Saltation Impact as a Means for Raising Dust on Mars By Ronald Greeley

Introduction Sand dunes and dust storms have been seen in spacecraft data images Winds of sufficient strength to entrain fine dust are rather infrequent. Numerous dust storms observed on Mars. Saltation (dynamic threshold) is an effective means under wind regimes that are insufficient to entrain particles.

Mars Orbiter Camera image of sand dunes in the north polar area of Mars. The dunes demonstrate sand- size particles on Mars.

Saltation impact on beds of dust Sand-sized grains were placed upwind from beds of dust. Winds were set 20% higher than threshold for sand saltation. The saltating grains then impacted the dust beds. Tests were done at a range of atmospheric pressures from Martian to Earth conditions.

Experiments Wind Tunnels ASU and NASA (MARSWIT) Both are open circuit atomospheric boundary layer wind tunnels suitable for simulating aeolian processes. ASU tunnel (1bar) at speeds of 40m/s. MARSWIT(1bar-3.5mbar) at speeds of 100 m/s.

Mixed particles Dust grains consisted of Xerox toner, which was about 8 um in diameter and had a density of about 1.0 g/cm³. Sand grains consisted of silica microspheres, which were 100 um in diameter and had a density of g/cm³. In all cases the wind velocity was set a few percent above threshold for the sand-size grains.

Results Sand-sized microspheres were set into saltation, at a wind speed slightly above threshold. After a few seconds sand grains on or near the surface were removed, leaving only the dust size grains, and particle movement seized. Increased wind speeds was not sufficient to entrain the dust.

A variety of materials were used under Mars Conditions. Sands used as saltation “triggers” included two sets of quartz particles, with average diameters of 200 and 500  m. These particles had threshold friction velocities of 2.0 to 3.0 m/s. The dust consisted of crushed basalt to a diameter of 16.6  m, and had a density of 2.83g/cm³ and a threshold friction of 5.5m/s. Experiments were also conducted using fly ash (14.8  m) in diameter. Coal-fired power plants and silicic pumice (13.5  m in diameter). There were no differences between fly ash and pumice observed.

Results g of dust is removed per gram of sand. Tests were conducted over a range of atmospheric pressures from Martian and Earth conditions. The trend suggests that the process might be more efficient of Mars under low atmospheric pressures than on Earth.

Three modes “Splashing” grains forming small craters and dislodging dust producing a diffuse cloud. Sand grains impacted the dust bed, some dust was dislodged as clumps or sheets which disaggregated into clouds of dust. Sand grains bounced off of the dust bed, leaving no imprint and failing to entrain dust.

Conclusions Saltation impact is known to inject particles into the wind under conditions which might other wise be inadequate for threshold. The greater mass of dust is set into motion as a function of impacting sand mass with lower atmospheric pressure. Martian conditions attributed to higher overall windspeeds required for saltation in the low-density atmosphere. Potential for injecting more dust into the atmosphere. Mixed sand and dust does not lead to significant dust entrainment. Saltation impact appear to be a viable mechanism for injecting dust into suspension on Mars when sands are upwind from dust beds.