West, R., Del Genio, A., Barbara, J., McEwen, A., Turtle, E.P., Porco, C., et al. (2005). "Initial Assessment of Titan's Atmosphere from Cassini's Imaging Science Subsystem." Spring 2005 Crete Titan/Cassini-Huygens meeting .


Abstract

Observations of Titan's atmosphere by Cassini's Imaging Science Subsystem (ISS) have revealed ephemeral clouds in the troposphere and multiple haze layers in the stratosphere1. The first measurements of wind speed in the troposphere were derived from cloud motions. The many stratospheric haze layers were unexpected. Analysis of early photometric and polarimetric observations of the stratospheric haze is in progress.
As of late February, 2005 the Cassini ISS instrument operated on three occasions within 106 km of Titan where Titan's angular diameter is comparable to the size of the narrow-angle camera (NAC) field of view (6 mrad). At this distance each NAC pixel maps to a spatial scale of 6 km which is sufficient to detect small clouds and thin haze layers in the stratosphere. The first of these (T0) occurred in early July, 2004 when the spacecraft flew over southern high latitudes and imaged a complex cloud field near the south pole. The second (TA) was in October, 2004 when south polar clouds were again seen. On the third occasion (TB in mid- February, 2005) no south polar clouds were seen. On each of these occasions ISS carried out a movie sequence to look for clouds, cloud motions, and evolution of cloud morphology.
Two types of clouds were observed. The dominant form occurs at high southern latitudes and appears as a cumulus cloud patch approximately 400 km in diameter with individual clouds having dimensions down to the resolution limit of the camera. These reside somewhere in the troposphere above much of the tropospheric methane because some contrast can be seen in filters which sample methane absorptions, whereas these same filters show no contrast at the surface. The second type of cloud is seen at southern mid-latitudes. These clouds are long (hundreds of km) in the east-west direction but narrow in the north-south direction, resembling volcanic plumes entrained in a strong vertical wind shear. A few images from the TB flyby are at a high enough resolution to show finer scale filaments, also aligned mostly in the east-west direction, as well as trains of small puffy clouds.
The first well-resolved images of the south polar cloud field showed morphological evolution on a time scale of hours. This kind of activity makes difficult the determination of wind speed. Nevertheless we reported wind speeds for 12 features1. Three of those provide evidence for super-rotating winds. Two features were stationary within measurement error.
A 'detached' haze layer appears around Titan's limb at latitudes less than about 70 degrees (outside of the north polar hood). A similar layer was seen in Voyager images, but at an altitude in the range 300-350 km, whereas in ISS images it is close to 500 km. This may be a condensate layer because the vertical temperature gradient is negative in this region, although a plausible composition has not yet been proposed. Stellar occultations from the ground2 show a local temperature inversion near 500 km, and the haze layer we see may be related. Particle sedimentation times are short at 500 km where the pressure is several microbars.
The detached haze layer is not seen or is very subtle at latitudes higher than about 70N. Instead, there is a series of haze layers at somewhat lower altitudes and the polar haze appears to dissipate near 500 km altitude. Approximately seven layers can be seen both in reflected light at low phase angle and in transmitted light at high phase. The wavelength of these layers decreases with altitude. If they are a manifestation of gravity waves the change in wavelength with altitude is most likely produced by the phase speed of the waves relative to the background zonal winds decreases upward as the super-rotation strengthens1.
In transmitted light at high phase angles many thin layers are seen near the top of the stratospheric haze at low and middle latitudes. These are most visible in the near-ultraviolet. Their profile along the limb is not uniform - there are undulations that may also be caused by gravity waves. More work on these and other aspects of the haze is underway.
[1] Porco, C. et al., Nature, in press. , [2] Sicardy, B. et al., Icarus 142, 357-390, 1999.