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The source of impactors capable of producing the cratering asymmetry on Triton is open to debate. While hyperbolic comets are the likely source of impactors, the theoretically predicted crater distribution due to impacts from comets is incompatible with the observed crater distribution on Triton. In this study, we adopted a dynamical approach to investigate the crater distribution produced by three sources of impactors: Neptune-centric impactors on prograde, low-inclination orbits, Neptune-centric impactors on high-inclination orbits such as its irregular satellites, and heliocentric impactors with hyperbolic orbits. We employ N -body simulations that take into account the dynamics of the impacts. We found that none of the impactor sources alone are able to reconcile with the crater distribution on Triton. The crater distribution produced by a low-inclination Neptune-centric source is narrowly concentrated on Triton's leading hemisphere, whereas the heliocentric impactors produce too many craters on the trailing hemisphere compared to the observed crater distribution. All three impactor populations produce craters on the trailing hemisphere of Triton. We derived the impact distribution of a mixed population of impactors using a Monte Carlo approach and found that if the population consists of mainly heliocentric impactors (70 per cent), the resulting impact distribution fits the observed crater distribution on Triton's leading hemisphere well. [ABSTRACT FROM AUTHOR]

The so-called Barbarian asteroids [from the name of the prototype of this class, asteroid (234) Barbara] are extremely interesting objects because they might be the survivors of a very early generation of planetesimals. We have identified in the past the first case of an asteroid family (the Watsonia family) whose members are Barbarians, all issued from the collisional disruption of a sizeable parent body. Here we report on the identification of another family, called after the name of its biggest member, asteroid (606) Brangäne, consisting of objects displaying the polarimetric properties characterizing the Barbarian behaviour. This is the first recognized case of a swarm of small Barbarians issued from a quite recent cratering event. [ABSTRACT FROM AUTHOR]

Rampino & Caldeira carry out a circular spectral analysis (CSA) of the terrestrial impact cratering record over the past 260 million years (Ma), and suggest a ~26 Ma periodicity of impact events. For some of the impacts in that analysis, new accurate and high-precision ('robust'; 2SE < 2 per cent) 40Ar-39Ar ages have recently been published, resulting in significant age shifts. In a CSA of the updated impact age list, the periodicity is strongly reduced. In a CSA of a list containing only impacts with robust ages, we find no significant periodicity for the last 500 Ma. We show that if we relax the assumption of a fully periodic impact record, assuming it to be a mix of a periodic and a random component instead, we should have found a periodic component if it contributes more than ~80 per cent of the impacts in the last 260 Ma. The difference between our CSA and the one by Rampino & Caldeira originates in a subset of 'clustered' impacts (i.e. with overlapping ages). The ~26 Ma periodicity seemingly carried by these clusters alone is strongly significant if tested against a random distribution of ages, but this significance disappears if it is tested against a distribution containing (randomly spaced) clusters. The presence of a few impact age clusters (e.g. from asteroid break-up events) in an otherwise random impact record can thus give rise to false periodicity peaks in a CSA. There is currently no evidence for periodicity in the impact record. [ABSTRACT FROM AUTHOR]

ABSTRACT Giant impacts by comets and asteroids have probably had an important influence on terrestrial biological evolution. We know of around 180 high-velocity impact craters on the Earth with ages up to 2400 Myr and diameters up to 300 km. Some studies have identified a periodicity in their age distribution, with periods ranging from 13 to 50 Myr. It has further been claimed that such periods may be causally linked to a periodic motion of the Solar system through the Galactic plane. However, many of these studies suffer from methodological problems, for example misinterpretation of p-values, overestimation of significance in the periodogram or a failure to consider plausible alternative models. Here I develop a Bayesian method for this problem in which impacts are treated as a stochastic phenomenon. Models for the time variation of the impact probability are defined and the evidence for them in the geological record is compared using Bayes factors. This probabilistic approach obviates the need for ad hoc statistics, and also makes explicit use of the age uncertainties. I find strong evidence for a monotonic decrease in the recorded impact rate going back in time over the past 250 Myr for craters larger than 5 km. The same is found for the past 150 Myr when craters with upper age limits are included. This is consistent with a crater preservation/discovery bias modulating an otherwise constant impact rate. The set of craters larger than 35 km (so less affected by erosion and infilling) and younger than 400 Myr is best explained by a constant impact probability model. A periodic variation in the cratering rate is strongly disfavoured in all data sets. There is also no evidence for a periodicity superimposed on a constant rate or trend, although this more complex signal would be harder to distinguish. [ABSTRACT FROM AUTHOR]

We study the orbital and physical properties of Trojan asteroids of Jupiter. We try to discern all the families previously discussed in the literature, but we conclude that there is only one significant family among the Trojans, namely the cluster around the asteroid (3548) Eurybates. This is the only cluster that has all of the following characteristics: (i) it is clearly concentrated in the proper-element space; (ii) the size-frequency distribution is different from that of background asteroids; (iii) we have a reasonable collisional/dynamical model of the family. Henceforth, we can consider it as a real collisional family. We also report the discovery of a possible family around the asteroid (4709) Ennomos, composed mostly of small asteroids. The asteroid (4709) Ennomos is known to have a very high albedo , which may be related to the hypothetical cratering event that exposed ice. The relation between the collisional family and the exposed surface of the parent body offers a unique means to study the physics of cratering events. However, more data are needed to confirm the existence of this family and its relationship with Ennomos. [ABSTRACT FROM AUTHOR]

Dawn space mission will provide the first, detailed data of two of the major bodies in the main asteroid belt, Vesta and Ceres. Through its connection with Howardite, Eucrite, Diogenite (HED) meteorites, Vesta is known as one of the first bodies to have accreted and differentiated in the solar nebula, predating the formation of Jupiter and surviving the violent evolution of the early Solar system. The formation time of Ceres instead is unknown, but it should not postdate that of Jupiter by far, since the perturbations of the giant planet stopped planetary accretion in the main asteroid belt. In this work we modelled the collisional histories of Vesta and Ceres at the time of the formation of Jupiter, assumed to be the first giant planet to form. In this first investigation of the evolution of the early Solar system, we did not include the presence of planetary embryos in the disc of planetesimals but concentrated on the role of the forming Jupiter and the effects of its possible inward migration due to disc-planet interactions. Our results clearly indicate that the formation of the giant planet caused an intense early bombardment in the orbital region of the main asteroid belt. We explored the effects of such bombardment on Vesta and Ceres assuming different size distributions of the primordial planetesimals. According to our results, Vesta and Ceres would not have survived the Jovian early bombardment if the disc was populated mainly by large planetesimals like those predicted to form in turbulent circumstellar discs. Discs dominated by small bodies, like those predicted to form in quiescent circumstellar discs, or with a varying fraction of the mass in the form of larger ( km) planetesimals, represent more favourable environments for the survival of the two asteroids. The abundance of planetesimals, especially the larger ones, proved a critical factor in this regard. The extent of Jupiter's radial migration due to disc-planet interactions proved itself another critical factor. In those scenarios where they survive, both asteroids had their surfaces saturated by craters as big as km and a few as big as 200-300 km. In the case of Vesta, the Jovian early bombardment would have significantly eroded (locally or globally) the crust and possibly caused effusive phenomena similar to the lunar maria, whose crystallization time would then be directly linked to the time of the formation of Jupiter. [ABSTRACT FROM AUTHOR]

Laboratory data and the results of modelling are combined to predict the possible size of craters in icy bodies such as a comet nucleus. This is done in particular for the case of a a 370-kg mass impacting a body the size of the nucleus of comet 9P/Temple-1 at 10 km s−1. This reproduces theDeep Impactcomet impact to occur in 2005, when a NASA spacecraft will observe at close range an impact on the comet nucleus of an object deployed from the main spacecraft. The predicted crater size depends not only on uncertainties in extrapolation from laboratory scale and the modelling in general, but also on assumptions made about the nature of the target. In particular, allowance is made for the full range of reasonable target porosities; this can significantly affect the outcome of theDeep Impactevent. The range of predicted crater sizes covers some 7–30 m crater depth and some 50–150 m crater diameter. An increasingly porous target (i.e. one with a higher percentage of void space) will increase the depth of the crater but not necessarily the diameter, leading to the possibility of an impact event where much of the crater formation is in the interior of the crater, with work going into compaction of void space and some possible lateral growth of the crater below the surface entrance. Nevertheless, for a wide range of scenarios concerning the nature of the impact, theDeep Impactevent should penetrate the surface to depths of a few tens of metres, accessing the immediate subsurface regions. In parallel to this, the same extrapolation methods are used to predict the size of impactors that may have caused the features observed on the surfaces of small bodies, e.g. the Saturnian satellite Phoebe and the nucleus of comet P/Wild-2. [ABSTRACT FROM AUTHOR]

Analysis is made of cratering records, whereby not only the crater ages, but also the sizes can be naturally taken into account. The method adopted is the periodogram analysis first discussed by Lomb. From the crater data based on Grieve's compilation, a period can be derived when a statistical weight of a crater is taken proportional to the size or when a weight proportional to the impact energy is adopted. The period so derived is 37.5 million years. A similar analysis based on Napier's list of craters yields almost the same result. To estimate the significance of the derived period, Monte Carlo experiments have been carried out. It turns out that the probability of occurrence of the derived period due to chance on the null hypothesis of a random distribution of crater ages lies between 0.02± 0.004 and 0.10± 0.009 depending on the data set adopted. The result shows that the level of significance obtained here is higher than in earlier investigations where large craters (with) only are taken into account. A brief discussion is made of the implication of the derived period for the estimates of the Galactic mass density by Stothers and byHipparcosdata analysis. [ABSTRACT FROM AUTHOR]

ABSTRACT The main influences on crater size distributions are investigated by deriving results for the four example target objects, (951) Gaspra, (243) Ida, (253) Mathilde and (433) Eros. The dynamical history of each of these asteroids is modelled using themercurynumerical integrator. An efficient, Öpik-type, collision code enables the distribution of impact velocities and the overall impact probability to be found. When combined with a crater scaling law and an impactor size distribution, using a Monte Carlo method, this yields a crater size distribution. The cratering time-scale is longer for Ida than either Gaspra or Mathilde, though it is harder to constrain for Eros due to the chaotic variation of its orbital elements. The slopes of the crater size distribution are in accord with observations. [ABSTRACT FROM AUTHOR]

A comparison between the terrestrial, Cytherean and lunar cratering records indicates that the large craters (diameters D>D[sub 0]) on these surfaces all have cumulative numbers that are proportional to D[sup -2.59±0.05] . Atmospheres have a negligible effect on the formation of D>D[sub 0] craters. It is shown that this limiting diameter is 45±3 km in the case of Venus, and 21.0±1.5 km in the case of Earth. In this large-diameter range, there are about 1.51±0.34 times more craters, per unit area, on Venus than on the Earth, and about 1350±310 times more craters, per unit area, on the Moon than on the Earth. [ABSTRACT FROM AUTHOR]

The recent cratering record of the surface of the Earth is re-examined using a new technique that concentrates on estimating the mean areas occupied by individual craters, together with the gradients of linear plots of crater numbers versus crater ages. This analysis indicates that the lower limit of the rate at which craters have been produced over the last 125±20 Myr is, for example, (12.0±0.7)×10[sup -15] km[sup -2] yr[sup -1] for D&ges;2.4 km craters, (9.5±0.6)×10[sup -15] km[sup -2] yr[sup -1] for D&ges;5.0 km craters, (6.5±0.5)×10[sup -15] km[sup -2] yr[sup -1] for D&ges;12 km craters, and (3.0±0.3)×10[sup -15] km[sup -2] yr[sup -1] for D&ges;22 km craters. These figures indicate that previous researchers have considerably overestimated the rate at which small (2.4