Early Mars: Oceans Away?

In the 1970’s, Mars’ first Orbiter had found a river bed that could easily date back 50 million years ago. As a result, scientists began to speculate whether life was on Mars or not. One theory these scientist had worked hard to explain is the need for an increase in temperature on the surface of Mars. Higher temperatures would allow there to be liquid water to be on the surface of Earth hence explaining the riverbed finding. Hence the first possible explanation is that Mars had a thicker atmosphere, allowing for a greenhouse effect to occur. However, Owen Toon states, "There was no way for Mars to ever have had this warm, moist greenhouse climate." An alternative explanation that arose was that the erosion like structures that resemble those caused by rivers on earth, may have been caused by massive asteroid impacts that had hit Mars. This may have also caused the atmosphere to warm up and allow solid ice to melt into liquid water.
http://www.space.com/scienceastronomy/astrobio_drymars_030130.html