Planet Four Talk

Fan appearance and particle size

  • wassock by wassock moderator

    Could the various fan colours/shades have anything to do with the particle size? This might also explain the fans formed early fade, but those later in the season persist thing mentioned by one of the scientists. The sub ice surface is dusty and the first couple of 'sneezes' will blow all this already loose material out onto the surface. once the exiting dust is gone subsequent eruptions will contain mainly dust eroded from the surface and I'm guessing that this will be fairly fine by comparison with a lot of what is in the normal surface layer. So the first eruption contains large dust particles which warm up in the sun and melt themselves back into the ice whereas the later dust is much smaller and doesn't have enough mass to support enough heat to melt the ice around it, so it just sits there.

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  • wassock by wassock moderator

    Is there any water ice in the underlying surface? If so then some of that is likely to be part of the fan as well?

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  • Portyankina by Portyankina scientist

    The two answers are yes and yes:

    1. The colors of fans could be due to different particles sizes. It is rather hard to pin down what combination you would need though. There is dust+CO2 ice combo, both can be of different sizes, both could influence the surface reflectivity. Dust size however would not change with time, even if it is frozen into the ice lattice - what counts for scattering is the smallest particle size. So such fans could have not disappeared. However there is still a possibility of compaction of dust deposits that also changes the surface scattering properties. But last time I checked, it could not be held responsible for the differences that we observe. This is why we often say that blue fans are CO2 frost = very small ice particles compared to CO2 ice - larger particles. This is the simpler explanation for bright fans.
    2. There is water ice in the underlying surface. Not directly in the top layer, but inside first 5-10 cm. But from spectrometer data we know that surface with fans have no/really few water ice and in addition the signal of water ice slowly decreases independently on the new fans. It is different in the northern hemisphere, but let us get to that when/if you'll get to look at those images 😃

    Anya

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

    What I was suggesting was the the first few eruptions scour the surface dust beneath the ice, once that is gone the only dust in subsequent fans will be what is being eroded which may have different properties from the normal surface dust

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  • Portyankina by Portyankina scientist

    Oh, I thought you wanted to explain the different appearances of fans. Like bright vs. dark, how that changes and so on.
    So, I got it now and fully agree with that theoretically, but I don't think we could see the difference. I mostly think so because when the ice is gone, there is no contrast between former fans and the surface.

    Anya

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  • wassock by wassock moderator

    Is there any reason why you should? I'm assuming that the fans are actually fairly thin, and pretty much the same colour as what's underneath, and made of fairly fine stuff which will get blow all over the place as soon as it's not 'stuck' to the ice. If the same fan forms in the same place and direction every year then maybe you'd get a feature build up but I'm guessing that once its back on the surface it's at the mercy of the wind all the time the sun is out?

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  • wassock by wassock moderator

    What I was trying to explain was what someone, maybe you, posted about fans from early season fading but fans from the back end persist.

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  • Portyankina by Portyankina scientist

    Ok, so what I was trying to tell you that if there is a considerable difference in particle sizes in the earlier and later fans, than there is a difference between later fans (larger particles) and undisturbed surface (covered with dust, same dust that goes up in first fans). But we do not observe such differences. So, as much as you might be right, we have no observation proving it.

    Anya

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

    My idea is that later fans have finer particles than early ones to explain why they dont melt them selves back into the ice. Are you able to gauge how much surface dust there is in different areas? It would all start to fit together nicely if the thick dark fans come from areas of (say) sand dunes and the more diffuse ones from harder less dusty areas

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