Two years ago, NASA made a big splash when it announced the discovery of flowing water on the surface of Mars. Unfortunately, according to new research from the U.S. Geological Survey, the surface features that NASA thought were made up of liquid water may actually be flowing grains of sand instead. The Verge reports: The features in question are dark streaks that show up periodically on Martian hills, known as recurring slope lineae, or RSLs. When one of NASA's spacecraft, the Mars Reconnaissance Orbiter, studied these lines more closely, it found that the RSLs were made up of hydrated salts -- meaning they were mixed with water molecules. At the time, NASA thought that was significant evidence that flowing liquid water caused these bizarre streaks. But researchers at the USGS say these features look identical to certain types of slopes found on sand dunes here on Earth. Those slopes are caused by dry grains of sand flowing downhill, without the help of any water. It's possible the same thing is happening on Mars, too. Since liquid water is key for life here on Earth, many thought these strange lines of flowing water may help support life on the Martian surface. But now these RSLs may not be the best place to look for life anymore.