Dr. Robert Duncan-Enzmann
|Image of Earth dust from: http://entropymag.org/on-dust/|
Dust enters the atmosphere of Mars from outside the planet as meteorite debris and from the surface of Mars due to atmospheric forces. In the lower 10,000 feet of the atmosphere the suspended dust is almost exclusively of surface origin. Dust is subtracted from the atmosphere of Mars by gravitational forces. The removal of suspended matter from the atmosphere to form surface deposits is accelerated by: 1) surface friction; 2) coagulation of dust particles; 3) vapor condensation on dust; and 4) adhesion of dust to ice (H20?) crystals.
The spread of suspended dust through the atmosphere of Mars is limited to several factors: 1) the balance of forces tending to add and subtract dust described in the above paragraph; 2) vertical extent and size of turbulences in the immediate ground environment; 3) vertical extent and size of turbulences above the immediate ground environment; 4) mean wind velocities; 5) adherence of particles to the surface at contact and as a function of time. Turbulence, which may be measured by the size of eddies or vortices, dominates transport and mixing of dust in an atmosphere. Instead of molecular kinetics, as expressed by molecular viscosity, conductivity, and diffusivity, the spread of particles is described on the basis of eddy viscosity, eddy conductivity, and eddy diffusivity.
To estimate the amount of dust in the atmosphere, it is necessary to know the factors outlined above and also something about the surface. For strong mixing to occur between the immediate ground level and higher levels it is necessary to have major turbulence, as introduced by topographic interference. Source and sink areas should also be considered. In the Earth environment sources are land areas and sinks are oceans. In the Mars environment there seems to be nothing like standing bodies of water. Adherence of particles to the surface may be enhanced by polar cap materials; adherence of particles may be enhanced by dark areas which could represent moisture. Generally conditions seem to be those of a cold, relatively flat desert with little interstitial water at the surface. Such an environment would parallel the worst desert conditions of Earth, with little tendency for particles to adhere to each other over relatively vast areas.
Using the Kaplan atmosphere and estimates made by Vaucouleurs, dust per unit volume of atmosphere in the immediate surface environment and also through the troposphere should be within one order of magnitude of terrestrial conditions.
In conclusion, although total dust content per unit volume of the Mars atmosphere may approximate or even exceed that of the Earth’s atmosphere, it should not cause any difficulties in communications or erosion. The average wind velocity is low, perhaps 20 mph average in the Trade Wind Belt, precluding significant wind erosion in anything but geological times. The low wind velocity also suggests a low frequency for static noise due to dust impacting on uninsulated antenna.
(Stay tuned for Mars Dust Particles, part 2: Effects of Suspended Dust)