Multispectral surface emissivity from VIRTIS on Venus Express

Abstract The surface composition of Venus is mostly inaccessible to remote observation due to the dense cloud cover. There are five spectral windows that show measurable thermal emission from the surface at night. The VIRTIS spectrometer on Venus Express observed three of these windows over much of the southern hemisphere of Venus. We use these data along with Magellan altimetry to map surface emissivity. The data are initially processed to correct for instrumental straylight from the dayside of Venus and to improve wavelength registration. These corrected data are then inverted to emissivity at 1020, 1100, and 1180 nm wavelength using lookup tables created by an atmospheric radiative transfer model. As in earlier studies we find residual trends of surface emissivity with respect to the Magellan altimetry that is used in the model to determine surface temperature and thickness of the atmosphere. A new observation is that these trends vary significantly from region to region, indicating some lateral variability of atmospheric parameters, most likely near surface atmospheric temperature. The trends are consistent over hundreds to thousands of km, thus it is possible to correct for them heuristically. In two regions studied in this paper there are significant deviations from the background emissivity which are associated with some geologic features. The high noise in 1100 and 1180 nm maps derived from VIRTIS data result in large uncertainties of spectral shape. The VIRTIS instrument was not designed for this task and future observations could provide high signal to noise ratio maps in at least 5 distinct bands diagnostic of major rock types and minerals.