NASA's Mars Reconnaissance Orbiter shows dark streaks flowing down Martian slopes. When the streaks are widest, there is evidence of water molecules. When the streaks shrink in the cold, water disappears. The water is not the same as a stream on Earth, but more like a hint of wetness. Although scientists are not entirely sure where the water source is, they say it could be from a salty underground reservoir or soaking up moisture from the atmosphere. Listen to hear how water can be important for future explorations on Mars.