By Paul Rincon
Science reporter, BBC News

Nasa and the European Space Agency (Esa) have taken a significant step towards teaming up to explore Mars.

The agencies have announced a joint initiative for robotic exploration of the Red Planet following a two-day summit in Plymouth this month.

But this means European space ministers now face tough decisions over the future of Esa’s ExoMars rover mission.

They need to decide whether to totally re-scope ExoMars in light of options opened up by collaboration with the US.

Nasa and Esa were eyeing a mutual arrangement on Mars exploration because of the growing cost of such missions and because of shared science goals.

The initiative is called the Mars Exploration Joint Initiative, or MEJI, and will investigate collaborative activities between the agencies for launch opportunities in 2016, 2018 and 2020 (launch opportunities to Mars come up roughly every two years).

Nasa and Esa envisage sending landers and orbiters to conduct astrobiological, geological and geophysical investigations on the Red Planet over the coming decade.

The ultimate goal of the collaboration is an international mission to bring back geological samples from the Red Planet.

Esa had previously envisaged ExoMars as a mission costing roughly 1.5bn euros. The rover is currently slated to launch in 2016, and will search for signs of past or present life on the Martian surface.

But last year, Esa’s director-general Jean-Jacques Dordain promised European governments that the cost of ExoMars would be kept as close as possible to 850m euros.

Esa officials believe the way forward is through collaboration with the US, which is now expected to make a major investment in ExoMars. In return, European money would be put into future Mars missions.

The US has its own concerns: Nasa is having to plan ahead with reduced money for Mars exploration, and, like Europe, is keen to discuss sharing the cost of future missions to the Red Planet.

Nasa is due to launch a $2bn (