NASA’s Mars Exploration Rover Opportunity has been puttering around the edge of the Endeavour Crater since 2011. It’s currently examining rocks just outside the crater rim’s crest, above a massive area called Perseverance Valley. The rover has been looking for signs that the area may have hosted water a billion years ago.

On Friday NASA released a color-enhanced image Opportunity captured of the edge of Endeavour Crater during a “walkabout” survey on June 3. With the rim crest of the valley just to the right of the image, a swath of dark rocks running east-west across the top of the photo may indicate what could have once been a channel into a lake perched against the edge of the crater. Alternately, the linear pattern of the rock piles could also have originated from radial fractures caused by the impact that created the crater. Either way, NASA is looking to get to the bottom of it.

Speaking about Opportunity’s current mission, Opportunity Deputy Principal Investigator Ray Arvidson says:

”We want to determine whether these are in-place rocks or transported rocks. One possibility is that this site was the end of a catchment where a lake was perched against the outside of the crater rim. A flood might have brought in the rocks, breached the rim and overflowed into the crater, carving the valley down the inner side of the rim. Another possibility is that the area was fractured by the impact that created Endeavour Crater, then rock dikes filled the fractures, and we’re seeing effects of wind erosion on those filled fractures.”

The surface above Endeavor Crater, captured by the Mars rover Opportunity.

To get a sense of the scale of what’s been captured, according to NASA the width of the swath near the center of the image is roughly 30 feet. Following the walkabout, Opportunity is scheduled to drive down into the valley itself for more exploration.

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Photos via NASA