Ancient Martian Ocean

The latest issue of The Planetary Report discusses polygons on Mars. These terrain features are in Mare Acidalium, in the planet's northern hemisphere. Measuring 1-12 miles across, the polygons have terrestrial analogs. Buried, undersea polygons have been found on Earth. They support an interpretation of the Martian features as having been formed in an ancient sea. This interpretation is almost certainly correct. Mare Acidalium was the destination for water and sediments from huge outflow channels. These channels are known as circumchrysean because they formed around the Chryse plain, south of Mare Acidalium. They were certainly vast. A single one, Kasei Vallis, dwarfed the Amazon river. Enough water flowed into Mare Acidalium to create an ocean there.
But could an ocean last on Mars? It almost certainly couldn't, especially not in liquid form. Even 2.5 billion years ago, Mars was essentially the same frozen world we know today. The ocean must've immediately acquired a thick ice cover. It probably froze completely, prior to gradual sublimation. After millions of years, the ice was all gone, the bulk of it ending up in the north polar area nearby.  Some water, however, seeped into the ground and froze there.
The brevity of the northern ocean argues against it being an abode of life.