impacts experiments onto heterogeneous targets simulating impact breccia implications for impact strength of asteroids and formation of the asteroid families abstract a series of impact experiments onto solid decimeter sized cylinders made of porous gypsum admixed with approximately 1 cm sized pebbles have been performed the target densities and their heterogeneous structures could be representative of those of the asteroids ida eros and many others because asteroid sub surface could be the consolidated boulders made by self compaction and or by impact compaction impact velocities in the experiments ranged from 2 0 km s to 6 7 km s collision velocity in the asteroid main belt is approximately 5 km s it was found that the slope of the cumulative number distribution of post impact fragments strongly depends on the specific energy of the impact the presence of pebbles strongly influences the impact strength of the target as well as the size distribution of the post impact fragments results of the experiments presented here are aimed at identifying the analogy between the laboratory results and the damage of small asteroids or their catastrophic disruption after impacts

the porosities of ordinary chondrites models and interpretation densities and porosities for 285 ordinary chondrites have been assembled and analyzed measured chondrite porosities are bimodal finds have an average porosity of 30 we conclude that mild degrees of weathering fill pore spaces lowering grain densities and porosities without significantly changing the bulk size or mass of the sample by assuming an original pristine grain density as a function of the meteorite s mineralogy determined by its class we can derive model pristine porosities these model porosities cluster around an average value of 10 for all classes of ordinary chondrites ordinary chondrites do not show any correlation of porosity model or measured with petrographic grade or sample size over a range from 0 2 g to 2 kg however we do see a correlation between shock state and porosity shock blackened meteorites are less porous than other meteorites furthermore less severely shocked meteorites show a much broader range of porosities with the maximum porosity seen among meteorites of a given shock class falling linearly as a function of that shock class this is consistent with the idea that shock compresses and closes pore space analysis of meteorite porosity provides a lower bound to the fine scale porosity of asteroids our densities even with 10 primordial porosity are significantly higher than inferred densities of possible asteroid parent bodies these asteroids are probably loose piles of rubble

gravity field of kuwait and its relevance to major geological structures regional gravity surveys were done in the state of kuwait during 1986 1988 the new gravity maps show a good correlation with geomorphological features as well as with deeper geological structures the free air anomaly map clearly reflects the topography of the jal az zor and ahmadi ridges and wadi al batin the bouguer anomaly map is dominated by two prominent gravity highs correlatable with subsurface structural arches the north trending gravity high in eastern kuwait represents the effect of a major structure the kuwait arch along which many important oil fields are located a smaller northwest trending high mapped in western kuwait indicates the presence of a second subsurface arch which in this paper is named the dibdibba arch the two gravity highs are separate by a wedge shaped gravity low presumably caused by thicker sediments of the dibdibba basin magnetic measurements along selected profiles show the two arches to be associated with 100 200 nt nannotesla anomalies apparently reflecting the positive subsurface relief of the crystalline basement along the length of the kuwait arch magnetic data also indicate lateral susceptibility variations possibly related to lithological variations within the basement

macroporosity and grain density of rubble pile asteroid 162173 ryugu rubble pile asteroids such as 162173 ryugu have large bulk porosities which are believed to result from void spaces in between the constituent boulders macroporosity as well as void spaces wit

bubble noise and wavelet spills recorded 1 m below the ocean surface a remote instrument has been used to record the sound and environment of small surface spills in light winds from a depth of approximately 1 m in the open ocean recordings from the instrument indicate that these small breaks have no correlation with the amplitude or phase of long period swells moving faster than prevailing winds the sound from the spills which is composed of a number of distinct resonant bubble oscillations is very similar to that described by medwin and beaky j acoust soc am 86 1124 1130 1989 for windless artificial wave breaks peak oscillation source pressures range up to 1 2 pa the average of several acoustic spectra from a single energetic spill has shown a slope of 5 db per octave over the frequency range of the instrument roughly 500 8000 hz the unique frequency for each oscillating bubble within a spill indicates that bubbles are rung as they are formed during entrainment die out exponentially within milliseconds and then no longer contribute actively to the a

global mapping of the degree of space weathering on asteroid 25143 itokawa by hayabusa amica observations we obtained color images of near earth asteroid 25143 itokawa by the hayabusa multiband imaging camera to characterize the regional color properties images were obtained for the whole disk from the gate position gp and home position hp at a spatial resolution of 0 8 3 7 m pixel whole disk spectra are adjusted to the telescopic data obtained by the university of hawaiis 88 inch telescope using the eight color asteroid survey ecas system the disk resolved measurements show large variations in the three visible channels we present a map of an index related to the degree of space weathering which has been newly developed based on laboratory measurements we find large variations in the degree of space weathering on itokawa fresh materials are observed in regions of steep slopes and craters whereas mature materials are ubiquitously distributed this result suggests that pristine ordinary chondrite like materials have been exposed through weathered layers by excavation by also examining close up images obtained during touchdown rehearsal we find that most rocks in itokawas rough terrains are weathered instead of a regolith blanket the surface of this small asteroid is covered with weathered rocks and gravels

modeling the structure of high porosity asteroids abstract a number of asteroids have been observed to have extremely low bulk densities in the 1 3 g cm 3 range given reasonable compositions based on meteorite analogues this bulk density implies bulk porosities of 40 60 suggesting that many asteroids are rubble piles held together by self gravity and containing significant empty space perhaps more empty space than solid material simple energy balance calculations and accretional models of disrupted asteroids suggest that materials in porous asteroids may be sorted by particle size the large irregular pieces and larger voids fractures are probably located deeper inside the asteroid and the fine particle size fractions that are observed on their surfaces are restricted to the surface regolith zone the large interior voids fractures are preserved from infilling by the effects of friction on the smaller size fractions friction tends to dominate the downward pull of gravity and prevents the fine fractions from filtering into the interior of the asteroid and infilling the large fractures and voids friction may also play a role in allowing shattered asteroids to maintain their relief features and shape by resisting the movements of pieces within the object in effect providing strength to non coherent objects

meteoritic and other constraints on the internal structure and impact history of small asteroids studies of the internal structure of asteroids which are crucial for understanding their impact history and for hazard mitigation appear to be in conflict for the s type asteroids eros gaspra and ida spacecraft images and geophysical data show that they are fractured coherent bodies whereas models of catastrophic asteroidal impacts family and satellite formation and studies of asteroid spin rates and other diverse properties of asteroids and planetary craters suggest that such asteroids are gravitationally bound aggregates of rubble these conflicting views may be reconciled if 10 50 km s type asteroids formed as rubble piles but were later consolidated into coherent bodies many meteorites are breccias that testify to a long history of impact fragmentation and consolidation by alteration metamorphism igneous and impact processes ordinary chondrites which are the best analogs for s asteroids are commonly breccias some may have formed in cratering events but many appear to have formed during disruption and reaccretion of their parent asteroids some breccias were lithified during metamorphism and a few were lithified by injected impact melt but most are regolith and fragmental breccias that were lithified by mild or moderate shock like their lunar analogs shock experiments show that porous chondritic powders can be consolidated during mild shock by small amounts of silicate melt that glues grains together and by friction and pressure welding of silicate and metallic fe ni grains we suggest that the same processes that converted impact debris into meteorite breccias also consolidated asteroidal rubble internal voids would be partly filled with regolith by impact induced seismic shaking consolidation of this material beneath large craters would lithify asteroidal rubble to form a more coherent body fractures on ida that were created by antipodal impacts and are concentrated in and near large craters and small positive gravity anomalies associated with the psyche and himeros craters on eros are consistent with this concept spin data suggest that smaller asteroids 0 6 6 km in size are unconsolidated rubble piles c type asteroids which are more porous than s types and their analogs the volatile rich carbonaceous chondrites were probably not lithified by shock

the secondary history of sutter s mill cm carbonaceous chondrite based on water abundance and the structure of its organic matter from two clasts sutter s mill is a regolith breccia composed of both heavily altered clasts and more reduced xenoliths here we present a detailed investigation of fragments of sm18 and sm51 we have characterized the water content and the mineralogy by infrared ir and thermogravimetric analysis tga and the structure of the organic compounds by raman spectroscopy to characterize the secondary history of the clasts including aqueous alteration and thermal metamorphism the three methods used in this study suggest that sm18 was significantly heated the amount of water contained in phyllosilicates derived by tga is estimated to be approximately 3 2 wt this value is quite low compared with other cm chondrites that typically range from 6 to 12 wt the infrared transmission spectra of sm18 show that the mineralogy of the sample is dominated by a mixture of phyllosilicate and olivine sm18 shows an intense peak at 11 2 lm indicative of olivine fig 1 if we compare sm18 with other cm and metamorphosed cm chondrites it shows one of the most intense olivine signatures and therefore a lower proportion of phyllosilicate minerals the raman results tend to support a short duration heating hypothesis in the id ig versus fwhm d diagram sm18 appears to be unusual compared to most cm samples and close to the metamorphosed cm chondrites pecora escarpment pca 91008 and pca 02012 in the case of sm51 infrared spectroscopy reveals that olivine is less abundant than in sm18 and the 10 lm silicate feature is more similar to that of moderately altered cm chondrites like murchison or queen alexandra range que 97990 raman spectroscopy does not clearly point to a heating event for sm51 in the id ig versus fwhm d diagram however tga analysis suggests that sm51 was slightly dehydrated as the amount of water contained in phyllosilicates is approximately 3 7 wt which is higher than sm18 but still lower than phyllosilicate water contents in weakly altered cm chondrites altogether these results confirm that fragments with different secondary histories are present within the sutter s mill fall the dehydration that is clearly observed for sm18 is attributed to a short duration heating based on the similarity of its raman spectra to that of pca 91008 because of the brecciated nature of sutter s mill and the presence of adjacent clasts with different thermal histories impacts that can efficiently fragment and heat porous materials are the preferred heat source

spectral properties and composition of potentially hazardous asteroid 99942 apophis abstract the known close approach of asteroid 99942 apophis in april 2029 provides the opportunity for the case study of a potentially hazardous asteroid in advance of its encounter the visible to near infrared 0 55 to 2 45 m reflectance spectrum of apophis is compared and modeled with respect to the spectral and mineralogical characteristics of likely meteorite analogs apophis is found to be an sq class asteroid that most closely resembles ll ordinary chondrite meteorites in terms of spectral characteristics and interpreted olivine and pyroxene abundances although we cannot rule out some degree of partial melting a meteorite analog allows some estimates and conjectures of apophis possible range of physical properties such as the grain density and micro porosity of its constituent material composition and size similarities of apophis with 25143 itokawa suggest a total porosity of 40 as a current best guess for apophis applying these parameters to apophis yields a mass estimate of 2 10 10 kg with a corresponding energy estimate of 375 mt for its potential hazard substantial unknowns most notably the total porosity allow uncertainties in these mass and energy estimates to be as large as factors of two or three

geological evolution of the afro arabian dome abstract the afro arabian dome includes the elevated continental regions enclosing the red sea gulf of aden and the ethiopian rift system and extends northwards as far as jordan it is more than an order of magnitude larger than other african uplifts both the structures and the igneous rocks of the dome appear to be products of the superimposition of two perhaps three semi independent generating systems initiated at different times but all still active a strain pattern dominated by nw trending basins and rifts first became established early in the cretaceous by the end of the oligocene much of the extensional strain had been taken up along the red sea and gulf of aden axes which subsequently developed into an ocean palaeogene trap volcanism of mildly alkaline to transitional character was related to this horizontal extension rather than to doming further west the east sahara swell has a history of intermittent alkaline volcanicity which began in the mesozoic and was independent of magmatism in the afro arabian dome volcanicity specifically related to doming began in the miocene along a n s zone of uplift extending from ethiopia to syria this elongated swell forms the northern termination of the east african system of domes and rifts characterized by episodic vertical uplift but very little extension superimposition of epeirogenic uplift upon structures formed by horizontal extension took place in the neogene volcanicity related to vertical tectonics is mildly alkaline in character whereas transitional and tholeiitic magmas are found along the spreading axes

the density and porosity of meteorites from the vatican collection abstract we have developed a nondestructive technique to measure the densities and porosities of large 50 g to several kilogram meteorite samples and have begun a systematic measurement program starting with meteorites from the vatican observatory collection our technique utilizes a modified archimedean method 40 m glass spheres substituting for the fluid to determine bulk density and an unusually large he pycnometer to determine grain density from these the porosity is calculated we report here our first results for 89 samples nearly doubling the published database on porosities of meteorite hand samples including porosities of 35 different meteorites these include several types not previously reported eight iron meteorites six stony iron meteorites an enstatite chondrite the ci carbonaceous chondrite orgueil and the shergottite nakhlatite chassignite snc meteorite nakhla

structural and photogeological evidence for the boundaries to an east arabian block computer enhanced landsat images reveal a 65 km arcuate lineament linking the concealed fault zone beneath wadi al batin to the central arabian graben and trough system detection of the link allows a continuous 1000 km arc of faults and troughs cutting the mesozoic cenozoic rocks of the arabian shelf to be recognized the arc here called the east arabian fault zone is interpreted as forming the western and southern boundaries of the east arabian block a tectonic unit underlying at least 200 000 km 2 of the arabian peninsula stratigraphical relationships within the 560 km central arabian graben and trough system demonstrate that its faults were active during the late cretaceous early palaeogene movements ceasing when the red sea began to open and the arabian subplate was established the principal hydrocarbon fields of the arabian peninsula lie within the boundaries of the east arabian block

near s flyby of 253 mathilde images of a c asteroid on 27 june 1997 the near earth asteroid rendezvous near spacecraft flew within 1212 kilometers of asteroid 253 mathilde mathilde is an irregular heavily cratered body measuring 66 kilometers by 48 kilometers by 46 kilometers the asteroid s surface is dark estimated albedo between 0 035 and 0 050 and similar in color to some cm carbonaceous chondrites no albedo or color variations were detected the volume derived from the images and the mass from doppler tracking of the spacecraft yield a mean density of 1 3 0 2 grams per cubic centimeter about half that of cm chondrites indicating a porous interior structure

collisional evolution of the asteroid belt abstract a new synthesis of asteroid collisional evolution is motivated by the question of whether most asteroids larger than 1 km size are strengthless gravitational aggregates rubble piles near found eros not to be a rubble pile but a shattered collisional fragment with a through going fracture system and an average of about 20 m regolith cover of four asteroids visited by spacecraft none appears likely to be a rubble pile except perhaps mathilde nevertheless current understanding of asteroid collisions and size dependent strength and the observed distribution of rotation rates versus size have led to a theoretical consensus that many or most asteroids larger than 1 km should be rubble piles is eros the best observed asteroid highly unusual because it is not a rubble pile is mathilde if it is a rubble pile like most asteroids what would be expected for the small asteroid itokawa the muses c sample return target an asteroid size distribution is synthesized from the minor planet center listing and results of the sloan digital sky survey an infrared space observatory survey the small main belt asteroid spectroscopic survey and the infrared astronomical satellite survey a new picture emerges of asteroid collisional evolution in which the well known dohnanyi result that the size distribution tends toward a self similar form with a 2 5 index power law is overturned because of scale dependent collision physics survival of a basaltic crust on vesta can be accommodated together with formation of many exposed metal cores the lifetimes against destruction are estimated as 3 gyr at the size of eros 10 gyr at ten times that size and 40 gyr at the size of vesta eros as a shattered collisional fragment is not highly unusual the new picture reveals the new possibility of a transition size in the collisional state where asteroids below 5 km size would be primarily collisional breakup fragments whereas much larger asteroids are mostly eroded or shattered survivors of collisions in this case well defined families would be found in asteroids larger than about 5 km size but for smaller asteroids families may no longer be readily separated from a background population moreover the measured boulder size distribution on eros is re interpreted as a sample of impactor size distributions in the asteroid belt the regolith on eros may result largely from the last giant impact and the same may be true of itokawa in which case about a meter of regolith would be expected there even a small asteroid like itokawa may be a shattered object with regolith cover

regional joint sets in the arabian platform as indicators of intraplate processes the eastern part of the saudi arabian craton is a platform underlain by gently tilted and horizontal mesozoic cenozoic sedimentary rocks everywhere more than 250 km distant from the zagros deformation front despite its tectonic simplicity the platform contains several laterally extensive but subdued structures and lineaments belonging to more than one system of fault zones and flexures new microtectonic observations of dominant sets of extension and conjugate hybrid joints permit regionally significant extension directions to be inferred and the origin of the macrostructures to be assessed the central arabian arch the e w crest of which coincides with an axis of mesozoic subsidence developed during the late cretaceous to eocene when the basin inverted as a consequence of the emplacement of ophiolite bearing nappes on the zagros and oman margins during arching there was strike parallel elongation of beds at the same time as the arch amplified the central arabian graben system evolved as a result of the northward displacement of an east arabian block or sheet that became detached across an arcuate separation zone which in the south is coincident with the crest of the arch an important neogene response in the arabian platform to ne sw shortening in the zagros ranges was the formation of a swarm of ne striking master joints and lineaments these structures reflect the diffuse and peripheral expansion of the foreland beyond the zagros deformation front

very low strengths of interplanetary meteoroids and small asteroids we have assembled data on thirteen cases of meteorite falls with accurate tracking data on atmospheric passage in all cases we estimate the bulk strength of the object corresponding to its earliest observed or inferred fragmentation in the high atmosphere and can compare these values with measured strengths of meteorites in the taxonomic class for that fall in all thirteen cases the strength corresponding to earliest observed or inferred fragmentation is much less than the compressive or tensile strength reported for that class of stony meteorites we find a more random relation between bulk sample strength and sample mass than is suggested by a commonly used empirical power law we estimate bulk strengths on entry being characteristically of the order of 10 1 to 10 2 times the tensile strengths of recovered samples

broadband parametric imaging of breaking ocean waves an acoustic array was deployed in the near surface layer of a fetch limited coastal inlet to image breaking waves using only the sound radiated in the band 400 hz to 2000 hz from the breaking region the breakers were assumed to possess predominantly spiller characteristics for this frequency band the wavelength of sound in bubble free water is much larger than the surface wave height and the depth of the breaker bubble plume so both were considered insignificant the 15 element array was configured as a sparse horizontal cross with an 8 m aperture bottom moored and positioned nominally 3 m beneath the surface propagation from the source to the array elements assumed dipole sources an acoustically flat surface and an acoustically thin bubble plume the radiating region was parameterized by a broadband two dimensional gaussian profile information from up to six independent frequencies was combined to yield a maximum likelihood image analysis shows that the images align closely with the wind and can be observed moving downwind with a speed roughly equal to 70 of the phase speed of the dominant wind waves a model of acoustic source strength which is linear in log frequency is found fit the data reasonably well and model parameters are provided for a single wind speed unlike other imaging experiments this technique provides measurements of the size and shape of the bubble creation region at or near the peak of the radiated autospectrum

simulations of impact ejecta and regolith accumulation on asteroid eros abstract we have carried out a set of monte carlo simulations of the placement of impact ejecta on asteroid 433 eros with the aim of understanding the distribution and accumulation of regolith the simulations consisted of two stages 1 random distribution of primary impact sites derived from a uniform isotropic flux of impactors and 2 integration of the orbits of test particle ejecta launched from primary impact points until their re impact or escape we integrated the orbits of a large number of test particles typically 10 6 per individual case for those particles that did not escape we collected the location of their re impact points to build up a distribution on the asteroid surface we find that secondary impact density is mostly controlled by the overall topography of the asteroid a gray scale image of the density of secondary ejecta impact points looks in general like a reduced scale negative of the topography of the asteroid s surface in other words regolith migration tends to fill in the topography of eros over time whereas topographic highs are denuded of free material thus the irregular shape of eros is not a steady state configuration but the result of larger stochastic events

compaction as the origin of the unusual craters on the asteroid mathilde the asteroid mathilde has suffered at least five giant impacts previous studies suggest that mathilde s giant craters should be surrounded by blankets of ejecta that are kilometres deep1 2 yet the craters show no evidence of filling by material excavated during later nearby impacts1 3 computer simulations of impacts have been used to suggest that the absence of ejecta arises because the impact energy is deposited in a small volume due to mathilde s unusually high porosity4 which produces ejecta velocities so high that nearly all of the material escapes mathilde s gravitational field5 here we report laboratory measurements of high velocity impacts into porous material which support an alternative explanation3 the crater is formed mainly by compaction not excavation the small amount of ejecta lofted in our experiments have velocities sufficiently low that nearly all of the material is redeposited within the crater bowl the crater itself results from material being compressed rather than ejected this type of cratering implies that highly porous asteroids are minor contributors of meteorites because essentially no material escapes the asteroids

sound generation and air entrainment by breaking waves in the surf zone this paper presents the results of acoustic and optical measurements of individual breaking waves in the surf zone two hydrophones horizontally separated and deployed in 2 m of water off la jolla shores beach california were used to measure the source spectrum of breaking surf and characterize propagation through the surf zone over length scales of order 10 m the acoustic data show an acoustically active region within a wave which propagates shoreward the production of sound within the active region is associated with the formation of bubble plumes above 500 hz the sound is consistent with the radiation from individual bubble oscillations pictures were taken of the bubble plumes formed beneath the breaking surf providing estimates of the plume size and bubble size distribution and void fraction within a plume the density of bubbles scales as a 2 5 for a 1 mm where a is bubble radius and total void fractions of 0 3 0 4 were measured theoretical calculations show that ra