Nature of Victoria's dark streaks, swept clean, deposited, or other? Options

[Stu]

Thanks for explaining that t'Other Doug; makes more sense now.

[dvandorn]

Here's another little gedankenexperiment for those of you who are having a hard time understanding why there are only streaks from some of the bays, and not all of them...

Prop a bicycle up so that one of its tires is free to rotate, without touching the ground. To take gravity out of this little experiment, set the bike on its side so that the wheel's axle is perindicular to the ground. Now take a garden hose with a spray attachment and spray a high-pressure stream tangential to the tire, to spin it up.

You'll find that the water which stays on the wheel as it turns will be sprayed off primarily from a fairly small arc of the wheel, about 120 degrees in the direction of rotation from where the main stream is hitting the tire.

It has to do with momentum and how long the material can be accelerated before the force holding it to the wheel (adhesion, mostly) is overcome by inertia. And it has to do with the original vector and location of the water stream.

Similarly, wind spun up inside a shallow bowl isn'g going to stream out of every available bay equally -- it will spin out where its momentum tosses it out, and since the pressures within the crater aren't even, that will be somewhere between 120 and 270 degrees "down-rotation" from where a majority of the wind enters the crater.

So -- the answer as to why Vicky streams her streaks out where she does is because of the direction of the prevailing wind, where the wind primarily sinks into the crater, and where the pressure bulge of that wind's entry tends to push it back out.

-the other Doug

[Guest_Edward Schmitz_*]

doug,

explain how the streaks in fit into your model...

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[fredk]

Dvandorn, there's certainly no difficulty explaining the asymmetry of the dark streaks given an asymmetric wind flow within the crater! But the problem is that the prevailing wind defines a single direction, and does not by itself give preference to "to the left" or "to the right" of that direction. You need to break that symmetry somehow. Ie, why should the wind flow counterclockwise inside the crater rather than clockwise? It's not obvious how a more-or-less circular crater could break that symmetry in a significant way, without an odd dependence of wind on subtle details of topography.

Coriolis could do it in principle, but should be important only on very large scales, like 10s of km and up.

[dvandorn]

I don't think the bays give the asymmetry here -- I think it has to do with the uneven height of the rim. Wind will enter the depression along the lowest points of the rim, and that likely determines how and where it spins up its racetrack pattern.

In fact, there are some indications that, at some times of year, the racetrack wind pattern in Victoria changes from counter-clockwise to clockwise. Look at the muted streaks coming off the western side of the crater -- those seem to indicate a spin in the opposite direction from the streaks off the north-northeast side of the crater.

Hey, maybe that explains why Victoria has bays and other, similar-sized craters in this same area do not. Victoria is situated on a slope, and that slope allows its internal wind pattern to shift seasonally. So, instead of a billion years of counter-clockwise rotation, it sees seasonal switches in wind direction, which enhances aeolian erosion along the crater rim. So, the same winds that would otherwise have reinforced the circularity of the depression by eroding everything in the same direction all of the time instead do this seasonal push-pull routine, which encourages greater and greater erosion along the rim. Probably also generated the overhangs and undercutting we see in the cape cliffs.

-the other Doug

[Juramike]

Thanks all, for an interesting discussion!

Victoria crater does seem to have an assymetry in the bowl shape. Looking from straight down, the S side seems to have a shallower slope then the north side of the bowl. (The sand dunes at the base also seem situated slightly to the North of center).

Could the assymetry in the bowl shape, coupled with a prevailing wind direction from a vector different to the N-S axis, be enough to cause the effects we are seeing?


-Mike

["Gedankenexperiment" must be German for "will be fun on a summer day!"]

[fredk]

Again, it certainly is possible that topography, including rim height, breaks the symmetry. But remember that Meridiani is extremely flat. From the newly released topo map the rim heights vary by only a few metres, compared to 70 metres or so total depth.

My appologies for sounding like a broken record one day after complaining we were going in circles with these arguments!

Here's a way to maybe break the deadlock. Perhaps someone could build a 3D model of Victoria, then perform some tests by letting wind blow over it and tracing the airflow with smoke. I don't know what would be involved in capturing the dynamics on such a different scale. Perhaps the test would need to be done underwater. Dip your model in a running stream and inject squirts of food colour or silt particles or something. Definitely a fun way to spend an afternoon!

[AndyG]

I don't think the bays give the asymmetry here -- I think it has to do with the uneven height of the rim.

I would almost be buying this. And I'd purchase the (wet?) wind tunnel tests too. Stu - time to dip the plasticine Victoria in the bath and pull the plug.

<rhetorical>How many forums could I meaningfully say that last line in???</rhetorical>

Doug - the RC issues. I personally think the pressure/gravity situation is a killer for realistic small flight on Mars. But there's always a good side: you'd be hard-pressed to find radio interference on your chosen channel.

Andy

Mars and UMSF: you've got to love this place!

[Juramike]

Here's a way to maybe break the deadlock. Perhaps someone could build a 3D model of Victoria, then perform some tests by letting wind blow over it and tracing the airflow with smoke. I don't know what would be involved in capturing the dynamics on such a different scale.

How 'bout a bike trip down tp to the beach on a day with a constant breeze?

We can build a model of Victoria in wet sand in the right orientation, then dribble sugar sand near the rim of the model and see where it ends up.

(Then we do the Gedankenexperiment to wash the sand off our bikes!)

-Mike

[Phil Stooke]

dvandorn:
"Here's another little gedankenexperiment ... Prop a bicycle up so that one of its tires is free to rotate..."

Edward Schmitz:
"explain how the streaks in fit into your model... "

Uh... They're the spokes?

Phil

[Juramike]

I just tried the mind-numbing exercise of going through the MI images and counting representative zones and putting grains in "bin-colors" in order to quantify if there was any difference between on-streak particle count and off-streak particle count.

I figured the ratio of brightest to darkest should be independant of lighting.

There may be a difference, but it might be really subtle. Again, I'll leave the particle color breakdown to image-meisters and their software toys.


I was almost able to convince myself that it "looks" like there is a concentration of dark particles in wind shadows of the beads. I was also able to convince myself that this has been seen in many other images, whether on-streak or off-streak. If this is real (I need a second opinion - I'm pretty much seeing inkblot images of my third-grade bully at this point), then perhaps there is a dark material being deposited all over in little pockets near berries. And perhaps this dark material is just a little bit more concentrated in the on-streak area than off, but just enough to make a difference. We're talking just a few % here.

Has anyone quantified the gross color difference between the two zones? Assuming that the color difference were due to jet black particles, how much % increase would be required?

Here's a picture that "seems" to indicate dark stuff in the windshadow of the berries or is it just shading.

Only the Shadow knows for sure....

-Mike

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[Juramike]

And while perusing MI's I also came across similarities in MI images between Victoria apron, and those near Vostok. The ("polished"?) aspect of the berries seems very similar.

-Mike

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[Stu]

Andy, I might have to get The Model down off the wardrobe...



Ah, happy days, happy days! When we were all still open-mouthed with wonder at the first real sight of Victoria...

So, I pour flour in, point a hairdryer at it from one side, and see what happens...?

[atomoid]

The angled plumes of the dark streaks are indicative of a "racetrack" wind flow within the crater that breaks out at the north and northeast bays. ...begin swirling the soup in the bowl -- in this case, counter-clockwise. ...why Vicky streams her streaks out where she does is because of the direction of the prevailing wind, where the wind primarily sinks into the crater, and where the pressure bulge of that wind's entry tends to push it back out.

Thats food for thought, but i took a different track here (but are there any meteorologists in the house that can help?):

* The racetrack winds are actually spinning clockwise: Duck Bay and Bottomless Bay form the preferential exits for any relative pressure building up in VC due to the prevailing winds, simply because they are gradually sloped and seem to be the lowest points. Since the air pressure primarily exits there, the air speed becomes faster on the left side of the crater, this causes a net clockwise rotation inside the crater which will tend to push the dark grains over the edge of the little humps formed by the debris from the cliffs and pile up the the rotational windshadow on the cliff slopes. there seems to be actual evidence of this process in this photo

* the reason why the dark stream changes course and veers to the west is merely due to the prevailing winds which are flowing in the 'general' direction the the streaks end up pointing. The air after exiting the crater becomes part of the larger windstream and follows that course, forming the deflected streak. From looking at the overhead shot showing the apparently different directional streams, it seems apparent that there is no deviation from this process since any air should exit the crater according to the crater local topography (radial from the crater) and then merge with the prevailing wind direction. The apparent deviation to this flow in the lower right looks more like a contrast artifact from the lighter evaporite dust that has blown onto the SE.

* so why arent there dark grains on the left side of the crater? There might actually be darker material eroding from the steeper cliffs on the NE than there is anywhere else in the crater for this reason: Note that the steep areas, which are probably not coincidentally associated with the dark streaks, all are formed along what is arguably the most linear feature in the crater as seen in the overhead image as well. his suggests this might be a fault or other feature that might have some concentration of one type or another material or might be more easily eroded.

[lyford]

So, I pour flour in, point a hairdryer at it from one side, and see what happens...?

Ah, following the rigorous experimental example of our illustrious founder?