Mars laser hygrometer

Abstract

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 microm to scan over a selected vibration-rotation line of both H2O and CO2 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 approximately 5% are calculated.

Line strengths of 12C16O2: 4550–7000cm−1

“…Other spectroscopic instruments are being deployed to monitor the Martian atmosphere [2,3]; one goal is to measure isotopic fractionation on Mars with 0.2% precision for a number of molecular species, including CO 2 [4] in order to trace the evolution of its atmosphere [5], as well as to provide ''early'' planet models for astrobiology [6]. Near infrared wavelengths are well-suited for these endeavors, but to achieve these goals, transition line positions and strengths must be known to very high precision and accuracy.…”

HAI, a new airborne, absolute, twin dual-channel, multi-phase TDLAS-hygrometer: background, design, setup, and first flight data

“…A high number of (mostly single channel and single H 2 O phase) hygrometers have been developed in the last decades with various advantages and drawbacks (Wiederhold, 1997). A non-exhaustive selection of instruments has been described (Buck, 1985;Busen and Buck, 1995;Cerni, 1994;Desjardins et al, 1989;Diskin et al, 2002;Durry et al, 2008;Ebert et al, 2000b;Gurlit et al, 2005;Hansford et al, 2006;Helten et al, 1998;Hunsmann et al, 2008;Karpechko et al, 2014;Kley and Stone, 1978;May, 1998;Meyer et al, 2015;Ohtaki and Matsui, 1982;Roths and Busen, 1996;Salasmaa and Kostamo, 1986;Sargent et al, 2013;Schiff et al, 1994;Silver and Hovde, 1994b, a;Thornberry et al, 2015;Webster et al, 2004;Zöger et al, 1999a, b;Zondlo et al, 2010). Consequently, the question should be raised from the opposite point of view: What are the important and required properties to be covered and combined for the near-universal Hygrometer for Atmospheric Investigation (HAI) to serve as an innovative and cutting-edge tool to explore open and new scientific questions related to atmospheric water vapor?…”

“…Compared to most airborne TDLAS instruments, HAI has a relatively low number of reflections and thus relatively short path lengths. Further categorizing distinguishes between wavelength-modulation schemes like single modulation frequency, called direct TDLAS or dTDLAS (Ebert and Wolfrum, 1994), or double modulation schemes like wavelength modulation spectroscopy (WMS) (Podolske and Loewenstein, 1993;Silver, 1992;Silver and Hovde, 1994a;Silver and Zondlo, 2006;Vance et al, 2011;Webster et al, 2004). WMS, often used for very compact sensors, provides on the first glance higher sensitivities by using lock-in technologies to efficiently filter noise.…”

Validation of an extractive, airborne, compact TDL spectrometer for atmospheric humidity sensing by blind intercomparison

“…Most important for field and particular for airborne instruments is the calibration strategy. WMS techniques [54][55][56][57] or [58] offer great sensitivity due to the small detection bandwidth and are very frequently employed in airborne hygrometers. However, they are expressly hampered by their complicated signal dependence on line width hence on air pressure and temperature and therefore need constant pressure extractive detection or complicated 2-D calibration matrixes.…”