| Summary: | 博士 === 國立中央大學 === 天文研究所 === 97 === 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.
|
|---|
