The US and European space agencies have signed the "letter of intent" that ties together their Mars programmes.

The agreement, which was penned in Washington DC, gives the green light to scientists and engineers to begin the joint planning of Red Planet missions.

The union will start with a European-led orbiter in 2016, and continue with surface rovers in 2018 and then perhaps a network of landers in 2018.

The ultimate aim is a mission to return Mars rock and soils to Earth labs.

The Washington document was signed by the heads of the agencies, Nasa administrator Charles Bolden and Esa director-general Jean-Jacques Dordain.

The Mars Joint Exploration Initiative (Meji) has been under discussion for several months, with the key elements - covering scope, division of responsibility and financing - gradually falling into place. The letter of intent puts the initiative on a more formal footing.

The US and Europe have taken the view that they can achieve more together scientifically at the Red Planet if they combine their expertise.

And with both parties' current Mars programmes also experiencing financial pressures, the shared approach means the exploration schedule of a mission every two years can be maintained.

Esa's member states have already pledged 850m euros towards a Red Planet venture. They will need to take that figure up to about a billion euros to properly fund Meji activities.

The existence of this extra funding, and which European nations might provide it, will have to be established at a council meeting of the agency in mid-December (although the subscription opportunity will officially stay open until the end of the year).