Regular ArticleImpact Craters on Asteroids: Does Gravity or Strength Control Their Size?
Abstract
The formation of kilometer-size craters on asteroids is qualitatively different from the formation of meter-size (laboratory- and weapons-scale) craters on Earth. A numerical hydrocode model is used to examine the outcomes of various-size cratering impacts into spheres and half-spaces. A shock wave fractures the target in advance of the crater excavation flow; thus, for impactors larger than 100 m, impacting at typical asteroid impact velocities, target tensile strength is irrelevant to the impact outcome. This result holds whether the target is initially intact or a “rubble pile,” even ignoring the effects of gravity. Because of the shock-induced fracture, crater excavation is controlled by gravity at smaller sizes than would otherwise be predicted. Determining the strength–gravity transition by comparing the physical strength of the material to the force of gravity will not work, because strength is eliminated by the shock wave.
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Itokawa has two distinct terrain types, rough highlands, and smooth lowlands. The lowlands formed by the movement of fine-grained materials from the highlands into topographic lows, covering up large boulders and producing a smooth surface. The topography of asteroids is a function of the shape, interior density, and spin rate. Itokawa, like many near-earth objects, may have experienced changes in its spin period due to YORP. Changes in spin period compared with the current 12.13 h period, may result in changes in the location of topographic lows and thus the concentration of fines in the lows. Under faster spin periods, 8 h or less, the northern topographic low, currently Sagamihara, changes location, but the southern lowland, Muses-Sea, stays in the same location. Above 8 h the topographic lows match the current geographic extent of the fine-grain lowlands. Current estimates of the timescale of regolith migration based on seismic shaking span several orders of magnitude. However, if these can be further refined, the location of the northern lowlands could be used as a constraint on the past spin rates of Itokawa The methods used in this study could be applied to other asteroids and may place an independent constraint on past spin periods.
This paper investigates the equatorial cavities found on asteroids 2008 EV5 and 2000 DP107 Alpha. As the likelihood of these cavities being impact craters is demonstrated to be low, the paper presents a fission mechanism that explains their existence as a scar of past fission events. The dynamical environment of “top-shaped” asteroids is such that, at high spin rates, an identifiable equatorial region enters into tension before the rest of the body. We propose hypothetical past shapes for 2008 EV5 and 2000 DP107, with mass added within the cavity to recreate a smoother equatorial ridge. The dynamical environment of these hypothetical parent bodies reveal that this modified region is indeed set in tension when spin is increased. The fission process requires tensile strength at the interface between the ejecta and the remaining body, at the moment of fission, between 0 and 2 Pa for 2008 EV5 and between 0 and 15 Pa for 2000 DP107, depending on the precise fission scenario considered. Going back to the spin-up deformation phase of the asteroids, the paper examines how kinetic sieving can form predominantly rocky equators, whose tensile strength could be much lower than that of the rest of the body. This process could explain the low cohesion values implied for this fission mechanism.
Morphology of the Morasko crater field (western Poland): Influences of pre-impact topography, meteoroid impact processes, and post-impact alterations
2017, GeomorphologySmall impact craters (< 1 km) developed in unconsolidated sediments are expected to be relatively common on Earth; however, only a few tens of them have been documented thus far. Among the reasons for this small number of documented craters are the post-impact erosion and sedimentation processes that modify craters and the lack of universal identification criteria to allow the differentiation of impact structures from landforms of other origins. Here, we focus on the well-preserved impact craters on the Morasko Hill push moraine in western Poland. These craters were formed by iron meteoroid impacts in unconsolidated sediments of glacial and fluvial origins ca. 5000–6000 years ago. We provide a new high-resolution topographic model of the crater field to identify the influences of the pre-impact topography, impact processes, and post-impact modifications on the final morphology of the craters. The topographic model obtained from airborne LiDAR data and total station surveying consists of DEMs related to the recent and reconstructed pre-impact topographies. Parameterization of recent topography in terms of slope gradients, slope curvatures, and roughness allowed us to delimit the boundaries of the craters and to calculate their Feret diameters, ellipticities, slope gradients, crater depths, and volumes. The novelty of our study lies in the estimation of the last two parameters based on the reconstructed pre-impact topography and modelled paraboloids related to each crater. The obtained results show that the studied craters are circular, bowl-shaped features displaying different cross-sectional asymmetries that resulted from the interplay between the trajectories of the bombarding projectiles and the topographies of the primary pre-impact glacial and post-glacial landforms. The oblique impacts likely influenced the asymmetric distribution of ejecta during the excavation of the craters and are considered as factors conditioning mass movements during the post-impact modification of the craters. The compilation of the existing data on terrestrial small-impact craters reveals that they are susceptible to post-impact geometry modification (shallowing and widening) and that many craters have depth/diameter ratios lower than are typical for simple impact craters.
A model for the asteroid resurfacing by regolith convection is built to estimate its timescale. In the model, regolith convection is driven by the impact-induced global seismic shaking. The model consists of three steps: (i) intermittent impact of meteoroids, (ii) impact-induced global vibration (seismic shaking), and (iii) vibration-induced regolith convection. In order to assess the feasibility of the resurfacing process driven by regolith convection, we estimate the resurfacing timescale as a function of the size of a target asteroid. In the estimate, a set of parameter values is assumed on the basis of previous works. However, some of them (e.g., seismic quality factor Q, seismic efficiency η, and seismic frequency f) are very uncertain. Although these parameter values might depend on asteroid size, we employ the standard values to estimate the representative behavior. To clarify the parameter dependences, we develop an approximated scaling form for the resurfacing timescale. According to the estimated result, we find that the regolith-convection-based resurfacing timescale is shorter than the mean collisional lifetime in most of the parameter uncertainty ranges. These parameter ranges are within those reported by previous works for small asteroids. This means that the regolith convection can be a possible mechanism for the asteroid resurfacing process.
Small crater populations on Vesta
2014, Planetary and Space ScienceThe NASA Dawn mission has extensively examined the surface of asteroid Vesta, the second most massive body in the main belt. The high quality of the gathered data provides us with a unique opportunity to determine the surface and internal properties of one of the most important and intriguing main belt asteroids (MBAs). In this paper, we focus on the size frequency distributions (SFDs) of sub-kilometer impact craters observed at high spatial resolution on several selected young terrains on Vesta. These small crater populations offer an excellent opportunity to determine the nature of their asteroidal precursors (namely MBAs) at sizes that are not directly observable from ground-based telescopes (i.e., below diameter). Moreover, unlike many other MBA surfaces observed by spacecraft thus far, the young terrains examined had crater spatial densities that were far from empirical saturation. Overall, we find that the cumulative power-law index (slope) of small crater SFDs on Vesta is fairly consistent with predictions derived from current collisional and dynamical models down to a projectile size of diameter (e.g., Bottke et al., 2005a, Bottke et al., 2005b). The shape of the impactor SFD for small projectile sizes does not appear to have changed over the last several billions of years, and an argument can be made that the absolute number of small MBAs has remained roughly constant (within a factor of 2) over the same time period. The apparent steady state nature of the main belt population potentially provides us with a set of intriguing constraints that can be used to glean insights into the physical evolution of individual MBAs as well as the main belt as an ensemble.
The cratering history of asteroid (2867) Steins
2010, Planetary and Space ScienceThe cratering history of main belt asteroid (2867) Steins has been investigated using OSIRIS imagery acquired during the Rosetta flyby that took place on the 5th of September 2008. For this purpose, we applied current models describing the formation and evolution of main belt asteroids, that provide the rate and velocity distributions of impactors. These models coupled with appropriate crater scaling laws, allow the cratering history to be estimated. Hence, we derive Steins’ cratering retention age, namely the time lapsed since its formation or global surface reset. We also investigate the influence of various factors—like bulk structure and crater erasing—on the estimated age, which spans from a few hundred Myrs to more than 1 Gyr, depending on the adopted scaling law and asteroid physical parameters. Moreover, a marked lack of craters smaller than about 0.6 km has been found and interpreted as a result of a peculiar evolution of Steins cratering record, possibly related either to the formation of the 2.1 km wide impact crater near the south pole or to YORP reshaping.
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E-mail: [email protected]
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Current address: Arecibo Observatory, PO Box 995, Arecibo PR 00613 USA.