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

new constraints on atmospheric co2 concentration for the phanerozoic earth s atmospheric co2 concentration ca for the phanerozoic eon is estimated from proxies and geochemical carbon cycle models most estimates come with large sometimes unbounded uncertainty here we calculate tightly constrained estimates of ca using a universal equation for leaf gas exchange with key variables obtained directly from the carbon isotope composition and stomatal anatomy of fossil leaves our new estimates validated against ice cores and direct measurements of ca are less than 1000 ppm for most of the phanerozoic from the devonian to the present coincident with the appearance and global proliferation of forests uncertainties obtained from monte carlo simulations are typically less than for ca estimates from other approaches these results provide critical new empirical support for the emerging view that large 2000 3000 ppm long term swings in ca do not characterize the post devonian and that earth s long term climate sensitivity to ca is greater than originally thought key points a novel co2 proxy calculates past atmospheric co2 with improved certainty co2 is unlikely to have exceeded 1000 ppm for extended periods post devonian earth s long term climate sensitivity to co2 is greater than originally thought

model evaluation for reconstructing the oxygen isotopic composition in precipitation from tree ring cellulose over the last century the oxygen isotope composition of tree rings is controlled by many factors including temperature amount of precipitation and changes in relative humidity in this study we present a modified leaf water model from dongmann et al radiation and environmental biophysics 11 1974 41 that can be used to calculate the isotopic composition of the source water and thus of precipitation that a tree used during the growing season the calibration of the model was accomplished by comparing a previously measured oxygen isotope tree ring chronology from central switzerland with the swiss network for isotopes in the hydrologic cycle station at bern from 1971 to 1995 and integrating temperature relative humidity data and ring width in particular our efforts focused on understanding the significance of the dampening factor f which we relate to changes in humidity and its variability over both the calibration period 1971 to 1995 and the study period 1913 to 1995 our results indicate that f ranging between 0 27 to 0 49 is variable based on correlation with relative humidity average daily temperature and ring width index using this model we have constructed a record of the oxygen isotope composition of precipitation during the growing season may through september for the last century in central western europe this approach can be potentially used in other locations where isotopic and meteorological data are available to extend records of the isotopic composition of precipitation back in time beyond observational records

possible long term decline in impact rates 1 martian geological data abstract quantin et al quantin c allemand p mangold n delacourt c 2004a icarus 172 555 572 tabulated crater count data for 56 landslides along the walls of valles marineris under the assumption of a constant cratering rate after about 3 gyr ago as used in the 1999 2005 iterations of the crater chronology isochron system of hartmann and in the hartmann and neukum system these data indicate a regularly increasing rate of landslides which would be difficult to explain we suggest that these data may support a decline in inner solar system cratering rates by about a factor of 3 since 3 gyr ago not unlike predictions based on asteroid belt collision models such a decline is also supported by our review of data on lunar impact melts and glass spherules in a companion paper hartmann w k quantin c mangold n 2007 icarus 186 11 23 such models produce not only a more uniform rate of landslides over the last 3 gyr but also a more uniform rate of resurfacing processes which also had an apparent increase under the assumption of a constant cratering rate

ejecta from impact craters an important feature of impacts into solar system bodies is the fate of crater ejecta the near surface material launched during the highly dynamic crater formation process laboratory measurements of impact crater ejecta from 18 studies are summarized the data are examined and used to assess our understanding of how the ejecta velocity and mass distributions depend on the conditions of an impact event the effects of impact speed on the ejecta are reasonably well understood but the dependences on target properties such as strength and porosity are only poorly constrained a point source scaling model for the ejecta mass and velocity distributions is developed and fit to the data for several classes of materials distinguished by porosity

do young martian ray craters have ages consistent with the crater count system abstract mcewen et al mcewen a s preblich b s turtle e p artemieva n a golombek m p hurst m kirk r l burr d m christensen p 2005 icarus 176 351 381 developed a useful test for the internal consistency of crater count chronometry systems they argued that certain multi kilometer fresh looking martian craters with prominent rays should be the youngest or near youngest craters in their size range the mcewen et al test is that the ages determined from crater densities of the smallest superimposed craters typically diameter d 5 20 m should thus be comparable to the expected formation intervals of the host primary mcewen et al concluded from moc data that crater chronometry failed this test by factors of 700 2000 we apply hirise and other imagery to eight different young craters in order to re evaluate their arguments we use existing crater chronology systems as well as the reported observed production rate of 16 m craters malin m c edgett k posiolova l mccolley s noe dobrea e 2006 science 314 1573 1557 hartmann w k quantin c mangold n 2007 icarus 186 11 23 kreslavsky 2007 seventh international conference on mars 3325 every case passes the mcewen et al test we conclude that the huge inconsistencies suggested by mcewen et al are spurious many of these craters show evidence of impact into ice rich material and appear to have ice flow features and sublimation pits on their floors as production rate data improve decameter scale craters will provide a valuable way of dating these young martian geological formations and the processes that modify them

theoretical analysis of secondary cratering on mars and an image based study on the cerberus plains the issue of crater retention age estimates on planetary surfaces is discussed with an attempt to quantify the effect of overlapping primary and secondary impact crater populations in restricted crater diameter ranges the approach to this problem is illustrated with a simple model production function where the secondary crater input is artificially enhanced extrapolation of such a secondary crater model distribution to a global record results in extraordinarily high crater frequencies that do not exist on mars and implies the need of detailed studies of the size frequency distribution for remote secondary craters to date poorly known a key case the martian crater zunil and its secondary crater field illustrate that reasonable predictions for the secondary crater size frequency distribution at small 100 m crater diameters affected the standard model crater retention age for the cerberus plains less than the statistical uncertainty these observations show that age determination based on appropriate crater counting statistics is valid in a wide primary crater diameter range

marked decline in atmospheric carbon dioxide concentrations during the paleogene the relation between the partial pressure of atmospheric carbon dioxide pco2 and paleogene climate is poorly resolved we used stable carbon isotopic values of di unsaturated alkenones extracted from deep sea cores to reconstruct pco2 from the middle eocene to the late oligocene approximately 45 to 25 million years ago our results demonstrate that pco2 ranged between 1000 to 1500 parts per million by volume in the middle to late eocene then decreased in several steps during the oligocene and reached modern levels by the latest oligocene the fall in pco2 likely allowed for a critical expansion of ice sheets on antarctica and promoted conditions that forced the onset of terrestrial c4 photosynthesis

early views of the martian surface from the mars orbiter camera of mars global surveyor high resolution images of the martian surface at scales of a few meters show ubiquitous erosional and depositional eolian landforms dunes sandsheets and drifts are prevalent and exhibit a range of morphology composition inferred from albedo and age as seen in occurrences of different dune orientations at the same location steep walls of topographic depressions such as canyons valleys and impact craters show the martian crust to be stratified at scales of a few tens of meters the south polar layered terrain and superposed permanent ice cap display diverse surface textures that may reflect the complex interplay of volatile and non volatile components low resolution regional views of the planet provide synoptic observations of polar cap retreat condensate clouds and the lifecycle of local and regional dust storms

subsurface exploration via satellite structure visible in seasat images of north sea atlantic continental margin and australia satellite observations of the sea surface height are sensitive to crustal structure overcoming the inability of previous remote sensing techniques to penetrate the earth s surface seasat gravity images have been compared with the structure of offshore sedimentary basins in northwestern europe seasat images display primary structural features e g the anglo dutch and smaller basins down to the scale of the manx furness basin and numerous secondary structural features e g the halibut horst additionally because of their synoptic viewpoint seasat images reveal features hard to distinguish i cite as an example a basement ridge extending from scotland to norway in position to have controlled marine circulation during deposition of the principal source rock wh ch is equivalent to the kimmeridge clay in north america and south america seasat images have been used to examine the relation of the atlantic fracture zones to the structure of the continental margin of the united states and brazil in australia known basins of the northwest continental margin and the bass strait have been compared with seasat projections of shallow structure in comparison with reconnaissance seismic magnetics and ship borne gravity surveys seasat images are valuable in discovering the presence of thick sediment accumulations seasat images are potentially more valuable than seismic and magnetics in the presence of basement masking by intrasectional volcanics or massive carbonates in known basins seasat data deliver a uniform synoptic view permitting the identification of large structural features in this form these images give the structural geologist a tool like that given the stratigrapher by regional seismic sections in remote unexplored regions seasat images provide a means of optimizing the layout of reflection seismic the gravity data from seasat offer the first of increasingly accurate structural images to be expected from altimetric satellites

the age of lunar mare basalts south of the aristarchus plateau and effects of secondary craters formed by the aristarchus event abstract using crater size frequency distributions csfds and careful discrimination of primary and secondary impact craters in the mare region south of aristarchus crater we reexamined the age of unit p60 determined previously by hiesinger et al 2003 to be one of the youngest basaltic surfaces on the moon owing to the apparently young age of these basalts this region is a potential site for future sample return missions we investigated this 55 000 km2 region with lunar reconnaissance orbiter camera lroc images to assess potential variations in csfds across the unit particularly in light of heavy contamination by secondary craters from aristarchus crater and to determine the age s of p60 over its full areal extent we are able to match within one standard error the previously determined age using approximately the same counting area in addition we defined twelve regions between the rays of secondaries to determine if and how the ages of p60 might vary across the unit for these inter ray regions we find a systematic progression of ages from west to east ranging from youngest 1 ga in the west to 1 8 ga southwest of aristarchus to 2 2 ga south of aristarchus and finally to 2 7 ga southeast of aristarchus this variation in ages is not solely attributable to secondary cratering indicating it must be at least partially due to volcanic resurfacing the northwestern most extension of p60 may belong to a different unit owing to topographic and crater distribution differences analysis of the summed csfd for p60 provides evidence for emplacement of the younger basalts

comprehensive analysis of glaciated martian crater greg abstract the 66 km diameter martian crater greg east of hellas hosts various distinctive features including dendritic valleys filled with chevron textured masses south wall and lobate tongues a few kilometers long north wall we analyze these features by various quantitative techniques to illuminate martian geologic and climatic history crater retention model ages indicate that greg is at least 1 3 gy old but surface layers of mantles and glacial features are orders of magnitude younger properties of the dendritic valleys combined with climate models suggest that fluvial activity began under a thicker warmer atmosphere soon after the crater s formation the oldest exposed fluvial systems have surface crater retention ages of a few hundred my indicating runoff in recent geologic time much of greg is covered by ice rich mantle deposits for which we infer gradual accumulation and depths of order 30 85 m they mask pre existing landforms the lobate tongues are interpreted as glaciers with mean slope of 10 2 2 3 and average thickness of 33 19 m our calculations and data suggest that these glaciers were originally ice rich and that their surface layers have been depleted by volatile loss the glaciers probably formed when ice rich mantle deposits reached critical thickness and flowed downhill the top 5 10 m of the mantle and glaciers show crater survival times of order a few my to 15 my which remarkably is the time since the last 1 4 episodes of obliquity 45 global climate models affirm that greg lies in one of two non polar areas with extremes of ice deposition during high obliquity epochs this match with observations supports the use of such models in studies of planetary climate change

lunar orientale impact basin secondary craters spatial distribution size frequency distribution and estimation of fragment size secondary impact craters features created by projectiles ejected from a primary impact contain important information about the primary cratering event and the nature and distribution of its ejecta the orientale impact basin d similar to 930km is the youngest and the least degraded large impact basin on the moon and has the most recognizable secondary impact craters we identified and mapped 2 728 secondary craters in the investigated area of similar to 1 66x10 7 km 2 covering an area from the rim of orientale to six radii secondary crater diameters range from similar to 2 to 27km and the median diameter decreases as distance increases secondary craters are concentrated predominantly in the northwest and southwest the ejecta deposit pattern inferred from secondary crater distribution suggests that the orientale basin was formed by an oblique impact in which the downrange direction was 240 degrees 265 degrees in azimuth and the incidence angle was steeper than 20 degrees the cumulative size frequency distribution of mapped secondary craters steepens as diameter increases and is very well approximated with a weibull distribution with an exponent 1 32 a widely used crater scaling relationship predicts that the fragments that produced the secondary craters were predominantly in similar to 0 5 2 km diameter range over the investigated area the diameter of the largest fragment however decreases with increasing distance from orientale on the basis of the diameter of the largest secondary crater of orientale and other craters and basins the largest secondary crater of the south pole aitken basin is estimated to be similar to 40km in diameter we explore the implications of these findings for the evolution of the megaregolith and future sample return missions

effect of topography degradation on crater size frequency distributions implications for populations of small craters and age dating whether or not background secondary craters dominate populations of small impact craters on terrestrial bodies is a half century controversy it has been suggested that small craters on some planetary bodies are dominated by background secondary craters based partly on the steepened slope of crater size frequency distribution csfd towards small diameters such as the less than 1 km diameter crater population on the lunar mare here we show that topography degradation enlarges craters and increases csfd slopes with time when topography degradation is taken into account for various aged crater populations the observed steep csfd at small diameters is uniformly consistent with an originally shallower csfd whose slope is undifferentiated from the csfd slope estimated from near earth objects and terrestrial bolides the results show that the effect of topography degradation on csfd is important in dating planetary surfaces and the steepening of csfd slopes is not necessarily caused by secondary cratering but rather a natural consequence of topography degradation

martian cratering 9 toward resolution of the controversy about small craters abstract the malin et al malin m c edgett k posiolova l mccolley s noe dobrea e 2006 science 314 1573 1577 report of ongoing production of decameter scale primary craters matches within an order of magnitude the crater production curves of hartmann hartmann w k 2005 icarus 174 294 320 and appears to contradict models that say our curves grossly overpredict primary production nonetheless such models may still predict correct order of magnitude secondary production the new observations if correct require geologically recent martian surface modification in some areas on timescales comparable to the cycles of obliquity changes suggesting episodic martian climatic environmental change driven by the obliquity cycles

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

geology of the northeastern flank of apollinaris mons mars constraints on the erosional history from morphology topography and crater populations abstract we have utilized themis mola ctx and hirise data sets to investigate the morphologic and topographic characteristics of the northeast flank of apollinaris mons a region with evidence for significant erosion of volcanic deposits using arcgis software we mapped the geology of the northeast flank and obtained age estimates using crater size frequency distributions from crater counts of the eight mapped units three apollinaris mons upper mid and lower flank units represent volcanic flank materials in various states of preservation the late noachian early hesperian upper flank unit is massive and contains joint like vertical structures along the scarp faces of two large plateaus whose upper surfaces likely represent the original surface or near surface deposits of apollinaris mons downslope facing plateau scarps have up to 500 m of relief positioned below this unit are the mid flank and lower flank units each representing eroded surfaces within apollinaris mons flank materials that have stabilization ages between the late hesperian to early amazonian mantling material in the region contains cavities and boxwork like textures that resemble patterns in terrestrial eroded pyroclastic flow deposits dome shaped mounds west of the mapping region have morphologic similarities to terrestrial fumarolic mounds or possibly inverted impact craters both the texture and features suggest widespread pyroclastic deposits adjacent to apollinaris mons using mola gridded topography we estimate that 308 km3 of materials have been eroded along the flanks of the volcano statistical and histogram data from the thickness values of eroded materials shows that up to 300 m of material has been removed from the majority of flank surfaces assuming steady state erosion of flank surfaces we estimate an area normalized loss rate of 0 859 nm yr for northeast apollinaris mons this erosion rate is within the long term range for mars 0 01 10 nm yr as estimated from mer landing site geology by golombek et al 2006

dating very young planetary surfaces from crater statistics a review of issues and challenges determining the ages of young planetary surfaces relies on using populations of small often sub km diameter impact craters due to the higher frequency at which they form smaller craters however can be less reliable for estimating ages as their size frequency distribution is more susceptible to alteration with debate as to whether they should be used at all with the current plethora of meter scale resolution images acquired of the lunar and martian surfaces small craters have been widely used to derive model ages to establish the temporal relation of recent geologic events in this review paper we discuss the many factors that make smaller craters particularly challenging to use and should be taken into consideration when crater counts are confined to small crater diameters establishing confidence in a model age ultimately requires an understanding of the geologic context of the surface being dated as reliability can vary considerably and limitations of the dating technique should be considered in applying ages to any geologic interpretation

high regional climate sensitivity over continental china constrained by glacial recent changes in temperature and the hydrological cycle the east asian monsoon is one of earth s most significant climatic phenomena and numerous paleoclimate archives have revealed that it exhibits variations on orbital and suborbital time scales quantitative constraints on the climate changes associated with these past variations are limited yet are needed to constrain sensitivity of the region to changes in greenhouse gas levels here we show central china is a region that experienced a much larger temperature change since the last glacial maximum than typically simulated by climate models we applied clumped isotope thermometry to carbonates from the central chinese loess plateau to reconstruct temperature and water isotope shifts from the last glacial maximum to present we find a summertime temperature change of 6 7 c that is reproduced by climate model simulations presented here proxy data reveal evidence for a shift to lighter isotopic composition of meteoric waters in glacial times which is also captured by our model analysis of model outputs suggests that glacial cooling over continental china is significantly amplified by the influence of stationary waves which in turn are enhanced by continental ice sheets these results not only support high regional climate sensitivity in central china but highlight the fundamental role of planetary scale atmospheric dynamics in the sensitivity of regional climates to continental glaciation changing greenhouse gas levels and insolation

middle eocene co2 and climate reconstructed from the sediment fill of a subarctic kimberlite maar eocene paleoclimate reconstructions are rarely accompanied by parallel estimates of co 2 from the same locality complicating assessment of the equilibrium climate response to elevated co 2 we reconstruct temperature precipitation and co 2 from latest middle eocene ca 38 ma terrestrial sediments in the posteruptive sediment fill of the giraffe kimberlite in subarctic canada mutual climatic range and oxygen isotope analyses of botanical fossils reveal a humid temperate forest ecosystem with mean annual temperatures mats more than 17 c warmer than present and mean annual precipitation 4 present metasequoia stomatal indices and gas exchange modeling produce median co 2 concentrations of 630 and 430 ppm respectively with a combined median estimate of 490 ppm reconstructed mats are more than 6 c warmer than those produced by eocene climate models forced at 560 ppm co 2 estimates of regional climate sensitivity expressed as mat per co 2 doubling above preindustrial levels converge on a value of 13 c underscoring the capacity for exceptional polar amplification of warming and hydrological intensification under modest co 2 concentrations once both fast and slow feedbacks become expressed

evidence for recent volcanism on mars from crater counts impact craters help characterize the age of a planetary surface because they accumulate with time they also provide useful constraints on the importance of surface erosion as such processes will preferentially remove the smaller craters earlier studies of martian crater populations revealed that erosion and dust deposition are important processes on mars they disagreed however on the age of the youngest volcanism these earlier studies were limited by image resolution to craters larger than a few hundred metres in diameter here we report an analysis using new images obtained by the mars global surveyor spacecraft of crater populations that extend the size distribution down to about 16 m our results indicate a wide range of surface ages with one region lava flows within the arsia mons caldera that we estimate to be no older than 40 100 million years we suggest that volcanism is a continuing process on mars

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

life at the top of the greenhouse eocene world a review of the eocene flora and vertebrate fauna from canada s high arctic early middle eocene ca 53 38 ma sediments of the eureka sound group in canada s arctic archipelago preserve evidence of lush mixed conifer broadleaf rain forests inhabited at times by alligators turtles and diverse mammals including primates tapirs brontotheres and hippo like coryphodon this biota reflects a greenhouse world offering a climatic and ecologic deep time analog of a mild ice free arctic that may be our best means to predict what is in store for the future arctic if current climate change goes unchecked in our review of the early middle eocene arctic flora and vertebrate fauna we place the arctic fossil localities in historic geographic and stratigraphic context and we provide an integrated synthesis and discussion of the paleobiology and paleoecology of these eocene arctic forests and their vertebrate inhabitants the abundance and diversity of tapirs and plagiomenids both rare elements in midlatitude faunas and the absence of artiodactyls early horses and the hyopsodontid condylarth hyopsodus well represented at midlatitude localities are peculiar to the eocene arctic the eocene arctic macrofloras reveal a forested landscape analogous to the swamp cypress and broadleaf floodplain forests of the modern southeastern united states multiple climate proxies indicate a mild temperate early middle eocene arctic with winter temperatures at or just above freezing and summer temperatures of 20 c or higher and high precipitation at times this high precipitation resulted in freshwater discharge into a nearly enclosed arctic ocean basin sufficient to cause surface freshening of the arctic ocean supporting mats of the floating fern azolla fluctuating arctic ocean sea level due to freshwater inputs as well as tectonics produced temporary land bridges allowing land plants and animals to disperse between north america and both europe and asia

secondary crater fields from 24 large primary craters on mars insights into nearby secondary crater production 1 crater statistics are used across a wide variety of applications on planetary surfaces one of the most notable being estimating relative and absolute ages of those surfaces this requires an assumed cratering rate over time and that craters be randomly distributed secondary craters craters that form from the ejecta of an impact event belie this assumption by creating greater crater density in a local area at a single time significantly affecting crater statistics there has been substantial debate over the relative importance of secondary craters and our findings in this mars study indicate that these events can be very significant and cannot be ignored when age dating surfaces we have analyzed secondary crater fields found close to 24 primary craters on mars among other findings such as terrain control over secondary crater field characteristics we conclude that a single large impact event 100 km can significantly affect crater statistics at the 1 5 km diameter level over a non trivial fraction of a planetary surface minimum secondary crater diameters examined were 0 9 km the minimum primary crater diameter was 20 km we also suggest a potential way to avoid significant contamination by the majority of secondary craters that occur close to the primary impact event without the need to manually classify every crater as primary or secondary our findings are specific to mars but further work may show the patterns are applicable to other solid bodies

a mechanism for the production of crater rays abstract a mechanism for the production of crater rays is proposed that is based on the interaction of impact induced shock waves with existing old craters numerical simulations are used to test this idea and to study the influence of inherited craters size and location on the rays parameters the results of the simulations show that crater rays at least some of them could be produced through this mechanism

the length of lunar crater rays explained using secondary crater scaling abstract the relationship between the length of crater rays and primary crater radius is still poorly understood in this study we mapped the ray systems of 27 lunar craters ranging from 10 m to 84 km in diameter for each ray system we measured the number length and optical maturity index omat of the rays our mapping effort shows that ray length scales to primary crater radius r with a power law of r1 22 except for the smallest and freshest rays

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

geomorphic analysis of small rayed craters on mars examining primary versus secondary impacts 1 twenty confirmed impacts over a 7 year time period on mars were qualitatively and statistically compared to 287 secondary craters believed to originate from zunil an 500 ka 10 km diameter primary crater our goal was to establish criteria to distinguish secondaries from primaries in the general crater population on the basis of their horizontal planforms recent primary impacts have extensive air blast zones distal ray systems 100 crater radii r and ephemeral ejecta recent primaries formed clusters of craters from atmospheric fragmentation of the meteoroid body secondary craters have ejecta blankets with shorter rays that are consistent with emplacement by low impact velocities near 1 km s the mean extent of the continuous ejecta blankets was less distal for secondaries 5 38 1 57r versus primaries 18 07 7 01r though primary ejecta were less fractal fractal dimension index fdi 1 30 and more circular on average circularity ratio cr 0 55 0 25 versus 0 27 0 13 for secondaries crater rims were remarkably circular primaries cr 0 97 0 02 secondaries at 0 94 0 05 though secondaries have the lowest values cr 0 9 secondary crater rims were elongated toward or orthogonal to their primary of origin uprange source directions for most secondaries determined by ejecta planform and crater rim ellipticity point toward zunil although contamination from other primaries is considered in some areas ejecta blanket discrepancies between recent primaries and zunil secondaries are attributable to differences in impact velocity and retention age after removal of the ejecta blanket crater rims are generally not diagnostic for determining crater origin fragmentation of primaries may play some role in steepening the size frequency distribution of crater diameters in the 5 m d 30 m range

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

size frequency distribution of different secondary crater populations 1 equilibrium caused by secondary impacts accumulation of impact craters is the major reason causing equilibrium of crater populations on airless planetary surfaces besides primary craters the effect of widespread secondaries on the equilibrium of local crater populations is little studied here the different secondary crater populations formed by the hokusai crater on mercury are systematically studied and they are compared with those on the moon to investigate their contribution to the evolution of local crater populations self secondaries cause equilibrium on continuous ejecta deposits in a short time and the equilibrium crater population has a differential sfd slope of about 3 background secondaries are abundant on mercury and equilibrium caused by a combination of primaries and potential background secondaries follows the same pattern on the moon and mercury the spatial dispersion of fragments that form both near field and distant secondaries is the major factor affecting the degree of mutual destruction and thus the final crater sfd some clustered distant secondaries on mercury are likely formed by individual fragments considering their large spatial dispersion and identical morphology with same sized primaries and the sfd rollovers of these secondaries possibly reflect the inherent sfd rollovers of the impact fragments near field secondaries and many other distant secondaries have morphology and spatial distribution that are consistent with being formed by clustered fragments and mutual destruction of secondaries may be the major reason causing the observed sfd rollovers heterogeneous secondary impacts are a potential explanation for both different crater densities within the equilibrium diameter range and different regolith thicknesses on coeval surfaces

formation of glaciers on mars by atmospheric precipitation at high obliquity surface conditions on mars are currently cold and dry with water ice unstable on the surface except near the poles however geologically recent glacierlike landforms have been identified in the tropics and the midlatitudes of mars the ice has been proposed to originate from either a subsurface reservoir or the atmosphere we present high resolution climate simulations performed with a model designed to simulate the present day mars water cycle but assuming a 45 obliquity as experienced by mars a few million years ago the model predicts ice accumulation in regions where glacier landforms are observed on the western flanks of the great volcanoes and in the eastern hellas region this agreement points to an atmospheric origin for the ice and reveals how precipitation could have formed glaciers on mars

evidence for glaciation in the northern hemisphere back to 44 ma from ice rafted debris in the greenland sea the widely accepted age estimate for the onset of glaciation in the northern hemisphere ranges between 2 and 15 million years ago ma however recent studies indicate the date for glacial onset may be significantly older we report the presence of ice rafted debris ird in 44 to 30 ma sediments from the greenland sea evidence for glaciation in the north atlantic during the middle eocene to early oligocene detailed sedimentological evidence indicates that glaciers extended to sea level in the region allowing icebergs to be produced ird may have been sourced from tidewater glaciers small ice caps and or a continental ice sheet

dependence of secondary crater characteristics on downrange distance high resolution morphometry and simulations on average secondary impact craters are expected to deepen and become more symmetric as impact velocity vi increases with downrange distance l we have used high resolution topography 1 2 m pixel to characterize the morphometry of secondary craters as a function of l for several well preserved primary craters on mars the secondaries in this study n 2 643 span a range of diameters 25 m d 400 m and estimated impact velocities 0 4 km s vi 2 km s the range of diameter normalized rim to floor depth d d broadens and reaches a ceiling of d d 0 22 at l 280 km vi 1 1 2 km s whereas average rim height shows little dependence on vi for the largest craters h d 0 02 d 60 m populations of secondaries that express the following morphometric asymmetries are confined to regions of differing radial extent planform elongations l 110 160 km taller downrange rims l 280 km and cavities that are deeper uprange l 450 500 km populations of secondaries with lopsided ejecta were found to extend to at least l 700 km impact hydrocode simulations with isale 2d for strong intact projectile and target materials predict a ceiling for d d vs l whose trend is consistent with our measurements this study illuminates the morphometric transition from subsonic to hypervelocity cratering and describes the initial state of secondary crater populations this has applications to understanding the chronology of planetary surfaces and the long term evolution of small crater populations

the planforms of low angle impact craters in the northern hemisphere of mars we have surveyed martian impact craters greater than 5 km in diameter using viking and thermal emission imaging system themis imagery to evaluate how the planform of the rim and ejecta changes with decreasing impact angle we infer the impact angles at which the changes occur by assuming a sin2 dependence for the cumulative fraction of craters forming below angle at impact angles less than 40 from horizontal the ejecta become offset downrange relative to the crater rim as the impact angle decreases to less than 20 the ejecta begin to concentrate in the crossrange direction and a forbidden zone that is void of ejecta develops in the uprange direction at angles less than 10 a butterfly ejecta pattern is generated by the presence of downrange and uprange forbidden zones and the rim planform becomes elliptical with the major axis oriented along the projectile s direction of travel the uprange forbidden zone appears as a v curving outward from the rim but the downrange forbidden zone is a straight edged wedge although fresh martian craters greater than 5 km in diameter have ramparts indicative of surface ejecta flow the ejecta planforms and the angles at which they occur are very similar to those for lunar craters and laboratory impacts conducted in a dry vacuum the planforms are different from those for venusian craters and experimental impacts in a dense atmosphere we interpret our results to indicate that martian ejecta are first emplaced predominantly ballistically and then experience modest surface flow

mapping rays and secondary craters from the martian crater zunil 1 zunil a 10 1 km rayed crater in elysium planitia mars produced more than 7 107 secondary craters 15 m in diameter we mapped zunil s rays from thermal ir themis nighttime images up to 1700 km from the primary crater and mapped bright and dark ejecta craters candidate secondaries up to 3600 km range ray segments were mapped up to 450 km east of zunil and up to 1700 km to the west both rays and bright ejecta craters are best detected over terrains with moderate thermal inertia which are abundant west of but not east of zunil nevertheless our interpretation is that zunil was created by a moderately oblique impact from the east zunil secondaries are abundant over all terrain types except the medusae fossae formation mff given the likely age of zunil 100 ma parts of the mff must be eroding at 0 08 m ma the size frequency distribution of the secondaries in zunil s rays and probable distant secondaries 750 1700 km to the west of zunil have a cumulative power law exponent near 5 whereas secondaries between the major rays have an exponent near 3 4 we modeled the size velocity relationship for zunil s ejected fragments it is consistent with the predictions of melosh 1984 for spallation of a strong surface layer and demonstrates that the inverse size velocity correlation continues up to at least 2 km s

secondary craters of tycho size frequency distributions and estimated fragment size velocity relationships 1 we examined the size frequency distribution and the spatial distribution of secondary craters around the lunar crater tycho secondary crater diameters were found to range from 0 55 to 4 0 km and their distance from tycho to range from 130 to 370 km the diameter and ejection velocity of the secondary forming fragments were also estimated from the crater size and the distance from the primary using the scaling relationships for cratering the power law index of the size frequency distribution of the tycho secondary craters ranges from 3 3 to 4 0 the same as the steep slope of the typical size frequency distribution for lunar craters of less than 4 km diameter the decay index of secondary crater densities with distance from the primary craters is not constant and is small at large distances less than 2 5 for r 2 5r the total number of secondaries forming ejecta fragments is more than twice larger than for other lunar primaries previously studied these results imply that there are many small secondary craters in fields far from the primary crater a dominant contribution of secondaries to the size frequency distribution of all lunar craters is thus strongly suggested the index range of 2 6 to 3 0 for estimated fragment size distributions is not only close to other natural and experimental craters but also to those of blocks on asteroids the relative fragment size distributions are independent of the ejection velocity this result is the first observational confirmation of the theoretical assumption and agrees with a recent result from laboratory experiments