Perhaps the Red Planet’s dry equatorial region is still a little wetter than we thought.
These recurring thin lines are typically less than 16 feet wide and can stretch for three quarters of a mile. They fade in cooler times and reappear in warmer ones – perhaps because in the warmer seasons or in direct sunlight the water would evaporate, leaving the salt marks behind. (Think of the sweat stains in clothing – those dark rings are the salt and minerals left from your sweat after the water has evaporated.)
Above and below, you can see the changes in these mysterious markings over time in these GIFs the Mars team put together.
Now, Mars has water ice and dry ice at its poles, and scientists working with the orbiter's HiRISE camera think that dry ice chunks break off and carve paths down slopes when the weather starts to warm up. But at the mid-latitudes near the equator, where chilly Mars gets relatively balmy, the air is too thin to support anything like the ancient lake that NASA's Mars rover Curiosity discovered in Gale Crater.
But the Mars Reconnaissance Orbiter, which has been taking shots of the Martian surface with HiRISE since it launched in 2005, has seen these features come and go and lengthen and shorten since it spotted them about two years ago.
These features have been particularly obvious in the canyons of Valles Marineris, and most active during the seasons when the slopes they mark tend to face the sun.
"Although the origin of the recurring slope lineae remains an open question, our observations are consistent with intermittent flow of briny water," the authors wrote in a study in Nature Geoscience. "Such an origin suggests surprisingly abundant liquid water in some near-surface equatorial regions of Mars."