Titan's surface observed with ground-based adaptive optics imaging

Athena Coustenis
Department for Space Research, Paris-Meudon Observatory,
France

Titan, Saturn's largest satellite is one of the most interesting and 
intriguing objects in our Solar System. Its uncanny resemblance to our 
own planet, revealed by the Voyager spacecraft in 1980, has motivated 
generations of scientists into studying it further from both the space 
(with ISO) and from the ground, and has instigated the Saturn-bound 
Cassini/Huygens ESA/NASA mission (arrival at Titan is scheduled for 
end of 2004).

Thus, we know today that Titan's thick atmosphere is essentially 
composed of nitrogen, with small amounts of methane and hydrogen. 
The combination among these mother molecules produces an exciting 
organic chemistry, with hydrocarbons and nitriles mixing with small 
amounts of oxygen (only traces of CO, CO2 and more recently, H2O 
have been discovered). Titan's surface, hidden under the veil of 
a thick aerosol cloud, affords a pressure close to our own planet's 
but for much lower temperatures (about 94 K). Its composition and 
morphology remain a mystery to this date. 

However, recent spectroscopy and imaging of the satellite, benefiting 
from the presence of weak regions of methane absorption (CH4 windows) 
in the near-IR, have shown that this surface is inhomogeneous, bright 
on the leading side and darker on the trailing one. 

I will discuss the images of Titan we have acquired since 1994 and 
up until a few months ago using two different adaptive optics systems 
(ADONIS at ESO and PU'EO at the CFHT) and with the spectrograph 
OASIS/CFHT, and associate them with a brief description of spectroscopic 
results. The images show a large, bright, equatorial region - possibly 
connected with relief - located on the leading hemisphere, while bright 
areas are also observed elsewhere near the poles. The exact nature of 
this ground remains to be discovered, but is probably an areal 
combination of ices (H2O, CH4, C2H6...), organic deposits, hydrocarbon 
liquid and rocks, connected with a relief (mountainous plateaux of 
some sort).