Possible Surface Anomalies on the Northern Hemisphere of Venus as Observed by VIRTIS/VEX: First Analyses for Comparative Planetologic Studies

Kurzfassung

Venus and Mars are characterized by basaltic surface composition. In contrast to Mars, very little is known about the Venusian surface material. Comparative studies of the planet’s evolution require a better knowledge about the composition of geological units on the surface of Venus in global scales. The radiation measurements of VIRTIS-M on Venus Express in the emission windows between 1.0 and 1.35 μm provide a first valuable database for systematic surface and near surface studies of the Earth’s sister planet. Measurements over the Northern hemisphere combined with new radiative transfer simulations have been used to extract information about the surface elevation and temperature using the 1.18 and 1.02 μm window ratios. In general, the ratio-based VIRTIS topography is in good agreement with the Magellan topography, but differences occur in localized areas. These “anomalies” may have quite different origins. This paper discusses some observed anomalies within the framework of atmospheric, surface and technically oriented processes and influences. Anomalous radiance ratios, i.e., ratios that significantly deviate from corresponding Magellan topography data can be a result of local surface and near surface temperature variations and local unstable atmospheric dynamics. Comparisons of measurement and simulation over the full VIRTIS-M-IR spectral range (1-5 μm) indicate inconsistencies of the cloud model that is used in the radiative transfer calculations to characterize the absorption and scattering features of the H2SO4 aerosols. Extreme opaque cloud layers may be formed by still unknown particles, which are able to violate the conservative scattering model applied in the spectral range of the atmospheric emission windows. Moreover, the Magellan radar data that represent the base of the topography information of the Venus surface result from a surface layer of about 1 m in thickness, whereas the VIRTIS-M-IR data describe the optical upper surface layer only. Local errors in the Magellan elevation data can also not be excluded. Last not least, the anomalies may be due to variations in the surface emissivity. Surface emissivity variations are important indicators of the nature of surface material. They may be due to variations in mineralogy and surface texture. While most of Venus' geologic units are thought to be basaltic in composition, some of them (tessera terrains) could be felsic. The 1 μm emissivity of felsic materials is lower compared to basalts at similar texture conditions. The anomalous areas located so far, however, comprise practically the same geologic units as adjacent non-anomalous terrains. The surface texture (grain size, packing density, surface roughness) is another important factor for emissivity variations. Grain size strongly affects the spectral characteristics of surface materials. Laboratory measurements of basalts and oxidized basalts show changes in the contrast of the 1 μm reflectivity band. The surface of Venus exhibits variations in surface roughness according to Magellan radar measurements. Tesserae and rifts have a higher surface roughness compared to other areas. The Venera spacecraft lander images as well as Magellan radiophysical data imply eolian and impact sediments in the meter to centimeter thick surface layer. The upper millimeter to micron thick surface may be dominated by dust, condensates and weathering products. Therefore, surface emissivity anomalies caused by variations of surface properties may result from different mineralogy/ chemistry of the surface layer or by processes of eolian sorting as it is observed in desert environments on Earth. Future improvements in the radiative transfer algorithm will contribute to eliminate the masking of the Venus nightside windows by far wing and pressure-induced absorptions of the deep atmosphere constituents. This will allow a better separation of deep atmosphere, temperature, emissivity contributions to the Venus nightside emission, and unambiguous global information about the Venusian surface properties for comparative planetologic analysis.