I noticed this anomalous feature in the sol 962 pancams from Cape Verde that came down today. I don't know exactly where it is in context, since it doesn't appear in any of the panoramas. A rover wheel is visible in the full sized image, so it is somewhere very near the rover's position at the time.

I have been staring at this darkly colored, circular patch with a very dark clast in its center for a quite a while, but I don't have a good explanation for what it could be. The dark circular part appears to be composed of a fine, granular sediment. I don't recall having seen anything similar to this before. It almost appears as if the dark central clast has somehow discolored the soil around it.
Click to view attachment
Who feels like speculating?

Oil leak?

Hehe, with all the previously speculated oil stains, this must surely be Opportunity's final attempt to purge all of it's lubricants.

I just realized that this had to be a specially targeted image, so I checked the pancam tracking site hoping to find additional information. I found "pancam_rf_wheel_scuff_L257," so it targeted the scuff, but this just happened to be nearby, or so it seems. It is difficult to imagine this could have been caused by the scuffing.

sandworm doo-doo....

I'm guessing there was a clod of material picked up who knows where that has been riding along in the wheel and it got jarred loose during the scuffing process.

I'll jump in, but before I speculate, I'll try to make some observations. Forgive me if this is too elementary.

I see a roughly circular spot that is darker than the area around it.
The center of the circle seems to be the darkest, although this may be a shadow.
There appears to be the impression of a wheel tread across part of the dark spot.
There are three "blueberries" within the dark spot that are ontop of the darker sediment.
A lighter toned grain between two of the "blueberries" appears to have some of the darker sediment on it.
There appears to be a small trough in the darker spot that is deepest near the center of the dark spot and is perpendicular to the goove made by the wheel tread.
There are other darker patches in the sediment in the vicintiy of the spot although none are as dark or a regularly shaped as the spot in question.
One portion of the wheel track near the edge of the image seems to have scuffed up some sediment that is of a similar tone and is almost equally dark.

After all of that I'm still scratching my head.

The questions seem to be how to explain the fairly regular shape and the dark tone of the sediment.

Did I miss anything?

ANTS ....

I would suggest a small dark, probably heavy, rock, perhaps ejected by an impact, that has almost all eroded to sand, leaving a remnant clast in the middle. If it is a heavy mineral and the grains are not too small most of them would remain in the vicinity.

tty

Given the proximity to the wheel track I vote strongly in favour of a pile of fine soil that came loose from the wheel and dropped down, as centsworth suggested. We've seen similar events before. If the berries in the dark patch don't seem to be covered by the dark soil, perhaps they also came down from the wheel and landed on top of the dark soil.

I must say to me it looks like some sort of vent with a halo deposit. Gas? Liquid? Powder?

Either those or bioturbation

Hehe, of all the suggestions made in jest, the ant hill was my favorite. Of the serious ones, I'd have to admit
that I am leaning more toward something like tty proposed.

Gray and tty seem to be thinking more along the same lines as I am. I could buy into the idea that it was simply dirt spilled from the passing wheel, but all of the many instances of such events we've seen have produced irregularly shaped piles of sediment. Even if the wheel passed over and flattened the pile, it is hard to imagine it resulting in such a nearly circular feature. The "fairly regular shape and the dark tone of the sediment" (as Gray noted) are somewhat difficult to explain with a rover-induced event. I would add to that summary, the observation of the large and very dark clast in the center of the ant hill.

I didn't know exactly where this thing was, but it seemed likely that it would have been visible in an earlier panorama. Thanks to a recent metadata update by our revered MMB hero, we now know pretty much where it is. Thanks, Mike. I'm not sure I have identified the location correctly in previous images, so some assistance from the mapping experts would helpful, but I think I see a dark spot in the vicinity, before Opportunity rolled by. I still find this thing curious.

fredk: It still might be wheel debris, but I think postulating berries also picked up by the wheel and fortuitously dropped onto the flattened ant hill is a little bit of a stretch.

I don't see it being a clod fallen off the wheel. Not only is it too precisely circular, the edges are too sharp. There is absolutely no "splat" effect which would likely result in irregular splays and tone gradations at the edge.

Another idea for the collection - it's the remains of a dirty ice-meteorite that has largely sublimed away leaving a carbon rich residue.

Too bad this wasn’t noticed and looked at more closely before it was left so far behind, since now speculation will run ahead of probability.

The cracks that run three quarters of the way across the dark spot, and down-picture at 90 degrees from that crack suggest some upward force. (But I doubt it was much - or maybe any.)

It could be a section of stone just under the surface, that was moved by the weight and passage of the rover wheel - possibly raising it just enough to cause apparent cracks - by passing over perhaps an edge or end of it.

If a piece of rock were moved slightly, it could disturb surface material and expose some darker material underneath - perhaps contributing to the effect of a ‘circle.’ (Though one would expect to see more surface cracking - - and the ‘cracks’ may not be cracks at all.)

Note that material below the ‘circle’ and spots to the upper left of it are also of the same or near the same color.

Note too, different colored spots frequently occur naturally here on earth that are sometimes near-perfect circles. They are most often caused by wind, and a stone or other object just underneath the surface. My best guess - this is a photo of the same type of action on Mars.

Although the image is very interesting, I don't think that there is much of a mystery to the dark spot.

Notice that there are a couple of fairly large blueberries on top of the dark spot. More importantly, at least one of those blueberries looks desturbed. There is also a small indentation in the spot that looks like part of a rover track. From the looks of the other identifiable rover tracks in the image, the rover made a slight turn near the dark spot. The tracks clearly show loose material to the left; yet, the ground to the right and near the dark spot looks much more firm or harder.

In my humble opinion, the spot is from a disintegrated dirt clot that may have originally come from an impact in the area. There are plenty of small rocks or dirt clots scatter throughout this area. The rover simply ran over a small one and it crumbled. Part of the center of the clot was actually pushed deaper into the surrounding soil. A couple of blueberries were kicked around and ending up on top of the disintegrated clot. Basically, it's rover created.

Just my opinion...so several of you must have had part of story correct.

The "cracks" are just wheel marks, I'd say.

Again, I think that a clump of soil dropped from a wheel and then possibly run over to flatten it is the best explanation. Given the proximity to the wheel tracks, it would be a bizarre coincidence if the dark spot wasn't related to the wheels.

I've located the spot on the sol 958 pancams, before we arrived at the spot, and there's no dark spot visible:
Click to view attachment

It is visible in the 959 front hazcams, and so are some patches of dark soil in the wheels, exactly what likely produced our spot:
Click to view attachment

The spot is towards the cliff from Oppy, so on sol 959 she edged a bit towards the cliff, then back again to make the scuff and the spot.

Fredk
Great detective work. You've convinced me.

Yes, good work indeed, fredk. Thanks for the help identifying the location in the earlier navcam. That is proof that the feature was not there before the wheel passed. I thought it was strange that the pile of soil was so regular, has berries on top, and a bullseye at it's center. But you're right, in retrospect it would be more bizarre for it to be so close to the wheel if it wasn't caused by it. I suppose the scuffing activity could explain the berries on top, and perhaps some other characteristics.