It is by far the largest moon of Saturn and until Ganymede took over the leadership, it was considered the largest in the Solar System. Of course, it is greater than Mercury. It is also the only moon in the entire Solar System that has a dense atmosphere ; in fact, the atmospheric pressure is 60% higher than on our own planet. It is also, along with Earth, the only atmosphere in which the main component is nitrogen (about 95%): the rest is methane followed by an eclectic mix of organic components.
The surface of Titan was a complete mystery until the arrival of the Cassini mission to Saturn . In fact, when Voyager 1 passed within 4,000 km of its surface in November 1980, we could only see it cloaked in an orange atmosphere , impenetrable to the probe’s camera lenses; It was then that observations made in the infrared and ultraviolet range revealed that this moon was submerged in a nitrogen atmosphere. It was even thought that there could be an ocean of liquid nitrogen on its surface.
But in June 2004 the Cassini mission, which carried the Huygens module – expressly designed to reach the surface of Titan (it was the first time we landed on a body in the outer Solar System) – we were able to partially lift that veil of mystery. Cassini mapped its surface and studied the chemical reactions taking place in its atmosphere , while Huygens took all kinds of data on its descent. This probe was a complete success because it not only survived the landing but was able to transmit data for an hour from the icy surface, until its batteries ran out. The information collected by the complex Cassini-Huygens mission has revealed to us a world surprisingly similar to Earth but the product of completely different chemistry and climatology. One of those similarities is, for example, that it has seasons that last 7.5 Earth years. We have even been able to witness a change of season: from autumn to winter at the south pole and the beginning of summer in the north.
On Titan it rains, but not water. Images from the VLT in Chile and the Keck telescope in Hawaii taken in 2005 and 2006 showed for the first time a huge cloud covering almost the entire moon as persistent drizzle fell west of the foothills of Titan’s largest continent, Xanadu. And it was a methane rain.
On Earth we usually think of methane as a gas, but on Titan’s cool surface this simple hydrocarbon (consisting of one carbon bonded to four hydrogens) is a liquid . There, instead of the water cycle, the methane cycle takes place. Rain rarely reaches the surface; normally the methane evaporates before it hits the ground, as it sometimes does in some deserts on Earth. On the other hand, downpours are very rare, only occurring once every few centuries. Of course, when it happens they usually last for months.
Titan is a world with lakes and seas of methane and ethane – the second simplest hydrocarbon, with two carbons and six hydrogens – near the poles, and vast arid regions with hydrocarbon-rich dunes around the equator. You can also see channels that, although dry most of the time, there is no doubt that they were carved by methane rivers. In certain places, steep valleys and deep cliffs can be seen, giving the moon a rugged appearance, while in others, such as the Huygens landing zone, they are flat and desert-like, with broken terrain made up of small icy cobblestones. .
With a ground level temperature of -180º C, the possibility of finding liquid water would be crazy , but surprisingly there is some circumstantial evidence that suggests that it may not be : certain brightly colored drainage patterns, totally different from the dark ones produced by hydrocarbon rivers. How is it possible? The only plausible explanation is that there is low-temperature volcanic activity, or cryovolcanism, caused by the tidal forces of Saturn.
In Titan’s atmosphere , dominated by nitrogen, a frenetic chemical activity is observed, produced both by ultraviolet radiation from the Sun and, above all, by the rain of highly energetic particles that are accelerated when they enter Saturn’s powerful magnetic field. This continuous bombardment causes the appearance of free radicals – fragments of molecules with free electrons – that end up joining together to form more complex molecules. The Cassini probe detected propylene (the second most used compound in the chemical industry worldwide), the poisonous hydrogen cyanide (better known as prussic acid, which smells like bitter almonds) and acetylene , a very energetic hydrocarbon that is composed of two carbons joined to each other by a triple bond and each of them, in addition, with a hydrogen. In the atmosphere, acetylene forms small solid particles that end up depositing on the surface.
And if on Earth this compound must be handled with extreme care because it is highly explosive, on the icy surface of Titan, where all chemical reactions take place at an unbearably slow rate, acetylene is the ideal compound to promote the appearance of organic molecules more complex. Some of these compounds, created from methane and nitrogen, cause Titan to be covered in a peculiar smog, an orange haze that hides its surface. Meanwhile, heavier hydrocarbons settle to the ground, playing the role of “sand” in equatorial dune fields. Methane, for its part, condenses in the atmosphere forming clouds, which end up unleashing storms.
But where does all that methane come from? Nobody knows. Why are there still large amounts of methane in the atmosphere? Sunlight continuously breaks down the molecules of this hydrocarbon, so there must be a reservoir of this compound somewhere, otherwise everything would have disappeared by now. Where is? This is another of the great mysteries of Titan.
From the gravimetric measurements carried out by the Cassini probe , planetologists think that there is an ocean of water and ammonia under its surface , at a depth of between 55 and 80 km. With all this, it is not uncommon for astrobiologists to have speculated about what life might be like there. Obviously it would be totally different from what we know, based on totally strange chemistry at very low temperatures and in absolutely unthinkable conditions. And the implications for our understanding of how life appears and evolves in the universe would be even more unthinkable.