Warren, Tristram - THE OXFORD SPACE ENVIRONMENT GONIOMETER

Abstract: 
Measurements of the light scattering properties of the regolith of airless bodies in the Solar system, across wavelengths from the visible to the far infrared are essential to understanding their surface properties. This presentation will describe a new experimental setup, the Oxford Space Environment Goniometer (OSEG). The OSEG allows phase function measurements of samples to be made under vacuum (<10-4 mbar) whilst enclosed by a cooled (<150 K) radiation shield. The cooled radiation shield reduces the thermal background allowing phase measurements from the visible to the thermal infrared to be made. This work was originally motivated by the need for new emission phase function measurements to support analysis of data currently being returned by the Diviner Lunar Radiometer (Diviner) instrument. Diviner is a nine-channel mapping radiometer onboard NASA’s Lunar Reconnaissance Orbiter. It has channels ranging from the visible to the far infrared (>400μm), with three mineralogy channels centered on the mid-infrared (8μm) [1].
To fully interpret the brightness temperatures measured by a thermal infrared instrument requires a 3D thermophyscal model [e.g. 2,3]. However these models are dependent on knowledge of the phase function of scattered and emitted radiation across the visible, near and thermal infrared. These models typically assume that infrared radiation is scattered isotropically from the lunar surface. Although generally the models are in very good agreement with the measured brightness temperatures of the lunar surface, there are some discrepancies [2]. One possible reason for these discrepancies is that the scattering properties of the regolith in the thermal infrared are incorrectly estimated by the models. Although significant progress is being made in determining the scattering properties of the lunar soil in the visible and near infrared [e.g. 4,5], there is still limited or no data available on the scattering properties in the thermal infrared. Therefore, we are developing an automated, vacuum compatible goniometer (angular measuring device) system capable of measuring both the bidirectional distribution reflectance function and directional emissivity in the thermal infrared of samples under simulated lunar thermal conditions in the laboratory. The first use of the system will be to provide support for measurements made by the Diviner instrument in the thermal infrared. Some very initial measurements of the directional emissivity for a variety of different surface textures will be shown.

References
[1] Paige, D. A. et al., Space Sci. Rev.150, 125-160, 2009. [2] Paige, D. A. et al., Science, 330, 479, 2010. [3] Vasavada, A.R. et al., Icarus, 141, 179, 1999. [4] Foote, E. J., LPSC XXXXIII abstract #2357, 2012. [5] Pommerol, A. et al., Planetary and Space Science., 59, 1601-1612, 2011