By Jonathan Amos
Science reporter, BBC News
NEW EXOMARS MISSION CONCEPT
Now likely to leave Earth in 2018; primary aim is to search for life
Current proposal is to use a US Atlas rocket to launch ExoMars
US also to look after the rover's entry, descent and landing
Hardware likely to be the same as for US 2013 rover, Curiosity
ExoMars would be targeted at key methane hotspots on the planet
It will have the capacity to drill 2m into the Martian ground
Esa states still have to sign off the final plan and a budget
European states have agreed in principle to a re-shaping of their plans to explore Mars.
Delegations to the European Space Agency (Esa) approved at their council meeting the idea of delaying the flagship ExoMars rover mission to 2018.
Its 2016 flight opportunity will be taken up instead by an orbiting satellite and a small static lander.
The changes were deemed necessary in order to make Europe's ambitions fit the available budget.
The member states of Esa had pledged 850 million euros to the ExoMars project at their triennial ministerial meeting last November - an inadequate sum to complete the mission as envisaged by scientists and engineers.
This prompted officials at the Paris-based agency to seek the participation of the US in a joint programme of exploration at the Red Planet, to, in essence, share the expense of exploring Mars.
The intention is that all Mars missions from 2016 onwards will now be badged as joint US-European ventures.
The council accepted this concept, but an assessment of the expected costs will be required before the delegations will give their full approval by the end of the year.
Although the Americans in their participation in ExoMars will be picking up some of the price tag, there is additional expenditure that now needs to be accounted for if Europe is going to pursue an orbiter and a static lander in 2016.
Member states will not want the figure to rise above a billion euros.
"The council was pretty good," said Professor David Southwood, Esa's director of science and robotics.
"The two-mission scenario is accepted by all and the long-term cooperation with the US is welcomed. For ExoMars, we have got an agreed technical basis that should work for everybody. For me, resource is now the fundamental issue," he told BBC News.
The 2016 orbiter would be designed to track down the sources of methane and other trace gases recently detected at Mars. The presence of methane is intriguing because its likely origin is either present-day life or geological activity.
Confirmation of either would be a major discovery. ExoMars could be targeted at one of the most interesting sources.
The 2016 mission would also have sufficient mass margin to put some sort of lander on the surface, although this would stay in just one location and may not be very long-lived.
Even so, Europe is keen to have a go at putting down some sort of instrument package on the planet to gain expertise in entry, descent and landing technologies.