Planet Four Talk

White Fans: A New Model?

  • Kitharode by Kitharode moderator

    The new model discussed here is not my idea. It's from a nasa presentation. I'm only interested in the process described here, not the fans themselves. The process is quite contrary to what I had imagined it to be (rather explosive and relatively short-lived) and I find it hard to accept this 'all day increasing flow' idea.

    Having said that, perhaps you like the new model ... ?

    From page 7 onwards: http://www.nasa.gov/pdf/205657main_AGU2007TitusPress-new.pdf

    Posted

  • mschwamb by mschwamb scientist, translator

    Hiya,
    I believe that this was one of the ideas but this is from 2007. My understanding is that we do think that frost is coming out of the vents and visible sometimes but that there is also evidence that the blueish fans or edges of fans are recrystallized carbon dioxide ice as the fans grains sink through the ice. So I think the latter is the explanation for when you see a whole fan or a large fraction of it in the blue-grey color compared to the black fans

    Cheers,

    ~Meg

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  • Kitharode by Kitharode moderator

    So are you saying that the 'new model' is now shelved and no longer relevant?

    I'm happy enough with the fans (wassock's 'Blue Fans...' discussion has these covered) but I was intrigued by the idea that the venting became gradually more forceful as the day progressed. This raises all sorts of interesting questions. Unfortunately I can't find any accompanying text for the 'new model' so I don't know what the science behind the idea is.

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  • Kitharode by Kitharode moderator

    PS to the above:

    For the new model to work, I think I'm right in saying that after the vent has opened, and gas begins to escape, then "the rate of sublimation below the icelayer must exceed the rate of expulsion through the vent". If the opposite were true then as the gas and dust escaped the venting would subside, not increase.

    So do we know anything about 'sublimation rates'? It seems to me that it would be an important consideration for any model.

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  • Kitharode by Kitharode moderator

    Doesn't answer my question directly, but there's some interesting 'dry ice' info here: http://www.dryiceinfo.com/science.htm

    Of particular interest perhaps (on the links Home Page): "As a general rule, Dry Ice will sublimate at a rate of five to ten pounds every 24 hours in a typical ice chest". Obviously the underside of the martian icelayer is not a typical ice chest, but it's an interesting piece of information.

    Also, "A pound of Dry Ice will sublimate into 8.3 cubic feet of carbon dioxide gas". So there.... 😛

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  • wassock by wassock moderator

    think it goes that a mole of an ideal gas occupies 22.4 litres at STP, think I did the sums a while back and at sublimation temp of 150 K and 7mbar pressure the molar volume is actually a bit less, around 19 litres. this paper has some info on the process and is where I got the numbers to put into pv=nRt to get me volume. Mole of CO2 is 44g

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  • Kitharode by Kitharode moderator

    Good stuff. As is often the case I understand what you've told me, but I'm not sure what to do with it. Chemistry is too hard for me 😦

    What do you think of the all day increasing flow idea?

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  • wassock by wassock moderator

    Makes sense to me, it's where I'm headed withe all the stuff about the shapes of fans, if they are ballistic then some should be affected by topography and so a multi fan would show different shapes, whereas a slower outgass will just go where the wind takes it.

    the conversion into litres is just cos I dont relate to what a cubic foot looks like, a litre is a lot easier to visualise, plus imperial units dont mix too well with moles. The next step you might like to try involves knowing that co2 density is about 1.5 g per cm3. Now you can work out how much gas you'll make per square meter for every mm of ice that sublimes. If you ignore all sorts of other stuff you can then figure out how long the ice layer lasts and come up with an average rate of gas production.

    Read another paper today which is pushing stored thermal energy in the ground, from the summer sun, causing sublimation even during the winter.

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  • Kitharode by Kitharode moderator

    I see now how some of your earlier questioning comes together. You make a good case for slower outgassing.

    Thanks for the guidance on the next step. I'll have a fiddle about with the numbers. I like the bit about "ignore all sorts of other stuff" - I can do that !!

    Stored thermal energy was in my mind when I raised my question about thermal inertia. Obviously I was asking the wrong question, but it's something that I can understand as being important. Any chance of seeing said paper?

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  • wassock by wassock moderator

    sorry thought I had posted it above here you go www.gps.caltech.edu/~oa/publications/conf/aharonson04_lpsc.pdf

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

    So do we know anything about 'sublimation rates'? It seems to me that it would be an important consideration for any model.

    Well that's something we'll hopefully get from your fan and blotch markings. With the shapes of the fans we can back out how much surface dirt was brought up by the jet and from that we can estimate how much gas would be needed to lost that much material. We have a guess of particle size the regolith on the Pole is from I believe thermal inertia measurements and a guess from what was at the Mars Polar Lander site.

    Cheers,
    ~Meg

    Posted

  • Kitharode by Kitharode moderator

    Cheers. I'll take the authors word that the equations all work out as stated, but I get the gist of the argument.

    The "requisite orbital parameters of Mars" paper mentioned on page 1 of the above is here: http://pubs.giss.nasa.gov/abs/al05000n.html (Heavy stuff, but it's more my area so might make some sense of it).

    I admit that the idea of a prolonged, slower outgassing (as you propose) is becoming more comfortable to me, but I'm still a little sceptical about the original 'increasing flow' model.

    Got to admit that it's all getting rather exciting from a citizen science point of view....

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  • wassock by wassock moderator

    Kith the gas needs to come out at a reasonable lick to pick up dust as it flows, I am just proposing that its mostly not an explosive decompression, which requires a mechanism to block the holes between events.

    I dont see a problem with there being more gas coming off in the warmth of the afternoon than in the chill of the morning

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

    What's your take on the opening salvo for these slow burners? Do they need an initial explosive event to open the vent before settling into the prolonged outgassing, or are you leaning towards some other idea?

    Are we treading on thin ice here? 😉

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  • wassock by wassock moderator

    Fnar, Fnar. Not committed either way just trying to fit what we see on the ground to the theory for it it all gets there (or not). At the back of my mind there's the ghost of a feeling that you may not actually get too much sublimation until the system is open to atmosphere - if there is no space then the pressure is likely to be higher than the vapour pressure and so nothing will happen, apart from the whole thing getting warmer. Or something.

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  • Kitharode by Kitharode moderator

    Oh, I know just what you mean about the 'ghost of a feeling' and I can see what you're trying to say. Very interesting idea.

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