Hot Rocks! Near-Infrared Reflectances (and Emissivities) or Rocks at Venus Surface Temperatures

Abstract

Venus surface can be viewed in emission through a few near-infrared (NIR) spectral windows (1 m) in its relatively opaque atmosphere [1]. Venus surface shows NIR emissivities that correlate with surface geology [2-4], and these emissivity variations are interpreted as differences in surface rock type (mafic vs. silicic) and/or extent of weathering (Fe2+ silicates vs. Fe3+-oxide-coated). To understand and quantify the observed variations in NIR emissivity, high-temperature (T) emissivity can be measured directly [5,6]. For example, emissivities of basalts in the wavelength range 0.85 1.2 m are ~0.95 [5-8]. This can be tested by measureing reflectance, because Kirchoffs Law holds that emissivity (e) = 1 reflectance (r). The r of basalt in the NIR is ~0.05 [o] consistent with a NIR e of ~0.95 [5-8]. High-T NIR es of silicic igneous rocks (granitic, rhyolite) have been reported to be 0.8-0.9 [5,6], which is inconsistent with r values of 0.3-0.8 of such rocks at 25C [9,10]. However, these measurements have been updated [7,8] and are consistent with the results here (see below and Fig. 3)