How do scientists know Mars had water in the past?

Mars now looks cold and dry, but many landforms show that liquid water once flowed there. Orbital images reveal ancient valley networks and delta deposits, while rovers have found clay minerals and other rocks that form in the presence of water. NASA’s Curiosity rover, which landed in 2012, examined sediments in Gale Crater that suggest a long-lived lake environment. Other missions have found evidence that Mars had a thicker atmosphere in the past, helping liquid water persist on the surface. These clues make ancient Mars one of the most important places to study planetary climate change.