The Mars Atmosphere and Volatile Evolution mission, or Maven, is bristling with instruments able to measure the effect of solar wind and analyse thin traces of gases, in order to help scientists model the process that left the planet so dry and barren. Its launch follows findings from Nasa's recent Mars rover mission which support growing evidence in rock samples that there was once water on the surface of the Mars, protected by a thick atmosphere that could have supported primitive life.
Orbiting between 3,864 miles and 77 miles above the desert surface, Maven is expected to reveal how Mars' atmosphere was gradually peeled away over billions of years, by the sun's radiation.
"Maven is going to focus on trying to understand what the history of the atmosphere has been, how the climate has changed through time and how that has influenced the evolution of the surface and the potential habitability – at least by microbes – of Mars," said lead scientist Bruce Jakosky. "Mars is a complicated system, just as complicated as the Earth in its own way. You can't hope, with a single spacecraft, to study all aspects and to learn everything there is to know about it. With Maven, we're exploring the single biggest unexplored piece of Mars so far."
Among the eight instruments and nine sensors on board the spacecraft is a magnetometer that will help scientists measure changes in the magnetic field around Mars that would once have protected its atmosphere from solar wind.
In September, decade-long preparations for the mission were briefly interrupted by the US government shutdown.
Maven will take nearly a year to reach the red planet and scientists estimate it should be sending back its first results by early 2015.
The evidence for a warmer, wetter, more Earth-like Mars has been building. Ancient rocks bear chemical fingerprints of past interactions with water. The planet's surface is riddled with geologic features carved by water, such as channels, dried up riverbeds, lake deltas and other sedimentary deposits.
"The atmosphere must have been thicker for the planet to be warmer and wetter. The question is where did all that carbon dioxide and the water go?" Jakosky said.
There are two places such an atmosphere could go: down into the ground or up into space.
Scientists know some of the planet's carbon dioxide ended up on the surface and joined with minerals in the crust. But so far, the ground inventory is not large enough to account for the thick atmosphere Mars would have needed to support water on its surface. Scientists suspect that most of the atmosphere was lost into space, a process that began about 4 billion years ago when the planet's protective magnetic field mysteriously turned off.
"If you have a global magnetic field, it causes the solar wind to stand off. It pushes it away so it isn't able to strip away atmosphere," Jakosky said.
Without a magnetic field, Mars became a ripe target for solar and cosmic radiation, a process that continues today.
Maven's prime mission is expected to last for one year, enough time for scientists to collect data during solar storms and other space weather events. Maven will remain in orbit for up to 10 years, serving as a communications relay for Curiosity, a follow-on rover slated to launch in 2020 and a lander that is being designed to study the planet's deep interior.
Following its successful launch on Monday, the spacecraft is due to reach Mars on 22 September 2014 – two days before India's Mars Orbiter Mission, which launched on 5 November. India's probe has been raising its orbit around Earth and should be in position to begin the journey to Mars on 1 December.
