Post by Nathan Bridges.
Definition and Significance of Ventifacts
Ventifacts are rocks abraded by windblown particles, generally or exclusively sand (Laity and Bridges, 2009). On Earth, they are found mostly in arid regions with little vegetation, a fairly abundant supply of sand (or an ancient supply for relict ventifacts), and winds capable of exceeding the threshold speed necessary for sand movement. Their form depends on original rock texture, shape, and composition, with common forms including facets sloping into the wind and wind-aligned elongated pits, flutes, and grooves. The direction of facet dip slopes and the long axes of textures serve as proxies for the predominant highest speed winds that carried the sand and thereby serve as paleowind indicators. It is common for ventifact textures to result from mineralogical or petrological hardness variations in the rock or from primary textures such as vesicles.
Ventifacts on Earth
As an example for Earth, we focus on three rock types that this author has studied in the Mojave Desert: Limestone, basalt, and diorite (Image 1). The linear fabric of marble, from the Little Cowhole Mountains, precludes the formation of abrasion pits. Abraded limestone textures therefore represent an endmember case in which hardness heterogeneities are distributed linearly, as opposed to point-to-point. The Cady Mountain basalts contain vesicles, identified as such because faces shielded from the wind contain pits. It seems that the vesicles serve as nucleation sites for abrasion and develop into flutes over time. These rocks are aphanitic and do not have hardness variations at a scale larger than an impacting sand grain (> 100 µm). The diorites at Silver Lake are another class. These rocks lack vesicles, but at the millimeter scale exhibit a pit- and mound-like texture that is probably due to differential mechanical weathering of the minerals. These may then develop into flutes. There are therefore 3 end member cases in which pitting may or may not play a role in rock abrasion: 1) Rocks with linear (not point-to-point) hardness variations and no primary pitted texture (Cowhole limestone), 2) Rocks with no hardness variations but primary textural heterogeneity in the form of vesicles (Cady basalt), 3) Rocks with point-to-point hardness variations and no primary pitted texture (Silver Lake diorite).
Ventifacts on Mars
Many rocks observed by the Viking and Pathfinder landers and MER rovers exhibit characteristics of ventifacts with various textures and facets. The finding of these rocks is not surprising given that the kinetic energies of saltating particles is predicted to be comparable or higher on Mars than on Earth [Bridges et al., 2005]. For example, rocks on the Gusev plains at MER Spirit site exhibit large scalloped or fluted upper edges and pits that are almost certainly abrasion features (Image 2). Compelling small flutes and grooves are seen on many other rocks (Image 3). Some of the best examples at the Spirit site are on and near the summit of the Columbia Hills [Thomson et al., 2008] (Image 4a, b, d-g) where topographic funneling and acceleration of wind flow has resulted in effective sand abrasion. At the Opportunity landing site, hard hematite concretions (“blueberries”) are more resistant to abrasion than the sulfate-rich rock matrix, thereby projecting into the windward direction [Sullivan et al., 2005] (Image 4c). In addition to serving as records of the efficacy of abrasion, the wind directions derived from ventifacts on Mars have served as vanes for current and past local winds [Bridges et al., 1999; Sullivan et al., 2005; Greeley et al., 2008; Thomson et al., 2008].
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