Hypervelocity impacts into ice‐topped layered targets: Investigating the effects of ice crust thickness and subsurface density on crater morphology

Abstract

Abstract-Many bodies in the outer solar system are theorized to have an ice shell with a different subsurface material below, be it chondritic, regolith, or a subsurface ocean. This layering can have a significant influence on the morphology of impact craters. Accordingly, we have undertaken laboratory hypervelocity impact experiments on a range of multilayered targets, with interiors of water, sand, and basalt. Impact experiments were undertaken using impact speeds in the range of 0.8-5.3 km s À1, a 1.5 mm Al ball bearing projectile, and an impact incidence of 45°. The surface ice crust had a thickness between 5 and 50 mm, i.e., some 3-30 times the projectile diameter. The thickness of the ice crust as well as the nature of the subsurface layer (liquid, well consolidated, etc.) have a marked effect on the morphology of the resulting impact crater, with thicker ice producing a larger crater diameter (at a given impact velocity), and the crater diameter scaling with impact speed to the power 0.72 for semi-infinite ice, but with 0.37 for thin ice. The density of the subsurface material changes the structure of the crater, with flat crater floors if there is a dense, well-consolidated subsurface layer (basalt) or steep, narrow craters if there is a less cohesive subsurface (sand). The associated faulting in the ice surface is also dependent on ice thickness and the substrate material. We find that the ice layer (in impacts at 5 km s À1 ) is effectively semi-infinite if its thickness is more than 15.5 times the projectile diameter. Below this, the crater diameter is reduced by 4% for each reduction in ice layer thickness equal to the impactor diameter. Crater depth is also affected. In the ice thickness region, 7-15.5 times the projectile diameter, the crater shape in the ice is modified even when the subsurface layer is not penetrated. For ice thicknesses, <7 times the projectile diameter, the ice layer is breached, but the nature of the resulting crater depends heavily on the subsurface material. If the subsurface is noncohesive (loose) material, a crater forms in it. If it is dense, well-consolidated basalt, no crater forms in the exposed subsurface layer.

Impact Generation of Holes in the Early Lunar Crust: Scaling Relations

“…We also compare our results with work on penetration of the ice shell of Europa (Bray et al, 2014;Cox & Bauer, 2015), finding an excellent match between our cratering-penetrating boundary and that for previous work on these bodies. Comparing with experimental data for ice (Cox et al, 2008;Harriss & Burchell, 2017) yields surprisingly close agreement given the large extrapolations necessary. Overall these comparisons suggest that our derived relations for the boundaries between different impact outcome regimes, and associated radii, are more broadly applicable than to just the rock-magma system of the very early Moon, with target density and material properties having relatively little influence on impact outcome.…”

“… (a): Comparison between the experimental ice impact results of Cox et al. (2008) (C08), Harriss and Burchell (2017) (HB17), and our regime transitions. The shading is identical to Figure 6: Pink = complete penetration, blue = partial penetration, orange = cratering with full‐depth fracturing, green = classical cratering.…”

“…Despite this, experimental data for impacts into ice over water are surprisingly sparse. The two most relevant studies are that of Cox et al (2008) (hereafter C08), Harriss and Burchell (2017) (hereafter HB17), both of whom performed impact experiments into planar ice sheets overlying water. Harriss and Burchell (2020) also investigate ice/water targets, but in that work the targets are spheres roughly 20 cm in diameter, comparable to the size of the craters produced, which is a somewhat different scenario to the planar targets of our simulations and the other comparisons.…”

Experimental and Simulation Efforts in the Astrobiological Exploration of Exooceans

“…Light gas guns can be used to simulate, under controlled laboratory conditions, both the ejection of material from beneath a layer of ice and to simulate the hypervelocity impacts that would occur in a Stardust type sample return mission. Previous work utilising light gas guns and ice targets has examined the crater morphology of impacts into ice (Grey et al 2001;Harriss and Burchell 2017;Sheng-wei et al 2017) and have shown the survival of organic compounds in ejecta material when solid ice targets experience hypervelocity impacts (Bowden et al 2009). However, using targets composed of a layer of ice over a liquid (which can be doped with potential organic compounds) could be used to generate a plume of liquid being ejected from beneath the ice.…”

“…However, to be able to interpret the data collected by instruments on board of missions, such as NASA's Cassini or ESA's JUICE (JUpiter ICy moons Explorer) probes, we have to conduct laboratory experiments to simulate the effects particles experience during such high-speed in situ sampling. For this purpose, both light gas guns (Grey et al 2001;Harriss and Burchell 2017;Shengwei et al 2017;Bowden et al 2009;Judge 2017;Fisher et al 2018;Fujishima et al 2016;Lexow et al 2013;Hibbert et al 2017;Carver et al 2008;Ramkissoon 2016) and laser induced liquid beam desorption (Postberg et al 2009b(Postberg et al , 2018bKlenner et al , 2020a) can be used. The resulting atomic, molecular, and macroscopic fragments are then analysed using mass spectrometers.…”