After almost 10 years exploring, NASA's Mars Exploration Rover Opportunity will now head for Solander Point.

Opportunity has made one of its greatest scientific discoveries so far. Clay minerals in a rock called Esperance clearly indicate that neutral water flowed across the rock some time in the first billion years of its existence.

The rock was found near Endurance Crater, and took seven attempts to analyse because it was partially covered in Martian dust.

The clay minerals are similar to one called montmorillonite. Formed under the influence of neutral water, this is significant because neutral water, which is similar to household tap water, is thought to be much more conducive to the chemistry needed for the origin of life.

Announcing the results, Steve Squyres, principal investigator for Opportunity, Cornell University, Ithaca, N.Y. said that, although they have talked in the past about finding water with Opportunity, really it was more like sulphuric acid.

As Mars turned into the desert planet we see today, so the water became more acidic. This happened because of evaporation, which left a higher concentration of minerals in the remaining water – rather like reducing a sauce to make it richer.

Neutral water dates from an earlier time on the planet, when it rained frequently bestowing a more Earth-like environment.

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