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1.
Vesta’s crater sizes that are based on images returned from the Dawn probe orbiting the asteroid 4 Vesta are compared with sizes of spots on its surface derived from hydrosilicate equivalent widths and asteroid color indices B-V and V-R observed spectrophotometrically on earth using a spectral-frequency method. The sizes of craters and spots at the asteroid poles prove to correspond to the sizes of spots that are assumed to be located at latitudes of 40°...45° N and S. Comparative results for the crater sizes ranging from 10 to 100 km are tabulated. We conclude that crater sizes on asteroid surfaces can be determined using the spectral-frequency method.  相似文献   

2.
The sizes of color patches on the surface of the asteroid 4 Vesta were estimated with the spectral-frequency method. We used the digital records of the asteroid spectra obtained on February 2, 3, 4, and 7, 2002, with the TV system and the slitless afocal spectrograph of the MTM-500 telescope at the Research Institute of the Crimean Astrophysical Observatory. The spectral resolution caused by the image quantity was about 40 Å and the exposure duration was 30 s. The energy calibration was performed both with the artificial light-emitting diode standard and three standard stars. The synthetic color indices B-V and V-R were calculated from the asteroid spectra taken out of the atmosphere. From these data, the changes with an asteroid rotation phase were eliminated. After that, the periods that were significantly smaller than the asteroid rotation period were searched for, and each time the data were whitened for the obtained period. Assuming that the patch sizes are determined by the half-obtained period and that the patches are located in the equatorial region of the asteroid, we estimated the sizes of 20 and 19 patches in the long-and short-wavelength ranges, respectively. The smallest found patch was about 9 km across. The statistical estimates of the reddish patches and the comparison with the statistics of old craters allowed us to suggest that the reddish patches on the asteroid Vesta surface are old formations.  相似文献   

3.
David P. O’Brien 《Icarus》2009,203(1):112-118
The near-Earth Asteroids Eros and Itokawa show a pronounced lack of small (?100 m) craters, the vast majority of which were formed during their time in the main belt, and this has been cited as possible evidence that small (?10 m) impactors are efficiently removed from the main belt by the Yarkovsky effect. Using well-tested models for the evolution of the main-belt size distribution and the evolution of crater populations on asteroid surfaces, I show that a pronounced lack of small impactors would require size-dependent removal far stronger than can result from the Yarkovsky effect (or any other known process). Furthermore, such strong removal would lead to wavelike perturbations in the main-belt and near-Earth asteroid size distributions that are inconsistent with their observed size distributions, as well as the cratering records on asteroid surfaces. A more likely explanation is that processes on asteroid surfaces, such as seismic shaking, are responsible for erasing small craters after they form.  相似文献   

4.
The spectral-frequency method (Busarev et al., 2007b) allowed us to obtain data on 16 sizes of hydrosilicate spots on the surface of the asteroid 4 Vesta. Large sizes (800 and 750 km) show clusters of small hydrosilicate spots near a well-known crater. Small spots of 50–13 km cover more than 50% of the surface. The predominant number of small-sized spots suggests their recent origin. Our data confirm the presence of water combinations on the surface of 4 Vesta and allow us to draw a conclusion on their recent appearance in collisions of this asteroid with primitive bodies arriving from the zone of Jupiter.  相似文献   

5.
Impact-induced seismic vibrations have long been suspected of being an important surface modification process on small satellites and asteroids. In this study, we use a series of linked seismic and geomorphic models to investigate the process in detail. We begin by developing a basic theory for the propagation of seismic energy in a highly fractured asteroid, and we use this theory to model the global vibrations experienced on the surface of an asteroid following an impact. These synthetic seismograms are then applied to a model of regolith resting on a slope, and the resulting downslope motion is computed for a full range of impactor sizes. Next, this computed downslope regolith flow is used in a morphological model of impact crater degradation and erasure, showing how topographic erosion accumulates as a function of time and the number of impacts. Finally, these results are applied in a stochastic cratering model for the surface of an Eros-like body (same volume and surface area as the asteroid), with craters formed by impacts and then erased by the effects of superposing craters, ejecta coverage, and seismic shakedown. This simulation shows good agreement with the observed 433 Eros cratering record at a Main Belt exposure age of 400±200 Myr, including the observed paucity of small craters. The lowered equilibrium numbers (loss rate = production rate) for craters less than ∼100 m in diameter is a direct result of seismic erasure, which requires less than a meter of mobilized regolith to reproduce the NEAR observations. This study also points to an upper limit on asteroid size for experiencing global, surface-modifying, seismic effects from individual impacts of about 70-100 km (depending upon asteroid seismic properties). Larger asteroids will experience only localized (regional) seismic effects from individual impacts.  相似文献   

6.
To study the mineralogical composition of an asteroid’s surface with a technique of colorimetry of small-scale details, we propose fast spectrophotometry with a low-resolution imaging spectrograph and a time resolution of a few minutes. In contrast to global tomography, our method allows only the features of small scale, as compared to the asteroid’s size, to be detected. As an example, the surface of asteroid 130 Elektra exhibits approximately 16 spots of different mineralogical composition; their sizes range from 13 to 30 km. Except for several cases, the absorption spectra of these features are typical of the olivine–pyroxene compounds. In principle, color characteristics of the spots in the UBVRI color system allow the mineralogical composition of the surface of a small celestial body to be identified.  相似文献   

7.
Preliminary measurements of craters and boulders have been made in various locations on Eros from images acquired during the first nine months of NEAR Shoemaker's orbital mission, including the October 2000 low altitude flyover. (We offer some very preliminary, qualitative analysis of later LAF images and very high-resolution images obtained during NEAR's landing on 12 February 2001). Craters on Eros >100 m diameter closely resemble the saturated crater population of Ida; Eros is more heavily cratered than Gaspra but lacks the saturated giant craters of Mathilde. These craters and the other large-scale geological features were formed over a duration of very roughly 2 Gyr while Eros was in the main asteroid belt, between the time when its parent body was disrupted and Eros was injected into an Earth-approaching orbit (probably tens of Myr ago). Saturation equilibrium had been expected to shape Eros' crater population down to very small sizes, as on the lunar maria. However, craters <200 m diameter are instead progressively depleted toward smaller sizes and are a factor of ∼200 below empirical saturation at diameters of 4 m. Conversely, boulders and positive relief features (PRFs) rise rapidly in numbers (differential power-law index ∼−5) and those <10 m in size dominate the landscape at high resolutions. The pervasive boulders and minimal craters on Eros is radically different from the lunar surface at similar scales. This may be partly explained by a major depletion of meter-scale projectiles in the asteroid belt (due to the Yarkovsky Effect: Bell 2001), which thus form few small craters and destroy few boulders. Additionally, the small size and low gravity of Eros may result in redistribution or loss of ejecta due to seismic shaking, thus preferentially destroying small craters formed in such regolith. Possibly Eros has only a patchy, thin regolith of mobile fines; the smaller PRFs may then reflect exposures of fractured bedrock or piles of large ejecta blocks, which might further inhibit formation of craters <10 m in size. Eros may well have been largely detached dynamically and collisionally from the main asteroid belt for the past tens of Myr, in which case its cratering rate would have dropped by two orders of magnitude, perhaps enhancing the relative efficacy of other processes that would normally be negligible in competition with cratering. Such processes include thermal creep, electrostatic levitation and redistribution of fines, and space weathering (e.g., bombardment by micrometeorites and solar wind particles). Combined with other small-body responses to impact cratering (e.g., greater widespread distribution of bouldery ejecta), such processes may also help explain the unexpected small-scale character of geology on Eros. If there was a recent virtual hiatus in cratering of Eros (during which only craters <∼300 m diameter would be expected to have formed), space weathering may have reached maturity, thus explaining Eros' remarkable spectral homogeneity compared with Ida.  相似文献   

8.
The surface topography of Asteroid 25143 Itokawa is explored using the LIght Detection And Ranging instrument (LIDAR). The data confirm the presence of a rough highland and a smooth lowland. The highland is dominated by boulders, but also possesses topography associated with surface lineaments and broad surface facets. The boulders ensure that the roughness of the highlands over short distances is typically greater relative to most surfaces on 433 Eros. Over larger distances, Itokawa is always smoother than Eros possibly because of its smaller size and weak rubble-pile structure. The lowlands of Itokawa are very smooth, and are typically devoid of boulders. Some transitional regions midway between the highlands and lowlands also exist. In these areas, craters that retain their regolith fill possess flat floors and resemble “ponds” seen on 433 Eros. Analyses of surface elevation, imagery and a quantitative measure of surface roughness are consistent with regolith flowing downhill from the highlands to fill in the low areas of Itokawa, probably covering up any pre-existing rough terrain. Using this interpretation, we find a minimum 2.3±0.4 m thick layer of regolith in the lowlands, which, if spread evenly across the entire asteroid, corresponds to a 42±1 cm thick layer. It is very difficult to generate this amount of regolith with the population of craters seen on Itokawa. However, an Itokawa composed of several large masses may have retained this regolith during its formation. The presence of such large masses could account for the observed lineaments and what appear to be exposures of bedrock on the largest steep slope observed.  相似文献   

9.
Abstract— The global high‐resolution imaging of asteroid 433 Eros by the Near‐Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft has made it possible to develop the first comprehensive picture of the geology of a small S‐type asteroid. Eros displays a variety of surface features, and evidence of a substantial regolith. Large scale facets, grooves, and ridges indicate the presence of at least one global planar structure. Directional and superposition relations of smaller structural features suggest that fracturing has occurred throughout the object. As with other small objects, impact craters dominate the overall shape as well as the small‐scale topography of Eros. Depth/diameter ratios of craters on Eros average ～0.13, but the freshest craters approach lunar values of ～0.2. Ejecta block production from craters is highly variable; the majority of large blocks appear to have originated from one 7.6 km crater (Shoemaker). The interior morphology of craters does not reveal the influence of discrete mechanical boundaries at depth in the manner of craters formed on lunar mare regolith and on some parts of Phobos. This lack of mechanical boundaries, and the abundant evidence of regolith in nearly every high‐resolution image, suggests a gradation in the porosity and fracturing with depth. The density of small craters is deficient at sizes below ～200 m relative to predicted slopes of empirical saturation. This characteristic, which is also found on parts of Phobos and lunar highland areas, probably results from the efficient obliteration of small craters on a body with significant topographic slopes and a thick regolith. Eros displays a variety of regolith features, such as debris aprons, fine‐grained “ponded” deposits, talus cones, and bright and dark streamers on steep slopes indicative of efficient downslope movement of regolith. These processes serve to mix materials in the upper loose fragmental portion of the asteroid (regolith). In the instance of “ponded” materials and crater wall deposits, there is evidence of processes that segregate finer materials into discrete deposits. The NEAR observations have shown us that surface processes on small asteroids can be very complex and result in a wide variety of morphologic features and landforms that today seem exotic. Future missions to comets and asteroids will surely reveal still as yet unseen processes as well as give context to those discovered by the NEAR Shoemaker spacecraft.  相似文献   

10.
Cratering rates on the Galilean satellites   总被引：1，自引：0，他引：1  
Zahnle K  Dones L  Levison HF 《Icarus》1998,136(2):202-222
We exploit recent theoretical advances toward the origin and orbital evolution of comets and asteroids to obtain revised estimates for cratering rates in the jovian system. We find that most, probably more than 90%, of the craters on the Galilean satellites are caused by the impact of Jupiter-family comets (JFCs). These are comets with short periods, in generally low-inclination orbits, whose dynamics are dominated by Jupiter. Nearly isotropic comets (long period and Halley-type) contribute at the 1-10% level. Trojan asteroids might also be important at the 1-10% level; if they are important, they would be especially important for smaller craters. Main belt asteroids are currently unimportant, as each 20-km crater made on Ganymede implies the disruption of a 200-km diameter parental asteroid, a destruction rate far beyond the resources of today's asteroid belt. Twenty-kilometer diameter craters are made by kilometer-size impactors; such events occur on a Galilean satellite about once in a million years. The paucity of 20-km craters on Europa indicates that its surface is of order 10 Ma. Lightly cratered surfaces on Ganymede are nominally of order 0.5-1.0 Ga. The uncertainty in these estimates is about a factor of five. Callisto is old, probably more than 4 Ga. It is too heavily cratered to be accounted for by the current flux of JFCs. The lack of pronounced apex-antapex asymmetries on Ganymede may be compatible with crater equilibrium, but it is more easily understood as evidence for nonsynchronous rotation of an icy carapace.  相似文献   

11.
The outcomes of asteroid collisional evolution are presently unclear: are most asteroids larger than 1 km size gravitational aggregates reaccreted from fragments of a parent body that was collisionally disrupted, while much smaller asteroids are collisional shards that were never completely disrupted? The 16 km mean diameter S-type asteroid 433 Eros, visited by the NEAR mission, has surface geology consistent with being a fractured shard. A ubiquitous fabric of linear structural features is found on the surface of Eros and probably indicates a globally consolidated structure beneath its regolith cover. Despite the differences in absolute scale and in lighting conditions for NEAR and Hayabusa, similar features should have been found on 25143 Itokawa if present. This much smaller, 320 m diameter S-asteroid was visited by the Hayabusa spacecraft. Comparative analyses of Itokawa and Eros geology reveal fundamental differences, and interpretation of Eros geology is illuminated by comparison with Itokawa. Itokawa lacks a global lineament fabric, and its blocks, craters, and regolith may be inconsistent with formation and evolution as a fractured shard, unlike Eros. An object as small as Itokawa can form as a rubble pile, while much larger Eros formed as a fractured shard. Itokawa is not a scaled-down Eros, but formed by catastrophic disruption and reaccumulation.  相似文献   

12.
William K. Hartmann 《Icarus》1980,44(2):441-453
Observational and theoretical data converge on the conclusion that planetesimals in Jupiter's region of the solar nebula were initially composed predominantly of a mixture of roughly 39–70% H2O ice by volume, and 30–61% dark stony material resembling carbonaceous chondrites. Recent observations emphasize a division of most asteroid and satellite surfaces in this region into two distinct groups: bright icy material and dark stony material. The present model accounts for these by two main processes: an impact-induced buildup of a dark stony regolith in the absence of surface thermal disturbance, and thermal-disturbance-induced eruption of “water magmas” that create icy surfaces. “Thermal disturbances” include tidal and radiative effects caused by nearness of a planet. A correlation of crater density and albedo, Ganymede's dark-ray craters, and other observed phenomena (listed in the summary) appear consistent with the model discussed here.  相似文献   

13.
Abstract— If impact stress reverberation is the primary gradational process on an asteroid at global scales, then the largest undegraded crater records an asteroid's seismological response. The critical crater diameter Dcrit is defined as the smallest crater whose formation disrupts all previous craters globally up to its size; it is solved for by combining relationships for crater growth and for stress attenuation. The computation for Dcrit gives a simple explanation for the curious observation that small asteroids have only modest undegraded craters, in comparison to their size, whereas large asteroids have giant undegraded craters. Dcrit can even exceed the asteroid diameter, in which case all craters are “local” and the asteroid becomes crowded with giant craters. Dcrit is the most recent crater to have formed on a blank slate; when it is equated to the measured diameter of the largest undegraded crater on known asteroids, peak particle velocities are found to attenuate with the 1.2–1.3 power of distance—less attenuative than strong shocks, and more characteristic of powerful seismic disturbances. This is to be expected, since global degradation can result from seismic (cm s?1) particle velocities on small asteroids. Attenuation, as modeled, appears to be higher on asteroids known to be porous, although these are also bodies for which different crater scaling rules might apply.  相似文献   

14.
We develop a physical model for the evolution of regoliths on small bodies and apply it to the asteroids and meteorite parent bodies. The model considers global deposition of that fraction of cratering ejecta that is not lost to space. It follows the build up of regolith on a typical region, removed from the larger craters which are the source of most regolith blankets. Later in the evolution, larger craters saturate the surface and are incorporated into the typical region; their net ejection of materials to space causes the elevation of the typical region to decrease and once-buried regolith becomes susceptible to ejection or gardening. The model is applied to cases of both strong, cohesive bodies and to bodies of weak, unconsolidated materials. Evolution of regolith depths and gardening rates are followed until a sufficiently large impact occurs that fractures the entire asteroid. (Larger asteroids are not dispersed, however, and evolve mergaregoliths from multiple generations of surficial regoliths mixed into their interiors.) We find that large, strong asteroids generate surficial regoliths of a few kilometers depth while strong asteroids smaller than 10-km diameter generate negligible regoliths. Our model does not treat large, weak asteroids, because their cratering ejecta fail to surround such bodies; regolith evolution is probably similar to that of the Moon. Small, weak asteroids of 1- to 10-km diameter generate centimeter- to meter-scale regoliths. In all cases studied, blanketing rates exceed excavation rates, so asteroid regoliths are rarely, if ever, gardened and should be very immature measured by lunar standards. They should exhibit many of the characteristics of the brecciated, gas-rich meteorites; intact foreign clasts, relatively low-exposure durations to galactic and solar cosmic rays low solar gas contents, minimal evidence for vitrification and agglutinate formation, etc. Both large, strong asteroids and small, weak ones provide regolith environments compatible with those inferred for the parent bodies of brecciated meteorites. But from volumetric calculations, we conclude that most brecciated meteorites formed on the surfaces of, and were recycled through the interiors of, parent bodies at least several tens of kilometers in diameter. The implications of our regolith model are consistent with properties inferred for asteroid regoliths from a variety of astronomical measurements of asteroids, although such data do not constrain regolith properties nearly as strongly as meteoritical evidence Our picture of substantial asteroidal regoliths produced predominantly by blanketing differs from earlier hypotheses that asteroidal regoliths might be thin or absent and that short surface exposure of asteroidal materials is due chiefly to erosion rather than blanketing.  相似文献   

15.
Abstract— Impact-induced comminution of planetary surfaces is pervasive throughout the solar system and occurs on submillimeter to global scales, resulting in comminution products that range from fine-grained surface soils, to massive, polymict ejecta deposits, to collisionally fragmented objects. Within this wide range of comminution products, we define regoliths in a narrow sense as materials that were processed by repetitive impacts to dimensional scales comparable to or smaller than that of component minerals of the progenitor rock(s). In this paper, we summarize a wide variety of impact experiments and other observations that were primarily intended to understand the evolution of regoliths on lunar basalt flows, and we discuss some of their implications for asteroidal surfaces. Cratering experiments in both rock and noncohesive materials, combined with photogeologic observations of the lunar surface, demonstrate that craters <500 m in diameter contribute most to the excavation of local bedrock for subsequent processing by micrometeorites. The overall excavation rate and, thus, growth rate of the debris layer decreases with time, because the increasingly thicker fragmental layer will prevent progressively larger projectiles from reaching bedrock. Typical growth rates for a 5 m thick lunar soil layer are initially (～≥3 Ga ago) a few mm/Ma and slowed to <1 mm/Ma at present. The coarse-grained crater ejecta are efficiently comminuted by collisional fragmentation processes, and the mean residence time of a 1 kg rock is typically 10 Ma. The actual comminution of either lithic or monomineralic detritus is highly mineral specific, with feldspar and mesostasis comminuting preferentially over pyroxene and olivine, thus resulting in mechanically fractionated fines, especially at grain sizes <20 μm. Such fractionated fines also participate preferentially in the shock melting of lunar soils, thus giving rise to “agglutinate” melts. As a consequence, agglutinate melts are systematically enriched in feldspar components relative to the bulk composition of their respective host soil(s). Compositionally homogeneous, impact derived glass beads in lunar soils seem to result from micrometeorite impacts on rock surfaces, reflecting lithic regolith components and associated mineral mixtures. Cumulatively, experimental and observational evidence from lunar mare soils suggests that regoliths derive substantially from the comminution of local bedrock; the addition of foreign, exotic components is not necessary to explain the modal and chemical compositions of diverse grain size fractions from typical lunar soils. Regoliths on asteroids are qualitatively different from those of the Moon. The modest impact velocities in the asteroid belt, some 5 km s?1, are barely sufficient to produce impact melts. Also, substantially more crater mass is being displaced on low-gravity asteroids compared to the Moon; collisional processing of surface boulders should therefore be more prominent in producing comminuted asteroid surfaces. These processes combine into asteroidal surface deposits that have suffered modest levels of shock metamorphism compared to the Moon. Impact melting does not seem to be a significant process under these conditions. However, the role of cometary particles encountering asteroid surfaces at presumably higher velocities has not been addressed in the past. Unfortunately, the asteroidal surface processes that seemingly modify the spectral properties of ordinary chondrites to match telescopically obtained spectra of S-type asteroids remain poorly understood at present, despite the extensive experimental and theoretical insights summarized in this report and our fairly mature understanding of lunar surface processes and regolith evolution.  相似文献   

16.
Calculations are made to determine the sizes of stone and iron meteoroids which could penetrate the atmosphere of Venus and cause hypervelocity impact craters on the planet's surface. Using scaling relationships based on kinetic energy, impact crater size is related to meteoriod size. Finally, it is determined that the smallest impact craters that might exist on Venus are on the order of 150 to 300 meters in diameter.  相似文献   

17.
The NEAR laser rangefinder (NLR) obtained more than 16 million range returns from asteroid 433 Eros. We present the first results from analyses of topographic profiles interpreted with the aid of simultaneous, boresighted images obtained by the NEAR multispectral imager (MSI). The location of the NLR boresight relative to that of MSI is determined by detailed correlations of ranging data and simultaneous images, including cases where the laser boresight slewed off and on the limb of the asteroid and cases where the laser illuminated a boulder close to the time of an image. In the data presented, the precision of the range measurements is about 1 m, with the minimum spot diameter under 5 m, and successive spots are contiguous or overlapping. Elevation on the irregular object Eros is given with respect to the gravitational and centrifugal potential. Landslides in craters are characterized. Possible crater benches are identified. Examples of infilled craters are presented. These observations suggest a depth of unconsolidated regolith, which is subject to sliding, of typically a few tens of meters. An example of structurally controlled cratering is presented. Examples of tectonic features are described. Surface roughness on Eros is approximately self-affine from scales of a few meters to hundreds of meters. A comparison of fractal statistics shows that Eros is extremely rough on observed scales, when compared to terrestrial a'a lava on submeter scales and undisturbed lunar regolith on subcentimeter scales.  相似文献   

18.
The maximum size of impact craters on finite bodies marks the largest impact that can occur short of impact induced disruption of the body. Recently attention has started to focus on large craters on small bodies such as asteroids and rocky and icy satellites. Here the large crater on the recently imaged Asteroid (2867) Steins (with crater diameter to mean asteroid radius ratio of 0.79) is shown to follow a limit set by other similar sized bodies with moderate macroporosity (i.e. fractured asteroids). Thus whilst large, the crater size is not novel, nor does it require Steins to possess an extremely large porosity. In one of the components of the binary Asteroid (90) Antiope there is the recently reported presence of an extremely large depression, possibly a crater, with depression diameter to mean asteroid radius ratio of ∼(1.4–1.62). This is consistent with the maximum size of a crater expected from previous observations of very porous rocky bodies (i.e. rubble-pile asteroids). Finally, a relationship between crater diameter (normalised to body radius) is proposed as a function of body porosity which suggests that the doubling of porosity between fractured asteroids and rubble-pile asteroids, nearly doubles the size (D/R value) of the largest crater sustainable on a rocky body.  相似文献   

19.
The factors required for estimation of the accuracy of the confidence region construction in the problem of asteroid orbit determination are considered. Blunders and large systematic errors occurring in asteroid observations increase the sizes of confidence regions and cause their noticeable shift in the space of determined parameters. We present the factors that, in addition to analysis of discrepancies (O–C), provide an opportunity to estimate the efficiency of screening observations containing gross systematic errors. The developed factors have been tested for efficiency using simulated observations. The observations have been simulated by parameters set by us and assumed true. It is shown how the sizes of systematic errors and the number of observations with these errors influence the results of screening. All calculations have been performed within the Keplerian model of asteroid motion.  相似文献   

20.
The relation between the size and velocity of impact crater ejecta has been studied by both laboratory experiments and numerical modeling. An alternative method, used here, is to analyze the record of past impact events, such as the distribution of secondary craters on planetary surfaces, as described by Vickery (Icarus 67 (1986) 224; Geophys. Res. Lett. 14 (1987) 726). We first applied the method to lunar images taken by the CLEMENTINE mission, which revealed that the size-velocity relations of ejecta from craters 32 and 40 km in diameter were similar to those derived by Vickery for a crater 39 km in diameter. Next, we studied the distribution of small craters in the vicinity of kilometer-sized craters on three images from the Mars Orbiter Camera (MOC) on board the Mars Global Surveyor (MGS). If these small craters are assumed to be secondaries ejected from the kilometer-sized crater in each image, the ejection velocities are of hundreds of meters per second. These data fill a gap between the previous results of Vickery and those of laboratory studies.  相似文献   

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