All looks good for an on-time launch of Europe's mission to Mars. A joint venture with Russia, the ExoMars Trace Gas Orbiter (TGO) is set to launch atop a Proton rocket from Baikonur, Kazakhstan, on Monday.

The satellite will try to detect and characterise the marginal constituents of the planet's atmosphere. A key quest is to understand methane, a gas that has an unexpected persistence and which some have speculated could hint at the presence of microbial life.

Lift-off of the Proton is scheduled for 15:31 local time (09:31 GMT). "We've had a good launch campaign to date - no real issues," said Walter Cugno from the lead European contractor, Thales Alenia Space.

"All the preparation milestones have been achieved as per the nominal schedule," he told BBC News. It will take 12 hours for the Proton's Breeze-M upper-stage to boost the ExoMars TGO into just the right trajectory to go to the Red Planet.

Controllers at European Space Agency's Operations Centre in Darmstadt, Germany, expect to pick up a signal from the satellite shortly after it has been released on to this cruise path, at 21:28 GMT. The journey time to Mars is seven months. Three days out from arrival, on 16 October, the satellite will release a small landing module known as Schiaparelli.

Once on the surface, on 19 October, its aim is to operate a few science instruments, but engineers are primarily interested to see how the module performs during the entry, descent and touchdown. "This technology development is very important," said Esa's director of science, Prof Alvaro Gimenez.

"If you want to be a partner in the future on more missions to Mars, you have to demonstrate your ability to land. If you can't, you are not at the right level. We have to show we can do it ourselves." In particular, Schiaparelli will showcase a suite of technologies - radar, computers and their algorithms - that will be needed to put a British-assembled rover safely on the planet.

This second-step ExoMars mission is supposed to leave Earth in 2018, although this looks increasingly in doubt. No final price with industry to build the rover venture has yet been agreed, and there is currently insufficient money within the programme to fund it to completion. But even if the budget was not a problem, there is now so little leeway in the development schedule that few people express optimism that the rover and its associated landing equipment can be made ready in time for what is a very narrow launch window in 2018.

"It's now marginal," said Mr Cugno. "We have some possibility of being ready on the European side, but on the Russian side it is even more marginal." A formal announcement of a delay to 2020 - when the planets next align - could come within a few weeks. But the difficulties with the rover part of the ExoMars project should not affect the orbiter.

Once it has dropped off Schiaparelli, the TGO will spend the better part of a year manoeuvring itself into a 400km-high circular orbit above the planet. From this vantage point, the TGO's state-of-the-art instruments will then make a detailed inventory of Mars' atmospheric gases.

The mission is concerned with the components that constitute less than 1 percent of the planet's air - chemical species such as methane, water vapour, nitrogen dioxide, and sulphur dioxide. Methane is the main focus. From previous measurements, its concentration is seen to be low and sporadic in nature. But the mere fact that it is detected at all is really fascinating.

The simple organic molecule should be destroyed easily in the harsh Martian environment, so its persistence - and the occasional spikes in its signal - indicate a replenishing source of the gas. The explanation could be geological. One idea is that it is the simple by-product from water interactions with particular rock minerals at depth. Another explanation is that the gas is ancient in origin and has been trapped in sub-surface ice. Periodic melting events then release the methane into the atmosphere.

But there remains the tantalising prospect that the source could be biological. Most of the methane in Earth's atmosphere comes from living organisms, and it is not a ludicrous suggestion that microbes might also be driving emissions on Mars. "Whatever the explanation, it all points to the existence of liquid water in the sub-surface, and that changes slightly our vision of Mars, because it means it is a planet that is a little more active than we've recognised," said Esa's ExoMars project scientist, Dr Jorge Vago.