Perseverance Valley, which cuts into the west rim of Endeavour Crater, a 22-kilometer (13.7-mile) diameter hole in the ground, was for years one of, if not the most anticipated geological features, not just of Opportunity’s expedition around Meridiani Planum, but of the entire mission. It was formed billions of years ago sometime after the crater was created, during the Noachian Period, an epoch when planetary scientists generally believe the planet was more like Earth, with lakes, rivers, underground water, hot springs, and volcanoes, and perhaps even an ocean.

Guided by her team of human colleagues on Earth, the robot pressed on through the depths of the mission’s eighth Martian winter in November still on the quest to ‘Follow the Water,’ as NASA directed so many years ago. The robot field geologist is starring as a Crater Scene Investigator in a kind of CSI Mars at Perseverance, sleuthing to uncover ancient clues to find out what exactly carved this unique valley feature into Endeavour’s rim billions of years ago, what formed its braided grooves or channels, and what scoured its outcrops.

Opportunity found signs of near neutral water in remnants of clay minerals in 2012-2013 at Cape York’s Matijevic Hill. Hopes are high that the rover’s forensic findings in Perseverance will uncover critical clues the scientists need to determine how much water was once here and whether it could have fed potentially habitable environments. “We’re doing the same thing we’d do if we were there as astronauts – walking around and looking for evidence,” said MER Deputy Principal Investigator Ray Arvidson, of Washington University St. Louis. “What we’re after is any indication of process.”

The MER scientists officially are considering multiple hypotheses – from flowing water to a muddy debris flow, ice, ice melting into water, water coming out of fractures, or wind or a combination of these forces – to explain how the valley and its distinctive features came to be. Perched over one of the bright, flat rocks on the northern outcrop of a site the team named La Bajada, the rover scored a small one for wind this past month.

La Bajada features “two different bedrock units in direct proximity” separated by rubbley terrain, “perhaps broken up rock that’s been filled in by sand,” said MER Project Scientist Matt Golombek, of the Jet Propulsion Laboratory (JPL), the birthplace of all of NASA’s Mars rovers. While the southern outcrop is darker in tone and lumpy with rocks, the northern outcrop is flatter and lighter toned.

Opportunity shot pictures of the site with her stereo Pancam “eyes” back in August as she drove by it on her way to the next, planned stop. The scientists were struck by what they saw: scouring on some of the northern outcrop rocks, specifically what appeared to be erosional tails, geological signs of an erosive force that seemed to be pointing up hill. “It looks pretty convincing in the Pancam images that these little tails actually are pointing uphill, though I wouldn’t say it was bulletproof,” said Squyres.

Since the favored theory is still that Perseverance itself was carved by water in some form, and since water doesn’t flow up hill, at least not a hill as steep as the grade of this valley, which descends from Endeavour’s rim to the crater floor, these little tails were just too intriguing to ignore. So in the final sols of September, the team decided to have Opportunity back up to La Bajada in October, as reported in last month’s MER Update.

“These erosional tails were the primary reason we decided to drive back up hill,” said Arvidson. “We wanted to really pin these down.” But even for the rover that loves to rove, backing upslope in Perseverance wasn’t easy. Opportunity popped two wheelies in October and struggled to get into a safe position over the chosen scoured rock and a target the team named Mesilla.

“The biggest trouble with this terrain is that it’s very steep,” said Rover Planner Ashley Stroupe, of JPL. Even so, if the scientists were going to effectively solve this scientific puzzle, they needed their robot to take close-up pictures of these erosional tails with her Microscopic Imager (MI) camera.