[P-PS06] Planetary Sciences

[PPS06-16] Numerical Study of the Formation Scenario of Mesosiderite Meteorites Through a Giant Impact Onto a Differentiated Asteroid

*Keisuke Sugiura¹, Makiko K. Haba², Hidenori Genda¹ (1.Tokyo Institute of Technology, Earth Life Science Institute, 2.Tokyo Institute of Technology, Department of Earth and Planetary Sciences)

Keywords:stony iron meteorites, mesosiderites, differentiated asteroid, giant impact, numerical simulation, SPH method

Mesosiderites are a kind of stony iron meteorites and contain similar amount of silicates and Fe-Ni metals. Chemical composition and other features of mesosiderites show that those silicates and Fe-Ni metals are, respectively, originated from a crust and a molten metal core of a differentiated asteroid (e.g., Mittlefehldt et al. 1979; Hassanzadeh et al. 1990). The metal components were molten when they mixed with silicates (e.g., Floran 1978). On the other hand, mesosiderites hardly contain olivine, which is main composition of mantle (e.g., Prinz et al. 1980). Measurements of cooling rate of mesosiderite metals show that mesosiderites experienced slow cooling at some depth in asteroids larger than several hundred kilometers (e.g., Haack et al. 1996). Thus, we expect that mesosiderites are formed through the following process: a molten core of a 100 km-sized differentiated asteroid is excavated through an impact of another asteroid and then excavated core mixes with surface crust of the differentiated asteroid. The parent body of mesosiderites should be about 500 km in diameter because its core was molten at the time of metal-crust mixing event (4.5 billion years ago: Haba et al. 2019). Moreover, the mesosiderites’ parent body is probably the asteroid Vesta because the O and Cr isotope composition of mesosiderites almost coincides with those of HED meteorites (e.g., Greenwood et al. 2006) and infrared spectra of HED meteorites are similar to those of Vesta (e.g., McCord et al. 1970).

Although excavation and reaccumulation process of a core of a differentiated asteroid through a giant impact are roughly investigated through low-resolution simulations (Scott et al. 2001), there is no work investigating whether a giant impact is possible to produce materials mainly containing crust and metal. In this study, we simulate giant impacts onto a Vesta-like asteroid using the Smoothed Particle Hydrodynamics (SPH) method and investigate detailed distribution of materials on the surface of the impact outcome. Based on the magma ocean model of Vesta (Mandler&Elkins-Tanton 2013), we set the body’s radius, core’s radius, mantle thickness, and crust thickness to 270 km, 110 km, 120 km, and 40km, respectively. We use an impactor with 0.1 times the target mass. We vary the impact angle from 10 degrees to 50 degrees and the impact velocity from 2 km/s to 5 km/s.
We find that destructive impacts with the mass of the impact outcome becoming half of the original body can excavate the metal core and mix them with the surface crust. Although mantle materials are exposed on about half of the surface of the impact outcome, there are some surface locations mainly containing crust and metal materials. It is possible to produce mesosiderites on such locations. We also find that less destructive impacts hardly excavate the metal core, and more destructive impacts mainly strip the surface crust so that such impacts are difficult to produce materials composed of crust and metal core.