Water Vapor Condensation in Optical Instruments on Mars

The interest of the scientific community in learning more about trace gasses in the Martian atmosphere motivates developing increasingly sophisticated instruments for flight. Modern high-sensitivity spectrometers, such as those that use the Cavity Ring-Down (CRDS), rely on photons reflecting thousands of times between extremely reflective mirrors to generate absorption pathlengths of kilometers. The absorption spectrum of trace gas species is further enhanced when cooling the gas sample, however, should any atmospheric volatiles condense within the gas cell, the highly-reflective mirrors will become marred, drastically cutting down the effective optical path length. In the Martian atmosphere, the first volatile to condense is typically water vapor, thus understanding the variability of the water frost point is essential for determining the performance limit of any optical instrument. Our presentation will examine these performance limits with respect to a model instrument being prepared for flight. The Martian Atmospheric Gas Evolution (MAGE) Experiment aims to better constrain the methane cycle and give more insight on the potential sources and sinks of the gas. MAGE is built around an Off-Axis Integrated Cavity-enhanced Output Spectrometer (OA-ICOS) gas analyzer manufactured by ABB Inc., which is designed to conduct hourly measurements of methane on the Martian surface. The variability of the frost point with time and solar longitude (Ls) therefore reveals areas of the Martian surface at which the accuracy of the ICOS instrument is optimized