Solving the Knowledge Gap between Ambient Reflectance and Venus-Temperature Emissivity Spectroscopy

Kurzfassung

The VERITAS, DAVINCI, and Envision Venus missions will utilize near-IR spectroscopy at wavelengths near 1 micron to map the surface. Previous missions like Venus Express observed a correlation between relative emissivity differences and geomorphic features. VERITAS and EnVision both carry 6-channel spectrometers capable of distinguishing between basalt and granitic rocks types, and all three missions may see emissivity variations caused by weathering. Data interpretation will be enabled by an understanding of spectra taken at Venus temperatures as calibrated emissivity data. Acquisition of such spectra is ongoing at the Planetary Spectroscopy Laboratory (PSL) at DLR, but the process is both meticulous and time-consuming. Sample selection and prioritization could be guided by drawing inferences about Venus-T spectra using existing VNIR spectral libraries collected at ambient temperatures using more conventional reflectance (R) methods. However, two issues must be appreciated. First, it is known that increased temperature results in changes in peak intensity and red-shifting depending on the material. Because Venus orbital spectroscopy can only use a limited set of atmospheric windows in the range between 0.86-1.18 mm, measuring those shifts correctly is crucial for the interpretation. Second, a vast majority of terrestrial spectral libraries at this wavelength were collected using bidirectional apparatus that are not directly comparable to (Venus) emissivity data using the conventional 1-R relationship. Here we show the magnitude of these effects with three modes of laboratory data collected on the same samples at the PSL: emissivity at 440°C, bidirectional data collected at 30° incidence angles, and hemispherical data. These results are compared with data on analogous minerals from the venerable RELAB database at Brown University. Understanding of these issues must inform studies of Venus-relevant spectra as the community prepares for the upcoming missions.