EMISSIVITY MEASUREMENTS FOR HOT PLANETARY SURFACES

As orbital missions explore ever more challenging environments on bodies in our solar system, there is a need to understand spectral properties on surfaces with extreme temperatures, for which little laboratory data exist. High and low surface temperatures affect band positions in mineral spectra as predicted by crystal field theory. In some cases where experimental data exist, temperature and pressure have opposing effects (i.e., shifts to higher and lower wavelengths), but such conclusions are difficult to generalize because they depend on crystal structures and site geometries of different mineral groups.

Venus presents special challenges for orbital spectroscopy because its atmosphere is opaque except for a few windows in the CO₂ spectrum (0.86, 0.91, 1.02, 1.11, and 1.18 μm). Proper interpretations of hot surface spectra in the near-IR require dedicated laboratory efforts; these are in progress at the Planetary Spectroscopy Laboratory at DLR in Berlin, Germany. The lab, funded partly by the European Union within the EuroPlanet consortium, is collecting a spectral library of rocks and minerals at temperatures up to 740K.

Initial spectral collection has focused on basalts and felsic rocks as proposed for Venus’ surface. Samples containing Fe oxides (rhyolite, granites, and the oxides themselves) have negative slopes between the 0.86 and 0.91 μm bands. The region from 0.99 to 1.02 μm allows felsic rocks to be distinguished from mafic ones because Fe and other transition metals in silicates cause elevated emissivity in that range. Tests using binary classifiers show that basalt spectra can be confidently discriminated from those of basaltic andesite, andesite, and rhyolite/granite. Moreover, recently acquired 740K basalt spectra match in situ reflectivity data from the Venera 9 and 10 landing sites.

Current studies are extending this work to the study of pure minerals, including hematite, magnetite, pyrite, feldspar, amphibole, pyroxene, and olivine with varying grain sizes. Results confirm older interpretations showing that the site geometry of transition metals has a first order effect on how peaks shift with high temperatures and pave the way for understanding the underlying physical phenomena that affect mineral spectra from extreme environments.