482 Encyclopedia of the Solar System
craters, dark plains with some brighter flows, mysterious
linear black features possibly related to winds, sand dunes,
snow dunes and a host of possible actors (solids, winds, liq-
uids, ices, volcanism, etc.). Titan has proven to be a much
more complex world than originally thought and much
tougher to unveil.
4. Looking Ahead
Much like Earth, a greenhouse effect exists on Titan; it is
produced essentially by methane, with contributions by ni-
trogen and hydrogen, which have important consequences
on the surface temperature. Methane is normally pho-
tolyzed in Titan’s atmosphere, and unless it can be replen-
ished by a large reservoir on or beneath the surface, it is
bound to disappear in a few million years. In such a case, the
surface temperature would drop below the condensation
point for nitrogen, and Titan’s atmosphere would collapse.
Should the absorptivity of the surface increase subsequently
(e.g., due to the accumulation of organics), the surface
temperature might once again rise and cause the reevapo-
ration of methane and nitrogen, thus rebuilding the atmo-
sphere. Such cycles have been hypothesized to occur on
Titan.
On the other hand, should the methane supply become
abundant, a small perturbation in the solar flux received
on Titan (such as is expected when the Sun becomes a red
giant and then a dwarf) would produce a dramatic warm-
ing of the climate, raising the temperature on the surface
and the pressure to values as high as 180 K (twice what we
have today) for several bars. It is not inconceivable to imag-
ine that some day in the distant future, conditions on Titan
one day may very closely resemble those found on our own
planet today.
In the meantime, theCassinimission has demonstrated
the complexity of this world and our need to further inves-
tigate it in order to better comprehend our solar system.
Beyond the extendedCassinimission (2010), discussions
on future missions to Titan are already underway.
Although future ideas for Titan missions are not mature
(after all,Cassiniis still on the spot), a prominent concept
is the use of “aerobots,” or intelligent balloons, to explore
a variety of Titan locations seems to be favored. Titan may
have many more surprises in store for us.Acknowledgments
The author wishes to thank D. Gautier, M. Hirtzig, F. Ferri,
T. Krimigis, D. Luz, and T. Tokano for inputs and discus-
sions.Bibliography
Coustenis, A., and Taylor, F. (2007). “Titan: An Earth-like
Moon,” 2nd Ed. World Scientific Publishers, Singapore.
Several articles inScienceandNatureissues of 2004 and 2005
describe in detail theCassini–Huygensmission first findings. In
particular:Cassiniarrives at Saturn;Science, 307: February 25,
2005, and Imaging of Titan from the Cassini spacecraft.Nature,
434: March 10, 2005, andNature, 438: December 8. 2005. Porco
et al.,Nature,10 March 2005, 434 ;Nature, 8 December 2005,
Imaging of Titan from theCassinispacecraft, 438.
Major Web sites: http://saturn.jpl.nasa.gov/home/index.cfm
http:// http://www.esa.int/SPECIALS/Cassini–Huygens/index.html