What amount of water needed to carve Martian valleys?

We know about Mars more than about all planets of the Solar System (excluding Earth of course!). However it i still planet of misteries. The greatest mistery of Mars is where its water gone. Satellite images and data from surface rovers reveal multiple traces of water erosion, thick sediment layers, left by flowing water and great valleys network, carved by water.

There are two main theories about watery past of Mars. First is that 3 billion years ago Mars was wet, watery place with great ocean, occupying almost all northern hemisphere,  with water evaporating from an ancient ocean, returning to the surface as rainfall and eroding the planet's extensive network of valleys. This valleys network (VN) is located in southern highlands.

Image:   Wei Luo, Northern Illinois University  - Mars 3 billions years ago

Second theory states that Mars always was cold and dry, suggesting accumulation of thick cold-based ice on the equatorial highlands with VN formation during short-lived episodes of top–down melting. This melting was caused by asteroid impacts or shield volcanoes explosions. Then ice caps were instantly melted and enormous tsunami rolled on the surface, filling craters and carving valleys. Then water was frozen again and sublimated first in the atmosphere and then molecules escaped Mars. This theory requires much less amount of water than first one and argues against an unfrozen ocean.

A new study led by Northern Illinois University geography professor Wei Luo calculates the amount of water needed to carve the ancient network of valleys on Mars and concludes the Red Planet's surface was once much more watery than previously thought. Luo and colleagues used an innovative algorithm to more precisely calculate the volume of cavity space within Mars valleys and the amount of water that would have been needed to create those cavities through erosion over time.

Image:  Luo, W. et al 

The team took into consideration not only the main VN an canyons, but also multiple tributaries and rivers draining into main canyons.

Luo said:

Our most conservative estimates of the global volume of the Martian valley networks and the cumulative amount of water needed to carve those valleys are at least 10 times greater than most previous estimate. That means water must have recycled through the valley systems on Mars many times, and a large open body of water or ocean is needed to facilitate such active cycling. I would imagine early Mars as being similar to what we have on Earth—with an ocean, lakes, running rivers and rainfall.

This is strong support of the first theory. But there is a problem, requiring explanation. Current climate models have not been able to reproduce an early Mars climate sufficiently warm enough to promote an active hydrologic cycle.

Luo:

Mars is much farther way from the sun than Earth, and when the sun was younger, it was not as bright as it is today. So there's still a lot to work out in trying to reconcile the evidence for more water.

Possible colonization and terraforming of Mars require deep understanding of Mars history. If the first theory is true, than to know what happened and what we have to do for not allowing water loss is of critical importance

Image: Dane Spangler 

Source:  Luo, W. et al. New Martian valley network volume estimate consistent with ancient ocean and warm and wet climate. Nat. Commun. 8, 15766 doi: 10.1038/ncomms15766 (2017)