New column simulations for the Viking landers: Winds, fog, frost, adsorption?

Abstract

Boundary layer simulations are shown for the first sols of the two Viking landers (VL1, VL2) on Mars. The column model (with cloud/radiation interaction and Prandtl slope wind terms), used successfully for Phoenix and Curiosity, is equipped here with an adsorption-diffusion scheme for water vapor transport in porous regolith. The model's 1.6 m temperatures and winds are quite close to those observed by the two landers; in particular the weak summer slope winds of VL2 are excellently reproduced.The model predicts for both sites diffusion and adsorption of water into regolith in the evening, very thin ground frost deposition from about midnight, and an early morning fog with an inflection in T1.6 m slightly weaker than observed. At the moister VL2 site fog increases optical depth as observed. Fogs and frosts sublimate away after sunrise, allowing desorption and diffusion of water off the sun-heated regolith. For porosity of 22% column water is approximately conserved from sol to sol at both sites with only little diurnal variation, as the depleted layer of air moisture is quite shallow.In simulations without adsorption frost forms early and it grows thick. At VL2 fog now forms earlier and the jump in optical depth is larger than observed. At the drier VL1 fog still forms nearly as with adsorption, so observations could also be explained without adsorption. On the other hand VL1 certainly landed onto porous regolith and frost was not observed. Hence it is suggested that adsorption is likely at both VL sites in summer.