Wind tunnel experimentation of ice particles transport in Martian-like environment

Introduction:  The transport of ice by wind plays a major role in the surface mass balance of polar caps [1, 2]. Ice can be redistributed by wind due to (1) transport of ice particles and/or (2) transport of water vapour associated with sublimation/condensation. On Mars, although the low atmospheric density is less favourable for the transport of particles than on Earth, both dust and sand have been observed to be transported by wind [3,4]. Despite ice aeolian landforms have been observed at the surface of the North Polar Cap of Mars [2, 5, 6], ice particle transport has not been directly observed on the Martian surface. Similarly, no laboratory studies of snow/ice particle transport under Martian-like conditions have been attempted thus far due to the complexity of the material. In this study we performed experiments of ice particle transport in a wind-flow under low temperatures and low pressures. From the experiments, threshold shear velocity of water ice particle transport is retrieved for different pressures and sizes in order to evaluate the plausibility of ice particle wind-driven transportation at the surface of Mars.

The North Polar Cap of Mars:  The Martian atmosphere is thin (7 mbar), cold (220 K) and dry (< 80 μm-pr) [7]. These conditions favoured ice sublimation/condensation processes. Spectral analyses [8, 9] suggested the optical ice grain sizes to vary between 10 μm to about 2000 μm for the seasonal frost and surface of the perennial North polar cap. But, the mechanisms of ice deposition are not well established. It can potentially come from vapour condensation directly onto the surface [9] or from snow fall [10]. This will affect the shape and size of ice particles and degree of ice sintering, which all influence the shear velocity threshold. The North polar cap experiences a permanent katabatic wind regime [11] with a typical friction shear velocity u_* about 0.2 m.s^-1. The complex interaction between the cryosphere and the wind leads to the formation of aeolian features at different scales [2, 5, 6].

Wind tunnel experiments:  We performed experiments using the environmental wind tunnel AWTSII at Aarhus University. It is a cylindrical vacuum chamber, housing a recirculating wind tunnel about 8 m long, 2 m wide and 1 m high [12]. The facility can achieve a turbulent boundary layer flow at both low temperature and low pressure. The ice samples were produced by using the Setup for production of Icy Planetary Analogues [13]. The ice samples were sieved (125 - 250 μm, 250 - 500 μm, 500 - 2000 μm) as a monolayer on a plate covered with volcanic regolith (125 μm). The fan speed was increased by steps (shear velocity u_* = 0 to 2 m.s^-1) and the wind flow characterized by laser Doppler anemometry. The removal of ice particles was monitored by webcam. We performed the experiments for the different particle shapes and sizes for 4 different air pressures; 40, 100, 500 and 1000 mbar. The air temperature was maintained low (~-25°C) close to the sample plate to prevent the ice melting, sublimating and sintering.

Threshold shear velocity calculation: The threshold shear velocity was determined from analysis of acquired images. When bright ice particles are removed from the dark volcanic regolith plate, the reflectance of the surface decreases. Black and white reference targets are placed close to the sample plate in the field of view of the webcam. The reflectance evolution of a region of interest (ROI) on the sample plate is calculated as follow:

reflectance = (ROI – black target)/(white target – black target)

The reflectance serves as a proxy for ice mass removal. For each image the reflectance is linked to the corresponding shear velocity. In most of the cases performed, the reflectance is constant until a certain wind speed and then decreases. To determine the threshold shear velocity u_th, we set the threshold reflectance at 10% decrease from the first image at u_* = 0 m.s^-1.

Results and conclusion: We have performed for the 1^st time experiments of ice particles transportation at low pressure in a planetary wind tunnel. The averaged threshold shear velocity obtained at 1000 mbar, u_th = 0.4 m.s^-1, is consistent with theoretical and experimental calculation of ice/snow at terrestrial condition [14, 15], from 0.3 m.s^-1 to 0.6 m.s^-1 for range of ice particles sizes selected, supporting our set-up reliability. The shear velocity increases significantly as the pressure decreases. The influence of the ice grain sizes is not clear and more experiments are required. The results should then be scaled to Martian gravity in order to compare the results to wind speed simulations and conclude about the likeliness of transport of ice particles by wind at the surface of Mars. 

References: [1] Das I. et al. (2013) Nature Geoscience, 6, 367-371. [2] Howard A. D. (2000) Icarus, 144, 267-288. [3] Cantor B. A. et al. (2010) Icarus, 208, 61-81. [4] Bridges B. A. et al. (2012) Geology, 40, 31-34. [5] Smith I. B. and Holt J. (2010) Nature, 465, 450-453. [6] Herny C. et al. (2014) EPSL, 403, 56-66.  [7] Pankine A. A. et al. (2010) Icarus, 210, 5871. [8] Langevin Y. et al. (2005) Science, 307, 1584-6. [9] Appéré T. et al. (2011) JGR : Planets, 116, E05001. [10] Spiga A. et al. (2017) Nature Geoscience, 10, 652-657. [11] Spiga A. et al. (2011) PSS, 59, 915-922. [12] Holstein-Rathlou C. et al. (2014) Am. Met. Society, 31, 447-457. [13] Pommerol A. et al. (2019) Space Sci. Rev., 215. [14] Shao Y. and Lu H. (2000) JGR, 105, 437-443. [15] Clifton A. et al. (2006) JoG, 52, 585-596. [16] Herny C. et al. (2016) 6^th MPSC, Abstract #6075. [17] Bordiec M. et al. (2018) ICAR X.

Acknowledgements: This work has been fund by Europlanet (Europlanet 2020 RI has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 654208). This work has been supported by the University of Bern. This work has been carried out within the framework of the NCCR PlanetS supported by the Swiss National Science Foundation.