Venus NIR Surface Emissivity estimated from VIRTIS on Venus Express Observations

Kurzfassung

Spectral window regions close to 1μm allow for the transfer of thermal radiation from the surface through the atmosphere of Venus. Results of radiative transfer modeling are used to invert VIRTIS images at 1.02, 1.10 and 1.18μm for thermal emission of the surface. Local atmospheric transmittance is derived from the VIRTIS band at 1.31μm. Several hundreds of VIRTIS images covering in total most of the southern hemisphere of Venus have thus been analyzed and stacked for improvement of signal to noise ratio. The results of this approach are to some extent ambiguous since neither surface emissivity nor surface temperature are well known. Furthermore aerosols or a gradient of absorbing gaseous constituents near the surface might affect the interpretation. But neglecting any effects of the near surface atmosphere and assuming parameters of the radiative transfer model within reasonable ranges it is possible to estimate either surface emissivity or surface temperature. Temperature of surface and atmosphere is mostly a function of altitude, no large diurnal, seasonal or latitudinal variations are expected in the lower atmosphere. The lapse rate is constrained by the adiabatic lapse rate. A hint for global average of surface emissivity is given by the dominance of probably basaltic volcanic plains on the southern hemisphere. It is however imaginable that temperature dependant weathering leads to a trend of emissivity with altitude similar to that seen in the Magellan radiothermal emissivity observations. Regardless of any global variations of lapse rate or emissivity with altitude, spatial variations of emissivity independent from topography can be examined by assuming constant emissivity and fitting surface temperature accordingly to the global relation of thermal emission to topography. This spatial variation of thermal emission is assumed to be due to variation of surface emissivity and shows correlation with some geological features known from Magellan radar images. In the Lada Terra region large lava streams, Cavillaca - and Juturna Fluctus, show increased emissivity with respect to neighboring regions of the same altitude. Other large lava streams in the region show a similar but less obvious relative emissivity. Large areas of tessera terrain on the contrary consistently show relative low emissivity. While this variation of emissivity might be related to content of mafic minerals it is also conceivable that weathering and thus age of the terrain in question is responsible. The accumulation of observations by VIRTIS during the Venus Express mission will allow us to study with a larger database these correlations and to further disentangle possible atmospheric from surface contributions and thus to increase understanding of surface composition as well as of composition and temperature of the atmosphere in contact with the surface.