Imagine an alien world, millions of miles from our planetary home.
Now imagine that same place somehow similar to Earth. A place with an atmosphere, with a landscape that would be recognizable to any human being. A place where you could see clouds and mountains in the horizon, where you could behold rivers and lakes and vast areas covered with sand dunes. Picture in your mind that place, where drops of methane rain fall smoothly to the ground like snowflakes on Earth.
Well, that world exists: It's called Titan, the largest moon of Saturn. Titan is one of the most Earth-like bodies found so far. It resembles a glacial version of our planet billions of years ago when there was no life on its surface. It is a primordial and extremely cold place filled with wonders.
The will to explore Titan increased when Pioneer and Voyager programs revealed parts of its atmospheric structure. The possibility of finding fair amounts of organic molecules on the atmosphere and soil of this saturnian moon and the desire to chart its covered surface set off NASA/ESA/ASI's Cassini-Huygens mission. This project's main goal is the observational study of the distant and complex Saturnian system. The launched probe was the combination of Saturn's orbiter Cassini and the Huygens lander; the last landed on Titan's surface on the 14th of January, 2005.
This mission brought us something that was never been seen by humankind. It unveiled an unique moon littered with then unseen astrogeological features, a truly remarkable place. And what makes this world so unique is its striking similarity with Earth: It's the only moon in the Solar System with a proper atmosphere, even a denser atmosphere than our own, composed mainly by nitrogen and traces of methane and ethane. Its orange colour is due to a thick hydrocarbon fog that may be formed in the upper atmospheric interaction with the Sun's ultraviolet light.
Cassini-Huygens also sent us pictures of a landscape carved and shaped by hydrocarbon flows and hydrocarbon lakes. Methane and ethane play a role much similar to water on Earth: Hydrocarbon molecules rain over the soil of Titan to form streams and lakes and then evaporates back into the atmosphere.
Wind and cryovulcanism may also play an important role on the formation of Titan's surface. Vast fields of dark sand dunes fill the moon's equatorial regions gathered there by tidal winds. Data also suggests that cryovulcanoes may have spawned plumes of liquid water and ammonia raising the possibilities for a more dynamic geological cycle on this distant world.
It is in fact wonderful to find such familiar natural processes in such a distant and cold moon. Still, Titan's inner working is very different from Earth's. The surface temperature on Titan is about -179º C. At this temperature water ice is hard as steel and methane, which exists as a gas at Earth's mean temperature, can flow as a liquid. And so the ground, the valleys, the mountains in Titan are mainly composed by water ice that behaves like solid rock.
Beneath this icy crust is thought to exist a global subsurface ocean of water and ammonia, fact that may be proved by a 30km shift on the surface scan made by the Cassini probe between October 2005 and May 2007. This information points to a decoupled crust and a liquid layer beneath it.
Titan is an extraordinary and complex world waiting to be explored. The human exploration of Titan is still in an initial stage even after all the information sent by Cassini-Huygens. I think new missions to this cold moon are much needed for our understanding of its nature and the broad nature of the Cosmos itself.
http://saturn.jpl.nasa.gov/
http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20120628.html
http://www.nasa.gov/mission_pages/cassini/whycassini/Saturns_Moon.html
http://www.esa.int/SPECIALS/Cassini-Huygens/
http://sci.esa.int/science-e/www/area/index.cfm?fareaid=12
