NASA's Opportunity rover appears to have reached another milestone in its amazing nine-year mission on Mars.
Scientists report the robot has been trundling over what they believe to be clay-bearing rocks on the edge of a wide bowl known as Endeavour Crater.
Clays are water-altered minerals, but very different to the ones seen by the rover so far on its travels.
A BBC science correspondent says those previous minerals were in contact with acidic water; clays are formed in the presence of neutral water.
The robot landed on Mars in 2004. Since August 2011, it has been driving across the western rim of the Endeavour depression, which is 22km wide.
Opportunity is trying to understand the sequence of rocks at the crater which is likely to have been formed when an asteroid hit the surface of the planet more than three billion years ago.
Two outcrops, within 100m of each other, have caught the attention. At the moment, it is not clear whether the two outcrops relate to a period from before or after the creation of the crater.
The BBC says Opportunity continues to exceed all expectations. A twin vehicle, Spirit, which was landed on the other side of Mars in 2004, succumbed to the dust and cold of the Red Planet in 2010. But Oppy keeps on rolling.
