Abrasion of windblown particles on Mars—Erosion of quartz and basaltic sand under simulated Martian conditions

Abstract

With the advent of the Viking Orbiters and Landers on Mars, a great deal of information has become available concerning eolian erosion on that planet. The surface textures or roughness of sand-, silt-, and clay-sized particles is one of the parameters which should lead to an understanding of this process. In order to study the effects of this parameter, a series of laboratory experiments was initiated to simulate Martian eolian erosion. To determine the validity of simulating eolian processes in the laboratory, quartz sand was artificially abraded and compared with eolian sand from deserts on Earth via scanning electron microscopy; similar surface textures were present on both the natural and laboratory specimens. Experiments were then conducted under Martian atmospheric pressures and compared to natural eolian sand produced on Earth. With all other parameters held equal (e.g., impact velocity, duration of erosion), the less dense atmosphere on Mars resulted in more energetic eolian erosion, manifested by a slightly higher rate of grain rounding and surface textures that included semicircular depressions termed “popouts.” Glassy basalt sand-sized particles were then abraded for various lengths of time at pressures and velocities corresponding to conditions characteristic of Earth and Mars; surface textures were again examined with the scanning electron microscope. The features observed varied mostly as a function of velocity. Textures at low velocities were similar to those found on eolian quartz sand grains from hot Earth deserts; however, at velocities above about 20 m sec −1, what appeared to be either plastic deformation or melting was observed. Micron-sized particles of glassy basalt were studied before and after abrasion; the “before” particles were angular and contained irregular surfaces whereas those examined after abrasion at 75 m sec −1 were rounded and contained rather smooth surfaces. Because the basalt particles were partially amorphous before abrasion and included different mineral species, it was decided to study a single mineral, quartz, with selected area electron diffraction. Micron sized quartz was compared with the same-sized particles after abrasion at 40 m sec −1. Prior to abrasion, the quartz particles consisted of undeformed single crystals whereas a good deal of structural disruption was observed afterwards, perhaps due to plastic deformation and/or melting. If this is a valid consideration for Mars, then physical and chemical weathering may proceed more rapidly on Mars than on Earth, given a sufficient supply of water vapor. Clay mineral formation should be facilitated on that planet by the presence of large amounts of disrupted material, again assuming some water vapor. In addition, the disrupted material could increase the ability of the soil to act as a reservoir for water provisionally explaining the large amount of bound water in the surface soil material over much of Mars.