Formation of hematite spherules from metallic spherules by oxidative hydrothermal alteration: Implications for hematite spherules on Martian surface

The mechanisms of hematite spherule formation on Mars have been widely evaluated to understand the Martian surface conditions and their history. These Martian hematite spherules was suggested to have a sedimentary origin, with the spherules predicted to have formed in solutions through precipitation. In this study, we investigated the formation mechanism of hematite spherules on Mars by performing oxidative hydrothermal alteration experiments using reproduced I-type cosmic spherules. Our results suggest that Martian hematite spherules were formed by oxidation of Ni-bearing metallic spherules produced by ablation of iron meteorites during their entry into the Martian atmosphere. After long-term exposure on the surface of Mars, most spherules were pulverized because of oxidation. Some hematite fine grains blended into the Martian soil and were moved by storms, whereas others maintained their initial spherical shape with a high content of Ni. Notably, the precursor of Martian hematite spherules may be metallic spherules derived from iron meteorites.