The above maps, on the top, show the areas of the crust which contained water ice during the most recent of the newly defined Martian eras, the Siderikan era, from 3.5 billion years ago to the present.

The map on the bottom shows the areas with hydrated minerals from the Phyllocian era, about 4 billion years ago.

It is believed that the presence of ice in different parts of the globe, even at mid-latitudes is caused by fluctuations in the planet's obliquity - the angle of tilt of its axis towards the sun. In this case it appears that the tilt was progressively more extreme further into the past, with a high obliquity of 60 degrees before 5 million years ago, during the Siderikan era, with a decrease to a 45 degree, 35 degree and finally 25 degree tilt through to the present.

This plotting of the movement of the water ice as based on changes in obliquity is, however, dependent on the absence of crustal movement and plate-tectonics on Mars, i.e. it presumes that the crust has not moved in over 5 million years. The positioning of areas of the crust on Earth varies greatly over millions of years.