Effects of rolling on wind-induced detachment thresholds of volcanic glass on Mars

Dunes in the northern lowlands on planet Mars are composed of volcanic sands with high contents of volcanic glass and these deposits are mobilised and transported by winds in the present-day surface environment. In this experimental study we measured fluid thresholds for detachment of Mars-analogue volcanic glass particles using a low-fluid density wind tunnel under wind shear stresses in the range of 0.1-0.6 N m(-2). Measured thresholds for larger particle diameters (> 150 mu m) were best-explained using a semi-empirical model of detachment which incorporates the effects of drag-induced rolling. Fitting of this semi-empirical model to obtained experimental data, combined with a residual analysis of the model fit with quantified particle properties made it possible to assess the sensitivity and validity of the model for predicting this type of particle detachment. This new model was used to predict the threshold shear stress for detachment of particles on Mars and indicated that larger particle diameters can detach by drag-induced rolling when subjected to present-day surface wind shears. A large morphological and granulometric variety of particles is therefore susceptible to this form of detachment, which provides a possible mechanism for the initial mobilisation of particles at lower wind speeds than required for detachment by saltation. Recent sand mobility may therefore have benefited from rolling as a contributing or as a saltation triggering process. (C) 2014 Elsevier Ltd. All rights reserved.