Mercury's Emission Phase Function (EPF) as Observed by BepiColombo MERTIS and Mariner 10 IRR

Rough planetary surfaces do not emit uniformly in all directions. Small-scale roughness results in a diverse range of surface temperatures, driven primarily by the orientation of slopes to the sun, and secondarily by scattering and re-emission between opposing slopes [1,2]. To a distant IR instrument, this unresolved, sub-pixel roughness expresses in two basic ways: 1) the relative proportions of warm and cool surfaces within the measurement’s field-of-view changes with viewing angle, leading to differences in apparent brightness temperature (TB); and 2) the mixture of sub-pixel temperatures results in an emission spectrum that deviates from that of a blackbody [2,5,7]. Understanding the Emission Phase Function (EPF) is critical to interpreting measured TB and emission spectra. While the EPF of the Moon has been well studied due to a wealth of telescopic [3] and spacecraft data sets [2,4,5,6,7], the EPF of Mercury has yet to be characterized in detail. However, new data acquired by the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) [8,9,10] instrument onboard BepiColombo [11] provide a new opportunity to study Mercury’s EPF. In this work, we leverage two complementary data sets: 1) Newly-acquired low phase angle data collected by BepiColombo MERTIS; and 2) high phase angle data collected by the Mariner 10 Infrared Radiometer (IRR). We compare these results using a global thermal model [12] that simulates EPF effects using lunar-like surface roughness [4,5].