Amorphization of S, Cl‐Salts Induced by Martian Dust Activities. Issue 12 (17th December 2020) - Wang, Alian
Abstract: 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 H2 O), 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. Plain Language Summary: Martian dust activities have been altering the look of the surface of Mars, by the blast of particles of various sizes. In addition, frictional electrification in these events would charge the particles, and the charged particles would be further spatially separated by wind at the same time, to form an electric field like those formed in the cloud layers on Earth. Electrostatic discharge (ESD) would occur when enough charges are accumulated. Unlike the lightning on Earth, another two types of ESD, Townsend dark discharge and normal glow discharge would more likely occur on Mars because of its thin atmosphere. The ESD would induce electrochemical reactions that could change Martian surface materials. Our new simulated ESD experiments revealed three types of phase changes can occur in S‐ and Cl‐salts. They are amorphization (damage of crystal structure), dehydration (loss of structural water), and oxidation of Cl, S, and Fe. Because of the high occurring frequency, the large area coverage, and long duration of dust events during the recent Amazonian period on Mars, our results imply that Martian dust activities may have generated and deposited a large quantity of S‐ and Cl‐rich amorphous materials all over the surface of Mars. Key Points: Amorphization of S and Cl salts was induced by electrostatic discharge in a Mars chamber that simulates Martian dust activities Amorphization is commonly accompanied by dehydration of salts and the oxidation of Fe, Cl, and S species Dust activities may have generated and deposited large quantities of S‐ and Cl‐rich amorphous materials all over the Martian surface