Ventifact

Abstract: Erosion rates derived from the Gusev cratered plains and the erosion of weak sulfates by saltating sand at Meridiani Planum are so slow that they argue that the present dry and desiccating environment has persisted since the Early Hesperian. In contrast, sedimentary rocks at Meridiani formed in the presence of groundwater and occasional surface water, and many Columbia Hills rocks at Gusev underwent aqueous alteration during the Late Noachian, approximately coeval with a wide variety of geomorphic indicators that indicate a wetter and likely warmer environment. Two-toned rocks, elevated ventifacts, and perched and undercut rocks indicate localized deflation of the Gusev plains and deposition of an equivalent amount of sediment into craters to form hollows, suggesting average erosion rates of approx.0.03 nm/yr. Erosion of Hesperian craters, modification of Late Amazonian craters, and the concentration of hematite concretions in the soils of Meridiani yield slightly higher average erosion rates of 1-10 nm/yr in the Amazonian. These erosion rates are 2-5 orders of magnitude lower than the slowest continental denudation rates on Earth, indicating that liquid water was not an active erosional agent. Erosion rates for Meridiani just before deposition of the sulfate-rich sediments and other eroded Noachian areas are comparable with slow denudation rates on Earth that are dominated by liquid water. Available data suggest the climate change at the landing sites from wet and likely warm to dry and desiccating occurred sometime between the Late Noachian and the beginning of the Late Hesperian (3.7-3.5 Ga).

Abstract: Abrasion by eolian transport was studied in three different types of wind tunnel. Limestone, feldspar, and quartz were tested in the shape of cubes, crushed crystals, or natural grains. Abrasion increases with size, wind velocity, angularity and roughness of surface. Polished medium-sized quartz grains sustain no loss. Wind abrasion of quartz is 100 to 1,000 times more effective over the same distance than the mechanical action of a river. But eolian abrasion is slight for fine quartz sand and drops to zero at 0.05 mm. in diameter. No quartz particles are produced in the size range dominating in loess. Experimental abrasion of ventifacts indicates that these are cut slowly in scores of years by medium strong winds, but in a matter of days by storm winds, and in a matter of hours if the cutting sand is coarse and angular. These results must be looked upon as first approximations, for many factors in nature are either unknown or not reproduced in the experiments. Experiments on frosting and defrosting will ...

Abstract: The observation that the Mars Pathfinder landing site looks very similar to its appearance after it was deposited by catastrophic floods around 1.8-3.5 Ga allows quantitative constraints to be placed on the rate of change of the site since that time. The abundance of erosional features such as an exhumed former soil horizon, sculpted wind tails, ripplelike and other tag deposits, and ventifacts (fluted and grooved rocks) all suggest the site has undergone net deflation or loss of 3-7 cm of material. The presence of barchan dunes and ventifacts argues for erosion by saltating crystalline sand-size particles entrained in the wind. Most ventifacts probably formed soon after the catastrophic flood, which likely introduced a large, fresh supply of sand-size particles distributed across the rocky plain. The strongest winds blew toward the northwest during this time, resulting in the sculpting of ventifacts, deflation of the surface, collections of dunes within Big Crater and other lows, and possibly preferentially eroding small crater rims. The predominant wind direction changed to blow toward the southwest, similar to today. These winds further deflated the surface, completed the deposition of sand-size material in dunes and ultimately trapped these dunes in lows. The erosional features observed by Pathfinder indicate extremely low long-term deflation rates of 0.01-0.04 nm/yr since the end of the Hesperian (1.8-3.5 Ga) similar to less precise rates of <1 nm/yr based on the preservation of craters at the Viking 1 and Pathfinder landing sites. Short-term redistribution rates (deposition and removal) of atmospheric dust at the Pathfinder landing site and preexisting dust and sand at other locations on Mars are up to 10 5 nm/yr. Estimates of erosion rates on Mars show a rapid decrease by 3-6 orders of magnitude from 10 2 -10 4 nm/yr in Noachian terrains (characterized by rimless, flat-floored craters and valley networks) to the exceedingly slow rates (10 -2 -10 - nm/yr) operating during the Hesperian and Amazonian. Noachian erosion rates comparable to low continental denudation rates on Earth are consistent with erosion by running water and perhaps a more clement climate. The rapid decrease in erosion rates is consistent with a major climatic change during the Noachian, at the tail end of heavy bombardment, and a cold, dry, desiccating climate similar to today's since that time.