Planet Four Talk

Spiders

  • wassock by wassock moderator

    Spider Formation. In the beginning there is a landsape, it is covered by ice all winter and in the spring the sun warms the surface beneath the ice creating a thin layer of gas between the surface and the bottom of the ice. For the pupose of this description lets say that the layer of gas covers a discrete area of 100 square meters which for want of a bbetter term we shal call the bubble (even though pancake miight be a better term). As spring prgresses the surfcace gets warmer and the gas pressure builds, then one morning something makes a hole in the ice somewhere above the bubble. The gas is now able to expand into the atmosphere until the pressure above and below the ice equalises.

    For this first eruption the gas space beneath the ice is very thin capillary even, and the gas finds it difficult to move towards the zone of low pressure. In some places the gap will be wider and the gas will move better neaning that a larger voume of gas will pass points where there is less constriction. In these areas the moving gas, and any particles it is carrying will wear the suface of mars and the ice above making the gas space larger wherever there is movement proportional to the volume of gas and speed of passage at any given point. Once the pressure has equalised the erruption stops and gas flow ceases, the hole seals (somehow) and the system recharges with gas.

    The next time a hole forms the escaping gas will again find the easiest way out and consequently any places where the surface/ice gap is slightly wider will see increased gas flow, and consequent erosion. This will hold everytime there is an eruption, the places with bigger gaps wil see more gas flow than their surroundings, and more erosion. the effect will be greater, for a single event, for features closer to the hole, but across the whole bubble there will be preferential flow to a greater or lesser degree everywhere in the bubble regardless of where the hole forms. Initially the process will form channels, largely independent of one another and will take a long time to leave any thing visible.

    The process might be accelerated if there is dust on the surface which both creates space by being vented and provides a medium for the gas to flow through.

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  • wassock by wassock moderator

    In subsequent season the process repeats, the bubble may cover the same area or there may be a different pattern of bubbles, or the whole surface my be one big bubble. Either way there is still preferential flow in places with a bigger gap.

    Surface dust as a medium for gas flow may be important as channels develop and dust collects.

    It matters not how the gas bubbles are arranged or where the vents form the channels will continue the get bigger at the "expense" of higher ground which is tight to the ice. Eventually individual channels will meet and intersect giving a 2 dimensional system which increases its potential for acting act a route for more gas.

    To visualise this consider a round bubble of gas containing two intersecting channels, which conveniently meet in the middle running up to the top and middle to left. So for any vent north of the bubble it is likely that the best path from the west will involve the L shaped channel and any gas from elsewhere that finds its way to the middle will also likely drop into the channel on its way out. Vents to the right and bottom will also draw gas along the channel to the bend where the gas will then seek the path of least resistance, thereby eroding a route out and starting the growth of a new leg.

    As the spider develops, a vent anywhere will result in a lot of the gas flowing down the channels to the centre and then out to the vent, consequently the middle of the spider sees much more gas flow than anywhere else and erodes much more.

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  • wassock by wassock moderator

    So there you have it a theory of how spiders form and grow new legs, and why they habe bigger bits in the middle. Not sure if any of it is new, but not seen the idea of increased wear regardless of vent location anywhere. Doesnt mean theres not one of course. It also doesnt require the ice to form the same each year, for gas bubbles to cover the same area each year or for vents to always form in the same place. The same basic mechanism allows formation of different spiders due to terrain, but thats probably enough theorising for now. The thread is yours

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  • Kitharode by Kitharode moderator

    Yo. I'm gonna read this soon. Busy elsewhere. If poss would appreciate paragraph breaks in long posts - old eyes and modern screens don't mix well. Speak later....

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  • wassock by wassock moderator

    The system closes everything up when you post, paragraphs, returns and even 2 spaces after a full stop, all dissapear

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  • JellyMonster by JellyMonster

    wassock, nice (long) article but it will take me a few reads.

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  • jellyhead by jellyhead

    Ok, I really like this. So it's the intersection of random channels that increases the probability (to bring in Anya's answer!) of erosion at the intersection and develops a 'centre' which then becomes progressively deeper as more legs join? Your explanation of one vent in one leg causing the gas to rush out of the entire spider, with the centre always eroded everytime one leg is eroded, is one I hadn't thought of but makes a lot of sense.

    The only issue that I can see is that the first stage requires an open system - random channels forming throughout the ice sheet, sublimed gas flowing under the whole ice sheet and escaping where it can but then it requires the spider to be a closed system, in order for one vent to empty the spider. Also, how would the closed system work with the 'web' type spiders?

    I wonder whether you've got two models in one there? So the first model is the intersecting channel one - the image here APF0000k2y would support your theory. This would lead to the web type spiders as they connect and join up. My hunch is that these are over flat terrain where the wider breakup of the ice sheet (into polygons) is a key variable.

    The second would be how the true spiders develop (closed system, one vent emptying whole spider), but not necessarily how they are initiated. I think we would see spider growth one leg at a time and not from a central point if the intersecting channel theory was applied here. I'm convinced that, at least for the larger true spiders, they develop from the central point. Not sure how! I guess either as a feature of the topography, they are basins and the dust filled gas moves into them (similar to the river bed dendritic drainage that Anya mentions in her post in 'ask the scientist') or from localised melting of a small spot under the ice - maybe the initial explosion from these hotspots venting is so intense it causes radial cracking in the bedrock, which then dictates the gas flow in later cycles?

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

    Actually I dont think it requires a spider to be either an open or a closed one, it will likely take many seasons for any thing to form and if one year only half the spider sees any gas flow then it won't matter too much.

    With you on the web type spiders - the crack will clearly form a path of least resistance for gas flow so it would make sense for there to be wear beneath, if the cracking is something to do with the landscape then maybe you get cracks in the same place often.

    The point is that when there's a vent anywhere then the gas moves across the whole sub ice surface with gas available that can get to the vent. The moving gas will use the path of least resistance where it can and will find the best way when there's no convenient channel (and thereby make the beginnings of a new leg, or finger.

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

    NB there are no cats on Mars so not at all sure what all that probability field stuff is about.

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  • jellyhead by jellyhead in response to wassock's comment.

    😃 ! Could it work as a thought experiment - is a cat on Mars dead, alive or both.....

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  • Kitharode by Kitharode moderator

    Wassock, Jellyhead. Just wanted to say how much I've enjoyed your discussion so far. It's thrilling stuff and all makes good sense to me (for what that's worth). You've got some real mileage in this one and I think you should keep picking away at the ideas. Lots of stuff you can take as fact, plenty of 'pretty sure that's right' moments. You're giving me a lot to work with because you're painting such a vivid picture of your theory. Congratulations.

    From the initial mellee of our earlier meetings we are now finding our own focus from amongst those ideas we kicked about and this is really good, especially because we keep in touch, but also I believe because when we put all our ideas and theories together way down the line, we're gonna be really chuffed at what's been acheived. I'm learning loads and having a whale of a time - I might even be contributing. Encore gentlemen, please.

    Many thanks for picking up and running with the 'true' and 'web' class names. Hope you'll expand and continue. (At which point he slipped in a quick advert about the SCS project thread being heavily updated ... exit stage left)

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  • michaelaye by michaelaye scientist in response to wassock's comment.

    wassock's reasoning is very sound here, well done. One point that helps with understanding is to realize what a 'channel' really means? When do you call something a 'channel'? And we believe it exactly like wassock said, that initially merely random movements will lead, over time and by erosion, to a defined channel, that then has the preferred path for the gas, rendering it stronger and stronger as a topographical feature. With gas movements that start from a center, the gas would have no reason no go to smaller units, therefore we don't believe jellyhead's model to build a spider from a central point works, we believe the other way around, from small towards big cavities makes more sense.

    Some interesting open questions: Why do some spiders seem to have a larger central-cavity to average arm diameter ratio? In other words, why for some spiders the central part looks far wider/thicker in relation to the arm width than for others? And the crown jewel question: Is that a different point in time for the general spider development, or a different class of spiders, caused by differences in ground erodability or other differences in the environment?

    Exciting stuff!! 😉

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  • jellyhead by jellyhead

    So channels are formed randomly, but their preferred path is to a central feature? I can see how this would be the case when random channels intersect, as that intersection/centre then becomes the most likely place for erosion for both channels as explained above by wassock but how does this explain the larger spiders such as APF00003bc? Why aren't there other random channels being formed around or in the spider, how do all these channels 'know' where the centre is? That's why I keep thinking that the centre must have some influence on the formation and development of spiders, and we see a lot of images which appear to indicate that spiders start small and grow radially outwards rather than one leg at a time as channels intersect.

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  • wassock by wassock moderator

    The problem I think with the idea that the body develops first and then the legs grow out from there is that I don't think I've seen a fan which comes from the body, the majority come from somewhere out in the 'leg field' and often are not associated with a visible part of the spider at all. The preferable path isn't to a central feature it's the path of least resistance from where the gas is to where it wants to be at (the vent) - so while the shortest route may be across the space between legs the easiest route will be through an 'open' channel down the nearest leg to the middle and then out again down the leg which will take it closest to the vent. Once the path runs out of easy ground it will take the hard route between surface and ice and in doing so it'll wear a bit away so that next time gas needs to pass that why there will be a route which is slightly easier to travel than the surroundings.

    I think that the differences in spider morphology will be down to differences in the surface hardness, its thermal properties and variations in slope.

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  • wassock by wassock moderator

    Anya just posted this on the classification thread - so much for fans not forming from the body

    https://dl.dropbox.com/u/356411/ESP_020597_0925_sf.jpg

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

    End of your last post but one you say "I think that the differences in spider morphology will be down to differences in the surface hardness, its thermal properties and variations in slope."

    From my recent posts on the landscape/morphology thread (begins at bottom of page 3 - quite long) you will find not only a glowing report of this thread but also;

    (After inspecting and discussing an image which apparently shows spider leg being 'forced' to change direction...) "All this strongly suggests that the top half of the image shows lower land sloping up to higher land, whereas the lower half of the image tells me that we are moving across a more even landscape. There is an implication here I think that ‘slope’ may influence leg growth in spiders, indicating perhaps that leg growth follows a path of least resistance. All this is very speculative, I agree, but I believe we can get a better feel for what we are dealing with here by this sort of scrutiny."

    Thought it might be of interest. (I'm in the 'starts in the middle and spreads' camp at the moment).

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  • eagiles by eagiles

    oo jellyhead the cat in the bag argument deep, i don't know about the closed spiders, if a vein is broken by a crevasse would it not give the same illusion, it is a shame that the images can't be viewed in stereo as the ordinance survey use to study there aerial photographic images.

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

    I'm not looking at slope with regard to the gas finding it easier to move downhill, rather that the undulations beneath the ice will both create spaces and affect the thickness of the ice above it. I would expect the ice to be thinner atop a ridge and also the ice to be closer to the surface than further down slope.

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  • Kitharode by Kitharode moderator

    You know me and landscapes, so your undulations theory sits well with me. The 'ice thinner atop a ridge, so closer to surface' idea rings true (Anya might know this) and I'm very much in favour of the 'path of least resistance' model that you mention earlier. My 'slope' post earlier was offered in support of the 'least resistance' idea. Rereading that post, I can see that perhaps I've not explained myself well enough.

    In relation to slope, I used the word 'influence' and perhaps this was not the best choice, you may be thinking that I'm suggesting a positive effect from a spider leg perspective; impede is better. My line would now read; "There is an implication here that ‘slope’ may impede leg growth in spiders, indicating perhaps that leg growth follows a path of least resistance". I thought that meeting an up-slope head on was not unlike meeting an undulation on the up. In the image I used this undulation looks to me like an up-sloping area, much wider than it is tall. On the assumption that a left or right turn would allow growth along the bottom edge of this bank, which is probably a 'flatter' route, then given a nook or cranny to work on it would take the left/right option, that is, it would try to follow the path of least resistance. Hope that's clearer even if it's still no use.

    In case you want to look again the bank and bent leg are in the top half of the image, in the middle. http://planetfour.org/subjects/standard/50e73ca15e2ed21240000b57.jpg

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  • wassock by wassock moderator

    There are legs running up the bank though.....

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  • BlueMoon58 by BlueMoon58

    Just been looing at the landscape of Greenland and what struck me was how similar the erosion of the landscape was to some of the images of Spiders on Mars. In the case of Greeniand the melting ice has carved fern like valleys into the bedrock as the ice flows into the sea. On Mar many of these Spider shapes seem to surround a vent or hole. Could these be a sink holes where Ice or Liquid water escapes back under ground ?

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  • Kitharode by Kitharode moderator

    I can see why you might think that, BlueMoon58. The Martian southern polar area, under investigation on this site, is way too cold for liquid water - lots of water ice, but nothing runny. http://www.esa.int/Our_Activities/Space_Science/Mars_Express/Water_at_Martian_south_pole

    Don't think there's any sinkholes or draining vents involved with spiders (but don't quote me on that one!).

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  • eagiles by eagiles in response to Kitharode's comment.

    the fact that there is a bank and a crevasse running along the top of it show the pressure cracking the ground the more pressure the deeper the crevasse as in the image
    http://planetfour.org/subjects/standard/50e73ca15e2ed21240000b57.jpg
    there is no crevasses on the slope as the expansion has not got there

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