JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. E1, 10.1029/2000JE001481, 2002
[49] On Earth, actively forming ventifacts are found in environments characterized by a supply of abradant (usually sand), sparse vegetation, strong winds, and topography that allows the free sweep of wind or that locally accelerates airflow. Ventifacts can form in a wide variety of rock types, including basalt, granite, marble, and limestone. Polishing and smoothing of rock surfaces occur in all lithologies.
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[50] The size of the original rock is an important control on the form of ventifacts. Classic faceted types are described as einkanter, zweikanter, and dreikanter for one-, two-, and three-ridged forms in fine-grained rocks (Figure 14). Surface features can include pits, flutes, grooves, or helical forms. The preservation of ventifacts varies with climate, rock type (with carbonate ventifacts being poorly preserved owing to chemical weathering), granular disintegration, and exfoliation. Ventifacts are generally well preserved in basaltic rocks.
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[51] Ventifact formation depends on the wind regime, the susceptibility of the rock to erosion, and the properties of the impacting particles (e.g., size, density, speed, and angle of incidence) [Greeley et al., 1982, 1985]. In general, saltating particles traveling at greater heights have faster velocities, owing to an increase in wind speed above the ground and longer saltation paths that allow more time for acceleration by the wind [Anderson and Hallet, 1986]. Thus, within a general zone of abrasion, erosion profiles develop with distinct maxima of mass removal (Figure 15), the height of which is determined by wind speed, sand flux, and saltation trajectory. As a consequence, many ventifacts develop semiplanar faces, with the upper part of the abrasion face receding more rapidly than the lower part. The height of maximum abrasion appears to be ~0.10 m above the surface on level terrain but may exceed 1.5 m on hillcrests or where local topography enhances wind velocity [Laity, 1995].
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[52] The angle of incidence is also important in ventifact formation and is a function of the particle trajectory and the incidence angle of the rock surface being struck. Although the exact values differ for brittle and ductile targets, in general steep incident angles of ~90° and grazing, low-angle impacts are less efficient than intermediate angle impacts, in which gouging increases the efficiency. This is demonstrated by Figure 16, which shows a metal plate around a power pole which has been abraded by sand into a pattern reflective of the incident angle. Controlled experiments show similar results for common rocks, which appear to behave as a combination of ductile and brittle materials [Greeley et al., 1982].
[53] The sizes of abrasion features, such as flutes and grooves, are also related to wind velocity and are subject to the same enhancement effects of geometry and topography described above. Thus these features are largest near the upper surfaces of very large ventifacts, especially those located on hillcrests and in swales. Under conditions of high wind velocities and ample sand supply, grooves in the Mojave Desert can exceed 100 cm long, 23 cm wide, and 13 cm deep. The depth of flutes and grooves is consistently less than their width.
3.2. Ventifacts at the Mars Landing Sites
[54] The surfaces of all three landing sites have abundant rocks, many of which show ventifact features [Bridges et al., 1999]. Rocks at each site exhibit differences, as a class, from rocks at the other sites. For example, rocks at VL 2 are generally pitted and have a “spongy” appearance [Viking Lander Team, 1978]. Rocks at VL 1 are also pitted, but commonly without the “spongy” texture. Rock textures at the MPF site include flutes, grooves, crusts, and knobby surfaces similar in appearance to sedimentary conglomerates [Rover Team, 1997; Bridges et al., 1999; Greeley et al., 1999; McSween et al., 1999; Moore et al., 1999]. Rocks at the MPF generally have rough surfaces, although some exhibit strongly forward-scattering photometric behavior (Figure 13), consistent with a varnished or polished surface [Johnson et al., 1999]. Rock textures at the MPF site probably include both primary and secondary features.
3.3. Ventifact Analog Comparisons
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[55] The primary shapes of Martian rocks probably reflect the original processes that emplaced them, including impact, fluvial, and volcanic events. However, some rocks at the MPF site have facets and sills suggesting modification by the wind and periods when the rocks might have been buried or partly buried by soils. For example, sills at the bases of some rocks suggest shielding from abrasion (Figure 17), and re-exposure by deflation [Greeley et al., 1999; Golombek and Bridges, 2000]. This interpretation is supported by the observation of the facets and erosional grooves. As noted by Bridges et al. [1999], erosional grooves on many of the rocks at the MPF site are found mostly on the upper parts of the rocks. We suggest that the lower parts of these rocks could have been buried, preventing the formation of well-defined facets, while the upper parts were exposed, enabling the development of the grooves. As discussed by Greeley et al. [2000], the surface at MPF was probably swept by duneforms repeatedly, with rocks buried, partly buried, exposed, and re-exposed; this would lead to highly variable wind abrasion on the exposed rock surfaces and inhibit the formation of well-developed facets.
[56] About half of the rocks at the MPF site exhibit elongated pits, flutes, and grooves (Figure 17). Similar to terrestrial ventifacts, pits are generally found on high-angle faces, while elongated pits and flutes occur on inclined upper surfaces, and the grooves are found on the upper surfaces of rocks. This is consistent with wind abrasion by generally unidirectional winds, in which the steep windward faces of the rocks are pitted by near-perpendicular collisions of saltating particles and the upper parts of the rock are gouged by grains hitting at shallower angles, forming grooves. Although these general relationships are similar for Martian and terrestrial ventifacts, there are notable differences in the morphology of specific features. Martian grooves generally have length to width ratios <4, with most being <2 [Bridges et al., 1999]. In contrast, grooves in the Mojave have ratios generally >5. Although the depths of Martian grooves are difficult to measure because of low image resolution, they appear to be shallower than many terrestrial grooves.
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[57] Pits are seen in rocks at all three landing sites (Figure 17). On Earth, pits form in the initial stages of wind abrasion on rock faces (Figures 18a, 18b, and 18c). Primary pits, such as vesicles, are often preferentially eroded by the wind, resulting in larger pits [McCauley et al., 1979]. The association of pits with flutes and grooves on many Martian rocks (e.g., Figures 18d and 18e) suggests that at least some pits result from wind abrasion. The fraction of all pits that form from abrasion versus other processes is, however, unknown.
[58] Some rocks in terrestrial ventifact environments display an outer crust consisting of a fluted fabric. The terrestrial example shown in Figure 18a is very similar in appearance to Chimp, Stimpy, and other rocks at the MPF site. The thickness of the crust on the Martian rocks is about equal to the penetration depth of the flutes. The surfaces of many of the Martian rocks, including the exposed substrate where the crust has spalled, are covered with a much finer fluted texture. This suggests a sequence of (1) intense fluting, (2) formation of crust and spallation, and (3) minor fluting. The large, outer flutes might have acted as repositories that trapped sand and dust.
[59] On Earth, weathering of feldspars into clays, nucleation of water onto trapped grains, or other chemical reactions could lead to the formation of rock crusts. Similar processes might have acted to form crusts at the MPF site. The formation of crusts might also have occurred during periods of burial, when chemical reactions could have occurred between the rock surfaces and the soils.
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[60] On Earth, differential erosion occurs on rocks as a function of resistance to abrasion. Softer materials in rocks are preferentially eroded, leaving the more resistant materials standing out in relief. For example, Figure 19a shows a basalt flow containing an ultramafic xenolith surrounded by a depression. Over time, wind has eroded the softer matrix surrounding the xenolith. Similarly, volcanoclastic rocks can contain rock fragments of different materials, as shown in Figure 19c. Similar relationships could explain the putative “conglomeritic” textures seen at the MPF site [Rover Team, 1997; Greeley et al., 1999; Moore et al., 1999], such as on the rock Squash (Figure 19b).
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[61] Compositional or textural bands can also result in differential erosion (Figures 20 and 21). Figure 20a shows a terrestrial basalt consisting of flow bands that are vesicle-rich, vesicle-poor, and feldspar-rich. The vesicle-rich bands have evolved into troughs, and the vesicle-poor zones have formed ridges. The vesicles act as sites preferentially abraded by windblown grains, whereas less erosion occurs where vesicles are scarce and there are more feldspar grains. This suggests that the apparent layering observed in some rocks at the MPF site [Parker, 1998; McSween et al., 1999] could be flow-banded lavas etched by wind abrasion. In some cases on Earth, aeolian flutes crosscut flow bands in volcanic rock (Figure 21a). Possible orthogonal linear features on rocks at MPF could be similar flutes superimposed on flow bands (Figure 21b).
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[62] Terrestrial rocks can become polished by the wind. Some rocks at the VL and MPF sites have specular reflections [Guinness et al., 1997; Johnson et al., 1999] (Figure 22), suggesting wind polish.
[63] Many of the best-developed ventifacts at the MPF site (Half Dome, Moe, Stimpy) are found near the crests of low ridges. This occurrence is consistent with terrestrial examples in which the most active ventifact formation is found on slopes where windflow is accelerated.
Citation: Terrestrial analogs to wind-related features at the Viking and Pathfinder landing sites on Mars, J. Geophys. Res., 107(E1), 10.1029/2000JE001481, 2002.
