NASA'S newest robotic explorer, Maven, has rocketed toward Mars on a quest to unravel the ancient mystery of the red planet's radical climate change.
The Maven spacecraft is due at Mars next autumn following a journey of over 700 million km.
Scientists want to know why Mars went from being warm and wet during its first billion years to cold and dry today.
The early Martian atmosphere was thick enough to hold water and possibly support microbial life. But much of that atmosphere may have been lost to space, eroded by the sun.
"We want to know: what happened?" said Michael Meyer, NASA's lead Mars scientist.
To help solve this environmental puzzle, Maven will spend an entire Earth year measuring atmospheric gases once it reaches Mars on September 22, 2014.
This is NASA's 21st mission to the planet since the 1960s. But it is the first one devoted to studying the Martian upper atmosphere. The mission costs $671m (€500m).
Maven – short for Mars Atmosphere and Volatile Evolution, with a capital 'N' in EvolutioN – bears eight science instruments. The spacecraft, at 2,454 million kgs, weighs as much as a 4X4. From solar wingtip to wingtip, it stretches 37.5 feet.