Quote: "There is no obvious explanation what causes the clouds - and they do not rain. It's probably too cold for rain."
Not obvious without a little research. I've looked into it. The explanation is that matter doesn't simply go solid to liquid and then to gas. There's something called a "
triple point". What it comes down to is there isn't enough atmospheric pressure to maintain a liquid state of water at any temperature except around the equator. Especially true in that huge canyon near the equator, and only at certain times of year..
There are a few occasional signs of liquid water flowing within a few weeks before photos have been taken. Namely, signs of previous flow where there was none before. This is probably due to weather systems creating high and low pressure like on Earth. Get the pressure high enough and suddenly the water vapor or ice becomes liquid.
But you're right, it doesn't rain. You need much higher pressure than Mars has for that to occur. But it does snow, and mostly around the poles. The clouds around the equator come mostly from the evaporation of ice straight into vapor. It then flows to the poles, condenses and falls as snow on the poles.
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