For NASA’s Mars 2020 rover, now being assembled at the agency’s Jet Propulsion Laboratory for launch in July 2020, the stakes could not be much higher. The $2.4 billion nuclear-powered rover is the most complex piece of machinery to ever make a ballistic beeline for the Red Planet. And after it lands in February 2021, its completion of one high-profile objective—collecting rock samples for eventual transport back to Earth—would ensure the rover sets the course for future Mars exploration for decades to come. But first, mission planners have to decide where on Mars this ambitious machine should actually go.

Last week, at a conference just north of Los Angeles, an international group of more than 150 scientists came together for two and a half days of deliberation on exactly that—their debates at times seeming less like a a careful scientific conversation and more a geological and astrobiological food fight. This contentious meeting is the fourth and final such workshop to be held since 2014, and considered four potentially life-friendly candidates for Mars 2020’s coveted landing site:

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