An American mission to Mars will focus on three key areas where alien life may once have thrived and gather samples to be returned to Earth, scientists have been told.
The Mars 2020 mission will launch a rover to the Red Planet in three years time to look for chemical relics of Martian bugs.
ExoMars Rover, a European mission also launching in 2020, will use similar techniques to search for life biosignatures.
However the Nasa craft will also collect 30 to 40 rock samples and seal them in titanium tubes on the Martian surface for future retrieval.
Deputy project scientist Dr Ken Williford was speaking at the Goldschmidt Conference geochemistry meeting in Paris, France.
He said: "Mars 2020 represents a crucial first step towards a possible Mars sample return.
"Our objective is to collect a diverse set of samples from our landing site with the best potential to preserve records of the evolution of Mars - including the presence of life if it was there.
"We'll use our onboard instruments to provide the critical field context that future scientists would need to understand the measurements made back on Earth."
Potential landing sites for Mars 2020 have been whittled down to three, said Dr Williford.
One site at Columbia Hills in Gusev crater was previously visited by the Nasa Spirit rover and features silica deposits thought to be evidence of hydrothermal springs, which on Earth are havens for life.
The two other sites are located close together on the edge of Isidis Planitia, one of the largest and oldest impact craters in the solar system.
Northeast Syrtis contains ancient exposed Martian crust that bears signs of alteration in the presence of liquid water.
Scientists believe the site could have hosted sub-surface life.
The remaining site, Jezero crater, is thought to be the location of an ancient river delta and lake which could have been a prime spot for living organisms.
Dr Williford added: "We've got some hard decisions in front of us.
"Because of the possibility of sample return, the selected site could have an outsized impact on the future of Mars science compared to a typical mission.
"We've been working hard to understand the scientific potential of the different sites and engaging the international scientific community for input on this consequential choice.
"The team is extremely excited about the opportunity to bring a powerful new payload to the surface of Mars and produce some spectacular results wherever we end up."
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