Amorphization of S, Cl-Salts Induced by Martian Dust Activities

A high proportion of noncrystalline (X-ray-amorphous) components has been found in all samples analyzed by CheMin on the Curiosity rover at Gale crater on Mars, and such X-ray-amorphous components probably occur at all sites that have been investigated thus far by landers and rovers. The amorphous material at Gale crater is rich in volatiles (S, Cl, and H₂O), as indicated by other science payload elements (APXS, SAM). We demonstrate here that amorphization of S and Cl salts can be induced by energetic electrons and free radicals generated in a medium-strength electrostatic discharge (ESD) process during Martian dust activities such as dust storms, dust devils, and grain saltation. Furthermore, we found that the amorphization is commonly accompanied by dehydration of the salts and oxidation of Cl, S, and Fe species. On the basis of experimentally observed rates of the above phase transformations and the mission-observed dust activities and wind speeds on Mars, we anticipate that similar phase transformations could occur on Mars within a time frame of years to hundreds of years. Considering the high frequency, long duration, and large areal coverage of Martian dust activities, our study suggests that the ESD induced by Martian dust activities may have contributed to some the S- and Cl-rich portion of X-ray amorphous materials observed in surface soils at Gale crater. Furthermore, dust activities in the Amazonian period may have generated and deposited a significant quantity of S- and Cl-rich amorphous materials all over Mars.