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| Titel |
Clouds and aerosols on Venus: an overview |
| VerfasserIn |
Dmitri Titov, Nikolay Ignatiev, Kevin McGouldrick, Valerie Wilquet, Colin Wilson |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250111856
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| Publikation (Nr.) |
 EGU/EGU2015-12002.pdf |
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| Zusammenfassung |
| The past decade demonstrated significant progress in understanding of the Venus cloud
system. Venus Express observations revealed significant latitudinal variations and temporal
changes in the global cloud top morphology. The cloud top altitude varies from ~72 km in
the low and middle latitudes to ~64 km in the polar region, correlated with decrease of the
aerosol scale height from 4 ± 1.6 km to 1.7 ± 2.4 km marking a vast polar depression. The
UV imaging shows the middle latitudes and polar regions in unprecedented detail. The eye of
the Southern polar vortex was found to be a strongly variable feature with complex
morphology and dynamics.
Solar and stellar occultations give access to a vertical profiling of the light absorption by
the aerosols in the upper haze. The aerosol loading in the mesosphere of Venus investigated
by SPICAV experiment onboard Venus Express between 2006 and 2010 was highly variable
on both short and long time scales. The extinction at a given altitude can vary with a factor of
10 for occultations separated by a few Earth days. The extinction at a given altitude is
also significantly lower towards the poles (by a factor 10 at least) compared to
the values around the equator, while there is apparently no correlation between
the extinction and the latitude in the region comprised between ±40°around the
equator.
Based on the Mie theory and on the observed spectral dependence of light extinction in
spectra recorded simultaneously in the UV (SPICAV-UV), in the near IR (SPICAV-IR), and
in the short-and mid-wavelength IR (SPICAV-SOIR), the size distribution of aerosols in the
upper haze of Venus was retrieved, assuming H2SO4/water composition of the droplets. The
optical model includes H2SO4 concentrations from 60% to 85%. A number of results are
strikingly new: (1) an increase of the H2SO4 concentration with a decreasing altitude (from
70-75% at about 90 km to 85% at 70 km of altitude) and (2) Many SOIR/SPICAV data
cannot be fitted when using size distributions found in the literature, with an effective radius
below 0.3 μm and a variance of about 2. The scale height of the upper haze is found to be 6.9
± 5.1 km.
The lower and middle cloud layers – those at 48 – 60 km altitudes – are difficult to
observe, as they are hidden by upper clouds. Nevertheless, both nightside near-IR sounding
and radio occultation has provided valuable insight into cloud processes in this region. Near
IR sounding reveals the morphology of the lower/middle clouds ‘backlit’ by thermally
emitted photons from the lower atmosphere. The morphology of these clouds changes on
timescales of order of 24 hours. The vertically integrated cloud optical depth is twice as great
in the polar collar (at 75 degrees latitude) compared to low latitudes. Spectral band ratio
analysis, if interpreted strictly in terms of Mode 1 / 2 / 2’ / 3 particles of H2SO4:H2O
mixtures, suggests that the acidity of the cloud particles is higher near the polar collar and in
regions of optically thick cloud. Particles in the centre of the polar vortex exhibit
anomalously high band ratios so are significantly larger and/or of different composition than
those at low latitudes.
Radio occultation from Venus Express confirms that the atmosphere is in convective
equilibrium from 50-60 km. Sulphuric acid vapour profiles calculated from the absorption of
the radio signals show an atmosphere saturated with sulphuric acid in the cloud layer. Both of
these results are consistent with the understanding of convective condensational cloud at
altitudes of 50-60 km.
Microphysical simulations of the aerosol populations in the atmosphere of Venus have
received a boost from the recent exploration of particle properties carried out by various
teams using Venus Express over the last decade or so. Numerous groups are applying separate
models to the coupled problems of the Venus clouds. Quasi-periodic variability of aerosol
population properties has been found in model simulations by several groups under
both forced and unforced conditions. Since the clouds play such a significant role
in the energy and momentum balance of the atmosphere of Venus – which then
feed back into variations in the aerosols themselves – constraining the magnitude
and timescales of these variations is a key to understanding the current, past, and
future Venusian environment. This paper gives a summary of new observations and
modelling efforts that will form the basis for a relevant chapter in the Venus III book. |
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