Experimental and analytical methods for thermal infrared spectroscopy of complex dust coatings in a simulated asteroid environment

The spectral and thermophysical effects of thin-continuous, macro-discontinuous, and micro-discontinuous dust cover are not understood and require relevant laboratory analyses to be deconvolved in an orbital setting. We have constructed a custom environment chamber that enables the controlled deposition of size-regulated dust particles in coatings with varying continuity and thickness. TIR spectra of coated substrates acquired in a simulated asteroid environment (SAE) are used to investigate the extent to which dust coatings of different thicknesses and arrangements contribute to orbital spectral signatures of airless body surfaces. TIR (5-50 𝜇m) spectra of each sample are acquired under SAE conditions using the Planetary and Asteroid Regolith Spectroscopy Environmental Chamber (PARSEC) at Stony Brook University. PARSEC is designed to measure samples under environmental conditions typical of airless bodies. The chamber houses a sample wheel with six sample cups and a calibration target coated with Nextel black. There is also a black body target under the wheel. All sample cup and black body targets can be individually heated and rotated into position from outside of the chamber. Temperature is controlled through two Eurotherm Mini8 Loop Controllers and managed on an in-lab computer with the Eurotherm iTools interface. Samples are illuminated at 55° incidence by a quartz halogen lamp connected to a Bentham 610 power source. Surrounding the sample wheel is a cold shield actively cooled by the input of liquid nitrogen into an internal dewar to reach temperatures < 150 K. Pressure in the chamber is controlled by a Pfeiffer HiCube turbo vacuum pump to reach 10^-6 mbar. In line with the vacuum chamber is a pressure regulated tank of N₂ used for purging and ambient pressure measurements. The PARSEC chamber is connected to a Nicolet 6700 FTIR spectrometer equipped with a Cesium Iodide (CsI) beamsplitter and a deuterated L-alanine doped triglycine sulfate (DLaTGS) detector with a CsI window. The spectrometer is actively purged with air scrubbed of CO₂ and water vapor and sealed at the interface with PARSEC. A total of 256 scans from 2,200 to 400 cm^-1 are integrated for a 10-minute measurement period, using a spectral sampling of 2 cm^-1. During each experimental session, all samples and the black body are measured under SAE conditions. Calibration measurements of the blackbody target at 70 and 100°C are acquired, then the integrated sample cup heaters and solar lamp are adjusted to achieve the desired sample brightness temperature of 80°C. Samples are allowed to reach temperature under the lamp for more than 45 minutes until the spectral maximum stabilizes and the calculated brightness temperature at the CF is within ~10 K of the target 353 K. This procedure is repeated for all samples while ensuring the chamber temperature remains stable under 150 K through continued addition of liquid nitrogen. The Radiance-to-emissivity conversion method used determines the maximum brightness temperature between 500 and 1700 cm-1 and divides the radiance by a Planck function of the same temperature. This assures the maximum brightness temperature is the kinetic temperature of the same, and its emissivity is unity at the frequency of this maximum. This dataset includes 15 different samples acquired under SAE and ambient pressure/temperature conditions. These samples range in layer thickness and continuity and are intended to test and demonstrate the range of the coating process.薄连续、宏观不连续及微观不连续尘埃覆盖层的光谱与热物理效应尚未明确，亟需在轨道环境下通过相关实验室分析进行解卷积处理。本研究搭建了一套定制化环境舱，可实现粒径受控的尘埃粒子在连续性与厚度各异的涂层上的可控沉积。在小行星模拟环境（Simulated Asteroid Environment, SAE）中采集的带涂层基底热红外（Thermal Infrared, TIR）光谱，被用于探究不同厚度与排布方式的尘埃涂层对无大气天体表面轨道光谱特征的贡献程度。本研究采用斯托尼布鲁克大学的行星与小行星风化层光谱学环境舱（Planetary and Asteroid Regolith Spectroscopy Environmental Chamber, PARSEC），在SAE条件下采集所有样品的5~50 μm波段TIR光谱。PARSEC专为在无大气天体典型环境条件下开展样品测试而设计。该舱内置带有6个样品杯的样品转盘，以及涂覆有Nextel黑涂层的校准靶；转盘下方还设有黑体靶标。所有样品杯与黑体靶标均可独立控温，并可从舱外旋转至指定测试位置。温度通过两台欧陆Mini8回路控制器进行调控，并通过搭载欧陆iTools操作界面的实验室内置计算机进行管理。样品以55°入射角接受石英卤钨灯的辐照，该灯连接至Bentham 610型电源。样品转盘外围设有冷屏蔽层，通过向内部杜瓦瓶注入液氮实现主动制冷，可将环境温度降至150 K以下。舱内压力通过普发HiCube涡轮真空泵进行调控，可达到10⁻⁶毫巴的真空度。真空舱配套设有调压氮气储气罐，用于吹扫作业与环境压力测量。PARSEC舱体连接至Nicolet 6700型傅里叶变换红外（Fourier Transform Infrared, FTIR）光谱仪，该光谱仪配备碘化铯（Cesium Iodide, CsI）分束器，以及带有CsI窗口的氘代L-丙氨酸掺杂硫酸三甘氨酸（Deuterated L-alanine doped Triglycine Sulfate, DLaTGS）探测器。该光谱仪采用脱除了CO₂与水蒸气的空气进行主动吹扫，并在与PARSEC的接口处实现密封。在10分钟的测量周期内，对2200~400 cm⁻¹波段共计256次扫描结果进行积分，光谱采样分辨率设为2 cm⁻¹。每次实验过程中，所有样品与黑体靶标均在SAE条件下完成测量。首先采集黑体靶标在70℃与100℃下的校准光谱，随后调整集成样品杯加热器与模拟太阳灯的参数，使样品达到目标亮温80℃。待样品在灯光辐照下升温超过45分钟，直至光谱峰值趋于稳定，且在CF处计算得到的亮温与目标353 K的偏差不超过约10 K。针对所有样品重复上述流程，期间通过持续注入液氮确保舱内温度稳定维持在150 K以下。本研究采用的辐亮度-发射率转换方法，首先确定500~1700 cm⁻¹波段内的最大亮温，再将辐亮度除以对应温度下的普朗克函数。该方法可确保该最大亮温等于样品的动力学温度，且在该峰值频率处的发射率为1。本数据集包含15种不同样品，均在SAE条件与环境压力/温度条件下完成采集。这些样品的涂层厚度与连续性各不相同，旨在验证并展示该涂层制备工艺的可调范围。