Europe begins Mars site selection

Landing sites 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.

Start Quote

I have to say, the height constraint is very taxing”

End Quote 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

Bridget in the Desert
  • 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.

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?

ExoMars drill 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."

Mars map Tight constraints: The site cannot be too high, nor too dusty, and it must afford good solar potential
Landing  ellipses
  • The above graphic uses Gale Crater to illustrate past landings
  • Engineers define an ellipse in which they can confidently put down
  • Curiosity had the precision to get into the crater safely
  • Viking's ellipse was 300km across - wider than Gale Crater itself
  • Phoenix (100km by 20km) could not confidently fit in Gale
  • ExoMars' ellipse is very similar, and so Gale would be off-limits
Jonathan Amos Article written by Jonathan Amos Jonathan Amos Science correspondent

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  • rate this

    Comment number 318.

    Robert Lucien
    re 317
    Apologies - mistake not deliberate

  • rate this

    Comment number 317.

    re 314
    Robert Julien
    While NERVA has its merits it has its insurmountable obstacles.
    Haven't studied it very closely but the UK government has pledged a mere £60m in two stages to REL towards the SABRE / SKYLON project with it in service early 2020's
    The economy in operation puts re-usable earth orbiting and inter continental vehicles truly a realiseable and realistic development.

  • rate this

    Comment number 316.

    4.inchindown “When so many Europeans are out of work...why on earth is the EU spending all this money on a project with very little benefit for the citizens of the EU”

    Look at that chap over there! Using an Ox to make a groove in the ground with a thingy, what a waste.

    No food in that, back to picking berries.

  • rate this

    Comment number 315.

    All this armchair expertise on a subject is amazing.
    My personal conrtibution would be a rocket sent to Mars with a few tonns of organic and chemical waste and let the rest look after itself!
    Then go back in ten years or so to see how the terraforming is going!

  • rate this

    Comment number 314.

    #313 peter goodland
    " ... I believe the REL Sabre engine."

    If you like that you'll love this. -

    The UK could easily fund a new NERVA type nuclear rocket program. It would cost about the same as the Olympics, less than 1/4 of HS2 and would not just be a train that goes to Birmingham. This would make us a true world leader in space technology. (lk Skylon)


Comments 5 of 318


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