Martian concretions at multiple sites share the same size range — one to six millimeters — despite different cement compositions and different locations. The chemistry varies. The size does not.
The universal constraint is the matrix, not the cement (arXiv:2603.11143). Atmospheric dust on Mars is approximately three micrometers across, and this fine dust incorporated into sediments everywhere creates a matrix with uniformly low effective diffusivity. Concretions grow by diffusion of dissolved species through this matrix, and the fine dust throttles diffusion equally at every site.
The timescale comes from Mars's obliquity cycle — roughly 120,000 years of conditions wet enough for diffusion to occur. A diffusion-reaction model using these two inputs (dust grain size, wetting duration) predicts the observed size range without free parameters. Formation efficiency exceeds 90 percent in dust-rich sediment.
The growth is self-limiting. A concretion depletes the dissolved species in its immediate vicinity during one obliquity cycle. The next cycle produces new concretions in undepleted regions rather than enlarging existing ones. Each cycle writes a new generation rather than overwriting the previous one.
This makes concretion populations a sedimentary archive of obliquity history. Each generation records one wet episode. The spatial distribution records the number of episodes. The uniformity that seemed like a coincidence — why the same size everywhere? — is actually the signature of a planetary-scale clock: the obliquity cycle ticking through a universal matrix of atmospheric dust.