Ion chromatography in Mars exploration rovers: an analytical technique to consider for future missions

The exploration of Mars has revealed the presence of a variety of soluble chemical compounds, both inorganic and organic, which offer key insights due to their relevance to geological and astrobiological processes.

In the case of chlorine-based compounds such as chloride, chlorite, chlorate, and perchlorate, these are common components on the Martian surface. The detection of these compounds may indicate volcanic, hydrothermal, or evaporative processes that occurred in the past of the planet. For example, perchlorates (ClO₄⁻) are highly oxidizing salts. Their presence has dual implications: on the one hand, they complicate the preservation of organic molecules by oxidizing them, but on the other hand, they provide clues about atmospheric and geological processes that have taken place on Mars.

Nitrates (NO₃⁻) and nitrites (NO₂⁻) are important because they can act as nitrogen sources, an essential element for life. The detection of nitrates in Martian sediments suggests that Mars may have had favourable conditions for microbial life. Sulphates (SO₄²⁻), in turn, could indicate the past presence of liquid water and evaporation processes, in addition to being potential energy sources for extremophile microorganisms.

Beyond inorganic anions, certain organic compounds may be fundamental in the search for biomarkers, as their presence could indicate prebiotic or biogenic processes that may have occurred in the past. For example, although their direct detection has not been confirmed, various studies have suggested the possible presence of oxalates on Mars, compounds that could have important implications for the carbon cycle and the detection of organic processes on the planet.

All the referred compounds form soluble salts that can be extracted with water solutions. Particularly noteworthy is the perchlorate ions discovered through analysis with ion-selective electrodes aboard the Phoenix Mars Lander after extracting the soils with an aqueous solution.  However, this method of analysis requires a separate electrode for each ion to be analysed.

Some missions have included gas chromatography systems, which allow analysis through thermal desorption while minimizing the use of solvents, thus reducing the rover’s weight and allowing the measurement of various compounds in the same sample. As early as the Viking mission in 1976, a system with a gas chromatograph coupled to mass spectrometry (GC/MS) was available, although it did not confirm the presence of organic compounds. With technological development, this became possible in more modern rovers like Curiosity in 2012, which detected organic molecules by GC/MS despite the presence of oxidizing molecules. Finally, the MOMA (Mars Organic Molecule Analyser) instrument in the rover Rosalind Franklin aims to overcome this issue to more accurately analyse the organic molecules present in Martian samples.

It is worth noting that there are non-chromatographic techniques, such as Raman spectroscopy, that enable perchlorate, sulfate, nitrate and other compound analysis. However, their identification is also complicated in the presence of complex saline mixtures, requiring a minimum concentration of their salts to be identified.

Taking all of this into account, ion chromatography (IC) may be a relevant technique for inclusion in future missions. In this sense, the IBeA group, with experience in implementation of analytical techniques for Martian rovers, is developing a method to analyse key soluble inorganic and organic ions in extraterrestrial samples with promising results. Thus, in a single analysis, it is possible to quantify fluoride, bromide, chloride, chlorite, chlorate, perchlorate, nitrite, nitrate, phosphate, sulfate, phthalate, oxalate, acetate, lactate, propionate, formiate, and glycolate in under 75 minutes (Figure 1). Implementing this technology would allow not only identification but also quantification of these key soluble species at concentrations lower than 1 mg/L. Moreover, the knowledge of the distribution and concentration of oxidizing compounds like perchlorates could help to design analysis and/or sampling strategies, minimizing the destruction of potential biosignatures by oxidative processes, and enhancing the performance of GC/MS instruments or the selection of sampling points for return missions. Regarding soluble low weight organic anions, IC would allow their quantification even in the presence of compounds such as perchlorate, without the oxidation problems present in thermal desorption-based techniques, requiring only aqueous extraction of the sample. This extraction is enhanced using ultrasound energy.

Figure 1.- Chromatogram obtained for the identification of the 17 proposed analytes, by gradient elution in a Metrohm IC in concentration between 1 and 0.5 mg/L depending on the ion. The retention time is included.

In contrast, IC does have a notable limitation: it requires the use of water in both the extraction and analysis processes. This can be considered a disadvantage, because it increases the technical complexity of sample treatment and the weight and volume of the required instrumentation due to water that must be included in the instrument as a consumable. Regarding sample preparation, extraction processes are already being carried out on current Mars rovers for example, with the MOMA instrument, so overcoming the challenge of aqueous extraction enhanced by ultrasonic energy is highly plausible. Moreover, water use on Mars has already been done, as the Phoenix rover conducted wet chemistry using its Wet Chemistry Lab (WCL). Therefore, the most important limitation for implementing IC in a future rover is the issue of weight and volume, which would limit the number of analyses to be performed but not the feasibility of implementing the technique.

Overall, given the current limitations in the analysis of organic compounds due to the presence of oxidizing agents, even so the identification of simple inorganic compounds. Thus, ion chromatography could be a potential alternative providing a quantitative analysis and enhancing the results obtained with the current analytical instruments on Mars, for example, supporting the selection of the analysis points.

Acknowledgements

This work has been supported through the PAMMAT project “Alteration processes in Mars and Moon Meteorites, and Terrestrial Analogues at different environments: Mars2020, Rosalind Franklin and Returned Samples from Mars and Moon” (Grant No. PID2022-142750OB-I00), funded by the Spanish Agency for Research (MICIU/AEI/10.13039/501100011033/FEDER/UE).