Thermal Analyzer for Planetary Soils (TAPS) Experiment, 2: Water Sensors

Introduction. The Thermal Analyzer for Planetary Soils (TAPS) is a proposed in situ experiment for identifying and measuring the abundances of volatile-bearing minerals and ices on planetary surfaces through a combination of calorimetric and evolved-gas sensors [I]. Because water is arguably the most important planetary volatile compound from both the geological and exobiological perspectives, TAPS must address analyses for water as a major priority. In this context, TAPS expands upon the Planetary Soil Water Analyzer (PSWA) [2] proposed by D. M. Anderson and collaborators in the early 1980s for Mars exploration. In the baseline TAPS design, water is evolved from planetary samples through programmed heating. Therefore, a successful water sensor is required to respond quickly to rapid changes in water vapor abundance while displaying specificity for water with minimal interference from other gases. Required Sensitivity for Water. The sensitivity required of a TAPS water sensor depends on the mass and water content of the sample but also on the heating and purge rates used in the experiment. For a well-crystallized mineral hydrate such as gypsum (CaS042H20), water release will occur over a limited and predictable temperature interval so that the water will appear as a concentrated "pulsen into the dry purge gas stream. In contrast, a poorly crystallized mineraloid hydrate such a palagonite (a product of the hydration of basalt glass) may release water in a more continuous fashion over a broad temperature interval. Therefore, for a given total abundance of water in a bulk sample, the abundance of evolved water vapor in the purge gas stream will be inversely related to the