A Proton rocket carrying the Trace Gas Orbiter to examine Mars's atmosphere and a descent module that will conduct a test landing on its surface launched into an overcast sky at the Russian-operated Baikonur cosmodrome in the Kazakh steppe at 0931 GMT (1501 IST).
The spacecraft carried out a series of engine burns to get it onto the right trajectory to Mars and is set to finally detach from its Briz-M rocket booster just after 2000 GMT before beginning its 496-million-kilometre voyage through the cosmos.
The Trace Gas Orbiter will examine methane around Mars while a lander dubbed Schiaparelli will detach and descend to the surface of the fourth planet from the Sun.
The landing of the module on Mars is designed as a trial run ahead of the planned second stage of the mission in 2018 that will see the first European rover land on the surface to drill for signs of life, although problems with financing mean it could be delayed.
"TGO will be like a big nose in space," said Jorge Vago, ExoMars project scientist.
Methane, the ESA said, is normally destroyed by ultraviolet radiation within a few hundred years, which implied that in Mars' case "it must still be produced today".
TGO will analyse Mars' methane in more detail than any previous mission, said ESA, in order to try to determine its likely origin.
A better insight into water on Mars could aid scientists' understanding of how the Earth might cope in conditions of increased drought.
Schiaparelli, in turn, will spend several days measuring climatic conditions including seasonal dust storms on the Red Planet while serving as a test lander ahead of the rover's anticipated arrival.
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