為了探究行星原始的狀態，科學家們將目標指向保留較多原始資訊的小行星，而曙光號正是為此目的而出發的，探測目標為穀神星，灶神星，及智神星。主小行星帶的位置在火星及木星的軌道之間，在這個區域中有一條有名的邊界，雪線，是劃分小行星殘餘水分與否的標的。目前為止已經有許多研究利用模擬，探討小行星上塵埃層以下是否能留有水冰（詳見費奈立等人在1987年發表的論文及巴南等人在1989年發表的論文及薛爾福等人在2008年發表的論文）。
本篇論文使用三維外球層模型，三維熱模形，及三維光線跟蹤模擬方法，來計算灶神星上水冰留存的機率。更甚者，由於灶神星的自轉軸傾角達二十一度，因此季節變化明顯，不同季節也會造成在灶神星上陰影及輻射量改變，這使得溫度改變同時也影響了水分子在表面的沾粘效應。

To detect the more original state of minor planets, now there is one mission exploring the main asteroids belt, Dawn Mission, and the destinations are Vesta, Ceres, and maybe Pallas. At the orbit between Mars and Jupiter, the region that main asteroids belt locates can be divided by the well-known boundary, snowline. Although the existence of water ice on Vesta hasn’t been found yet, many researches (cf. Fanale and Savail, 1987; Bar-Nun et al., 1989; Schorghofer, 2008) simulate the amount of water ice survived under dust mantle in the main asteroids belt.
This thesis applies the 3D exospheric model, 3D thermal model and ray tracing to simulate the survival rate of water ice on Vesta. Moreover, the seasonal effect on Vesta makes the diurnal temperature different, which also changes the region of adsorption and cold traps. The proposed ray tracing model can be used to compute the seasonal change of solar radiation flux which dominates the diurnal temperature. However, the mean free path of molecules in the exosphere of Vesta is dominated by the gravity. Since the shape of Vesta is not spherical, the proposed exospheric model also applies the elliptical gravity.

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