashes for both southern storms and the giant storm range up to 8 10 9 j which is larger than the previous 2009 equinox estimate of 1 7 10 9 j cassini radio measurements at 1 16 mhz suggest that assuming lightning radio emissions range up to 10 ghz lightning radio energies are of the same order of magnitude as the optical energies southern storms flash at a rate 1 2 per minute the 2011 storm flashes hundreds of times more often 5 times per second and produces 10 10 w of optical power based on this power the storm s total convective power is of the order 10 17 w which is uncertain by at least an order of magnitude and probably is underestimated this power is similar to saturn s global internal power radiated to space it suggests that storms like the 2010 2011 giant storm are important players in saturn s cooling and thermal evolution

a revised surface age for the north polar layered deposits of mars this work was funded by nasa grant nnx13ag72g m e l was supported by the national science foundation graduate research fellowship program grant dge 1143953 hirise images referenced are available on the instrument s public website https hirise lpl arizona edu the crater catalog used in this work is included with this paper as supporting information the authors thank s sutton for help with socet set software m m sori for useful discussion on viscous relaxation and m e banks for useful discussion on the impact population the authors additionally thank j a skinner p becerra d laikko m sori n barlow and an anonymous reviewer for helpful comments on the manuscript

saturn lightning recorded by cassini rpws in 2004 during 2004 the cassini rpws radio and plasma wave science instrument recorded about 5400 seds saturn electrostatic discharges which were organized in 4 storm systems and 95 episodes a computer algorithm with different intensity thresholds was applied to extract the seds from the rpws data and a statistical analysis on the main characteristics of these seds is performed compared to the seds recorded by the voyagers in the early 1980s some characteristics like sed rate intensity signal duration or power spectrum are similar but there are also remarkable differences with regard to time occurrence and frequency range the first appearance of seds storm 0 was recorded by rpws from a distance of more than 300 saturn radii at the end of may 2004 followed by storm a in mid july storm b at the beginning of august and the most prominent storm c throughout most of september there were also significant intervals of time with no detectable sed activity e g seds were practically absent from october 2004 until june 2005 no clear indication for seds below a frequency of 1 3 mhz could be found we suggest that the sed storms a b c and possibly also storm 0 originate from the same storm system residing at a latitude of 35 south which lasted for several months waxed and waned in strength and rotated with the voyager radio period of saturn the sed source might be located in the updrafting water clouds beneath the visible cloud features detected in the cassini images

analysis of fault slip inversions do they constrain stress or strain rate fault slip data commonly are used to infer the orientations and relative magnitudes of either the principal stresses or the principal strain rates which are not necessarily parallel or equal at the local scale of an individual fault the shear plane and slip direction define the orientations of the local principal strain rate axes but not in general the local principal stress axes at a large scale the orientations of p and t axes maxima for sets of fault slip data do not provide accurate inversion solutions for either strain rate or stress the quantitative inversion of such fault slip data however provides direct constraints on the orientations and relative magnitudes of the global principal strain rates to interpret the inversion solution as constraining the global principal stresses requires that 1 the fault slip pattern must have a characteristic symmetry no lower than orthorhombic 2 the material must be mechanically isotropic and 3 there must be a linear constitutive relationship between the global stress and the global strain rate isotropic linear elastic constitutive equations are appropriate to describe the local deformation surrounding an individual slip discontinuity fault slip inversions however constrain the characteristics of a large scale cataclastic flow which is described by constitutive equations that are probably but to an unknown degree anisotropic and nonlinear such material behavior would not strictly satisfy the requirements for the stress interpretation thus at the present state of knowledge fault slip inversion solutions are most reliably interpreted as constraining the principal strain rates

small crater modification on meridiani planum and implications for erosion rates and climate change on mars a morphometric and morphologic catalog of 100 small craters imaged by the opportunity rover over the 33 5 km traverse between eagle and endeavour craters on meridiani planum shows craters in six stages of degradation that range from fresh and blocky to eroded and shallow depressions ringed by planed off rim blocks the age of each morphologic class from 50 200 ka to 20 ma has been determined from the size frequency distribution of craters in the catalog the retention age of small craters on meridiani planum and the age of the latest phase of ripple migration the rate of degradation of the craters has been determined from crater depth rim height and ejecta removal over the class age these rates show a rapid decrease from 1 m myr for craters 1 ma to 0 1 m myr for craters 10 20 ma which can be explained by topographic diffusion with modeled diffusivities of 10 6 m2 yr in contrast to these relatively fast short term erosion rates previously estimated average erosion rates on mars over 100 myr and 3 gyr timescales from the amazonian and hesperian are of order 0 01 m myr which is 3 4 orders of magnitude slower than typical terrestrial rates erosion rates during the middle late noachian averaged over 250 myr and 700 myr intervals are around 1 m myr comparable to slow terrestrial erosion rates calculated over similar timescales this argues for a wet climate before 3 ga in which liquid water was the erosional agent followed by a dry environment dominated by slow eolian erosion

changes in blast zone albedo patterns around new martian impact craters abstract blast zones bzs around new martian craters comprise various albedo features caused by the initial impact including diffuse halos extended linear and arcuate rays secondary craters ejecta patterns and dust avalanches we examined these features for changes in repeat images separated by up to four mars years here we present the first comprehensive survey of the qualitative and quantitative changes observed in impact blast zones over time such changes are most likely due to airfall of high albedo dust restoring darkened areas to their original albedo the albedo of adjacent non impacted surfaces although some sites show drastic changes over short timescales nearly half of the sites show no obvious changes over several mars years albedo changes are more likely to occur at higher latitude sites lower elevation sites and at sites with smaller central craters no correlation was seen between amount of change and dust cover index relative halo size or historical regional albedo changes quantitative albedo measurements of the diffuse dark halos relative to their surroundings yielded estimates of fading lifetimes for these features the average lifetime among sites with measurable fading is 15 mars years the median is 8 mars years for a linear brightening however at approximately half of sites with three or more repeat images a nonlinear function with rapid initial fading followed by a slow increase in albedo provides a better fit to the fading behavior this would predict even longer lifetimes the predicted lifetimes of bzs are comparable to those of slope streaks and considered representative of fading by global atmospheric dust deposition they last significantly longer than dust devil or rover tracks albedo features that are erased by different processes these relatively long lifetimes indicate that the measurement of the current impact rate by daubar et al daubar i j et al 2013 icarus 225 506 516 http dx doi org 10 1016 j icarus 2013 04 009 does not suffer significantly from overall under sampling due to blast zones fading before new impact sites can be initially discovered however the prevalence of changes seen around smaller craters may explain in part their shallower size frequency distribution

the current martian cratering rate abstract the discovery of 248 dated impact sites known to have formed within the last few decades allows us to refine the current cratering rate and slope of the production function at mars we use a subset of 44 of these new craters that were imaged before and after impact by mars reconnaissance orbiter s context camera a thoroughly searched data set that minimizes biases from variable image resolutions we find the current impact rate is 1 65 10 6 craters with an effective diameter 3 9 m km2 yr with a differential slope power law exponent of 2 45 0 36 this results in model ages that are factors of three to five below the hartmann hartmann w k 2005 icarus 174 294 320 and neukum et al neukum g ivanov b a hartmann w k 2001 space sci rev 96 55 86 ivanov ivanov b a 2001 space sci rev 96 87 104 model production functions where they overlap in diameter the best fit production function we measure has a shallower slope than model functions at these sizes but model function slopes are within the statistical errors more than half of the impacts in this size range form clusters which is another reason to use caution when estimating surface ages using craters smaller than 50 m in diameter

constraints on ripple migration at meridiani planum from opportunity and hirise observations of fresh craters 1 observations of fresh impact craters by the opportunity rover and in high resolution orbital images constrain the latest phase of granule ripple migration at meridiani planum to have occurred between 50 ka and 200 ka opportunity explored the fresh resolution crater cluster and concepcion crater that are superposed on and thus younger than the ripples these fresh craters have small dark pebbles scattered across their surfaces which are most likely fragments of the impactor suggesting that the dark pebbles and cobbles observed by opportunity at meridiani planum are a lag of impactor derived material either meteoritic or secondary impactors from elsewhere on mars two larger fresh rayed craters in meridiani planum bracket ripple migration secondaries from ada crater are clearly superposed on and secondaries from an unnamed 0 84 km diameter crater have been modified and overprinted by the ripples three methods were used to estimate the age of these craters and thus when the latest phase of ripple migration occurred the inactivity of the ripples over the past 50 ka at meridiani is also consistent with other evidence for the stability of the ripples the lack of observed eolian bed forms in craters that formed in the past 20 years and little evidence for much dune motion in the past 30 yr on mars observations of crater morphology and their interaction with the ripples allow the development of a general time scale for craters in meridiani planum over the past million years

collision versus sliver transport in the hanging wall at the middle america subduction zone constraints from background seismicity in central costa rica earthquake focal mechanism solutions for 135 small magnitude events are inverted for best fitting partial strain rate tensors that characterize contemporary strain in five areas that span the western margin of the panama microplate in central costa rica the results indicate the predominance of subhorizontal maximum stretching subparallel to the middle america trench mat and provide constraints on the role of cocos ridge collision at the mat the trajectory of maximum stretching changes 25 45 over several tens of kilometers from the central costa rica deformed belt ccrdb where it is nearly e w to the area inboard of the cocos ridge where it is nw se this change suggests that background seismogenic deformation reflects the transition from the trailing edge of a fore arc sliver to an area of the upper plate affected by ridge collision this diffuse deformation may be localized in part on conjugate strike slip faults of the ccrdb

effects of subducting seafloor roughness on upper plate vertical tectonism osa peninsula costa rica 1 subduction of seamounts and ridges along thinly sedimented convergent margins results in deformation of the overriding forearc exposures of newly recognized late pleistocene shallow water deposits i e marenco formation record intervals of rapid subsidence and uplift across the costa rican forearc inboard of the subducting cocos ridge in general the marenco formation is a fining upward fossiliferious late pleistocene marine sand disconformably overlying beveled surfaces cut across the competent osa melange basement the 50 to 27 ka age of the marenco formation is constrained by 12 accelerator mass spectrometry and two conventional 14c dates obtained on marine macrofossils the deposition of this sequence coincident with a general fall in sea level during oxygen isotope stage 3 requires 6 mm yr 1 subsidence inboard of the northwest flank of the subducting cocos ridge presently exposures of the marenco formation are found at 75 m above sea level requiring uplift rates in excess of 6 mm yr 1 we interpret the down and up history of vertical tectonism recorded by the marenco formation as the response of the upper plate to variations in the elevation of the subducting cocos ridge on the basis of a model where the upper plate deforms through bends because of roughness on a rigid downgoing plate the rate duration and spatial distribution of vertical tectonism across the forearc are determined by the magnitude of the orthogonal component of the relative convergence vector and the bathymetry of the underthrusting plate application of this model to bathymetric data for the cocos plate offshore yields a broad agreement between predicted future rates of subsidence and uplift and rates over the last 50 kyr recorded by the marenco formation furthermore analysis suggests that the arrival of the blunt tipped leading edge of the cocos ridge 0 5 3 ma resulted in an initial period of very rapid 30 mm yr 1 uplift

analysis of a giant lightning storm on saturn on january 23 2006 the cassini rpws radio and plasma wave science instrument detected a massive outbreak of seds saturn electrostatic discharges the following sed storm lasted for about one month and consisted of 71 consecutive episodes it exceeded all other previous sed observations by cassini as well as by the voyagers with regard to number and rate of detected events at the same time astronomers at the earth as well as cassini iss imaging science subsystem detected a distinctive bright atmospheric cloud feature at a latitude of 35 south strongly confirming the current interpretation of seds being the radio signatures of lightning flashes in saturn s atmosphere in this paper we will analyze the main physical properties of this sed storm and of a single small sed storm from 2005 the giant sed storm of 2006 had maximum burst rates of 1 sed every 2 s its episodes lasted for 5 5 h on average and the episode s periodicity of about 10 66 h exactly matched the period of the iss observed cloud feature using the low frequency cutoff of sed episodes we determined an ionospheric electron density around 10 4 cm 3 for the dawn side of saturn

the march 25 1990 mw 7 0 ml 6 8 earthquake at the entrance of the nicoya gulf costa rica its prior activity foreshocks aftershocks and triggered seismicity on march 25 1990 a large earthquake mw 7 0 ml 6 8 occurred at the entrance of the nicoya gulf costa rica at 1322 55 6 utc producing considerable damage in central costa rica and generating much interest about whether or not the nicoya seismic gap nishenko 1989 had broken the local country wide seismographic network recorded 6 years of activity prior to this large earthquake 16 hours of foreshocks the mainshock and its aftershocks this network is operated jointly by the costa rica volcanological and seismological observatory at the national university ovsicori una and the charles f richter seismological laboratory at the university of california santa cruz cfrsl ucsc we obtained high resolution locations from this network and located the mainshock at 9 38 5 n 84 55 6 w depth is 20 0 km and the largest foreshock mw 6 0 march 25 1990 at 1316 05 8 utc at 9 36 4 n 84 57 1 w depth is 22 4 km we find that the aftershock zone abuts the southeast boundary of the nicoya seismic gap suggesting that the seismic gap did not rupture since the installation of the local network in april 1984 to march 24 1990 nearly 1900 earthquakes with magnitudes from 1 7 to 4 8 318 with magnitude 3 0 or larger have been located at the entrance of the nicoya gulf one of the most active regions in costa rica the march 25 earthquake occurred at the northwest edge of this region where a sequence of foreshocks began 16 hours prior to the mainshock the spatial temporal distribution of aftershocks and directivity analysis of the mainshock rupture process using teleseismic records both indicate a southeast propagating rupture the mainshock ruptured an asperity of approximately 600 km2 of area with this area expanding to 4000 km2 after 7 days we present evidence that suggests that the ruptured asperity is produced by the subduction of a seamount inversion of teleseismic broadband and long period p and sh waves yields a thrust faulting mechanism with the shallow plane striking 292 dipping 26 and with a rake of 88 in agreement with the subduction of the cocos plate under the caribbean plate local first motions for the largest foreshock and the mainshock agree with this solution we also present evidence suggesting that the march 25 1990 earthquake triggered and reactivated several seismic swarms in central costa rica and temporally decreased the activity in the epicentral area of the july 3 1983 ms 6 2 perez zeledon earthquake

hubble space telescope observations of the 1990 equatorial disturbance on saturn images albedos and limb darkening in september 1990 a major equatorial eruption on saturn produced a disturbance that spread in longitude until it completely girdled the planet we report here on 150 images recorded in six passbands with the wide field planetary camera wf pc aboard the hubble space telescope hst on 17 and 18 november 1990 for comparison we used hst wf pc observations of saturn obtained in three colors on 26 august 1990 before the onset of the disturbance and in six colors on 5 and 6 june 1991 when almost no evidence of the disturbance remained at both of those times the equatorial belt was normal in appearance four of the passbands with mean wavelengths of 336 435 546 and 716 nm were selected for photometric analysis and a patch of the b ring near the central meridian was used for photometric calibration using deconvolved images from all three epochs of observation we measured reflectivities i f of the disk along parallels of latitude as a function of longitudinal distance from the central meridian and also along the central meridian as a function of latitude from 0 to 90 the longitudinal measurements cover essentially the whole visible disk they were made at 1 intervals of planetographic latitude from 0 to 80 and the results are expressed in terms of minnaert coefficients k and minnaert albedos i f 0 we find that the cloud particles associated with the disturbance must differ in character from those that normally make up the visible cloud deck on saturn they were brighter and bluer they had greater limb darkening and their limb darkening was spectrally more neutral the mutual relationship of those properties is such that features which stand out strongly near the meridian fade to invisibility when near the limb

active thrusting in the inner forearc of an erosive convergent margin pacific coast costa rica 1 structural and geomorphic analyses of the fila costena thrust belt in southwest costa rica indicate active thrusting within the inner forearc the fila costena exposes three major thrust faults that imbricate the late tertiary forearc basin sequence of the terraba basin the frontal thrust of the fila costena marks the boundary between an uplifting inner forearc and a subsiding outer forearc with only local uplift astride the indenting cocos ridge on the basis of surface constraints a cross section across the thrust belt suggests that all three thrusts flatten into parallelism with a low angle decollement horizon near the contact between the basement and the cover sequence of the terraba basin this decollement lies at a depth of 4 km the minimum shortening recorded by restoration of fault related folds is 17 km or 45 observations of late tertiary marine sediments tilted and faulted late quaternary fluvial terraces and raised holocene marine terraces indicate that fila costena uplift was likely initiated in the quaternary and is ongoing given that the coastal mountains are separated from the talamanca range by a valley the decollement that delaminates the forearc basin from the underthrusting forearc must continue as a flat beneath the valley but must link with the plate boundary along a crustal scale ramp system a structural geometry that has resulted in uplift of the talamanca range the highest peaks in central america the dichotomy between uplift in the inner forearc and subsidence in the outer forearc is explained in terms of the response of an arcward thickening wedge to rough subducting crust index terms 8150 tectonophysics plate boundary general 3040 8123 tectonophysics dynamics seismotectonics 8010 structural geology fractures and faults 8015 structural geology local crustal structure 8005 structural geology folds and folding

the production of small primary craters on mars and the moon abstract we model the primary crater production of small d d 10 20 cm well below the downturn observed in the distribution of fresh craters detected by the mars global surveyor mgs mars orbiter camera moc or the mars reconnaissance orbiter mro context camera ctx crater counts are conducted on the ejecta blanket of zunil crater and the interior of pangboche crater on mars and north ray and cone craters on the moon our model isochrons produce a similar slope and age estimate for the formation of zunil crater as the hartmann production function 1 ma we derive an age of 35 1 ma for pangboche when accounting for the higher elevation 20 km higher than zunil a factor 2 younger than estimated using the hartmann production function which assumes 6 mbar surface pressure we estimate ages of 52 3 ma and 23 9 ma for north ray and cone crater respectively consistent with cosmic ray exposure ages from apollo samples our results indicate that the average cratering rate has been constant on these bodies over these time periods since our monte carlo simulations demonstrate that the existing crater chronology systems can be applied to date young surfaces using small craters on the moon and mars we conclude that the signal from secondary craters in the isochrons must be relatively small at these locations as our monte carlo model only generates primary craters

predicted detection rates of regional scale meteorite impacts on mars with the insight short period seismometer abstract in 2016 nasa will launch the insight discovery class mission which aims to study the detailed internal structure of mars for the first time short and long period seismometers form a major component of insight s payload and have the potential to detect seismic waves generated by meteorite impacts large globally detectable impact events producing craters with diameters of 100 m have been investigated previously and are likely to be rare teanby n a wookey j 2011 phys earth planet int 186 70 80 but smaller impacts producing craters in the 0 5 20 m range are more numerous and potentially occur sufficiently often to be detectable on regional scales 1000 km at these distances seismic waves will have significant high frequency content and will be suited to detection with insight s short period seismometer seis sp in this paper i estimate the current martian crater production function from observations of new craters malin m c et al 2006 science 314 1573 1577 daubar i j et al 2013 icarus 225 506 516 model results williams j p pathare a v aharonson o 2014 icarus 235 23 36 and standard isochrons hartmann w k 2005 icarus 174 294 320 these impact rates are combined with an empirical relation between impact energy source receiver distance and peak seismogram amplitude derived from a compilation of seismic recordings of terrestrial and lunar impacts chemical explosions and nuclear tests the resulting peak seismogram amplitude scaling law contains significant uncertainty but can be used to predict impact detection rates i estimate that for a short period instrument with a noise spectral density of 10 8 ms 2 hz 1 2 in the 1 16 hz frequency band approximately 0 1 30 regional impacts per year should be detectable with a nominal value of 1 3 impacts per year therefore small regional impacts are likely to be a viable source of seismic energy for probing mars crustal and upper mantle structure this is particularly appealing as such impacts should be easily located with orbital imagery increasing their scientific value compared to other types of events with unknown origins finally comparison of the empirical results presented here with the modelling study of teanby and wookey teanby n a wookey j 2011 phys earth planet int 186 70 80 provides constraints on the seismic efficiency suggesting that values of 5 10 4 may be appropriate for impact generated seismic waves comparing explosion and impact datasets indicate that buried explosions are 10 times more efficient at generating seismic waves than impacts

present day impact cratering rate and contemporary gully activity on mars the mars global surveyor mars orbiter camera has acquired data that establish the present day impact cratering rate and document new deposits formed by downslope movement of material in mid latitude gullies on mars twenty impacts created craters 2 to 150 meters in diameter within an area of 21 5 106 square kilometers between may 1999 and march 2006 the values predicted by models that scale the lunar cratering rate to mars are close to the observed rate implying that surfaces devoid of craters are truly young and that as yet unrecognized processes of denudation must be operating the new gully deposits formed since august 1999 are light toned and exhibit attributes expected from emplacement aided by a fluid with the properties of liquid water relatively long extended digitate distal and marginal branches diversion around obstacles and low relief the observations suggest that liquid water flowed on the surface of mars during the past decade

motion of a fragmented meteoroid through the planetary atmosphere meteor observations and crater field investigations support the view that some meteoroids undergo fragmentation into a finite number of splinters which move a long distance without further disruption the motion of these fragments cannot be described in the frame of liquid like models the primary purpose of this study is to achieve some success in improving the alternative model of separate fragments which is based on studying the motion of a finite number of fragments interacting with each other through the air pressure the three dimensional 3 d numerical technique is elaborated for the direct modeling of the flow around several arbitrarily arranged fragments the model allows us to calculate the ablation mass rate and aerodynamic loading for each fragment the new approach hybrid model based on the simultaneous solution of the 3 d hydrodynamic equations for the airflow and the differential equations for the motion of discrete particles is proposed to study the debris cloud evolution taking into account successive fragmentation the simplified version of this model was applied to the sikhote alin meteorite shower benesov bolide and small meteoroids impacts against the martian surface

seismic detection of meteorite impacts on mars abstract meteorite impacts provide a potentially important seismic source for probing mars interior it has recently been shown that new craters can be detected from orbit using high resolution imaging which means the location of any impact related seismic event could be accurately determined thus improving the constraints that could be placed on internal structure using a single seismic station this is not true of other seismic sources on mars such as sub surface faulting which require location using multiple seismic stations this study aims to determine the seismic detectability of meteorite impacts and assess whether they are a viable means of probing deep internal structure first we derive a relation between crater diameter and equivalent seismic moment based on observational data compiled from impact tests controlled explosions and earthquake seismology second this relation was combined with updated cratering rates based on newly observed craters to derive the impact induced seismicity on mars which we estimate to total 10 13 10 14 n m per year finally seismic waveform modelling was used to determine the detectability of these impacts based on reasonable assumptions about likely seismometer performance and background noise levels for our nominal noise instrument case we find that detectable impacts at teleseismic distances source receiver offsets greater than 60 are very rare and occur approximately once every 10 years for our most optimistic noise instrument case approximately one such event occurs each year this suggests that using solely meteorite impacts is not a reliable way of probing the martian interior although local impacts are more frequently detectable and could provide important constraints on near surface seismic properties

saturn s cloud structure inferred from cassini iss abstract using high resolution cassini iss images with wavelengths ranging from the ultraviolet to the near infrared we have retrieved saturn s atmospheric aerosol structure and properties for a broad range of latitudes in the southern hemisphere the observations are consistent with two distinct layers of haze above a scattered cloud each layer is characterized by a vertical location an optical thickness and an effective mean particle size all of which vary with latitude the tropospheric haze is optically thickest and extends to the greatest heights 40 mbar over the equator its top surface is at significantly greater depths 150 mbar at mid latitudes the height of the haze correlates well with position of the tropopause as indicated by the temperature field beneath this haze we find a scattered denser cloud responsible for small scale contrasts at an average depth of 1 75 0 4 bar with some features as deep as 2 5 bar or greater

event and conditions that produced the iron meteorite block island on mars 1 the mars exploration rover opportunity has discovered four large iron nickel meteorites that range in size from 50 to 240 kg dispersed over 10 km of meridiani planum mars because these meteorites are covered with hollows that resemble regmaglypts their surfaces record their ablation through the atmosphere and they must have landed at speeds below hypervelocity 2 km s 1 to survive slowing massive iron meteorites requires a minimum atmospheric density which was quantified using a numerical model that integrates the equations of motion for incoming meteoroids through an atmosphere of a given surface density and scale height and records their outcomes as direct generally hypervelocity impacts that form craters longer over the horizon and fallback flight paths and skip outs the present atmosphere of mars is sufficient to slow iron meteoroids as large as block island the most massive meteorite via drag and significant ablation on long flight paths although for standard distributions of entering meteoroid masses velocities and entry angles such events are rare 0 007 of incoming iron meteoroids such events require entry angles of 10 13 entry velocities of 6 18 km s 1 and entry masses of 225 710 kg the absence of large stony meteorites is probably at least partially because they are much weaker and thus broken up into smaller fragments on impact although differential drag deceleration on long flight paths could disperse fragments of an entering meteoroid by tens of kilometers dynamic pressures are too low to break up an iron meteorite leaving the possibility that they are paired an open question