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The Question

(Submitted April 03, 1998)

Reading about Mars, I noticed that it has .03% of water vapor in it's atmosphere, and my question was if it has water vapor why doesn't it have precipitation?

The Answer

Mars does have precipitation.

A volume at a given temperature can hold a certain amount of water vapor. (The amount of water vapor the volume can hold is almost independent of any other gases the volume also holds, so if you are looking at the amount of water in the atmosphere, you can ignore the atmosphere and look only at the water.) The colder it is, the less water it can hold. The ratio of the amount of water it does hold to the amount of water it CAN hold is called the Relative Humidity (R.H.). When the volume is cooled so that the R.H. would be greater than 100%, the water vapor in the volume turns into solid or liquid water, depending on whether the temperature is above the freezing point.

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.

A part of the polar ice caps of Mars is made of precipitated water ice (the rest is made of carbon dioxide as 'dry ice').

David Palmer and Kevin Boyce
for Ask an Astrophysicist

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