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.