Effect of target properties and impact velocity on ejection dynamics and ejecta deposition

Abstract

Impact craters are formed by the displacement and ejection of target material. Ejection angles and speeds during the excavation process depend on specific target properties. In order to quantify the influence of the constitutive properties of the target and impact velocity on ejection trajectories, we present the results of a systematic numerical parameter study. We have carried out a suite of numerical simulations of impact scenarios with different coefficients of friction (0.0–1.0), porosities (0–42%), and cohesions (0–150 MPa). Furthermore, simulations with varying pairs of impact velocity (1–20 km s−1) and projectile mass yielding craters of approximately equal volume are examined. We record ejection speed, ejection angle, and the mass of ejected material to determine parameters in scaling relationships, and to calculate the thickness of deposited ejecta by assuming analytical parabolic trajectories under Earth gravity. For the resulting deposits, we parameterize the thickness as a function of radial distance by a power law. We find that strength—that is, the coefficient of friction and target cohesion—has the strongest effect on the distribution of ejecta. In contrast, ejecta thickness as a function of distance is very similar for different target porosities and for varying impact velocities larger than ~6 km s−1. We compare the derived ejecta deposits with observations from natural craters and experiments.

Experimental impact cratering: A summary of the major results of the MEMIN research unit

“…, ) comparable to funneled craters in highly porous snow (Luther et al. , Forthcoming). Saturation of pore space with water at 50 and 90%, respectively, set off the effects of porosity, with crater size increasing with progressive saturation of target pore space (Dufresne et al.…”

“…Complementary numerical modeling of the ejection process shows that the ejection angle and velocity depend on porosity, friction, and cohesion (Luther et al. ). Increasing the coefficient of friction and the porosity of the target material leads to a decrease of ejection velocities and the formation of smaller craters.…”

Are the Moon's Nearside‐Farside Asymmetries the Result of a Giant Impact?

“…For example, for ejecta a with a launch velocity of V , we calculate its normal velocity ( V r ) and tangent velocity ( V t ), from which the launch angle is estimated to be θ = atan ( V r / V t ; see Figure a). This method has been used previously in several modeling studies on the ejecta distribution (e.g., Luther et al, ; Wünnemann et al, ). Although the ejecta behave more as a continuous flow with partly fluidized melt, they move along parabolic trajectories and land on the lunar surface with approximately the same velocity as launched.…”

The effects of asteroid layering on ejecta mass-velocity distribution and implications for impact momentum transfer

“…During the excavation stage of crater formation, a large amount of material is ejected ballistically out of the crater as ejecta (Oberbeck, 1975;Housen et al, 1983). Previous laboratory (Housen and Holsapple, 2011) and numerical studies (Jutzi and Michel, 2014;Luther et al, 2018;Raducan et al, 2019) of impact events into homogeneous targets have shown that the speed and mass of ejecta depends sensitively on target material properties, such as cohesive strength, porosity and the coefficient of internal friction. However, most bodies in the Solar System are not homogeneous, as shown by past missions to asteroids, such as the NEAR-Shoemaker (Veverka et al, 2001), the OSIRIS-REx (Lauretta et al, 2019;Walsh et al, 2012) or the Hayabusa missions (Yano et al, 2006;Watanabe et al, 2019), as well as Earth-based thermal infrared observations (Delbo et al, 2014).…”