  The Venus thermal radiation spectrum exhibits the signature of $CO_2$
absorption bands. By means of inversion techniques, those bands enable the
retrieval of atmospheric temperature profiles. We have analyzed VIRTIS-M-IR
night-side data obtaining high-resolution thermal maps of Venus south polar
region between 55 and 85 km altitudes for three dynamical configurations of the
vortex. The cold collar is clearly distinguishable at $\sim 62$ km altitude
level, and it is more than 15 K colder than the pole on average. The South
Polar Vortex appears as a vertically extended hot region close to the pole and
squeezed by the cold collar between altitudes 55 and 67 km but spreading
equatorward at about 74 km. Both the instantaneous temperature maps and their
zonal averages show that the top altitude limit of the thermal signature of the
vortex is at $\sim 80$ km altitude, at least on the night-side of the planet.
The upper part of the atmosphere (67 - 85 km) is more homogeneous and has
long-scale horizontal temperature differences of about 25 K over horizontal
distances of $\sim 2,000$ km. The lower part (55 - 67 km) shows more fine-scale
structure, creating the vortex' morphology, with thermal differences of up to
about 50 K over $\sim 500$ km horizontal distances. We also study the vertical
stability of different atmospheric layers within the 55 - 85 km altitude range
for the three vortex configurations. It is always positive, but the cold collar
is the most vertically stable structure at polar latitudes, while the vortex
and sub-polar latitudes show lower stability values. Furthermore, the hot
filaments present within the vortex exhibit lower stability values than their
surroundings. The layer between 62 and 67 km resulted to be the most stable.
These results are in good agreement with conclusions from previous radio
occultation analyses.
