A new study suggests that dark markings on the planet Mars represent sand, not water. Research in 2015 had suggested that lines on some Martian hills were evidence of water. Yet American scientists now say these lines appear more like dry, steep flows of sand. If water is present, they said, it is likely to be a small amount. Water in liquid form would be necessary for microbial life. NASA, the American space agency, said more research is needed.