Hyperspectral imaging of convective CO2 ice clouds in the equatorial mesosphere of Mars
Hyperspectral imaging of convective CO2 ice clouds in the equatorial mesosphere of Mars
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火星赤道中间层对流二氧化碳冰云的高光谱成像
DOI：
10.1029/2007je002944
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发表时间：
2007
影响因子：
--
通讯作者：
T. Fouchet
中科院分区：
文献类型：
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作者：
F. Montmessin;B. Gondet;J. Bibring;Y. Langevin;P. Drossart;F. Forget;T. Fouchet
[1]火星气候的一个独特之处是，大气中的主要成分二氧化碳有可能凝结成冰。二氧化碳冰通常被检测为霜，但也被称为云。本文介绍了第一个明确的观测火星上的二氧化碳冰云。这些图像由火星快车上的可见光和近红外成像光谱仪OMEGA获得。数据集包括19个不同的事件。成分识别是基于对4.26 μm附近的诊断光谱特征的探测，该特征是由中层CO2冰粒对太阳光子的共振散射产生的，该光谱间隔否则由饱和气体吸收主导。观测到的云表现出很强的季节和地理依赖性，在北方夏至（Ls 45°和135°）之前和之后的两个时期集中在近赤道地区。辐射传输模型表明，4.26 μm特征对云的高度、不透明度和粒子大小非常敏感，从而解释了与云图像相关的光谱的多样性。在两个轨道上，同时检测云及其阴影提供了云属性的直接和可靠的估计。这些图像证实了从模拟中得出的结论：云层很厚，在近红外线上的正常不透明度大于0.2，并且在80公里以上的中间层中漂浮。发现CO2冰晶的平均半径超过1 μm，考虑到这个海拔范围，这是一个意想不到的值。这一发现意味着高空大气上升气流的存在，其强度足以抵消粒子的快速重力下落。这一说法与云的积云形态一致，这可能与二氧化碳凝结过程中释放的潜热产生的潮湿对流起源有关。
[1] A unique feature of the Martian climate is the possibility for carbon dioxide, the main atmospheric constituent, to condense as ice. CO2 ice is usually detected as frost but is also known to exist as clouds. This paper presents the first unambiguous observation of CO2 ice clouds on Mars. These images were obtained by the visible and near-infrared imaging spectrometer OMEGA on board Mars Express. The data set encompasses 19 different occurrences. Compositional identification is based on the detection of a diagnostic spectral feature around 4.26 μm which is produced by resonant scattering of solar photons by mesospheric CO2 ice particles in a spectral interval otherwise dominated by saturated gaseous absorption. Observed clouds exhibit a strong seasonal and geographic dependence, concentrating in the near-equatorial regions during two periods before and after northern summer solstice (Ls 45° and 135°). Radiative transfer modeling indicates that the 4.26 μm feature is very sensitive to cloud altitude, opacity, and particle size, thereby explaining the variety of spectra associated with the cloud images. On two orbits, the simultaneous detection of clouds with their shadow provides straightforward and robust estimates of cloud properties. These images confirm the conclusions established from modeling: clouds are thick, with normal opacities greater than 0.2 in the near infrared, and are lofted in the mesosphere above 80 km. The mean radius of CO2 ice crystals is found to exceed 1 μm, an unexpected value considering this altitude range. This finding implies the existence of high-altitude atmospheric updrafts which are strong enough to counteract the rapid gravitational fall of particles. This statement is consistent with the cumuliform morphology of the clouds which may be linked to a moist convective origin generated by the latent heat released during CO2 condensation.