Seasonal Dark Streaks on Mars . ... Credit Copyright: HiRISE , MRO , LPL (U. Arizona) , NASA Explanation: What is causing these dark streaks on Mars? A leading hypothesis is flowing -- but quickly evaporating -- water. ... These are not the first markings on Mars that have been interpreted as showing the effects of running water, but they are the first to add the clue of a seasonal dependence. ... NASA Web Site Statements, Warnings, and Disclaimers . NASA Official: Jay Norris. ...