How old is the oldest water ice in Mars' polar caps?

Question

Mars has permanent water ice caps at both poles, which are seasonally covered by carbon dioxide ice.

What knowledge do we have regarding the age of the oldest ice in those caps?

Answer 1

There are no direct measurements of this, but the caps are probably only a few million years old. All we have are estimated cratering ages, which indicate the surfaces of the polar caps are very young, and model estimates, which place special emphasis on a transition in the axis tilt behavior about 5 Myr ago.

This is discussed in Byrne 2009 The Polar Deposits of Mars (also downloadable in research gate and here) and the age estimates have grown younger since this review was written.

Answer 2

According to this paper by Laboratoire de Météorologie Dynamique, IPSL, CNRS, Université Paris,

Mars undergoes cycles of colder and warmer climate, as the orbital obliquity cycles over a period of roughly 120 000 years.

The south polar regions appear to consist of multiple layers of alternate lighter and darker material, the result of accumulations during these cyclic variations. Thus there is both CO₂ ice and H₂O ice on the south pole that has persisted largely intact through "multiple" cycles of 120 000 years each.
The north pole does not survive through a warm summer during a warmer period within the cycle though.

There may also be large areas of dust-covered permafrost at lower latitudes, which are a remnant of the previous cycle (thus "only" somewhat less than 120 000 years old)

Finally, there is some evidence that "it is likely that the large pore volume of the Martian subsurface regolith might be partly filled by ice"
This would be ice remaining from when the average subsurface temperature of Mars first went below freezing, and can thus be up to some billions of years old.

So in summary:

• ON TOP of the north pole: 1 (martian) year old. Older is evaporated or covered by CO₂ frost and dust layers.
• IN the north polar cap: multiple years, but no more than one obliquity cycle, absolute max 120 000 (earth) years
• IN the south polar cap: many multiple cycles of 120 000 years of accumulated layers.
• UNDER the surface, shallow: up to one obliquity cycle as permafrost.
• DEEP under the surface: as ice in porous rock, some billions of years.