After almost a decade of exploring the surface of Mars, Nasa's rover has found the strongest evidence yet for the presence of water on the red planet.
The data comes from "Esperance", one of the oldest rocks studied on Mars. The clays inside the rock point to the presence of abundant water early in Mars' history.
To obtain this data the American space agency's rover Opportunity scraped at the rock's surface to reveal its fresh interior, then used x-rays to analyse what the chemicals the minerals inside the rock were made of.
The results showed the interior was high in aluminium and silicon, but low in calcium and iron. The minerals look like swelling clays, just like those found in a muddy puddle on Earth.
Such clays act like sponges in their capacity to soak up water. Actually, the clays form by addition of water and weathering of "primary" silicates, commonly found as volcanic rocks on the surface of Mars.
Opportunity's results mirror data from Martian rocks at Gale crater obtained by Curiosity rover earlier this year. But they contradict the vast majority of previous mineral analyses, which showed most hydrated rocks were formed of sulphates. Those minerals, such as gypsum, formed in dilute sulphuric acid. Instead, the chemistry reported this week from Esperance implies that the clays formed there had formed in waters that would have been drinkable.
"This is powerful evidence that water interacted with this rock to change its chemistry and mineralogy in a dramatic way," Steve Squyres of Cornell University, science team leader of the Opportunity mission, told the New York Times.
It is the strongest evidence yet for a past Martian environment that would have been conducive to life.
Speculation linking the origins of life on Earth to the presence of clay minerals has been something of a theme since first suggested in the early 1950s. Swelling clays, like those seen at Esperance, demonstrate the presence of neutral water early in Mars' history. But at the molecular scale the inter-layer structure of the clay can also act as a template to any organic molecules present and, potentially, promote replication of enzymes and proteins, which are necessary for life.
The findings back up earlier theories that the Martian surface once hosted an ocean, covering much of its Northern Hemisphere.
After the landing of Curiosity rover in 2012 with its much upgraded sets of analysis tools, Opportunity's work acts as independent verification of some of Curiosity's findings. Opportunity launched on July 7, 2003 and was designed to operate on Mars only for 90 days after landing. But it has far exceeded expectations, even when its twin rover, Spirit, ground to a halt back in 2010.
As its 10th anniversary nears and the rover trundles on its explorations by moving to new rocks, these new results demonstrate the ageing rover's value and make it worthy of celebration.
Simon Redfern is Professor of Mineral Physics at University of Cambridge.