Oxia Planum, an area on Mars which preserves clues from the planet's wetter past, has been recommended as the landing site for the ESA-Roscosmos Mars rover -- scheduled to be launched in 2020.
The proposal, by ExoMars Landing Site Selection Working Group, will be reviewed internally by the European Space Agency (ESA) and Roscosmos with an official confirmation expected mid-2019.
At the heart of the ExoMars programme is the quest to determine if life has ever existed on Mars, a planet that has clearly hosted water in the past, but has a dry surface exposed to harsh radiation today.
While the ExoMars Trace Gas Orbiter, launched in 2016, began its science mission earlier this year to search for tiny amounts of gases in the atmosphere that might be linked to biological or geological activity, the rover will drive to different locations and drill down to two metres below the surface in search of clues for past life preserved underground.
It will relay its data to Earth through the Trace Gas Orbiter.
Both landing site candidates -- Oxia Planum and Mawrth Vallis -- preserve a rich record of geological history from the planet's wetter past, approximately four billion years ago.
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They lie just north of the equator, with several hundred kilometres between them, in an area of the planet with many channels cutting through from the southern highlands to the northern lowlands.
Since life as we know it on Earth requires liquid water, locations like these include many prime targets to search for clues that may help reveal the presence of past life on Mars.
"With ExoMars we are on a quest to find biosignatures. While both sites offer valuable scientific opportunities to explore ancient water-rich environments that could have been colonised by micro-organisms, Oxia Planum received the majority of votes," said Jorge Vago, project scientist at ESA's ExoMars 2020.
"Mawrth Vallis is a scientifically unique site, but Oxia Planum offers an additional safety margin for entry, descent and landing, and for traversing the terrain to reach the scientifically interesting sites that have been identified from orbit," Vago said.
The Landing Site Selection Working Group also emphasised that the discoveries generated during the landing site selection process are essential to guide the science operations of the ExoMars rover.
The recommendation was made following a two-day meeting held at the National Space Centre in Leicester, UK, which saw experts from the Mars science community, industry, and ExoMars project present and discuss the scientific merits of the sites alongside the engineering and technical constraints.
The quest to find the perfect landing site began almost five years ago, in December 2013, when the science community was asked to propose candidate locations.
Eight proposals were considered in the following April, with four put forward for detailed analysis in late 2014.
In October 2015, Oxia Planum was identified as one of the most compatible sites with the mission requirements -- at that time with a 2018 launch date in mind -- with a second option to be selected from Aram Dorsum and Mawrth Vallis.
In March 2017, the down-selection identified Oxia Planum and Mawrth Vallis as the two candidates for the 2020 mission, with both undergoing a detailed evaluation over the last 18 months.
The ESA-led rover and Roscosmos-led surface science platform will launch in the 25 July-13 August 2020 launch window on a Proton-M rocket from Baikonur, Kazakhstan, and cruise to Mars in a carrier module containing a single descent module, arriving at Mars 19 March 2021.
The descent module will separate from the carrier shortly before reaching the Martian atmosphere, and will use two large parachutes, along with thrusters and a damping system, to slow its descent to land on the red planet.
While the rover will drive to different locations to analyse the surface and subsurface, the stationary platform will provide context imaging at the landing site, and long-term climate monitoring and atmospheric investigations.