Very Short Transport Distance

Here is a press release from the JPL site that posts an image of angular bedform armour particles at the summit of Husband Hill.

[link]

The relatively unrounded particles are noted and explained: They have had few collisions that would cause rounding because they have not travelled far to reach their present location. I think that the bedform armour particles never move in Mars' wind, so this idea is on the right track, but doesn't go far enough.

In this paper, Sullivan et al suggest that a wind speed of 80 m/sec (288 km/hr), at one meter, would be required to saltate the bedform armour particles. This is 2.6 times the highest wind speed ever observed near the surface (at 1.6 m) and if Andy and Lavache had their formulae right that's 17 times the highest wind power density ever observed. I think that it is an interesting hypothesis to consider that these extreme winds never occur, and that the distribution of sand and granule sized particles on the surface of Mars has been achieved by other processes. I think that the bedform armour particles have formed from airfall dust or soil within a few millimeters of where we find them.

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

I am not particularly comfortable this far from the mainstream hypothesis. I expect that my audacity is probably annoying to some people, but I feel compelled to put this hypothesis forward. Even those who think that this idea is without merit, might find it interesting to consider just how infrequently these proposed extreme wind events occur. Global dust storms were recorded by the Viking landers and the maximum wind speeds never came close to the necessary intensity to move sand. Would we have to wait for another part of the 124,000 year obliquity cycle to see them? These extreme storm events are currently mythical, but also central to the mainstream explanation of the Martian landscape.

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

To paraphrase Edward Bullard, when he wrote it I can't remember:
"In geological time, the improbable happens often and the impossible occasionally".
This has only recently been incorporated into mainstream 'uniformitarian' geology on Earth as human life spans don't cut it for rare or long cycle events.
While I wouldn't argue that high winds speeds are either improbable or impossible on Mars, we can see that the large dunes present at Meridiani point to higher sustained wind speeds than have been seen/recorded by our sensors over a short period. So what?
The point is that very long periods of time are available on a planet with very slow weathering and erosion.
Finally, Kye, why do you have a problem with this, once again, simple explanation, which by the way came from a paper on Meridiani rather than Gusev?
'Mythical' is a very harsh comment on geological argument. There are many rocks where all we can do is make our best estimate of past conditions of deposition, because no-one was there say 50 million years ago! I certainly have never accused you of myth, only insufficient user of geological evidence. I do worry sometimes about your implacable opposition to geology. But your ideas are always fun and provocative.

Newboy, I am most certainly not opposed to Geology. I think that I am one of the most serious geological contributors to this site, though I think it is time that we started calling it Martology. I have always stayed firmly rooted in chemistry and physics.

It is not correct to call the aeolian structures on the Meridiani plains dunes. If for no other reason: they do not have slipfaces. I think that I am going to start calling them what I think they really are: dust ripples. They are made up primarily of fine particles too small to be resolved with the mic-imager.

A problem that I have with extremely rare storm events shaping what we see on Mars, is that there are clearly gradual processes at work every day. In time the slow processes like dust movements and chemical weathering would erase the effect of the rare extreme events, so there is some limit on how much time can pass between extreme storms, if they are going to explain the current conditions. This is more important at a small scale where the gradual processes would have the most noticible effect.

Yes, that paper is about Meridiani, and only a small part of Meridiani at that, the Eagle and Endurance crater area. It is now about a year old, and is, as far as I know, the only paper published on aeolian processes since the MERs landed. It was actually published before the larger plains ripples south of Endurance had been discovered. The small ripples between Eagle and Endurance bear more resemblance to Earth sand ripples or granule ripples, though I think that this is a superficial resemblance, because they form in a completely different way. All the aeolian bedforms on Mars form from dust movements. Their surfaces are elaborated by chemical weathering processes.

Yes, many geological processes are more or less mythical in the sense that we have never seen an example of them in action. I do not wish to be harsh, but I do want to draw attention to the fact that these events are very important to the mainstream understanding, but at the same time, almost nothing is known of them. They are entirely theoretical and in my opinion the evidence for their occurence is weak. If typical global dust storms do not produce them, what does?

Kye&New
Being a geologist I am not totally comfortable with Kye's view of how the berries formed but unfortunately I have at this point no alternative explanation .

I will say that many geological features resulting from relatively rapid actvity usually represent the result of the last major event. Dunes being a good example.

You only see the results of the last storm .

Don't the dust devils, de facto, virtually rule out any assertion the winds on Mars can't significantly transport the soil. The numerous dust devil tracks crisscrossing the surface seen in photos
seem to cover a generous amount of surface area and would lead me to expect that,by this process alone, the entire surface layer gets "reworked" at least once at every few years (not to mention every Milankovich[sp?] cycle,or better, millions+ years). Didn't both rovers experience "dust cleaning" events? Particulate matter is observably being transported in the air in the devils.
Best guess at wind speed anyone?

Ben, Yes, I hear you on that last point: You only see the results of the last storm. Its not that I doubt the superstorms occurrence because we haven't seen one. Its more like my doubt really started because the scene at Meridiani does not look like the raw aftermath of a storm. It looks highly developed and complex, like it has taken a long time to achieve all the consistent patterns and details.

I have looked at a lot of pictures of bedforms on Earth lately and ours are simple and boring compared with Mars'. Even if every 124,000 years or so there were a superstorm and the rippleforms moved a few meters or got re-oriented, I do not think that this could account for the details that we see now. There is just too much else going on for the landscape to be that static. I do not think that 1 mm bedform armour particles were perched in a thin layer along the crest of the ripples centuries ago and are still there, unburied and unweathered for us to see today.

Kye, re your 2, 5. IT'S NOT JUST A STORM!!!
Dunes - please let's not get tautological. They are dunes under any landform classification system, .i.e. 'ridge of sand created by wind'. Where's the ambiguity?
The 'dunes' (Kye, 'quotes' are for you) could not just be the 'last storm'. They have formed as a result of a prevailing wind over a long period.
And it's really annoying to have you dismiss decades of geological study (I'm not being greedy here - it's longer) as just wrong. I repeat: I admire your interest in seeking to explain what we see by your own lights. but please take account of some previous experience on the part of others, despite the fact they don't agree with you.
Martology - Fundamentally will not challenge the observational experience of Geology. So far, I have been thrilled (NB the word THRILLED) by the power of our science on another planet.

robert, Yes, I agree completely with what you wrote in Reply 4. The SUSPENSION of dust in the atmosphere moves a great deal of material. The question that I am asking is how often does saltation (hopping) and creep move larger, heavier particles than dust. The dust particles are not only smaller than sand, they are also now known to be composites rather than little pieces of rock, so they could be somewhat "fluffy".

Most of the soil surfaces that we have seen are crusted over in one way or another and the Viking global dust storms did not disturb those crusts. It was during one of these storms that a wind speed of 31 m/sec was measured at 1.6 m above the ground, which I think is the maximum ever recorded by a lander. That is about 112 km/hr. What this speed amounts to as a big-particle-mover in the near-vacuum that is Mars' atmosphere is harder to say. There was a poster named Andy here a few weeks back who sounded like he designed wind turbines and I arrived at an Earth equivalency in wind power density of about 24 km/hr with his help. This is near the minimum wind speed that can saltate medium sand on Earth, so it really doesn't settle the issue. The real calculations describing particle saltation are much more complicated and there are lots of uncertainties going in. The only authoritative estimate that I have found is in that Sullivan paper I posted. It would take an 80 m/sec wind at 1 m to make the 1 mm armour particles hop.

I haven't looked for an estimate of dust-devil wind speeds, good point. If dust devils are responsible for the movement of larger particles then we would not expect them to form ripples or rock-tail ventifacts because of the lack of a prevailing direction for the mobilising wind.

Newboy, I didn't want to be tautological, but a few general references that I read on aeolian forms described dunes as having slipfaces. That is why the rover scientists refer to the Meridiani forms as ripples, which do not have slipfaces. I have a much better reason to question the use of "dune": We do not know how these Martian structures form.

"previous experience on the part of others" ?? What previous experience? This is an alien planet being explored for the first time. We have no experience with the aeolian movement of larger particles on Mars. There have always been theories, but the issue of how Mars wind bedforms form has never been settled, because remote-sensing has never been able to determine their particle sizes.

The particles on the surface of the Meridiani ripples, the "bedform armour" particles cannot be moved by the prevailing winds. That is my point.

First, your last point. Agreed. The armour is evidence for no movement and itself provides more resistance to future movement. That is why I have stated for quite some time that the climatic conditions that created the Meridiani landscape are not active today. And so have others, including the reference you introduced at the top here.

I think that strong prevailing winds that acted in the past (when, we don't know) created the dune field. The dunes are there so we have to explain them. And calling them ripples doesn't change my view of what they are, a type of dune. JPL has also called them 'drifts'. But they are all sand piles moved by wind.

From remote sensing we can see a cornucopia of dunes on Mars, including ones that are enormous by Earth standards. The geometries observed display the range of dunes types seen on Earth, plus some pretty strange ones that are not so easily explained. I will agree with you on that: in detail there will be differences of opinion on how exactly a dune formed, but there is no doubt that it is sediment moved by wind.

Such generalisations are examples of 'previous experience'. Wind and sand particles will behave the same anywhere in the universe, may I be so bold as to suggest?

Excuse me for my late night capital letters in 6 above - not intentional.

Newboy

Lets don't say wind and sand particles will behave the SAME because their action will also be affected by gravity, atmospheric pressure and temperature to mention a few.
Obviously somewhat SIMILAR though.

Newboy, thanks for the agreement. Your use of the term "sand" is unclear. "Sand" has a fairly explicit meaning in geology, defined usually by particle diameter. I have also come across an aeolian definition of sand: Particles that can be moved by the wind through saltation or creep, but which cannot be suspended in the atmosphere. In this sense, I think that Mars' aeolian bedforms were not created by the movement of sand, but rather by the transport of dust in suspension.

Kye, Let's follow your idea through.
But first, sand: typically 0.1 to 1 mm grain diameter, rounded to sub-rounded grains. Do a google of 'sand silt grain size' for examples of schemes of classification.
So you want to have all the sediment in suspension as a result of dust storms?
Check the diameters - 'dust' is coarse to fine silt and clay, i.e. not sand. At Meridiani we have a mixture but sand predominates. The mixture includes the spherules. This tells us that to move and accumulate these bedforms, we need significant winds to move them over long periods of time. It could indeed be short bursts over millions of years or sustained strong winds over long periods. Note my relativistic language - I am not doing any research here, just summarising on the back of observation and experience.
If the dunes were just dust i.e. finer than sand, I would suggest that Opportunity would have disappeared in a 'ripple' long ago.
But I do fancy one question which you have raised indirectly Kye. Where is the dust, and where does it finish up? If recent winds are less strong, then they are only capable of lifting the finer material. So where is the fine dust coating one would expect on every surface?

Newboy, Yes, I would like you to follow my idea through. I do not think that sand-sized particles are being suspended, I think that sand-sized particles are not moved by the wind at all. The spherules have never been moved by the wind, and the bedform armour particles have never been moved by the wind. These particles have all formed within millimeters of where there are now.

Your last question is one that is really exciting to me right now. I have just started to look for an answer to the question of how much dust is being deposited. The best source I have found yet is this abstract based on Pathfinder results:

[link] &format=

It suggests 20 to 45 microns per Earth year, which strikes me as a huge amount: a millimeter in 30 years, a centimeter in 300, 30 meters in a million years. Dust could pile up to the depth of the Columbia Hills in a tiny fraction of Mars' history. But OK, I am going way too far based on that abstract, which does not make it clear to me how densely packed that dust is, but even if we knock a power of 10 off that estimate, dust deposition would be a major geological force.

What happens to the dust that is deposited? A great many different things happen, depending on where it falls. Some is re-lofted into the atmosphere. Some colects as deep deposits. Some is the main raw material for the rock coatings. Dust is the main input to the soil. Dust is the source of the bedform armour, the spherules, and the cobbles, and dust is the source of the sulphates on the surface at Meridiani.

Where does the dust come from? I think that it comes from the chemical weathering of minerals, including those that it has formed. There is a dust cycle of some sort, and it is powered not just by the wind, but also by a chemical cycle, in which minerals are both accreted and eroded by processes that involve water films and the oxidation-reduction potential of the surface environment.

Once again Kye our debate breaks down when something that is unambiguous to geologists is disputed or rejected by you. And that's that.
So how do you make a sand particle? How do you build the drifts? Where does the sand that has different compositions from the underlying bedrock come from? Just a few sample questions which need answering and your model cannot solve. Logic dictates that all the evidence must fit your explanation, not just part of it.
If you insist on ignoring the obvious clues to the origins of the heaps of sand, then we are wasting our time debating.

Newboy, Those are not heaps of sand. They are heaps of dust and chemically altered dust.

This recent paper on Mars' soil concludes that the local rocks are probably not contributing much to the soils at either Meridiani or Gusev. The soil mostly derives from material that has been wind-trasported.

http://www.mars.asu.edu/christensen/docs/yen_mer_nature_05.pdf

Great Discussion and you both make good points.

A comment however. Is the angle of repose for sediments on Mars the same as on Earth? The conditions which determine the angle of repose would determine if slip faces could or do form on Mars. As Ben states, lets not assume the geologic process on Mars duplicate those on earth. The present may not be the key to the past

Great Discussion and you both make good points.

A comment however. Is the angle of repose for sediments on Mars the same as on Earth? The conditions which determine the angle of repose would determine if slip faces could or do form on Mars. As Ben states, lets not assume the geologic process on Mars duplicate those on earth. The present may not be the key to the past

Jim, Thanks. Mars has surface gravity about 3/8 the strength of Earth's, so the angle of repose should be substantially steeper overall.

A more difficult issue concerns the properties of the particles that are being piled up? Particle shape would affect slope stability, as would any tendency for the particles to stick together. Many of the aeolian surfaces are crusted over. Here is an image from the top of Husband Hill. The soil material in the foreground appears to be wind-deposited dust. In the lower left of the image a gully in the dust and rocks rises toward the viewer. This becomes a narrow gap at the top just where the gully reaches the main dust terrace that fills the foreground. On the far side of that narrow gap, beside a rock, the dust deposit seems to achieve an angle greater than 90 degrees to the horizontal.

There are structures on Mars, visible from orbit, that have been interpreted as slip faces, but so far, no slip faces have been spotted by the MERs, unless we count that tiny landside at Larry's Lookout.

Kye, Christensen concludes just as you say, but ”soil” comprises only the top few cm of a dune. Beneath the very fine grained layer is coarser grained material, closer to home, let there be no mistake on that. Dunes can’t form without. A global depositional dust unit has been the working model since planet wide dust storms were first observed, so no surprises there. The attached MSSS view shows nice barchan development on mars. Since barchans are common on earth and mars (and other familiar dune forms), what can we conclude? Similar grain size distribution, similar wind patterns? there's more....

http://www.msss.com/mars_images/moc/2004/07/20/2004.07.20.E0200732.gif

Kye:

That was another question I had. What is the cohesion of this material and is there an ectrical charge on it that one would expect the finer grain material to have, and would these charges affect the angle of repose, especially in a cold, dry environment.

The photos seem to show much of the finer grain material is rounded to subrounded which would seem to indicate some transport distance have taken place, but there also appears to be subangular to angular material present. One would think the rounded material would be on the flatter slopes and the angular material would form the steeper faces. As you point out, where are the slip faces that you would expect to find on wind formed dunes.

Also, what is the source of the chemical alteration. Being from the old school I'm kinda hung up on slightly acid water breaking down the mineral grains, especially the feldspars. Obviously we don't have the free water available, so what else is going on at the chemical level?

When the rovers first came off the transport my first thought was how in the world do tracks like we saw form in soils which I initially thought would have no cohesion?

Actually do we even know what the USCS classification of this material would be. SP, SM? If were looking at silt to clay size particles could we have SM or CL. My soil classification training seems to completely break down on these soils.

Knowing the classification could at least help to explain the formation of the soil and how they behave when subject to the influence of wind.

Anyway, I think this is probably one of the best discussions I have seen on the blog in a long time.