Retrieval of thermal and compositional structure of Venus surface and lower atmosphere based on VIRTIS results from the Venus Express mission
Zusammenfassung der Projektergebnisse
The long-time radiation measurements performed by the VIRTIS instrument aboard ESA's Venus Express mission have provided the first global database for systematic surface and atmosphere studies. VIRTIS-M-IR data offer a major potential for surface science. Although the data were identified to be in generally very good and useful condition, qualitative and quantitative analyses in comparison with simulation results and ground based observations as well as internal data consistency tests revealed issues regarding the instrument calibration that severely affect the retrieved results. Thus, an extremely time-consuming development of sophisticated refinements of calibration procedures became necessary to ensure that the extracted surface information will be reliable. This was not predictable at the time of the basic project application. Furthermore, new approaches for an effective straylight removal technique, an update of surface topography data, and inclusion of the atmospheric blurring effect were considered. An automatic procedure is now able to generate application specific archives of high quality Venus spectra. Qualitative comparisons of synthetic Venus spectra over the full range of available spectral measurements were performed to determine any spectrum responses to atmospheric as well as model input parameter variations. These studies include the thermal infrared, where spectral data from the precursor experiment PMV on Venera-15 are available. Many non-linear parameter relationships were studied in depth. Such a 'library of experience' is very important prior to the development of quantitative procedures to retrieve atmospheric and surface parameters from measured planetary radiances. Gaseous absorption features in the atmosphere of Venus have been investigated in detail including several spectral line databases. As a result and very important precondition for quantitative analyses, three-dimensional data bases of gaseous absorption cross-sections were created over the spectral range from 0.8-50 µm at different spectral resolufions and for the full range of temperature and pressure conditions on Venus. The investigation of the cloud composition in the Venus middle atmosphere was not the primary focus of this project, but surface temperature and emissivity retrieval results are seriously influenced by the high variability of the cloud cover and the resulting radiance changes. Modeling of cloud features is absolutely essential even for surface investigations, therefore. Many important results have already been gained, for example the definition of individual mode contrasts for each atmospheric emission window, but further studies are urgently required. Since the microphysical parameters of H2SO4 aerosols differ strongly at the corresponding wavelengths, a comparison between VIRTIS-M-IR and PMV-TIR temperature retrieval results in the middle atmosphere of Venus would assist in future work to improve the cloud model. Different radiative transfer models have been investigated and tested with respect to their effectiveness and accuracy under the environmental conditions on Venus and an optimum model was selected (including DISORT as the central core part). First retrieval algorithms with respect to cloud opacity, atmospheric and surface temperature as well as surface emissivity have been developed and successfully applied, but several improvements are still required. Comparisons of VIRTIS surface window radiance ratios and surface elevation obtained from Magellan topography records have proven a high correlation between both datasets, but deviations are detected in localized areas. Such ratio-based anomalies give hints on surface temperature as well as emissivity anomalies. Surface emissivity variations that may be due to changes in the chemical composition (mineralogy) and surface texture are important indicators of the nature of the surface material. In spite of the high qualitative correlation of the two independent acquisitions it seems to be impossible to inter-relate the datasets in a quantitative way. Quantitative results on surface emissivity require sophisticated radiative transfer simulafion and retrieval tools, therefore. Preliminary surface feature retrievals along a number of complete VIRTIS northern orbits that were based on the superseded data pre-processing revealed a trend towards lower values of highland surface emissivity compared to surrounding lowlands. This could be an indication of more felsic components of highlands giving a hint to older surface forming processes. New data calibration refinements as well as first application tests of a multi-spectrum retrieval technique, however, yield slightly different results and indicate that these previously established trends may become less pronounced or even vanish. Surface emissivity is the most critical parameter to be retrieved from orbital radiance measurement, but methodical improvements with respect to a better decoupling of atmospheric and surface parameters will certainly enhance the accuracy of future emissivity retrievals.
Projektbezogene Publikationen (Auswahl)
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(2009), Detection of Venus surface anomalies in the Northern hemisphere by VIRTISA/EX and discussion of retrieved results, EPSC Abstracts 4, EPSC2009-475
Arnold, G., Haus, R., Kappel, D., and Basilevsky, A.
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(2010), Radiative transfer in the atmosphere of Venus and application to surface emissivity retrieval from VIRTISA/EX measurements. Planet Space Sci. 58, 1578-1598
Haus, R., and Arnold, G.
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(2010), Results from multispectrum retrieval of VIRTIS-M-IR measurements of Venus' nightside, EPSC Abstracts 5, EPSC2010-390
Kappel, D., Arnold, G., Haus, R., Piccioni, G., and Drossart, P.
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(2010), Retrieval of surface emissivity from VIRTISA/EX radiation measurements over the Northern hemisphere of Venus, Geophys. Res. Abstracts 12, EGU2010-264
Haus, R., Arnold, G., and Kappel, D.
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(2012) Refinements in the data analysis of VIRTIS-M-IR Venus nightside spectra. Advances in Space Research
Kappel, D.. Arnold, G., Haus, R., Piccioni, G., and Drossart, P.
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(2012) VIRTIS/VEX observations of Venus: overview of selected scientific results, J. Appl. Remote Sens. 6, 063580
Arnold G., Haus, R., Kappel, D., Piccioni, G., and Drossart, P.
