What indicates that Mars had liquid water in its past?

The presence of valley networks on Mars is a significant indicator that the planet had liquid water in its past. These networks resemble river valleys on Earth and extend through many regions of the Martian surface, which suggests that water once flowed over the terrain, shaping the landscape much like rivers do here. The formation of these valleys would require a reliable source of liquid water, implying that Mars was at one time warmer and had conditions suitable for sustaining liquid water.

The morphology of these valley networks provides evidence for erosion caused by flowing water, which is a key feature of a hydrological cycle. Such features are typically associated with environments that have flowing liquid water, making them a strong clue to Mars' wetter past.

While other factors like salt deposits, flood plains, and ice caps do relate to water on Mars, they do not provide the direct geological evidence of past flowing water that valley networks illustrate. Salt deposits may suggest past evaporation of water, and ice caps indicate the presence of water ice, but they do not directly confirm that liquid water was present in the same way valley networks do. Flood plains could suggest past flooding events, but they are less definitive than valley networks, where extensive erosion and transport of materials are evident.