Washington - Liquid water has been observed on the planet Mars, the US space agency Nasa said on Monday."Mars is not the dry, arid planet we thought of in the past," Jim Green, Nasa's planetary science director, told a press conference."Under certain circumstances, liquid water has been found on Mars."Scientists have long believed that water once flowed freely across the red planet and was responsible for forming its valleys and canyons.Major climate change about three billion years ago is believed to have changed all that, Green said."Today we're revolutionising our understanding of this planet," Green said."Our rovers are finding there's a lot more humidity in the air."The rovers searching the planet's surface have also found that the soil is much more moist than anticipated.Dark streaks running down slopes on the Martian surface were observed about four years ago.Scientists did not have proof, however, that these streaks - which would form in spring, grow by summer and then disappear by autumn - were actually water.But after careful study and analysis, they are ready to say that these streaks are, in fact, water.