Question

Why do scientists believe that Mars once had running water?

Answer 1

Photographs of mars surface shows dried river beds.

Explanation:

Both the ice caps have some water other than carbon di oxide ice..There is very small percentage of water in mars atmosphere.
picture heritika.com.

Answer 2

There are also geological signs e.g sedimentary rocks

Explanation:

Some minerals (and hence rocks) only form in th e presence of water, and some formations show evidence of long-term variations in the level of water in a lake/sea.

The photos here show clear evidence of this, and look superficially like common sedimentary rocks here on Earth.

I'm really hoping you won't take this as evidence that the photos were taken in Hollywood and that the US Space programme was just mocked up in Hollywood!