I just did a test with a bag of Gold Medal, and indeed, the stuff does cling together into tiny clumps all by its own self. They fly apart if the flour is thrown about the room, the clumps become the finest dust, and it's a mess to clean up, as anyone who's tried it knows. But the dry flour clumps are very small, barely perceptible, and they are very fragile. They do not hold together under any stress at all.

This is dry flour under ordinary room conditions, average temperature and sea-level pressure, "normal" relative humidity (about 30%) for here at this time of year.

Dampening the flour with water, even very lightly, causes much larger clumps to spontaneously form, and these clumps hold together under considerable stress. They do not merge without persuasion. Nor do they fly apart.

Depending on other factors, water ice can also cause spontaneous larger clumpage, but I didn't have any dry ice to see how flour would behave in its presence. I don't know what would happen. Maybe Nick has tried it.

On the other hand, the near-vacuum and the fierce cold at the surface of Mars argues against the surface soil routinely responding as flour might under terrestrial conditions.

The absence of any water in the test samples that made it into the ovens is interesting. Could the initial sample have been "wet" -- by Mars standards, which would be parched indeed by our own -- perhaps "wet" with ices of some sort, and might it have thoroughly dried out through sublimation as it sat exposed to the sun and all that UV and got bounced around for a while until a sample could be obtained?

Just idly wondering...