What we need is a 16 mbar wind tunnel, with 10m/s wind and a pile of dust in it.

Doug

Doug:

It's called, er, 'Mars'!

Bob Shaw

The Mars exhibition I saw at the Manchester Museum thingie was good - it has a little cylindical unit with a load of dust on the bottom, a clear lid, and a fan you could turn - and in the space of just 5 minutes, the entire dune system totally changed....not to mention sand slips on the sides of dunes that looked JUST like the sort of thing some say is water seepage on crater walls

We need an aerodynamicist....given a sphere of radius 0.1mm - what is velocity required at 1000mbar to produce the same drag as 20m/sec at 15mbar

I've seen various equations, some have the velocity squared, some don't.

Doug

Am not an aerodynamicist me but one of the important equations is the standard drag force.
Drag=0.5*(Drag Coeff)*(atmospheric density)*(Cross Sectional Area)*(velocity squared).
Viscosity, turbulence and high speed shock effects and other things modify this a lot but for low speeds it should be good enough for a comparison.
Drag Coefficient and cross sectional area stay the same, Earth's air density at 1000mbar is ~1.2kg/cubic meter, Martian atmospheric density is around 12g/cubic meter.
So a 20m/sec wind on Mars exerts (12*20*20)/(1200*10*10)=4% of the force of a 10m/sec wind on earth.

The lower gravity then increases the effectiveness of that force by a factor of 3 over an equivalent particle on earth so overall a 20m/sec wind on Mars (45mph for those who haven't gone metric) would have a roughly the same effect as a ~3.5m/sec (~8mph) wind on Earth.

From the Beaufort scale: 8-12mph is Force 3: Gentle Breeze Leaves and small twigs in constant motion; wind extends light flag.

Very interesting stuff.

Next conundrum - we have seem elongated depressions that look like the sapping - where the sand is falling into cracks in the evaporite. If the ripples are moving, even over many years, then why are these depressions visible? Moving sand would surely fill in the cracks over time, and then there would be no depressions.

The cracks can't consume an infinite amount of sand, which suggests some event made it happen? Marsquake? Meteorite impact shaking the plain?

Chris

Thats just it, the cracks should have filled up long ago at the supposed rate of dune propogation, unless that is, that the cracks are still (or just recently) opening up. Therefore, if the 'microcraters' or 'sinks' are caused by sapping in the cracks this apparently makes sense only in that same context.

i still think however, that the dunes are essentially fossils with a light veneer of dust and loose lighter grains blowing around on top of the desert pavement-like surface. There might be more activity in summer, when the atmosphere gets (was it 20%?) thicker due to polar sublimation, but most movement probably occurs episodically when the planet warms episodically due to 'other factors'. The tiger stripes show the wind has been pretty directionally constant during the time that the erosion was taking palce, which seems to be general direction it is leading today.

We still need to dig int a tiger stripe to see if the coloring extends below the surface or if its just an illusion created by darker grains 'sticking' in the places where the grain-gap size is a fit for the material blowing around.

Is that a laden or unladen sphere?

Well, if it's the Holy Hand-Grenade of Antioch then it would be at least equal to a European Sparrow.

Run away! Run away!

Bob Shaw

Take thee to Denmark ...
http://evolution.skf.com/zino.aspx?articleID=14896

First let me take a moment to say thanks to Alan (and Doug?) for getting this discussion on its own thread.

If there is one thing that stands out about Meridiani, it is the seemingly endless mega-ripples – deserving more discussion of this kind.

And, thanks Cosmic, for your most interesting contribution to the discussion! (BTW, a better link to earthsciences mega-rippling is here.)

I’m skeptical that the features of Meridiani were entirely aeolian formed, primarily because where sand and wind exist in abundance over geological time, we see pictures (like here, and here) where much larger, even mountainous, dunes form. In fact, Mars has its own ‘desert’ areas with similar mountainous dunes. But not in Meridiani? Then why not?

Secondarily, we know (or think we do) that this is an old seabed – but little discussion has centered on what would happen when that sea reached a depth of only a few feet, and then ever lower over time (as it dried up) across its entirety.

It seems more probable to me that the dynamics of an ever-shallower sea engaged two forces, both wind and water, formed what we see today – adding even more powerful evidence of it being a sea in the first place.

I don’t see water alone causing these mega-ripples, because as has been amply discussed, we don’t commonly see ripples this large under bodies of water – except near coastal areas where seas become shallower, in tidal areas and often after storms – however we do have a working knowledge of how mega-ripples form. This naval study provides much insight, as does this study of Puget Sound mega-rippling.

This NASA piece on cross-bedding concentrates on ‘flowing’ water. (Huh? One must ask, from whence to where?) Shallow standing bodies of water subjected to wind and other forces are at issue here, and that seems to have had much less attention than it ought to have had - particularly with respect to how static Meridiani has been and is today.

If we're looking for mathematical evidence of a sea on Mars, I think that some of the strongest evidence to date may be going almost entirely undocumented.

- -
Bob, lyford (others too) - keep the humor coming...

Thanks for your kind comments, folks.

sattrackpro: I'm afraid you may have misunderstood me. I wasn't suggesting that the Meridiani ripples were mega-ripples. "Mega-ripple" is a dangerous term to use, as it has been used for some very different things over the years. I was only trying to point out that one cannot distinguish between subaerial bedforms and subaqueous ones based only on size. The large currrent ripples seen in the channeled scablands, named mega-ripples by Bretz, were only one spectacular example. There are others who have used the term "megaripple" for some very different things.

It really seems that the water that was present when the bedrock formed did eventually disappear, and the thinly bedded layers were later deformed and eroded. The deformed and eroded surface was later covered by the ripple field. We don't know how much of the upper section of rock has been eroded away, but the missing section will forever hide the last part of the story. In other words, there is an erosional surface between the top of the bedrock and the base of the ripples. That part of the story is missing in this area.

A lot of the scientists studying this data really are looking for evidence about the coming and going of water. You can't simply assume that the water was here once, and then it went away. The infamous Burns section in Endurance suggested it once was dry, and that it later became wet, then possibly began to dry again. Obviously, it went dry again at some point, but those details are the lost part of the story. Hopefully, the team identified the highest part of the section that was available on their traverse, and looked at it carefully.

>Hopefully, the team identified the highest part of the section

Yep, that's why I was jumping up and down and waving my arms when we reached Heck-of-a-View. Since the strata are essentially flat-lying, that topo high would have been the strat high.

OTOH, when we do Victoria, that will be the chance to see that Holy Grail of "the basal unit" and see the beginning.

--Bill

0
Oh cool, my university...

Getting into this a little late, but... guys, think of the hard basalts and other lavas we've seen at the other landing sites, that have been carved and polished by the winds over the aeons into perfectly smooth surfaces. If y'all have ever seen chunks of this kind of rock, you'll note that in its more natirual state it doesn't feature such nice, gently curved, smooth surfaces.

These ventifacts, which are ubiquitous wherever you find hard rock, are even more compelling mute testimony to the powerful, if slow, erosive force of the Martian winds than are the ripples/dunes.

-the other Doug

Looking at the bedding in the ripples, you can see that there are some compositional changes over time in the grain deposition. That's the only way you get this kind of bedding.

So, the layers in the ripples (which seem relatively evenly spaced) speak to epochs of ripple deposition, I think. The way that the layers tend to have partially-cemented surfaces (similat to the cementing we see on the surfaces of the ripples today) hint that each of these layers was once the surface of the ripple.

So, what could occur in epochs on Mars that would result in the building of material on top of existing, surface-cemented (and therefore probably relaytively static for a time) ripples?

Cyclic, drastic climate change, due to Mars' extreme wobble, could just fit the model. I think.

So, perhaps we are seeing layering in these ripples that is built up every time Mars goes through a given portion of its extreme climate cycle. Possibly, you only get rapid ripple/dune deposition during those times when the maximum amount of Mars' atmosphere is gaseous for the greatest amount of time during the year. I'm not sure which portion of the climate cycle would offer these conditions -- but I bet the planetary climatologists will have it all figured out in a decade or two...

-the other Doug

Exactly, Doug. The Universe has a cyclic nature about it. In Terran geology there is even a term for this phenomenon: cyclothem.

--Bill

oDoug:

I quite agree, too - and two more points: the cycles are *not* going to be close together (unless things add up that way for a brief moment of synchronicity) but will be well-distributed in time; and, the rest of the time, there'll be precious little going on other than a smear of talc-like dust being blown about.

We're currently in a 'rest of the time' period, with nothing much happening...

Bob Shaw

I have been pondering this dune moving/not moving thread for a while now and for the most part have been sitting on the fence until now. There have been some good arguments supporting both sides, but I think I have to agree with Bob Shaw (or is it *Bruce* ) on this one. Every time we cut into these dunes, whether accidentally or intentionally, we always see the hard clast on the outside and soft powdery material on the inside. If these dunes were indeed moving presently, I would think that we should see multiple layers of the harder clast material embedded inside the dunes as softer/newer material covers over older/harder layers.
The only way to explain this is if the winds were once strong enough to erode away the harder outer layers and start the process of moving the dunes. I'm not sure what the time frame for these intermittent movements would be, it could be in the hundreds of years, thousands of years or even longer. We have only been looking at this for a very short time frame. The best analogy I can give is if you were suddenly plopped down on earth next to a small streambed that runs through a canyon in the middle of summer. Your first thought would be that it must have taken this little stream millions or billions of years to cut this big canyon. But as you watch this stream over the course of a full year, you see that every spring there are floods that swell the stream into a small river. And as you watch it over the course of a few hundred years, you see that there are periods when this little stream becomes a raging river, able to move large boulders with ease. And as you watch this over longer and longer time scales you will see that the canyon is being formed not by the little stream that runs through it in the summer time, but by the periodic events that happen over ling time scales.
I think we have the same thing going with these dunes. We are looking at this event on a very small time scale. If we could look at it over much longer time scales, I think that we would see events that are catastrophic enough to start moving these dunes.
It's my understanding that about every 125,000 years the poles swing from about 10 degrees to about 35 degrees and every few million years they can swing as far a 60 degrees off axis. It's not hard to imagine under some of these extreme conditions there being fierce winds blowing across this plain. With the planet at a 60-degree tilt, we could have the polar ice caps encroaching all the way to the equator. I would expect there to be strong winds blowing where the ice/sand boundaries meet. I believe the areas that you currently see dunes forming on an annual basis are in the northern and southern hemispheres not far from the polar cap boundaries.
Anyways, that's my two cents on this topic. Comments certainly welcome.

Gary

Happy with my easy chair in the peanut gallery, I found myself nodding my head in agreement with everything you've mentioned so far.

Looking at these latest pics such as
http://www.flickr.com/photo_zoom.gne?id=185510642&size=l
I'd suggest that the even Beagle/Corner Crater was in place before the majority of the "dune events" got started. Traces of ejecta from the crater in the evaporite "bedrock" look to be missing, and there's even what looks like to be additional solution generated deposits rising above the "bedrock" as seen in earlier Oppy pictures. This should be confirmed as Oppy makes a closer approach, but I suspect that only larger chunks of evaporite are in the litter around the crater.

I'd even suspect that the base of some of the evaporite ejecta blocks might show a unique pattern if we can see any in direct contact with the "bedrock" -- perhaps there was a "wet" episode during/after some of these craters. I just have a hard time seeing enough ejecta blocks in the "in between" sizes between block sizes presumably around the crater edge to "powdered evaporite" which we might presume has been blown and eventually incorporated into the dune features we see. (The "light" stripes of the tiger striping of the dunes??)

I can't help but notice how the "Hillock" seems to overlay the dunes. Am I reading this correctly?


1N206667393EFF74LVP0722L0M1.JPG

1N206752410EFF74N8P0735L0M1.JPG

I mentioned this in the "Moving South to Victoria" thread, Gary. I see it that way, too -- but I was concerned that I was seeing the contact wrong. The fact that it looks that way to you, too, makes me feel a little better.

It really does appear that the hillock overlays the ripples, doesn't it? Question is, does the entire hillock overlay ripples, or has the hillock just crumbled and slumped on top of ripples that are, indeed, more recent than the emplacement of the main body of the hillock?

-the other Doug

Does the hillock overly the ripples?

I don't think so. It may look superficially as if that is the case but if you look very carefully at the edge of the hillock, then offshoots of the dunes interfinger with it.

I would interpret the hillock as being too tall for the dune material to be transported up and over it. Instead, the material is going round and past it. (or perhaps the winds are simply stronger with altitude - an observed phenomenon on Mars - and the hill crest is swept clean of fine dust.

If the hillock was imposed on the dunes, you'd expect some sort of evidence of the (ballistic?) emplacement mechanism, and some loose fragments spilling outside the main mound.

I also think the rubble pile predates the ripples. But I will admit that it is not entirely obvious from the imagery. Upon close inspection, my best guess is that the finer grained ripples are encroaching the rubble pile. It seems that some areas can be seen where the coarser clasts are being buried by more recent and finer ripples. But one could argue that the larger clasts are spilling onto the fine stuff. Clearly there are fine-grained, mini-ripples migrating over the top of this mound at a slightly different orientation than that of the older ripples.

It won't be settled to anyone's satisfaction unless they do a wheel scuff, but I don't know if that is necessary. As Nick mentioned, you'd expect to see some evidence of drift disruption if the original object fell onto them.

It's an odd formation but let's be honest there is absolutely no way the hillock could have dropped into place on top of pre-existing dunes _and_ still have the interface look clean. Well unless there are Martian rubble\regolith moving teams racing around on Airspeeders.
I think Nick's explanation is pretty solid - the dune\ripple forming\moving process is disrupted by this mound because it is higher than the surface zone in which the dune processes are stable\semi stable. Above that zone the conditions are sufficiently different, probably higher wind velocity but it could be something else, to prevent the dunes forming so we get a nice clean rubble pile.
I'm quite glad they are going to poke around in this for a short while, should be very interesting.

I generally agree with the reasoning, but do not know how old these ripples are. I think it might still be possible for the hummock to be part of the VC ejecta blanket, as yet uneroded. We've seen small amounts of dust moving about from the wheeltracks and DDs but this might just be light stuff and could account for the small ripples on top of the hummock.

Roy

The rover needs to get in a position <maybe after the Vctoria visit> where it can image a good bit of the edge of the smooth terrain (ejecta blanket?) around Victoria. Park on etched terrain maybe 10 meters from a good sample of the edge and take a 180 deg multispectral and binocular panorama of the edge.

I've wondered if the continuous blanket around Victoria is an area where the ejecta contained enough hard-to-erode bits <blueberries and whatever> that they formed an erosion-inhibiting lag deposit on the remains of the blanket after much of it's been stripped away.

That is a good thought, Ed. The ejecta from VC contains both evaporite and basalt fragments, which are erodable and resistant rocks at this site, so it makes sense that a lag deposit was left behind.

I'll leave this to someone with 3D mapping and gridding capabilities, but I'd be interested in finding out what the thicknesses and therefore the volumes of the evaporite and basalt rocks that were removed from VC.

--Bill

I agree with many of you that it is hard to fathom how the hillock could be older than the ripples, but if the dune were formed after the hillock, I would expect that the mound would disrupt the local airflow enough that the dune would make a curve near the hillock. Instead it looks like it goes straight into it. I'm keeping an open mind about it until we can get a scuff at the transition point. As we skirt along the edge of the ejecta blanket we should see more examples that will either support or dismiss the idea that the hillocks overlays the dunes. I think these dunes are quite old, but I can't imagine a weathering mechanism that would preserve the dunes while at the same time crumble basaltic rocks.

I think we are essentially on the same page, in that the rubble pile came first and the ripples later. Whether it is an outlying ejecta block or a remnant of the ejecta blanket is not clear at the moment. But I can't buy into the idea that the hillock is too tall for ripples to form. Haven't we seen ripples as tall and even taller than this mound?