Why does Mars have a dusty atmosphere?
The Martian atmosphere is rather thin, but fine (dust) particles still remain in suspension for a very long time, not to mention the famous Martian dust storms. So, how come dust particles do not settle faster on Mars?
It is true that similarly sized dust will settle from the thinner Martian atmosphere sooner than it would on Earth. For example, the dust suspended by the 2001 global dust storms on Mars only remained in the Martian atmosphere for 0.6 years, while the dust from Mt. Pinatubo took about 2 years to settle. However, we shouldn’t forget that even the 2001 global dust storms on Mars moved only the equivalent of a very thin dust layer – about 3µm thick if deposited with uniform thickness between 58 degrees N and S of the equator.
The difference in the concentration of dust in Earth's atmosphere and that of Mars stems from a key factor: On Earth, dust that leaves atmospheric suspension usually gets aggregated into larger particles through the action of soil moisture or gets suspended in oceanic waters. It helps that most of earth's surface is covered by liquid water. Neither process occurs on Mars, leaving deposited dust available for suspension back into the Martian atmosphere. In fact, the composition of Martian atmospheric dust – very similar to surface dust – as observed by the Mars Global Surveyor Thermal Emission Spectrometer, may be volumetrically dominated by composites of plagioclase feldspar and zeolite which can be mechanically derived from Martian basaltic rocks without chemical alteration. Observations of the Mars Exploration Rovers’ magnetic dust traps suggest that about 45% of the Fe in atmospheric dust is maximally (3+) oxidized and that nearly half exists in titanomagnetite, both consistent with mechanical derivation of dust with aqueous alteration limited to just thin films of water. Collectively, these observations support the absence of water-driven dust aggregation processes on Mars. Furthermore, wind activity dominates the surface of Mars at present, and the abundant dune fields of Mars can easily yield particles into atmospheric suspension through effects such as larger grains disaggregating fine particles through collisions.
The Martian atmospheric dust particles are generally 3µm in diameter. It is important to note that while the atmosphere of Mars is thinner, Mars also has a lower gravitational acceleration, so the size of particles that will remain in suspension cannot be estimated with atmospheric thickness alone. Electrostatic and Van der Waal’s forces acting among fine particles introduce additional complexities to calculations. Rigorous modeling of all relevant variables suggests that 3µm diameter particles can remain in suspension indefinitely at most wind speeds, while particles as large as 20µm diameter can enter suspension from rest at surface wind turbulence as low as 2ms^-1 or remain in suspension at 0.8ms^-1. Incidentally, dust deposition at the two rover sites has proceeded at a rate of about the thickness of a grain every 100 sols.
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