a low temperature transfer of alh84001 from mars to earth the ejection of material from mars is thought to be caused by large impacts that would heat much of the ejecta to high temperatures images of the magnetic field of martian meteorite alh84001 reveal a spatially heterogeneous pattern of magnetization associated with fractures and rock fragments heating the meteorite to 40 c reduces the intensity of some magnetic features indicating that the interior of the rock has not been above this temperature since before its ejection from the surface of mars because this temperature cannot sterilize most bacteria or eukarya these data support the hypothesis that meteorites could transfer life between planets in the solar system

meteorite accumulations on mars abstract we have modeled single body meteoroid atmospheric entry speeds at mars and the effect of drag and ablation and identify a narrow range of small masses 10 50 g that should impact mars at survivable speeds the rate of oxidative weathering is much lower than that on earth so this small flux of meteorites could give rise to significant accumulations ca 5 10 2 to 5 10 5 meteorites greater than 10 g in mass per square kilometer given that extremely large numbers of meteorites may be present on mars future sample return missions should consider the real possibility that they may recover meteoritic material due to the low weathering rate meteorites may survive on the surface of mars for more than 10 9 years preserving a record of the temporal variability of the meteoroid flux and the compositional evolution of the meteoroid complex intact carbonaceous chondrites may also preserve organic compounds from degradation by ultraviolet radiation terrestrial meteorites may be present but would probably be sterile

martian meteorite dhofar 019 a new shergottite dhofar 019 is a new martian meteorite found in the desert of oman in texture mineralogy and major and trace element chemistry this meteorite is classified as a basaltic shergottite olivine megacrysts are set within a groundmass composed of finer grained olivine pyroxene pigeonite and augite and maskelynite minor phases are chromite ulvospinel ilmenite silica k rich feldspar merrillite chlorapatite and pyrrhotite secondary phases of terrestrial origin include calcite gypsum celestite fe hydroxides and smectite dhofar 019 is most similar to the elephant moraine eeta 79001 lithology a and dar al gani dag 476 489 shergottites the main features that distinguish dhofar 019 from other shergottites are lack of orthopyroxene lower ni contents of olivine the heaviest oxygen isotopic bulk composition and larger compositional ranges for olivine maskelynite and spinel as well as a wide range for pyroxenes the large compositional ranges of the minerals are indicative of relatively rapid crystallization modeling of olivine chemical zonations yield minimum cooling rates of 0 5 0 8 c h spinel chemistry suggests that crystallization took place under one of the most reduced conditions for martian meteorites at an f o2 3 log units below the quartz fayalite magnetite qfm buffer the olivine megacrysts are heterogeneously distributed in the rock crystal size distribution analysis suggests that they constitute a population formed under steady state conditions of nucleation and growth although a few grains may be cumulates the parent melt is thought to have been derived from partial melting of a light rare earth element and platinum group element depleted mantle source shergottites eeta79001 lithology a dag 476 489 and dhofar 019 although of different ages comprise a particular type of martian rocks such rocks could have formed from chemically similar source s and parent melt s with their bulk compositions affected by olivine accumulation

terrestrial ages of ordinary chondrites from the lewis cliff stranding area east antarctica abstract we determined terrestrial ages of ordinary chondrites from the lewis cliff stranding area east antarctica on the basis of the concentrations of cosmogenic 10be t 1 51 ma 26al t 0 705 ma and 36cl t 0 301 ma after an initial 26al ray survey of 91 meteorites suggested that many have terrestrial ages 0 1 ma we selected 62 meteorites for 10be and 26al measurements by accelerator mass spectrometry ams and measured 36cl in twelve of those low terrestrial ages 2 ma welten et al 1997 our major conclusions are 1 the lewis cliff h chondrites show similar ages to those from the allan hills icefields but the l chondrites are about a factor of 2 younger than those from allan hills which indicates that lewis cliff is a younger stranding area 2 the terrestrial age distributions at different parts of the lewis cliff stranding area generally agree with simple meteorite concentration models although differences in weathering rate may also play a role 3 we confirm that meteorites with natural thermoluminescence tl levels 80 krad are associated with low terrestrial ages benoit et al 1992 but conclude that natural tl levels 80 krad can not be used to calculate the terrestrial age of a meteorite natural tl levels do seem useful to estimate relative terrestrial ages of large groups of meteorites and to determine differences in the surface exposure age of paired meteorite fragments 4 of the 62 meteorites measured with ams 31 were assigned to 11 different pairing groups mainly on the basis of their cosmogenic nuclide record the meteorites are estimated to represent between 42 and 52 distinct falls

quasi periodic atmosphere regolith cap co2 redistribution in the martian past our earlier mars regolith atmosphere cap co2 distribution model fanale et al 1982 icarus 50 381 407 has been improved revised and extended back over mars mid to late history the present model takes into account four new factors 1 a more realistic long term obliquity cycle 2 thermal conduction as it affects the surface energy balance 3 the changing solar constant and 4 atmospheric erosion 3 5 byr ago to the present solar insolation and temperatures are computed for the full range of obliquities latitudes and epochs and a co2 adsorption relation is used together with a conservation of mass constraint to calculate atmospheric pressures and exchangeable co2 mass as functions of obliquity and epoch for the regolith atmosphere and polar caps for two assumed thicknesses of a basalt regolith it is found that the heat conduction term in the surface boundary condition has an important effect in reducing the range of atmospheric pressures over the obliquity cycle at all epochs its main effect is to maintain the atmospheric co2 pressure near 1 mb at very low obliquity 10 as opposed to 0 01 mb without this term fanale et al 1982 icarus 50 381 407 at low to medium obliquities when a perennial co2 polar cap was present atmospheric pressures increased toward the present due to the increasing solar constant at intermediate to high obliquities at which there was no perennial co2 polar cap atmospheric pressures decreased toward the present to a point due to the decreasing co2 inventory and then increased to the present due to the increasing solar constant in past times up to the point considered in this study co2 pressures have been as low as 0 6 mbar at the lowest obliquity and only slightly higher than at present at the highest obliquity atmospheric co2 pressures 0 1 mbar are still suggested by the model but only in cases where near zero obliquity occurs episodically or chaotically as predicted by recent models of the obliquity and eccentricity variation at low to intermediate obliquities the polar caps contained most of the exchangeable co2 at intermediate to high obliquities the regolith contained most of the co2 at least in more recent epochs while the polar caps may have been the dominant reservoir during earlier epochs in physical equilibrium the atmosphere contained a very small portion of the exchangeable co2 for most cases

lunar and martian meteorite delivery services launch mechanisms for lunar and martian meteorites have been investigated by integrating physical modeling constraints geochemical cosmic ray exposure cre constraints and petrologic constraints the potential source region for lunar meteorites is remarkably small compared to the final crater volume cre constraints indicate that most launches start at depths of 3 2 m and cratering theory implies derivation of suitably accelerated objects from a subvolume with diameter only about 0 3 the final crater diameter the shallow depth provenance is probably related to shock wave interference enhanced by the lunar regolith s extremely low compressional wave velocity cre constraints alone imply that four to five separate launch events are represented among the eight well studied lunar meteorites most of the lunar meteorites are regolith breccias which tend to show only limited compositional diversity within any kilometer scale region of the moon several others are polymict breccias which also show relatively subdued compositional diversity compared to igneous rocks the observed diversity among these samples in terms of abundances of mare basalt and kreep and in mg mg fe ratio implies that among eight well studied lunar meteorites only two potential source crater pairings are plausible between asuka 881757 y 793169 most probable and between y 793274 eet875721 altogether these eight lunar meteorites apparently represent at least six separate source craters including three in the past 105 years and five in the past 106 years cre constraints imply that snc meteorites are launched from systematically greater depths than lunar meteorites sncs are also systematically bigger and all nine well studied sncs are uncommonly young by martian standards mafic igneous rocks comparison between viking and apollo results reveals that rocks the size of common meteorites are remarkably scarce in the martian regolith probably due to pervasive weathering a plausible explanation for these trends is that most old small and shallow rocks on mars have been weakened by a two stage process of brecciation followed by pervasive weathering to the point where they seldom survive the stresses of spallation off the planet the scarcity of source crater pairing among the lunar meteorites implies that these objects can be launched from craters much smaller than previously estimated and tends to support the suggestion of rabinowitz 1993 astrophys j 407 412 427 that present day cratering rates for the earth moon region may be higher than previously estimated

terrestrial 14c age of the antarctic shergottite eeta 79001 abstract the terrestrial age of the elephant moraine shergottite eeta 79001 lithology a has been determined from measurement of its cosmogenic 14 c content as 12 2 kyr the results on saturated and blank samples of l g or less are also discussed the age calculated for eeta 79001 is compared to exposure and terrestrial ages of other shergottites in the light of possible origins of these meteorites on mars

the 1989 submarine eruption off eastern izu peninsula japan ejecta and eruption mechanisms the submarine eruption of a new small knoll which was named teishi knoll off eastern izu peninsula behind the izu mariana arc occurred in the evening of 13 july 1989 this is the first historic eruption of the higashi izu monogenetic volcano group the eruption of 13 july followed an earthquake swarm near ito city starting on 30 june there were subsequent volcanic tremors on 11 and 12 july and the formation of the teishi knoll on the 100 m deep insular shelf 4 km northeast of ito city there were five submarine explosions which were characterized by intermittent domelike bulges of water and black tephra jets which occurred within 10 min on 13 july ejecta of the eruption was small in volume and composed of highly crystalline basalt scoria highly vesiculated pumice and lithic material petrographical features suggest that the pumice was produced by vesiculation of reheated wet felsic tuff of an older formation the teishi knoll before the eruption was a circular dome 450 m across and 25 m high with steep sides and a flat summit considerations of submarine topographic change indicate the knoll was raised by sill like intrusion of 106 m3 of magma beneath a 30 m thick sediment blanket this shallow intrusion is assumed to have started on 11 july when volcanic tremors were observed for the first time but there was no indications of violent interaction between wet host sediments and intruding magma the submarine eruption of 13 july appears to have been friggered by a major lowering of the magma column the basalt scoria having crystal contents of more than 60 is assumed to be derived from the cooled plastic margin of the shallow intrusive body however glassy scoria which would indicate the interaction between hot fluidal magma and external water was not observed a scenario for the 1989 submarine eruption is as follows when rapid subsidence of the hot interior of the intrusive magma occurred reduced pressure caused the implosion of cooled plastic magma adjacent pressurized hot host material and wet sediment the mixing of these materials triggered the vigorous vapor explosions

numerical simulation of temperature effects at fissures due to shock loading the localized appearance of specific shock features in target rocks and meteorites such as melt veins and high pressure polymorphs suggests that regions with a local increase in pressure and temperature exist as a shock wave propagates through an inhomogeneous rock in this paper we investigate the effect of planar fissures on the local temperature distribution using numerical simulations time dependent parameters such as temperature pressure and displacement are evaluated the simulation model is based on a shock equation of state for the involved materials dunite and quartzite and simulates geometries that were also used in shock loading experiments an artificial gap between the materials simulates an open fissure at the interface a strong temperature increase occurs at a gap size of 0 1 mm which potentially can cause melting in a thin layer at the interfaces the temperature decreases with decreasing gap size temperature and pressure excursions at the interface are induced by the closure of the gap which causes a second shock wave to superpose the primary wave open fissures and fractures which occur ubiquitously in shallow buried target rocks and projectiles thus act as local pressure and temperature amplyfiers and may be responsible for thin melt vein formation in shocked rocks

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

shock experiments with the h6 chondrite kernouve pressure calibration of microscopic shock effects shock recovery experiments were carried out on samples of the h6 chondrite kernouve at shock pressures of 10 15 20 25 30 35 45 and 60 gpa and preheating temperatures of 293 k low temperature experiments and 920 k high temperature experiments using a calculated equation of state of kernouve pressure pulse durations of 0 3 to 1 2 s were estimated the shocked samples were investigated by optical microscopy to calibrate the various shock effects in olivine orthopyroxene oligoclase and troilite the following pressure calibration is proposed for silicates 1 undulatory extinction of olivine gpa 2 weak mosaicism of olivine from 10 15 gpa to 20 25 gpa 3 onset of strong mosaicism of olivine at 20 25 gpa 4 transformation of oligoclase to diaplectic glass completed at 25 30 gpa low temperature experiments and at 20 25 gpa high temperature experiments 5 onset of weak mosaicism in orthopyroxene at 30 35 gpa low temperature experiments and at 25 30 gpa high temperature experiments and 6 recrystallization or melting of olivine starting at 45 60 gpa low temperature experiments and at 35 45 gpa high temperature experiments and completed above 45 60 gpa in the high temperature experiments troilite displays distinct differences between the samples shocked at low and high temperatures in the low temperature experiments the following effects can be observed in troilite 1 undulatory extinction up to 25 gpa 2 twinning up to 45 gpa 3 partial recrystallization from 30 to 60 gpa and 4 complete recrystallization 35 gpa whereas in the high temperature experiments troilite shows 1 complete recrystallization from 10 up to 45 gpa and 2 melting and crystallization above 45 gpa localized shock induced melting is observed in samples shocked to pressures 15 gpa in the high temperature experiments and 30 gpa for the low temperature experiments in the form of feni metal and troilite melt injections and intergrowths and as pockets and veins of whole rock melt obviously the onset and abundance of shock induced localized melting strongly depends on the initial temperature of the sample

the rocks of mars from far and near award of the meteoritical society 2001 h y mcsween jr the leonard medal address the rocks of mars from far and near

ejection of martian meteorites we investigated the transfer of meteorites from mars to earth with a combined mineralogical and numerical approach we used quantitative shock pressure barometry and thermodynamic calculations of post shock temperatures to constrain the pressure temperature conditions for the ejection of martian meteorites the results show that shock pressures allowing the ejection of martian meteorites range from 5 to 55 gpa with corresponding post shock temperature elevations of 10 to about 1000 c with respect to shock pressures and post shock temperatures an ejection of potentially viable organisms in martian surface rocks seems possible a calculation of the cooling time in space for the most highly shocked martian meteorite allan hills alh 77005 was performed and yielded a best fit for a post shock temperature of 1000 c and a meteoroid size of 0 4 to 0 6 m the final burial depths of the sub volcanic to volcanic martian rocks as indicated by textures and mineral compositions of meteorites are in good agreement with the postulated size of the potential source region for martian meteorites during the impact of a small projectile 200 m as defined by numerical modeling artemieva and ivanov 2004 a comparison of shock pressures and ejection and terrestrial ages indicates that on average highly shocked fragments reach earth crossing orbits faster than weakly shocked fragments if climatic changes on mars have a significant influence on the atmospheric pressure they could account for the increase of recorded ejection events of martian meteorites in the last 5 ma

shock metamorphism and petrography of the shergotty achondrite the shergotty subsamples 1 and 12 consist of augite and pigeonite 67 5 maskelynite 24 ilmenite and titanomagnetite 2 pyrrhotite 0 4 whitlockite 1 8 apatite 0 1 quartz 0 5 baddeleyite trace fayalite 0 4 mesostasis 3 and shock induced local polymineralic melt products 0 6 the overall modal composition is similar to other shergotty samples except for the rather high whitlockite content the shock effects observed in the mineral constituents include mosaicism deformation bands planar fractures and mechanical twin lamellae in clinopyroxene isotropization of plagioclase with very rare remnants of birefringence planar deformation structures mosaicism and strongly reduced birefringence in quartz mechanical twinning of ilmenite localized in situ melting of neighbouring minerals at the contact of low and high density phases based on the refractive index of maskelynite average 1 5467 with average an content of 49 and the degree of isotropization of the plagioclase an equilibrium shock pressure of 29 1 gpa is derived the inferred post shock temperature is 200 20 c no heating event could have exceeded 400 c duke 1968 local stress and temperature concentrations reach 60 80 gpa and 1600 2000 c the observed shock effects can be explained by a single shock event a second weaker shock event as found by others appears to be highly improbable equilibrium shock pressures and post shock temperatures for the other known shergottites are 31 2 gpa and 220 50 c zagami 43 2 gpa and 400 800 c alha 77005 the pressure estimate for eeta 79001 by lambert 1985 is confirmed 34 1 gpa the post shock temperature is 250 50 c the abundance and textural setting of localized melt products in these meteorites confirm increasing shock pressures in the sequence shergotty zagami eeta 79001 alha 77005 undoubtedly the melts could have been formed by the same single shock which produced the equilibrium shock effects e g maskelynitization in these meteorites the shock induced particle velocities inferred from hugoniot data of basalts are in the 1 5 2 0 km s range for the parental rocks of the shergottites ejection velocities are therefore in the order of 3 4 km s special ejection mechanisms are required in order to exceed the escape velocity of a planet like mars without producing higher degrees of shock e g melting than those observed in the shergottites

anatomy of black veins in zagami clues to the formation of high pressure phases abstract black melt veins in the martian basaltic meteorite zagami were investigated using analytical transmission electron microscopy with the goal to constrain the mechanisms and ptt conditions that result in the formation of high pressure phases therein our study reveals the existence of a number of high pressure minerals stishovite k and na ca rich hollandites akimotoite silicate ilmenite as well as amorphous grains of silicate perovskite composition crystals of a ferro magnesian silicate titanite with a low ca content have been identified for the first time the veins formed by rapid shear melting and solidified in extremely short times as a consequence the vein melt could not homogeneously mix and it partially preserves the chemical heterogeneity of precursor minerals accounting for the large diversity of high pressure minerals most of the tiny phases directly crystallized from the melt under high pressure at very high temperatures silicate perovskite probably amorphized upon decompression and evolved at its grain boundaries by solid state diffusion toward the lower pressure assemblage wustite stishovite not only do these observations help to understand the formation of high pressure phases in shock events but they also provide clues to the mineralogical complexity that can be expected in planetary mantles

oblique impact a process for obtaining meteorite samples from other planets cratering flow calculations for a series of oblique to normal 10 to 90 impacts of silicate projectiles onto a silicate halfspace were carried out to determine whether or not the gas produced upon shock vaporizing both projectile and target material would form a downstream jet that could entrain and propel snc meteorites from the martian surface the difficult constraints that the impact origin hypothesis for snc meteorites has to satisfy are that these meteorites are lightly to moderately shocked and yet have been accelerated to speeds in excess of the martian escape velocity more than 5 kilometers per second two dimensional finite difference calculations were performed that show that at highly probable impact velocities 7 5 kilometers per second vapor plume jets are produced at oblique impact angles of 25 to 60 and have speeds as great as 20 kilometers per second these plumes flow nearly parallel to the planetary surface it is shown that upon impact of projectiles having radii of 0 1 to 1 kilometer the resulting vapor jets have densities of 0 1 to 1 gram per cubic centimeter these jets can entrain martian surface rocks and accelerate them to velocities greater than 5 kilometers per second this mechanism may launch snc meteorites to earth

a new natural high pressure na ca hexaluminosilicate caxna1 x al3 xsi3 xo11 in shocked martian meteorites a ca na hexaluminosilicate whose ca end member was previously synthesized in numerous high pressure experiments has been identified by raman spectroscopy in heavily shocked martian meteorites this mineral has a structural formula close to caxna1 x al3 xsi3 xo11 and is similar to the calcium aluminum silicate phase previously synthesized in high pressure experiments performed on anorthite and rocks of basaltic composition this new mineral occurs in shock melt pockets in two distinct settings and is intimately intergrown with sio2 stishovite the first setting encountered in zagami consists of idiomorphic equant crystals overgrown by acicular stishovite that crystallized from a melt of labradorite composition the second setting contains the na ca hexaluminosilicate phase intergrown with stishovite and hollandite and was formed during partial melting at high pressures the mineralogical association na ca hexaluminosilicate stishovite was observed in shock melt pockets which have distinct bulk compositions in seven martian shergottites this new mineral represents after majorite the second natural occurrence of a silicate mineral with silicon in both four and six coordination the assemblage stishovite na ca hexalummosilicate sets constraints on the pressure and temperature conditions that prevailed during shock in some of the studied meteorites the na ca hexaluminosilicate mineral is a potential carrier of al and na during subduction of oceanic crust in the lower mantle of the earth c 2003 elsevier b v all rights reserved

petrogenesis of allan hills 84001 constraints from impact melted feldspathic and silica glasses abstract compositional and textural relationships of shock melted glasses in the allan hills alh 84001 meteorite have been examined by optical microscopy electron microprobe analysis and compositional mapping the feldspathic and silica glasses exhibit features which constrain the relative timing of shock events and carbonate deposition in alh 84001 the feldspathic glasses are stoichiometric and have compositions plausibly described as forming from igneous plagioclase an27 39ab58 68or3 7 or sanidine or51ab46an3 or from a mixture of these phases mixed feldspar glasses these observations argue against prior interpretations of feldspathic glasses as unflowed maskelynite hydrothermal precipitates or alteration products or shock melts that have undergone alkali volatilization carbonate was deposited around previously formed mixed feldspar glass clasts suggesting that carbonate deposition occurred after the shock event that formed the granular bands crushed zones in this meteorite sio2 rich glasses appear to be silica remobilized during shock with little addition of other material a petrogenetic history of alh 84001 consistent with the observations of feldspathic and silica glasses is 1 igneous crystallization and cumulate formation 2 a pre carbonate shock event that formed the granular bands crushed zones and sheared chromites and melted igneous plagioclase and sanidine to form mixed feldspar glasses 3 carbonate and silica deposition in the granular bands veining of plagioclase glasses by sio2 and deposition of carbonate around mixed feldspar and plagioclase glass clasts 4 a post carbonate shock event that resulted in invasion of carbonate by feldspathic melts shock faulting and decarbonation of carbonate high temperature mobilization of silica melts and minor dissolution of orthopyroxene by silica melts

the history of allan hills 84001 revised multiple shock events the geologic history of martian meteorite allan hills alh 84001 is more complex than previously recognized with evidence for four or five crater forming impacts onto mars this history of repeated deformation and shock metamorphism appears to weaken some arguments that have been offered for and against the hypothesis of ancient martian life in alh 84001 allan hills 84001 formed originally from basaltic magma its first impact event i1 is inferred from the deformation d1 that produced the granular textured bands crush zones that transect the original igneous fabric deformation d1 is characterized by intense shear and may represent excavation or rebound flow of rock beneath a large impact crater an intense thermal metamorphism followed d1 and may be related to it the next impact i2 produced fractures fr2 in which carbonate pancakes were deposited and produced feldspathic glass from some of the igneous feldspars and silica after i2 carbonate pancakes and globules were deposited in fr2 fractures and replaced feldspathic glass and possibly crystalline silicates next feldspars feldspathic glass and possibly some carbonates were mobilized and melted in the third impact i3 microfaulting intense fracturing and shear are also associated with i3 in the fourth impact i4 the rock was fractured and deformed without significant heating which permitted remnant magnetization directions to vary across fracture surfaces finally alh 84001 was ejected from mars in event i5 which could be identical to i4 this history of multiple impacts is consistent with the photogeology of the martian highlands and may help resolve some apparent contradictions among recent results on alh 84001 for example the submicron rounded magnetite grains in the carbonate globules could be contemporaneous with carbonate deposition whereas the elongate magnetite grains epitaxial on carbonates could be ascribed to vapor phase deposition during i3

mineralogy and petrology of the dar al gani 476 martian meteorite implications for its cooling history and relationship to other shergottites dar al gani 476 the 13th martian meteorite was recovered from the sahara in 1998 it is a basaltic shergottitic rock composed of olivine megacrysts reaching 5 mm 24 vol set in a finegrained groundmass of pyroxene 59 vol and maskelynitized plagioclase 12 vol with minor amounts of accessory phases spinel merrillite ilmenite dar al gani 476 is similar to lithology a of elephant moraine a79001 eeta79001 in petrography and mineralogy but is distinct in several aspects low ca pyroxenes in the dar al gani 476 groundmass are more magnesian en76fs21 wo3 en58fs30wo12 than those in lithology a of eeta79001 en73fs22wo5 en45fs43wo12 rather similar to pyroxenes in lherzolitic martian meteorites en76fs21 wo3 en63fs22wo15 dar al gani 476 olivine is less magnesian and shows a narrower compositional range fo76 58 than eeta79001 olivine fo81 53 and is also similar to olivines in lherzolitic martian meteorites fo74 65 the orthopyroxene olivine chromite xenolith typical in the lithology a of eeta79001 is absent in dar al gani 476 it seems that dar al gani 476 crystallized from a slightly more primitive mafic magma than lithology a of eeta79001 and several phases olivine pyroxene chromite and ilmenite in dar al gani 476 may have petrogenetic similarities to those of lherzolitic martian meteorites olivine megacrysts in dar al gani 476 are in disequilibrium with the bulk composition the presence of fractured olivine grains in which the most mg rich parts are in contact with the groundmass suggests that little diffusive modification of original olivine compositions occurred during cooling this observation enabled us to estimate the cooling rates of dar al gani 476 and eeta79001 olivines giving similar cooling rates of 0 03 3 c h for dar al gani 476 and 0 05 5 c h for eeta79001 this suggests that they were cooled near the surface burial depth shallower than about 3 m at most probably in lava flows during crystallization of groundmass as is proposed for lithology a of eeta79001 it may be possible to consider that dar al gani 476 has an impact melt origin a mixture of martian lherzolite and other martian rock queen alexandra range 94201 nakhlites

refraction and dispersion effects compensation for uwb sar subsurface object imaging it is a trend to use airborne or vehicle borne ultrawideband synthetic aperture radar uwb sar to quickly detect subsurface objects i e landmines or minefields over large areas from a safe standoff distance the traditional image formations without considering the refraction and dispersion of electromagnetic wave will lead to locating error and defocusing for subsurface objects in this paper an efficient refraction and dispersion effects compensation method is proposed which can be integrated with the traditional three stage automatic target detection framework for detection of subsurface landmines the compensation method can focus and correctly locate multiple objects buried in different soils with different depths over large areas which satisfies the operating requirement of an airborne or vehicle borne uwb sar system the proposed method is proved efficient for subsurface object imaging by the field data collected by a uwb sar system

experimental generation of shock induced pseudotachylites along lithological interfaces to understand the mechanism of formation of shock induced pseudotachylites and particularly the role that rock heterogeneities and interfaces play in their formation shock recovery experiments were carried out on samples consisting of two distinct lithologies dunite and quartzite it was possible to generate melt veins of 1 6 m width along lithological interfaces at moderate shock pressures 6 to 34 gpa the magnitudes of displacement along the interface strain rate and the kinetic heat production indicate that friction is an important heat source that largely contributes to the energy budget of the melt veins the experimentally produced veins resemble natural s type pseudotachylites the geometry of the veins depends on the orientation of the interface with respect to the shock front and includes strong variations in thickness formation of melt pockets and injection veins sudden changes in vein orientation and sharp vein margins two types of melt were observed vesicle free and vesicular melts dense vesicle free melt rock is likely to represent high pressure melts vesicular melts were also generated during shock compression but they remained in a molten state during pressure release and continued shearing intermingling of comminuted olivine and melt suggests that ultracataclasis of olivine induced by a dynamic tensile failure is a precursor stage to frictional melting shock wave interferences at the lithological interface provide the necessary stress conditions to start dynamic failure of olivine the composition of the frictional melts ranges from olivine normative to enstatite normative and is thus largely determined by olivine melting the validity of the sequence of friction melting susceptibilities of rock forming minerals inferred from tectonically produced pseudotachylites is confirmed and can now be applied to ultra high strain rates during shock compression

seasonal carbon dioxide exchange between the regolith and atmosphere of mars experimental and theoretical studies measurements of the rate of penetration of a co2 pressure wave through basalt clay soils have been conducted under conditions of pco2 6 mbar pco2 2 mbar soil density 1 3 and t 40 to 70 c which simulate the penetration of the cap induced seasonal co2 pressure wave through the topmost regolith on mars results suggest that existing theoretical models for diffusion of a gas through a porous highly adsorbing medium may be reliably used to assess the importance of martian seasonal regolith atmosphere co2 exchange results also suggest that although mars topsoils with a substantial component of fine grained weathering products could harbor within a few meters of the surface a substantial fraction of the atmospheric co2 inventory seasonal cap atmosphere co2 exchange is still much more important than seasonal regolith atmosphere exchange this is so because the high adsorptive capacity of such soils greatly slows co2 diffusion through them owing to the typically high ratio of mean residence time for co2 in the static adsorbed phase to that in the pore gas at t 40 c at such temperatures typical of the martian regolith the co2 diffusivity of such soils may be 1 10 2 cm2 s 1 only somewhat higher than the thermal diffusivity and not high enough to allow seasonal isothermal regolith buffering of the cap induced co2 pressure wave to be effective on the other hand even the adsorption slowed diffusivities are still high enough to allow thermally driven regolith atmosphere exchange to be effective on a time scale of the obliquity cycle 105 yr we also examined the maximum effect of thermally driven exchange between the topmost seasonally thermally affected regolith and the atmosphere while of somewhat greater potential importance than the isothermal exchange the thermally driven exchange would be recognizable only if the pressure wave from co2 thus exchanged at high latitudes did not propagate though the atmosphere at a faster rate than that at which the exchange itself occurred an unreasonable assumption our results are insensitive to soil geometry and temperature and suggest that both isothermal and thermally driven regolith atmosphere seasonal co2 exchange effects are likely to be greatly dominated by cap atmosphere exchange in principle it is conceivable that a more profound understanding of the cap atmosphere regolith interaction could eventually allow us to use the externally imposed pressure waves as a natural experiment to set limits on global or regional soil properties however the regolith effects while quite possibly dominant for h2o are seemingly too weak and too complex for co2 to allow this to be accomplished

exposure history of shergottites abstract the cosmogenic nuclides 36 cl t 1 2 3 0 10 5 years 26 al 7 05 10 5 years 10 be 1 6 10 6 years and 53 mn 3 7 10 6 years were measured in three shergottites alha 77005 eeta 79001 and shergotty by accelerator mass spectrometry and neutron activation the cosmogenic nuclide data constrain the history and origin of shergottites alha 77005 was ejected from its parent body about 2 7 my ago and was exposed to cosmic rays as a body of 5 6 cm radius solar cosmic ray produced 26 a1 and 53 mn were first clearly observed in this meteorite the exposure age of eeta 79001 was 0 6 my and the preatmospheric radius was about 15 cm the exposure age of shergotty was calculated to be 2 2 my and the preatmospheric size was about 10 15 cm in radius the terrestrial age of alha 77005 was 0 2 my and that of eeta 79001 was less than 0 06 my all cosmogenic nuclide data indicate that the three shergottites were never exposed to cosmic rays in their parent bodies whether that parent body was mars or an asteroid all shergottites were ejected from greater than a 3 m depth in their parent bodies the ejection of eeta 79001 was a different event from that of the other shergottites

zagami product of a two stage magmatic history abstract large specimens of the zagami shergottite show highly varied grain sizes and mineral abundances on a cm scale and preferred alignment of pyroxene laths we document the presence of whitlockite and of melt veins and pockets of shock origin pyroxene crystals have homogeneous mg rich pigeonite and augite cores overgrown by fe rich zoned pyroxene mostly pigeonite rims amphibole bearing magmatic melt inclusions occur exclusively in the cores we conclude that zagami experienced a twostage crystallization history the first stage occurred in a deep seated slowly cooling magma chamber there the homogeneous mg rich cores of the pyroxenes crystallized during relatively slow cooling the deep seated origin of the cores is also indicated by the presence of amphibole within them which requires pressures of formation equivalent to crystallization at depths 7 5 km on mars modest abundance of homogeneous mg rich cores 15 20 indicates that crystal settling did not play a significant role in this part of the magma chamber during the second stage the mg rich pyroxenes were entrained into a magma that either intruded to the near surface and cooled in a relatively thin dike or sill or extruded to the surface and crystallized in a lava flow 10 m thick again without indications for crystal settling this scenario is suggested by estimates of cooling rates of 0 1 0 5 c h based on sizes of the plagioclase maskelynite crystals and of 0 02 c h based on the width of pyroxene exsolution lamellae brearley 1991 irregular shapes of the mg rich cores result from resorption and possible solid state diffusion and the shapes and sizes of the pyroxenes after crystallization of fe rich pigeonite rims onto the cores were strongly controlled by the shapes and sizes of the cores the finer grained areas of the rock inherited smaller and more numerous mg rich pyroxene cores from the first stage than did the coarser grained areas crystallization of both augite and pigeonite cores at depth but mostly pigeonite rims in the nearsurface environment may be the result of a phase boundary shift and expansion of the pigeonite stability field at lower pressures the estimated depth of the magma chamber for zagami of 7 5 km and thickness of the putative lava flow of 10 m are consistent with calculations and observations of volcanic constructs and flows in the tharsis region of mars

survival of bacteria exposed to extreme acceleration implications for panspermia abstract we studied the effect of extreme acceleration and change in acceleration or jerk on bacteria to determine if they could survive impact ejection from a planet computer simulations based on the spallation model h j melosh icarus 59 1984 234 260 h j melosh nature 363 1993 498 499 for ejecting material from planetary surfaces provided estimates for acceleration rise time and jerk for material accelerated to escape velocity for ejection from mars the maximum acceleration predicted was 3 106 m s2 or 3 105 g with a rise time of 0 5 ms and a corresponding jerk of 6 109 m s3 we tested the resistance of bacillus subtilis spores and deinococcus radiodurans cells to high acceleration and jerk by 1 subjecting b subtilis spores to the forces of an ultracentrifuge and 2 firing both bacteria from a rifle into a plasticene target we measured the survival of b subtilis spores at extreme acceleration in an ultracentrifuge operated at its highest speed 100 000 rpm corresponding to an acceleration of 4 27 106 m s2 or 4 36 105 g approximately 107 spores were centrifuged in phosphate buffered saline for 24 48 50 and 72 h spores were inactivated with simple exponential kinetics and 65 h of centrifugation was required to inactivate 90 of the spore population to test for resistance to jerk spores of b subtilis or cells of d radiodurans were loaded into the rear cavities of lead pellets fired from a compressed air pellet rifle into a target consisting of plasticene modeling clay previously chilled to 4 c the velocity of each pellet was measured using a chronograph and the depth of penetration of each pellet into the target was measured before removing the pellet from the clay using sterile forceps two different rifles were used one with a measured pellet velocity of 100 m s and the other with a velocity of 300 m s these correspond to estimated accelerations of 1 5 106 and 4 5 106 m s2 and jerks of 1 5 1010 and 1 5 1011 m s3 respectively the percent survival for both organisms ranged from 40 to 100 the samples in the ballistic experiments were subjected to jerks and accelerations 2 5 25 times larger than those estimated to prevail during ejection according to the computer simulation we therefore conclude that acceleration and jerk are not important lethal factors during the ejection of viable microorganisms from planetary surfaces

a 1 gyr climate model for mars new orbital statistics and the importance of seasonally resolved polar processes new results from a 1 gyr integration of the martian orbit are presented along with a seasonally resolved energy balance climate model employed to illuminate the gross characteristics of the long term atmospheric pressure evolution we present a new analysis of the statistical variation of the martian obliquity and precession prior to and subsequent to the formation of the tharsis uplift and explore the long term effects on the martian climate we find that seasonal polar cycles have a critical influence on the ability for the regolith to release co2 at high obliquities and find that the atmospheric co2 actually decreases at high obliquities due to the cooling effect of polar deposits at latitudes where seasonal caps form at low obliquity the formation of massive permanent polar caps depends critically on the values of the frost albedo afrost and frost emissivity frost using our model with values of afrost 0 67 and frost 0 55 matched to the nasa ames general circulation model gcm results haberle et al 1993 j geophys res 98 3093 3123 and haberle et al 2003 icarus 161 66 89 we find that permanent caps only form at low obliquities 13 suggesting that any permanent deposits on the surface of mars today may be residuals left over from a period of very low obliquity or are the result of mechanisms not represented by this model thus contrary to expectations the martian atmospheric pressure is remarkable static over time and decreases both at high and low obliquity also from our one billion year orbital model we present new results on the fraction of time mars is expected to experience periods of low obliquity and high obliquity

lewis cliff 86360 an antarctic l chondrite with a terrestrial age of 2 35 million years we measured the concentrations of the cosmogenic radionuclides 10be half life 1 51 106 a 26al 7 05 105 a and 36cl 3 01 105 a in lewis cliff lew 86360 an l chondrite from the lewis cliff stranding area east antarctica in addition the concentrations and isotopic compositions of he ne and ar were measured the combined results yield a terrestrial age of 2 35 0 15 ma only one other stony meteorite with a similar terrestrial age 2 ma is known from the allan hills stranding area alh 88019 whereas all previously dated stony meteorites from antarctica are younger than 1 ma we argue that lew 86360 spent most of its terrestrial residence time deep inside the ice near the base of the glacier where ice flow rates are much lower than at the surface the terrestrial ages of lew 86360 and alh 88019 are consistent with existing hypotheses concerning the stability and persistence of the east antarctic ice sheet

evidence for recent climate change on mars from the identification of youthful near surface ground ice ground ice in the crust and soil may be one of the largest reservoirs of water on mars1 2 3 near surface ground ice is predicted to be stable at latitudes higher than 40 ref 4 where a number of geomorphologic features indicative of viscous creep and hence ground ice have been observed5 mid latitude soils have also been implicated as a water ice reservoir6 the capacity of which is predicted to vary on a 100 000 year timescale owing to orbitally driven variations in climate7 it is uncertain however whether near surface ground ice currently exists at these latitudes and how it is changing with time here we report observational evidence for a mid latitude reservoir of near surface water ice occupying the pore space of soils the thickness of the ice occupied soil reservoir 1 10 m and its distribution in the 30 to 60 latitude bands indicate a reservoir of 1 5 6 0 104 km3 equivalent to a global layer of water 10 40 cm thick we infer that the reservoir was created during the last phase of high orbital obliquity less than 100 000 years ago and is now being diminished