Nasa's Opportunity rover on Mars identified rock laden with what scientists believe to be clay minerals. Their presence indicates that the rock called Esperance was altered at some point in the past through prolonged contact with water. NASA has been discovering evidence for water on Mars since it first landed back in 2004.
Clay minerals only tend to form at a more neutral pH. This is water that was much more favourable for things like pre-biotic chemistry , the kind of chemistry that could lead to the origin of life.