the origin of vesta s crust insights from spectroscopy of the vestoids high quality vnir spectra of 15 vestoids small asteroids that are believed to originate from vesta were collected and compared to laboratory spectra and compositional data for selected hed meteorites a combination of spectral parameters such as band centers and factors derived from modified gaussian model fits band centers band strengths calculation of the low to high ca pyroxene ratio were used to establish if each vestoid appeared most like eucrite or diogenite material or a mixture of the two howardite this resulted in the identification of the first asteroid with a ferroan diogenite composition 2511 patterson this asteroid can be used to constrain the size of diogenite magma chambers within the crust of vesta the vestoids indicate that both large scale homogeneous units 5 km and smaller scale heterogeneity 1 km exist on the surface of vesta as both monomineralogic eucrite or diogenite material alone and mixed both eucrite and diogenite spectra are observed the small scale of the variation observed within the vestoid population is predicted by the partial melting model which has multiple intrusions penetrating into the crust of vesta it is much more difficult to reconcile the observations here with the magma ocean model which would predict much more homogeneous layers on a large scale both at the surface and with depth

mega ejecta on asteroid vesta 1 asteroid 4 vesta sometimes called the smallest terrestrial planet will be orbited next july by nasa s dawn mission this will be the first time a small planet is visited by a spacecraft and novel geological structures are expected a key issue regarding vesta mean diameter 530 km is to what extent its geology is dominated by the 460 km diameter impact basin on its southern hemisphere we model the basin s formation using a very high resolution 3d smooth particle hydrodynamics simulation to establish some of the major impact related aspects of vesta s geology the goal is to provide a framework for interpreting anticipated observations of landforms a collision of this magnitude a 50 km diameter impactor at 5 km s exposes many deep strata from within the planet and offsets the center of mass by 10 km from the center of figure vesta spins every 5 3 hr so that a hemispheric scale impact evolves in a non inertial frame and deposits variably shaped multiply folded and abruptly terminated ejecta sequences of regional scale since little of this ejecta would have been molten these massive sequenced deposits could be mistaken in images of vesta for other geologic forms such as thrusts and folds detailed mapping and the piecing together of mega ejecta via impact models will enable an informed understanding of the interior geology of vesta

compositional maps of saturn s moon phoebe from imaging spectroscopy the origin of phoebe which is the outermost large satellite of saturn is of particular interest because its inclined retrograde orbit suggests that it was gravitationally captured by saturn having accreted outside the region of the solar nebula in which saturn formed by contrast saturn s regular satellites with prograde low inclination circular orbits probably accreted within the sub nebula in which saturn itself formed here we report imaging spectroscopy of phoebe resulting from the cassini huygens spacecraft encounter on 11 june 2004 we mapped ferrous iron bearing minerals bound water trapped co2 probable phyllosilicates organics nitriles and cyanide compounds detection of these compounds on phoebe makes it one of the most compositionally diverse objects yet observed in our solar system it is likely that phoebe s surface contains primitive materials from the outer solar system indicating a surface of cometary origin

new cassini radar results for saturn s icy satellites abstract cassini radar tracks on saturn s icy satellites through the end of the prime mission in 2008 have increased the number of radar albedo estimates from 10 ostro et al 2006 to 73 the measurements sample diverse subradar locations and for dione rhea and iapetus almost always use beamwidths less than half the target angular diameters thereby constraining the satellites global radar albedo distributions the echoes result predominantly from volume scattering and their strength is thus strongly sensitive to ice purity and regolith maturity the combination of the cassini data set and arecibo 13 cm observations of enceladus tethys dione rhea black et al 2007 and iapetus black et al 2004 discloses an unexpectedly complex pattern of 13 to 2 cm wavelength dependence the 13 cm albedos are generally smaller than 2 cm albedos and lack the correlation seen between 2 cm and optical geometric albedos enceladus and iapetus are the most interesting cases we infer from hemispheric albedo variations that the e ring has a prominent effect on the 13 cm radar lightcurve the uppermost trailing side regolith is too fresh for meteoroid bombardment to have developed larger scale heterogeneities that would be necessary to elevate the 13 cm radar albedo whereas all of enceladus is clean and mature enough for the 2 cm albedo to be uniformly high for iapetus the 2 cm albedo is strongly correlated with optical albedo low for the optically dark leading side material and high for the optically bright trailing side material however iapetus 13 cm albedo values show no significant albedo dichotomy and are several times lower than 2 cm values being indistinguishable from the weighted mean of 13 cm albedos for main belt asteroids 0 15 0 10 the leading side s optically dark contaminant must be present to depths of at least one to several decimeters so 2 cm albedos can mimic the optical dichotomy however it does not have to extend any deeper than that the fact that both hemispheres of iapetus look asteroid like at 13 cm means that coherent backscattering itself is not nearly as effective as it is at 2 cm since iapetus entire surface is mature regolith the wavelength dependence must involve composition not structure either the composition is a function of depth everywhere with electrical loss much greater at depths greater than a decimeter or two or the intrinsic electrical loss of some pervasive constituent is much higher at 13 cm than at 2 cm ammonia is a candidate for such a contaminant if ammonia s electrical properties do not depend on frequency and if ammonia is globally much less abundant within the upper one or two decimeters than at greater depths then coherent backscattering would effectively be shut down at 13 cm explaining the asteroid like 13 cm albedo

discovery of a basaltic asteroid in the outer main belt visible and near infrared spectroscopic observations of the asteroid 1459 magnya indicate that it has a basaltic surface magnya is at 3 15 astronomical units au from the sun and has no known dynamical link to any family to any nearby large asteroid or to asteroid 4 vesta at 2 36 au which is the only other known large basaltic asteroid we show that the region of the belt around magnya is densely filled by mean motion resonances generating slow orbital diffusion processes and providing a potential mechanism for removing other basaltic fragments that may have been created on the same parent body as magnya magnya may represent a rare surviving fragment from a larger differentiated planetesimal that was disrupted long ago

determination of planetary basalt parentage a simple technique using the electron microprobe many basaltic meteorites are being discovered in old and new meteorite suites including those from cold deserts e g antarctica and hot desert environments it is important to establish the specific planetary body source proven techniques for establishing planetary parentage include stable isotopic signatures especially oxygen certain elemental ratios in bulk samples and certain elemental ratios in specific minerals some of these techniques are expensive require considerable sample preparation and are adversely affected by weathering processes on the parent body or on earth we have been seeking key major and minor elemental ratios in pyroxene olivine and feldspar that can be measured by the electron microprobe on standard thin sections these ratios may be preserved in unweathered portions of mineral grains and thus see through weathering processes in addition if the sample is too small to provide a representative bulk composition it may still have key information recorded in individual minerals we have found that some of the most useful chemical parameters are fe mn atomic in olivine or pyroxene and the percent anorthite an in plagioclase solid solutions a plot of fe mn in pyroxene and or olivine verses an defines compositional fields that are significantly different for earth mars moon 4 vesta and the angrite parent body this method may be especially powerful when combined with oxygen isotope data

formation of iapetus extreme albedo dichotomy by exogenically triggered thermal ice migration the striking appearance of saturn s moon iapetus half black and half white has puzzled astronomers for over three centuries now spencer and denk p 432 1 published online 10 december present an explanation for this asymmetry a thermally controlled runaway migration of water ice triggered by exogenic deposition of dark material on the moon s leading darker side which faces the direction of motion of the moon in its orbit around saturn this mechanism is unique to iapetus because it rotates slowly enough to allow large temperature variations to arise it is small enough to allow long range migration of water and there is a source of dust to trigger the process in a related paper denk et al p 435 2 published online 10 december present data derived from the cassini imaging science subsystem that reveal that both dark and bright materials on the leading side of iapetus are redder than their trailing side counterparts this asymmetry results from the deposition of dust and debris from other moons in the saturnian system the very same process that initiates the thermal segregation proposed above 1 lookup doi 10 1126 science 1177132 2 lookup doi 10 1126 science 1177088

infrared spectroscopic characterization of the low albedo materials on iapetus abstract iapetus one of the large satellites of saturn has been studied over the centuries for its signature brightness contrast light on one side and dark on the opposite it has recently been suggested that the dark material is a combination of native and exogenous materials with distinct histories we present an analysis of parts of the cassini regio the darkest region on the leading hemisphere of iapetus focusing on the hydrocarbon signature with a view to detect and investigate differences in the material s we find variations in the hydrocarbon bands with geographic location one type predominantly located on the leading hemisphere a comparison with the equivalent spectral features on phoebe and hyperion reveals a predictable resemblance between the leading hemisphere material and phoebe and an unexpected likeness between hyperion s darkest material and iapetus trailing hemisphere surface an analysis of the slope in the visible part of the spectrum is strongly affected by a rise in the continuum 0 35 0 65 m attributed to rayleigh scattering from nano size particles on the surface the continuum rise varies in strength with the albedo and h 2 o ice content and when it is properly accounted for the overall slope in all the identified spectral units is the same over the interval 0 35 2 3 m independent of albedo or ice abundance the interpretation of current and previous results offers two different scenarios illustrated by the presence of one vs two dark materials distributed over the iapetus surface we describe the scenarios and their implications the appearance of the aromatic and aliphatic absorption bands together in their measured relative strengths makes this spectral signature unique and thus enables the comparison among the three satellites

dawn the vesta hed connection and the geologic context for eucrites diogenites and howardites the dawn mission has provided new evidence strengthening the identification of asteroid vesta as the parent body of the howardite eucrite and diogenite hed meteorites the evidence includes vesta s petrologic complexity detailed spectroscopic characteristics unique space weathering diagnostic geochemical abundances and neutron absorption characteristics chronology of surface units and impact history occurrence of exogenous carbonaceous chondritic materials in the regolith and dimensions of the core all of which are consistent with hed observations and constraints global mapping of the distributions of hed lithologies by dawn cameras and spectrometers provides the missing geologic context for these meteorites thereby allowing tests of petrogenetic models and increasing their scientific value

a worldwide comparison of predicted s wave delays from a three dimensional upper mantle model with p wave station corrections abstract this paper is a comparison on a worldwide scale between p station corrections deduced from isc residuals dziewonski and anderson and synthetic s station corrections computed using a three dimensional upper mantle model obtained from mantle wave data woodhouse and dziewonski the upper mantle s velocity model is described by a spherical harmonic expansion up to degree 8 the p station corrections are smoothed using a similar expansion in order that the two data sets can be compared correlations between p station corrections t p and synthetic s station corrections t s relevant to various depths of integration indicate that a station correction contains information about structures down to at least 670 km for this depth of integration the correlation coefficient of the two data sets is 0 59 the slope a of the relation t s a t p b obtained for the worldwide distribution of stations is in good agreement with results of previous regional studies using direct readings of p and s arrival times a 3 61 0 13 an analysis of regional variations of the relation t s a t p b is carried out on the basis of two published global tectonic patterns okal jodan results for oceanic regions are not reliable due to the lack of data on continental areas a significant difference appears between mountains a 2 7 0 3 and shields a 4 5 0 4 for okal s pattern a 5 7 1 5 for jordan s pattern the largest a value for shields rules out an explanation by partial melting as proposed in previous studies thermal heterogeneities lead to low a values undulations of the lithosphere asthenosphere boundary appear to be the most feasible explanation of the high slope beneath shields they are also able to explain the range of variation of the station corrections the lowest values of the station corrections correspond to a total vanishing of the low velocity zone beneath the oldest shields for mountains the mean values of the station corrections as well as the low a value can be accounted for by a slight increase of poisson s ratio together with a significant density increase

spectroscopic classification of icy satellites of saturn ii identification of terrain units on rhea abstract dione is one of the largest and densest icy satellites of saturn its surface shows a marked asymmetry between its leading and trailing hemispheres the leading side being brighter than the trailing side which shows regions mantled by a dark veneer whose origin is likely exogenic in order to identify different terrain units we applied the spectral angle mapper sam classification technique to dione s hyperspectral images acquired by the visual and infrared mapping spectrometer vims onboard the cassini orbiter in the infrared range 0 88 5 12 m on a relatively limited portion of the surface of dione we first identified nine spectral endmembers corresponding to as many terrain units which mostly distinguish for water ice abundance and ice grain size we then used these endmembers in sam to achieve a comprehensive classification of the entire surface the analysis of the infrared spectra returned by vims shows that different regions of dione have variations in water ice bands depths in average ice grain size and in the concentration of contaminants such as co 2 and hydrocarbons which are clearly connected to morphological and geological structures generally the spectral units that classify optically dark terrains are those showing suppressed water ice bands a finer ice grain size and a higher concentration of carbon dioxide conversely spectral units labeling brighter regions have deeper water ice absorption bands higher albedo and a smaller concentration of contaminants we also considered vims cubes of the small satellite helene one of the two dione s trojan moons and we compared its infrared spectra to those of the spectral units found on dione we observe that the closest match between the spectra of the two satellites occurs for one of the youngest and freshest terrain units on dione the creusa crater region

finding the trigger to iapetus odd global albedo pattern dynamics of dust from saturn s irregular satellites abstract the leading face of saturn s moon iapetus cassini regio has an albedo only one tenth that on its trailing side the origin of this enigmatic dichotomy has been debated for over 40 years but with new data a clearer picture is emerging motivated by cassini radar and imaging observations we investigate soter s model of dark exogenous dust striking an originally brighter iapetus by modeling the dynamics of the dark dust from the ring of the exterior retrograde satellite phoebe under the relevant perturbations in particular we study the particles probabilities of striking iapetus as well as their expected spatial distribution on the iapetian surface we find that of the long lived particles 5 m most particle sizes 10 m are virtually certain to strike iapetus and their calculated distribution on the surface matches up well with cassini regio s extent in its longitudinal span the satellite s polar regions are observed to be bright presumably because ice is deposited there thus in the latitudinal direction we estimate polar dust deposition rates to help constrain models of thermal migration invoked to explain the bright poles spencer j r denk t 2010 science 327 432 435 we also analyze dust originating from other irregular outer moons determining that a significant fraction of that material will eventually coat iapetus perhaps explaining why the spectrum of iapetus dark material differs somewhat from that of phoebe finally we track the dust particles that do not strike iapetus and find that most land on titan with a smaller fraction hitting hyperion as has been previously conjectured such exogenous dust coupled with hyperion s chaotic rotation could produce hyperion s roughly isotropic moderate albedo surface

diogenites as polymict breccias composed of orthopyroxenite and harzburgite abstract a few relatively unbrecciated olivine rich diogenites consist of an equilibrium assemblage of olivine and magnesian orthopyroxene harzburgite more common diogenites with smaller amounts of olivine are breccias containing two distinct orthopyroxenes one magnesian and one ferroan these diogenites are mixtures of a harzburgite lithology that is more magnesian with the normal orthopyroxenite lithology that is ferroan and may contain small amounts of plagioclase both lithologies likely formed by fractional crystallization in multiple plutons emplaced within the crust of asteroid 4 vesta minor element trends in orthopyroxenes indicate that these plutons exhibited a range of compositions we propose a revised taxonomy for the hed howardites eucrites and diogenites suite where all ultramafic samples are referred to as diogenites within this group the prefixes dunitic harzburgitic and orthopyroxenitic are used to distinguish diogenites consisting of more than or equal to 90 olivine olivine orthopyroxene and more than or equal to 90 orthopyroxene respectively the prefix polymict is used to describe brecciated mixtures of any of these rock types the recognition that olivine is a significant phase in some diogenites is consistent with spectral interpretations of olivine in a deeply excavated crater on vesta and has important implications for the bulk composition and petrogenesis of that body

magnesium oxide iron oxide mass balance constraints and a more detailed model for the relationship between eucrites and diogenites according to a currently popular model for petrogenesis on the howardite eucrite and diogenite hed parent asteroid the diogenites are not comagmatic with most eucrites but instead formed in separate orthopyroxenite dominated plutons this model can be tested for consistency with mass balance for mgo and feo assuming the overall diogenite diogenite eucrite ratio d of the parent asteroid is at least comparable to the average d for the eucrite diogenite dominated howardite regolith breccias average mg mgo mgo feo is much lower for eucrites especially noncumulate eucrites than for diogenites unless the diogenite parent magmas eventually produced a large proportion of low mg residual basalt and gabbro rbg the implied initial magma s mg is vastly higher than that of any noncumulate eucrite starting from a source previously depleted by putative primary eucrite genesis melt mg can be estimated as a function of the exchange reaction kd and degree of melting using several very conservative assumptions e g assuming that the total mgo feo concentration is nearly the same in the nascent melt as in the residual solids the degree of melting required to yield a melt with mg high enough to satisfy mass balance without implying an rbg component that accounts for 50 of all eucrites is an implausibly high 60 80 wt the separate orthopyroxenitic plutons sop model also seems inconsistent with the uniform density of melts across the diogenite eucrite compositional spectrum 2 77 2 82 g cm3 which implies that diogenitic magmas should have been as capable as eucrites of extruding to form lavas this difficulty cannot be reduced by simply assuming that later formed magmas were systematically both more plutonic and more mgo rich than earlier ones because the plutonic cumulate eucrites equilibrated with melts systematically lower in mg than noncumulate eucrites conceivably the bulk mg of the asteroid s silicate system was increased between primary melt eucrite genesis and sop diogenite genesis by graphite fueled reduction of feo however the graphite oxidation process generates a huge proportion of gas which would have enhanced the buoyancy of the nascent diogenite parent magmas thus exacerbating the difficulty of achieving the implied high degrees of partial melting to avoid these difficulties but still form most eucrites as rapidly cooled extrusives i propose the nerd noncumulate eucrites as extruded residua of diogenites model in this model the diogenites form as early cumulates from a large magma system probably a global magma ocean that yields a large proportion of eucritic melt as residuum this residual melt zone undergoes relatively little crystallization during a period when it is episodically tapped to produce extrusions dikes and sills of rapidly cooled noncumulate eucrites slight 5 10 porosity in the nascent eucritic crust keeps it marginally buoyant over the residual melt zone the common thermal metamorphism of noncumulate eucrites results from baking by superjacent flows during the episodic venting of the melt zone the nerd model s greatest advantage is that it does not require implausibly high degrees of localized melting in the mature stages of igneous evolution of the hed asteroid

al sm eu sr systematics of eucrites and moon rocks implications for planetary bulk compositions abstract analysis of the eu and sr anomalies of eucrites and lunar rocks allows constraints to be placed on the bulk compositions of the eucrite parent body epb and the moon the elements al ree and sr all are essentially incompatible with the major minerals of these small low o 2 bodies except for plagioclase into which al sr and eu tend to concentrate therefore the hypothesis that al ree and sr in the epb and the moon are all in proportions close to those in the bulk solar system i e chondrites leads to certain predictions about the concentrations of these elements in samples affected by plagioclase fractionation the predictions are almost ideally fulfilled by eucrites and lunar samples for the epb the ratios ree al sr al and sr ree are constrained to be probably within 10 almost certainly within 20 of the chondritic ratios for the more complicated moon the constraints are less precise ree al is very probably within 25 of chondritic sr al and sr ree are probably within 35 of chondritic these findings are proof that there is a strong similarity between the bulk compositions of the planets and the compositions of chondritic meteorites the eucrites sm eu sr systematics are also valuable sources of constraints on the distribution coefficients for eu and sr into plagioclase at low o 2 from the slope of data for noncumulate eucrites on a eu sm plot d eu pl liq can be inferred to be 1 1 0 1 0 2 from the slope on a sr sm plot d sr pl liq can be inferred to be 1 5 0 3 in the case of d eu this is in excellent agreement with experimental data in the case of d sr the empirical result is probably more appropriate for eucritic systems than most experimental data which due to compositional effects scatter widely

surface composition of hyperion hyperion saturn s eighth largest icy satellite is a body of irregular shape in a state of chaotic rotation the surface is segregated into two distinct units a spatially dominant high albedo unit having the strong signature of h2o ice contrasts with a unit that is about a factor of four lower in albedo and is found mostly in the bottoms of cup like craters here we report observations of hyperion s surface in the ultraviolet and near infrared spectral regions with two optical remote sensing instruments on the cassini spacecraft at closest approach during a fly by on 25 26 september 2005 the close fly by afforded us the opportunity to obtain separate reflectance spectra of the high and low albedo surface components the low albedo material has spectral similarities and compositional signatures that link it with the surface of phoebe and a hemisphere wide superficial coating on iapetus

distinctive space weathering on vesta from regolith mixing processes whereas space weathering of some airless bodies such as the moon occurs through the accumulation on regolith of nanophase metallic particles spectroscopic data show that space weathering of the asteroid vesta occurs through the small scale mixing of diverse surface components which gradually generates locally homogenized upper regolith between 16 july 2011 and 5 september 2012 nasa s space probe dawn was orbiting vesta a protoplanet thought to have survived virtually intact since an early phase of solar system formation in this issue of nature two groups report on the encounter carle pieters and co workers find that space weathering on vesta has followed a different course from that observed on the moon and on itokawa the asteroid sampled in an earth return mission on vesta weathering involved fine scale regolith soil mixing that has removed clear traces of recent impact deposits there are no signs of the nanophase metallic particle deposits seen on the moon and itokawa thomas mccord and co authors describe two main types of material on vesta s surface bright and dark the bright material may be uncontaminated indigenous vesta basaltic soil with the darker material derived from low albedo impactors dawn has now moved on and is due to rendezvous with the protoplanet ceres in february 2015 the surface of the asteroid vesta has prominent near infrared absorption bands characteristic of a range of pyroxenes confirming a direct link to the basaltic howardite eucrite diogenite class of meteorites1 2 3 processes active in the space environment produce space weathering products that substantially weaken or mask such diagnostic absorption on airless bodies observed elsewhere4 5 and it has long been a mystery why vesta s absorption bands are so strong analyses of soil samples from both the moon6 and the asteroid itokawa7 determined that nanophase metallic particles commonly nanophase iron accumulate on the rims of regolith grains with time accounting for an observed optical degradation these nanophase particles believed to be related to solar wind and micrometeoroid bombardment processes leave unique spectroscopic signatures that can be measured remotely8 9 10 but require sufficient spatial resolution to discern the geologic context and history of the surface which has not been achieved for vesta until now here we report that vesta shows its own form of space weathering which is quite different from that of other airless bodies visited no evidence is detected on vesta for accumulation of lunar like nanophase iron on regolith particles even though distinct material exposed at several fresh craters becomes gradually masked and fades into the background as the craters age instead spectroscopic data reveal that on vesta a locally homogenized upper regolith is generated with time through small scale mixing of diverse surface components

composition of the rheasilvia basin a window into vesta s interior 1 the estimated excavation depth of the huge rheasilvia impact basin is nearly twice the likely thickness of the vestan basaltic crust so the mantle should be exposed spectral mapping by the dawn spacecraft reveals orthopyroxene rich materials similar to diogenite meteorites in the deepest parts of the basin and within its walls significant amounts of olivine are predicted for the mantles of bulk chondritic bodies like vesta and its occurrence is demonstrated by some diogenites that are harzburgite and dunite however olivine has so far escaped detection by dawn s instruments spectral detection of olivine in the presence of orthopyroxene is difficult in samples with 25 olivine and olivine in rheasilvia might have been diluted during impact mixing or covered by the collapse of basin walls the distribution of diogenite inferred from its exposures in and around rheasilvia provides a geologic context for the formation of these meteorites but does not clearly distinguish between a magmatic cumulate versus partial melting restite origin for diogenites the former is favored by geochemical arguments and crystallization in either a magma ocean or multiple plutons emplaced near the crust mantle boundary is permitted by dawn observations

spectrophotometry of the small satellites of saturn and their relationship to iapetus phoebe and hyperion abstract we have obtained broadband spectrophotometric observations of four of the recently discovered small satellites of saturn gladman et al 2001 nature 412 163 166 the new data enable an understanding of the provenance composition and interrelationships among these satellites and the other satellites of saturn particularly iapetus phoebe and hyperion temporal coverage of one satellite s21 tarvos was sufficient to determine a partial rotational lightcurve our major findings include 1 the satellites are red and similar in color comparable to d type asteroids some kbos iapetus and hyperion 2 none of the satellites including those from the phoebe group has any spectrophotometric relationship to phoebe and 3 s21 tarvos exhibits a rotational lightcurve although the data are not well constrained and more observations are required to fit a definitive period dust created by meteoritic impacts and ejected from these satellites and additional undiscovered ones may be the source of the exogenous material deposited on the low albedo side of iapetus recent work which states that the small irregular satellites of saturn have impacted phoebe at least 6 7 times in the age of the solar system nesvorny et al 2003 astron j 126 398 429 suggests that such collisions may have propelled additional material from both phoebe and the small irregular satellites toward iapetus the accretion of material from outer retrograde satellites may be a process that also occurs on callisto and the uranian satellites

petrologic and textural diversity among the pca 02 howardite group one of the largest pieces of the vestan surface abstract nine howardites and two diogenites were recovered from the pecora escarpment icefield pca in 2002 cosmogenic radionuclide abundances indicate that the samples are paired and that they constituted an approximately 1 m diameter meteoroid prior to atmospheric entry at about 1 m in diameter the pca 02 hed group represents one of the largest single pre atmospheric pieces of the vestan surface yet described mineral and textural variations were measured in six of the pca 02 howardites to investigate meter scale diversity of the vestan surface mineral compositions span the range of known eucrite and diogenite compositions additional non diogenitic groups of mg and fe rich olivine are observed and are interpreted to have been formed by exogenic contamination and impact melting respectively these howardites contain olivine rich impact melts that likely formed from dunite and harzburgite rich target rocks containing the first recognized olivine rich hed impact melts these samples provide meteoritic evidence that olivine rich lithologies have been exposed on the surface of vesta finally we present a new method for mapping distributions of lithologies in howardites using 8 elemental x ray maps proportions of diogenite and eucrite vary considerably among the pca 02 howardites suggesting they originated from a heterogeneous portion of the vestan surface while whole sample modes are dominated by diogenite the finer grain size fractions are consistently more eucritic this discrepancy has implications for near infrared spectral observations of portions of vesta s surface that are similar to the pca 02 howardites as the finer grained eucritic material will disproportionately dominate the spectra

an endogenic origin of cerean organics abstract localized deposits of aliphatic organic rich material have been found on the surface of dwarf planet ceres and based on mineralogical context it has been suggested that these organics formed within ceres de sanctis et al 2017 however no obvious source and exposure mechanism can be inferred from the geological settings of the organic rich terrains the alternative hypothesis is that the organics were delivered from an exogenic source via impact pieters et al 2018 we numerically simulate the thermal degradation of aliphatic organics delivered to ceres during hypervelocity impacts as well as the thermal degradation and excavation of organics from the subsurface our models show that exogenic delivery of aliphatic organics is inefficient as most of the spectral signature of the organic species would be thermally degraded and diluted by mixing with target material in the ejecta blanket of a given crater instead our models suggest that the formation of ernutet crater excavated a subsurface reservoir of aliphatic organics from a depth of 3 6 km bringing it close to the surface where it could be subsequently exhumed by small craters and landslides compatible with observations

iapetus surface variability revealed from statistical clustering of a vims mosaic the distribution of co2 abstract we present a detailed study of an iapetus mosaic of vims data with high spatial resolution 0 5 0 5 or 6 4 km pixel the spectra were taken in august 2007 and provide the highest vims spatial resolution data for this object during cassini s primary mission we analyze this set of data using a statistical clustering approach to reduce the analysis of a large number of data 10 4 spectra from 0 35 to 5 10 m to the study of seven representative groups accounting for 99 6 of the surface covered by the original sample we analyze the spectral absorption bands in the spectra of the different clusters indicative of different composition over the observed surface we find coherence between the distribution of the clusters and the geographical features on the surface we give special attention to the study of the water ice and co 2 bands we find that co 2 is widespread over the entire surface being studied including the bright and dark areas on iapetus surface and is probably trapped at the molecular level with other materials the strength of the co 2 band in the areas where both h 2 o and carbon bearing materials exist gives support to the hypothesis that this volatile is formed on the surface of iapetus as a product of irradiation of these two components finally we also compare the iapetus co 2 with that on other satellites confirming that there are evident differences on the center depth and width of the band on iapetus and phoebe where co 2 has been suggested to be endogenous

iapetus near surface thermal emission modeled and constrained using cassini radar radiometer microwave observations abstract since its arrival at saturn the cassini spacecraft has had only a few opportunities to observe iapetus saturn s most distant regular satellite these observations were all made from long ranges 100 000 km except on september 10 2007 during cassini orbit 49 when the spacecraft encountered the two toned moon during its closest flyby so far in this pass it collected spatially resolved data on the object s leading side mainly over the equatorial dark terrains of cassini regio cr in this paper we examine the radiometry data acquired by the cassini radar during both this close targeted flyby referred to as ia49 3 and the distant iapetus observations in the radar s passive mode the receiver functions as a radiometer to record the thermal emission from planetary surfaces at a wavelength of 2 2 cm on the cold icy surfaces of saturn s moons the measured brightness temperatures depend both on the microwave emissivity and the physical temperature profile below the surface down to a depth that is likely to be tens of centimeters or even a few meters combined with the concurrent active data passive measurements can shed light on the composition structure and thermal properties of planetary regoliths and thus on the processes from which they have formed and evolved the model we propose for iapetus microwave thermal emission is fitted to the ia49 3 observations and reveals that the thermal inertias sensed by the cassini radiometer over both cr and the bright mid to high latitude terrains namely ronceveaux terra rt in the north and saragossa terra st in the south significantly exceed those measured by cassini s cirs composite infrared spectrometer which is sensitive to much smaller depths generally the first few millimeters of the surface this implies that the subsurface of iapetus sensed at 2 2 cm wavelength is more consolidated than the uppermost layers of the surface in the case of cr a thermal inertia of at least 50 j m 2 k 1 s 1 2 and most probably 200 j m 2 k 1 s 1 2 is inferred this suggests a gradient in density with depth or more likely that the radiometer has probed the icy substrate underlying the dark layer furthermore the measured thermal emission is found to arise from the upper few meters of the subsurface which points to tholins rather than iron oxide compounds as the primary contaminants of the dark material we also find that although there is a latitudinal decrease probably related to the thinning of the dark layer away from the equator the cr region exhibits a high 2 2 cm emissivity 0 87 in average which is close to the emissivity of phoebe a putative source of the dark matter in the case of rt st model fitting points to a mean thermal inertia of 160 j m 2 k 1 s 1 2 along with the possible presence of an absorbing compound in the regolith of the bright terrains nevertheless this layer is transparent enough for the radiometer to capture the seasonal contrast between the northern and southern hemispheres lastly a global decline of the microwave emissivity with latitude is revealed it is probably indicative of a progressive increase of the water ice content in the near surface