Phoenix also detected calcium carbonate, a chief component of limestone rock, which, like the phyllosilicates that characterise the Phyllocian era, can only form in the presence of liquid water. The orbiters have found such minerals in other parts of Mars, but in regions that have clear evidence of former flowing water. The Phoenix landing site is an open plain without obvious features carved by flowing water. This suggests that ice melted in-situ and stayed in that area.

Glacial deposits near the equator on Mars are believed to be remnants of ancient snow. Ice -rich landforms and scattered boulders appear to have been caused by glaciers between 350,000 years to 4 million years ago, the end of the Theikian Era.