Fig. 3. Mixing model between an incoming flux of outer solar system chondritic impactor and the crust of Mars.

The model describes a scenario where the observed Cr isotopic composition of the crust (μ⁵⁴Cr of 6.8 ± 1.5 ppm) can be explained by mixing the anomalous composition of incoming impactor flux before 4.1 Ga modeled to be CR chondrite-like (μ⁵⁴Cr of 128 ± 4 ppm) and requires a fraction of impactor material of 24% to satisfy the observed composition (see Materials and Methods). Two models are considered, namely, the old, buried basins (model 1) (32) and the dichotomy-forming impact (model 2) (28). Pink vertical bar represents the range of average crustal thickness from the results of seismic data from the InSight mission (35). Both cases can account for the observed μ⁵⁴Cr of the crustal endmember if the impactor mass is admixed to the first 5 km of the crust (model 1) or the entire thickness of the crust (model 2).