According to the story published in the Proceedings of the National Academy of Sciences and covered in Space.com, it appears that some of the landforms in dry valleys on Mars are the result of glaciation...advancing and retreating of glaciers, formed from the accumulation of snow into compacted ice over tens or hundreds of thousands of years.
There is no return to the perchlorate discussion yet. However, this implies that the climate was far more earth-like than it is now, since it supported H-2-0 precipitation, which mean atmospheric pressure was much higher than it is now, and temperatures must have been in the narrow band (relatively speaking) when snow falls.
The thicker atmosphere would have deflected ultraviolet radiation.
If it's too hot, there will only be water vapor and maybe liquid water, if it's too cold - it won't snow. On earth, snow does not fall below temperatures of -40 C. Heaviest accumulations of snow occur when there is significant atmospheric mixing of low and high temperatures and surface temperatures may be anywhere from around 0 C (32 F) to -30 C or so. Glaciers form from yearly accumulation of snow that doesn't melt completely. On earth the glacier accumulation range at present is sea level in the arctic to 6000 meters high at the equator.
This has varied in the past. So, possibly Mars would have been habitable by forms of organisms recognizable on the earth today. With all the ice and snow, water - and cloud cover - Mars would have been much brighter in the sky than it's present iron-rich desert appearance and would have looked more like earth, an azure marble, through a small telescope.
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