dactyl galileo observations of ida s satellite abstract galileo s flyby of 243 ida in august 1993 led to the discovery of a small satellite dactyl some 85 km from the asteroid s center from earth at mean opposition the satellite is a v 20 3 mag object some 6 7 magnitudes fainter than ida forty seven images of the satellite at 18 different observing times were played back including one multicolor sequence in which the satellite is resolved adequately to distinguish surface markings 105 m pxl and three higher resolution single color views 89 39 and 24 m pxl the satellite mean radius 0 7 km is an elongated but not angular body with principal diameters of 1 6 1 4 1 2 km in the highest resolution view the longest axis points approximately in the direction of ida and its shortest axis is perpendicular to the orbital plane the spin period is slow 8 hr and may be synchronous the satellite shows no conspicuous sharp edges and is much less irregular in shape than ida limb profiles are remarkably smooth over distances of 200 300 m the geometric albedos of the two objects are similar 0 20 vs 0 21 as are the 0 4 1 0 m colors like ida dactyl is an s asteroid but has a slightly deeper 1 m band than ida by 5 8 while no identical regions in color are seen on ida the color difference is consistent with color variations reported within the koronis family and may be due to a slightly higher pyroxene olivine ratio on the satellite more than a dozen craters ranging from 90 to 280 m diameter are visible in the best image 39 m pxl at 47 phase the largest contains an off centered positive relief feature some 75 m across the image includes an intriguing crater chain but no grooves ridges or sharp edges are evident in terms of limb roughness dactyl is much smoother than ida but comparable to the two satellites of mars phobos and deimos while the satellite s origin is uncertain a likely scenario would have the satellite date from the breakup of the koronis family it is interesting that crater densities on the satellite are similar to those on ida itself

the shape of ida abstract ida s shape is more irregular than that of any solar system object previously encountered by spacecraft approximately 95 of the surface was imaged by the galileo spacecraft at better than 1 km pixel and 30 at better than 40 m pixel ida s volume is 16 100 1900 km 3 mean radius of 15 7 0 6 km the surface area is 3900 300 km 2 the maximum and minimum dimensions are 55 8 and 14 8 km viewed from the poles ida has a somewhat crescent shape smaller scale overlapping depressions along much of the 110 and 260 e longitudes give the appearance of a waist a sharp ridge about 40 km in length spans nearly 150 of longitude the distinctive topography on either side of the waist suggests a difference of mechanical properties along the length of the asteroid these differences might arise from structures inherited in place from a larger precursor object or might indicate a two component object the spin pole aligns with the model maximum principal moment of inertia axis to within 2 this relationship rules out extreme density asymmetries inside ida but does not limit modest changes in porosity or composition across the asteroid the irregular shape low mean density 2 6 0 5 gm cm 3 and rapid spin p 4 63 hr of ida mean that surface accelerations vary greatly 0 3 to 1 1 cm sec 2 gravity is lowest at the greatest and minimum radii due to the effects of rotation and the amount of mass interior to surface location maximum slopes are about 50 although very little of the surface area has slopes in excess of 35

the lunar far uv albedo indicator of hydration and weathering 1 we present an analysis of lunar reconnaissance orbiter lro lyman alpha mapping project lamp measurements of the dayside lunar surface at far ultraviolet wavelengths we use the strong 165 nm h2o absorption edge to look for diurnal variations in hydration we find that diurnal variations in spectral slope are indeed present they are superimposed on latitudinal and spatial variations related to composition and weathering we use two different spectral regions 164 173 nm and 175 190 nm to separate out these effects highlands and mare regions have distinct reflectance spectra with mare regions being spectrally bluer than highlands regions a consequence of the greater abundance of opaque minerals in mare regions bright ray terrains and areas known to be young such as giordano bruno crater are found to be relatively spectrally flat or red in the far uv this is consistent with a lack of space weathering which tends to make the far uv spectrum bluer due to the spectral behavior of nanophase iron large scale latitudinal variations in fuv slope are distinct and are likely due to a gradient in space weathering the diurnal variation in hydration is consistent with a solar wind origin and with loss of h2o at temperatures above 320 k far uv spectroscopy is thus shown to represent a viable method for mapping aqueous alteration even on the dayside of the moon and potentially elsewhere in the solar system

correlation between magnetic fabrics strain and biotite microstructure with increasing mylonitisation in the pretectonic wyangala granite australia abstract the wyangala granite is a foliated porphyritic silurian granite from the palaeozoic circum pacific type eastern lachlan orogen elo of australia it is a paramagnetic ilmenite bearing s marginal i type two mica to mainly biotite granite with different biotite contents and local chlorite alteration very highly strained quartz epidote bands are present in this contribution anisotropy of magnetic susceptibility ams is compared with independently measured intensity and 3d style of strain biotite microstructure and degree of mylonitisation for low strain granites with weak s foliations through medium strain protomylonitic granites with moderate s and c foliations to a high strain altered granite with a strong single foliation the samples are further analysed for possible contributions from sample heterogeneity magmatic flow and sub magmatic deformation a good correlation p ams 1 02 0 04 ln p e qtz is obtained between site average degree of ams p ams in the granite and degree of finite strain anisotropy p e qtz from aspect ratios of quartz aggregates in s foliations in hand specimen and outcrop p ams 1 03 1 14 p e qtz 1 4 19 the magnetic fabric ellipsoids agree with a kinematic regime between neutral and pure oblate predicted by the march model the observed quartz strains however exceed the ams march strains and are near neutral plano linear character the geological factors that may have contributed to these differences include intra and inter crystalline deformation of biotite and bimodality in s and c magmatic fabric is not clearly evident in either the ams or the biotite data new data for synkinematic oligoclase low titanium biotite and low sodium k feldspar show that conditions during deformation were approximately transitional greenschist amphibolite facies i e well below solidus this agrees with published age data that put the granite emplacement in an extensional back arc setting in already deformed country rocks and before later convergent regional deformation in middle devonian tabberabberan event to early carboniferous kanimblan event time

mechanical and geological effects of impact cratering on ida asteroids respond to impact stresses differently from either laboratory specimens or large planets gravity is typically so small that seismic disturbances of a few cm s 1can devastate unconsolidated topography yet the presence of regolith and the likelihood that many asteroids are gravitational assemblages tell us that gravity cannot generally be ignored we use numerical models for impact fracture in solids to examine the initial stage of crater formation on asteroid 243 ida up to the cessation of fracture and the establishment of the cratering flow at this stage we can infer final crater diameters but not profiles we find that a modified strength scaling applies for craters up to a few 100 m in diameter forming in rock subject to ida s gravity and that gravity controls all craters larger than 1 km bright annuli around a number of intermediate craters may be the result of low velocity surface disturbances rather than bright proximal ejecta deposits we also consider large impactors to which ida presents a curved finite target surface with irregular gravity these can excavate asymmetrical concavities stresses from large events can refocus and cause fracture far from the crater using the shape of ida as a basis for 3d hydrocode simulations we show that impact genesis of the vienna regio concavity can cause fracture in pola regio where grooves are observed in spacecraft images other simulations indicate that the formation of the 10 km crater azzurra might have reopened these fractures which may account for their fresh appearance this mechanism of groove formation requires an interior which coherently transmits elastic stress while this precludes a classic rubble pile asteroid it does allow well joined fault planes and welded blocks or pores smaller than the stress pulse

near encounter with asteroid 253 mathilde overview abstract on june 27 1997 the near spacecraft carried out the first ever encounter with a c type asteroid flying by 253 mathilde at a distance of 1212 km we summarize findings derived from 330 images obtained by near s msi camera which cover about 60 of the surface of the asteroid the highest resolution achieved was about 160 m pixel mathilde is a low reflectance object geometric albedo 0 047 with principal diameters of 66 48 44 km the mean radius of 26 4 1 3 km is somewhat smaller than the value of 30 km suggested by previous telescopic data mathilde s surface morphology is dominated by large craters at least four of which have diameters comparable to the radius of mathilde the two largest ishikari and karoo have diameters of 29 3 and 33 4 km respectively no evidence of layering is exposed in the crater walls but suggestions of downslope movement are present the surface density of craters in the diameter range from 0 5 to 5 km is close to equilibrium saturation a situation in which as many craters are being destroyed as are being produced observed depth to diameter ratios for craters in this size range are close to those observed on the lunar surface a disruption lifetime of about 4 billion years has been estimated for mathilde based on the mass determination obtained from doppler tracking d k yeomans et al 1997 science 278 2106 2109 and the volume derived from msi images the average density of mathilde is remarkably low 1 3 0 3 g cm 3 a value consistent with a rubble pile structure for the interior assuming that mathilde s rock type is similar to that found in cm meteorites the porosity of the interior must be some 50 shock and seismic disturbances associated with major impacts are expected to be transmitted very poorly by mathilde s underdense interior a fact which may explain the remarkable degree to which surface morphology and topography have been preserved in spite of later major collisional events except for the lower geometric albedo 0 047 0 005 the photometric properties of mathilde are closely similar to those of phobos the surface is extremely homogeneous in terms of both color or albedo specifically no color or albedo variations associated with craters have been identified

s type asteroids ordinary chondrites and space weathering the evidence from galileo s fly bys of gaspra and ida new observations of the s type asteroids gaspra and ida especially by the galileo spacecraft demonstrate that a space weathering process operates which modifies the reflectance spectra of fresh material to be redder straighter and have shallower absorption bands the weathering process appears similar to but less potent than that observed on the moon it operates in the sense that it would tend to convert spectra of ordinary chondrites oc to having the spectral traits of s type asteroids these results appear to resolve the major obstacles of the long standing s type conundrum about the provenance of ordinary chondrite meteorites a wide body of recent research which is reviewed here builds on previous meteoritical evidence to support a new developing consensus that the larger s type asteroids are a diverse assortment of silicaceous assemblages which includes the ordinary chrondite parent bodies recent fairly realistic laboratory simulations of space weathering processes have changed oc spectra to resemble s type spectra j f bell s highly regarded earlier paradigm that oc parent bodies would be found only among sub 10 km main belt asteroids has been tested by an extensive observational survey by r binzel and his proteges the result is that no small main belt analog for the ocs has been found not even the somewhat oc like object boznemcova this article reviews the history of the s type conundrum which set the stage for galileo s historic flybys i review the findings about gaspra and ida including results about their geology their geophysical structure and probable origins and about ida s small moonlet dactyl density constraints on ida set by dactyl s orbit rule out for ida at least the classic view of s types as metallic stripped cores of differentiated precursor asteroids new analysis of the galileo spectral images of ida is presented that provides strong evidence that space weathering occurs on ida and that ida and the koronis family asteroids in general are plausibly oc like in composition after reviewing 1990s developments on the s type conundrum i advocate a new perspective that the ordinary chondrite parent bodies are among the s type asteroids a diverse grouping that also contains other silicaceous and silicate metal assemblages presumably including various stony irons and primitive achondrites represented in meteorite collections gaspra may be such a metal rich assemblage

irradiation of olivine by 4 kev he simulation of space weathering by the solar wind 1 we have studied the effects of 4 kev he ion irradiation on olivine while measuring in situ changes in the near infrared nir reflectance and in the chemical composition of the surface the observed changes in reflectance are reddening and the attenuation of the fe 3d absorption bands in the nir spectral reddening of irradiated olivine powder 45 m correlates with the amount of metallic iron formed by ion impact consistent with the idea that space weathering effects in the reflectance of olivine bearing s type asteroids are due to the formation of metallic iron the metallization rate of the powder is about half that of a sectioned rock of olivine which we propose is a consequence of redeposition of sputtered material the nir spectral changes observed in ion irradiation experiments are similar to those observed in our previous experiments on vapor redeposition indicating that different space weathering mechanisms can lead to similar final effects on reflectance finally we estimate that at 1 au the spectral reddening caused by the solar wind saturates approximately 2 orders of magnitude faster than comparable reddening caused by micrometeorite impacts

size and density sorting of metal and silicate grains under microgravity conditions and the origin of chondrites 1 we report here experiments on nasa s kc 135 microgravity facility aimed at investigating metal silicate fractionation in chondrites metal and sand particle mixtures with sizes and relative proportions approximating chondritic values were allowed to undergo density and aerodynamic sorting as air was passed through 310 ten centimeter long 2 54 cm diameter columns under microgravity conditions metal and silicate fractionation was found to occur when gases passed through the mixtures and when the particle diameter ratio was large 3 0 with sand being enriched near the surface of the beds this is contrary to experiments performed under terrestrial gravity and the semi empirical ergun equation while the reason for this discrepancy is not well understood the present results do suggest that separation of metal and silicates will readily occur under microgravity conditions on the surface of asteroids

cratering on ida abstract craters on ida have a size distribution consistent with equilibrium under conditions of crater saturation over the range from 100 m to 1 km in diameter the density of craters is high and they follow a 3 differential slope the craters show a full spectrum of degradational morphologies at both large and small sizes craters 2 km in diameter and especially those 5 km are more prevalent on one part of ida a sub equilibrium production population is seen on the other part which rises steeply and reaches equilibrium for diameters

analysis of lunar samples implications for planet formation and evolution the analysis of lunar samples returned to earth by the apollo and luna missions changed our view of the processes involved in planet formation the data obtained on lunar samples brought to light the importance during planet growth of highly energetic collisions that lead to global scale melting this violent birth determines the initial structure and long term evolution of planets once past its formative era the lunar surface has served as a recorder of more than 4 billion years of interaction with the space environment the chronologic record of lunar cratering determined from the returned samples underpins age estimates for planetary surfaces throughout the inner solar system and provides evidence of the dynamic nature of the solar system during the planet forming era

spatially resolved spectral observations of asteroid 951 gaspra on october 29 1991 the galileo spacecraft encountered asteroid 951 gaspra with a telescopic ccd camera and a near infrared mapping spectrometer that provided the first optically resolved views of any asteroid data from these two sensors were combined to detect the spectral signature of iron bearing minerals on this s type asteroid a minimum of two spectral units were identified on 951 gaspra both containing a higher relative abundance of olivine than those found in ordinary chondrites these data indicate that this s asteroid is an object that has undergone igneous differentiation processes a 2 7 m spectral feature was also detected on the surface of 951 gaspra and may be due to the presence of structural oh

spectral properties and rotational spectral heterogeneity of 433 eros composite spectra of different rotational aspects of the s asteroid 433 eros were assembled from previous studies covering the wavelength range 0 33 2 5 m these data allow us to measure significant attributes of eros s spectrum its mafic mineral absorptions and the degree and nature of compositional heterogeneity of its surface we also evaluated previous s asteroid taxonomies using independent methods and from this test determined that the taxonomy of gaffey et al 1993 provides a sound basis for relating eros to other s asteroids on average eros resembles the s iv asteroids which bear the closest resemblance to ordinary chondrites in mafic mineralogy however eros also exhibits distinct rotational spectral variations comparable to the greatest yet recognized on an s asteroid one side of the asteroid has a shorter wavelength 1 m absorption and a stronger 2 m absorption than the global average and is interpreted to be richer in pyroxene the opposite face has a longer wavelength 1 m absorption and a weaker 2 m absorption and is interpreted to be richer in olivine the pyroxene rich side coincides with a facet like face identified in radar studies and the olivine rich side coincides with a convex shaped face each side of the asteroid exhibits spectral affinities with a different spectral class of s asteroids demonstrating that such subdivisions of s asteroids represent different parts of a compositional continuum high spatial resolution measurement of the mineralogic and elemental compositions and especially the geologic settings of these different materials by the near earth asteroid rendezvous mission will provide a great advance in the understanding of the origin and evolution of a candidate ordinary chondrite parent body

oxygen and asteroids hundreds of thousands of asteroids have been discovered in the asteroid belt and in near earth space oxygen is an abundant element in meteorites and presumably in most asteroids spectral reflectance measurements of asteroids in the visible and near infrared can identify oxygen bearing minerals such as those found in the olivine pyroxene and serpentine groups due to their distinctive absorption features interpretation of the mineralogy of asteroids is complicated by the effects of space weathering which tends to redden and darken the surfaces of asteroids asteroids are primarily classified into a number of taxonomic classes and subclasses according to their spectral properties in the visible wavelength region however asteroids with similar spectral properties in the visible may have different spectral properties in the near infrared and therefore different interpreted mineralogies definite trends in the abundances of different taxonomic classes versus heliocentric distance are apparent these trends appear to be a function of both composition differences and degree of heating however dynamical processes have significantly affected these trends close observation of asteroids by spacecraft such as near shoemaker to 433 eros and hayabusa to 25143 itokawa are currently the best way to discern the mineralogies of individual asteroids

the geology of gaspra the surface of gaspra can be divided into several facets separated by ridges superimposed on the facets and ridges are two populations of craters type 1 craters are crisply defined and constitute a production function of impact origin type 2 craters are mostly irregular shallow depressions and probably of various origins some of the type 2 craters may have formed by impact and be remnants of a crater population that predates the type 1 population the number of type 1 craters suggests that they started to accumulate 2 107 to 3 108 years ago the freshest craters have a depth diameter ratio of 1 7 as compared with 1 5 typically observed on other bodies the craters appear to have become shallower with age at a rate of 10 6 to 10 7 m year the shallow depth of newly formed craters and the progressive degradation with age are ascribed to downslope movement of poorly coherent surface materials the movement being aided by seismic shaking as a result of impact the surface shows subtle color differences the most prominent differences are observed around craters on ridges where the surface has a stronger 1 m absorption than elsewhere the contrast is not observed around craters on the facets the color differences are attributed to alteration of the materials brought to the surface the unaltered material is estimated to be located at least 50 m below the surface on the facets only on the ridges is unaltered material at a shallow enough depth to be excavated by the observed craters the rate of degradation of craters the rounded form of the ridges the depth of altered material on the facets and the presence of old degraded craters all suggest that gaspra is covered with a regolith a few tens to several tens of meters thick

hydrogen isotopic composition of water from fossil micrometeorites in howardites we have measured the hydrogen isotopic composition d h ratios of the water from 13 carbonaceous chondritic microclasts ccms size 1 mm trapped in two howardites kapoeta and yamato 793497 early in the evolution of solar system the division into tochilinite rich magnetite rich olivine poor magnetite rich olivine rich ccm types is corroborated by the hydrogen isotopic compositions both mineralogy and hydrogen isotopic compositions demonstrate that tochilinite rich ccms represent cm2 chondritic matter in contrast there is no good match between the isotopic and mineralogical properties of the magnetite rich ccms and the known groups of carbonaceous chondrites suggesting that magnetite rich ccms represent a new kind of chondritic matter not yet sampled in meteorite collections this demonstrates that the view of the asteroid belt revealed by the collection of meteorites is incomplete the study of micro clasts offers a unique opportunity to better decipher the nature and relative abundance of asteroids the average hydrogen isotopic composition of water belonging to ccms d h 152 0 4 8 x 10 6 1 sigma m is similar to that of antarctic micrometeorites amms d h 161 2 3 8 x 10 6 1 sigma m the similarity in terms of mineralogy and hydrogen isotopic composition between ccms and amms demonstrates that the composition of the micrometeorites has not been modified over the whole history of the solar system it indicates that the composition of the micrometeorite flux onto earth has been and is dominated by a mixture of cm2 like magnetite rich olivine poor magnetite rich olivine rich carbonaceous chondritic matter exemplified by ccms found in howardites because ccms have not suffered atmospheric entry they provide an abundant source of pristine micrometeorites the average d h ratio of the whole population of ccms is identical within errors to that of the earth 149 3 x 10 6 the match between the ccms d h ratio and that of the earth is especially remarkable because 1 three different populations of ccms are needed to make the d h ratio of the earth 2 there is no single carbonaceous chondrite group for which a similar match exists this observation suggests that ccms population might be representative of the late veneer agent s that delivered water to the earth

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

a spectral analysis of ordinary chondrites s type asteroids and their component minerals genetic implications three salient features of visible and infrared reflectance spectra of ordinary chondrites ocs and s type asteroids are 1 albedo at 0 56 m 2 continuum slope and 3 depth of the electronic absorption band due to octahedrally coordinated fe2 in olivine and pyroxene these quantities were numerically extracted from the spectra of 23 ocs representing all metamorphic grades and 39 s type asteroids to be plotted in a three dimensional coordinate system the spectral characteristics of three ocs which were comminuted melted recrystallized and recomminuted are also presented in the same format the results show that although laboratory simulation of melt alteration in an asteroidal regolith does alter oc spectra the spectral parameters of these altered meteorites do not change enough to leave the parametric region defined by unaltered oc spectra when the region containing the 39 s asteroid spectra is compared with that of the altered and unaltered ocs it is found that not one of the ocs falls within the s asteroid region the differences may be largely attributed to spectral differences between the respective metallic components the range of s asteroid parameters is then compared with potential pure end member components most likely to result from magmatic differentiation of a chondritic protoasteroid olivine orthopyroxene clinopyroxene and fe ni meteorite metal alternatively represented by the m asteroids it is found that the s asteroid array is consistent with random mixtures of the differentiated components except for a notable dominance of the spectral characteristics of the opaque metallic component these results suggest that the m asteroids may form a composition continuum with the s asteroids we discuss a scenario consistent with this analysis and with the bell et al theory of the geological structure of the asteroid belt ordinary chondritic protoasteroids of all sizes were probably the dominant primary condensates in the inner portion of the main asteroid belt these were later heated by electromagnetic induction or by 26al nuclide decay as a result the smaller ones were subjected to various degrees of metamorphism while the larger ones were subjected to large scale magmatic differentiation petrological domains of the s asteroids beneath a mixed regolith may be large and supply achondrites irons and stony irons to earth rather than well mixed breccias of these components the smaller oc protoasteroids may still be abundant in the asteroid belt but if small enough to escape differentiation may also be small enough to escape earth based identification

geology of 243 ida abstract the surface of 243 ida is dominated by the effects of impacts no complex crater morphologies are observed a complete range of crater degradation states is present which also reveals optical maturation of the surface darkening and reddening of materials with increasing exposure age regions of bright material associated with the freshest craters might be ballistically emplaced deposits or the result of seismic disturbance of loosely bound surface materials diameter depth ratios for fresh craters on ida are 1 6 5 similar to gaspra results but greater than the 1 5 ratios common on other rocky bodies contributing causes include rim degradation by whole body ringing relatively thin ejecta blankets around crater rims or an extended strength gradient in near surface materials due to low gravitational self packing grooves probably represent expressions in surface debris of reactivated fractures in the deeper interior isolated positive relief features as large as 150 m are probably ejecta blocks related to large impacts evidence for the presence of debris on the surface includes resolved ejecta blocks mass wasting scars contrasts in color and albedo of fresh crater materials and albedo streaks oriented down local slopes color data indicate relatively uniform calcium abundance in pyroxenes and constant pyroxene olivine ratio a large relatively blue unit across the northern polar area is probably related to regolith processes involving ejecta from azzurra rather than representing internal compositional heterogeneity a small number of bluer brighter craters are randomly distributed across the surface unlike on gaspra where these features are concentrated along ridges this implies that debris on ida is less mobile and or consistently thicker than on gaspra estimates of the average depth of mobile materials derived from chute depths 20 60 m grooves 30 m and shallowing of the largest degraded craters 20 50 m minimum 100 m maximum suggest a thickness of potentially mobile materials of 50 m and a typical thickness for the debris layer of 50 100 m

the role of meteoritics in spaceflight missions and vice versa research on extraterrestrial materials plays a critical role in formulating the science rationale and design for spacecraft missions and conversely spaceflight holds great promise for solving perplexing problems in meteoritics the connections between meteoritics and sample return missions are obvious meteorite research can define sampling strategies the capabilities of sampling devices acceptable levels of chemical contamination and physical alteration of samples and the conditions under which samples are stored prior to and during recovery for their part sample return missions can provide geologic context for meteorites increased sampling diversity including materials not sampled as meteorites such as unconsolidated regolith ices and atmosphere calibration for crater counting chronology and ground truth for remote sensing measurements of meteorite parent bodies meteoritics also relates to spacecraft flyby rendezvous and lander missions that do not necessarily return samples specific illustrations of this mutual relationship based on a selection of recent or planned spacecraft missions include identifying source asteroid classes for ordinary and carbonaceous chondrites and reconstructing their thermal and collisional histories galileo near clementine ii and muses c determining the extent to which cometary dust and interstellar grains are found as interplanetary dust particles and assessing volatile abundances isotopic compositions and molecular species in cometary nuclei stardust and rosetta understanding the compositions of ancient martian crust and the mantle sources for snc meteorites as well as inventorying the planet s volatile reservoirs and interactions mars pathfinder mars global surveyor and mars volatiles and climate surveyor assessing whether lunar meteorites provide a more representative chemical sampling of the highlands crust and of mare volcanism than do apollo samples galileo clementine and lunar prospector spaceflight is the first priority of the space agencies that fund most research on extraterrestrial materials and the continued level of support for such research may be linked in part to its use in exploration by spacecraft

lithification of gas rich chondrite regolith breccias by grain boundary and localized shock melting we studied the fine grained matrices 150 m of 14 gas rich ordinary chondrite regolith breccias in an attempt to decipher the nature of the lithification process that converted loose regolith material into consolidated breccias we find that there is a continuous gradation in matrix textures from nearly completely clastic class a to highly cemented class c breccias in which the remaining clasts are completely surrounded by interstitial shock melted material we conclude that this interstitial material formed by shock melting in the porous regolith in general the abundances of solar wind implanted 4he and 20ne are inversely correlated with the abundance of interstitial shock melted feldspathic material chondrites with the highest abundance of interstitial melted material class c experienced the highest shock pressures and temperatures and suffered the most extensive degassing it is this interstitial feldspathic melt that lithifies and cements the breccias together those breccias with very little interstitial melt class a are the most porous and least consolidated

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

ida and dactyl spectral reflectance and color variations abstract galileo ssi color data between 0 4 and 1 0 m demonstrate that both ida and dactyl are s type asteroids with similar but distinct spectra small but definite color variations are also observed on ida itself and involve both the blue part of the spectrum and the depth of the 1 m pyroxene olivine band ida s surface can be classified into two color terrains terrain a has a shallower 1 m absorption and a steeper visible red slope than does terrain b qualitatively the color albedo systematics of these two terrains follow those noted for color units on gaspra and the variations in 1 m band depth with weathering described by gaffeyet al gaffey m j j f bell r h brown t h burbine j piatek k l reed and d a chaky 1993 icarus106 573 602 terrain a with its slightly lower albedo its shallower 1 m band and its slightly steeper visible red slope relative to terrain b could be interpreted as the more processed more mature or the more weathered of the two terrains consistent with this interpretation is that terrain a appears to be the ubiquitous background on most of ida while terrain b is correlated with some small craters as well as with possible ejecta from the 10 km azzurra impact structure because of these trends it is less likely that differences between terrains a and b are caused by an original compositional inhomogeneity within the body of ida although they do fall within the range known to occur within the koronis family the spectrum of dactyl is similar to but definitely different from that of terrain b on ida it does not conform to the pattern that obtains between the colors and albedos of terrains a and b the satellite s 1 m band is deeper than that of terrain b but its albedo is lower rather than higher by itself the deeper band depth could be interpreted following gaffeyet al to mean that dactyl is a less weathered version of terrain b on ida but such an interpretation is at odds with dactyl s redder spectral slope thus the explanation for the color difference between dactyl and ida is likely to be different from that which accounts for the differences between the two terrains on ida given that dactyl and ida have very similar photometric properties helfenstein p j veverka p c thomas d p simonelli k klassen t v johnson f fanale j granahan a s mcewen m j s belton and c r chapman 1996icarus120 48 65 thus ruling out any dramatic texture differences between the two surfaces the most likely explanation is that the satellite has a slightly different composition more pyroxene than ida the spectral difference is within the range reported by binzelet al binzel r p s xu and s j bus 1993 icarus106 608 611 for members of the koronis family and could be caused by compositional inhomogeneities of the koronis parent body rather than by post breakup regolith processes

geologic map of the northern hemisphere of vesta based on dawn framing camera fc images abstract the dawn framing camera fc has imaged the northern hemisphere of the asteroid 4 vesta at high spatial resolution and coverage this study represents the first investigation of the overall geology of the northern hemisphere 22 90 n quadrangles av 1 2 3 4 and 5 using these unique dawn mission observations we have compiled a morphologic map and performed crater size frequency distribution csfd measurements to date the geologic units the hemisphere is characterized by a heavily cratered surface with a few highly subdued basins up to 200 km in diameter the most widespread unit is a plateau cratered highland unit similar to although of lower elevation than the equatorial vestalia terra plateau large scale troughs and ridges have regionally affected the surface between 180 e and 270 e these tectonic features are well developed and related to the south pole veneneia impact saturnalia fossae trough unit elsewhere on the hemisphere they are rare and subdued saturnalia fossae cratered unit in these pre rheasilvia units we observed an unexpectedly high frequency of impact craters up to 10 km in diameter whose formation could in part be related to the rheasilvia basin forming event the rheasilvia impact has potentially affected the northern hemisphere also with s n small scale lineations but without covering it with an ejecta blanket post rheasilvia impact craters are small

ejecta blocks on 243 ida and on other asteroids abstract seventeen positive relief features 45 150 m across are identified as probable blocks in galileo high resolution images of ida their presence provides direct evidence for regolith retention on asteroids the spatial distribution maximum size and integrated volume of the blocks are consistent with those of blocks associated with craters on the earth the moon phobos and deimos the concordance suggests that the features are impact ejecta fragments and that cratering mechanics on ida an object of average diameter 32 km are similar to those applying on previously studied rocky bodies the blocks that lie within or near the rims of craters lascaux and mammoth were likely mobilized in the low velocity tail portion of the excavation flow that formed those craters a few blocks located near smaller craters may have migrated some distance away from their source possibly by impact induced spallation hopping rolling and or sliding some blocks on ida could be surviving fragments from the koronis parent body accreted after its breakup the lifetime of 102 m sized boulders against collisional disruption is estimated to be in the 108 109year range consistent with ages considered for the largest and oldest craters on ida extrapolation of successful ejecta scaling laws to other asteroids suggests that blocks 15 and 70 m across could be present on dactyl and gaspra respectively in both cases too small to be identified in available galileo images blocks 100 m in size could be present on 433 eros and km sized megablocks on 4 vesta

optical maturation of asteroid surfaces abstract analysis of laboratory experiments simulating space weathering optical effects on atmosphereless planetary bodies reveals that the time needed to alter the spectrum of an ordinary chondrite meteorite to resemble the overall spectral shape and slope of an s type asteroid is about 10 5 yr the time required to reduce the visible albedo of samples to 0 05 is 10 6 yr since both these timescales are much less than the average collisional lifetime of asteroids larger than several kilometers in size numerous low albedo asteroids having reddish spectra with subdued absorption bands should be observed instead of an s type dominated population this is not the case because asteroid surfaces cannot be considered as undisturbed unlike laboratory samples we have estimated the number of collisions occurring in the time of 10 5 yr between asteroids and projectiles of various sizes and show that impact activated motions of regolith particles counteract the progress of optical maturation of asteroid surfaces we suppose that the maturation level of asteroid surfaces may be a compromise resulting from a competition between impact resurfacing and solar wind darkening and that after reaching some steady state after a relatively short time 7 10 5 yr thereafter depends only slightly on time spectroscopic analysis using relatively invariant spectral parameters such as band centers and band area ratios can determine whether the surface of an s asteroid has chondritic composition or not in this sense the bodies with the ordinary chondrite composition cannot be masked among s asteroids differences in the environment of the main asteroid belt versus that at 1 au and the physical difference between the moon and main belt asteroids i e size can account for the lack of lunar type weathering on main belt asteroids

bulk density of asteroid 243 ida from the orbit of its satellite dactyl during its reconnaissance of the asteroid 243 ida the galileo spacecraft returned images of a second object 1993 243 1 dactyl1 the first confirmed satellite of an asteroid sufficient data were obtained on the motion of dactyl to determine its orbit as a function of ida s mass here we apply statistical and dynamical arguments to constrain the range of possible orbits and hence the mass of ida combined with the volume of ida2 this yields a bulk density of 2 6 0 5 g cm 3 allowing for the uncertainty in the porosity of ida this density range is consistent with a bulk chon dritic composition and argues against some but not all classes of meteoritic igneous rock types that have been suggested as compositionally representative of s type asteroids like ida

spectral properties of near earth asteroids evidence for sources of ordinary chondrite meteorites although ordinary chondrite oc meteorites dominate observed falls the identification of near earth and main belt asteroid sources has remained elusive telescopic measurements of 35 near earth asteroids 3 kilometers in diameter revealed six that have visible wavelength spectra similar to laboratory spectra of oc meteorites near earth asteroids were found to have spectral properties that span the range between the previously separated domains of oc meteorites and the most common s class asteroids suggesting a link this range of spectral properties could arise through a diversity of mineralogies and regolith particle sizes as well as through a time dependent surface weathering process

galileo encounter with 951 gaspra first pictures of an asteroid galileo images of gaspra reveal it to be an irregularly shaped object 19 by 12 by 11 kilometers that appears to have been created by a catastrophic collisional disruption of a precursor parent body the cratering age of the surface is about 200 million years subtle albedo and color variations appear to correlate with morphological features brighter materials are associated with craters especially along the crests of ridges have a stronger 1 micrometer absorption and may represent freshly excavated mafic materials darker materials exhibiting a significantly weaker 1 micrometer absorption appear concentrated in interridge areas one explanation of these patterns is that gaspra is covered with a thin regolith and that some of this material has migrated downslope in some areas

optical effects of space weathering the role of the finest fraction the optical properties of lunar softs are different than those of rocks from which they are derived as a consequence of lunar space weathering soils are darker and exhibit a distinctive red sloped continuum and weaker mineral absorption bands the accumulation of dark glass welded aggregates agglutinates has been thought to account for these optical effects of space weathering on lunar soils spectroscopic analyses of agglutinate separates and size fractions for a suite of lunar softs presented here indicate that the agglutinate paradigm is insufficient to fully account for lunar optical alteration it is the finest fraction of lunar soils 25 gm which constitute 25 wt that dominates the optical properties of the bulk soil unlike size fractions of most silicates for which the finest fraction is the brightest the lunar soil size fractions all have comparable albedos in the short wavelength visible in the near infrared however it is the finest fraction that exhibits the steep red continuum and weak absorption bands the properties of the finest fraction cannot be duplicated by preparing a fine fraction by grinding larger agglutinate rich soil particles these results suggest space weathering on airless bodies is dominated by surface correlated processes perhaps associated with the development of fine grained fe o on or near the surface of grains and the accumulation of the larger agglutinates is not necessarily required to account for lunar optical alteration

original structures and fragmentation and reassembly histories of asteroids evidence from meteorites abstract if chondritic meteorites were internally heated after accretion had ended then the hottest material would have been buried the deepest and should have cooled the slowest if this is correct there ought to be a correlation between cooling rate and petrographic type a measure of the extent to which chondrites were metamorphosed i e heated published and new cooling rates derived from the compositions of metallic iron nickel grains do not display this correlation implying either that chondrite parent asteroids never had onion shell structures or that bodies with onion shell structures were broken up and reassembled prior to cooling to below 500 c the temperature at which cooling rate information is recorded in metallic iron nickel chondritic regolith breccias formed from materials that resided on the surfaces of their parent asteroids metallic iron nickel grains in h and l chondrite regolith breccias indicate that the breccia constituents cooled at rates ranging from 1 to 1000 k myr based on thermal calculations these cooling rates suggest that the materials spread out on the surfaces of h and l chondrite parent asteroids originated at depths ranging from about one kilometer to several tens of kilometers craters deep enough to excavate tens of kilometers cannot form on typical asteroidal bodies only 100 to 300 km in diameter without disrupting them therefore it appears that at least some asteroids namely the parent bodies of h and l chondrites were disrupted after cooling to below 300 c and then reassembled to create surfaces containing rocks that originated at a wide range of depths these results support theoretical calculations suggesting that many asteroids were broken up and subsequently reassembled into gravitationally bound rubble piles

meteorite and asteroid reflectance spectroscopy clues to early solar system processes although earth has a magnetic shield that protects it from energetic ions and a cushioning atmosphere that catches the tons of small particles i mm that bombard it daily solid material from extraterrestrial sources nevertheless reaches the surface of earth many times a year dodd 1 98 1 when the largest of these bodies impact the surface with great velocity they release immense energy usually form a crater and often disrupt the terrestrial biosphere fortunately such events occur relatively infrequently although individual impacts are currently unpredictable beyond a statistical accounting shoemaker et al 1979 grieve 1 982 cosmic material between the size of small dust particles and the larger crater forming bodies is slowed by the terrestrial atmosphere upon entry the outermost portion of these objects is melted and ablated during passage forming a fusion crust which protects the interior and they land

space weathering on eros constraints from albedo and spectral measurements of psyche crater we present combined multi spectral imager msi 0 95 m and near infrared spectrometer nis 0 8 2 4 m observations of psyche crater on s type asteroid 433 eros obtained by the near earth asteroid rendezvous near shoemaker spacecraft at 5 3 km in diameter psyche is one of the largest craters on eros which exhibit distinctive brightness patterns consistent with downslope motion of dark regolith material overlying a substrate of brighter material at spatial scales of 620 m spectrum psyche crater wall materials exhibit albedo contrasts of 32 40 at 0 946 m associated spectral variations occur at a much lower level of 4 8 2 4 we report results of scattering model and lunar analogy investigations into several possible causes for these albedo and spectral trends grain size differences olivine pyroxene and troilite variations and optical surface maturation we find that the albedo contrasts in psyche crater are not consistent with a cause due solely to variations in grain size olivine pyroxene or lunar like optical maturation a grain size change sufficient to explain the observed albedo contrasts would result in strong color variations that are not observed olivine and pyroxene variations would produce strong band correlated variations that are not observed a simple lunar like optical maturation effect would produce strong reddening that is not observed the contrasts and associated spectral variation trends are most consistent with a combination of enhanced troilite a dark spectrally neutral component simulating optical effects of shock and lunar like optical maturation these results suggest that space weathering processes may affect the spectral properties of eros materials causing surface exposures to differ optically from subsurface bedrock however there are significant spectral differences between eros proposed analog meteorites ordinary chondrites and or primitive achondrites and eros freshest exposures of subsurface bright materials after accounting for all differences in the measurement units of our reflectance comparisons we have found that the bright materials on eros have reflectance values at 0 946 m consistent with meteorites but spectral continua that are much redder than meteorites from 1 5 to 2 4 m most importantly we calculate that average eros surface materials are 30 40 darker than meteorites

coupled organic synthesis and mineral alteration on meteorite parent bodies the hypothesis that the soluble fraction of the organic compounds present in carbonaceous chondrite meteorites was formed during aqueous alteration of the parent body was tested with masstransfer reaction path calculations in these calculations we start with likely compositions of the original parent body and asteroidal fluids that are far from thermodynamic equilibrium and metastable and stable equilibrium constraints are imposed as the total gibbs free energy of the parent body environment is minimized the results of these calculations suggest that the classes of soluble organic compounds present in carbonaceous chondrite meteorites could have formed during relatively low temperature aqueous alteration of the meteorite parent body or bodies the main controls on the potential for synthesis and transformation of organic compounds were the oxidation state of the rock fluid system the bulk composition of that system and the temperatures that were achieved during the alteration event or events it also appears that the alteration mineral assemblages were influenced by the presence of soluble organic compounds and reaction among them

asteroid 6 hebe the probable parent body of the h type ordinary chondrites and the iie iron meteorites the s iv type asteroid 6 hebe is identified as the probable parent body of the h type ordinary chondrites and of the iie iron meteorites the ordinary chondrites are the most common type of meteorites falling to earth but prior to the present study no large mainbelt source bodies have been confirmed hebe is located adjacent to both the v6 and 3 1 resonances and has been previously suggested as a major potential source of the terrestrial meteorite flux hebe exhibits subtle rotational spectral variations indicating the presence of some compositional variations across its surface the silicate portion of the surface assemblage of hebe is consistent both in overall average and in its range of variation with the silicate components in the suite of h type chondrites the high albedo of hebe rules out a lunar style space weathering process to produce the weakened absorption features and reddish spectral slope in the s type spectrum of hebe linear unmixing models show that a typical ni fe metal spectrum is consistent with the component that modifies an h chondrite spectrum to produce the s type spectrum of hebe on the basis of the association between the h chondrites and the he iron meteorites our model suggests that large impacts onto the relatively metal rich h chondrite target produced melt bodies sheets or pods that differentiated to form thin laterally extensive near surface layers of ni fe metal fragments of the upper silicate portions of these melt bodies are apparently represented by some of the igneous inclusions in h chondrite breccias alternately masses of metal could have been deposited on the surface of hebe by the impact of a core or core fragment from a differentiated parent body of h chondrite composition subsequent impacts preferentially eroded and depleted the overlying silicate and regolith components exposing and maintaining large masses of metal at the optical surface of hebe in this interpretation the nonmagmatic iie iron meteorites are samples of the ni fe metal masses on the surface of hebe whereas the h chondrites are samples from between and or beneath the metal masses

discovery and physical properties of dactyl a satellite of asteroid 243 ida observations of stellar occultations by asteroids have suggested that some may have satellites1 but given the absence of any confirmatory evidence the prevailing view has been that although such satellites probably do exist they are likely to be rare2 here we report the discovery3 by the galileo spacecraft of a satellite associated with the asteroid 243 ida although the satellite dactyl is only 1 6km across it has been imaged with sufficient resolution for geological analysis we describe the physical properties of dactyl with emphasis on its notably smooth shape its crater population which includes a crater chain and its photometric properties we find that spectroscopically dactyl resembles both ida and the other members of the koronis asteroid family implying a similar composition small spectral differences may reflect a space weathering process that slightly alters the colours with time we argue that dactyl originated during the breakup of the koronis parent body and that satellites could be common around other asteroids particularly members of asteroid families

identification of the projectile component in the impact structures rochechouart france and saaksjarvi finland implications for the impactor population for the earth a set of 11 impact melt rock samples from the rochechouart impact structure france and nine impact melt rock samples from saaksjarvi impact structure finland were studied for their major and trace element compositions including the abundances of the platinum group elements the main goal of this study was to identify the projectile type s responsible for the formation of these two impact structures the results confirmed previous studies that suggested extraterrestrial contamination in both the rochechouart and saaksjarvi impact melt rocks the projectile types found for rochechouart and saaksjarvi are quite similar and compatible with the composition of non magmatic iron meteorites ia and iiic this interpretation is based on identical platinum group element patterns as well as peculiar ni cr ni ir and cr ir ratios which can be explained by mixing of the different components of non magmatic iron meteorites this result indicates that besides ordinary chondrites non magmatic iron may be among the most common material impacting the earth as they also represent the majority of the projectiles for craters smaller that 1 5 km the abundance of non magmatic irons as projectiles as well as their composition olivine pyroxene and iron supports the assumption that a fraction of the s type asteroids could by related to this type of material

first images of asteroid 243 ida the first images of the asteroid 243 ida from galileo show an irregular object measuring 56 kilometers by 24 kilometers by 21 kilometers its surface is rich in geologic features including systems of grooves blocks chutes albedo features crater chains and a full range of crater morphologies the largest blocks may be distributed nonuniformly across the surface lineaments and dark floored craters also have preferential locations ida is interpreted to have a substantial regolith the high crater density and size frequency distribution 3 differential power law index indicate a surface in equilibrium with saturated cratering a minimum model crater age for ida and therefore for the koronis family to which ida belongs is estimated at 1 billion years older than expected

rotational spectral variations of asteroid 8 flora implications for the nature of the s type asteroids and for the parent bodies of the ordinary chondrites abstract the new interpretive calibrations olivine pyroxene abundance spectral signature of chondritic metal grains have been applied to visible and near infrared spectral reflectance data for the typical s type asteroid 8 flora chondritic metal grains do not exhibit the steeply reddened spectral signature previously associated with a nickel iron nife metal phase the conditions required to produce a s type spectral curve from a chondritic material are much more restrictive and improbable than previously supposed five mineralogic and patrologic parameters have been derived for the surface material of flora from the analysis of the spectral reflectance and rotational spectral variations of this object these include bulk mineralogy nife metal olivine pyroxene mafic mineral abundance ol py 2 8 metal nature abundant 50 and coarse grained in the substrate pyroxene composition orthopyroxene f s 50 and or abundant clinopyroxene and spatial mineralogic variations olivine pyroxene ratio and pyroxene composition do not covary in a chondritic manner all of the diagnostic mineralogic properties of the flora surface material indicate that this is a differentiated body none of the derived surface properties unambiguously support a chondritic type material and several contradict any type of undifferentiated assemblage flora is most probably the residual core of an intensely heated thermally evolved and magmatically differentiated planetesimal which was subsequently disrupted the present surface samples layers formed at and near the core mantle boundary in the parent body several lines of evidence suggest that the s type asteroids as a class are predominantly the core fragments of disrupted differentiated planetesimals a main belt source of the ordinary chondrites remains elusive

galileo photometry of asteroid 243 ida galileo imaging observations over phase angles 19 5 to 109 8 are combined with near opposition earth based data to derive the photometric properties of ida to first order these properties are uniform over the surface and well modeled at 0 55 m by hapke parameters 0 0 22 h 0 020 b0 1 5 g 0 33 and 18 with corresponding geometric albedop 0 21 0 030 01and bond albedoab 0 081 0 0170 008 ida s photometric properties are more similar to those of average s asteroids p helfenstein and j veverka 1989 asteroids ii univ of arizona press tucson than are those of 951 gaspra two primary color units are identified on ida terrain a exhibits a spectrum with relatively shallower 1 m absorption and a relatively steeper red spectral slope than average ida while terrain b has a deeper 1 m absorption and a less steep red slope the average photometric properties of ida and terrain a are similar while those of terrain b differ mostly in having a slightly higher value of 0 0 22 versus 0 21 suggesting that terrain b consists of slightly brighter more transparent regolith particles galileo observations of ida s satellite dactyl over phase angles 19 5 to 47 6 suggest photometric characteristics similar to those of ida the major difference being dactyl s slightly lower albedo 0 20 compared to 0 21