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[PPS03-P02] A long-term evolution model of comets to asteroids considering the blowing off of rocks
Keywords:comets, asteroids, long-term evolution, numerical calculations
Comets and asteroids are small bodies in the solar system, and it has been considered that there is a clear distinction between them; namely, the former consisting mainly of ice and the latter consisting mainly of rock. When a comet approaches the sun, the ice sublimates and the cometary nucleus releases gas and rocks, which scatter sunlight and form coma and long tails (cometary activity). Contrarily, asteroids do not exhibit such activity. However, recent discoveries of active asteroids with cometary activity have blurred this distinction [1]. One possible cause is the evolution of comets into asteroids.
ause is the evolution of comets into asteroids.
Our previous evolutionary model [2] considered the process that the porous spherical cometary nucleus, which uniformly contains water ice and rock fragments, loses the water ice due to sublimation, and rocks accumulate on the surface of the nucleus due to its gravity to form a layer consisting only of rocks (dust mantle), which eventually becomes asteroids consisting only of rocks. However, the process of rocks being blown away by the outflowing water vapor was not considered. If the drag force exerted on a rock by the water vapor flow is stronger than the gravity by the cometary nucleus, the rock is blown away from the comet nucleus. Since this process affects the formation of dust mantle and cometary activity, it is important to consider the rock-blown-off process in the long-term evolution model of cometary nuclei.
In this study, we modeled the movement of rock entrained in a vapor flow within a cometary nucleus and its ejection from the surface of the cometary nucleus, and incorporated these processes into a numerical model of the long-term evolution of comets to asteroids. Since the gravity on the rock is proportional to its mass but the gas drag force is proportional to the cross-sectional area, smaller particles are more likely to be blown away and larger particles are less likely to be. We considered rocks of different sizes and calculated the gas drag and gravity for each rock to determine whether the rock would be blown away or not. Rocks inside the cometary nucleus can move outward through the void due to the gas drag force. Rocks smaller than the void can move, but larger cannot. We permitted the outward movement when a rock of a size that can pass through the void is determined to be blown away. The criterion for determining whether a rock of a certain size can move outward was determined by the size distribution of the dust outside. Numerical calculations of the long-term evolution of comets are performed considering the above processes, and the timescale of evolution into asteroids and the size distribution of rocks left on the surface of the object were investigated.
Numerical calculations showed that as ice sublimates inside the cometary nucleus, smaller rocks are caught in the water vapor flow and blown away, while larger rocks are not blown away and remain near the surface of the cometary nucleus. As the water-ice sublimation progressed and the dust mantle thickened, the water vapor flow weakened and had less power to blow away the rocks, so small rocks remained in the center of the cometary nucleus after the ice was completely depleted. This result suggests that the surfaces of asteroids that evolved from cometary nuclei are covered with larger particles than those that did not.
[1] Wakita, S. et al. (2019), Journal of the Planetary Society of Japan Yu-Sei-Jin, Vol 28, No.2, pp.124-139.
[2] Miura, H. et al. (2022), Astrophys. J. 925, L15.
ause is the evolution of comets into asteroids.
Our previous evolutionary model [2] considered the process that the porous spherical cometary nucleus, which uniformly contains water ice and rock fragments, loses the water ice due to sublimation, and rocks accumulate on the surface of the nucleus due to its gravity to form a layer consisting only of rocks (dust mantle), which eventually becomes asteroids consisting only of rocks. However, the process of rocks being blown away by the outflowing water vapor was not considered. If the drag force exerted on a rock by the water vapor flow is stronger than the gravity by the cometary nucleus, the rock is blown away from the comet nucleus. Since this process affects the formation of dust mantle and cometary activity, it is important to consider the rock-blown-off process in the long-term evolution model of cometary nuclei.
In this study, we modeled the movement of rock entrained in a vapor flow within a cometary nucleus and its ejection from the surface of the cometary nucleus, and incorporated these processes into a numerical model of the long-term evolution of comets to asteroids. Since the gravity on the rock is proportional to its mass but the gas drag force is proportional to the cross-sectional area, smaller particles are more likely to be blown away and larger particles are less likely to be. We considered rocks of different sizes and calculated the gas drag and gravity for each rock to determine whether the rock would be blown away or not. Rocks inside the cometary nucleus can move outward through the void due to the gas drag force. Rocks smaller than the void can move, but larger cannot. We permitted the outward movement when a rock of a size that can pass through the void is determined to be blown away. The criterion for determining whether a rock of a certain size can move outward was determined by the size distribution of the dust outside. Numerical calculations of the long-term evolution of comets are performed considering the above processes, and the timescale of evolution into asteroids and the size distribution of rocks left on the surface of the object were investigated.
Numerical calculations showed that as ice sublimates inside the cometary nucleus, smaller rocks are caught in the water vapor flow and blown away, while larger rocks are not blown away and remain near the surface of the cometary nucleus. As the water-ice sublimation progressed and the dust mantle thickened, the water vapor flow weakened and had less power to blow away the rocks, so small rocks remained in the center of the cometary nucleus after the ice was completely depleted. This result suggests that the surfaces of asteroids that evolved from cometary nuclei are covered with larger particles than those that did not.
[1] Wakita, S. et al. (2019), Journal of the Planetary Society of Japan Yu-Sei-Jin, Vol 28, No.2, pp.124-139.
[2] Miura, H. et al. (2022), Astrophys. J. 925, L15.