Image copyright ESA Image caption The ExoMars "longlist". Two proposals were received for Mawrth, but these were virtually the same

The European Space Agency (Esa) has published the "longlist" of eight sites it is considering as a destination for the ExoMars rover.

The 300kg vehicle will be put on the surface of the Red Planet in January 2019 to search for evidence of past or present life.

It should operate for at least seven months and will carry a drill to probe up to 2m underground.

The sites are generally clustered in a relatively tight zone close to the equator. They are: Hypanis Vallis, Simud Vallis, Mawrth, Oxia Planum (x2), Coogoon Valles, Oxia Palus and Southern Isidis.

The ExoMars Landing Site Selection Working Group is meeting now in Madrid to begin the process of downselection. The teams that proposed these locations will make their case during the Spanish gathering (two, virtually identical proposals were received for Mawrth).

It is hoped to have a shortlist of no more than four locations in June or July. These will then be intensively studied, calling on new high-resolution pictures and mineralogical data acquired by satellites in orbit at Mars.

I have to say, the height constraint is very taxing Dr Jorge Vago, ExoMars project scientist

A final decision is likely to be announced in 2017. This will probably take the form of a first choice and a back-up.

We've been talking about ExoMars for a long time. The project has had several ups and downs, but it is now moving positively in the right direction.

The venture is a joint undertaking with the Russians, who, as well as providing the launch rocket in May 2018, and some of the instrumentation, will also build the landing system. This will see the rover enter the Martian atmosphere in 2019 in a protective shell, deploying parachutes and retro-rockets to reduce the descent velocity.

The robotic vehicle will arrive at the surface on a legged lander, driving down a ramp to begin its grand traverse.

Everything hinges on a safe touchdown, of course. However, scientifically, it's vital ExoMars goes to the right place.

Euro-Russian mission to Mars Image copyright E.Allouis/Astrium Image caption The Chilean desert is remarkably Mars-like ExoMars rover set for launch in May 2018

Proton rocket will send it on its way

Russians also building landing system

Rover will look for biosignatures on Mars

Initial mission duration will be 218 Mars days

I have used two maps on this page to help explain how the final decision will be made.

They are both Mercator projections of Mars which will be familiar from Earth maps that also pull the 360-degree globe on to a flat surface.

For reference, I've marked the locations of the two current American rovers - Curiosity and Opportunity - on the top map.

Choosing a site is a trade-off between what's scientifically desirable and what's achievable with the available engineering.

ExoMars wants to look for life markers. Its best chance of finding these will be to go to places where there is abundant evidence for long-duration, or frequently reoccurring, water activity.

This will exist on the old terrains of Mars - ones that are billions of years old.

These are places where you would hope to roll across recently exposed fine-grained sediments; the kind of clay-bearing mudstones that Curiosity has been enjoying in Gale Crater.

If you get lucky, you just might hit upon preserved organic molecules that hint at some past biology.

But choosing the places you'd love to go is the easy part; having the capability to reach them is another question.

Media playback is unsupported on your device Media caption Mars on Earth: A guided tour of Stevenage's indoor version of the Red Planet

Engineers describe an ellipse of confidence into which they can put a Mars lander.

With the extraordinary skycrane used on Curiosity, this ellipse measured just 20km by 7km. It meant the US space agency had the belief to shoot for one of the deepest holes on Mars.

ExoMars, by comparison, will have a landing ellipse that measures about 105km by 15km - not dissimilar to Nasa's Phoenix mission of 2008.

So, this immediately rules out places like Gale Crater, for example. The greater uncertainty in landing performance would put ExoMars in danger of slamming into the crater's walls or its big central mountain.

Can the eight sites accommodate the Russian landing ellipse?

Image copyright TAS Image caption ExoMars will use a drill to get up to 2m below the surface of Mars

Another consideration: You need to give your parachutes time to work in Mars' thin atmosphere. This means that any terrain that lies above minus 2km datum (what we on Earth would think of as sea level) is out of reach.

And yet another: Although ExoMars will have a sizeable drill, it doesn't want to be probing through decimetres of dust, and there are places where Mars' ubiquitous "red dirt" is worse than others. These will be avoided.

In addition, energy is an important consideration. ExoMars will use radioisotope heating units to cope with the cold, but it will rely on solar panels for day-to-day power. Given that it is landing in the northern summer, optimal operations will be found in a band between latitudes of 5 degrees South and 25 degrees North.

And boulder fields - you need to avoid those, too. Although, the landers' legs will give a surface clearance of about 30-50cm, you want to try to avoid coming down on big rocks.

I could go on. The point is the scientists on the mission will be working very closely with the engineers to pick a destination that balances the highest return with the lowest risk.

"The eight sites all have possibilities," ExoMars project scientist Dr Jorge Vago told me.

"Some have more problems than others and we'll see how the discussions turn out. I have to say, the height constraint is very taxing. There is very little of Mars that is scientifically interesting that also fits the elevation bill."

And working group member Dr John Bridges of Leicester University added: "We've got some serious things to discuss, for sure. But I'm very positive about it; I think we've got some cracking sites on the list."

Image copyright ESA Image caption Tight constraints: The site cannot be too high, nor too dusty, and it must afford good solar potential