Correlations between Venus nightside near infrared emissions measured by VIRTIS/Venus Express and Magellan radar data

Kurzfassung

<b>Background</b> <p>The Venus Express Spacecraft images the nightside thermal emissions using the VIRTIS imaging spectrometer. At 1.02 micron thermal emission from the surface is penetrates the atmosphere but the signal is attenuated by scattering and absorption [1, 2]. Although the measured flux at top of the atmosphere is nonlinearly related to the original emission of the surface, it is still positively correlated with the product of surface temperature and surface emissivity [3]. The surface temperature of Venus is relatively well constrained as a monotonous function of altitude. Emissivity at 1 micron depends strongly on surface composition, in particular abundance of mafic minerals [3]. Mapping the thermal emission of the surface of Venus therefore supplements radar data as it allows to infer relative variation of surface composition.</p> <b>Data Processing</b> <p>This study examines the correlation of VIRTIS images showing a signal of the surface with all known parameters that govern radiance and applies semi empirical relations to remove the respective influences.</p> <ol><li> Stray sunlight is removed by subtraction of a spectrum template scaled to fit radiance at 1.4 µm [2]</li><li>Limb darkening is accounted for using a linear phase function consistent with results of radiative transfer modeling [4].</li><li>Cloud opacity is determined from 1.31 µm and applied to 1.02 µm while accounting for multiple reflections between lower atmosphere and clouds [3]. Result is brightness temperature of thermal emission below the cloud deck but above the lowest 20 km of the atmosphere.</li><li>Influence of surface temperature and lower atmosphere absorption is determined by correlation of VIRTIS declouded brightness temperature and Magellan Topography data [5].</li></ol> <p>To further reduce the influence of cloud contrast and increase the signal of the surface, all suitable VIRTIS observations are map projected and stacked to create a map of the southern hemisphere of Venus.</p> <b>Observations and Interpretation</b> <p>As expected from the small diurnal, latitudinal and seasonal variations of temperature in the atmosphere of Venus, the map created from all retrieved brightness temperatures is highly correlated with Magellan altimetry. Local deviation from the globally averaged brightness to topography relation can be either ascribed to surface emissivity or unexpected temperature variations. Temperature variations e.g. due to active volcanism are unlikely to be persistent over the time of observations. The stacked data is here interpreted in terms of surface emissivity variation by removal of the influence of topography.</p><p> The emissivity variation found is correlated with geomorphological features established from Magellan radar images. It is generally lower at tessera terrain. This might indicate felsic surface composition of tessera highlands, e.g. anorthosite or granite [6, 7]. Creation of felsic crust is unlikely under current conditions.</p><p> Some, but not all volcanic edifices show increased emissivity. Large lava flows in the Lada terra - Lavinia planitia region also show an increased thermal emission. In particular Cavilaca and Juturna fluctus, emanating from Boala corona (70S 0E) inside Quetzalpetlatl corona, are characterized by an increased IR flux. This might be consistent with the large scale extrusive volcanism of ultramafic composition considered by [8] in the context of chemical differentiation in the upper mantle.</p> <b>Discussion</b> <p>These observations are however highly sensitive to errors in the altimetry applied. A known systematic error in the Magellan dataset stemming from spacecraft orbit determination uncertainty is qualitatively confirmed by comparison with VIRTIS data (see longitude -120 in fig. 1 and 2. Tessera terrain is known to strongly scatter radar waves which might influences accuracy of altimetry. An quantitative analysis and search for small scale systematic errors is in progress during the submission of this abstract.</p> <b>References</b> [1] Lecacheux, J., P. Drossart, P. Laques, F. Deladerriere, and F. Colas (1993), Planetary and Space Science, 41, 543–549.<br /> [2] Meadows, V. S., and D. Crisp (1996), Journal of Geophysical Research, 101, 4595–4622.<br /> [3] Hashimoto, G. L., and S. Sugita (2003), Journal of Geophysical Research (Planets), 108, 13–18.<br /> [4] Tsang, C. C. C., P. G. J. Irwin, F. W. Taylor, and C. F. Wilson (2008), Journal of Quantitative Spectroscopy &amp; Radiative Transfer, In press.<br /> [5] Ford, P. G., and G. H. Pettengill (1992), Journal of Geophysical Research, 97, 13,103.<br /> [6] Nikolaeva, O. V., M. A. Ivanov, and V. K. Borozdin (1992), Venus Geology, Geochemistry, and Geophysics - Research results from the USSR.<br /> [7] Hashimoto, G. L., M. Roos-Serote, S. Sugita, M. S. Gilmore, L. W. Kamp, B. Carlson, and K. Baines (this issue), Journal of Geophysical Research, submitted.<br /> [8] Head, J. W., E. M. Parmentier, and P. C. Hess (1994), Planetary and Space Science, 42, 803–811.<br />