We have designed and built a miniature near-IR tunable diode laser (TDL) spectrometer for measuring in situ the water vapor mixing ratio either in the Martian atmosphere or thermally evolved from Martian soil or ice samples. The laser hygrometer uses a thermoelectrically cooled single-mode distributed-feedback TDL at 1.87 μm to scan over a selected vibration-rotation line of both H_(2)O and CO_(2) near 5327.3 cm~(-1). A working prototype that weighs only 230 g has been built and used to generate spectra whose analysis demonstrates precision sensitivities as fine as 1 part in 10~(6) by volume in 1 s or 0.1 part in 10~(6) in 10 s at Martian pressures and temperatures. Absolute uncertainties of ～5% are calculated.
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