I’ll probably leave this one for Sara because she is bound to know more about space than I do! What I will say is that pictures of ancient mars are obviously hypotheses of what it “may” have looked like as you said so it does not necessarily mean that it did!
Anyway lets see if Sara can shed any light on it! 🙂
Oh dear. Sara got voted off before she answered! I’ll do my best as a poor substitute for you. The Mars Odyssey spacecraft took some pictures that show shapes that might have been made by oceans. This matches what people have thought before, but our hopes are dwindling that there was ever any actual life. So pictures that show any green on Mars are just the ideas of artists.
Remember that Mars is further away from the Sun than the earth, and it is smaller. That means that it is naturally colder, and harder for it to keep an atmosphere. It has very little atmosphere now, with much less CO2 than us, so it is harder for what little atmosphere it has to trap heat.
All this gives you a combination of low pressure and cold temperature, which makes it hard to keep a liquid ocean. It either freezes or vaporises.
The oceans may just be wishful thinking though. Everyone _wants_ there to have been lots of water on Mars, so it is easy to believe it if we get the faintest hint.