Planet Four Talk

Breathe In, Breathe Out, or What?

  • Kitharode by Kitharode moderator

    I've got a feeling that I've seen an explanation to my query elsewhere, either here or in some article, but try as I might I cannot now find that explanation. However, I think the problem is worth discussing here and hopefully you can help me out.

    "During each year on Mars as much as a third of Mars' thin carbon dioxide (CO2) atmosphere "freezes out" during the winter in the northern and southern hemispheres." (Wikipedia).

    Fair enough. But as the north 'breathes in' the south 'breathes out'. As the icelayer forms over one pole (depleting the atmosphere) the icelayer at the other pole is subliming (replenishing the atmosphere).

    Why does the one not negate the other?

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  • wassock by wassock moderator

    Suspect that come late summer all the ice at one end has gone and the freeze at the other is not yet started

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  • angi60 by angi60 in response to Kitharode's comment.

    This might be a bit simplistic, but could it be due to the fact that the winters and summers in the North and south differ due to the eccentricity of Mars orbit e.g winter in the south is longer and colder, whilst at the same time the summer in the north is shorter and cooler.

    I'm probably way off beam, but it was just a thought! :-X

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  • Kitharode by Kitharode moderator

    Wassock. There probably is some 'seasonal drag', if we may call it that, and if the figure had been say 5% then I probably wouldn't have questioned it. But I find a 30% atmospheric depletion, caused by 'drag/delay' alone, a little hard to accept.

    Angi60. (Getting good at this astronomy lark, eh?). Eccentricity must indeed play some part here and you are correct when you say that southern winter is longer, but I'm yet yet sure that it's colder. We know that the north pole is several kilometers lower in altitude than the south and I think this makes it colder for longer than it would otherwise be. Need more info on that.

    If 'seasonal drag due to orbital eccentricity' is true, I'm happy enough. But I've a feeling there's more to it than that. We'll see....

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  • wassock by wassock moderator

    I Googler this yessedie found a paper about variability of the atmosphere over big time. As the tilt of the axis varies the length and depth of the winters varies so in a warm part the co2 in the atmosphere is higher than when its cold, corresponding to the size of the winter ice caps

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  • Kitharode by Kitharode moderator

    Yes indeed. We've talked elsewhere about the massive changes in axial tilt of Mars and it is easy to see how polar caps would shrink and grow (and relocate?) across the polar areas in line with the changing tilt of Mars. Also, it's not hard to imagine that over these long timescales there will be periods with less of an icecover, therefore denser atmosphere. At other times the opposite would be true. It may even be that the amount of atmospheric depletion in winter was greater or lesser than the current 30%-ish figure.

    But at present the tilt and atmosphere of Mars are whatever they are, so for now we must consider them 'fixed'. Eccentricity for sure plays a big part in the speed and duration of icelayer formation and sublimation, but on its own is this enough to allow one polar icelayer to almost fully form before the opposite pole begins to sublime? I'm not sure.

    I've been thinking about the areas beyond and between (as well as below) the icelayers. I've heard of something known as the "thermal inertia of rocks" and I believe there are areas on Mars recognised as having high/low thermal inertia. On the assumption that thermal inertia has something to do with 'how much heat a rock can hold for how long', then perhaps this could play a part in producing the 'seasonal drag'?

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  • Kitharode by Kitharode moderator

    PS: Changes in axial tilt might well be responsible, at least in part, for the location and extent of the polar layered deposits ... ?

    http://www.nasa.gov/images/content/171411main_pia09224-thick-annot-516.jpg

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  • angi60 by angi60 in response to Kitharode's comment.

    HAHA, you'll be waiting a long time before I'm good at Astronomy! But at least I'm plucking up the courage to make comments now, even if they're at a basic level 😃 (something I wouldn't have done three months ago). 30% is a large amount isn't it? All very interesting!

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  • wassock by wassock moderator

    Thermal inertia is, I think, mostly about amount of stuff. So if you have a big lump of stuff it takes longer to warm up/cool down than it would if the same amount of stuff was spread out. Think of the snowman on your snowy lawn. Different types of stuff have different thermal properties but in general a thin layer will change temperature quicker than a ball. In the cryptic region on mars, which h seems to stay colder than it ought, I think the theory is that there is a big deposit of ice covered with a blanket of surface dust.

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  • wassock by wassock moderator

    If anyone's up for some tricky physics - at home there's a huge reserve of co2 stored up in the ocean, what happens when the sea freezes? Is the Co2 released or is it trapped in the water ice? If there's lots of water ice on Mars does it also contain a proportion of CO2?

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

    Yes, I see the subtle difference and I get what you're saying. Good stuff - Thanks.

    I'll get back to you on the tricky physics. 😛

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  • Kitharode by Kitharode moderator

    I'm beginning to think that I'm asking the wrong question, or asking my question in the wrong way.

    From what we're told it's clear enough that a fully formed icelayer 'freezes out' about 30% of the atmosphere. Seasonal icelayers at both poles are similar is thickness and extent, so this 30% depletion occurs at both poles during their respective winters.

    What I haven't yet seen is an explanation of the 'transport' of this 30% from one pole to the other. It may well be that an icelayer fully sublimes before the opposite icelayer begins to form, but it could easily be otherwise.

    Therefore my question is: Does one polar icelayer fully sublime before the other one starts to form? If yes, problem solved. If no, I've got a project. 😉

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  • wassock by wassock moderator

    Probably depends on iffing the planet is warming or cooling. There's a permanent ice cap at both ends so figuring when ALL the seasonal stuff is gone will be tricky. If mars is warming then on average a bit more than what formed over winter will go, cooling then a bit less

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  • ngheavens by ngheavens

    CO2 sublimation or condensation is governed by an energy balance equation:

    I'll summarize it as:

    The amount of solar energy absorbed by the surface+the amount of infrared radiation emitted by the surface+any downwelling infrared radiation emitted from the atmosphere+ vertical transport of heat from or into the ground = the energy to sublimate or condense carbon dioxide.

    When the CO2-ice covered surface is a net absorber of energy, ice will sublimate and the remaining ice will remain at the sublimation temperature of CO2 until there is no CO2 left. When CO2 sublimes, there is a local pressure perturbation, which drives a little bit of wind away from the cap. If there were no condensation (technically deposition, since it's a gas to solid change) of CO2 ice anywhere else, the total atmospheric mass and mean atmospheric pressure would increase.

    The question Kitharode asks is a good one. CO2 is likely to sublimate when the pole emerges out of its Sun-less season, which may boost solar absorption at the surface and thus turn But that's the time when the other pole is entering its Sun-less season. Wouldn't its energy balance shift to deposit CO2 ice at the other pole, exactly balancing the loss at the other pole and resulting in no change of mean atmospheric pressure?

    The first answer is that it doesn't. Pressure observations by instruments on landers from Viking to the present Mars Science Laboratory show significant pressure oscillations.

    Of course, that's no proof. Perhaps, the changes in CO2 ice at the poles aren't responsible for the pressure changes. Except that they are: the Mars Orbiter Laser Altimeter made measurements of polar ice cap height changes that line up with the pressure records: http://mola.gsfc.nasa.gov/snow.html.

    However, when scientists try to model the physics of the process (it's very important for the models used to study Mars's weather and climate), they get the "lag" discussed here wrong. And that's because incoming solar radiation isn't always the dominant energy term in the equation. When a seasonal cap erodes away, the water ice can store incoming solar energy as heat. When the Sun falls over the horizon, that heat can conduct toward the surface again. Carbon dioxide ice can condense into clouds in the atmosphere. These clouds can precipitate snow, adding CO2 ice to the surface but also adding downwelling infrared radiation to the equation. Moreover, the absorption of solar radiation at the surface depends on the albedo: a complex function of the grain size of the ice and any dust involved.

    The complexities of energy balance at each pole set the unequal balance of sublimation and deposition, producing the two minima and maxima in Mars's seasonal surface pressure curves .

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  • wassock by wassock moderator

    Clear as trust then 😉

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  • Kitharode by Kitharode moderator

    Wassock: And as grust (which I'm sure is what you meant).

    Ngheavens: That's gonna take some thinking about, but it's all good stuff. Although I'm more of a 'bigger picture' kinda guy, I'm always happy to learn from someone else's more detailed approach. Many thanks for posting.

    We've met the 'MOLA globes' diagram before, along with some more detailed plots from the science papers in 'Spiders by Latitude' here: http://talk.planetfour.org/#/boards/BPF0000008/discussions/DPF0000c2e

    Going back to the 'bigger picture' - I've spun my football round my lightbulb, drawn up my diagrams, and had a good think about the situation and I'm now convinced that, give or take a little, one polar icelayer will fully form and sublime before the opposite pole does the same. Astronomically, I'm happy with that.

    Let's say for now that that's how it is. How does this work from the atmosphere's point of view? Does the global density gradually increase as the subliming icelayer replenishes the atmosphere? Or is there a 'seasonal transport cycle' thingy going on?

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

    Grust indeed kith, damn preditictive text.

    Ngheavens I'm with kith, seasonal frost at one end is gone before t'other end starts forming .

    Couple of questions/points about your post: Is all this infrared radiation different from the solar? If so where's it coming from?

    You seem to be suggesting that the whole body of ice needs to be at sublimation temp for sublimation to happen?

    The winters are not equal at each pole so even if they were nicely synched to start and stop they wouldn't balance
    .

    When its dark we can get snow which "adds downwelling IR...." how?

    We are working on the premise that the ice is effectively transparent and that any dust in it melts its way out fairly quickly once the sun gets on it so albedo,as in reflectance, may not be much of an issue with most of the solar gain happening at the surface beneath the ice

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

    Thanks Nick for answering. Nick and I were in grad school together. He studied Mars' atmosphere and was part of a big effort to model the Martian atmosphere.

    Cheers,
    ~Meg

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  • Kitharode by Kitharode moderator

    Good to know. Thanks Meg (and Nick).

    Wassock and I were in grust school together. He's part of a big effert to get me some psychiatric treatment. ** 😛 **

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  • Kitharode by Kitharode moderator

    The article mentioned in Anya's recent blog post is very relevant to this discussion. If you haven't already, take a look at the first few paragraphs and the excellent graph there: http://www.planetary.org/blogs/guest-blogs/2014/20140116-dry-ice-snowfall-at-the-poles-of-mars.html

    Here's the Solar Longitude (Ls) diagram if you need it: http://planetfourzoo.files.wordpress.com/2013/02/orbit.png?w=450&h=342

    The primary minima is around Ls=150deg (southern winter) with a secondary minima at around Ls=340deg (northern winter). The results are very interesting and somewhat surprising. For example, I would have expected the primary minima to be at, or later than, Ls=180deg (deep southern winter).

    At least the original question is answered: Yes, there is a 'breathe in, breathe out' cycle.

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

    Sorry, all beyond me, but I was not aware of big changes in axial tilt over short periods. A geologically short time ago , there was no 'ice cap', now there is a much bigger ice cap at the south pole.as the axial tilt has changed. The 'lag' , 'inertia' (or whatever other terms are used) would seem to mean that the smaller northern ice cap will react more quickly to seasonal change than the southern ice cap. just a thought,as I said to begin with, beyond me really. I'll follow the thread with interest. ~Pete.

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

    Well in our oceans the carbon dioxide gets converted to carbonates rocks at the bottom of the oceans and stored there or at least it is one of the sinks for carbon. Also the full ocean never freezes only the top layer so the carbon dioxide remains dissolved in. But yes people have searching for the signature of large amounts carbonates of Mars from orbit, because that would be a big signature that there was a huge amount of standing water. There has been some discovery of carbonates (and http://www.nature.com/news/2008/081219/full/news.2008.1329.html) on Mars.

    Cheers,
    ~Meg

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  • Kitharode by Kitharode moderator

    An interesting abstract which sheds some light on the situation.

    Giuranna, M., Formisano, V., Grassi, D., Maturilli, A., 2007. Tracking the edge of the south seasonal polar cap of Mars. Planetary and Space Science 55, 1319-1327.

    The advance and retreat of the polar caps were one of the first observations that indicated Mars had seasons. Because a large portion of the atmosphere is cycled in and out of the seasonal caps during the year, the frost deposits play a significant role in regional and global atmospheric circulation. Understanding the nature of the seasonal polar caps is imperative if we are to understand the current Martian climate.

    In this study, we track the seasonal cap edges as a function of season and longitude for the fall and winter seasons (MY27), using data from the Planetary Fourier Spectrometer (PFS) onboard the Mars Express (MEX) ESA mission. Making use of the rapid rise (decrease) in surface temperature that occurs when CO2 ice is removed (deposited), in a first approach, we defined the advancing cap edge to be where the surface temperature drops below 150 K, and the retreating cap edge where the surface temperature rises above 160 K.

    In this case, starting from Ls∼50°, the edge progression speed start to be longitude dependent. In the hemisphere that extends form the eastern limit of the Hellas basin to the western limit of the Argyrae basin (and containing the two) the edges progression speed is about a half than that of the other hemisphere; the cap is thus asymmetric and, unexpectedly, no CO2 ice seems to be present inside the basins. This is because the above mentioned surface temperatures used in this approach to detect the cap edges are not adequate (too low) for the high-pressure regions inside the basins where, following the Clausius–Clapeyron's law, the CO2 condensation temperature can be several degrees higher than that of the adjacent lower-pressure regions.

    In the second, final approach, special attention has been given to this aspect and the advancing and retreating cap edges are defined where, respectively, the surface temperatures drop below and rise above the CO2 condensation temperature for the actual surface pressure values. Now, the results show an opposite situation than the previous one, with the progression speed being higher and the cap more extended (up to −30° latitude) in the hemisphere containing the two major Martian basins.

    During the fall season, up to Ls∼50° the South Martian polar cap consists of CO2 frost deposits that advance towards lower latitudes at a constant speed of 10° of latitude per 15 degrees of Ls. The maximum extension (−40° latitude) of the South polar cap occurs somewhere in the 80°–90° Ls range. At the winter solstice, when the edges of the polar night start moving poleward, the cap recession has already started, in response to seasonal changes in insolation. The CO2 ice South polar cap will recede with a constant speed of ∼5° of latitude every 25° degrees of Ls during the whole winter. The longitudinal asymmetries reduce during the cap retreat and completely disappear around Ls=145°.

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