Surely there might have been a clement era on Early Mars.

The discovery of a more widespread runoff channel system in the Southern Highlands would seem to clinch the argument, yes? It had to have been made by rainfall, yes? And that rainfall could only have occurred under a thicker atmosphere at higher temperatures than now are found, yes?

Could be, and surely worth further study, but I'd resist jumping to conclusions.

If Early Mars was substantially colder than now -- at least generally speaking -- due to the dimmer sun or other reasons, then even with a thicker atmosphere, it's hard to maintain sufficient surface temperatures to allow significant amounts of rainfall over extended time periods.

Occasional rainfall following impact events or as a consequence of volcanic eruptions is likely. But the runoff channels identified in these papers are far more widespread and complex than would be expected from the odd occurrance.

If rainfall is the cause, what could have sustained it over extended periods? If they were not caused by rainfall, or only rarely caused by rainfall, what alternative mechanisms could have helped create these drainage systems?

Could water have been forced to the surface from the interior? And could drainage systems have formed from such surface outbursts, mimicking rainfall runoff? Could such outbursts cause rainfall, if only temporarily? For example, in some cases, it appears that water once filled craters and flowed out of them. Could the water that once apparently filled those craters have originated from below?

I'm just toying with some questions here. How could an otherwise frozen Mars, in other words, develop widespread runoff channels without rainfall, and apparently without substantial reliance on groundwater sapping?

Are we seeing part of the answer today in the examples of crater wall channeling?

Hmmm.... Interesting!

Cheers, BRFelix