Martian concretion sizes predicted from two independently constrained inputs: atmospheric dust grain size and obliquity-forced wetting duration

Abstract

Diagenetic concretions have been identified at multiple widely separated sites on Mars, including Meridiani Planum (Opportunity), Gale crater (Curiosity), and Jezero crater (Perseverance). Solid concretions at all sites fall within the millimetre size range (typically 1-6 mm diameter), despite differing cement mineralogies. The one substantial outlier -- centimetre-to-decimetre-scale hollow concretions on Bradbury Rise -- formed in coarser basaltic sandstone via a distinct mechanism. I propose that this size convergence reflects a common physical control: the globally uniform fraction of ultra-fine (~3 um), amorphous, equant atmospheric dust incorporated into sediments at all sites. I derive the diagenetic timescale from Mars' ~120 kyr obliquity cycle, which drives periodic subsurface wetting: each high-obliquity pulse (~10^4-10^5 yr) sets the available growth time. Using a diffusion-reaction model with nucleation competition, I show that the low effective diffusivity imposed by the fine dust matrix limits concretion growth to the observed millimetre scale, independent of local fluid chemistry. Formation efficiency in dust-rich sediment exceeds 90%, making concretion formation essentially inevitable wherever liquid water contacts the dust. This mechanism depends on the non-phyllosilicate, equant-grain mineralogy of Martian dust, which maintains connected pore networks unlike terrestrial clays. Growth is self-limiting: the first wetting pulse exhausts reactive phases in the depletion halo, so successive obliquity cycles produce new concretions in fresh sediment rather than enlarging existing ones. Each concretion records a single wetting episode. The narrow size distributions at all sites suggest that Martian concretion populations may constitute a sedimentary archive of the planet's obliquity history.

Figures (3)

• Figure 1: Sensitivity of predicted concretion diameter to (left) the sorption parameter $k_{d0}$ across two orders of magnitude and (right) the nucleation density coefficient $n_0$. Green band: observed range for solid Martian concretions in fine-grained sediment (1--6 mm diameter; Table \ref{['tab:observed']}). At 3$\,\mu\text{m}$ grain size (red dashed line), the prediction remains within the observed range across the full plausible parameter space. The hollow Bradbury Rise concretions (1--23 cm) plot far above this band, consistent with formation in coarser host sediment (Section \ref{['sec:outlier']}).
• Figure 2: (Left) Predicted concretion diameter for different nucleation density scaling exponents $\alpha$. For $\alpha \leq 1$, diffusion controls at the Mars dust grain size and predictions match observations (green band). For $\alpha \geq 1.5$, nucleation dominates even at 3$\,\mu\text{m}$, predicting concretions well below the observed range, ruled out by the data. (Right) Crossover grain size as a function of $\alpha$ (log scale). Green region: diffusion-limited zone (crossover above Mars dust at 3$\,\mu\text{m}$). Pink region: nucleation-limited zone. The grain-contact derivation (dotted line, $\alpha = 1$) sits within the diffusion-limited zone, consistent with observations.
• Figure 3: (A) Predicted concretion diameter vs. host-sediment grain size. Blue curve: model prediction for the reference wetting timescale $t_\text{wet} = 10^5$ yr. Red dashed: diffusion limit. Green dashed: nucleation spacing limit. Shaded blue envelope: range of obliquity forcing durations ($10^4$--$10^6$ yr). Numbered diamonds: observed solid Martian concretions in dust-rich sediment (Table \ref{['tab:observed']}); open circle (B): hollow Bradbury Rise concretions in coarser basaltic sandstone; squares: terrestrial analogues. See inset key for site identifications. The Mars dust band (red shading) overlaps the clay-size range but, unlike terrestrial phyllosilicate clays, maintains connected pore networks that permit diffusion-limited cementation. (B) Concretion formation efficiency $\eta$ as a function of host grain size. Blue curve: capture efficiency at the reference pressure gradient ($\nabla P = 1$ Pa m$^{-1}$). Shaded envelope: range of site-specific topographic gradients (0.1--100 Pa m$^{-1}$). At the Mars dust grain size, $\eta > 0.9$: concretion formation is essentially inevitable. Efficiency drops steeply through the silt range and falls below a few per cent at medium sand sizes, where advective flushing dominates.