Was Mars' ocean 3.5 billion years ago salt water or fresh water?
On a related note: would we be able to drink the water that is on Europa?
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The article "Water Activity and the Challenge for Life on Early Mars" finds that the early Martian ocean would have been acidic and almost as salty as the dead sea.
The estimates for salinity of the Europan ocean vary widely, but most place the salinity above 50 g/kg, more than that of sea water (about 45 g/kg).
You couldn't drink it, but there are plenty of life forms that are adapted to this level of salt.