Iron meteorites are composed of iron-nickel alloys and classified structurally into hexahedrites (5–6.5 wt% Ni), octahedrites (6–12 wt% Ni), and ataxites (~10 to >20 wt% Ni). Iron meteorites are also classified chemically in terms of the concentrations of trace elements, for example Ge and Ga, into several groups. Comparisons of the chemical trends within groups suggest that there are two different types: magmatic and non-magmatic. Magmatic groups are thought to be derived from the metallic cores of differentiated bodies whereas non-magmatic groups may come from bodies that were not heated enough to form metallic cores. The cores formed in the parent bodies of magmatic iron meteorites less than ~1.0 Myr after the formation of calcium-, aluminum-rich inclusions (CAIs), whereas chondrule ages are about 2–4 Myr after CAIs.
In 2006, Bottke et al. suggested that the parent bodies of iron meteorites are considered to have formed early in the terrestrial planet region before migrating to the main asteroid belt by gravitational interactions with protoplanets. Asteroid 16 Psyche is the largest metal-rich asteroid in the main asteroid belt, measuring 232×189×279 km, and might be such a remnant. The equilibrium temperature of the iron meteorites’ parent bodies decreased from about 300 K to 160 K during migration from the terrestrial planet region to the main asteroid belt. Some iron meteorites undergo a transition from ductile to brittle behavior as the temperature decreases. Moreover, the most probable collision velocity in the main asteroid belt is 4.4 km/s, however, it could have been higher in the terrestrial planet region early in the solar system. Such a velocity difference may also cause a difference in crater shape on metallic bodies.
Now, Ogawa and colleagues conducted impact experiments on room- and low-temperature iron meteorite and iron alloy targets (carbon steel SS400 and iron-nickel alloy) with velocities of 0.8–7 km s¯1, using a two-stage hydrogen-gas gun installed at the Institute of Space and Astronautical Science (ISAS) and a vertical powder gun at Kobe University, to investigate the dependence of crater shape on temperature, velocity and impactor density.
The projectiles were rock cylinders and metal spheres and cylinders. They also conducted Oblique impact experiments using stainless steel projectiles and SS400 steel targets which produced more prominent radial patterns downrange at room temperature than at low temperature. Crater diameters and depths were measured and compiled using non-dimensional parameter sets based on the π -group crater scaling relations. They also performed iSALE-2D simulations using the Johnson– Cook (JNCK) strength model assuming JNCK parameters for the Gibeon iron meteorite, SS400 and SUS304.
“The laboratory and numerical results collectively show that the depth/diameter (d/D) values of metallic targets are more dependent on velocity (U) than are those of rocky targets. The ratio is smaller under low velocity and low-temperature conditions; however, the ratio is more sensitive to U than it is to the temperature of the target.”— told Nakamura, author of the study
Both experimental and numerical results showed that the crater depth and diameter decreased with decreasing temperature, which strengthened the target, and with decreasing impact velocity. The decreasing tendency was more prominent for depth than for diameter, i.e., the depth/diameter (d/D) ratio was smaller for the low temperature and low velocity conditions. The depth/diameter ratios of craters formed by rock projectiles were shallower than those of craters formed by metallic projectiles. Their results imply that the d/D values of craters on metallic surfaces contain information about the past impact environment of metallic bodies.
Featured image: The definitions of crater diameter, crater depth and rim height used in this study. On the left is the numerical result and on the right is the experimental result for an SUS-Gibeon 5.1-km/s impact (Gs4). The black and gray parts of the numerical result represent the projectile and target materials, respectively © Nakamura et al.
Reference: Ryo Ogawa, Akiko M. Nakamura, Ayako Suzuki, Sunao Hasegawa, “Crater shape as a possible record of the impact environment of metallic bodies: Effects of temperature, impact velocity and impactor density”, ArXiv, pp. 1-63, 4 Mar 2021. https://arxiv.org/abs/2103.03128
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