How much water did Mars actually have when it was young and how did it lose it?
A team of researchers at Nasa’s Goddard Space Flight Center have used infrared telescopes on Earth to study water molecules in the Martian atmosphere. The findings indicate that early Mars would have looked much different than it does today with a significant proportion of it’s surface covered by water. So the really interesting question is ” could it form a sea or an ocean?”, and indeed it would. In the northern planes which is a relatively flat region but depressed from the rest of the planet it would form an ocean that was approx 20% of the planet’s surface area.
Source: Nasa Goddard Space Flight Center