With the arrival of Europe’s first interplanetary probe at Mars and two more U.S. spacecraft on the way, the red planet will be under intense scrutiny for months as scientists attempt to figure out why a world flecked with evidence of an Earth-like past appears dead and dry. An even more compelling question is whether indigenous life ever took root on Mars, as many suspect but cannot prove. “If you look at the surface of Mars today, it’s a desolate place. It’s dry. It’s cold. It’s barren,” said Cornell University astronomer Steven Squyres, who heads the science teams for two NASA rovers scheduled to land on Mars beginning next month. “It’s not an inviting environment for life, and yet we see these tantalizing clues,” he said.