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1.
New mapping reveals 100 probable impact craters on Triton wider than 5 km diameter. All of the probable craters are within 90° of the apex of Triton's orbital motion (i.e., all are on the leading hemisphere) and have a cosine density distribution with respect to the apex. This spatial distribution is difficult to reconcile with a heliocentric (Sun-orbiting) source of impactors, be it ecliptic comets, the Kuiper Belt, the scattered disk, or tidally-disrupted temporary satellites in the style of Shoemaker-Levy 9, but it is consistent with head-on collisions, as would be produced if a prograde population of planetocentric (Neptune-orbiting) debris were swept up by retrograde Triton. Plausible sources include ejecta from impact on or disruption of inner/outer moons of Neptune. If Triton's small craters are mostly of planetocentric origin, Triton offers no evidence for or against the existence of small comets in the Kuiper Belt, and New Horizons observations of Pluto must fill this role. The possibility that the distribution of impact craters is an artifact caused by difficulty in identifying impact craters on the cantaloupe terrain is considered and rejected. The possibility that capricious resurfacing has mimicked the effect of head-on collisions is considered and shown to be unlikely given current geologic constraints, and is no more probable than planetocentrogenesis. The estimated cratering rate on Triton by ecliptic comets is used to put an upper limit of ∼50 Myr on the age of the more heavily cratered terrains, and of ∼6 Myr for the Neptune-facing cantaloupe terrain. If the vast majority of cratering is by planetocentric debris, as we propose, then the surface everywhere is probably less than 10 Myr old. Although the uncertainty in these cratering ages is at least a factor ten, it seems likely that Triton's is among the youngest surfaces in the Solar System, a candidate ocean moon, and an important target for future exploration. 相似文献
2.
Andrew F. Cheng 《Icarus》2006,184(2):584-588
Three Jupiter family comets have now been observed by spacecraft with the surprising result that these comets lack unambiguous impact craters. Large-scale topography generally appears to be softened on these comets, although sharp topography is preserved at small scales. We find that viscous relaxation of water ice may explain these observations, given reasonable assumptions about ice grain size and temperatures attained in the interiors. We suggest that both the shapes and the cratering records of Jupiter family comets may be substantially modified and no longer reflect cometary formation processes or collisional evolution in the Kuiper Belt. 相似文献
3.
Resolution of Voyager 1 and 2 images of the mid-sized, icy saturnian satellites was generally not much better than 1 km per line pair, except for a few, isolated higher resolution images. Therefore, analyses of impact crater distributions were generally limited to diameters (D) of tens of kilometers. Even with the limitation, however, these analyses demonstrated that studying impact crater distributions could expand understanding of the geology of the saturnian satellites and impact cratering in the outer Solar System. Thus to gain further insight into Saturn’s mid-sized satellites and impact cratering in the outer Solar System, we have compiled cratering records of these satellites using higher resolution CassiniISS images. Images from Cassini of the satellites range in resolution from tens m/pixel to hundreds m/pixel. These high-resolution images provide a look at the impact cratering records of these satellites never seen before, expanding the observable craters down to diameters of hundreds of meters. The diameters and locations of all observable craters are recorded for regions of Mimas, Tethys, Dione, Rhea, Iapetus, and Phoebe. These impact crater data are then analyzed and compared using cumulative, differential and relative (R) size-frequency distributions. Results indicate that the heavily cratered terrains on Rhea and Iapetus have similar distributions implying one common impactor population bombarded these two satellites. The distributions for Mimas and Dione, however, are different from Rhea and Iapetus, but are similar to one another, possibly implying another impactor population common to those two satellites. The difference between these two populations is a relative increase of craters with diameters between 10 and 30 km and a relative deficiency of craters with diameters between 30 and 80 km for Mimas and Dione compared with Rhea and Iapetus. This may support the result from Voyager images of two distinct impactor populations. One population was suggested to have a greater number of large impactors, most likely heliocentric comets (Saturn Population I in the Voyager literature), and the other a relative deficiency of large impactors and a greater number of small impactors, most likely planetocentric debris (Saturn Population II). Meanwhile, Tethys’ impact crater size-frequency distribution, which has some similarity to the distributions of Mimas, Dione, Rhea, and Iapetus, may be transitional between the two populations. Furthermore, when the impact crater distributions from these older cratered terrains are compared to younger ones like Dione’s smooth plains, the distributions have some similarities and differences. Therefore, it is uncertain whether the size-frequency distribution of the impactor population(s) changed over time. Finally, we find that Phoebe has a unique impact crater distribution. Phoebe appears to be lacking craters in a narrow diameter range around 1 km. The explanation for this confined “dip” at D = 1 km is not yet clear, but may have something to do with the interaction of Saturn’s irregular satellites or the capture of Phoebe. 相似文献
4.
We address impact cratering on Io and Europa, with the emphasis on the origin of small craters on Europa as secondary to the primary impacts of comets on Io, Europa, and Ganymede. In passing we also address the origin of secondary craters generated by Zunil, a well-studied impact crater on Mars that is a plausible analog to impact craters on Io. At nominal impact rates, and taking volcanic resurfacing into account, we find that there should be 1.3 impact craters on Io, equally likely to be of any diameter between 100 m and 20 km. The corresponding model age of Europa's surface is between 60 and 100 Ma. This range of ages does not include a factor three uncertainty stemming from the uncertain sizes and numbers of comets. The mass of basaltic impact ejecta from Io to reach Europa is found to meet or exceed the micrometeoroid flux as a source of rock-forming elements to Europa's ice crust. To describe impact ejecta in more detail we adapt models for impact-generated spalls and Grady-Kipp fragments originally developed by Melosh. Our model successfully reproduces the observed size-number distributions of small craters on both Mars and Europa. However, the model predicts that a significant fraction of the 200-500 m diameter craters on Europa are not traditional secondary craters but are instead sesquinary craters caused by impact ejecta from Io that had gone into orbit about Jupiter. This prediction is not supported by observation, which implies that high speed spalls usually break up into smaller fragments that make smaller sesquinary craters. Iogenic basalts are also interesting because they provide stratigraphic horizons on Europa that in principle could be used to track historic motions of the ice, and they provide materials suitable to radiometric dating of Europa's surface. 相似文献
5.
We study the dynamical evolution of the Hilda group of asteroids trough numerical methods, performing also a collisional pseudo-evolution of the present population, in order to calculate the rate of evaporation and its contribution to the cratering history of the Galilean satellites. If the present population of small asteroids in the Hilda's region follows the same size distribution observed at larger radii, we find that this family is the main contributor to the production of small craters (i.e., crater with diameters d∼4 km) on the Galilean system, overcoming the production by Jupiter Family Comets and by Trojan asteroids. The results of this investigation encourage further observational campaigns, in order to determine the size distribution function of small Hilda asteroids. 相似文献
6.
Cratering rates on the Galilean satellites 总被引：1，自引：0，他引：1
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
7.
We use conventional numerical integrations to assess the fates of impact ejecta in the Saturn system. For specificity we consider impact ejecta launched from four giant craters on three satellites: Herschel on Mimas, Odysseus and Penelope on Tethys, and Tirawa on Rhea. Speeds, trajectories, and size of the ejecta are consistent with impact on a competent surface (“spalls”) and into unconsolidated regolith. We do not include near-field effects, jetting, or effects peculiar to highly oblique impact. Ejecta are launched at velocities comparable to or exceeding the satellite's escape speed. Most ejecta are swept up by the source moon on time-scales of a few to several decades, and produce craters no larger than 19 km in diameter, with typical craters in the range of a few km. As much as 17% of ejecta reach satellites other than the source moon. Our models generate cratering patterns consistent with a planetocentric origin of most small impact craters on the saturnian icy moons, but the predicted craters tend to be smaller than putative Population II craters. We conclude that ejecta from the known giant craters in the saturnian system do not fully account for Population II craters. 相似文献
8.
The Oort Cloud, the Kuiper belt and the Scattered Disk are dynamically distinct populations of small bodies evolving in the outer regions of the Solar System. Whereas their collisional activity is now quiet, gravitational interactions with giant planets may have shaped these populations both dynamically and collisionally during their formation. Using a hybrid approach [Charnoz, S., Morbidelli, A., 2003. Icarus 166, 141-166], the present paper tries to couple the primordial collisional and dynamical evolution of these three populations in a self-consistent way. A critical parameter is the primordial size-distribution. We show that the initial planetesimal size distribution that allows an effective mass depletion of the Kuiper belt by collisional grinding, would decimate also the population of comet-size bodies that end in the Oort Cloud and, in particular, in the Scattered Disk. As a consequence, the Oort Cloud and the Scattered Disk would be too anemic, by a factor 20 to 100, relative to the estimates achieved from the observation of the fluxes of long period and Jupiter family comets, respectively. For these two reservoirs to have a sufficient number of comets, the initial size distribution in the planetesimal disk had to be such that the mass depletion by collisional erosion of the Kuiper belt was negligible. Consequently the current mass deficit of the Kuiper belt needs to be explained by dynamical mechanisms. 相似文献
9.
The terrestrial cratering record for the Phanerozoic has a size-frequency distribution of NαD?2.05 for D > 22.6 km and NαD?0.24 for D < 11.3 km. This shallowing of the distribution slope at D > 22.6 km reflects the removal of small terrestrial craters by erosion. The number of large craters on the North American and East European cratons provide estimated terrestrial crater production rates for D > 20 km of 0.36 ± 0.1 and 0.33 ± 0.2 × 10?14 km?2 year?1, respectively. These rates are in good agreement with previous estimates and astronomical observations on Apollo bodies. Comparisons with the lunar rate, taking account of the effects of variations in impact velocity, surface gravity, and gravitational cross section, indicate that the lunar and terrestrial rates overlap, if the cratering flux has been constant during the last 3.4 by. If the early (pre 4.0 by) high-flux rate did not decay to a constant value until 3.0 to 2.5 by then the rates differ by a factor of 2 and the Phanerozoic can be interpreted as a period of higher than normal cratering. 相似文献
10.
A. Morbidelli J. Chambers J. I. Lunine J. M. Petit F. Robert G. B. Valsecchi K. E. Cyr 《Meteoritics & planetary science》2000,35(6):1309-1320
Abstract— In the primordial solar system, the most plausible sources of the water accreted by the Earth were in the outer asteroid belt, in the giant planet regions, and in the Kuiper Belt. We investigate the implications on the origin of Earth's water of dynamical models of primordial evolution of solar system bodies and check them with respect to chemical constraints. We find that it is plausible that the Earth accreted water all along its formation, from the early phases when the solar nebula was still present to the late stages of gas‐free sweepup of scattered planetesimals. Asteroids and the comets from the Jupiter‐Saturn region were the first water deliverers, when the Earth was less than half its present mass. The bulk of the water presently on Earth was carried by a few planetary embryos, originally formed in the outer asteroid belt and accreted by the Earth at the final stage of its formation. Finally, a late veneer, accounting for at most 10% of the present water mass, occurred due to comets from the Uranus‐Neptune region and from the Kuiper Belt. The net result of accretion from these several reservoirs is that the water on Earth had essentially the D/H ratio typical of the water condensed in the outer asteroid belt. This is in agreement with the observation that the D/H ratio in the oceans is very close to the mean value of the D/H ratio of the water inclusions in carbonaceous chondrites. 相似文献
11.
Jacek Leliwa-Kopystynski Marcin Banaszek Ireneusz Wlodarczyk 《Planetary and Space Science》2012,60(1):181-192
Crater's density distribution versus satellitographical longitude was searched for seven icy satellites: two of Jupiter (Ganymede and Callisto) and five of Saturn (Mimas, Tethys, Dione, Rhea and Iapetus). Craters were classified according to their size. Four bins of the craters' diameter were used. Density distributions were found in the longitudinal sectors of the near-equatorial stripes that circumscribe the satellites. The size distributions (R-plots) were done independently for each of the eight longitudinal sectors of the satellites. Searching for the leading/trailing (apex/antapex) and the near-side/far-side asymmetry was done. It was found that the crater density is longitudinally asymmetric for all seven satellites being studied. However, the apex–antapex asymmetry is much less pronounced than predicted by theory of Zahnle et al. (2003), for impacts on the satellites by ecliptic comets. We conclude that the impact craters observed on the considered satellites are mostly originated from planetocentric swarm of debris. In that case longitudinal asymmetry is not expected, as stated by Horedt and Neukum, 1984a, Horedt and Neukum, 1984b. However, cratering longitudinal asymmetry that we observe for Mimas perfectly agrees with calculations of Alvarellos et al. (2005). It is very likely that important part of craters on Mimas were formed due to impacts of ejecta originated from crater Herschel. 相似文献
12.
《Icarus》1987,70(3):517-535
The cratering record at Uranus shows two different crater populations of different ages. The old crater population occurs on the heavily cratered surfaces of Oberon, Umbriel, and Miranda, while the younger one is found on Titania, Ariel and the resurfaced areas of Miranda. Since only the young population occurs on Titania, this satellite must have experienced a global resurfacing event which obliterated the older population prior to the impact of objects causing the younger one. The old crater population is characterized by an abundance of large craters and a relative paucity of small ones. The young crater population, however, has an abundance of small craters and a paucity of large ones relative to the old population. Furthermore, the abundance of small craters and the paucity of large craters increases with decreasing density. This change in the size distribution is consistent with a population of impactors that evolved with time by mutual collision, and therefore was probably in planetocentric orbits. In fact, both crater populations may be the result of accretional remnants in planetocentric orbits that evolved with time by mutual collisions. If so, then the higher crater density on Miranda compared to Oberon and Umbriel suggests that both Oberon and Umbriel were also resurfaced early in their histories.A comparison of the Solar System cratering record from Mercury to Uranus (19 AU) shows different crater populations at different locations in the Solar System. Computer simulations using a modified Holsapple-Schmidt crater scaling and short-period comet impact velocities to recover the projectile diameters from the cratering record produce different projectile populations in different parts of the Solar System. Furthermore, adjusting the Jovian crater curve to match that in the inner Solar System requires differences in the impact velocities that are unrealistic for objects in heliocentric orbits. These results suggest that the Solar System cratering record cannot be explained by a single family of objects in heliocentric orbits, e.g., comets. One possible explanation is that the cratering record is the result of different families of objects (possibly accretional remnants) indigenous to that region of the Solar System in which the different crater populations are found. Thus, in the inner Solar System, the impactors responsible for heavy bombardment were in heliocentric orbits with semimajor axes less than 3 AU. In the outer Solar System, they may have been in planetocentric orbits around each of the Jovian planets. 相似文献
13.
G. W. Wetherill 《Meteoritics & planetary science》1989,24(1):15-22
Abstract— Absolute and relative cratering rates on the terrestrial planets have been calculated using the same asteroidal collision model and Monte Carlo program used for previous studies of the terrestrial meteorite flux, the steady-state number of Apollo-Amor objects, and the orbital distribution of both meteorites and Apollo-Amor objects. The most straightforward result is that projectiles from the asteroid belt appear to provide about one-third the observed present-day production of terrestrial craters larger than 10 km in diameter. When uncertainties in the calculations and observations are included, it cannot be excluded that the entire terrestrial cratering flux is asteroidal. On the other hand, assumption of an additional Apollo-Amor source of extinct comets, in the same quantity permitted by Apollo-Amor observations, provides better agreement with the observed cratering rate. In addition, a significant (e.g., ～30%) terrestrial contribution from active long and short period comets is acceptable within the uncertainties of the assumptions required. The ratios of the cratering rates on the different terrestrial planets are somewhat sensitive to the assumed source. A purely asteroidal source predicts a martian cratering rate per unit area about four times that on Earth, whereas the difference is reduced to about a factor of two for the mixed asteroid-extinct comet source. The opposite effect is found for Mercury. As discussed by previous authors, the predicted lunar cratering rate is significantly higher than that observed. It is not clear whether this is a result of scaling to impacts on a body considerably smaller than Earth, or if it indicates an increase in the cratering flux during the Phanerozoic. 相似文献
14.
The hypothesis on the genetic connection of near-parabolic comets with Jupiter, Saturn, and the transPlutonian region (5–3000
AU) proposed by E.M. Drobyshevskii is considered. It has been shown that, on average, 5.6 comets per an area of 106 AU2 passed through the transPlutonian region during the whole history of observations. Six-hundred nineteen comets crossed the
ecliptic at heliocentric distances ranging from 0 to 2 AU. As has been shown, from the total number of 945 near-parabolic
comets, eight comets closely approached Jupiter and five closely approached Saturn. The Kreutz comets, 1277 objects, did not
approach Jupiter closer than 3 AU. Their minimal distance to Saturn was 5.5 AU. The minimal distance of the Kreutz comets
from the edge of the transPlutonian region was 28.8 AU. The analysis led to the conclusion that the concept on the origin
of the near-parabolic comets suggested by Drobyshevskii is groundless. 相似文献
15.
Previous analyses of Galileo images showed the small (≈1 km and smaller) crater population on Callisto to be lower than had been expected (Moore, J.M. et al. [1999]. Icarus 140, 294-312; Bierhaus E.B. et al. [2000]. Lunar Planet. Sci. 31. Abstract #1996). In this paper we examine the small crater population using high-resolution imagery from Callisto flybys during Galileo orbits C3, C10, C21, and C30, including several C30 regions not previously analyzed. Our findings confirm that most small craters are depleted relative to a presumed equilibrium of R = 0.22, and we find that there is significant variability in the small crater counts. While some of the variability in the small crater population on Callisto can be attributed to secondary cratering, some variability also may be explained by resetting of portions of Callisto’s surface by larger impactors. This is expected where the differential size frequency distribution of the crater production population b < 3 (where b represents the exponent of a differential power-law crater-size distribution), such that large impacts affect a greater planetary surface area than smaller craters. 相似文献
16.
William K. Hartmann 《Icarus》1984,60(1):56-74
Similarity is found in crater densities on the most heavily cratered surfaces throughout the solar system. This is hypothesized to result from a steady-state “saturation equilibrium” being approached or achieved by cratering processes. This hypothesis conflicts with some recent interpretations. However, it accounts for (1) a similarity in maximum relative crater density, below certain theoretically predicted values, on all heavily cratered surfaces; (2) a leveling off at this same relative density among 100-m scale (secondary?) craters in populations on lunar maria and other sparsely cratered lunar surfaces; (3) the approximate uniformity of maximum relative densities on Saturn satellites (in spite of dramatic variations predicted from nonsaturation models assuming heliocentric impactors). The lunar frontside upland crater population, sometimes described as a well-preserved production function useful for interpreting other planetary surfaces, is found not to be a production function. It was modified by intercrater plains formed (at least partly) by early upland basaltic lava flooding, recently confirmed spectrophotometrically. Consistent with this, counts in “pure uplands” (those lacking intercrater plains) match the proposed saturation equilibrium density. Variations among large (D > 64 km) crater populations are found, but these may involve several hypothesized mechanisms that rapidly obliterate large craters, especially on icy surfaces. Recent models, in which different populations of interplanetary bodies hit different planets, need further appraisal. 相似文献
17.
New three-dimensional hydrodynamic simulations of hypervelocity impacts into the crust of Titan were undertaken to determine the fraction of liquid water generated on the surface of Saturn's largest moon over its history and, hence, the potential for surface—modification of hydrocarbons and nitriles by exposure to liquid water. We model in detail an individual impact event in terms of ejecta produced and melt generated, and use this to estimate melt production over Titan's history, taking into account the total flux of the impactors and its decay over time. Our estimates show that a global melt layer at any time after the very beginning of Titan's history is improbable; but transient melting local to newly formed craters has occurred over large parts of the surface. Local maxima of the melt are connected with the largest impact events. We also calculate the amount of volatiles delivered at the impact with various impact velocities (from 3 km/s for possible Hyperion fragments to 11 km/s for Jupiter family comets) and their retention as a possible source of Titan's atmosphere. We find the probability of impact ejecta escaping Titan with its modern dense and thick atmosphere is rather low, and dispersal of Titan organics throughout the rest of the Solar System requires impactors tens of kilometers in diameter. Water ice melting and exposure of organics to liquid water has been widespread because of impacts, but burial or obscuration of craters by organic deposits or cryovolcanism is aided by viscous relaxation. The largest impactors may breach an ammonia-water mantle layer, creating a circular albedo contrast rather than a crater. 相似文献
18.
The Voyager spacecraft discovered that small moons orbit within all four observed ring systems coincident with the discovery of narrow and dusty rings around Jupiter, Saturn, Uranus and Neptune. These moons can provide the source for new rings if they are catastrophically disrupted by a comet or large meteoroid impact. This hypothesis for ring origins provides a natural mechanism for the ongoing creation of planetary rings. While it relieves somewhat the problem of explaining the continued existence of rings with apparently short evolutionary lifetimes, it raises the problem of explaining the continued existence of small moons, and the coexistence of moons and rings at comparable locations within the Roche zones of the giant planets. This problem has been studied in some detail recently, and the present work is a review of our current understanding of the processes in satellite disruption that pertain to the creation of planetary rings and the collisional cascade of circumplanetary bodies. Significant progress has been made. Narrow rings are produced by disruption of small moons in numerical simulations, and a self-consistent model of the collisional cascade can explain present-day moon populations. Absolute timescales and initial moon populations remain uncertain due to our poor knowledge of the impactor population and uncertainties in the strength of planetary satellites. More pressing are the qualitative issues that remain to be resolved including the nature of reaccretion of the debris and the origin of Saturn's rings. 相似文献
19.
P. Nurmi M.J. Valtonen J.Q. Zheng 《Monthly notices of the Royal Astronomical Society》2001,327(4):1367-1376
Time variation in impact probability is studied by assuming that the periodic flux of the Oort Cloud comets within 15 au arises from the motion of the Sun with respect to the Galactic mid-plane. The periodic flux clearly shows up in the impact rate of the captured Oort Cloud cometary population, with a phase shift caused by the orbital evolution. Depending on the assumed flux of comets and the size distribution of comets, the impact rate of the Oort Cloud comets of 1 km in diameter or greater is from 5 to 700 impacts Myr−1 on the Earth and from 0.5 to 70 impacts per 1000 yr on Jupiter. The relative fractions of impacts are 0.09, 0.11, 0.26 and 0.54 for long-period comets, Halley type comets, Jupiter family comets and near-Earth objects, respectively. For Jupiter, the corresponding fractions in the first three categories are 0.18, 0.31 and 0.51. If we consider physical fading of comet activity that is compatible with the observations, then the impact rates of active comets are two orders of magnitude smaller than the total impact rates by all kinds of comets and cometary asteroids of size 1 km or greater. 相似文献
20.
An investigation of low-resolution ratio spectra of Jupiter, Saturn, and Titan in the region 5400–6500 Å has permitted new evaluations of ammonia absorption bands. The distribution of ammonia over the disk of Jupiter is very inhomogeneous. The carbon-to-nitrogen ratio is distinctly different from the solar value, but this is probably a result of uneven mixing of methane and ammonia, as suggested previously by Kuiper, rather than a compositional anomaly. The abundance of ammonia on Saturn also shows spatial variations, but appears constant in time over a 3-yr period. Two weak, unidentified absorptions were discovered in the red region of Titan's spectrum, in the absence of any detectable ammonia. The new upper limit is ηN < 120 cm-am. 相似文献