Water vapor condensation in optical instruments on Mars

Modern high-sensitivity spectrometers rely on photons reflecting thousands of times between extremely reflective mirrors to generate absorption pathlengths that are kilometers long. The absorption spectrum of trace gas species is enhanced when cooling the gas sample, however, should any atmospheric volatiles condense within the gas cell, the highly-reflective mirrors may 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. We will examine these performance limits and map the maximum temperatures at which water vapor will condense on the Martian surface with respect to a model instrument being prepared for flight. The variability of the frost point with time and solar longitude (L_s) therefore reveals areas of the Martian surface at which the precision of a high-sensitivity spectrometer is optimized. We find that the optimal performance is achieved during northern fall and winter (L_s = 180°–360°) at polar and mid-latitude locations in the northern hemisphere, and during northern spring (L_s = 0°–90°) at equatorial and mid-latitude locations in the southern hemisphere when the frost point reaches the lowest maximums.