摘 要 火星南北極冰冠對大氣結構與動力上扮演重要的角色。經由火星冰冠的昇華與凝結有效的成為全球氣候的指標。極區包含季節性乾冰，此冰層在冬季會延伸到中緯度，而在夏季永久冰層會持續存在於極區中。在北半球的永久冰層主要是由水冰所組成，在南半球則是由乾冰與水冰共同組成。南半球的永久冰冠形狀是不對稱的，在永久冰冠的對面存在一種非常特殊的地形名為「Cryptic Region」。這種地形的特徵是低反照率與低溫，而且只發生在火星南半球。許多公里尺度的扇形(fans)結構顯然是由於氣體噴流從未完全昇華的冰層中噴出，再加上風的作用所造成的。在南半球從Ls＝180度開始，季節性的冰層開始消融，扇形與蜘蛛形(spider)結構依序出現，而且這些地形特徵幾乎每年都會出現在相同的區域中。 火星雷射測高儀(MOLA)是火星全球測量者號(MOC)其中的一個探測儀器，提供地形測量、地面光滑度以及冰層的高度變化更精細的觀測。使用火星雷射測高儀收集超過一整個火星年的資料(1999-2001)，利用這些資料來探測冰層的高度變化以及二氧化碳在南北半球極區季節性演化。在北半球極區，最大的高度變化發生在Ls =330度，變化值是1.09公尺，在南半球則是發生在Ls= 150度，高度變化是0.78公尺。 我們計算出四維(三維空間加上時間)的季節性乾冰厚度變化以及cryptic 區域與非cryptic 區域凝結與昇華隨著季節的差異。結果顯示這兩個區域的乾冰高度變化隨著時間是相當類似的。最大的乾冰高度變化在這兩個區域大約是0.76-0.78 公尺，發生的時間是在Ls=150度。 在這份研究中我們使用火星軌道相機(MOC)窄視野影像來做扇形及蜘蛛形地形的時間分布與地形關係分布的系統研究。我們發現大部分的扇形特徵分布在春天早期，Ls 小於230度，扇形與蜘蛛形地形會共同發生於Ls=250± 20度的時間內。同時這些特殊地形顯然與緯度及高度有很密切的關係，例如大部分發生於緯度83度以南，高度2500公尺以上。統計結果也指出扇形特徵的出現與乾冰厚度有密切的關係，這也支持了排氣模式(Venting Model)的說法。 Abstract The polar caps of Mars play an important role on the atmospheric structure and dynamics of this volatile-rich planet. The processes of sublimation-condensation of Martian polar caps can be a sensitive indicator of global weather. The polar regions include seasonal CO2 frost caps which will stretch to mid latitudes in winter and residual frost deposits close to the pole which will remain also in the summer. The permanent ice cap on the northern cap is composed of water ice and the southern cap includes both carbon dioxide ice and water ice. The South Pole of Mars is characterized by an asymmetric residual ice cap. On the opposite side of the south residual cap, there exists a distinctive terrain named Cryptic Region which is characterized by regions of low albedo and low temperature has been identified on the Martian south polar cap. Many fan-shaped km-scale structures apparently caused by a wind-blown system of dust-laden gas jets occurred dozens degrees of Ls before the complete sublimation of the CO2 frost layer. During the warming period starting at Ls~180o in the south hemisphere, the seasonal ice cap regresses and fans and spiders appear subsequently. These surface features are repeatable events that tend to occupy the same areas from year to year. The Mars Orbital Laser Altimeter (MOLA), an instrument on the Mars Global Surveyor（MGS）, provided detailed mapping of the topography, surface roughness and the height of volatile deposits. Using the MOLA topography data collected over one Martian year (1999-2001), we have studied the temporal elevation change and the seasonal cycle of the carbon dioxide frost on the northern and southern polar caps. In the north polar cap, the maximum elevation changes occurred at Ls=3300 and the value was 1.09 m and about 0.78 m at the opposite Ls=1500 in the south. . We have produced four-dimensional (3D plus time) mappings of the seasonal CO2 frost thickness variation and examined the seasonal cycles of condensation and sublimation in the cryptic and non-cryptic regions. It is found that the time variations of the CO2 frost thickness in these two regions are quite similar. The greatest thickness of the CO2 frost layer is about 0.76-0.78 meter in both places and occurs at Ls=1500. In this study, we also use the Mars Orbiter Camera (MOC) narrow angle images to produce a statistical study of the time distributions of the fans and spiders as functions of Ls and as functions of the topography. We have documented that most of the fans are found in the early spring with Ls < 230o and the fans and spiders coexist at Ls=250o ± 20o. It is also found that there is a strong dependence on latitude and altitude with fans and spiders most often observed at high latitude (>83oS) and high altitude (>2500 m). Our statistical result also indicates that the occurrence of fans is highly correlated with the thickness of the CO2 frost thus providing support for the venting model.
