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The NASA Goddard science education page answer to 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? says

Mars does have 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').

However another educational page Precipitation (Weather) says:

Precipitation in meteorology refers to all forms of liquid or solid water particles that form in the atmosphere and then fall to the earth's surface. Types of precipitation include hail, sleet, snow, rain, and drizzle. Frost and dew are not classified as precipitation because they form directly on solid surfaces.

Mars has polar ice caps and a 25.19° inclination similar to that of Earth's. We see the polar caps shrink and grow regularly with the seasons each year.

Questions:

  1. Can we say that the changes in the polar ice caps is due to "frost"?
  2. Can we say that the changes in the polar ice caps is due to "precipitation"?
  3. Is there really precipitation on Mars? Does frost count?
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    $\begingroup$ For me, frost is frozen condensation, where atmospheric moisture has directly condensed onto a surface. In effect, frost is frozen dew. I also regard weather related precipitation as something that as fallen from a height. However, chemical precipitation is the formation of a solid (or insoluble) substance from a solution due to a decrease in the solubility of the substance. Thus if atmospheric gases are a solution & solid frost forms at the bottom of the solution maybe frost can be considered to be a precipitate. $\endgroup$
    – Fred
    Nov 6 '20 at 20:18
  • $\begingroup$ @Fred aha! I didn't connect the two usages of the term. Don't precipitates generally form in solution and only fall after they get large enough to sink at a reasonable rate? If they form directly and conformally on to all surfaces (including the sides of the container) I don't think that's called precipitation in Chemistry. But let's find out: Are precipitation and crystallization the same things in Chemistry and Meteorology? $\endgroup$
    – uhoh
    Nov 7 '20 at 0:34
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    $\begingroup$ Thanks :-) I know cold winters, but not frozen ones. Snow can sometimes be a novelty. $\endgroup$
    – Fred
    Nov 7 '20 at 1:20
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    $\begingroup$ It sounds more inviting than having ones shoes getting stuck to roads when bitumen melts due to seemingly countless days of plus 40 C degree temperatures. $\endgroup$
    – Fred
    Nov 7 '20 at 1:29
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    $\begingroup$ In addition to the technical answer of what is a precipitate, I think there may be a slight technical difference. If you consider the first website, it is called "ask an astrophysicist." An astrophysicist may not draw a distinction between frost and precipitation in the same sense a meteorologist might. Its also worth considering that inaccuracies can form as a result of simplification in communication, especially for the general public. $\endgroup$ Nov 7 '20 at 21:54
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See this answer.

Actually already snows on Mars. The snow, made of carbon dioxide, has been observed and studied by orbiting spacecraft. Aside from the different chemical composition from the stuff we shovel on Earth, Martian $\text{CO}_2$ snow also is very finely divided, microscopic in size in fact; likely this is related to the low density of available material and the very fine nature of the dust on which the snowflakes nucleate. In fact studying the snow could provide clues about the atmospheric Martian dust on which it nucleates.

Details are published in the Journal of Geophysical Research.

In addition:

Indirect evidence for water ice snow has also been found. According to an article published in Nature, mixing layers have been identified from orbiting spacecraft and water-ice precipitation signatures have been found by the Phoenix lander. Simulations show that such signatures are consistent with snow formation, with the water-ice snow expected to fall at night (as temperatures cool and the atmosphere can retain less water vapor). The arcticle does not claim any direct detection of water-ice snow."

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    $\begingroup$ Done that @uhoh. $\endgroup$ Mar 17 at 14:29
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    $\begingroup$ looks great, many thanks! $\endgroup$
    – uhoh
    Mar 17 at 14:33

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