How are rover landing zones selected?

Feb 19, 2021
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It seems each rover mission has a goal to search for signs of "ancient life" around the land zones near the equator.

There is a water ice cap at the pole. Mars has measurable seasonal cycles and atmosphere at the water sublimation pressure. At the margin of the ice cap, there is almost certainly periodic liquid water present.

It seems that a mission to search for evidence of life would chose a landing zone where there may sometimes be liquid water. Why look for signs of ancient life and not look for signs of current life?

On earth, we have discovered life in every environment from deep sea vents to under a mile of ice.

So, why don't missions target the polar ice boundary areas as landing zones?