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In this article: https://users.physics.ox.ac.uk/~pierrehumbert/papers/BAMS_SFatm.pdf, Raymond Pierrehumbert discusses (among other things) how Kim Stanley Robinson deals with the terraformation of Mars. Pierrehumbert points out that while some ideas are good, others are not. Among the bad ones is, in his own words:

"Silliest of all are the windmills, which are supposed to heat the planet by using wind-generated electricity to drive heating coils. (I won’t insult the reader’s intelligence by spelling out why this wouldn’t work.)"

Please, feel free to insult my intelligence because I don't see why this couldn't happen. I don't think it's a perpetuum mobile, not from the 1st or 2nd class: indeed what I think is that the wind will produce electricity and by Joule effect this electricity dissipates heat. This heat, even small, would increase the planet's temperature and that would imply a positive feedback because with higher temperatures the winds will be stronger (although in the end the temperature in all the planet will reach an equilibrium), and stronger winds imply more electricity, and more heat dissipated by Joule effect and so on...

What do you think? Where am I wrong?

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  • $\begingroup$ What is the atmospheric pressure on Mars? At this pressure, how strong do you think the winds are? $\endgroup$ – Chet Miller Mar 27 '17 at 12:24
  • $\begingroup$ Yes, the gradient pressure will be small, but also the density, and a wind like the geostrophic wind is directly proportional to the gradient pressure but also inversely proportional to the density... so they can compensate each other... $\endgroup$ – David Mar 27 '17 at 14:08
  • $\begingroup$ Indeed, in this comment the author affirms that winds on Mars are faster than on Earth: space.stackexchange.com/questions/12237/… $\endgroup$ – David Mar 27 '17 at 14:18
  • $\begingroup$ Thinking a little bit more, it seems that to exert pressure on a windmill you need high density, so even with very fast winds in Mars, the pressure that can exert is really small. (That's dynamic pressure: en.wikipedia.org/wiki/Dynamic_pressure). $\endgroup$ – David Mar 27 '17 at 14:33
  • $\begingroup$ That may be, but the density of the atmosphere is extremely low, so the kinetic energy of the winds will be very low. $\endgroup$ – Chet Miller Mar 27 '17 at 14:35
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Wind is a form of kinetic energy, and even though it seems to cool you down in humid weather, somewhere there is a source of heat required to create this wind in the first place. Kinetic energy generally heats things up, even if it is widely spread out, low grade heat energy. Most of the energy Mars gets is, like Earth, derived from the Sun.

Putting windmills up eventually slows the wind down, (assuming enough windmills). Slower winds means less electrical power to the heaters.

As noted in the comments above,the potential energy from winds is not very significant, much slower speeds and reduced density compared to Earth. I am not sure however, how dense is the dust in the wind storms.

A better, fuller answer is given at Windmills on Mars and my above answer is based on this article.

According to NASA, the maximum windspeed on Mars is 100 km/h and combining this with the extremely low atmospheric density on Mars, would not provide enough power to make any windmill project worthwhile.

Because of these factors, building the windmills would probably be considered as wasted time, energy and resources.

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    $\begingroup$ The dust density in windstorms might be pretty detrimental as well. I can't imagine they are designed to take a beating from high density dust. You would need to specially design them to probably try and take away the dust momentum if you wanted energy; but that raises the question of how much could that affect the atmosphere. Definitely not as easy as slapping up a windmill and thinking "yeah this will get me energy". The same applies on Earth too I guess. $\endgroup$ – JMac Mar 27 '17 at 13:01
  • $\begingroup$ But if you have enough windmills and as you said you finally slow the winds, that's because the energy has been employed in heating the planet. But, as I wrote in my question, with the heating of the planet the pressure gradients will be greater because the local differences in temperature will be greater, and so the wind... $\endgroup$ – David Mar 27 '17 at 14:14
  • $\begingroup$ Thanks for the link, and yes, without something to capture the (small) heat generated by the windmills it is just radiated back to space. It seems so obvious but I just missed it. Perhaps you could add the link to your main answer. And yes, I agree, Robinson is a top hard sf writer, among my tops with Egan, Chiang and so on... $\endgroup$ – David Mar 27 '17 at 14:52
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The wind is going to be slowed down anyway. Slowing it down prematurely using windmills is just a way of concentrating the energy dissipated by slowing it down into a single place on the surface. So there would be no net heat input into the atmosphere. It would only make a difference if the atmosphere was moving as an ideal fluid without friction. Surely this must be the argument he had in mind.

This is a simplification however. Actually, as Lee Weinstein wrote in his blog post "Windmills on Mars", there would be a very minute, but temporary, warming effect. By slowing down the winds with the windmills, this reduces the kinetic energy of the Mars atmosphere due to wind, and by conservation of energy, this has to mean a slight increase in the temperature of Mars. However this is just a temporary effect while they reduce the speed of the wind. Once the average speed of the wind has reached a new, lower, equilibrium, then Mars returns to thermal equilibirum witth the rest of the universe. So there would be a really tiny increase in temperature for a short while after the windmills are deployed, after which the temperature resturns to normal. When you stop the windmills, the opposite happens, it cools down then returns to normal.

You could use the same argument for the moholes. The temperature of Mars can only increase temporarily as a result, because no heat is being created. The heat from the interior is just being lost more quickly at that point. The rest of the crust of Mars must be getting slightly less heat radiated through it, so eventually this is going to cause Mars to cool down slightly elsewhere, by tiny immeasurable amounts but probably only on long timescales.

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