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| Titel |
Synthetic images of Venus surface based on VMC images |
| VerfasserIn |
Eugene Shalygin, Alexander Basilevsky, Dmirti Titov, Wojciech J. Markiewicz |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250045906
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| Zusammenfassung |
| The Venus Monitoring Camera (VMC) is a part of the Venus Express payload. It takes images
in four channels, one of which centered at 1.01 micron registers the night side thermal
emission from the planet surface. On the night side VMC maps thermal emission of the
surface in the 1.01μm spectral transparency “window”. These measurements are at the limit
of instrument capability. Faintness of the surface emission and low efficiency of the CCD
detector at 1.01μm result in that even at maximum exposure of 30 s the measured signal does
not exceed - 200 digital units (DNs) which is  3% of the CCD full well. The second
difficulty of the surface observations results from the solar stray light. In order to
cope with this problem VMC observes the night side when the spacecraft is in
eclipse. This limits the observations to low latitudes (±- 40-). Formal spatial
resolution of these images taken from the working distances (2000–8000 km) is 1 to 5
km, but because the surface radiation on its way to the camera passes through the
dense scattering atmosphere and cloud layer, the actual spatial resolution is about 50
km.
Thermal emission of the surface is the only source of radiation on the Venus night side.
The radiation intensity depends on the surface temperature thus giving a hope to register the
ongoing volcanic eruptions. Also the radiation intensity depends on the emissivity of the
surface material, which is a function of a number of parameters including surface texture in
micron to millimeter scale and mineralogical composition.
To calculate synthetic VMC images we used the Magellan topography derived from
Magellan Radar Altimeter, Monte-Carlo based simulations of light scattering to
get blurring function and atmosphere reflectance/transmittance for two streams
approximation and DISORT calculations for gaseous absorption. We used the vertical
structure of clouds and their optical properties from VIRA. The topography data were
converted into the maps of temperature assuming thermal equilibrium with the
atmosphere. Comparison of model images with VMC once give us maps of relative
emissivity.
We analyzed four couples of two base model parameters: temperature lapse rate: -8.1
(adiabatic) and -5.6 K/km (to compare with VIRTIS) and emissivity of basaltic plains (0.8
and 0.58). Obtained results for region that covers Chimana-mana tesserae, part of Hinemoa
planitia and part of Beta regio show that emissivity of tesserae and emissivity of planitia are
statistically unequal (emissivity of tesserae is 10% higher) in models with temperature
lapse rate -8.1 K/km. Besides on correlation diagram between surface altitude and
emissivity there are two clusters that correspond to tesserae and plain regions. Top of
Tuulikki volcano also shows different emissivity comparing to surrounding plains
in models with temperature lapse rate -8.1 K/km. If these differences caused by
weathering, then they can be results of stronger winds at higher altitudes which
removed smallest dust particles from surface. For models with temperature lapse rate
-5.6K/km the tesserae’s emissivity appears to be a bit lower than emissivity of plains
(same as in VIRTIS results), but difference is inside the range of statistical errors. |
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