Dunes inside Victoria Options

[ngunn]

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.

[Cugel]

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.

[AndyG]

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

[ustrax]

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?...

[Bill Harris]

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

[MarkL]

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)

[climber]

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.

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

[ngunn]

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.

[ngunn]

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.

[CosmicRocker]

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.

[Cugel]

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

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

[Bill Harris]

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

[ngunn]

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?

[kenny]

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

[ngunn]

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