Nearly a decade ago on a mission on Mars, the spacecraft Opportunity found clay minerals in a rock that clearly indicate that drinking water has flowed on the Mars once, probably sometime in its first billion years of existence. This rock was discovered near a crater called Endurance, and clay contained in it was formed in a chemical environment conducive to the emergence of life.
According to The Guardian, minerals are found similar to montmorillonite, which is formed on earth under the influence of water with neutral pH, or the same type of fluid flowing through our taps. This means that, potentially, this substance provides the best conditions for the origin of life forms. The evidence on the existence of water found on Mars so far have always been much more acidic.
According to the publication, as the planet was turning in the desert it is today, the process of evaporation of water was making this substance increasingly acidic due to the high concentration of minerals that grew in the remaining liquid. The evidence found by Opportunity date from a time prior to that process when frequent rains occur on Mars and the environment there was more similar to Earth.
This is not the first trace of drinkable water to be existed on the Mars in the past, since the Curiosity found in March this year similar evidence during one of its missions in Gale Crater.