Why does any of this matter? Because it gets to the question of when the habitable environment documented in the Sheepbed mudstones formed. Is it the toe end of the alluvial fan that comes down from the northern rim of Gale crater? If so, it's relatively late in Mars' geologic history, and it postdates the mound of rocks in the center of Gale. Is it, instead, entirely unrelated to that fan? If so, does the Sheepbed unit lie underneath the Gale mound? That would make it the oldest thing around. Did its sediments derive from the Gale mound? Then it's younger than the rocks in the mound, and of indeterminate relative age to the fan.

The only way that Curiosity might be able to answer these relative-age questions is to get moving. But the results of a casual survey I performed of friends on the Curiosity team is that the relative age of the Sheepbed unit might not be easy to figure out, even when (if?) Curiosity ever does get driving.

Over on the other side of Mars, there is (as Eldar Noe Dobrea said to introduce his talk on Tuesday afternoon) "a perfectly good rover sitting on ancient Noachian clays." He showed that there are smectites -- the same kind of clay minerals seen by Curiosity in the Sheepbed unit -- all over Endeavour crater. However, it developed in the rest of the Opportunity session that the evidence for Opportunity having found those smectites, while suggestive, isn't yet conclusive. But I'm getting ahead of myself.

Opportunity, unlike Curiosity, is traversing all over the place in order to get a good grip on the sequence of stratigraphic units at Cape York, on the rim of Endeavour crater. Larry Crumpler gave a very enthusiastic talk about this, about how Opportunity has essentially been walking along the contacts between sedimentary layers in order to map them out, like any good field geologist. The challenge is that Opportunity only has two functioning science instruments left: the APXS, which can count abundances of major elements heavier than sodium, and Pancam, whose multispectral color imaging can identify different rock units and give inconclusive hints as to what is going on mineralogically to make those rock units distinct.

Opportunity has spent the last many years rolling across the mostly aeolian sulfate sands of the Burns formation. The rim of Cape York is composed of older rocks that clearly underlie the Burns formation; they were raised into their current position by the Endeavour impact event. The Burns formation postdates the impact, so those sands "onlap" the rocks of Cape York.

Intriguingly, there is a thin layer of rock called the Grasberg formation that is basal to the Burns formation (meaning, it's a flat layer that underlies Burns everywhere and so formed before it) whose composition has "nothing in common" with Burns, Crumpler said. It's not a sand; it's a very fine-grained material (so fine that the Microscopic Imager can't see the grains), and it's harder than the soft Burns formation. It has "lots of alkalis and chlorine" and is "very similar to Clovis rocks" that Spirit saw at the Columbia Hills. Crumpler suggested maybe it was an airfall material (so, made of volcanic ash, maybe). Whatever it is, it's not a windblown sandstone.

Next down in the sequence is the suevite breccia of Shoemaker Ridge. That's an impact breccia (a rock made up of busted-up angular bits of rock broken in a major impact). This rock is probably but not necessarily associated with the formation of Endeavour crater. Below that is Whitewater Lake.

But what is Whitewater Lake? It's very fine-grained, and there's nothing about its texture (at least, nothing that Opportunity is capable of seeing) that tells us how it formed in the first place. Its fine-grainedness means that if it is an impact-derived sediment, it didn't have anything to do with the formation of Endeavour crater, because it's too close; sediment so close to the crater would be a breccia like Shoemaker Ridge. Therefore, it has to be older than Endeavour, which makes it very old indeed.

Steve Squyres and Barbara Cohen and Ray Arvidson all argued that Whitewater Lake is where the clay signal seen from orbit is coming from. But there was a puzzling lack of iron observed by APXS, meaning that their case, while strong, is still only circumstantial. So orbital data suggests that Opportunity is sitting on clay minerals, but Opportunity has not yet conclusively proven that. No matter what, Squyres said, Opportunity is sitting on "the most habitable environment that it's seen" throughout the mission. The Whitewater Lake rock is very old, and records an age before Endeavour crater.

Another thing that remains mysterious even after all of Opportunity's hiking around the outcrop is what the "newberries" are made of. Whitewater Lake is absolutely full of these round things that look a lot like the "blueberries" of the Burns formation, but they are absolutely not made of hematite. At least, not entirely. Rocks full of newberries are indistinguishable in composition from rocks mostly lacking them. I asked Matt Golombek how that can be, and he explained to me that these berries have a rind to them (the team likes to call them "crunchy on the outside and soft in the middle"), and that the rind likely has a distinct composition (enriched in iron) but that the interior does not. They will be trying a couple of tricks with the rock abrasion tool and APXS to try to figure out the story of the newberries.