Fine Layering Effects on Thermal Infrared Emissivity of CI Simulant Materials

Introduction: Thermal infrared emissivity measurements of asteroid regolith analogs are challenging owing to atmospheric water vapor absorption, sample heating requirements, and the need for controlled atmospheric conditions [1], yet they provide fundamental constraints on surface thermal properties that cannot be obtained from reflectance spectroscopy alone [1]. While diffuse reflectance measurements have demonstrated that minimal fine dust coverage can dominate spectral signatures [2], spacecraft-based thermal emission instruments like the OSIRIS-REx Thermal Emission Spectrometer (OTES) observe different physical processes related to thermal emission rather than scattered light [3]. The disconnect between laboratory studies and spacecraft observations has thus limited our ability to interpret thermal infrared spectra of asteroid surfaces. Previous work using Space Resource Technology's CI simulant showed that 7-10 wt% fine dust coverage could impose fine-dominated reflectance features on coarse substrates [2], but the corresponding thermal emission properties remained uncharacterized. To bridge this gap, we conducted systematic thermal emissivity measurements of layered CI simulant materials using Oxford’s PASCALE instrument [4] under nitrogen atmosphere, constraining how dust deposition mechanisms affect the thermal emission processes observed by spacecraft instruments at airless bodies like asteroid (101955) Bennu. 

Methods: We measured thermal emission of layered CI simulant [5] samples using PASCALE under nitrogen atmosphere across 2000-400 cm⁻¹ (5-25 µm), eliminating atmospheric water vapor interference. Six layering configurations were tested, using 10 wt% fines (<25 µm) and a coarse (250-500 µm) substrate, outlined in [2]: KBr (simulating porosity effects), sprinkled fines (simulating electrostatic deposition), liquid-deposited layers (isopropyl alcohol suspension), mechanically mixed samples (simulating gardening), lofted particles (gravitational settling from 1m), and directly sieved deposits. Spectra were acquired at 4 cm⁻¹ resolution with 150 scans using a Bruker 70v FTIR spectrometer, achieving signal-to-noise sufficient to identify 2% spectral contrast features [4].  

 

Figure 1: Emissivity spectra for each layering mechanism, continuum-corrected by using the thermal gradient derived from an internal calibration target. Spectra are then normalized to 1.  We note the shortwave end is subject to more noise, owing to instrument constraints, than the longwave end.

Results: PASCALE emissivity measurements reveal two distinct spectral groupings. “Fluffy” deposition methods (lofted, sieved, sprinkled) cluster together with similar spectral behavior, exhibiting prominent absorption features at ~1600, ~1400, and ~1000-1100 cm⁻¹ corresponding to carbonate and silicate vibrational modes. In contrast, KBr, liquid, and mixed samples form a second group with systematically different emissivity characteristics, reflecting porosity and compaction effects. Liquid samples display the most pronounced spectral deviations (>5% emissivity variations from unity), while the fluffy group shows more subdued but consistent spectral signatures. All method-dependent variations exceed the 2% measurement precision, demonstrating that dust deposition mechanism leaves diagnostic thermal emission signatures that can distinguish (and potentially identify) natural surface processes on airless body surfaces. 

Discussion: The separation between fluffy and compact layering methods demonstrates that thermal emission spectroscopy can distinguish surface formation processes on airless bodies. These results provide constraints missing from reflectance-only studies, by characterizing thermal emission properties relevant to spacecraft observations like OTES. The ability to spectrally distinguish between natural deposition processes offers new frameworks for understanding regolith evolution and thermophysical properties on asteroid surfaces. 

Summary: This study establishes thermal emissivity as a diagnostic tool for identifying dust deposition mechanisms on asteroid surfaces, demonstrating that layering processes leave distinct spectral signatures. 

References: [1] Salisbury et al. (1991) Icarus 92, 280-297. [2] Belhadfa et al. (2026) MaPs, In Prep. [3] Christensen P. R. et al. (2018) Space Science Reviews (Vol. 214, Issue 5). [4] Donaldson Hanna et al. (2019) Icarus 319, 701-723. [5] Landsman Z. et al. (2020) EPSC.