Physics – Optics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p44a..02m&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P44A-02
Physics
Optics
[0305] Atmospheric Composition And Structure / Aerosols And Particles, [0319] Atmospheric Composition And Structure / Cloud Optics, [0343] Atmospheric Composition And Structure / Planetary Atmospheres, [6295] Planetary Sciences: Solar System Objects / Venus
Scientific paper
The Venus Monitoring Camera (VMC) on Venus Express (VEX) spacecraft has been observing the upper cloud layer since April 2006. To date nearly two hundred thousand images have been acquired. VEX has a highly elliptical orbit allowing for global as well as close up views with resolution down to 200 meter per pixel. The VMC is a CCD camera with four channels in the UV, visible and near IR, with centre wavelengths at 365, 513, 965 and 1010 nanometers respectively. The VMC UV wavelength corresponds to the spectral feature of a, so far unidentified, absorber. We have started to model the VMC data in all channels to infer the physical properties of the upper clouds as well as the haze which in most cases lies above the clouds. When looking at any individual orbit we are always limited to a short range of phase angles. Our initial results indicated that i) the phase dependence of brightness measured at small phase angles points to the presence of submicron particles in the upper cloud layer in many places, ii) the optical thickness of this haze varies and grows to the south pole by several times, iii) the phase gradient of the normalized brightness measured in the 20°-50° phase range suggests the presence of large, up to 4 μm in radius, spherical particles (droplets) located in clouds in the regions of dark UV features at middle latitudes, iv) the size of these droplets decreases to 1 μm at higher latitudes, v) at phase angles < 30° and not in the UV dark regions, the phase behavior of the visible brightness agree with the model of 1 μm cloud particles, while the near IR profiles suggest the cloud particles a bit larger in size. Probably, this is caused by the altitude gradient of particles sizes, since with IR channel we probe the clouds somewhat deeper than at the UV wavelength, vi) the UV contrasts can be caused not only by the absorption variations in clouds but also by the variations in size of cloud particles. Simultaneous variations in both parameters are possible. These initial efforts led to a rather simple and compelling picture of the upper clouds. It appeared that cloud particles nucleate at the equator with a droplet size of about 1 μm. They subsequently grow as they are transported by the flow of the planetary vortex to higher latitudes. At about 40° latitude they reach size of 4 μm and start precipitating. At higher latitudes only the smaller (mostly 1 μm) particles remain. This relatively simple scenario has not been completely substantiated by further modeling at different phase angles. In some regions the obtained phase gradient of brightness is even steeper than that described by sulfuric-acid droplets and requires a higher refractive index and probably a very narrow size distribution, both of which can be provided by sulfur crystals of about 1.5 micron across. We are now in the process of looking at statistics of the phase function of the clouds from all orbits.
Ignatiev Nikolai
Markiewicz Wojciech J.
Petrova Elitza
Shalygina O.
Titov Dimitry
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