Presentation Information
[PPS04-P09]Experimental study on crater formation and droplet generation inducted by projectile impact into molten basalt
*Kazunari Habara1, Masahiko Arakawa1, Minami Yasui1, Hatsune Okawa1, Anzu Ikoma1 (1.Kobe University)
Keywords:
Magma ocean,Chondrule,Crater,Planetary impact
Unraveling the origin of chondrules (spherical solid particles of a few hundred μm) is crucial for understanding the early environment of planetary formation. Chondrules formed 1-4 Ma after CAIs (Pape et al., 2019), when many planetesimals likely possessed molten interiors due to the decay of 26Al. The “splash” model proposed by Asphaug et al. (2011) suggests that low-velocity collisions (≈30-100 m/s) between molten planetesimals eject magma that breaks into droplets to form chondrules. However, this model has been tested only by numerical simulations; impact experiments using molten rock have not been conducted, and it remains uncertain whether chondrule-sized spheres can form through impacts onto molten mantles.
We performed impact experiments on molten basalt targets simulating planetesimal magma oceans to investigate the effects of impact velocity and target temperature on crater formation, crater relaxation, and droplet ejection. Fuji basalt sand (bulk density 1.18 g/cm3) used as a target was placed in a graphite crucible and melted by induction heating at 1200-1250 ℃ in air. Target temperature was varied in 50 ℃ increments and measured using a two-color pyrometer. Target viscosity was estimated using the empirical model of Giordano et al. (2008) based on the composition reported by Ishibashi & Amano (2017). Impact experiments were conducted using a vertical single-stage powder gun at Kobe University under atmospheric pressure. Impact velocities were ∼50 m/s and ∼300 m/s. Projectiles were aluminum cylinders (7 mm in diameter, 10 mm in height) containing a 5 mm iron sphere for low-velocity impacts, and 7 mm polycarbonate spheres for medium-velocity impacts. High-speed cameras recorded the impacts from horizontal and oblique directions. Ejecta produced during impact were collected and examined under a stereomicroscope to identify droplets.
Low-velocity impacts produced overturned-rim craters at ≧1350 ℃, whereas impacts at 1300 ℃ produced smooth depressions, indicating a transition near 1300-1350 ℃ (viscosity ≈21-11 Pa s). Medium-velocity impacts also produced depressions at 1200-1300 ℃ (≈101-21 Pa s), indicating that high viscosity suppresses crater formation even at higher impact energy. Impact craters formed at ≧1300 ℃ relaxed over time, with crater diameter decreasing exponentially as D∝exp(-t/τ). The relaxation time τ was 0.26 s at 1350 ℃ and 0.31 s at 1400 ℃, about one order of magnitude longer than predicted by simple viscous relaxation theory, suggesting that the theoretical model may underestimate the effective viscosity, possibly due to compositional differences in SiO2 content or crystallization effects.
Microscopic observations of recovered ejecta revealed that glass spherules ranged from 80-200 µm at low velocity and 30-60 µm at medium velocity, indicating that the maximum size decreased with increasing velocity. Aerodynamic breakup theory (Pilch & Erdman, 1987) predicts that droplet size decreases with increasing velocity at constant viscosity. Therefore, assuming the spherules originated from an ejecta curtain fragmented by aerodynamic forces, these observations are consistent with aerodynamic breakup theory. These results suggest that collisions between molten planetesimals produce magma ejecta curtains that can generate chondrule-sized droplets under nebular gas ram pressure; thus, our results do not exclude the splash model proposed by Asphaug et al. (2011).
Reference: Pape et al. (2019) Geoch. Cosmo. Acta 244, 416-436; Asphaug et al. (2011) Planet. Sci. J. 2, 200; Giordano et al. (2008) EPSL 271, 123-134; Ishibashi & Amano (2017) report (Shizuoka Univ.), 44, 17-29; Pilch & Erdman (1987) Int. J. Multi. Flow, 13, 741-757.
We performed impact experiments on molten basalt targets simulating planetesimal magma oceans to investigate the effects of impact velocity and target temperature on crater formation, crater relaxation, and droplet ejection. Fuji basalt sand (bulk density 1.18 g/cm3) used as a target was placed in a graphite crucible and melted by induction heating at 1200-1250 ℃ in air. Target temperature was varied in 50 ℃ increments and measured using a two-color pyrometer. Target viscosity was estimated using the empirical model of Giordano et al. (2008) based on the composition reported by Ishibashi & Amano (2017). Impact experiments were conducted using a vertical single-stage powder gun at Kobe University under atmospheric pressure. Impact velocities were ∼50 m/s and ∼300 m/s. Projectiles were aluminum cylinders (7 mm in diameter, 10 mm in height) containing a 5 mm iron sphere for low-velocity impacts, and 7 mm polycarbonate spheres for medium-velocity impacts. High-speed cameras recorded the impacts from horizontal and oblique directions. Ejecta produced during impact were collected and examined under a stereomicroscope to identify droplets.
Low-velocity impacts produced overturned-rim craters at ≧1350 ℃, whereas impacts at 1300 ℃ produced smooth depressions, indicating a transition near 1300-1350 ℃ (viscosity ≈21-11 Pa s). Medium-velocity impacts also produced depressions at 1200-1300 ℃ (≈101-21 Pa s), indicating that high viscosity suppresses crater formation even at higher impact energy. Impact craters formed at ≧1300 ℃ relaxed over time, with crater diameter decreasing exponentially as D∝exp(-t/τ). The relaxation time τ was 0.26 s at 1350 ℃ and 0.31 s at 1400 ℃, about one order of magnitude longer than predicted by simple viscous relaxation theory, suggesting that the theoretical model may underestimate the effective viscosity, possibly due to compositional differences in SiO2 content or crystallization effects.
Microscopic observations of recovered ejecta revealed that glass spherules ranged from 80-200 µm at low velocity and 30-60 µm at medium velocity, indicating that the maximum size decreased with increasing velocity. Aerodynamic breakup theory (Pilch & Erdman, 1987) predicts that droplet size decreases with increasing velocity at constant viscosity. Therefore, assuming the spherules originated from an ejecta curtain fragmented by aerodynamic forces, these observations are consistent with aerodynamic breakup theory. These results suggest that collisions between molten planetesimals produce magma ejecta curtains that can generate chondrule-sized droplets under nebular gas ram pressure; thus, our results do not exclude the splash model proposed by Asphaug et al. (2011).
Reference: Pape et al. (2019) Geoch. Cosmo. Acta 244, 416-436; Asphaug et al. (2011) Planet. Sci. J. 2, 200; Giordano et al. (2008) EPSL 271, 123-134; Ishibashi & Amano (2017) report (Shizuoka Univ.), 44, 17-29; Pilch & Erdman (1987) Int. J. Multi. Flow, 13, 741-757.