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The carbon dioxide ice on the Martian poles are known to evaporate every summer and re-deposit every winter.

Where does this carbon dioxide 'go' and does it mean there is a seasonal variation in the atmospheric pressure on Mars?

Is there any known estimate on how much CO2 gets evaporated or how thick the layer of ice is that gets sublimated and re-deposited each season?

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According to https://marsed.asu.edu/mep/atmosphere

The deep cold southern polar winter removes CO2 gas from the atmosphere by freezing it directly onto the south polar cap. As temperatures drop below –123° Celsius (–189° Fahrenheit), CO2 freezes out as frost, snow, or ice. This causes air pressure all over Mars to drop by 25% to 30%.

Wikipedia says something similar:

In summer, the polar dry ice cap can undergo sublimation and release the CO2 back to the atmosphere. As a result, significant annual variability in atmospheric pressure (≈25%) and atmospheric composition can be observed on Mars.

https://www.msss.com/http/ps/seasons/seasons.html gives a graph: seasonal pressure graph

A prettier graph is available from https://www.planetary.org/space-images/atmospheric-pressure-data-viking-landers:

viking 1, viking 2 pressure graph

If you want the raw data it's available from NASA

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    $\begingroup$ I know you didn't produce it, and from the materials you found it's the appropriate choice, but wow what an awful graph that is. Once I looked at the context I was reasonably sure my first interpretation was correct, but wow. $\endgroup$
    – Erin Anne
    Nov 7 at 20:36
  • $\begingroup$ @ErinAnne I mean, both axis are described. The notches are clearly 1 mb vertical and 200 days horizontal. Is there something else off about it? $\endgroup$
    – Yakk
    Nov 8 at 1:07
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    $\begingroup$ @Yakk the X axis actually isn't described, only its extent is; you've inferred (as have I) that they're Mars days. Having to infer the labels and increments of the graph is bad too, though. $\endgroup$
    – Erin Anne
    Nov 8 at 1:51
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    $\begingroup$ @ErinAnne I dunno, a graph about atmosphere of Mars as it cycles through the year with 687 in one axis is pretty self explanatory. :) It would be like a graph talking about traffic in a major city with 7 days listed. Sure, knowing where 0 is might be useful, but here it wouldn't help, as few people know the Mars high holidays, let alone when Mars new year is. ;) (I'm just kidding; but I'd guess most of their intended audience was planetary scientists, and they would tend to have 687 memorized: I sure didn't) Also, that was a 1994 graph: I'm surprised it is in a usable image format! $\endgroup$
    – Yakk
    Nov 8 at 5:39
  • $\begingroup$ The HTML page says: it reaches a maximum during southern spring and summer, has its deepest minimum near the end of southern winter, and has a smaller minimum at the start of northern spring This could mean that $x=0$ is the day of the northward equinox? $\endgroup$
    – Jeppe Stig Nielsen
    Nov 8 at 15:30
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More a comment to the answer above, than an answer on its own: the referenced web page provides the raw data of the diagram as a tab separated ASCII:

0   8.0
57  8.25
114 8.4
172 7.75
229 7.0
286 6.9
343 7.25
400 8.0
458 8.8
515 8.9
572 8.5
629 8.0

which, saved as data.tsv, I used to reprint the diagram with gnuplot as

enter image description here

by the script below.

#!/usr/bin/gnuplot

set terminal pngcairo;
set output "test.png";
set term png size 800,400

set xrange [0:650];
set format y "%3.1f";

set xlabel "day of year";
set ylabel "pressure [mbar]";
set title "variation of Martian atmospheric pressure";

set mxtics 5;
set mytics 5;
set ytics 0.5;

set grid;

set key left;
plot "data.tsv" u 1:2 pt 7 t "original data", \
     "" w l t "smoothed with csplines" smooth csplines;
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    $\begingroup$ Upvoted. But in your fit, the slope at the $8.0$ point at day 0 is different from the slope at the $8.0$ point at day 629. You could wrap the sheet on which the data is plotted around a cylinder with circumference 629, if you wanted to identify the two points. Then if a fit respected this, the graph should have no "bend" one the northward equinox. $\endgroup$
    – Jeppe Stig Nielsen
    Nov 9 at 9:41
  • $\begingroup$ there's a local minima around day 600 in the original graph which isn't shown in yours $\endgroup$
    – Alan Birtles
    Nov 9 at 10:10
  • $\begingroup$ @JeppeStigNielsen I agree with the observation however lack experience how to advise gnuplot to perform the suggested «fold around a cylinder». Maybe visually it would improve the plot to copy paste the assumed full sweep for one additional period prior, and one additional period after the time frame displayed (just a matter to adjust xrange) but this is an addition of data I don't assume right. The msss.com page possibly omitted some readings print in the table or/and used a different smoothing/interpolation algorithm. $\endgroup$
    – Buttonwood
    Nov 9 at 18:41
  • $\begingroup$ @AlanBirtles Perhaps a cause is the missing «folding around the cylinder» suggested by JeppeStigNielsen. $\endgroup$
    – Buttonwood
    Nov 9 at 18:42

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