Calibrating Venus Orbiters Spectral Data: Comparisons Among Emissivity, Bidirectional, and Hemispherical Reflectance Spectra

Laboratory measurements of emissivity (E) acquired at hot Venus surface temperatures will be key to calibrating and interpreting data from upcoming missions equipped with near-IR instruments, such as VERITAS, DAVINCI, and EnVision, using 440°C spectra acquired at the Planetary Spectroscopy Laboratory at DLR (Berlin). This paper presents spectra of samples (basalt and granodiorite) measured in the MIR spectral range. Comparisons of these data to high-temperature and ambient hemispherical and biconical reflectance (R) data amply demonstrate that the latter cannot be used for quantitative calibrations because the resultant intensities are only qualitatively related to emissivity. This result is expected because biconical reflectance is measured only along a single viewing angle. In order to be representative of emissivity, the reflected radiation must be measured over all angles of reflection (as it happens for hemispherical reflectance). This effect was also documented by Pieters (1999, New Views of the Moon, #8025), who observed a 0.05 difference in reflectance between bidirectional spectra take at i = 30° and e = 0° and diffuse directional hemispheric data acquired by John Adams for an Apollo 16 soil (62231). The graph below compares data on two slab samples of granodiorite and basalt, all of them measured in emissivity at Tsample=200°C, biconical reflectance, and hemispherical reflectance (both measured at room temperature). This comparison serves as a reminder of the experimental difficulties in calibrating flight instruments for Venus spectroscopy. Common laboratory measurements of reflectance using calibrated but bidirectional spectrometers produce qualitative data useful only to understanding relative differences between rocks and minerals (e.g., Treiman et al., 2021, PSJ, abd546). However, bidirectional spectra may inform questions about band shifts at high temperature predicted by crystal field theory, such as the shift of Fe-O charge-transfer absorptions to longer wavelengths seen by Pieters et al. (Science, 234, 1379-1383). However, only high temperature hemispherical reflectance and/or emissivity spectra are useful to derive maximum science benefit for effective calibration of Venus orbital data.