Measuring Soluble Properties of Planetary Science Samples: Sensor and System Development Since the Wet Chemistry Laboratory

Abstract

The measurement of the soluble properties (pH, eH, conductivity, specific ion concentrations, etc.) of planetary science samples provides important information on the habitability of a location, the bulk components of a sample, the potential for undesired effects on analysis protocols, and the history/preservation of the sample (e.g., geologic weathering, radiation processing, etc.). Electrochemical sensors are an ideal means to measure soluble properties because they excel at detecting ionic species in solution. The Wet Chemistry Lab (WCL) on the 2007 Mars Phoenix Lander was the first planetary science instrument to focus on soluble inorganic ion measurements and used an array of electrochemical sensors. WCL’s discovery of significant quantities of perchlorate salts in the Martian regolith helped reshape our understanding of Martian chemistry and the search for past or present life there. As NASA starts on a new chapter of exploration with potential missions to further out locations like the Ocean Worlds of Europa and Enceladus, it is a natural fit to adapt the successful WCL instrument for operation on bodies where the samples could be ice or liquid. Since the successful operation of WCL on Mars there have been a number of projects to improve sensor capabilities, develop microfluidic implementations, and prepare for Ocean World mission scenarios. This work has led to a number of developments, including: 1. Production of Ag/AgCl reference electrodes that are stable for use during flight missions. 2. Cyclic voltammetry methods for the specific detection of SO4 2- and NO3 -. 3. Adaptation of hydrogel-based ion-sensitive electrodes (ISEs) for use in microfluidic arrays. 4. The development of solid-contact ISEs for use during flight missions. 5. Radiation testing of ISE ionophores. 6. The development of microfluidic infrastructure based on successful small satellite payloads. The combination of all this work has led to a significant increase in sensor/system performance and robustness in preparation for potential missions to Ocean Worlds.