小行星 25143 Itokawa 表面的物質分布成因

The origin of the surface material distribution on asteroid 25143 Itokawa

孫凱隆(Kai-Lung Sun)

指導教授 ： 葉永烜

國立中央大學/理學院/天文研究所/碩士(2006年)

摘要

小行星的表面物質能對太陽系統的形成與演化提供重要的線索。日本的太空船鷹隼號 (Hayabusa)在西元2003年的時候發射,於2005年的九月到達了糸川(Itokawa)小行 星,對這小行星做了一些觀測,並且採集樣本,暫定在2010年會將樣本帶回地球。 觀測中發現,糸川小行星為狹長的形狀,將其套成近似的橢圓球體則軸長為540 × 270 × 210公尺。鷹隼號對糸川小行星拍的照片中可看出此小行星有一個特別之處,就是表面很明顯地有粗糙而充滿碎石的地區以及平滑而充滿細塵的地區,而平滑區大約都集中在靠近小行星的自轉軸附近。
本論文試著用數值方法解釋這個現象。我們假設糸川小行星是由鄰近雙星所組成的,而雙星間的空隙則是被隕石撞擊小行星表面時產生的拋出物所填滿;在填滿的過程中,小塵埃因為比較輕而容易受到震波的影響而從高重力位的地方流動到低重力位的地方(Cheng et al. 2002),因而形成一個由小塵埃組成的等重力位的平面,大石礫則被掩埋住,形成鷹隼號在糸川小行星上看到的自轉軸附近的平滑區。
要驗證這個假設,得要模擬雙星系統內小行星表面的隕石坑噴出物的軌跡,並對這些噴出物做一些簡單的統計。我們在三度空間上用格點建立出一個鄰近雙星的模型,其質量、大小與公轉的週期都與糸川小行星相當,利用此模型可以計算粒子在小行星附近的受力,並利用四階的 Runge-Kutta 方法計算粒子軌道。粒子是從雙星系統中其中一顆小行星的地表平均射出,粒子的仰角為45度而方向是隨機的,速度是在0 ～ 15cm/s之間隨機分佈,目前為止共試驗了25600個粒子。
對這些粒子做統計之後,發現全部粒子的重新碰撞點在靠近雙星系統的公轉軸附近有一個最大值,尤其是粒子的初速度在9 ～ 12cm/s之間,若再加上震波使得表面物質重新分佈的機制,或許可以解釋糸川小行星的物質分佈。

關鍵字

模擬 ； 雙星 ； 碎屑層 ； 小行星 ； 震波 ； 糸川小行星

並列摘要

The surface material on asteroids plays important roles on the understanding of the formation and evolution of the solar system. The Japanese spacecraft Hayabusa launched in 2003 and arrived at asteroid 25143 Itokawa in 2005. The purpose of the mission is to observe the asteroid, collect samples from the surface, and return to Earth with these precious samples in 2010 if everything goes well.
The observations by the imaging camera onboard the Hayabusa spacecraft found that asteroid Itokawa is an ellipsoidal-shaped asteroid with its three axes measured to be close of 540 × 270 × 210 meters. From pictures obtained by Hayabusa, people may easily found that the surface of the asteroid could be divided into two regions, the rough regions which are full of boulders and the smooth regions which have only centimeter and sub-centimeter sized particles. The smooth areas are located near the rotation axis.
In this work, we try to explain why there is such a difference in surface material distribution. We assume that asteroid 25143 Itokawa is composed of a contact binary system, with the slit between the two components filled with collisional fragments and crater ejecta. Seismic resurfacing effect caused by impact events could mobilize the lighter particles. The small particles could therefore be moved to a potential minimum area (Cheng et al. 2002) and formed a ponded deposit area.
In order to prove this assumption, tra jectories of crater ejecta have been simulated. We build a contact binary model using 1918 3-D grid points in our program with its total mass, size, and rotation period taken to be the same as asteroid Itokawa. The tra jectories of test particles are calcaulated by using the fourth order Runge-Kutta algorithm. In our simulation, all test particles are ejected randomly with elevation angle of 45 degrees. The ejection velocity is distributed between 0 and 15 cm/s. Tra jectories of 25,600 test particles are simulated.
Results show that there is a concentration of re-impact sites near the rotation axis, especially for particles with initial velocity between 9 ～ 12cm/s. Thus the surface material distribution of asteroid Itokawa could be explained by combining our results with the seismic resurfacing mechanism.

並列關鍵字

asteroids ； regolith ； simulation ； binary ； Itokawa ； seiemic resurfacing

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