"From looking at the surface, you can see the channels were there, but you have no idea how deep it was, which is obviously important because the depth tells you something about the amount of erosion and how much water was there," lead researcher Gareth Morgan, with the Smithsonian Institution in Washington DC, told Discovery News.
"We see similar channels elsewhere on Mars and they are not filled with lava so it's important to be able to compare different channel systems, and also similar systems on Earth, to give us clues about how they formed," Morgan said.
The finding indicates that about 500 million years ago, a time when Mars already was presumed to be cold and dry, massive flooding was still under way, perhaps contributing to a global climate change.
The radar maps also for the first time point to the source of the flooding, a now-buried portion of a fractured region known as Cerberus Fossae, located west of Marte Vallis.
The scientists did not give an estimate of how much water was needed to carve the channels, which are at least as deep as between 75 meters, (246 feet) to 110 meters (361 feet) beneath the surface.