This week, researchers reported that they had evidence showing that periodically-appearing dark streaks on the sides of Martian hills (known as “recurring slope lineae,” or RSLs) were patches of moist soil. The researchers believe that the water may have been absorbed out of the air into salts in the soil through a process known as deliquescence—though they say there is a slight possibility of the soil being moistened by underground water sources. The water, though both salty and limited in quantity, could be an important ingredient both in future human exploration of Mars and in projects hunting for traces of life on the Red Planet. Georgia Tech planetary scientist James Wray, one of the authors of a paper discussing the research in the journal Nature Geoscience, describes the evidence for present-day liquid water on Mars.