Relative 1 micron emissivity of tesserae and regional plains from the Venus Monitoring Camera observations

We analyzed the relative emissivity of several distinct and extensive surface units portrayed in the global geological map of Venus using the new release of the relative emissivity map derived from the VMC/ Venus Express night side imaging at 1 micron. Specifically, we analyzed the emissivity of tessera and the most widespread unit of regional plains. There is some circumstantial evidence that tesserae may represent massifs of non-basaltic, granite-like materials while regional plains are undoubtfully composed of basaltic lava flows. To characterized these units, we have selected massifs of tessera and fields of regional plains in two longitudinal zones: 60-120W and 120-150E. These include tesserae at the eastern flank of Beta Regio, in Thetis Regio and north of it. We analyzed 16 tesserae massifs that comprise ~5% of the total tessera population. The tesserae relative emissivity values vary from 0.25 to 0.63. This range is almost completely overlapping with the emissivity values range for regional plains, from ~0.31 to ~0.64. The emissivity values of tessera appear to form three clusters. The lowest values, 0.25-0.27, characterize tessera in Thetis Regio and its immediate surroundings (2 massifs). The intermediate values, 0.30-0.50, are typical almost for all other massifs. The highest value of ~0.63 occurred in one tessera northward of Thetis at the edge of reliable emissivity data coverage. The low-emissivity tesserae are topographically high (mean elevation is 3.92±0.08 km), whereas all other massifs are at significantly lower elevations (1.95±0.44 km). The low surface emissivity on tesserae might indicate their non-basaltic (granite) composition. However, since the studied tesserae are elevated terrains, the retrieved low emissivity could also result from uncertainties in the model of the lower atmosphere. The emissivity values for the rest of the analyzed tesserae do not provide any supporting evidence for their non-basaltic composition.