The Venus Express VIRTIS and MESSENGER teams

Abstract

Because Venus Express has been in orbit around Venus since April 2006, MESSENGER's flyby of Venus on 5 June 2007 permitted a coordinated campaign of two-spacecraft observations of that planet. Here we report initial results from joint observations of the surface of Aphrodite Terra performed by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on Venus Express and the Mercury Dual Imaging System (MDIS) on MESSENGER. VIRTIS observed the region 13 hours before and 9 hours after the time of MESSENGER's closest approach (when Venus Express was on the opposite side of the planet). This timeline of nearly 24 hours allows the disentangling of variable atmospheric conditions from surface signals. Both instruments were able to observe the surface in the 1.02 μ m atmospheric window. At this wavelength the CO₂ atmosphere allows about 95% of the thermal radiation from the surface to escape. In addition VIRTIS could make observations in the atmospheric windows at 1.10 and 1.18 μ m, which are less transparent. Radiative transfer models were used to invert VIRTIS images at 1.02, 1.10, and 1.18 μ m for the thermal emission of the surface. Local atmospheric transmittance is derived from VIRTIS images at 1.31 μ m. The combined MDIS and VIRTIS data set covers almost the entire extent of southern Aphrodite Terra. Both instruments observed as expected a correlation between topography and thermal radiation, as the surface temperature on Venus is dominantly a function of altitude. There are, however, tentative indications of possible surface emissivity differences between highland material and the adjacent lowlands. Detailed data analysis and interpretation within the framework of the geology of the region are currently underway.