Design and Implementation of the Chamber for Simulating Asteroid and Lunar Environment(ChaSALE)

Introduction: China's Tianwen-2 mission is designed to achieve orbital exploration and sample return from the near-Earth asteroid (2016 HO₃), as well as a flyby exploration of the main-belt comet (311P), in a single launch [1]. Equipped with an Asteroid Thermal Emission Spectrometer (ATES), the mission aims to determine the surface mineral composition and thermophysical properties of both targets, thereby shedding light on their formation and evolutionary mechanisms [2]. However, on airless bodies like asteroids [3], the lack of interstitial gas limits heat transfer within the shallow subsurface to inefficient inter-particle radiation and contact conduction. This creates a steep thermal gradient within the top few hundred micrometers, which significantly alters thermal emission spectral features (e.g., shifting the Christiansen Feature (CF) to higher wavenumbers and increasing spectral contrast) [4-9]. Consequently, remote sensing thermal emission spectra cannot be directly interpreted using standard spectral libraries acquired under terrestrial conditions. To accurately interpret thermal infrared remote sensing data from airless bodies, laboratory thermal emission spectroscopy systems that simulate the environments of airless bodies are strictly required. Several such systems have been established internationally to simulate airless surface environments, including Brown University's ALEC [10], Oxford University's SLEC [11] and PASCALE [12], and Stony Brook University's PARSEC [13]. However, there is currently no comparable facility in China. To address this gap, this abstract introduces a newly, independently developed thermal emission spectroscopy measurement system (ChaSALE) designed for airless bodies such as the Moon and asteroids.

Design and Implementation of the ChaSALE:  The ChaSALE primarily consists of a vacuum vessel, a 60K cryogenic helium circulation system, a sample cup assembly, a cryogenic off-axis parabolic mirror, a solar simulator and a control unit (Figure 1). The vacuum vessel provides the necessary vacuum environment for sample testing and includes a chamber, a pumping system and vacuum gauges. The system is capable of achieving a high vacuum of 8×10^-9 bar inside the chamber. The 60K cryogenic helium circulation system cools the chamber to provide a cold background environment of < 100K, consisting of a thermal shroud, a cryocooler and associated piping. The sample cup assembly is used to hold and heat the test samples. From top to bottom, it comprises the sample receptacle, a heating module, a thermal insulation block, a support rod and a base. The heating module controls the sample temperature to simulate the varying thermal conditions on a planetary surface. The sample cup is designed with dimensions of Φ44 mm×5 mm. The cryogenic off-axis parabolic mirror collimates the divergent emission signals from the blackbody and the sample, directing the parallel beam into the spectrometer. A gold-coated aluminum mirror is utilized. To prevent the mirror's own thermal emission from contaminating the measurements, it is housed within a cold shield (Figure 1), which cools the mirror to approximately 150K during testing. The solar simulator is designed to replicate solar irradiation on the target body. The chamber is coupled to a Bruker Vertex 80V vacuum Fourier Transform Infrared (FTIR) spectrometer, which is equipped with a CsI beamsplitter and a liquid nitrogen-cooled Mercury Cadmium Telluride (MCT) detector. The operational capabilities and technical specifications of this system, alongside a comparison with similar international facilities, are summarized in Table 1.

Figure 1: Schematic diagram of the thermal emission spectrum measurement system and its optical path.

Table 1: Configurations and specifications of thermal emission measurement systems for the airless celestial bodies.

Samples Test: To validate the capability of our newly developed system to reproduce thermal emission spectra, we selected a particulate mixture of olivine and calcite (70 wt.% olivine and 30 wt.% calcite) with well-characterized spectral features as a test sample. We measured and derived the sample's emissivity under cryogenic vacuum conditions, and the results are presented in Figure 2. By comparing our data with standard mineral spectra for olivine and calcite, key diagnostic features can be clearly identified. For olivine, the CF at ~1100 cm^-1, the Transparency Feature (TF) at ~790 cm^-1, and the Reststrahlen Bands (RB) located between them are distinctly resolved. Similarly, for calcite, the prominent spectral features at ~875 cm^-1 and ~710 cm^-1, alongside the broad absorption feature between 1400 cm^-1 and 1200 cm^-1, are readily apparent. These findings successfully demonstrate the system's reliability in accurately reproducing the thermal emission spectra of geological samples under simulated cryogenic vacuum environments.

Figure 2: Validation results of the system’s ability to replicate sample thermal emission spectra.

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