4:30 PM - 4:45 PM
[PPS06-05] Collisional Disruption of Core-mantle Bodies Simulating Differentiated Planetesimals with Morten or Solid Cores.
Keywords:differentiated bodies, impact disruption
Planetesimals in the early solar system were heated and melted by radioactive nuclides like 26Al. This caused them to undergo gravitational differentiation, forming layered bodies with an iron core and a rocky mantle. M-type asteroids and iron meteorites, such as 16 Psyche, may have originated from the iron cores or fragments of these differentiated planetesimals. Observations indicate that Psyche has an average density of about 4 g/cm³, lower than iron's 7.9 g/cm³, suggesting it could be a high-porosity rubble pile object. Since such objects form by gravitational accumulation of fragments, it is essential to consider planetary body disruption. However, numerical simulations and laboratory experiments show that an iron core is rarely disrupted due to the protective rocky mantle and the high impact strength.
For an iron core to disrupt and form rubble pile bodies, it must be in a state prone to disrupt. This study hypothesizes that iron core fragments resulted from the breakup of differentiated planetesimals with molten cores. To test this, we experimentally determined the impact disruption strength of molten and solid core-mantle targets. We also measured the ejection velocity of fragments to assess whether they could reaccumulate into rubble piles.
Impact disruption experiments were conducted using spherical core-mantle targets to simulate differentiated planetesimal impacts. The targets consisted of a bismuth-based alloy core (melting point 70℃) and a gypsum mortar mantle. The alloy was poured into a spherical mold, then placed inside another mold, where mortar was poured around it. Each target had a mass of about 100 g, a 4 cm diameter, a 2 cm core diameter, and a 1 cm mantle thickness. Additional bismuth-based alloy spheres with a mass of about 45 g and a diameter of 2 cm and mortar spheres with a mass of about 210 g and a diameter of 6 cm were prepared for comparison. Targets were suspended in a vacuum chamber, and polycarbonate spheres (7 mm or 4.7 mm) were used as projectiles at velocities of 2–6 km/s, utilizing a horizontal two-stage light gas gun at Kobe University and ISAS. The impact events were recorded with high-speed cameras and flash X-ray imaging to determine fragment velocities.
Experimental results showed that in a collision with a solid core-mantle target at the impact velocity of 2–4 km/s, the largest fragment was the iron core, which remained intact but deformed, while the mantle was catastrophically disrupted. This corresponds to type 2 classification in [2], indicating that the mantle preferentially broke apart while shielding the core. At 6 km/s, both core and mantle experienced catastrophic disruption. Flash X-ray images revealed that a crater formed in the mantle 5 µs after impact, continued growing at 100 µs, and led to total mantle disruption by 150 µs. In all cases, the mantle completely separated from the core within 500 µs. Fragment velocities were measured from the recorded images.
The relationship between the energy density and the maximum fragment mass was analyzed to estimate impact strength. The impact strength Q* of the mantle part covering a solid core was found to be approximately 400 J/kg. Future work includes impact experiments with molten cores to investigate the impact strength of molten-core target, comparing results with the impact strength of solid-core target.
[1] Elkins-Tanton, L. T. et al. Space Science Reviews 218. 17.
[2] Okamoto C. & Arakawa M. (2008) Icarus 197, 627-637.
For an iron core to disrupt and form rubble pile bodies, it must be in a state prone to disrupt. This study hypothesizes that iron core fragments resulted from the breakup of differentiated planetesimals with molten cores. To test this, we experimentally determined the impact disruption strength of molten and solid core-mantle targets. We also measured the ejection velocity of fragments to assess whether they could reaccumulate into rubble piles.
Impact disruption experiments were conducted using spherical core-mantle targets to simulate differentiated planetesimal impacts. The targets consisted of a bismuth-based alloy core (melting point 70℃) and a gypsum mortar mantle. The alloy was poured into a spherical mold, then placed inside another mold, where mortar was poured around it. Each target had a mass of about 100 g, a 4 cm diameter, a 2 cm core diameter, and a 1 cm mantle thickness. Additional bismuth-based alloy spheres with a mass of about 45 g and a diameter of 2 cm and mortar spheres with a mass of about 210 g and a diameter of 6 cm were prepared for comparison. Targets were suspended in a vacuum chamber, and polycarbonate spheres (7 mm or 4.7 mm) were used as projectiles at velocities of 2–6 km/s, utilizing a horizontal two-stage light gas gun at Kobe University and ISAS. The impact events were recorded with high-speed cameras and flash X-ray imaging to determine fragment velocities.
Experimental results showed that in a collision with a solid core-mantle target at the impact velocity of 2–4 km/s, the largest fragment was the iron core, which remained intact but deformed, while the mantle was catastrophically disrupted. This corresponds to type 2 classification in [2], indicating that the mantle preferentially broke apart while shielding the core. At 6 km/s, both core and mantle experienced catastrophic disruption. Flash X-ray images revealed that a crater formed in the mantle 5 µs after impact, continued growing at 100 µs, and led to total mantle disruption by 150 µs. In all cases, the mantle completely separated from the core within 500 µs. Fragment velocities were measured from the recorded images.
The relationship between the energy density and the maximum fragment mass was analyzed to estimate impact strength. The impact strength Q* of the mantle part covering a solid core was found to be approximately 400 J/kg. Future work includes impact experiments with molten cores to investigate the impact strength of molten-core target, comparing results with the impact strength of solid-core target.
[1] Elkins-Tanton, L. T. et al. Space Science Reviews 218. 17.
[2] Okamoto C. & Arakawa M. (2008) Icarus 197, 627-637.