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Actually the entire red planet is very cold, cold enough for ice and frost to form not only on the poles. Temperature rises above 0 deg Celsius in summer only. Why then are just the poles frozen, and not lower latitudes during most of the Martian year? Is it because most of these areas perhaps aren't within the triple point of water, unlike the poles?

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    $\begingroup$ What kind of ice are the frozen caps of Mars made of? $\endgroup$
    – uhoh
    Commented Nov 5, 2020 at 7:34
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    $\begingroup$ Maybe tilt is the reason. At equator, the temperature varies between 20 C to -73 C but at poles, there is always sub-zero temperature and so ice caps is more likely to reside at the poles. $\endgroup$
    – Nilay Ghosh
    Commented Nov 5, 2020 at 7:59
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    $\begingroup$ @NilayGhosh The bottom image of Mars is similar to one I'd expect of winter Mars. $\endgroup$
    – John
    Commented Nov 5, 2020 at 8:23
  • $\begingroup$ Important to note that there is seasonal frost down to about 40° latitude in both the north and south hemispheres. Just that the pictures you usually see are for those hemispheres' summers, when the frost is gone. $\endgroup$
    – Stuart Robbins
    Commented Mar 3, 2021 at 17:13

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Just like a cold winters night on Earth does not guarantee snow, so does a cold Mars not guarantee ice everywhere.

In order for ice or snow to form, the moisture that is later solidifying has to be available at the cold location. Moisture in this case is $CO_2$ with traces of water, leading to an ice cap made of $CO_2$-ice, with a ring of $H_2O$-ice 'trailing' around its outer radius.

The atmosphere is in fact only cold and dense enough for $CO_2$ to solidify at the poles, so no mystery there. Otherwise the entire atmosphere would condense out into a thin layer of $CO_2$-ice. To understand the formation of the $H_2O$ ice, we have to take into account how the water is transported inside the martian atmosphere.

Mars possesses only one global circulation cell (see early modelling on this by Haberle et al., (1993), and a recent review on the martian climate for the interested reader by Read et al., (2015)).

Haberle et al., (2015) fig. 2 ((Figure from Haberle et al.,(2015))

The reason for the single cell is a combination of Mars' small size and rotation period, as outlined in this answer. The effect of the single cell is that the condensable moisture is only transported essentially pole-to-pole directly, with only small variations of ~10-15 degrees latitude during the martian year.

Hence, no other location on Mars than the poles can accumulate a significant enough vapour pressure in order to form ice.

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