THE ROLE OF ROCK STRENGTH IN VENTIFACT FORMATION ON EARTH AND MARS

A topic of long-standing interest to Nathan Bridges was the interaction of planetary atmospheres and surfaces. Key features that result from these interactions are bedforms, which are accumulations of wind-mobilized sediment, and ventifacts, which are erosional scour marks on rocks resulting from the passage of this sediment. Ventifacts are important recorders of the effects of wind-driven sand since they can capture the effects of both modern and paleo winds. Nathan and colleagues studied these features using observational, theoretical, experimental, and analog methods.

Although ventifacts can form in almost any rock type given a sufficient supply of sand and wind velocities needed for saltation, they are perhaps best expressed in aphanitic rocks, e.g., basalts, limestones, or tuff deposits. What is the role of rock strength in the rate of ventifact formation? Experimental and terrestrial field studies indicate that ventifacts develop more readily in weaker rocks. This leads us to the related question of how strong are the rocks on Mars? We attempted to address this question using engineering data from the Rock Abrasion Tool (RAT) on the Mars Exploration Rovers in Gusev crater and Meridiani Planum. Results indicate that basalts in Gusev are weaker than fresh terrestrial basalts [Thomson et al., 2013], suggesting that they have experienced significant weathering-induced weakening. Drill data from the Mars Science Laboratory of sedimentary layers in Gale crater also suggests low rock strength values [Peters et al., 2018], a factor which would permit more rapid ventifact development. The ability of the surface rovers to directly measure exposure ages [Farley et al., 2014] permit better temporal constraints on ventifact development rates in the future.

Farley, K. A and 33 others (2014), In situ radiometric and exposure age dating of the Martian surface, Science, 343(6169), 1247166.

Peters, G. H. and 10 others (2018), Uniaxial compressive strengths of rocks drilled at Gale crater, Mars, GRL, 45, 108-116.

Thomson, B. J., Bridges, N. T., Cohen, J., Hurowitz, J. A., Lennon, A., Paulsen, G., and Zacny, K. (2013) Estimating rock compressive strength from Rock Abrasion Tool (RAT) grinds, JGR, 118, 1233-1244.