If So... Can They Do It Here, Too?
The European Space
Agency stated Monday that by using recent observations from the
Cassini and Huygens probes, as well as those closer to home,
scientists have been able to create a computer model which explains
the formation of several types of ethane and methane clouds on
Titan.
Sources at ESA tell Aero-News that clouds have been observed
recently on Titan, Saturn’s largest moon, through the thick
haze, using near-infrared spectroscopy and images of the south pole
and temperate regions near 40 degrees South. Recent observations
from Earth-based telescopes and the NASA/ESA/ASI Cassini spacecraft
are now providing an insight into cloud climatology.
A European team, led by Pascal Rannou of the Service
d’Aeronomie, IPSL Universite de Versailles-St-Quentin,
France, has developed a general circulation model which couples
dynamics, haze and cloud physics to study Titan climate and enables
us to understand how the major cloud features which are observed,
are produced.
The climate model also allows scientists to predict the cloud
distribution for the complete Titan year -- equivalent to 30 Earth
years -- and especially in the next years of Cassini
observations.
NASA's Voyager missions of the early 1980s gave the first
indications of condensate clouds on Titan. Because of the cold
temperatures in the moon’s atmosphere (tropopause), it was
assumed that most of the organic chemicals formed in the upper
atmosphere by photochemistry would condense into clouds while
sinking. Scientists also believed methane would condense at high
altitudes, having been transported from the surface.
Since then, several one-dimensional models of Titan’s
atmosphere, including sophisticated microphysics models, were
created to predict the formation of drops of ethane and methane.
Similarly, the methane cycle had been studied separately in a
circulation model, but without cloud microphysics.
These studies generally found that methane clouds could be
triggered when air parcels cooled while moving upward or from
equator to pole. However, these models hardly captured the fine
details of the methane and ethane cloud cycles.
Rannou’s team combined a cloud microphysical model into a
general circulation model. The team can now identify and explain
the formation of several types of ethane and methane clouds,
including the south polar and sporadic clouds in the temperate
regions, especially at 40 degrees South in the summer
hemisphere.
The scientists found that the predicted physical properties of
the clouds in their model matched well with recent observations.
Methane clouds that have been observed to date appear in locations
where ascending air motions are predicted in their model.
The observed south polar cloud appears at the top of a
particular ‘Hadley cell’, or mass of vertically
circulating air, exactly where predicted at the south pole at an
altitude of around 20-30 kilometers.
The recurrent large zonal (longitudinal direction) clouds at
40° S and the linear and discrete clouds that appear in the
lower latitudes are also correlated with the ascending part of
similar circulation cell in the troposphere, whereas smaller clouds
at low latitudes, similar to the linear and discrete clouds already
observed by Cassini are rather produced by mixing processes.
"Clouds in our
circulation model are necessarily simplified relative to the real
clouds, however the main cloud features predicted find a
counterpart in reality.
"Consistently, our model produces clouds at places where clouds
are actually observed, but it also predicts clouds that have not,
or not yet, been observed," said Pascal Rannou.
Titan’s cloud pattern appears to be similar to that of the
main cloud patterns on Earth and Mars. The puzzling clouds at
40° S are produced by the ascending branch of a Hadley cell,
exactly like tropical clouds are in the Intertropical Convergence
Zone (ITCZ), as on Earth and Mars.
Polar clouds -- produced by 'polar cells' -- are similar to
those produced at mid-latitudes on Earth, although Titan's clouds
only appears at some longitudes. This is a specific feature of
Titan clouds, and may be due to a Saturn tidal effect. The dynamic
origin of cloud distribution on Titan is easy to test.
Cloudiness prediction for the coming years will be compared to
observations made by Cassini and ground-based telescopes. Specific
events will definitely prove the role of the circulation on the
cloud distribution.