大角度成像光谱法反演火星大气尘埃不透明度
Retrieving Atmospheric Dust Opacity on Mars by Imaging Spectroscopy at Large Angles
摘要：
提出了一种利用OMEGA和CRISM高光谱图像反演1 μm参考波长下火星气溶胶光学厚度的新方法。即使下垫面完全由矿物组成，对应于表面和大气尘埃之间的低对比度，我们的方法也有效，同时以固定的几何形状进行观察。最大限度地减少表面反射属性对AOD反演的影响是我们方法的第二个主要资产。该方法是基于粒子和气体之间的辐射耦合的参数化确定，与当地的测高，采集几何形状，和气象情况下，气态CO2的吸收带深度。由于后三个因素在一定程度上可以预测，因此我们可以定义一个新的参数β，该参数直接取决于AOD，具体表示气溶胶耦合的强度。结合从2 μm波段CO2气体观测光谱中提取的β和大气辐亮度顶值的估算，利用辐射转移反演了大气光学厚度和地表反射率。应该注意的是，实际上β可以在各种矿物或冰表面上进行估计，但CO2冰除外，因为它的2 μm固体带没有充分饱和。验证所提出的方法表明，它是可靠的，如果满足两个条件：（i）观测条件提供大的入射角或/和出射角（ii）气溶胶在大气中垂直混合良好。在OMEGA天底观测和入射角分别大于65°和33°的CRISM EPF采集上进行的实验产生了非常令人满意的结果。最后，在一个配套文件中，该方法被应用到监测大气尘埃春季活动在火星南部高纬度地区使用欧米茄。
英文摘要：
We propose a new method to retrieve the optical depth of Martian aerosols (AOD) from OMEGA and CRISM hyperspectral imagery at a reference wavelength of 1 μm. Our method works even if the underlying surface is completely made of minerals, corresponding to a low contrast between surface and atmospheric dust, while being observed at a fixed geometry. Minimizing the effect of the surface reflectance properties on the AOD retrieval is the second principal asset of our method. The method is based on the parametrization of the radiative coupling between particles and gas determining, with local altimetry, acquisition geometry, and the meteorological situation, the absorption band depth of gaseous CO2. Because the last three factors can be predicted to some extent, we can define a new parameter β that expresses specifically the strength of the gas-aerosols coupling while directly depending on the AOD. Combining estimations of β and top of the atmosphere radiance values extracted from the observed spectra within the CO2 gas band at 2 μm, we evaluate the AOD and the surface reflectance by radiative transfer inversion. One should note that practically β can be estimated for a large variety of mineral or icy surfaces with the exception of CO2 ice when its 2 μm solid band is not sufficiently saturated. Validation of the proposed method shows that it is reliable if two conditions are fulfilled: (i) the observation conditions provide large incidence or/and emergence angles (ii) the aerosol are vertically well mixed in the atmosphere. Experiments conducted on OMEGA nadir looking observations as well as CRISM EPF acquisitions with incidence angles higher than 65° and 33° respectively produce very satisfactory results. Finally in a companion paper the method is applied to monitoring atmospheric dust spring activity at high southern latitudes on Mars using OMEGA.
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