Oxygen Isotope Exchange between Carbon Dioxide and Iron Oxides on Mars' Surface.

An investigation of the fundamental processes leading to the incorporation of 18 O isotopes in carbon dioxide and in iron oxides is critical to understanding the atmospheric evolution and geochemistry of Mars. Whereas signatures of 18 O have been observed by the Phoenix Lander and the sample analysis at Mars for carbon dioxide, the underlying isotopic exchange pathways with minerals of the crust of Mars are still elusive. Here, we reveal that reactions of gaseous 18 O-carbon dioxide over goethite (FeO(OH)) and hematite (Fe 2 O 3 ) lead to an 18 O transfer from the atmosphere that enriches the 18 O content of the iron oxides in the absence of water and light. This proof-of-concept study shows that isotopic enrichment processes on Mars not only are limited to the atmosphere but also proceed via chemical interaction with dry iron oxides. These processes are decisive to comprehending the 18 O cycle between the atmosphere and the surface on the planetary scale.