WASSR: WAter-vapor Sounding Short-range Radar for mapping local atmospheric humidity over the Martian surface from an in situ platform (WASSR)

We propose to build a new kind of in situ radar for short-range mapping of near-surface atmospheric water vapor on Mars. Dubbed WASSR (WAter Sounding Short-range Radar), the instrument features a highly compact and low-power differential absorption radar that operates near the 557 GHz water absorption line to measure absolute humidity along its beam path with as good as few-ppm accuracy. WASSR takes advantage of a unique and fortuitous condition in the Martian atmosphere wherein water vapor exhibits an extremely strong, identifiable, and narrow molecular rotation absorption feature centered at 557 GHz that will modulate the strength of radar echoes from nearby terrain. The magnitude of beam attenuation depends on the precise frequency of the radar transmission and the water vapor abundance along the radar beam's line of sight. By taking ratios of echo powers between different frequencies along the wing of the water absorption line, and between ground targets at different distances, WASSR can obtain, for the first time ever, range-resolved, absolute humidity estimates. Furthermore, WASSR requires no calibration because its ratio measurements are independent of the ground reflectivity, the radar's transmit power, and its receiver sensitivity – all common mode factors that drop out in a frequency- and range-differential measurement. The central measurement objectives of WASSR are to accurately measure local water vapor mixing ratios over diurnal, seasonal, and yearly time scales; to probe the vertical distribution of water vapor up to 2 km in altitude; and to map ground-level humidity with sufficient range resolution to discern differences in atmospheric water vapor exchange with localized geologic features. Mounted on a two-axis gimbal, WASSR will be able to generate maps of local absolute humidity at ground level by scanning its beam over the terrain at low grazing angles. This functionality may reveal localized regions of higher or lower humidity, perhaps arising from different soil composition, evaporation, or even brine flows. Likewise, beam-scanning up the slopes of nearby topography can validate or constrain models about the variation of humidity with altitude. WASSR can also operate with high accuracy at all times of the day or night in any season, overcoming the limitations of the past relative humidity measurements made by the Curiosity rover. WASSR is thus an excellent candidate for future Mars missions that seek to understand the processes by which water travels in the Martian atmosphere, is exchanged with its soil, and affects astrobiological potential. WASSR's design will be based on low-power millimeter- and submillimeter-wave radar systems that JPL has built over the last decade, especially a 95 GHz Doppler radar prototype for comet jet observations and a 183 GHz humidity-sounding cloud radar for Earth science applications. Special enabling technologies for WASSR include a low-power W-band synthesizer, efficient Schottky diode frequency-multipliers and mixers, and an FPGA-based digital processor. At the conclusion of this MatISSE-18 effort, WASSR will be near TRL 6 and thus well positioned to compete for selection in prime Mars rover or lander missions as soon as the 2020s. The development of WASSR will also open up a whole new class of compact flight instruments for emerging submillimeter-wave radar applications in space, such as water vapor detection from orbit around Mars or on comets or icy moons, and short-range spacecraft navigation for landing or docking.