Venus nightside emission measurements of VIRTIS on Venus Express provide the opportunity of surface studies
in the narrow near infrared atmospheric windows. The measurements as well as detailed new radiative transfer
simulations show that radiance ratios in the emission windows between 1.0 and 1.35 micron with respect to the 1.02 micron
window can be used to extract information about the surface elevation and temperature. Based on these analyses,
first surface and near surface anomalies are identified on the Northern hemisphere of Venus, which are due to
deviations of the elevation - temperature correlation in certain small areas.
The data are selected from VIRTIS-M-IR nightside measurements. To ensure minimal atmospheric influence on
the measured signatures, only pushbroom observations with small observation angles close to nadir are taken into
account. The radiative transfer simulation technique considers absorption, emission, and multiple scattering by
gaseous and particulate constituents of the atmosphere. Look-up tables of quasi-monochromatic gas absorption
cross-sections are calculated using appropriate spectral line data bases and line profiles and a line-by-line procedure.
Empirical continuum absorption coefficients are determined from a ’VIRTIS reference spectrum’. In order to
derive the parameters of the H2SO4 clouds, Mie theory is applied. Multiple scattering is considered by a Successive
Order procedure. The synthetic quasi-monochromatic intensity spectra at the model top level of the atmosphere
are convolved with the VIRTIS spectral response function.
The surface windows at 1.02, 1.10 and 1.18 micron exhibit a clear dependence of transmitted radiation on topographical
features and, thus, on surface thermal emission, since an elevation change of 12 km results in a temperature
change of 100 K. In the first approximation, the radiance ratios are affine linear functions of the surface temperature.
This is demonstrated by both measurements and simulations. In general, the ratio-based VIRTIS topography
correlates well with the Magellan topography, but differences occur in localized areas.
Different local surface anomalies do exist. These anomalies are probably a result of the lower atmosphere dynamics,
errors in Magellan elevation determinations, or variations in the surface emissivity. Surface emissivity
variations are important indicators of the nature of surface material. They may be due to variations in mineralogy
and surface texture. While most of Venus’ geologic units are thought to be basaltic in composition, some of them
(tessera terrains) could be felsic. The 1 micron emissivity of felsic materials is lower compared to basalts at similar
texture conditions. Nevertheless, we found that anomalous areas comprise practically the same geologic units as
adjacent non-anomalous terrains.
The surface texture (grain size, packing density, surface roughness) is another important factor for emissivity
anomalies. Grain size affects the spectral characteristics. Laboratory measurements of basalts and oxidized basalts
show significant changes in the contrast of the 1 micron reflectivity band. Although most of the surface of Venus is
not very rough, roughness variations exist. Tesserae and rifts show a higher surface roughness compared to other
areas.
Finally, the Magellan radar data that represent the base of the topography information of the Venus surface result
from a surface layer of about 1 m in thickness, whereas the VIRTIS-NIR data describe the optical upper surface
layer only.
The radiative transfer simulations show the capability of our algorithm to investigate the surface of Venus. Based on
these simulations and the VIRTIS/VEX measurements, the extracted anomalies are discussed in the framework of
these processes and influences mentioned above. Future improvements will contribute to eliminate the masking of
the Venus nightside windows by far wing and pressure-induced absorptions of the deep atmosphere constituents.
This will allow a better separation of deep atmosphere, temperature, and emissivity contributions to the Venus
nightside emission
