THEMIS-VIS observations of clouds in the martian mesosphere: Altitudes, wind speeds, and decameter-scale morphology
THEMIS-VIS observations of clouds in the martian mesosphere: Altitudes, wind speeds, and decameter-scale morphology
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THEMIS-VIS 火星中间层云观测：高度、风速和十米尺度形态
DOI：
10.1016/j.icarus.2010.07.021
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发表时间：
2010
期刊：
影响因子：
3.2
通讯作者：
P. Christensen
中科院分区：
文献类型：
--
作者：
T. McConnochie;J. Bell;D. Savransky;M. Wolff;A. Toigo;Huiqun Wang;M. Richardson;P. Christensen
本文介绍了火星奥德赛号（ODY）航天器的热发射成像系统可见光成像子系统（THEMIS-VIS）获得的35个成像序列中中间层云的高度和向东速度分量的测量结果。我们利用高空特征的视差漂移来测量高度，利用THEMIS-VIS成像序列的时间延迟来测量速度。我们观测到两种不同类型的中间层云：0°和180°l之间观测到的赤道中间层云；北部中纬度的云只在200-300°lperiod的暮光中观测到。赤道中间层云在THEMIS-VIS数据集中是相当罕见的。在总共2048个多波段赤道成像序列中，我们仅在5个成像序列中检测到它们。这五个地方都在南纬20°到0°之间，东经260°到295°之间。中纬度的中间层云显然更为常见；对于这些，我们从210个北方冬季中纬度暮光成像序列中找到了30个例子。观测到的中纬度云只有一个例外，在Acidalia地区，但这很可能是THEMIS-VIS图像瞄准模式的人工产物。将THEMIS-VIS图像与火星轨道飞行器广角相机（MOC-WA）图像生成的每日全球地图进行比较，我们发现一些中纬度中间层云特征与MOC-WA通常观测到的云特征相对应。将我们的中间层云的速度与GCM进行比较，我们发现中纬度北部云的速度很符合，但也发现GCM与赤道云的强东风不匹配。将一个简单的辐射传输模型应用于一些赤道中间层云，我们发现两种不同的成像序列都有很好的模型拟合。通过使用观测到的云和无云区域在多个可见波段波长上的亮度对比，这些拟合同时约束了云的光学深度和颗粒大小。颗粒大小主要受波长相对对比的限制，在reff=0.1μm和reff=1.5μm两种成像序列下，颗粒大小存在较大差异。光学深度（受绝对对比度的限制）是实质性的：分别为0.22和0.5。这些光学深度意味着质量密度大大超过中间层温度下水蒸气的饱和质量密度，因此气溶胶粒子可能主要由co2组成。我们的简单辐射传输模型不适用于观测中纬度中间层云时的黄昏，因此我们将这些云的性质留作进一步工作的问题。
We present measurements of the altitude and eastward velocity component of mesospheric clouds in 35 imaging sequences acquired by the Mars Odyssey (ODY) spacecraft’s Thermal Emission Imaging System visible imaging subsystem (THEMIS-VIS). We measure altitude by using the parallax drift of high-altitude features, and the velocity by exploiting the time delay in the THEMIS-VIS imaging sequence. We observe two distinct classes of mesospheric clouds: equatorial mesospheric clouds observed between 0° and 180°Ls; and northern mid-latitude clouds observed only in twilight in the 200–300°Lsperiod. The equatorial mesospheric clouds are quite rare in the THEMIS-VIS data set. We have detected them in only five imaging sequences, out of a total of 2048 multi-band equatorial imaging sequences. All five fall between 20° south and 0° latitude, and between 260° and 295° east longitude. The mid-latitude mesospheric clouds are apparently much more common; for these we find 30 examples out of 210 northern winter mid-latitude twilight imaging sequences. The observed mid-latitude clouds are found, with only one exception, in the Acidalia region, but this is quite likely an artifact of the pattern of THEMIS-VIS image targeting. Comparing our THEMIS-VIS images with daily global maps generated from Mars Orbiter Camera Wide Angle (MOC-WA) images, we find some evidence that some mid-latitude mesospheric cloud features correspond to cloud features commonly observed by MOC-WA. Comparing the velocity of our mesospheric clouds with a GCM, we find good agreement for the northern mid-latitude class, but also find that the GCM fails to match the strong easterly winds measured for the equatorial clouds. Applying a simple radiative transfer model to some of the equatorial mesospheric clouds, we find good model fits in two different imaging sequences. By using the observed radiance contrast between cloud and cloud-free regions at multiple visible-band wavelengths, these fits simultaneously constrain the optical depths and particles sizes of the clouds. The particle sizes are constrained primarily by the relative contrasts at the available wavelengths, and are found to be quite different in the two imaging sequences: reff=0.1μm and reff=1.5μm. The optical depths (constrained by the absolute contrasts) are substantial: 0.22 and 0.5, respectively. These optical depths imply a mass density that greatly exceeds the saturated mass density of water vapor at mesospheric temperatures, and so the aerosol particles are probably composed mainly of CO2ice. Our simple radiative transfer model is not applicable to twilight, when the mid-latitude mesospheric clouds were observed, and so we leave the properties of these clouds as a question for further work.