Paper Review — Claim-Level

• New measurements: Ti, Zr, Mo isotopes across 17 martian meteorites, reporting a consistent bulk-sampled Mars mantle (within measurement scatter). Mars isotope dataset and core results
• Multielement isotope correlations among NC meteorites imply an “inner-disk gradient” and an unsampled component enriched in s-process nuclides (inferred from where Earth/Mars lie relative to NC trends). NC correlations and “unsampled” component
• Three-component mixing model + Monte Carlo uses Mo (siderophile; last stages dominate) together with lithophile isotope correlations to infer bulk CC fractions (Earth and Mars peak near ~4% by mass). Monte Carlo inference of bulk CC ~4%
• Interpretation: low CC fraction implies an efficient, persistent barrier limiting inward transport of outer-disk pebbles/dust, consistent with Jupiter acting as such a barrier. Persistent dust/pebble drift barrier interpretation

5.1 Strengths

• Measurement program directly targets Mars with multiple reservoirs and reports internally consistent Ti and Zr isotope results across 10/6 samples (respectively), plus Mo isotope measurements that indicate both NC and CC contributions in ε95Mo–ε94Mo space. Tight isotope clustering motivates mixing model
• Model uses explicit uncertainty propagation (Monte Carlo with 5 million trials) rather than a single deterministic mixing solution, which helps avoid over-interpreting a single best-fit point. Monte Carlo uncertainty propagation
• Clear physical bottleneck for discrimination: Mo is siderophile and therefore sensitive to later accretion stages; the paper explicitly uses this to separate what Mo says vs what bulk CC says. Physical rationale for separating Mo-only vs bulk CC

5.2 Potential blind spots / assumptions (what could change the conclusion)

• Endmember and line-slope dependence: the inference depends on the geometry of NC and CC in multielement isotope space (e.g., whether NC and CC lines are parallel, and how slopes are estimated from meteorite data). The paper states parallelism issues cause smaller than uncertainty effects for Mo-derived mass fractions, but this is still a model structural assumption. NC vs CC slope effect estimated <0.1
• Unsampled inner-disk material interpretation: Earth and Mars may lie on/near NC correlation lines because their precursor mixtures followed that gradient, but this requires that the inferred “lost” component really dominates the offset in Mo/Zr vs Fe-group behavior. The paper’s discussion suggests several checks (evolution with age not obvious; heliocentric gradient), but any mismatch between assumed precursor heterogeneity and actual disk dynamics would be a blind spot. Unsampled inner-disk component rationale
• Representativeness of meteorites as disk tracers: using meteorite distributions as proxies for inner/outer reservoirs inherits sampling bias (meteorite survival, excavation, and classification). While the paper directly addresses “unsampled” material, the mapping from meteorite groups to disk reservoirs remains underdetermined. Motivation: meteorites may not sample all building blocks

1. New isotopic constraints that force Earth/Mars to require much larger bulk CC fractions (≳10–30%) across multiple lithophile/siderophile tracers, not just one (e.g., reconciling with silicon-isotope interpretations that infer ~26% CI-like for proto-Earth and ~10% for Mars would be a major target). Si isotopes imply larger outer-disk fractions
2. Endmember revision (NC/CC line geometry) that systematically shifts inferred CC fractions. This would require either better characterization of the NC-line slope/curvature or revised CC endmember isotope variability. Bulk CC inference depends on NC/CC line constraints
3. Atmosphere/core-timing confounds that alter how Mo records late accretion (e.g., if Mo partitioning/removal to the core behaved differently than assumed). Mo records last ~20% accretion