Flowing Barchan Sand Dunes on Mars . ... Credit Copyright: HiRISE , MRO , LPL (U. Arizona) , NASA Explanation: When does Mars act like a liquid? Although liquids freeze and evaporate quickly into the thin atmosphere of Mars, persistent winds may make large sand dunes appear to flow and even drip like a liquid. ... Over the past few weeks, winds on southern Mars have been kicking up dust and are being watched to see if they escalate into another of Mars' famous planet-scale sand storms . ...