FOLLOW THE WATER
In September 2015, NASA’S Mars Reconnaissance Orbiter had a better reason to grab headlines: it found evidence of flowing water. MRO’S spectrometer showed that dark streaks on the slopes of some craters and canyons were composed of salts left by evaporating water. It also identified clay sediments on a canyon floor in the Nili Fossae region (far left), a candidate site for a future landing mission. Two older orbiters, the 2001 Mars Odyssey and Mars Global Surveyor, identified ancient lakes and snowmelt-fed streams in the Arabia Terra region (left: white represents lowest elevation, yellow highest). Based on the number of craters, scientists estimate they held water until 2-3 billion years ago – a billion years later than previously thought.