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Farmers use dry ice(CO2) to seed the cloud on Earth, can we do the same for Mars? The acidity doesn't matter it can be any element or compound as long as we can recognize it as precipitation, as usual ban magic and FTL tech. (Actually I'm trying to do a selfie there with a rainbow in the background.)

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No.

How cloud seeding works.

Cloud seeding cannot make it rain if the air isn't already saturated with water vapor. Rain requires a tiny, tiny, tiny nuclei for the water dissolved in the air to condense around. This can be dust or tiny particles of ice. Cloud seeding artificially introduces those tiny nuclei into humid air to make the water fall out of solution.

For a dramatic demonstration, drop Mentos into Diet Coke. The Mentos contains many nucleation sites (microscopic nooks and cranies on the shell) and causes the supersaturated carbon dioxide in the water to rapidly exit the water. But if you drop another Mentos in nothing happens. The soda is flat. There's not enough dissolved CO2 left.

Cloud seeding can't make rain if there's no dissolved liquid to precipitate.

The Martian atmosphere is too thin, too cold, and made of the wrong stuff.

The atmosphere of Mars is very thin. The average pressure is 0.006 bars (1 bar is the average Earth atmosphere). Best we can get is 0.011 bars in the depths of Hellas Planitia. Martian temperatures vary from 130 to 300 Kelvin (you'll see why I'm doing this in Kelvin in a moment). It's 96% carbon dioxide with 2% argon and 2% nitrogen.

At these temperatures and pressures, there's no dissolved liquids in the Martian atmosphere to make fall out of solution. They will immediately sublimate, convert from solid to gas. To understand why we need to look at some phase diagrams: the temperature and pressures at which various substances are solid, liquid, gas, or other.

Carbon Dioxide

enter image description here

Martian pressure is so low it's off the bottom of the scale here. That means CO2 can never be a liquid.

Argon

enter image description here

Argon is a noble gas and very difficult to liquefy. While Mars does get cold enough to make argon a liquid, again, the pressure is far too low.

Nitrogen

enter image description here

(Sorry, this one is in Celsius). Again, the pressure on Mars is so low it's off the bottom of this graph. Nitrogen cannot be liquid at Martian pressures. It's also too warm on Mars for liquid nitrogen.

Liquids are pretty rare in the universe. Most substances have a relatively narrow temperature/pressure window in which they're liquid. They much prefer to be solid, gaseous, or plasma. You might be able to make it snow, but you won't be able to make it rain.

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Divert a small comet so it impacts Mars. I’ve seen this idea used to make an atmosphere for the moon, and it ought to be applicable to Mars too.

Imagine that it rains quite a lot for a few years as things settle down.

The easiest way to get a rainbow though is inside a domed habitat. Perhaps it’s not a huge domed city, but just a domed park/garden attached to an underground habitat. Water is sprayed via irrigation and could be arranged to make nice rainbows viewed from a picnic area.

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  • $\begingroup$ Well Mars doesn't have much of an atmosphere for a reason, but I guess a rainbow or two might help. $\endgroup$ – Skye Oct 11 '16 at 12:19
  • $\begingroup$ @Skye I don’t understand your point. $\endgroup$ – JDługosz Oct 11 '16 at 12:21
  • $\begingroup$ My thought was that the gravity of Mars which couldn't contain its atmosphere well would lose water vapor in a few hundred years or so. Which I'm probably wrong right? $\endgroup$ – Skye Oct 11 '16 at 12:26
  • $\begingroup$ More like millions of years. I added a link to earlier moon discussion, and Mars has twice the surface gravity. $\endgroup$ – JDługosz Oct 11 '16 at 12:38
  • $\begingroup$ See this presentation. Suppose that it applies anew to a newly introduced atmosphere. $\endgroup$ – JDługosz Oct 11 '16 at 12:41

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