The European Space Agency (Esa) has selected four candidate sites on the surface of Mars for the landing of its ExoMars rover in 2019.
The four locations, chosen from an earlier shortlist of eight sites, are all close to the Martian equator and include the Mawrth Vallis, Oxia Planum, Hypanis Vallis and Aram Dorsum.
As the main goal of the ExoMars mission will be to search for signs of the past and present life, the four potential landing sites have been selected with regard to the probability of them having hosted life in the past and maintaining evidence of it until the present day.
"The present-day surface of Mars is a hostile place for living organisms, but primitive life may have gained a foothold when the climate was warmer and wetter, between 3.5 billion and 4 billion years ago," said Jorge Vago, Esa's ExoMars project scientist.
"Therefore, our landing site should be in an area with ancient rocks where liquid water was once abundant.”
However, as the mission will only get one shot to success, the team also had to evaluate the terrain at each of those sites to determine whether it is safe for the ground-breaking landing attempt and subsequent operation of the 310kg rover.
"Technical constraints are satisfied to different degrees in each of these locations and, although our preliminary evaluation indicates that Oxia Planum has fewer problems compared to the other sites, verification is still ongoing," Vago added.
The team behind the project will now run simulations to predict the probability of a landing success for each site and make the final decision by the end of 2017.
The search for a suitable landing site for the ExoMars rover began in December 2013, when the science community was asked to propose candidates.
The area around Mawrth Vallis and nearby Oxia Planum contains one of the largest exposures of rocks on Mars that are older than 3.8 billion years and rich in clay, indicating the region was once abundant with water. Mawrth Vallis lies on the boundary between the highlands and lowlands and is one of the oldest outflow channels on Mars.
The exposed rocks at both Mawrth Vallis and Oxia Planum have varied compositions, indicating a variety of deposition and wetting environments. In addition, the material of interest has been exposed by erosion only within the last few hundred million years, meaning the rocks are still well preserved against damage from the planet's harsh radiation and oxidation environment.
By contrast, Hypanis Vallis lies on an exhumed fluvial fan, thought to be the remnant of an ancient river delta at the end of a major valley network. Distinct layers of fine-grained sedimentary rocks provide access to material deposited about 3.45 billion years ago.
Finally, the Aram Dorsum site receives its name from the eponymous channel, curving from northeast to west across the location. The sedimentary rocks around the channel are thought to be alluvial sediments deposited much like those around Earth's River Nile.
This region experienced both sustained water activity followed by burial, providing protection from radiation and oxidation for most of Mars' geological history, also making this a site with strong potential for finding preserved biosignatures.
The ExoMars mission is Europe’s second attempt to land on object on the surface of Mars after the failed British Beagle 2 spacecraft. Originally proposed as a cooperation with Nasa, the project was nearly scrapped after the American space agency pulled out after budget cuts in 2012. Esa saved the mission by turning to Russia’s Roscosmos. Russia committed to provide two Proton rockets for the two planned ExoMars launches, delivering to the Red Planet first an orbiter and a test landing module in 2016, followed by the rover two years later. Russia will also develop the sophisticated landing mechanism that should safely lower the precious rover on the Martian surface. The landing concept will be tested in the 2016 mission.