space weathering of asteroid surfaces abstract visible and near infrared spectra of reflected sunlight from asteroid surfaces exhibit features that hold the promise for identifying surface mineralogy however the very surfaces that are observed by remote sensing are also subject to impingement by micrometeoroids and solar wind particles which are believed to play the dominant role in space weathering which is the time dependent modification of an asteroid s reflectance spectrum such space weathering has confused the interpretations of telescopic spectra of asteroids especially concerning the possible association of common ordinary chondritic meteorites with so called s type asteroids recent spacecraft studies of asteroids especially of eros by near shoemaker have documented aspects of space weathering processes but we still do not understand the physics of space weathering well enough to confidently assay mineralogy of diverse asteroids by remote sensing a review of the intellectual history of this topic reveals the complexity of

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

polarimetric observations of small asteroids preliminary results abstract a campaign of polarimetric observations of small d h p min describing the phase polarization curve are used the main goal of this research is to obtain an independent check on the size dependence of the asteroid albedo distribution exhibited by the iras data at the same time we can look for the presence of possible systematic effects influencing the radiometric albedo estimates for objects close to the iras detection limit although some indication that iras derived albedos tend to be on the average slightly higher than polarimetric albedos preliminary results are not sufficient to draw definitive conclusions an evident discrepancy has been found only for the a class asteroid 863 benkoela

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

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

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

mineralogical composition of 25143 itokawa 1998 sf36 from visible and near infrared reflectance spectroscopy evidence for partial melting in march 2001 asteroid 25143 itokawa the target of the japanese hayabusa spacecraft mission was in a favorable viewing geometry for ground based telescopic study visible nearinfrared vnir spectra 0 48 to 0 9 m obtained on march 24 26 and 27 ut and near infrared nir spectra 0 75 to 2 5 m obtained on march 10 11 12 23 and 24 ut collectively show absorption features centered near 1 0 and 2 0 m which are indicative of olivine and pyroxene analyses of these absorption features indicate an abundance ratio of olivine to pyroxene of approximately 75 25 5 respectively with no significant variation in the relative abundance of these minerals across its surface on a regional scale the band center positions indicate that the mean pyroxene chemistry is wo 14 5 fs 43 5 there appear to be at least two pyroxene components primarily a low ca orthopyroxene accompanied by a spectrally significant 15 20 high fe rich pigeonite phase the mean pyroxene composition is significantly more fe rich than the fs1426 range found in ordinary chondrites these pyroxene compositions are suggestive of phases crystallized from partial melts this would indicate that the parent body of 25143 itokawa reached temperatures sufficient to initiate partial melting 1050 to 1250 c but that it did not attain the degree of melting required for significant melt mobilization and efficient segregation of the basaltic melt component from the unmelted residual olivine portion itokawas spectral band parameters place it near the s iii s iv boundary but within the s iii taxonomic field in meteoritic nomenclature itokawa would be most analogous to an olivine rich primitive achondrite alternatively if the high fs value is not related to partial melting then itokawa could also represent a rare atypical ll chondrite or a previously unsampled oxidized fe rich chondritic like assemblage

analysis of a microcrack model and constitutive equations for time dependent dilatancy of rocks summary based on experimental observations and theoretical analyses the author introduces an ideal microcrack model in which an array of cracks with the same shape and initial size is distributed evenly in rocks the mechanism of creep dilatancy for rocks is analysed theoretically initiation propagation and linkage of pre existing microcracks during creep are well described also the relationship between the velocity of microcrack growth and the duration of the creep process is derived numerically the relationship agrees well with the character of typical experimental creep curves and includes three stages of creep then the damage constitutive equations and damage evolution equations which describe the dilatant behaviour of rocks are presented because the dilatant estimated value is taken as the damage variable the relationship between the microscopic model and the macroscopic constitutive equations is established in this way the mechanical behaviour of rocks can be predicted

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

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

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

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

refining the asteroid taxonomy by polarimetric observations abstract we present new results of polarimetric observations of 15 main belt asteroids of different composition by merging new and published data we determined polarimetric parameters characterizing individual asteroids and mean values of the same parameters characterizing different taxonomic classes the majority of asteroids show polarimetric phase curves close to the average curve of the corresponding class we show that using polarimetric data it is possible to refine asteroid taxonomy and derive a polarimetric classification for 283 main belt asteroids polarimetric observations of asteroid 21 lutetia are found to exhibit possible variations of the position angle of the polarization plane over the surface

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

fault creep growth model and its relationship with occurrence of earthquakes summary a fault model is used to analyse creep growth process of microcracks in damage zone near the tips of fault by means of the rheology fracture mechanics when microcracks in the zone link each other and their deformation quickly intensifies it means that an earthquake is taking place to a certain degree the relationships among some key parameters that are correlated to the growth of microcracks are analysed theoretically these parameters include failure time of fault number of microcracks in unit area stresses field yield strength and rheological properties of medium the results indicate that the model proposed here is reasonable to describe the pregnant process of an earthquake in the meantime this research shows that with the results of in situ surveys and lab experiments it is possible to estimate the time span for a fault expanding

magma driven subcritical crack growth and implications for dike initiation from a magma chamber 1 the purpose of this paper is to explore a viscoelastic energy dissipation theory for subcritical dike growth from a magma chamber the theoretical relationship between the dike growth velocity and dike length is established using the viscoelastic subcritical crack growth theory proposed by the first author and the solutions of stress intensity factor at the crack tip derived by a perturbation method effects of magma chamber over pressure buoyancy and viscoelastic properties of the host rock on the subcritical growth rate are included in the model the numerical results indicate that the viscous energy dissipation of the host rock could allow a short dike to slowly grow on the order of 10 7 10 5 m s under modest over pressure and to accelerate when the stress intensity factor increases close to the fracture toughness followed by the unstable dike propagation the proposed theory provides a reasonable understanding of dike initiation process from a magma chamber

space weathering from mercury to the asteroid belt the variety of evidence bearing on the nature of space weathering is reviewed the effects of space weathering include spectral darkening reddening and subdued absorption bands and the distinctive magnetic electron spin resonance caused by single domain metallic iron particles ever since the apollo missions two paradigms have dominated the thinking of the planetary science community concerning space weathering 1 the optical effects are caused by impact vitrified glass in agglutinates and 2 the submicroscopic metallic iron results from the reduction of ferrous iron by the impact melting of minerals whose surfaces have been saturated with hydrogen from the solar wind however studies carried out since the apollo program showed that both of these paradigms are invalid a hypothesis first suggested by the author and his colleagues 26 years ago but not generally accepted at that time now appears to be essentially correct both the optical and magnetic effects are caused by metallic iron particles smaller than the wavelength in ubiquitous vapor deposited coatings on soil particle surfaces and inside agglutinates the vapor is generated by both solar wind sputtering and micrometeorite impact vaporization and injected preferentially downward into the porous regolith the iron is reduced by a physical process the selective loss of oxygen that occurs during deposition of the vapor and does not require heating melting or a reducing environment a mathematical theory that describes the optical effects of the submicroscopic iron quantitatively is derived and applied to the regoliths of the moon mercury and an s asteroid

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

comments on soil development parameters in the absence of a chronosequence in a glaciated basin of the white mountains california nevada by t w swanson d l elliot fisk and r j southard

solar wind as the origin of rapid reddening of asteroid surfaces a comparison of the laboratory reflectance spectra of meteorites with observations of asteroids revealed that the latter are much redder with the spectral difference explained by space weathering though the actual processes and timescales involved have remained controversial a recent study of young asteroid families concluded that they suffered only minimal space weathering here we report additional observations of those families revealing that space weathering must be a very rapid process the final colour of a silicate rich asteroid is acquired shortly after its birth within 10 6 years of undergoing a catastrophic collision this rapid timescale favours solar wind implantation as the main mechanism of space weathering as laboratory experiments have shown that it is the most rapid of several competing processes we further demonstrate the necessity to take account of composition when evaluating weathering effectiveness as both laboratory and asteroid data show an apparent dependence of weathering on olivine abundance the rapid colour change that we find implies that colour trends seen among asteroids are most probably due to compositional or surface particle size properties rather than to different relative ages apparently fresh surfaces most frequently seen among small near earth asteroids may be the result of tidal shaking that rejuvenates their surfaces during planetary encounters

glaciation in the great basin of the western united states abstract forty individually named ranges plateaus and massifs draining wholly or partly into the great basin of the western united states show definite evidence of pleistocene glaciation the most obvious deposits are a family of moraines designated among other names tioga angel lake and pinedale such moraines generally can be traced from range to range away from described type moraines these deposits have been numerically assigned to late wisconsinan glaciation in the wasatch range white mountains boulder mountain and sierra nevada on the basis of radiocarbon and surface exposure ages and have been assigned to late wisconsinan time in several other ranges on the basis of relative age studies the type angel lake moraine and most other equivalent moraines across the great basin are thick hummocky lobate piles of till rather than looping ridges the thicknesses of the moraines often 60 m can be explained by heavy debris loads and or glacial advance retreat and readvance to the same positions a number of times which is consistent with recent evidence that multiple late wisconsinan advances possibly related to heinrich and dansgaard oeschger events occurred in the sierra nevada pre angel lake deposits occur in many great basin ranges but it is currently difficult or perhaps impossible to determine if these deposits are equivalent to each other and what their relationship is to pre tioga deposits in the sierra nevada numerical ages are rare and relative age studies suggest that pre angel lake deposits may be products of more than one glaciation mapped pre angel lake glaciers were longer than their angel lake counterparts but the length differences do not translate into large differences in ela depression there is evidence of two minor latest pleistocene or early holocene advances in some ranges judging from the presence of overlying mazama tephra and or weathering comparisons to local angel lake moraines in the latter part of the holocene ela s were sufficiently high that only the highest wettest ranges developed neoglacial glaciers there does not appear to be a consistent pattern of latest pleistocene holocene glacial fluctuations along an east west transect through the cordillera or even through the great basin

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

subcritical dyke propagation in a host rock with temperature dependent viscoelastic properties summary in this paper we examine the effects of temperature dependent viscoelastic properties of the host rock on the subcritical growth of a dyke from a magma chamber a theoretical relationship between the velocity of subcritical dyke growth and dyke length is established using a perturbation solution of stress intensity factor at the dyke tip and a viscoelastic crack growth theory in which the temperature dependent creep properties are taken into account the temperature field around the dyke is calculated using an analytic solution the numerical results for a dyke subcritically propagating from a magma chamber indicate that while the general dyke growth characteristics are similar to those with constant creep properties the subcritical dyke growth velocity is increased by an order of magnitude by considering the temperature dependence of the creep properties hence the subcritical growth duration before the dyke reaches the unstable growth state is significantly shortened

asteroid photometric and polarimetric phase curves empirical interpretation abstract a method for interpretation of asteroid phase curves based on empirical modeling and laboratory measurements is outlined and preliminary results are presented a linear exponential function is used to describe the opposition peaks and negative polarization surges of various asteroids and laboratory samples and a statistical algorithm is used in parameter estimation the linear exponential function describes well the phase curves but dense phase angle coverage particularly at small phase angles must be obtained to improve the results major emphasis should also be put on laboratory study with an extensive library of laboratory measurements a stronger connection between the phase curve properties and surface characteristics is possible

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

a polarimetric study of asteroid 25143 itokawa abstract the near earth asteroid 25143 itokawa the target of the japanese space mission hayabusa was observed in june 2004 with the torino photopolarimeter attached at the 2 15 m telescope of the el leoncito observatory argentina the degree of linear polarization in five colors was measured over a wide range of phase angles between 40 and 80 the data obtained are sufficient to derive an estimate of the asteroid s albedo of 0 24 0 01 which is in good agreement with the s type taxonomic classification of this object the phase polarization curve has been sampled in ubvri colors covering a wide range of phase angles that cannot be reached by earth based observations of main belt asteroids

asteroid polarimetric observations using the torino ubvri photopolarimeter abstract a long term project of polarimetric observations of minor planets has been carried out since 1995 at the 2 15 m telescope of the el leoncito observatory san juan argentina using the torino photopolarimeter we present here an updated summary of the results including many measurements obtained during the most recent observing runs the main purpose of the observations has been to obtain albedo estimates and compare them with previous iras radiometric determinations mainly for objects having small

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

a constitutive law for low temperature creep of water saturated sandstones an accurate predictive model for the long term strength of the continental lithosphere is important in a range of geophysical and geodynamic problems while laboratory experiments are consistent with mohr coulomb brittle faulting in the cold upper continental crust there is increasing evidence that time dependent processes may also be important in these rocks even at low temperature however there is some ambiguity as to the exact form of the constitutive law for describing time dependent behavior of upper crustal rocks here we present results of room temperature creep experiments on a suite of water saturated sandstones spanning a range of petrophysical and rheological properties and underlying deformation mechanisms on physical and microstructural grounds our analysis suggests that a modified power law creep of the form a d f where d is the differential stress and f is the long term failure fundamental strength provides a more complete description of the experimental data in particular the parameters can be used to differentiate between sandstone types with a f and varying systematically with cementation state rock rheology and confining pressure the fundamental strength f for time dependent deformation varies much more than the other parameters of the distribution making it a potentially sensitive indicator of underlying creep mechanisms further tests would be needed to prove the constitutive law on a wider range of rock types and to prove that the three parameter model is statistically better in the general case

soil development parameters in the absence of a chronosequence in a glaciated basin of the white mountains california nevada abstract detailed mapping and provisional numerical age determinations of glacial deposits in the south chiatovich creek basin of the white mountains provide an opportunity to evaluate the ability of conventional soil parameters to discriminate first and second order glacial events sampling and analytical procedures were designed to minimize variation in climate and lithology when lithology and climate are similar among sites age trends are more pronounced in both field and chemical soil properties profile development indices pdis adjusted by removing melanization and ph systematically increase with greater soil age and discriminate first order but not second order glacial events the best fit curve for adjusted pdi data assumes an exponential form and suggests that the rate of soil formation in this region decreases over time similar to the rate of weathering rind development variation in eolian influx and surface erosion which are dominant processes affecting soils of the basin cause major uncertainties in establishing soil age and hence soil development rates even on the youngest glacial deposits soil age is probably significantly less than deposit age due to these geomorphic processes soil and weathering parameters imply that these field techniques can be inexpensively employed to define relative chronologies and to assess surface degradation and its impact on surface exposure ages results from this study indicate that site selection strategy for establishing glacial chronologies should be reevaluated working with stable residual bedrock surfaces and associated low relief outwash fans and terraces may prove more productive than focusing on relatively unstable moraine surfaces in tectonically active mountain systems

muses c target asteroid 25143 1998 sf36 a reddened ordinary chondrite abstract near earth asteroid 25143 1998 sf36 is a planned target for the japanese muses c sample return mission high signal to noise and relatively high resolution 50 a visible and near infrared spectroscopic measurements obtained during this asteroid s favorable 2001 apparition reveal it to have a red sloped s iv type spectrum with strong 1 and 2 m absorption bands analogous to those measured for ordinary chondrite meteorites this red slope which is the primary spectral difference between 25143 1998 sf36 and ordinary chondrite meteorites is well modeled by the spectrum of 0 05 nanophase iron npfe0 proposed as a weathering mechanism by pieters et al 2000 asteroid 1998 sf36 appears to have a surface composition corresponding to that of ordinary chondrite meteorites and is most similar in spectral characteristics and modeled olivine pyroxene content to the ll chondrite class

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

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

polarimetric evidence of close similarity between members of the karin and koronis dynamical families we present the results of a campaign of polarimetric observations of small asteroids belonging to the karin and koronis families carried out at the eso cerro paranal observatory using the vlt kueyen 8 m telescope the karin family is known to be very young having likely been produced by the disruption of an original member of the koronis family less than 6 myr ago the purpose of our study was to derive polarimetric properties for a reasonable sample of objects belonging to the two families in order to look for possible systematic differences between them to be interpreted in terms of differences in surface properties in particular albedo in turn systematic albedo differences might be caused by different times of exposure to space weathering processes experienced by the two groups of objects the results of our analysis indicate that no appreciable difference exists between the polarimetric properties of karin and koronis members we thus find that space weathering mechanisms may be very efficient in affecting surface properties of s class asteroids on very short timescales this result complements some independent evidence found by recent spectroscopic studies of very young families

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