The program is being supported by 14 European nations and Russia.
This will be the European Space Agency's first attempt to land on Mars.
The orbiter would map the variable distributions of trace gases in Mars' atmosphere, including methane and its isotopes.
The Entry, descent and landing Demonstrator Module (EDM) is a key test in delivering Europe's first rover to the surface of Mars in 2018.
The lander will survive only a short time on the surface due to battery limitations.