Presentation Information
[PPS04-P11]Experimental study of the impact strength of differentiated planetesimals with molten metallic cores
Saki Hayashi1, Masahiko Arakawa1, *Minami Yasui1, Hatsune Okawa1, Sunao Hasegawa2 (1.Department of Planetology, Graduate School of Science, Kobe University, 2.ISAS/JAXA)
Keywords:
molten metallic core,M-type asteroids,impact experiments,impact strength,antipodal velocity
Planetesimals experienced internal heating and melting due to the decay heat of short-lived radioactive isotopes such as 26Al, and developed differentiated structures consisting of a metallic core and a rocky mantle. 16 Psyche, one of the M-type asteroids, is expected to have a rubble-pile structure composed of metallic and rocky components with porosity [Elkins-Tanton et al. (2020)]. To clarify the origin of M-type asteroids, it is essential to understand the collisional accretion processes of differentiated planetesimals composed of metal and rock.
Katsura et al. (2014) showed that the impact strength of iron meteorites is two orders of magnitude higher than that of rock. Furthermore, Davis and Ryan (1990) and Okamoto and Arakawa (2008) conducted impact experiments on two-layered spheres with a solid core and solid mantle, and showed that the solid core was difficult to disrupt because it was protected by the mantle. Therefore, it is expected that the formation of M-type asteroids through the collisional disruption of differentiated planetesimals with solid cores and solid mantles would be difficult.
In this study, we propose that the collisional disruption of differentiated planetesimals with a molten core and solid mantle can produce metallic impact fragments, leading to the formation of metal-rich rubble-pile asteroids such as M-type asteroids. To examine this hypothesis, we conducted impact experiments on two-layered spheres with either a molten or solid core surrounded by a solid mantle, using various core sizes. We investigated the effect of core state on impact strength.
We prepared two-layered spherical targets consisting of a low-melting-point metallic core surrounded by a porous mortar mantle. The core diameters were 17, 20, and 30 mm, and the total target diameter was 40 mm. In the molten-core targets, thermocouples were inserted into both the core and the mantle. The targets were heated with a halogen lamp for approximately 10 minutes prior to impact until the core temperature exceeded 100 °C. Impact experiments were conducted using two-stage light-gas guns at Kobe University and ISAS/JAXA. The impact velocities ranged from 1 to 6 km/s. The projectiles were polycarbonate spheres with diameters of 4.7 and 7 mm. Two high-speed cameras were used to observe the impact phenomena, and an infrared high-speed camera was used to measure the temperature distribution of the impact fragments after the collision.
For the solid-core targets, the mantle disruption increased with increasing specific energy, whereas the core was not disrupted even at high specific energies due to the protective effect of the mantle. In contrast, the mantle of the molten-core targets was more significantly disrupted at the same specific energy compared with the solid-core targets. Furthermore, immediately after impact, the molten core spread into a sheet-like structure at all specific energies. This sheet subsequently fragmented due to surface tension, and the fragments then solidified. We recovered metallic fragments with various morphologies, including spherical droplets, sheets, and aggregates. The impact strength of these molten-core targets was significantly lower than that of the solid-core targets, with a reduction of more than two orders of magnitude. Furthermore, it was one order of magnitude lower than that of the homogeneous porous mortar targets.
Using the results of impact strength and antipodal velocity, we discussed the conditions for impact disruption and reaccumulation of differentiated planetesimals with either molten or solid metallic cores and rocky mantles.
Katsura et al. (2014) showed that the impact strength of iron meteorites is two orders of magnitude higher than that of rock. Furthermore, Davis and Ryan (1990) and Okamoto and Arakawa (2008) conducted impact experiments on two-layered spheres with a solid core and solid mantle, and showed that the solid core was difficult to disrupt because it was protected by the mantle. Therefore, it is expected that the formation of M-type asteroids through the collisional disruption of differentiated planetesimals with solid cores and solid mantles would be difficult.
In this study, we propose that the collisional disruption of differentiated planetesimals with a molten core and solid mantle can produce metallic impact fragments, leading to the formation of metal-rich rubble-pile asteroids such as M-type asteroids. To examine this hypothesis, we conducted impact experiments on two-layered spheres with either a molten or solid core surrounded by a solid mantle, using various core sizes. We investigated the effect of core state on impact strength.
We prepared two-layered spherical targets consisting of a low-melting-point metallic core surrounded by a porous mortar mantle. The core diameters were 17, 20, and 30 mm, and the total target diameter was 40 mm. In the molten-core targets, thermocouples were inserted into both the core and the mantle. The targets were heated with a halogen lamp for approximately 10 minutes prior to impact until the core temperature exceeded 100 °C. Impact experiments were conducted using two-stage light-gas guns at Kobe University and ISAS/JAXA. The impact velocities ranged from 1 to 6 km/s. The projectiles were polycarbonate spheres with diameters of 4.7 and 7 mm. Two high-speed cameras were used to observe the impact phenomena, and an infrared high-speed camera was used to measure the temperature distribution of the impact fragments after the collision.
For the solid-core targets, the mantle disruption increased with increasing specific energy, whereas the core was not disrupted even at high specific energies due to the protective effect of the mantle. In contrast, the mantle of the molten-core targets was more significantly disrupted at the same specific energy compared with the solid-core targets. Furthermore, immediately after impact, the molten core spread into a sheet-like structure at all specific energies. This sheet subsequently fragmented due to surface tension, and the fragments then solidified. We recovered metallic fragments with various morphologies, including spherical droplets, sheets, and aggregates. The impact strength of these molten-core targets was significantly lower than that of the solid-core targets, with a reduction of more than two orders of magnitude. Furthermore, it was one order of magnitude lower than that of the homogeneous porous mortar targets.
Using the results of impact strength and antipodal velocity, we discussed the conditions for impact disruption and reaccumulation of differentiated planetesimals with either molten or solid metallic cores and rocky mantles.