INVESTIGATION OF THE SURFACE OF VENUS WITH VIRTIS ON VENUS EXPRESS

摘要

The VIRTIS-M instrument on Venus Express is a mapping infrared spectrometer intended for the study of the atmosphere of Venus. As there are several atmospheric windows in the near-infrared through which radiation from the surface leaks into space, it is possible to use data from VIRTIS-M to investigate the surface of Venus. Previous ground-based investigations e.g. by Meadows and Crisp (1996) found no significant variations in the surface infrared emissivity. Hashimoto and Sugita (2003) showed that these previous studies omitted to account for multiple surface-cloud reflections and that by doing so a much higher emissivity contrast might be expected. They further considered the possibility to distinguish between granitic and basaltic rock when given a sufficient accuracy of infrared emissivity measurements. Basaltic volcanic flood plain is the most common type of surface on Venus but the composition of the highly deformed highlands is still unknown. Campbell and Taylor (1983) speculated that these highlands might contain granitic rock and thus 'resemble terrestrial Archean continental nuclei'. Confirmation of this supposed composition would strengthen the interpretation of the high D/H ratio in the atmosphere as a remnant of an ocean because liquid surface water is required for the formation of granites. For my diploma thesis I am implementing the method from Hashimoto and Sugita (2003) to calculate maps of surface emissivity from VIRTIS-M data. After the arrival of Venus Express at Venus in April we will test and enhance my algorithm using the VIRTIS-M data. By the time of October we should be able to present some first results on the retrieval of emissivities and a preliminary discussion of the implication for the surface composition. The VIRTIS-M data will greatly improve the existing data on Venus surface infrared emissivity as previous studies were either earth-based or, in case of Galileo-NIMS, of limited spectral resolution. The data might put important constraints on our knowledge of the atmospherical and geological evolution on Venus and in general on the habitability of planets. Literature: Meadows and Crisp (1996): Ground based near-infrared observations of the Venus nightside: The thermal structure and water abundance near the surface, J. Geophys. Res., 97, 15,967-15,976 Hashimoto and Sugita (2003): On observing the compositional variability of the surface of Venus using nightside near-infrared thermal radiation, J. Geophys. Res., 108(E9), 5109 Campbell and Taylor(1983): No water no granites - No oceans, no continents, Geophys. Res. Lett., 10, 1061-1064