If Neptune gave us some real surprises, the other bodies in the Neptunian system were just as amazing. For example, there are now six other satellites besides the two already known (which have been temporarily "christened" with the codes 1989N1 to 1989N6), the largest of which has a diameter of no less than 420 kilometers (about 20 times that of Phobos, the largest of Mars' moons). And Triton, the largest of the two known satellites, turned out to be one of the most extraordinary bodies of the entire Voyager mission. This moon, with a diameter of 2 800 kilometers, was famous for an unusual characteristic: though only 350 000 kilometers away from Neptune (less than the distance between our Moon and the Earth), unlike all the other satellites in the solar system, it turns in the opposite direction to the planet's rotation. A strange "dance" that probably did not occur at the beginning of time and probably will not last forever. In fact, in the past, Triton was a wandering body, perhaps a comet, "captured" by Neptune when the solar system was still young. Since it follows an ever-decreasing spiral around the planet, within a period of 10 to 100 million years it should literally break apart due to the increasing intensity of the tidal forces exerted by the larger body. Voyager encountered Triton immediately after its on-time passage under the south pole of Neptune, and the pictures which reached the Earth were the most beautiful and puzzling of the entire mission. This moon has a very thin atmosphere, consisting mainly of nitrogen, and is a thousand times more rarefied than the Earth's atmosphere. It is not dense enough to hide the surface, which appears in the first pictures to be speckled like marble, with delicate shades of pink in the equatorial areas and a light blue strip in the polar cap. "This is the first time that we have observed the color blue on a satellite from when we launched the Voyager", exclaimed with surprise Bradford Smith, another mission manager. "The most probable hypothesis is that this color is generated by the diffusion of sunlight through frozen methane crystals". But the discoveries didn't end there. As Voyager got closer, the scientists continued to be shocked as they gazed at that strange moon on their screens. Even Carl Sagan, the astronomer who was directly involved with some of Nasa's most important planetary missions and who is one of the "fathers" of the small probe, appeared astonished. "Simply incredible", was all that he could whisper, while all eyes focused on the surface of Triton. As the seconds ticked, various landscapes appeared on the screen: valleys and hills, immense plains and long linear structures similar to the large faults on the Earth's crust. Everyone tried to locate some craters which, being produced by the impact with meteorites when the solar system was still young, would make it possible to determine the age and the behavior of any planetary surface: the presence of many craters, like on the Moon, means that that world is inactive, and has been geologically "dead" for a long time, otherwise it means that it is "alive", i.e. that its surface was re-shaped in relatively recent times. Well, Triton showed right away that it belongs to the second category: there are very few impact craters and instead there are immense smooth surfaces around which the ground seems to have "thickened" into many different types of structures. "It creates the impression", says Smith, "that it's history is similar to ice cream taken home in August: frozen, melted and then frozen again". What happened on this moon that, with its 240 degrees below zero, is the coldest place in the solar system? According to Nasa researchers, Triton has intense internal activity which, in recent times, brought to the surface large outflows of ice: this activity, which was defined as "cryovolcanic" (from the Greek kryos, which means cold) was probably fed by an interior heat source, related to a semi-fluid core that is continuously being remixed. This would be demonstrated by the gigantic calderas discovered by Voyager, with a diameter of hundreds of kilometers, the interior of which, until now, are hiding enormous quantities of water in a viscous state. Instead, other volcanic structures seem to be explosive: this would lead to the emission of nitrogen, a part of which would be responsible for this moon's weak atmosphere. Dark traces in the form of plumes, with a length of sixty kilometers, are the remains of such activity on the pink surface.
If Neptune gave us some real surprises, the other bodies in the Neptunian system were just as amazing. For example, there are now six other satellites besides the two already known (which have been temporarily "christened" with the codes 1989N1 to 1989N6), the largest of which has a diameter of no less than 420 kilometers (about 20 times that of Phobos, the largest of Mars' moons). And Triton, the largest of the two known satellites, turned out to be one of the most extraordinary bodies of the entire Voyager mission. This moon, with a diameter of 2 800 kilometers, was famous for an unusual characteristic: though only 350 000 kilometers away from Neptune (less than the distance between our Moon and the Earth), unlike all the other satellites in the solar system, it turns in the opposite direction to the planet's rotation. A strange "dance" that probably did not occur at the beginning of time and probably will not last forever. In fact, in the past, Triton was a wandering body, perhaps a comet, "captured" by Neptune when the solar system was still young. Since it follows an ever-decreasing spiral around the planet, within a period of 10 to 100 million years it should literally break apart due to the increasing intensity of the tidal forces exerted by the larger body. Voyager encountered Triton immediately after its on-time passage under the south pole of Neptune, and the pictures which reached the Earth were the most beautiful and puzzling of the entire mission. This moon has a very thin atmosphere, consisting mainly of nitrogen, and is a thousand times more rarefied than the Earth's atmosphere. It is not dense enough to hide the surface, which appears in the first pictures to be speckled like marble, with delicate shades of pink in the equatorial areas and a light blue strip in the polar cap. "This is the first time that we have observed the color blue on a satellite from when we launched the Voyager", exclaimed with surprise Bradford Smith, another mission manager. "The most probable hypothesis is that this color is generated by the diffusion of sunlight through frozen methane crystals". But the discoveries didn't end there. As Voyager got closer, the scientists continued to be shocked as they gazed at that strange moon on their screens. Even Carl Sagan, the astronomer who was directly involved with some of Nasa's most important planetary missions and who is one of the "fathers" of the small probe, appeared astonished. "Simply incredible", was all that he could whisper, while all eyes focused on the surface of Triton. As the seconds ticked, various landscapes appeared on the screen: valleys and hills, immense plains and long linear structures similar to the large faults on the Earth's crust. Everyone tried to locate some craters which, being produced by the impact with meteorites when the solar system was still young, would make it possible to determine the age and the behavior of any planetary surface: the presence of many craters, like on the Moon, means that that world is inactive, and has been geologically "dead" for a long time, otherwise it means that it is "alive", i.e. that its surface was re-shaped in relatively recent times. Well, Triton showed right away that it belongs to the second category: there are very few impact craters and instead there are immense smooth surfaces around which the ground seems to have "thickened" into many different types of structures. "It creates the impression", says Smith, "that it's history is similar to ice cream taken home in August: frozen, melted and then frozen again". What happened on this moon that, with its 240 deg
