Discovered by Huygens in the spring of 1655, the same year in which the Dutch astronomer had deduced the presence of the rings around Saturn, Titan is undoubtedly a unique object in the solar system. In fact, except for the Earth, it is the only body whose surface is covered by liquids. Its name, which was chosen by John Herschel only two centuries after its discovery, came from the fact that it was believed to be the largest satellite, but the results from Voyager 1 eliminated such a distinction: in fact, it is the thickness of its atmosphere that makes it seem to be bigger. In reality, the diameter of its solid part is 5 150 kilometers, compared to the 5 280 of Ganymede. But scientists want to solve this mystery: what is the composition of this mysterious body hidden by such dense veils? And, more than anything else, what does it hide? Astronomical observations had already shown that Titan was surrounded by an atmosphere that is three times as dense as the one on Mars, but its composition and its structure were only the object of what were often conflicting theories. At this point, a very close "encounter" was needed to find out more. Therefore, the Nasa technicians decided that Voyager 1 would be "sacrificed" to complete this assignment: it would be flown very close by Titan, giving up any possibility of using the gravitational effect of Saturn to continue the voyage towards Uranus and Neptune. Only Voyager 2 would continue the mission towards the outer planets. On November 12, 1980, Voyager 1 reached its rendez-vous and passed less than 7 000 kilometers from Titan. But scientists were very disappointed by the first pictures: the satellite, opaque and uniform, looked like a ball of pink cotton, and there was not even one opening in the atmosphere to catch a glance of the surface. So, the chemical data transmitted by the probe was the only information that could be used to try to discover its composition. First, scientists discovered that the atmosphere on this moon is denser than that of the Earth and has maintained the conditions which are very similar to those which must have existed on all planets immediately after their formation: in other words it contains nitrogen, argon, hydrogen and carbon in the form of traces of hydrocarbons. It has a truly unusual structure: a layer at an altitude of 400 kilometers from the ground is transparent to visible light, but it does absorb the ultraviolet radiations from the Sun; below this layer, at an altitude of 300 kilometers, there is a layer of fog which covers a layer of aerosol, i.e. solid suspended particles. During Titan's evolution, these particles probably aggregated gradually into large bodies which fell to the surface. Today, scientists believe that the surface of this moon, which has a temperature of -179 degrees Celsius, is covered by a layer of hydrocarbons (probably methane) in the liquid state, with a thickness that varies from 100 to 500 meters. Like the Earth is rich in water oceans, Titan might be literally covered by an ocean of methane which performs the same function as water does on our planet. And perhaps the same lower layers of the atmosphere might even have methane clouds which, when saturated, would produce real rain. All these considerations led to this extraordinary conclusion by scientists: under these conditions, the chemical reactions which today occur in the atmosphere of the mysterious satellite might one day lead to the formation of some of the organic molecules considered to have been the base of development of life on Earth. If our great-great-grandchildren keep an eye on Titan long enough (but on an evolutionary scale this may mean millions of years), they may witness the birth of new organisms in the solar system.
Discovered by Huygens in the spring of 1655, the same year in which the Dutch astronomer had deduced the presence of the rings around Saturn, Titan is undoubtedly a unique object in the solar system. In fact, except for the Earth, it is the only body whose surface is covered by liquids. Its name, which was chosen by John Herschel only two centuries after its discovery, came from the fact that it was believed to be the largest satellite, but the results from Voyager 1 eliminated such a distinction: in fact, it is the thickness of its atmosphere that makes it seem to be bigger. In reality, the diameter of its solid part is 5 150 kilometers, compared to the 5 280 of Ganymede. But scientists want to solve this mystery: what is the composition of this mysterious body hidden by such dense veils? And, more than anything else, what does it hide? Astronomical observations had already shown that Titan was surrounded by an atmosphere that is three times as dense as the one on Mars, but its composition and its structure were only the object of what were often conflicting theories. At this point, a very close "encounter" was needed to find out more. Therefore, the Nasa technicians decided that Voyager 1 would be "sacrificed" to complete this assignment: it would be flown very close by Titan, giving up any possibility of using the gravitational effect of Saturn to continue the voyage towards Uranus and Neptune. Only Voyager 2 would continue the mission towards the outer planets. On November 12, 1980, Voyager 1 reached its rendez-vous and passed less than 7 000 kilometers from Titan. But scientists were very disappointed by the first pictures: the satellite, opaque and uniform, looked like a ball of pink cotton, and there was not even one opening in the atmosphere to catch a glance of the surface. So, the chemical data transmitted by the probe was the only information that could be used to try to discover its composition. First, scientists discovered that the atmosphere on this moon is denser than that of the Earth and has maintained the conditions which are very similar to those which must have existed on all planets immediately after their formation: in other words it contains nitrogen, argon, hydrogen and carbon in the form of traces of hydrocarbons. It has a truly unusual structure: a layer at an altitude of 400 kilometers from the ground is transparent to visible light, but it does absorb the ultraviolet radiations from the Sun; below this layer, at an altitude of 300 kilometers, there is a layer of fog which covers a layer of aerosol, i.e. solid suspended particles. During Titan's evolution, these particles probably aggregated gradually into large bodies which fell to the surface. Today, scientists believe that the surface of this moon, which has a temperature of -179 degrees Celsius, is covered by a layer of hydrocarbons (probably methane) in the liquid state, with a thickness that varies from 100 to 500 meters. Like the Earth is rich in water oceans, Titan might be literally covered by an ocean of methane which performs the same function as water does on our planet. And perhaps the same lower layers of the atmosphere might even have methane clouds which, when saturated, would produce real rain. All these considerations led to this extraordinary conclusion by scientists: under these conditions, the chemical reactions which today occur in the atmosphere of the mysterious satellite might one day lead to the formation of some of the organic molecules considered to have been the base of development of life on Earth. If our great-great-grandchildren keep an eye on Titan long enough (but on an evolutionary scale this may mean millions of years), they may witness the birth of new organisms in the solar system.
