The science team is “just about convinced” that these wind features are revealing that the fine-scale grooving, the scouring that we are seeing in the valley was actually “caused by wind that came much later,” Squyres said. “It kind of overprinted the original erosion [and carving] of the valley. But we want to be able to prove that so it stands up to peer review. We want to really nail this issue and do it in a way that is definitive. If we’re right about this all it says is don’t interpret these grooves as in terms of flowing water, because that ain’t what did it,” he said. “If we’re wrong, then the MI pictures might show us that too, so we’ll see.”

From her location about 3 meters downslope from the cluster of wind tail targets on the bright outcrop at La Bajada, Opportunity put it into gear on Sol 4890 (October 25, 2017) and climbed back up the slope. She managed to put 4.18 meters (13.71 feet) in her rear view mirror before she stopped, her odometer clicking up to 45.044 kilometers (27.98 miles).

Success! The robot field geologist pulled up into the perfect position on the outcrop with “a beautiful one of these features we think is a wind tail right within reach of the arm,” said Squyres. And – she popped another wheelie. This time, the left rear wheel.

“It’s up in space,” Golombek said, deadpan.

“Mars is making our life hard,” reported RP Paolo Bellutta.

Or it could be that Opportunity is just putting a mechanical exclamation on the point.

Either way, the vehicle is “perfectly stable,” assured Bellutta. However, he added: “It makes a bit tricky to use the IDD in this configuration.”

That of course would be out of the question. “None of us wants to try start waving the arm around with one wheel in the air,” said Squyres. “That is not a prudent thing to do with a half-billion-dollar rover.”

So the MER ops team spent all day Friday, October 27th trying to figure out a way to unwind the wheelie without moving the vehicle. “We’re trying to get all six wheels back on the ground and still have that target we’re after still be in reach,” said Squyres. “And we’re on very steep terrain and unwinding the wheelie without moving the vehicle is not an easy thing to do, so we wanted to have a lot of eyes on the screen, a lot of eyes on the code and everybody taking their time and thinking it through carefully.”

It was a long, long and particularly challenging day.

The plan, as always with rover wheelies, was to "flatten" the suspension. But the rover would not drive or do much of anything else because limited power. And the MER scientists didn’t want the rover to move, because she was ideally positioned to begin close-up research on an ideal target.

The RPs would have the difficult task of adjusting the position of the vehicle after returning the left rear wheel to the ground. “Very accurate maneuvers are always quite challenging, even a tiny pebble can make a big difference,” said Bellutta.

That task was slated for the final weekend of October. It was a simple plan: undo the wheelie on the right rear wheel on sol 4893 (October 28, 2017), rest and recharge. During the final sol of October, as ghosts and goblins donned their finest and began to hit the streets and cobblestone roads here and there on Earth, Opportunity will focus on wind tails at La Bajada.

On Earth, the MER scientists are working the working hypotheses and things are evolving. Remember talk of a possible lake or catchment in previous editions of The MER Update, to the west of Endeavour’s west rim?

Well, scratch that.

“Never existed,” said Arvidson. “There was not a big lake to the west. There is not a source of water coming to the rim from the west. We have now finished all the calculations and demonstrated that you just can’t have anything to the west that would produce a local lake.”

Whatever water was involved had to be local. It had to come from right there, right at the rim crest, Arvidson proffered. “I think what is clear now, from all the analyses that we’ve done, is that any water that was involved in making Perseverance Valley would be local, and that’s a take-home point.”

So what local source off water could there have been?

How about ice? Packed on the top of the rim?