Planet Four Talk

Conundrum

  • wassock by wassock moderator

    If you have a vertical vent on flat ground in still air conditions the dust settles back evenly around the vent giving a circular blotch.

    If the vent is on, say, a 45 degrees slope but still vents vertically wrt gravity what shape will the resultant blotch be when viewed from a point normal to the surface at the vent point?

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  • Kitharode by Kitharode moderator

    Honestly - I don't know.

    Instinctively I'd say it would be an ellipse, like the orbit of a comet. Like a comet's orbit there would be a 'peri-venting' point above the vent where the edge of the blotch is closest to the vent, with an 'aph-venting' point below where the edge of the blotch is farthest from the vent. The ellipticity of the blotch would be related to the amount of slope. Greater the slope, greater the ellipticity.

    It would be good to know the truth of the matter. Back to fridges and vacuum cleaners maybe? * 😃 *

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  • p.titchin by p.titchin in response to wassock's comment.

    A mind bender indeed! I think the 'blotch will be an oval with the length of the longer ( up- down slope axis increasing as the gradient increases (vent still central). When viewed from a point normal to the vent, perspective will reduce the long axis, so the blotch will appear more circular again. If the falling column of dust is still expanding, then the blotch will become more egg shaped, as the dust on the downslope side will have more time to expand outwards. The vent will be nearer to the upslope side.This shape again being shortened when viewed from point normal to the vent. I'm sure a mathematician will have simple formulae, but sadly I am no mathematician! (I would guess you can tell!). 😃

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  • wassock by wassock moderator in response to p.titchin's comment.

    Result, thanks Pete. I have been visualising the thing as being longer down slope and shorter up but couldn't figure out how to make it so. Yes, so for a plume that rises and spreads before it falls you'll get an ellipse evenly distributed about the vent. And if it is still spreading as it falls then the ground pattern will be shifted down slope. On a steep slope you'll get what looks like a fan but with a bluntish up wind end and no obvious source? - Seen a few of them. And the colour will be darker up slope and more diffuse down.

    Does this give us some sort of tool to use I wonder - an oval blotch on a slope is indicative of still air, the width along the contour of the slope at the vent point will give a measure of the plume size and the difference between that and the size of the up and down wind lobes will be determined by a combination of how high the plume was and the slope (higher plume gives bigger up and down bits)?

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  • Kitharode by Kitharode moderator in response to wassock's comment.

    It might well act as some sort of 'measuring tool' as you suggest. A scientists take on what we all think so far would be good. Maybe it can't be the way we see it because of ... whatever.

    Let's say you do get an ellipse, with the down-side increasing in size/area in line with an increase in slope. You'd have to be sure there was slope in the image and I'm not sure how accurately we can do that. On flat ground, would a gentle breeze not produce a similar effect?

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  • wassock by wassock moderator

    Ah, now we get to the interesting bit - on flat ground you'd get the same sort of thing getting more eccentric as wind speed increases, up to the point that we get a fan. On a slope you'd only get the same thing if the wind is blowing exactly downslope any wind across the slope will distort that basic pattern.
    All of which I think adds up to the bendy winds can't be down to terrain?

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  • Kitharode by Kitharode moderator

    I was with you up until the last sentance, but now I'm losing it. Help me out. Are you saying that the 'bendy fans' can't be down to terrain, or the 'bendy winds' can't be down to terrain (or both or neither)?

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  • wassock by wassock moderator

    Bendy winds, as shown by lots of small straight fans (which have point sources) all with slightly different directions

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  • Kitharode by Kitharode moderator

    Hmmm ... Ok. I'd like to know more about lie of the land in the Bendy Winds images. I've a feeling that there's a 'valley' across the middle of those images (the dark area with the dendritics) with 'high ground' to the left and right of image.

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  • wassock by wassock moderator

    The original bendy area is manhattan and one of the early papers Meg/Anya provided (series of 4) uses the same image And has a traverse line showing topography

    Hansen et al HiRise observation of gas driven sublimation -driven activity in mars' southern polar regions: I. Erosion of the surface

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  • Kitharode by Kitharode moderator

    I thought I'd seen the image before. Thanks. There's lot of similar scenarios around Inca City, which is where I'd thought the image might come from. There's some 'wind bending' going on there too, with occasional near-90deg turns.

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  • Kitharode by Kitharode moderator

    Not Manhattan by a long way. Closer to Inca City, but hardly next door. I won't say there's no wind, but if there is it doesn't appear significant. Touch of bendyness involved. Black blobs can be seen inside the halo's. Really interesting in the top left area. Pears and teardrops to be added to eggs and ellipses? http://viewer.mars.asu.edu/planetview/inst/hirise/ESP_020592_0955_RED

    I could live with 'slope makes teardrops'. 😉

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