Wind

I have a general question about the wind on Mars. There has been much talk about wind erosion and ripples and dunes caused by wind.
Since the air pressure on Mars is only 1/100th of that on Earth, it would seem that it would take an enormous wind to move a grain of sand. Does anyone know what the wind speeds are like? We have seen dust devils, and "red outs" that stir things up pretty good.

Stan

Phoenix made some measurements which suggest wind speed is around 20 to 30 MPH up there. The core vortex of a Dust Devil will be very fast but short lived. Future missions will carry better equipment for climate studies.

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More here, I am not sure if I have got the speed right in reply 1 over to you Fred

http://www.planetary.brown.edu/m42/m42_29.pdf

One thing you have to understand is that there is a difference - a big one - between force and speed. Wind speeds may not be that different but the force of the wind is much, much lower than on Earth. That said, it doesn't take much force to move a grain of sand. Try a gentle blow with your own mouth. Doesn't take much force at all. One presumes however that the erosion process on Mars is much slower because the wind is much less forceful.

I'm now almost a 5 year veteran of this discussion, and in my opinion Mars science really doesn't yet know the details about how sand-sized or granule-sized particles may move in the wind. Dust moves all the time for sure. I think that the longstanding assumption that aeolian processes on Mars are much like those on Earth has not been confirmed by the MER results.

Kye -

Don't know why you say that. There is evidence everywhere - dust storms,holes in rocks, weathered rocks, dunes etc.

What precisely do you say has not been observed on Mars? This is a simple matter of the laws of physics. There is no way that wind blow particles could NOT erode.

field, re your 5, I didn't say that Mars has no aeolian processes, or that the laws of physics are different on Mars. When we understand the details, Mars' aeolian processes will be considered very different from Earth's. Take the big plains ripples at Meridiani for example - they're heavily crusted and dotted with impact craters. Chances are they're something like 1-3 million years old:

http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1973.pdf

I'm just realizing that almost all of the types of ripple that the MERs have seen DO NOT MOVE downwind. The big Meridiani ripples are ancient and sometimes include cobbles, the smaller cross-ripples are often preferentially situated where they incorporate rocks which means they are probably formed around rocks and are "anchored" to rocks. This is also happens a lot with smaller ripples at Gusev. The many ripple sets in craters at Meridiani are clearly not going anywhere, ditto Eldorado and the other ripple patches on the Columbia Hills which, like the crater ripples, seem to depend on topography, particular slopes with the right orientation, for their existence. (Sorry for the undocumented outburst.)

Dunes, No. The MERs haven't imaged anything properly called a dune as I understand the term, because we have seen no slip-faces where sand has avalanched on a downwind slope. Some ripples might once have been dunes, but if they were, the evidence of slip-faces has been erased. There are aeolian forms visible from orbit that appear to have slip-faces, but who knows what they'll turn out to be on the ground. The MERs have seen one recent little soil avalanche near Larry's Lookout in the Columbia Hills, which is likely the collapse of a wind-loaded slope, and it was an avalanche of mostly small soil crust chunks.

http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1973.pdf

Good question Stan. Because the atmospheric pressure on Mars is so low the kinetic energy for a given wind speed in m/s is around an order of magnitude less than on Earth. So the 30 mph measured by Kevin equals 13.4/10 or 1.34 m/s equivalent on earth. Not much force there. But Mars has a lower value for a (or g) of around 1.6 m/s I think. So this reduces the number of Newtons needed to overcome friction and move a particle.

The science returned by the MER and from orbit seems to point to a wetter and far more violent past for Mars. Those comfortable with a steady state universe will point to a Mars static since creation napart from impact surge, despite the clear evidence of river deltas and the constant stream of MER science results that contradict this scenario, at least for Meridiani. My version. The sand supply reduces, the ripples shrink and then freeze as the atmospheric pressure dropped to a vacuum billions of years ago.

The erosion patterns on the Victoria cliffs are an indication of what occurred and you might like to have a close look ar some of the Duck Bay shots in particular. Oppy has been roaming for years and it can be real fun putting all the pieces of the jigsaw together.

I wish I was better at finding old posts but I am not however I do remember Hort posting a sequence from Spirit I think during the storms a couple of years back and it clearly showed the soil being blown around the Rover.

Dust Devils leave dark trails that eventually dissapear when lighter material more red coloured covers up the track. Spirit went back over some old tracks and various changes were noticed on some of them.

Dunes exist but move very slowly

" >[link]

Kevin, re your 8, Horton created animations of dust storm changes at Gusev. Go to any of Horton's images and use the 'Search' function with the terms Gusev dust storm. These are well worth a long careful study. There are also navcam images from the same period worth considering, Before:

And After:

Kye -

I accept that the patterns of wind erosion may differ from Earth. Depends what you mean by "much like".

From a lay person's point of view, clearly there has been erosion similar to that on Earth, revealing sedimentary rocks, or so we are told by NASA, and that appears to be the case from photos.

I accept however that a wind erosion expert will no doubt see telling differences between processes on Earth and processes on Mars, related fundamentally to the thin atmosphere, movement on axis, difference in length of seasons, nature of particles etc.

Thanks Kye

According to Serpens' estimates, it would appear that a 30 mph wind on Mars has about the same ability to directly move surface particles as a 10 mph wind on the Earth.

I'm wondering if there could be an indirect effect where the wind directly moves dust particles that in turn could push larger particles?

Another effect could be where larger particles are thrown up by an impact, and then could be moved by the wind since there would no longer be resistance from surface friction.

I guess we also have little knowledge about what the coefficient of friction on Martian surfaces under Martian conditions would be.

field, re your 10, I wasn't referring so much to wind erosion as to the great variety of "ripples" that the MERs have imaged, but yes I am also suspicious that greater scepticism about wind erosion is in order. We have seen very little of the commonest sort of rock ventifact, parallel elongated pits and flutes. Maybe the action is mostly from dust devils, in which case the wind would be essentially omni-directional and ventifacts from a prevailing wind would not be expected. The bay slopes of Victoria Crater appear to me to have parallel flutes (troughs) in places running up and down the slope. This is also the only place at Meridiani where we can be fairly sure that erosion has removed meters depth of material. This is odd considering that wind speeds might be expected to be lower in the crater depression than on the plains. I suspect that Victoria is some sort of enormous weathering pit that is widening and deepening itself due to processes that are as yet unexplained. It seems that the bays have eroded orders of magnitude faster than the adjacent plains.

There is a lot of material on line about aeolian ripples and dunes, for example:

http://www.agc.army.mil/research/products/desert_guide/lguide1.htm

More Google book excerpts are appearing all the time too, Yay Google! I find that using a fairly technical search term like "granule ripples" gets one to the "meatier" sources. If you read about Earth's dunes, sand ripples, and granule ripples in detail and then compare these forms sceptically to what we have seen on Mars, the fit just isn't very good for any of them, in my opinion.

Barsoomer. Slight error in your reply 12. 30 mph on Mars would be equivalent kinetic energy to 3 mph on Earth.

There have been plenty of examples of 'wind tails' behind the hematite berries in the lightly cemented sedimantary rocks of meridiani, and ventifacts at the Pathfinder site and in Gusev. For instance Mazatzal.

Kye Goodwin's link is a great resource. Thanks Kye. The explanation under the 'soft when dry - ripples' link really put the Meridiani ripples into context for me. Although I believe that the equilibrium state was reached long, long ago.

Thanks for the responses. I had forgotten to factor in the reduced gravity in thinking about movement. The lesser gravity would make a difference in the force needed to get an object moving, slow down the rate it returns to the surface (if airborne) and also reduce friction when the particles start to move, if they are in contact with the ground.

Stan

So now I have another question. In thinking about the lesser gravity on Mars, I was wondering how old the rovers are. Since time moves faster in less gravity, the rovers have aged more than we have. Anyone here now how to do the math on time dilation due to gravity?

Stan

Stan,
Regarding your #16 above, if I remember my "Theory of Relativity" right, when you're talking about gravitational time dilation under various gravities, you're just talking nanoseconds difference at the Mars/Earth gravity comparison scale. Could be wrong, as it's been awhile, but that's how I remember it.

Which parts were cast first in the factory, which nuts and bolts were turned first, or when they first plugged a given rover into its power source would have more of an effect on rover "age" than Relativity would. But again, that's based on my fading recollections of physics courses completed long ago.

> Barsoomer. Slight error in your reply 12. 30 mph on Mars would be equivalent kinetic energy to 3 mph on Earth.

Serpens, re: your reply 14:

I was trying to take into account the reduced gravity, which is 1/3 that on Earth. As you pointed out, the frictional resistance to motion is proportional to the gravity, so the momentum of the wind molecules could be 1/3 of that required to move surface particles on Earth. Is this reasoning flawed?

Yes, I've been through it all before, in discussion with a poster going by "aldebaran" a few years ago. Here's something to consider if you think that aeolian processes are so well understood that all we have to do is sit down and calculate what MUST happen given any particular combination of gravity, wind speed, atmospheric density, particle size and so on:

http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1942.pdf

This was one of the first items published on aeolian processes at Meridiani. I'm calling attention here specifically to the section on page 2 titled Saltation/Suspension Transition. To paraphrase: In a given aeolian environment some smaller particles will easily become suspended in moving air while other larger particles will only saltate (hop) briefly and then return to the surface. Calculations were done to determine the particle size at which this transition should take place on Mars. Soon after landing however, researchers were confronted by ripples in Eagle Crater made up of substantially smaller particles than expected, particles that should by theory be moved in suspension. There is a doctrine in aeolian research that only saltating and creeping particles can form ripples or dunes. The theory had to be changed in the face of observation. Was the right "fudge" chosen, that is that the smaller particles have moved by saltation, or could it be that because we do not understand the saltation-suspension transition on Mars, the general doctrine that suspension CANNOT participate in ripple formation might be inaccurate.

If we interpret the tiny ripples that formed during the dust storm as sand ripples, where then is all that dust that settled out of the atmosphere during the storm? It should not be part of the "sand ripples" if they are a close analog with terrestrial sand ripples.

These little ripples sure don't look like terrestrial sand ripples. (See Reply 9) Their orientations, sizes and spacings are much more irregular. Maybe this can be attributed to the uneven surface in the vicinity but I haven't seen any MER-imaged ripples that look like terrestrial sand ripples.

Weather data from Mars is sparse to say the least. We all saw the tail-tail blow in the wind at the Phoenix site and have seen dust plumes rise off the northern scarp, mislabeled Avalanche, bless their heart. Along the thermal gradient in what is called the sand sea on Mars winds are estimated to reach over 150 mph as gust fronts should be common here and sand should move about with no problem.

The Equatorial regions are a big unknown with no weather instruments on the Rovers. Dust Devils s may kick up a little sand as well as the atmospheric fines that make up the short wave ripples visible in the Rover images.

You would probably have little trouble walking in a 150mph wind on Mars. Does the wind blow? Yes, yes indeed.

Fred