The detailed exploration of Titan with space missions began a couple of years ago and the presence of bright polar clouds and dry riverbeds on this satellite of Saturn has intrigued astronomers ever since. Doctors Ricardo Hueso and Agustín Sánchez Lavega from the Planetary Sciences Group of the University of the Basque Country (UPV-EHU), based at the Basque Country School of Engineering in Bilbao, have put forward an explanation for the phenomena in the July 2006 edition (Nº 27) Nature. The clouds and dry beds are due to giant storms of methane that occur on Titan. The satellite of Saturn would thus have a "methane cycle", probably similar to the Earth's water cycle.
Titan is the largest satellite of the planet Saturn and the second in size of the whole solar system at 5,150 kilometres diameter. It is the only moon in the system with an atmosphere dense in nitrogen, similar to that of the Earth. Titan's atmosphere has a thick orange cloud of hydrocarbons that impede visibility of its surface, which also makes it different from the rest of the satellites. Moreover, given the enormous distance separating it from the Sun - some 1,500 million kilometres -, the surface temperatures there are icy at 180ºC below zero, any water on it surface will be completely frozen.
However, the pictures taken by the Cassini spaceship, orbiting Saturn since July 2004, and the measurements obtained from the Huygens probe, installed in a satellite launched in January 2005, point to the existence of recently formed canals and dried riverbed structures on the surface of this frozen world. If this is the case, we have the first example in the whole solar system - apart from the Earth, that is - where we can regularly find surface deposits of liquid. How can flowing structures be formed in such a frozen atmosphere? Are they formed by rains? If so, what kind?
The research published in Nature by Doctors Ricardo Hueso and Agustín Sánchez Lavega and entitled, 'Methane storms on Saturn's moon, Titan' provides an explanation to these questions that intrigue astronomers. According to these Basque Country University scientists, huge clouds of methane vapour form storms on Titan´s surface, this hydrocarbon compound playing a similar role to that of water on Earth. From calculations carried out, these heavy storms, which can reach a vertical height of 35 kilometres above the surface, produce dense clouds of methane and copious precipitation of liquid drops of the gaseous compound, similar to the intense downpours of rain we experience on Earth. The precipitation generates accumulations and rivers of liquid methane on Titan, producing the canals observed.
The hypothesis formulated by the researchers of the existence of heavy methane storms on Titan is based on the ongoing observations over the last few years from the Cassini craft and by the largest terrestrial telescopes of very localised and bright clouds. One of the most prolific regions where these wide masses of cloud have been found is at the South Pole of the satellite. Despite the low mean temperatures of Titan, the polar region is currently in summer, thus receiving more heat than the rest of the planet and raising the temperature slightly but sufficiently to provide the energy needed to for the stormy episodes. The researchers have found that one of the keys in the development of the storms are the so-called "condensation nuclei" particles that form the orangey cloud and reach the lower atmosphere of Titan. Methane drops form around these particles and give rise to the storm clouds.
The researchers suggest that the methane, present in small quantities in the atmosphere, play a similar role to that of water on Earth although, to date, they have not yet discovered liquid-state deposits on Saturn's moon. Titan, in this sense, would have a "methane cycle", similar perhaps to the Earth's water cycle. In order to confirm their hypothesis of liquid methane formation causing the storms, the UPV/EHU investigators suggest the systematic radar observation with Cassini of the surface in those regions where the formation of the bright white clouds have been observed.