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Morning Frost on Mars. (NASA/JPL/Caltech/University of Arizona/Texas A&M University)

Does It Rain On Mars?

by Adam Frank
Aug 14, 2011

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Adam Frank

Yesterday my post on the presumptive eruption of groundwater on Mars prompted this comment by Delvan Ramey.

"If the water [from the runoffs] evaporates into the thin atmosphere, then does it rain?"

Good question and worth a second to ponder. There is lots of evidence that Mars did have running water on the surface at one time when the atmosphere was warmer, wetter and thicker. That means Mars would have likely supported a full hydrospheric/atmospheric cycle including rainfall. Now however there is simply too little water and the atmosphere is too cold for rain to fall. Frost however is another story.

A nice explanation comes from Ask An Astronomer

At around -75 Celsius, a volume can hold 0.03% of 7 millibars = 2 microbars worth of water, which is about the typical content of Mars's atmosphere. (It is more complicated than that because the amount of water vapor and atmospheric pressure vary a lot from place to place, just like on Earth). At about this temperature, the relative humidity reaches 100%. So when it gets cold, Mars has precipitation.

However, this precipitation most likely takes the form of frost, rather than rain or snow. The ground is likely to be colder than the air (especially on cold clear nights), and so air hitting the ground cools and the water freezes to the ground as frost. Viking II (a Mars lander in the 1970's) saw frost on the ground some mornings.

The Mars Phoenix Lander also saw a frost of "Diamond Dust" falling in the mornings. Finally, it's worth noting that fog has also been seen collecting in martian canyons and craters.

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