# How much atmospheric pressure (CO2) is required to carry iron oxide dust in the wind?

It is common knowledge that there are dust devils and dust storms on Mars. But can we demonstrate that the atmospheric pressure on Mars, which is 0.6% of the pressure we experience on Earth, provides enough atmospheric mass to be able to suspend solid particles in the air for long periods of time, resisting the forces of gravity and natural buoyancy?

It is easy to assume the atmosphere of Mars is responsible for the dust devils and dust storms, but I'm concerned it may not be possible for such a thin atmosphere to produce these forces and effects, and I believe it may be necessary to pursue other possible causes for the dust devils and dust storms, such as static or other electromagnetic events.

NASA has shown that Martian dust is incredibly reactive to any type of electrical or magnetic forces, such as perhaps the solar wind or other forms of magnetic influence from the sun, Earth, or even Jupiter.
Martian dust is primarily iron oxide after all, therefore strong electromagnetic interaction should be expected.

Here is a good anecdote, citing observations of Mars from the Viking spacecraft:

"... the wind speeds picked up considerably—indeed, within only an hour of the storm's arrival they had increased to 17 m/s (61 km/h), with gusts up to 26 m/s (94 km/h). Nevertheless, no actual transport of material was observed at either site, only a gradual brightening and loss of contrast of the surface material as dust settled onto it."

The main question is, can we demonstrate whether or not the ultra-thin CO2 atmosphere of Mars actually has the physical ability to pull dust into the air, without the aid of other forces, under normal or even high wind velocities?

Update:
I read through some of the literature provided by Russell McMahon in his excellent response to this question. (Thank you!)
I'd like to provide some feedback, but I won't be able to fit all of this into a comment, so I'll put it up here in the question.

I started off reading the Space.com article Russell mentioned, and right off the bat I started seeing suspicious language:

"The new images from the Mars Reconnaissance Orbiter show wind-blown sand dunes moving across the Martian surface, sometimes up to several yards at a time, scientists said."

"'Mars either has more gusts of wind than we knew about before, or the winds are capable of transporting more sand,' explained planetary scientist Nathan Bridges of the Johns Hopkins University's Applied Physics Laboratory in Laurel, Md., who led the new study on the shifty sand dunes on Mars. 'We used to think of the sand on Mars as relatively immobile, so these new observations are changing our whole perspective.'"

I say this is suspicious language, because it sounds like circular or otherwise flawed logic:

1. We know the atmospheric pressure, and the chemical composition of both the atmosphere and the Martian surface.
2. We've observed and measured wind speeds, and determined that the wind should not be able to move sand dunes, presumably based on equations such as the classic drag equation etc.
3. We then observe that sand dunes do move quite rapidly, despite the lack of sufficient wind composition.
4. Therefore, the Martian wind must be able to move sand really easily, despite our earlier observations that the Martian wind really can't move much of anything at all.
5. Therefore, maybe the particles bounce off the ground after they get carried into the wind, causing chain reactions over hundreds of millions of years etc. etc.

The problem with this logic, is where they say, "Martian sand dunes are moving rapidly, therefore the Martian wind must be able to rapidly move sand dunes, even though the Martian wind shouldn't be able to move sand dunes at all based on our understanding of wind drag."

For all we know, the Martian winds have little or nothing to do with sand moving around the planet.

Our previous observations and measurements of the atmospheric conditions on Mars support the idea that the wind cannot be much of a factor, even over cosmic time periods.

After all, we see dust storms begin within a period of hours/days, cover the planet for up to a month, then subside again over a period of hours/days and return to normal clear weather patterns - clearly this doesn't take hundreds of millions of years to build up a chain reaction of dust in the wind.

All the talk of dust particles bouncing on the ground and starting chain reactions sounds like faulty speculation, since it is based on the idea that there is not enough gravity to keep the dust on the ground, but somehow plenty of gravity when the dust eventually slams back into the ground with enough force to send several more dust particles back into the air. This sounds like a perpetual motion machine, i.e. a violation of the law of energy conservation.

As a scientific community, I feel like we need to acknowledge the fact that other forces must be in play, electromagnetic forces in particular. Any wind-based explanation we come up with just seems to sound like nonsense, and the dust storms and dust devils look eerily similar to various formations of electrified plasma filaments that can be reproduced in labs.

What do you think, Russell?
And what do you think, StackExchange?

• Rushing past .... . Lots of "maybes" here. I don't know, but: Electromagnetic seems unlikely (but I don't know). Electrostatics may help - very dry climate - some very fine grains likely. You (may) get charging of dust from triboelectric effects which aids it's ability to separate from adjacent particles and be lofted by the wind. | A (possibly) important aspect is that square-cube law applies. Mass increases with radius (or side) cubed but area with radius squared. When a drag force is exerted the force per mass decreases with increasing radius - and equally increases as particle size falls. – Russell McMahon Mar 19 '15 at 21:55
• This leads to the common sense observation that fine dust is more liable to be blown by wind - larger grains are "too heavy" even though made of the same density material. Jump chuteless from an aircraft at altitude and you die. Drop a Fieldmouse or an ant and they live (impact wise anyway). For both square-cubed law saves them. Scale and ant to human size and it would destroy itself mechanically. | Must fly (metaphorically) - more anon maybe. – Russell McMahon Mar 19 '15 at 21:58

The force exerted by an airflow is given to a good first approximation by the classic drag equation:

Force = 0.5 x Rho x Cd x A x V^2

Rho = gas density
Cd = drag coefficient wrt flat plate drag ( 0 < Cd <= 1)
A = projected area relative to flow
V = velocity

Mainly opinion: I am unaware of the particle size of Martian dust, but this information will be available, and the absence of substantial surface water for 'rather a long while' is liable to have led to a substantial amount of small particles due to wind driven abrasion. Even very minor disturbances of larger particles will lead to major "milling" after a few (hundred) million years.

Mainly fact: Mars "wind" forces will be altered relative to earth's by lower density due to reduced atmospheric pressure, higher density per pressure due to CO2 and generally increased velocities. For known velocities, such as those cited in your example, it will be possible to calculate the resultant force per area compared to those from typical terran winds.

Wind is almost wholly thermally driven. Martian insolation is well known and models of Martian wind speeds will be available.

In older literature Martian 'wind' speeds of hundreds of km/h have been suggested. As force rises with velocity squared the low pressure CO2 atmosphere needs ~~= 10 x terran wind speed to exert the same force per area. So a 200 km/h Martian wind would exert about the same force per area as a 20 kph terran wind. Without checking references that feels to be on the low side for substantial disturbance of dust but enough to cause eddies and some dust movement. Increase that to 300 kph and you probably have a dust storm. Especially so if the "fines" are as fine as you'd expect from the environment.

However, such speeds are above those so far reported.
So, see below -

Good discussion of mechanisms and magnitude - quotes from Nathan Bridges, a planetary scientist with Johns Hopkins University and Jasper Kok, with the Earth and Atmospheric Sciences department at Cornell University.

• Scientists suspect it takes a big wind to get sand particles airborne, but once launched from the surface, they bounce around with ease, thanks to the planet's thin atmosphere and low gravity.

"It's kind of like playing golf on the moon -- (the sand) goes really high and far compared to what it does on Earth. When it lands it can pick up really large speeds -- even with low wind speeds -- and splash a whole bunch of other particles to keep the process going,"

Once sand gets moving on Mars, wind speeds can drop by a factor of 10 and still be strong enough to transport as much sand as what moves around many places on Earth, the new study shows.

and

• "We show a mechanism where this blowing sand can actually erode rock, can erode landscapes. It can be a fairly active process in the current environment," Bridges told Discovery News.

"Up to a few years ago, it was uncertain how much the sand dunes were moving on Mars, or even if they were moving at all," Bridges added.

Good populist level discussion suggests that wind based sand movement is greater than had been expected.

Space.com - Good discussion here - 130 km/h said to be required for sand movement.

• Thank you so much for this fabulous detailed response! I'm enjoying reading through the material. I've provided some lengthy initial feedback, in an edit to the question (much too long for a comment). Let me know what you think. – Giffyguy Mar 19 '15 at 3:01
• I would guess that for dust grains the drag is proportional to $v$ not $v^2$. The change from $v$ to $v^2$ occurs when the flow becomes turbulent, and I would guess that the velocity of the dust grain relative to the atmosphere is small enough for the air flow to be laminar. – John Rennie Mar 19 '15 at 10:04
• @JohnRennie If the delta-Vis small then the grain is presumably going with the flow - mission accomplished. If the particles are stationary and waiting to be perturbed then even if only some see turbulent flow, it's potentially enough. For laminar flow the assumption of Cd=1 is probably poor (almost by definition) and modelling "gets hard. – Russell McMahon Mar 20 '15 at 11:10