Since 2011, the solar-powered six-wheel Opportunity has explored the Endeavour Crater, the largest of the five craters already examined by the rover.
In Endeavour, the rover found minerals dating back to the first billion years of Martian geological history. When the rover scraped away the top layer of a light-colored rock after several attempts, it found traces of clay minerals with an aluminum-rich chemical signature, suggesting they were formed through interaction with pH-neutral water.
In 2004, twin rovers Opportunity and Spirit landed on opposite sides of the Red Planet. They were supposed to work for three months, but both operated for years.
Spirit operated until 2010 when it got stuck in sand, and later stopped communicating with mission control. Opportunity continues to gather valuable information as it moves further on the Mars surface, though its hardware is aging – it recent experienced a problem with its flash memory, but the operators were able to reboot the system.