McKeeby B E, Ramsey M S, Tai Udovicic C J, Haberle C, Edwards C S
Department of Geology and Environmental Science University of Pittsburgh Pittsburgh PA USA.
Department of Astronomy and Planetary Science Northern Arizona University Flagstaff AZ USA.
Earth Space Sci. 2022 Oct;9(10):e2022EA002430. doi: 10.1029/2022EA002430. Epub 2022 Oct 12.
Surface heterogeneities below the spatial resolution of thermal infrared (TIR) instruments result in anisothermality and can produce emissivity spectra with negative slopes toward longer wavelengths. Sloped spectra arise from an incorrect assumption of either a uniform surface temperature or a maximum emissivity during the temperature-emissivity separation of radiance data. Surface roughness and lateral mixing of different sub-pixel surface units result in distinct spectral slopes with magnitudes proportional to the degree of temperature mixing. Routine Off-nadir Targeted Observations (ROTO) of the Thermal Emission Imaging Spectrometer (THEMIS) are used here for the first time to investigate anisothermality below the spatial resolution of THEMIS. The southern flank of Apollinaris Mons and regions within the Medusae Fossae Formation are studied using THEMIS ROTO data acquired just after local sunset. We observe a range of sloped TIR emission spectra dependent on the magnitude of temperature differences within a THEMIS pixel. Spectral slopes and wavelength-dependent brightness temperature differences are forward-modeled for a series of two-component surfaces of varying thermal inertia values. Our results imply that differing relative proportions of rocky and unconsolidated surface units are observed at each ROTO viewing geometry and suggest a local rock abundance six times greater than published results that rely on nadir data. High-resolution visible images of these regions indicate a mixture of surface units from boulders to dunes, providing credence to the model.
热红外（TIR）仪器空间分辨率以下的地表非均质性会导致等温线异常，并能产生向更长波长方向斜率为负的发射率光谱。斜率光谱源于在辐射数据的温度-发射率分离过程中对均匀表面温度或最大发射率的错误假设。地表粗糙度和不同亚像素地表单元的横向混合会导致明显的光谱斜率，其大小与温度混合程度成正比。热发射成像光谱仪（THEMIS）的常规离天底定向观测（ROTO）首次在此用于研究THEMIS空间分辨率以下的等温线异常。利用当地日落刚过获取的THEMIS ROTO数据，对阿波利纳里斯山的南麓和墨杜萨槽沟组内的区域进行了研究。我们观察到一系列斜率不同的热红外发射光谱，这取决于THEMIS像素内温度差异的大小。针对一系列具有不同热惯量值的双组分表面，对光谱斜率和与波长相关的亮温差异进行了正向建模。我们的结果表明，在每个ROTO观测几何条件下，观察到的岩石和未固结地表单元的相对比例不同，并且表明当地岩石丰度比依赖天底数据公布的结果大六倍。这些区域的高分辨率可见光图像显示了从巨石到沙丘的地表单元混合情况，为该模型提供了可信度。
