Updates to the Oxford Space Environment Goniometer to measure visible wavelength bidirectional reflectance distribution functions in ambient conditions.

Understanding how the surfaces of airless planetary bodies-such as the Moon-scatter visible light enables constraints to be placed on their surface properties and top boundary layer inputs to be set within thermal models. Remote sensing instruments-such as Diviner onboard the Lunar Reconnaissance Orbiter-measure thermal emission and visible light scattering functions across visible (∼0.38-0.7 µm) to thermal infrared (TIR) wavelengths (∼0.7-350 µm). To provide ground support measurements for such instruments, the Oxford Space Environment Goniometer (OSEG) was built. Initially, the OSEG focused on measuring TIR directional emissivity functions for regolith and regolith simulant samples in a simulated space environment, but it has recently been modified to measure visible wavelength Bidirectional Reflectance Distribution Functions (BRDFs) of samples in ambient conditions. Laboratory-measured BRDFs can be used (1) to test and to help interpret models-such as the Hapke photometric model-and (2) as visible scattering function inputs for thermal models. This paper describes the modifications to and initial calibration measurements taken by the Visible Oxford Space Environment Goniometer with a 532 nm laser, and details how this setup can be used to measure BRDFs of regolith and regolith simulant samples of airless planetary bodies.

Spectral Characterization of Bennu Analogs Using PASCALE: A New Experimental Set-Up for Simulating the Near-Surface Conditions of Airless Bodies.

We describe the capabilities, radiometric stability, and calibration of a custom vacuum environment chamber capable of simulating the near-surface conditions of airless bodies. Here we demonstrate the collection of spectral measurements of a suite of fine particulate asteroid analogs made using the Planetary Analogue Surface Chamber for Asteroid and Lunar Environments (PASCALE) under conditions like those found on Earth and on airless bodies. The sample suite includes anhydrous and hydrated physical mixtures, and chondritic meteorites (CM, CI, CV, CR, and L5) previously characterized under Earth- and asteroid-like conditions. And for the first time, we measure the terrestrial and extra-terrestrial mineral end members used in the olivine- and phyllosilicate-dominated physical mixtures under the same conditions as the mixtures and meteorites allowing us better understand how minerals combine spectrally when mixed intimately. Our measurements highlight the sensitivity of thermal infrared emissivity spectra to small amounts of low albedo materials and the composition of the sample materials. As the albedo of the sample decreases, we observe smaller differences between Earth- and asteroid-like spectra, which results from a reduced thermal gradient in the upper hundreds of microns in the sample. These spectral measurements can be compared to thermal infrared emissivity spectra of asteroid (101955) Bennu's surface in regions where similarly fine particulate materials may be observed to infer surface compositions.

The Oxford space environment goniometer: A new experimental setup for making directional emissivity measurements under a simulated space environment.

Measurements of the light scattering behaviour of the regoliths of airless bodies via remote sensing techniques in the Solar System, across wavelengths from the visible to the far infrared, are essential in understanding their surface properties. A key parameter is knowledge of the angular behaviour of scattered light, usually represented mathematically by a phase function. The phase function is believed to be dependent on many factors including the following: surface composition, surface roughness across all length scales, and the wavelength of radiation. Although there have been many phase function measurements of regolith analog materials across visible wavelengths, there have been no equivalent measurements made in the thermal infrared (TIR). This may have been due to a lack of TIR instruments as part of planetary remote sensing payloads. However, since the launch of Diviner to the Moon in 2009, OSIRIS-Rex to the asteroid Bennu in 2016, and the planned launch of BepiColombo to Mercury in 2018, there is now a large quantity of TIR remote sensing data that need to be interpreted. It is therefore important to extend laboratory phase function measurements to the TIR. This paper describes the design, build, calibration, and initial measurements from a new laboratory instrument that is able to make phase function measurements of analog planetary regoliths across wavelengths from the visible to the TIR.