Four potential landing sites have been shortlisted for the 2018 ExoMars mission. ExoMars is a joint venture between the ESA and the Russian Federal Space Agency Roscosmos, with the ultimate goal of scouring the Red Planet for any signs of life, past or present. The mission will consist of multiple spacecraft in addition to two rovers. The ExoMars rover will be one of the final assets to launch, with an expected arrival date on Mars estimated at January 2019.

The ESA is currently in the process of testing new and innovative approaches for safely deploying the rover onto the surface of the Red Planet. ESA and its partner Airbus are striving to make the testing conditions as realistic as possible, going so far as to create a 30 x 13 m (98 x 42 ft) "Mars yard," representing the closest approximation to the Martian surface available here on Earth.

The process of selecting a suitable landing site began late last year, with a call for the scientific community to propose candidate landing zones with the greatest chance of yielding clues to the existence of life on Mars. Eight of these proposals were considered by a Landing Site Selection Working Group in April, with a successive ESA-appointed panel officially selecting four locations for further analysis.

"The present-day surface of Mars is a hostile place for living organisms," states Jorge Vago, project scientist for the ExoMars mission. "Primitive life may have gained a foothold when the climate was warmer and wetter, between 3.5 billion and 4 billion years ago."

All four sites are located close to the equator, with each representing a potential treasure trove of scientific discovery. The candidate sites are named Aram Dorsum, Hypanis Vallis, Mawrth Vallis and Oxia Planum. As the mission's primary goal is to seek out clues to life on the Red Planet, each of the suggested mission areas sit in or around locations that are believed to be the site of substantial water activity in ancient times, as they represent the ideal place for life to gain a foothold.

Aram Dorsum sits in a channel littered with what appear to be sedimentary rocks. It is believed that the area may contain well-preserved biosignatures, as all signs point towards the area experiencing sustained water activity in the planet's past. Similarly, Hypanis Vallis is situated in what is believed to be an ancient river delta. Deploying the rover at this location would allow scientists to analyze materials deposited at the site roughly 3.45 billion years ago.

Finally two of the selected sites, Mawrth Vallis and Oxia Planum, are located near one of the largest exposures of rock present on the Red Planet. The site is of particular scientific curiosity, as it has only been exposed to the harsh climate of Mars relatively recently – "recently" in cosmological terms meaning over the last few hundred million years. Therefore the high levels of radiation, as well as the oxidizing elements present in the Martian atmosphere, have not altered the nature of potential samples, allowing scientists a glimpse at a relatively well-preserved piece of the ancient Martian landscape.

Moving closer to the 2018 launch date, the team will attempt to ascertain which of the four shortlisted sites have the best balance in terms of scientific output versus the risks of a potential landing. Therefore, the next step in the process will involve a detailed risk assessment on each site, considering factors such as surface composition, atmospheric qualities and entry profile. A final decision on the landing site is slated to be made sometime in 2017

Source: ESA

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