Since it's already Monday morning for me here, I'll draw my scattered wits together and see what we seem to have learned over the last few exciting days in Opportunity Crater. (BTW, does it have an "official" name, or are we out of dead astronauts?).

The Meridiani bedrock consists of exquisitely finely layered pale rock. It appears to contain embedded within it spheres of bluish grey material(? haematite - bearing ?). We've seen a few x-sections of these - they break neatly and evenly without obvious growth rings or radial features. We still have no real idea about the chemistry of the pale rock nor that of the spherules, other than the hints of haematite - but remember, this could just be a surface coating on beads of something else (but that would, of course, require the haematite to be forming continuously but very slowly as a reaction rim with the modern atmosphere and radiation environment).

I venture to suggest that the association we see here between pale rock and dark debris is the same as seen in many places on Mars - the "white" rock seems to often be associated with dark "sand".

Whatever has happened here at Meridiani, may well have happened at other places all over Mars in craters, basins, and plains. I admit that we do not see a good haematite signature in all those other places. Right now I don't have a slick answer for that. So let's put aside global implications and try to understand what's happening here.

Several people have mentioned oolites - spheroids produced by organic or inorganic precipitation in a wave-rich shallow aqueous environment on Earth. This is certainly "possible", but doesn't fit well with the paper-thin layers of the pale rock. To explain it, there would have to be a nearby shallow shoal where the oolites actually grew and then a quiet deeper-water basin with the paper-thin layering. This is "possible", but requires the Meridiani area to have been an enclosed lacustrine or marine basin. The topography argues against this. However, if oolites *are* the explanation, one should be very attentive since on Earth algae and bacterial slime are implicated in the growth of oolites. (they are essentially miniature, mobile, spherical stromatolites)

Another possibility is that we are seeing layers of volcanic ash with spheroids embedded in them. I would encourage readers to learn about lapilli - peas sized spherical aggregates of volcanic ash that form in some ash eruptions related either to static electricity or moisture, or both. On Earth, we see beds rich in lapilli and others where they are sparse. One would expect the concentration of spheroids in these pale rocks on Mars to also fluctuate (as would also be true for the oolite case). So there may be beds richer in spheroids, or they may simply be a lag deposit.

There are also pisolites: http://schwehr.org/yellowstone/Explore/Beads.html (thanks, Marcia, for the link). This would obviously be a nice outcome for watery enthusiasts and of exobiological interest.

Some wilder ideas are that the spheroids have been transported a considerable distance. I think this idea fails (or becomes unneccesary) when we see them eroding out of the local pale rock. Clearly, there is some transport of them - they spill down into the crater. It "may" be that they were transported in to their current location in the past (The oolite and lapilli models have elements of this, with the juxtaposition of coarse spheroids within fine layers). But long-distance transport does not seen to be neccesary.

One puzzle - what is the density of these spheroids? (wrt the surrounding sand) Solid haematite would be quite dense, and I have a few problems with the beads spilling around as freely as they appear to, if they were denser than the sand around them. They "look" as if they are lower density than the surrounding fines, but that's an interpretation, so put it back in the box for now.

In summary, I have no idea what these things really are, or in what environment they formed, but it's *really* important that we understand how they came to be here, and their chemistry and stratigraphy. We need to retain open minds as we interpret new data and not leap to conclusions. I'm as guilty of this as the next man, but I'm working hard on being more restrained and letting the data accumulate to "critical mass", while trying to point out observations, rather than interpretations, at this stage.

I'm avidly looking forward to Mossbauer and APDX results, to more Mini-TES, and to the results of RAT borings of the pale rock. As I said earlier, even if we never get out of this one crater, we'll have had full value for money. I think this one site alone will teach us more about Mars than all of the Viking and Pathfinder landers together (but of course, we wouldn't be here without their lead, so that isn't a fair comparison)