9:30 AM - 9:45 AM
[PPS03-03] Evolution of boulder size distribution on near-Earth asteroids
Keywords:near-Earth asteroid, boulder, size distribution evolution, simulation, Ryugu
The recently explored near-Earth asteroids Ryugu and Bennu are considered to be rubble-piles reaccumulated after the destruction of the parent bodies. The size-frequency measurements of rocks and particles on the surfaces of rubble-pile asteroids are fundamental to understand their surface evolution. The size distributions of surface boulders of Ryugu and Bennu have been measured from images taken by spacecraft [1,2], and the size distribution of sample particles collected from Ryugu has also been measured [3]. Since the size distributions of surface rocks and particles evolve through multiple processes such as fragmentation by micrometeoroid impact and thermal fatigue, the modeling of size distribution evolution is necessary for interpreting the surface evolution of rubble-pile asteroids from the observed size distribution.
A numerical simulation of the size distribution evolution of relatively small particles (<1m), including the effect of micro-particle dissipation due to electrostatic forces, has shown that boulder-dominated surfaces were formed on small asteroids because of the deficiency of micro-particles [4]. However, the effect of the disruption of boulders larger than 1m by millimeter- to centimeter-sized impactors, which are important in the disruption of meter-sized boulders, have not been taken into account in the simulation. In this study, we constructed a numerical model for the boulder size distribution evolution on the surface of near-Earth rubble-pile asteroid that incorporates the size distribution of impactors and the disruption of boulders by those to understand the main factors and timescale of boulder disruption process by comparing the simulation results with the boulder size distribution on the actual asteroid.
Two types of boulder disruption processes were incorporated in the evolution model: collisional disruption by impacts and thermal fatigue of boulders due to diurnal thermal cycles. The impact flux in the near-Earth orbit inferred from observations of lunar impact flashes and small fireballs was applied to the model [5,6]. We will report the results of simulations of the evolution of the boulder size distribution on the Ryugu surface using the constructed model and will discuss the timescale of boulder disruptions by comparing to the actual boulder size distribution and the boulder surface age measured using small craters on the boulders.
References: [1] Michikami et al., 2019, Icarus, 331, 179-191.[2] DellaGiustina et al., 2019, Nat. Astron., 3, 341-351. [3] Yada et al., 2022, Nat. Astron., 6, 214-220. [4] Hsu et al., 2022, Nat. Astron., 6, 1043-1050. [5] Brown et al., 2002, Nature, 420, 294-296. [6] Suggs et al., 2014, Icarus, 238, 23-36.
A numerical simulation of the size distribution evolution of relatively small particles (<1m), including the effect of micro-particle dissipation due to electrostatic forces, has shown that boulder-dominated surfaces were formed on small asteroids because of the deficiency of micro-particles [4]. However, the effect of the disruption of boulders larger than 1m by millimeter- to centimeter-sized impactors, which are important in the disruption of meter-sized boulders, have not been taken into account in the simulation. In this study, we constructed a numerical model for the boulder size distribution evolution on the surface of near-Earth rubble-pile asteroid that incorporates the size distribution of impactors and the disruption of boulders by those to understand the main factors and timescale of boulder disruption process by comparing the simulation results with the boulder size distribution on the actual asteroid.
Two types of boulder disruption processes were incorporated in the evolution model: collisional disruption by impacts and thermal fatigue of boulders due to diurnal thermal cycles. The impact flux in the near-Earth orbit inferred from observations of lunar impact flashes and small fireballs was applied to the model [5,6]. We will report the results of simulations of the evolution of the boulder size distribution on the Ryugu surface using the constructed model and will discuss the timescale of boulder disruptions by comparing to the actual boulder size distribution and the boulder surface age measured using small craters on the boulders.
References: [1] Michikami et al., 2019, Icarus, 331, 179-191.[2] DellaGiustina et al., 2019, Nat. Astron., 3, 341-351. [3] Yada et al., 2022, Nat. Astron., 6, 214-220. [4] Hsu et al., 2022, Nat. Astron., 6, 1043-1050. [5] Brown et al., 2002, Nature, 420, 294-296. [6] Suggs et al., 2014, Icarus, 238, 23-36.