Space.com (November 23, 2009)
"Several lines of evidence point to the possibility of a past ocean on Mars, from apparent ancient shorelines to chemicals in the soil.
"Add to the list a new map of the red planet's valley. The map shows extensive valley networks around the equator and in the southern hemisphere, suggesting a warmer mars long ago, with extensive rainfall that would have feed an ocean in the northern hemisphere.
" 'All the evidence gathered by analyzing the valley network on the new map points to a particular climate scenario on early Mars,' Northern Illinois University geography professor Wei Luo said. 'It would have included rainfall and the existence of an ocean covering most of the northern hemisphere, or about one-third of the planet's surface.'
"The valley networks are more than twice as extensive (2.3 times longer in total length) than had been previously mapped out...."
There's a bit more detail in the article, about the data and how it was analyzed. I think that a scientist, who said that a single large northern ocean on Mars - a very long time ago - would explain patterns that are emerging as we study drainage channels that are still visible.
On the other hand, there's a big question that I'm not sure has been answered yet: why do Mercury, Venus, Earth and Mars all have the same general sort of terrain? A section of lowlands, and a section of highlands. On Earth, the lowlands are filled with a water-salt solution that we call oceans. Venus and Mars don't have that much water - now. Mercury probably never did.
Even more interesting, Venus - and even Mars - seem to have fairly clearly-defined continents. ("Venus Continents," "Exploratour - Comparing the Surfaces of Earth and Mars," Windows to the UniverseWindows to the Universe®, University Corporation for Atmospheric Research (UCAR). © The Regents of the University of Michigan)
"Just Add Water?"If someone put a tub of liquid water on Mars, the air there is so thin that it would quickly boil away - cold as it is. Venus isn't much better as a vacation spot: it's hot enough to melt lead in some places on the surface.
Still, Venus does have continents: two larger ones, Ishtar Terra, and Aphrodite Terra. Ishtar Terra is near the north pole and none too large, really - roughly the size of the continental United States. Aphrodite Terra is near the equator and about half the size of Africa. There are two other highland/continental regions: Alpha Regio near the equator, and Beta Regio in the southern hemisphere.
As things stand, nobody's likely to walk on Venus. Not anytime soon. On the other hand, if we thinned out the atmosphere of Venus, cooling the place off in the process, introduced water and plants to start producing oxygen, after a while we'd have a planet that looks a little like Earth.
I wouldn't live to see it: but some cultures are able to plan projects that span generations. And, in the meantime, we just might be able to float cities in the clouds of Venus. ("Cloud Cities of Venus ," (July 25, 2008))
As for Mars, if or when we do settle the place, it'll probably be easier to make micro-habitats, instead of trying to terraform the whole planet in one go. Besides, there's a lot to study on Mars, just the way it is.
Other posts, about "Mars, Mostly."