Planet Four Talk

Blue fans and the Low pressure physics of CO2

  • wassock by wassock moderator

    We're tagging the blue bits as frost, is it possible that where we are seeing blue fans we are actually looking at snow? As I understand the process; The fans are caused by gas coming out of holes and fissures carrying dust from beneath, It there's no wind the plume of gas and dust goes up and the drops back on itself causing ablotch. If theres wind the gas and dust gets carried along a bit before it settles back leaving a fan shaped stain on the ground. The distance the fan travels from the source is determined by the height of the plume and strength of the wind, so tall plume strong, one directional wind gives a long narrow plume, shorter plume with a weak/blustery wind gives a short fat fan. The fact that the gas/dust is ejecting from the surface means that it's moving from, comparitively, high pressure to low. back home if you do that to CO2, by opening a gas cylinder say, as the gas expands into the lower pressure region it turns solid forming dry ice. So could the gas coming out of the surface be similarly condensing to a solid once it's in the open, giving a plume of solid CO2 particles which then settle as snow? Do the blue fans persist or do they fade with time and turn back to black ones as the CO2 sublimes? If it is snow then it suggests a mechanism for the blue edged fans with the central frost/snow melting quicker than that at the edges as its mixed up with light absorbing dust. The outer fringes of the plumes would contain less dust than snow if the dust particles are heavier than the snow and drop out faster.

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  • Wounded_Knee by Wounded_Knee

    i think i remember a scientists reply about the colors are not always what they seem due to something or other, maybe filter balance, i would think that frost is something that allows surface features to be seen through it, snow would appear as a blanket feature.
    edit...thinking about it if it flies in the air an settles then its snow.

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  • wassock by wassock moderator

    I'm reading frost as CO2 freezing out onto a surface direct from the air, that may not be what was meant when giving the #frost tag but I've then made a leap and assumed that "frost" is a different, separate, process from fan formation

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  • Wounded_Knee by Wounded_Knee

    thats what i was thinking, frost would have the properties of collecting in relative depressions without following wind direction, I assume that frost would show a different spectrum ie more scattered light hence the blue tinge.....( not sure that makes sense 😃..)

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  • Wounded_Knee by Wounded_Knee

    Its times like this you need a Scientist on call, wait a minute........ 😃

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  • michaelaye by michaelaye scientist

    We indeed are not sure if 1. the former dark fan altered the CO2 ice surface in a way that made refrosting of CO2 from the atmosphere on that place favourable, (and fresh CO2 frost is bluer) or 2. if the dark fan was actually very thin and sunk through the ice, leaving a transformed co2 ice shape that looks bluer/fresher. But what we know from looking at time series, that so far the blue fans basically ALWAYS are positioned exactly where a former black/dark fan was located.

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  • wassock by wassock moderator

    Are the former black fans there in the same season? given that it seems common for several fans to be formed from a vent could the explanation for blue fans always following dark ones be that you've not caught a first venting snow fall as yet?
    Just been back and reread the post on bright fans http://talk.planetfour.org/#/boards/BPF0000002/discussions/DPF00000d3. There you are defining "frost" as falling - the meaning is actually snow, as in airborne particulate CO2 which settles from the atmosphere? So the critical point is the chronology of the fans - do the blue fans turn into dark ones ~(CO2 sublimes leaving the dirt behind) or does a pre-existant dark fan on the ice surface somehow encourage CO2 to crystallise onto it to become a true "frost"? Doe you have any time sequenced images which would show which it is?

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  • Wounded_Knee by Wounded_Knee

    As the dark material sinks through the ice it turns the smooth clear ice into blue slush puppy?. 😃

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  • mschwamb by mschwamb scientist, translator in response to wassock's comment.

    Learn more about the blue frost here

    Cheers,

    ~Meg

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  • wassock by wassock moderator

    Meg as I said I went back and read what you wrote on the bright fans discussion. My point is that what you are calling frost is described there more like snow. Frost, on Earth, is a layer of ice which 'condenses' there from the atmosphere. This is not what you are describing as frost, you are using the term frost to describe solid CO2 settling to ground which is snow in Earth terms. The two processes are quite different and understanding what is going on surely requires an understanding of which process is producing the blue ice. If as you state the blue fans ONLY form in the position of an existing fan then frost, by my definition, seems more likely as snow would sometimes result in a blue fan on clear ground because the wind direction changes.

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  • wassock by wassock moderator

    this image http://talk.planetfour.org/#/boards/BPF0000002/discussions/DPF00007ig seems to show blue/green fans on a clear surface. are there any previous images which show these as being on top of dark fans?

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  • Portyankina by Portyankina scientist

    Absolutely right, that snow is when ice condenses in the atmosphere and then falls to the surface and frost is when it condenses directly to the surface.

    "if fans ONLY form in the position of an existing fan then frost, by my definition, seems more likely as snow would sometimes result in a blue fan on clear ground because the wind direction changes" - another point we want to learn from the statistics.
    I saw a lot of examples where dark fan turns with time into bright fan, I even saw the stages of this transformation from outside to inside - like all those blue halos people find. So, frost, as you say, makes more sense than snow. But there is a problem too: dust is dark, dark means warm, because it absorbs more sun light. So why would frost even condense on the warmer dust?

    Anya

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  • wassock by wassock moderator

    Depends whats going on when the fans are active.its a spring thing? So there is day and night? Frost forms at night then thaws again maybe and having a solid substrate of dirt my encourage frost whenits cold. This would explin the dark fans with blue edges. The sides have less dirt and would thaw slower than the dirty middle.

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  • wassock by wassock moderator

    Just realised i dont know how the seasons work on mars and the above is assuming its like at home. So whats spring on mars mean?

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  • Portyankina by Portyankina scientist

    The seasons are pretty much like on Earth, just twice longer, as martian year is twice longer. (while martian day is almost same 24h). Just remember that we are looking at polar areas, where day-night change only happens in spring and fall, while during summer there is polar day and polar night in winter. This also what limits us: we have no idea what happens there in winter, because we can not observe it - no light.

    Anya

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  • Portyankina by Portyankina scientist

    And regarding your previous idea: if the dust layer is thin enough so that it completely cools at night, it might work. I like this idea a lot, maybe I can do some modeling of this to see if it works...

    Anya

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  • wassock by wassock moderator

    Any theories on how the layer of ice forms in the winter - is it a condensation feature, just 'sucking' the CO2 from atmosphere, or is snow, or both? Or just dunno?

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  • Kitharode by Kitharode moderator

    Wassock: For what it's worth, I think this is a great piece of citizen science. Congratulations on inspiring a possible new model for these most fascinating objects.

    By now, you can probably answer your questions above far better than I can answer them for you. (Thank goodness!).

    I'd like to know more about what happens under the icelayer at the end of a venting session. If a volume of gas and dust is expelled from underneath the ice, there is the possibility at least of an empty chamber being left behind, eg spider leg. If there is such a chamber then how empty can it become? How, and with what, does it fill itself up again? Might there be a 'sucking in' of atmosphere after the venting and, if there was, how would this affect the area (fans) around the vent hole?

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