Looking at the Victoria dunefield it strongly resembles the patterns of sand grains on a vibrating tin lid. I have started to wonder if resonance could indeed be involved. Something along the lines of standing sound waves within the crater producing displacement nodes and antinodes that could cause very light particles to migrate to the nodes. Next question: given the dune wavelengths what would be the predominant frequencies of these sound waves? Would they be audible or infrasonic? At this point I realised I have no idea what the speed of sound is at Victoria. Any relevant facts or opinions to constrain my rambling thoughts would be most welcome. Likewise any pointers to previous discussion along these lines.

The speed of sound on Mars at "Sea" level is

801.3 ft/s
244.2 m/s
546.4 mph
879.3 km/h
474.8 knots
etc...

I have serious doubts about your theory as the pretty irregular windfalls in Victoria would quickly and easily destroy any such intricate patterns, so it seems.

Hi ngunn,

Mars' speed of sound is around 240 m/s, depending on the altitude and temperature. The transmission of sound would be very much poorer than in the Earth's atmosphere, due to the lower density at Mars. Some work on this subject is reported here - there was a brief flurry of work dating to the MPL days, which was destined to have recorded in-situ audio.

I suspect the dune formations have little to do with "sound", and much more to do with wind-driven rotors and vortices within the crater.

Andy

!Silly question alert!

Can't they be related with the relief of the ground beneath? Wind erosion acting but still keeping a rough pattern of what once was the floor of the crater?...

The interior dunes in many Martian craters are classed as Star Dunes. The craters disrupt the local winds and create persistent multi-directional wind regimes and eddies that create star dunes. Star dunes are pyramidal, radially symmetrical sand mounds with slipfaces on three or more arms that radiate from the center, and grow upward rather than laterally.

Nothing mysterious; this is the way of sand. This is a very good puzzle-piece telling us about the nature of the winds inside the crater.

Do a google on 'star dunes' and 'dune morphology' for more info.

--Bill

The dune field as a whole shows a lot of regularity. The locations of bays play a role as well it seems. You can see that the five near-parallel "spines" (trending WSW to ENE) seem to connect to bays on opposite sides of the crater and there are a nice series of transverse "ribs" which might have been formed by Westerlies blowing from bay to bay. The bays must have a significant influence on the patterns of wind within the crater, regularizing them considerably. The saltation process would be complex though and may rely on the particles of dust being extremely uniform and small as well as wind directions being consistent over a great many years. The structures within the field appear delicate so as to be seriously altered if the wind was blowing randomly across them. Perhaps they are well protected against the strongest winds because they sit in a depression.

Please post any photos of dunes you may have or come across that appear similar to these. I've seen several examples in Meridiani craters (including the one we agreed was similar to Victoria), but can't remember seeing any in other places.

Edit: (Article on Dune Geology)

OT, but do you know if Oppy hitted the sound barrier during EDL ?

An excellent bunch of replies so far, thanks to you all. I've had a very quick look at the Dune Geology article and the briefest of googles on star dunes and a few points come to mind.
1/ There is nothing about star dunes as such that would cause them to line up into regular arrays such as we see in Victoria, so there is still a question - where does the regularity of the pattern come from?
2/ The patterns within a particular field of terrestrial star dunes presumably change quasi-randomly over time. Looking at the Victoria pattern suggest something relatively fixed, as if the pattern were self-sustaining in some way, perhaps in response to fixed boundary conditions (the crater walls?).
3/ The typical longest wavelengths within the Victoria dunes are of the same order as the dimensions of the surounding bays and promontories, and are much longer than the characteristic wavelengths of dunes outside the crater (which anyhow appear to be a different colour and could have a completely different origin).

So, whilst not wishing to pretend that there is a mystery where none exists, I'm not content to say 'This can be easily explained by well understood sand dynamics' and then move on. I think we might learn something if we could understand the reason why the pattern is as it is and not some other way. In other words how does the pattern emerge from the process? Of course standing sound waves are a long shot. I just liked the thought of Victoria crater singing it's own deep song, like the wind blowing over an enormous bottle, and those particles jiggling around in response. I'm sure there are other ways in which the overall form of the crater could constrain the dune pattern.

Almost forgotten those probably fictitious sound frequencies - I get about 4-5 Hz for the lowest (North-South) mode, about 10 Hz E-W, and plenty of higher frequencies extending into the audible range at other angles.

Note: frequencies edited to correct silly mistake - node separation is only half a wavelength.

This is an interesting topic. I really don't know a lot about this type of dune/ripple but I know I've seen them in many Martian craters. I once came across an article on Martian dunes/ripples that attempted to classify the kinds of dunes observed on Mars. They described this variety as "network dunes." All I remember about them was that it was thought they formed in environments where there was a bidirectional wind flow regime (which has already been documented in Meridiani) and that they require sediment with a very uniform grain size distribution. It seems notable that the direction of the dominant linear trend in these is perpendicular to the NW/SE seasonal wind directions. From what we saw in Endurance, we can be confident these sediments are uniformly fine-grained. It helps to use quotes when googling "network dunes" to eliminate the many false hits.

It deployed its main parachute well above the speed of sound.
This eventually slowed it down to subsonic speeds.

Good find, Tom. Network Dunes is the exact description. I also suspect that the dunes we've seen in Endurance and Victoria are composed of very fine particles of eroded evaporite that accumulate in the 'wind shadow' created by the crater bowl. You recall how Oppy nearly got stuck in soft material near Wopmay.

--Bill

I'm having trouble finding anything on network or akle dunes other than papers I don't have access to. Can anyone find some good free picture links? I would expect dunes formed in confined spaces to behave differently from dunes on an open plain but again I can't find anything on this topic. Have we any terrestrial craters that contain dunefields? Quarries? Sandpits? Bill, Tom, have you any thoughts on the wavelength disparity between the light coloured (presumed evaporite) dunes on the crater floor and the darker (basaltic sand?) dunes elsewhere?

My recollection is that barchans (cresecent dunes) on earth the direction of travel is along the axis of symmetry - i.e the ends of the crescents point in the direction of travel and the main body of the dune follows them along. Some other dunes become "anchored" at one end and string out, and end up alinged with the prevailing wind direction.

My thought on the "net" of dunes in Victoria is that each section of the mesh is made up of min-barchans all joined up. There are broadly speaking 2 orinetations of dune crests insdie Victoria - the dominant SW-NE and less prominent NW-SE. Perhapos these reflect two sets of previaling seasonal wind directions perpedicular to the dune crest trend lines.

Of course the inside of a crater is very constrained environment for wind formed features and is not typical of the open plains around, where we have broadly north-south oriented dune crests.

Perhaps any wind entering the crater, irrespective of its direction out on the plains, turns into a gust which rotates around the inside of crater, conforming to its circular shape. Maybe the whole net is slowly rotatiing over long periods of time, and the 2 predominant dune crest directsion we see in Victoria at the moment are just a "snap-shot" in time, and unrelated to the prevailing wind direction on the plains outside.

Kenny

Another nice example part way down this fractals page - look in 'Other Fractals and Patterns':
http://www.miqel.com/index.html

Click to view attachmentthe picture I tried to link to above:

An amazing picture, when you inspect the crater bottom. Obviously Mars, but where?

Here's a thought - under certain conditions winds can create "standing waves" upwards in air which become visible as linear ripple clouds. Could the same effect operate downwards into an enclosed depression like Victoria, and create ripples in the dust? Australia's Morning Glory is one, and the initial large cloud is often followed by subsequent parallel crests of cloud interspersed with clear air.

http://www.cloudappreciationsociety.org/a/glory/glory1.html

http://www.greenhorsesociety.com/Clouds/Glory.htm

At sea, of course, steadily blowing winds raise ripples in the form of sea swells, manifested in regular ripple crests.

Kenny

Just so. At sea travelling waves. On a small pond or birdbath standing waves. But this is reasoning by analogy - it's not so easy to understand the actual mechanism for the sand dune case.

Also consider that the sediment in the bottom of Victoria is probably not sand, but finer-grained particles which may carry an electrostatic charge.

Great photo. Thanks. Perhaps a HiRise candidate? Is there a higher resolution version?

Sorry, I've no idea where the image came from - I found it by chance on a maths website. I expect many similar examples will come from HiRISE (perhaps there are even some already).

OK folks, eye strain time. Here is a long baseline colour anaglyph of the Victoria dune field. The baseline is a bit long really and the repetitive and low contrast nature of the dunes make this a little tricky.

I'm going to upload three scaled versions. The full resolution one is great for viewing the dunes on the left (which are stunning!), but I find the right side very difficult. In the half resolution version it is much easier. I've also included a small quarter scale one which is good for seeing the whole dune field at once.

Full res version - start by looking at the dunes 1/3 of the way from the left.
Click to view attachment

Half res.
Click to view attachment

Quarter res.
Click to view attachment

Enjoy,

James

Thanks James.

Hmmm...is the melange scoop on Oppy's arm functional? Might need it in there!

Andy

If they are forming reipples and dunes then the particles composing them must have been transported and deposited as sand sized grains. Of course, they may (or may not) consist of electrostatically bound aggregates, but these are still functionally sand grains.

Jon

When I used the term 'sand', I was simply referring to the size of the grains. The standard scale that I had in mind classifies sand-sized grains as those particles whose diameter is between 2mm and 1/16 mm. Grains between 1/16 mm and 1/256 mm are called silt-sized grains. The smallest grain sizes fall into the clay size range.
Certainly the sediments composing the drifts in the bottom of Victoria were transported as a bedload, via saltation or perhaps as very low-level suspended load. All I meant by my comment was that the finer grain sizes may behave differently than the sand-sized grains, and that terrestrial sand dunes might not provide the perfect analogy for the martian drifts. There may be some important similarities, between the two types of deposits, but there may be some significant differences.
Those differences could make the origin of the martian drifts that much more difficult to decipher.

After writing all this I vaguely remember a discussion some time ago about how the different grains sizes behave in the less dense atmopshere and lower gravity of Mars ... but the wheels of my memory these days are turning about as fast as spirit's right front wheel...

That's one rough washboard! They work great for me, but then I can contort my eyes pretty much any which way.

While different atmospheric density and gravity have an infuence, the difference between sand forming dunes and other bedforms and finer particles forming mantling deposits of parna still hold true - AFAIK.

Jon

I think the difference is grain/particle size. Martian "sand" probably fits mostly into the silt or clay terrestrial categories. You'd never get it out of your spacesuit if you rolled in it! Note to future Marsonauts. Bring lots of wet wipes.

http://www.agu.org/cgi-bin/wais?dd=H31G-02

Thanks for the link, Mark. That helped clear up a few of my questions.

Please note that the linked abstracts says:

However, recent field evidence collected by MER is inconsistent with these predictions, revealing well-formed, active ripples of 100 micron basaltic sand

If we see ripples or dunes or dunes on Mars then they are composed of sand sized particles, just as they are Earth. Finer particles will not form ripples or dunes, instead they will form coatings and mantles, again, just as they do here.

As far as space suits go, the ease with which the rovers are cleaned by wind gusts suggests that cleaning of future suits will not be a problem. Brushes, compressed air jets, small vacuum cleaners should suffice.

Jon

Ron Greeley and his crew of grad students and postdocs have systematically studied eolian sedimentary physics on other planets, starting with clues from Mariner 9 images and building the Mars Wind Tunnel (to start with). We have a pretty good idea of the physics of different sized (and density) particles on Mars, Venus (I presume they've been doing Titan) etc in those "planets" gravity, surface pressure, mean atomic weight, temperatures, wind-regimes, etc. etc. etc.

Mars dust is probably a lot less of a problem than lunar dust, due to chemical weathering, though acid sulfates and peroxides could be a problem. Lunar dust is full of utterly sharp glass microshards and unweathered mineral grain fragments, plus reactive nanoscale iron and vapor deposited products. It's not super toxic. Biological tests with Apollo 11 and 12 (mostly) samples showed that, but it is nasty.

Good info, Ed. Thanks!

I'd sure like to see their Titan studies, if for no other reason than their model might set some composition constraints on the dune material(s).

Not to nitpick but, the full quote is

"The same physics applied to Mars predicted the smallest particles capable of saltation will be four times larger---about 200 microns---than on Earth, and that, analogous with Earth, the mean particle size for Martian dunes should be several times greater still (i.e., coarser than is typical for terrestrial dunes). However, recent field evidence collected by MER is inconsistent with these predictions, revealing well-formed, active ripples of 100 micron basaltic sand".

Observation trumps theory. What has formerly been noodled/theorized has been proven wrong by what the rovers have seen. Bang goes another theory!

Replicating a dynamic 7.5 Millibar CO2 atmosphere accurately on Earth is outrageously difficult even on a small scale. It's not like we have warehouse-sized rooms to test dune formation in. Sure the Mars Surface Wind Tunnel takes up a warehouse, but only about a cubic metre's worth of that is useful.

Dust on Mars goes well below 20 microns, and it "looks" (apologies for being so normative) as though particles of that size are well integrated into Meridiani dunes. Whether this resulted from a saltation process or a suspension-settling process combined with saltation of larger grains doesn't really matter. There is plenty of very fine dust in dunes on the surface of Mars, and Opportunity has not just seen it but gotten stuck in it.

As we've said before, processes on Mars are similar to those on Earth, but the details are different. Until we get the composition and a particle size distribution on these dunes we won'y know anything for sure.

--Bill

There are several ways that dust can be incorporated into dunes without violating the fact that the dune forms themselves are constructed of material that was tranported and deposited as sand sized particles.

The dust particles can be transported as sand sized aggregates of clay-sized particles. You can get interstratifed sand from salatation and dust from fall out. Sand grains can also be converted into much finer grains post depositionally. Dust aggregates may fall apart, sulphate grains can disinegrate from hydration-dehyration reactions.

None ofthis changes the fact that dunes are constructed primarily by the transport and deposition of sand grains.

Jon

The boundaries of phenomena certainly change in sublte and interesting ways. The abstract linked by an earlier poster illustrates this nicely. But the fact that dunes are constructed by sand is either subtle nor a detail. This is something that can be safely predicted without getting close to them. Determining whathe dunes are actually made of, their detailed size fraction, grain structure and composition, and post depositional modification (if any) is what will require up close observation.

Jon

Agree that Martian dust is less likely to be a problem from anengineering perspective, partly because it is chemically weathered and also extensively tranported. Acidity should not be a problem either there are plenty of environments with acidic soils and other regolith materials with similar pHn Earth than can be dealt with provided people are aware of the issue. As for peroxides, does anyone have any details on the concentrations? The only source I have brave enough to give a concentration suggests that the levels are actually quite low.

Lunar fines are nasty, but whether they are very much nastier than say fine volcanic ash remains to be seen. Volcanic ash is also full of glass microshards and unweathered mineral grain fragments, although it does lack reactive nanoscale iron and vapor deposited products.

Aerospace engineers are just going to have to get used to the fact that their nice clean spacecraft are going to get dirty.

Jon

No, it doesn't.

One of the "surprising" -- at least somewhat to me -- discoveries of the rovers is that while micrometer sized dust is everywhere, it doesn't stay micrometer sized except as thin layers on stuff. There are no thick dust deposits that the microscopic camera has looked that that are unresolved dust particle accumulations. There's been plenty of ideas that there are dust-aglomeration (nice word... I can probably work that into a conversation once a decade, if I spelled it right...) processes that build up dust pellets that can be strong enough to act like sand. It's entirely possible that light toned dunes found in many areas are dust-pellet-sand dunes. We just don't know yet.

Though everybody wants a bit, sophisticated sampling rover, there still is an enormously great scientific case for a absolutely minimal cost "grab-bag" Mars sample return. I'd be entirely satisfied with a Viking-like lander with crawler tracks like the proposed Viking III mission... Just enough mobility to get 1 gram samples of different soils and rock-chips from a 10 meter radius of a landing spot. (I would NOT trust a landing to not come down in a sand ripple bed or something like sleepy hollow, and not be able to move a minimal amount.)

We see this on Earth as well. Clay-sized particles in parna often form aggregates (agglomerates if you prefer, although that term is already used with respect to coarse-grained volcanic rocks formed near vents) up to 60 microns. Sand sized aggregates of clay and silt make up many dunes, especially source bordering ones. the really interesting questions are to do with what does this tell us about the soil forming processes on Mars. On Earth such aggregates are mostly deflated from bare smectictic arid soils, dry lake beds, and flood plains.



Definitely. Even a single scoop of the globally distributed fines (~100 g) would tell us about the global crustal composition of Mars and by inference the mantle and perhaps the core. It would also give lots of information of soil-atmosphere exchanges, weathering, granulometry and thus wind behaviour, allow a range of geochronological techniques to be carried out and detailed surfaces for the missing organics and the putative super oxides. Lastly understanding the physical and chemical properties of the fines would be very useful for designing future missions, both robotic and crewed. It would be very expensive but is long over due.

Jon

Well, Purgatory sure looked to me to be made predominantly of grains about the size of fine wheat flour -- whatever size that is. Imprint something smooth on it, and it retains a very smooth surface. It looked just like when I used to play with flour as a kid.

Scott

Have we noticed that there appears to be another area of ripple dune deposits outside the main area in the centre of Victroia...

http://marsrovers.jpl.nasa.gov/gallery/all...FP1997L0M1.HTML

Kenny

I saw those too. They show up as little "wrinkles" in the HiRise image and seem to be of a different orientation that the more prominent dunes.Click to view attachment

They look quite nice looking down from the edge...

Click to view attachment

Click to view attachment

Certainly, Purgatory might well consist of flour sized particles now , but if it was a dune then they must have been deposited as sand grains. If the majority of the dune sand was composed of sulphates and/or aggregates then diagenesis/pedogenesis since they were deposited could resulted in major changes in grainsize.

Jon

Just out in the most recent Earth and Planetary Science Letters:

Claudin, P., and Andreotti, B., 2006, A scaling law for aeolian dunes on Mars, Venus, Earth, and for subaqueous ripples: Earth and Planetary Science Letters, v. 252, p. 30-44.

Read, discuss, and enjoy!

Ambitious title and very interesting abstract. If only I had 30 dollars in my pocket . . .

It's truly unfortunate that it is so expensive for people to access scientific journals these days. It just isn't right. The scientists who did all of the hard work to advance human knowledge didn't intend for their discoveries to be hidden behind a veil of money. I suppose it could be argued that free access is available to those who have access to a good library. If you dig around on the net though, sometimes you can find what you are looking for. We need a new paradigm for the distribution of knowledge.

Gottit! Thanks CosmicRocker
tdemko thanks for your kind offer too.

It is a bit hard to take. The cost of distribution is very low so why charge $30? Limited market perhaps. Most authors have rights to their own articles as well and can distribute them. We don't need these barriers.