This plot shows that levels of the element chlorine rise dramatically in the deeper rocks lining the walls of the crater dubbed "Endurance." The data shown here were taken by the Mars Exploration Rover Opportunity's alpha particle X-ray spectrometer at Endurance and "Eagle Crater," the site where Opportunity first landed at Meridiani Planum.
Opportunity has been inching down the walls of Endurance Crater, investigating distinct layers of rock as it goes for clues to Mars' buried past. The various Endurance layers have been informally labeled "A" through "F." Targets within these layers are listed on the graph along with previous targets from Eagle Crater. All the rocks listed here were observed after they had been drilled by the rover's rock abrasion tool.
The observations indicate that the elements making up the shallow rock layers of Endurance Crater resemble those of Eagle, while the deeper layers of Endurance possess increasingly higher concentrations of the element chlorine.
Opportunity will continue to roll deeper into Endurance to see if this puzzling trend continues. Scientists hope the new data will help them figure out how the presence of chlorine fits into the history of water at Endurance Crater.