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| Titel |
Venus surface and near surface anomalies on the Northern hemisphere observed by VIRTIS/VEX: First results |
| VerfasserIn |
G. Arnold, R. Haus, D. Kappel, A. Basilevski, P. Drossart, G. Piccioni |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250026017
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| Zusammenfassung |
| 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 μm with respect to the 1.02 μm 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 μm 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 μm 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
μm 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. |
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