Years of in situ investigation by Mars landers and rovers, plus analysis of Martian meteorites, have shown that oxychlorine species (ClO_x^-) are ubiquitous on the Martian surface.

Two stable phases, perchlorate (ClO₄^-) and chlorate (ClO₃^-), are suggested to present in the samples where oxychlorine species are detected. However, the factors that control the ClO₄^-/ClO₃^- generation ratios are not well understood.

A research group led by Dr. ZHAO Yuyan from the Institute of Geochemistry of the Chinese Academy of Sciences (IGCAS) evaluated how the Fe secondary minerals, oxidants and oxidation pathways, and presence of liquid water would influence the generation ratios of ClO₄^-/ClO₃^- on Mars.

Their findings were published in Nature Astronomy on Feb. 7.

The ClO₄^-/ClO₃^- abundance ratio has critical implications for the redox conditions, aqueous environments and habitability on Mars due to their different physical and chemical characteristics. For instance, ClO₄^- can reduce the eutectic points to a lower level than ClO₃^-. ClO₃^- can act as an effective oxidizer on the Martian surface and serve as an energy source for microorganisms. In comparison, ClO4- is relatively stable under ambient conditions.

Therefore, constraining the primary species, distribution characteristics and environmental effects of oxychlorine species on Mars is critical for the chlorine cycle at the soil-atmosphere interface on Mars, exploration of organics on Mars, hazard assessment of future human exploration and in situ resource utilization.

The researchers found that Fe secondary mineralogy is the dominant factor controlling the ClO₄^-/ClO₃^- generation ratio: the Fe sulfates and Fe³⁺-montmorillonite mixing with NaCl produce much higher yields of ClO₄^- than of ClO₃^-, whereas the opposite is true for the NaCl-Fe (hydr)oxide mixtures.

The study further indicated that the physical state (solid, liquid or gas) of chloride (Cl-) and the characteristics (for example, semiconductivity, specific surface area, acidity) of the co-occurring minerals have the most significant influence, whereas oxidation sources (ultraviolet radiation or ozone) and atmospheric composition induced only secondary effects.

The researchers highlighted ClO₄^-/ClO₃^- generation ratios in different surficial environments relevant to Mars.

During the Noachian and Hesperian, when phyllosilicates and sulfates predominated, fluid activities were widely distributed, and UV radiation was relatively faint, ClO_x^- production would have been less efficient than in later geological periods. The aqueous environments may produce more ClO₃^- than ClO₄^-, but some local regions dominated by Fe sulfate evaporites may produce much more ClO₄^- than ClO₃^-.

Since the Amazonian, when a (hyper)arid climate prevailed with stronger UV irradiation and ubiquitous Fe (hydr)oxides and other oxides, ClO_x^- yields higher by orders of magnitude would have been produced with much more ClO₃^- than ClO₄^-.

Therefore, ClO₃^- rather than ClO₄^- should be the key focus of future oxychlorine-related studies for Mars, including the redox environment, habitability, in situ organic analysis and in situ resource utilization on Mars.