Planetary laser spectrometer for sensitive in situ detection of water at 1881 nm

We describe the design and capabilities of a highly sensitive prototype tunable diode laser spectrometer for detecting water vapor and its isotopologues in the 1881 nm region. It is a compact instrument based on wavelength modulated cavity enhanced absorption spectroscopy and can measure a fractional optical absorption of ~10−5 for a pathlength of ~4 km, corresponding to ~10−8 mbar of water vapor (improvable to 10−9 mbar). The spectrometer would be suitable for searching for fingerprints of ancient climates on Mars and potential habitats of life in the Solar system. It would be useful for appraising water deposits on the Moon and asteroids, and characterizing the isotopic composition. Lunar deployment could provide ground truth to the recent orbital measurements, and help to discover records of the early bombardment history of the inner Solar system buried at the poles and clarify the mechanism for the generation of water in the illuminated regions.