Hematite Coatings Match TES Spectra of Sinus Meridiani, Mars
Hematite Coatings Match TES Spectra of Sinus Meridiani, Mars
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赤铁矿涂层与火星子午线窦的 TES 光谱相匹配
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发表时间：
2003
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通讯作者：
J. Salisbury
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作者：
L. Kirkland;K. Herr;P. Adams;J. Salisbury
导论. Lane等人[1]和Christensen等人[2]得出结论，1996年全球勘测者热发射光谱仪（TES，~1700-200 cm）在子午线窦区域的光谱仅与粗颗粒赤铁矿相匹配。在这里，我们展示了与TES特征相匹配的赤铁矿涂层的光谱。涂层的选择是重要的，因为（1）赤铁矿涂层可能需要很少的水来形成，这可能会大大改变目前的解释，并解释明显缺乏其他蚀变矿物;（2）2003年的一个漫游者计划在这个网站上，所以研究人员需要准备可能测量赤铁矿涂层的漫游者仪器。这包括将涂层特征添加到解释库，并解决与赤铁矿涂层相关的测量问题。光谱对比度。赤铁矿有三个吸收带，中心在18、23和33 μm附近。这些强带被称为“剩余辐射带”。当剩余辐射带被观察为具有良好光谱对比度的槽时，这表明存在两种广泛形式之一的光滑表面材料：（1）紧密堆积或胶结的细颗粒（例如，涂层或硬锈）;（2）大颗粒。有两种广泛的选择，因为材料可以通过两个过程散射光：表面和体积散射[3]。强带是由表面的高反射率（表面散射）产生的，当带内的高不透明度使其具有镜面性质时。这会产生称为“剩余辐射带”的反射峰。在发射中，表面在剩余辐射带处向内反射辐射，导致发射率谷（例如图1“固体表面”）[4]。当未固结的颗粒足够小，光可以通过颗粒，体积吸收（体积散射）发生[5]。当体积散射占主导地位时，剩余辐射带表现为发射峰（图1，下部迹线）。粗糙与涂层。对于光学薄的材料，体积散射占主导地位，并且大多数材料在小颗粒尺寸下变得光学薄。这导致非常小的赤铁矿颗粒具有非常低的光谱对比度（例如，图1上迹线），这与TES观测相反。这种推理已被用来排除细颗粒赤铁矿的存在作为所观察到的TES签名的来源。然而，当光学薄颗粒靠近（~波长）在一起时，它们相干地散射，并且表现得好像它们是较大的颗粒[5]。这种情况发生在胶结的细颗粒中（例如，可能发生在硬锈和沙漠清漆中）。表面散射和体积散射的相对贡献的差异将影响带的深度、形状和宽度。
Introduction. Lane et al. [1] and Christensen et al. [2] conclude that the 1996 Global Surveyor Thermal Emission Spectrometer (TES, ~1700–200 cm) spectra of regions in Sinus Meridiani match only coarsely particulate hematite. Here we show spectra of hematite coatings that match the TES signatures. The coating option is important because (1) hematite coatings can require little water to form, which may significantly change current interpretations and explain the apparent absence of other alteration minerals; and (2) one 2003 rover is planned for this site, so researchers need to prepare for possible measurement of hematite coatings by the rover instruments. This includes adding coating signatures to interpretation libraries and addressing measurement issues related to a hematite coating. Spectral contrast. Hematite has three absorption bands centered near 18, 23, and 33 μm. These strong bands are called “reststrahlen bands.” When reststrahlen bands are observed as troughs with good spectral contrast, that indicates the presence of smooth-surfaced material in one of two broad forms: (1) closely packed or cemented fine particles (e.g., a coating or duricrust); (2) large particles. There are two broad options because material can scatter light through two processes: surface and volume scattering [3]. A strong band is produced by high reflectance from the surface (surface scattering) when high opacity within the band gives it a mirrorlike property. This produces reflectance peaks called "reststrahlen bands." In emission, the surface reflects radiance inward at reststrahlen bands, causing an emissivity trough (e.g. Fig. 1 “solid surface”) [4]. When unconsolidated particles are small enough for light to survive passage through the grain, volume absorption (volume scattering) occurs [5]. When volume scattering dominates, the reststrahlen bands appear as emission peaks (Fig. 1, lower trace). Coarse vs. coating. Volume scattering dominates for optically thin materials, and most materials become optically thin at small particle sizes. This causes very small hematite particles to have very low spectral contrast (e.g., Fig. 1 upper trace), which is contrary to the TES observations. This reasoning has been used to rule out the presence of finely particulate hematite as the source of the observed TES signatures. However, when optically thin particles are close (~wavelength) together, they scatter coherently, and behave as if they were larger particles [5]. This occurs for cemented, fine particles (e.g. as may occur in duricrust and desert varnish). Differences in the relative contribution from surface and volume scattering will affect the band depths, shapes, and widths.