ESA ExoMars: Mars Landing Site selection

The ESA ExoMars "longlist". Two proposals were received for Mars' Mawrth Vallis, but these were virtually the same

The 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, Shalbatana (Simud) Vallis, Mawrth Vallis, 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 Vallis).

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.

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.

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 search 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 i.e. 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.