# How can a remote planet with little to no sunlight have high wind speeds?

I saw this question here: Can I have wind turbines on my base? and someone else asked about how the planet can have 60-100mph winds without very much sunlight to cause the energy differential, to which OP never gave a response. I'm now curious if there's any ways for this to occur naturally to a planet with little to no sunlight.

Background (Copied from OP)

A colony of humans has settled on a remote planet where there is little to no sunlight, but there is a plentiful amount of earth metals. E.g.- Iron, aluminium, Titanium, etc. There are high speed consistent winds that make it ideal for harvesting electricity from wind (speeds of 60 - 100mph.)

Question

How can this planet have high winds with little to no sunlight? What else could be causing high winds on a metal-rich planet like the one described?

• I wondered this too. Good question! – Willk Aug 29 '18 at 18:06
• Neptune's wind speeds reach 1500 mph, and gets about 1/1000th the sunlight as Earth, due to being 30 times further away. Of course, the process on a gas giant is different from a rocky planet that humans might build a base on. space.com/21157-uranus-neptune-winds-revealed.html – Ghedipunk Aug 29 '18 at 18:06
• If the planet has very little sunlight, what would make its atmosphere gaseous in the first place? – Alexander Aug 29 '18 at 18:07
• @Alexander, generally the further from a star that a planet sits, the thicker its atmosphere, because the solar wind wouldn't be stripping it away as quickly. Of course, this is just one of MANY factors that matter; for instance Venus has a thick atmosphere because it's largely CO2 based, and Mars has a very thin atmosphere because it doesn't have a magnetic field to keep the solar wind at bay. – Ghedipunk Aug 29 '18 at 18:10
• Can I summon @kingdelion? King, pull across your sweet math from here worldbuilding.stackexchange.com/questions/74099/… to show how a very fast rotation speed could produce the desired winds. – Willk Aug 29 '18 at 18:17

To generate winds you need a temperature differential within the planet.

Normally this is provided by the sun, heating at different temperatures different places.

However, you can have differential heating for several other reasons:

• geological activity: a lake of molten material erupted from below the surface will be hotter than the surroundings, generating a strong wind
• astronomical impact: the impact crater will be also really hot immediately after the impact, with again the result of generating winds
• tidal heating: local increase of temperature due to strong tidal deformation can transfer heat to the atmosphere, again generating wind

It's not entirely true that you need a lot of sunlight to generate high speed winds; and example of this is Neptune. If you're willing to wait around for a while (geologically speaking of course) then even on a low energy planet, windspeeds will pick up.

Why? Because it not only takes energy to start the wind, but it also takes energy to stop it. All it takes is a minor differential in temperature over a long time to start the wind but there won't likely be a lot of energy left in the system to stop it.

Ultimately, this is less likely to happen on a planet that humans find comfortable (because our comfort involves a lot of energy) but it certainly can happen if you have a terrestrial planet in an orbit further out. Whether that is likely given our current understanding and models of planet formation I'll leave for another question.

• Tim, your claim that it "takes energy" to stop wind seems counter-intuitive to me. Isn't friction with the surface / with air masses of different speeds all it takes to slow down the wind? I feel pretty certain that, on the contrary, it would take energy to keep the wind going rather than to stop it. Is there something I'm missing? – Qami Sep 7 '18 at 16:03
• @Qami you're right that it seems counter-intuitive and it is. My first thought when I saw the data about Neptune is that it had to be wrong. After all, the Coriolis effect alone would cause the winds to stop through turbulence, right? But, it seems if you're only adding a very small amount of energy over a very long time, the Coriolis effect doesn't happen. This could also be due to the fact that out that far, the difference between solar energy at the equator and at the poles would be far less than it is for (say) Earth. – Tim B II Sep 7 '18 at 22:19

Do you need high windspeeds everywhere, or just in localized areas where power generation will be situated? If the latter, you just need convenient geology to concentrate otherwise low-power winds. E.g., a canyon that concentrates wind through a mountain range next to a seacoast with a consistent current that keeps the air temperature over the water (or whatever liquid your oceans are made of) just slightly warmer / colder than the neighboring land.

If you want consistent global windstorms, such as occur on Mars... well, the mechanism that creates them on Mars clearly works! Just so long as you have some small amount of sunlight, you can make use the seasonal variations in that sunlight to periodically condense and evaporate large portions of the atmosphere. On Mars, this means freezing out CO2 as dry ice on the south pole during southern winter, resulting in prevailing winds towards the south, and evaporating it again during southern summer, resulting in prevailing winds towards the north. A similar process operates on Pluto with nitrogen ice, so you really don't need a ton of sunlight to drive it.

• I'd say high windspeeds in localized areas would be sufficient. An area that has relatively good traits for human survival, with the added high wind speeds for power generation is ideal. – J0hn Sep 10 '18 at 12:57

To figure out how wind speeds work on a planet far from the sun, we can use two real-life examples- Neptune and Uranus. This answer has some of what was mentioned in other answers, but it also gives a good source that explains why those winds are so strong.

Neptune clearly doesn't have enough sunlight to cause wins. One possible answer suggests that these huge windstorms exist because of the lack of sunlight reaching the planets. On Earth, sunlight changes the speed and direction of winds through convection currents- air rises and falls instead of always blowing in one direction. Convection currents slow down the atmosphere. This is clearly true looking at other planets in the solar system. Jupiter and Saturn, which are also far away from the sun, have huge wind storms blowing across them.

Other factors that also contribute to these planet's wind speeds include their hydrogen-rich composition which is much easier to move than our heavier oxygen and carbon dioxide.

A big factor in these winds is also geological activity. Without much interference from the sunlight, heating patterns can speed up the wind to the hurricane-grade storms that you see on the exoplanets. The air starts spinning naturally either from the creation of the planet or form the planet's spin, and then geological activity in hot spots of the planet expand the air that passes by. Then the air radiates heat and shrinks again. The air that is now passing over a hot spot pushes on the cold air as it expands and gradually speeds up the air. Combine that with very little air resistance and convection currents and you can cook up some monster storms.

That said, because we don't really know for sure how these storms form, any realistic answer is probably acceptable.

# Planet spin can't drive wind speeds for a rocky planet

Let's assume the planet in question is rocky and generally Earth-sized. In this case, the rotation speed of the planet will not be directly related to its wind speeds.

First, the Coriolis effect is not a force that can drive an object to move. Instead is it a deflection of a moving object. Acceleration due to the Coriolis effect is given by

$$\mathbf{a}_C = \mathbf{v} \times \mathbf{\Omega}.$$ The bold symbols mean we are talking about vectors. If the velocity vector is zero, then acceleration is zero.

Second, we can see from our solar system, that rotation speed doesn't control wind speeds. The rotation Venus's surface is about 7 km/h while the Earth's is 1700 km/h. Meanwhile, Venus has constant wind speeds around 300 km/h at the cloud tops, circling the planet every 5 hours or so. Earth's winds are obviously not that strong.

But let's use the Coriolis effect to help with driving fast winds...

While the planet's spin can't really drive winds the way we want it to, the spin of the core can do some neat things. Specifically, we could get it to make a ferocious magnetic field, and then use that field to do some stuff.

A powerful magnetic field can induce charge particles to move. Ocean water is full of such particles, specifically the ions of dissociated salts (and water's hydrogen bonds make even electrically neutral water molecules act like they are charged in some circumstances).

Magnetic field lines on Earth look like the diagram above. The magnetic field lines point from north to south. For this planet to work, we imagine that water is circulating along the green line (magnetic equator), from east to west (right to left on the screen). Along the green line, the direction of the flow is perpendicular to the direction of the magnetic field lines.

Lorentz force magnetic acceleration of a charged particle is

$$\mathbf{J}\times\mathbf{B}$$.

$J$ is current flow in our ocean. We need this to be in a direction such that a strong magnetic field will force acceleration in the east to west direction as indicated.

This is the basis of magnetohydrodynamic propulsion systems. Current systems are limited by the conductivity of seawater. On our planet, we'd want to add lots of salts to the seawater to make this work. One way to do this is to make the planet mostly land area; with a smaller ocean, runoff from the land will be more significant and the ocean will be much saltier.

We get the ocean accelerated to high speeds around the magnetic equator, and it then 'drags' the atmosphere along with it. This is the same principle in reverse of how it works on Earth. Now we have a constant, high speed west to east wind flowing on Earth.

The Hadley cell is caused by differential heating between the equator and subtropical areas within about 30 degrees of the equator. It depends on warm moist air converging near the equator. So lets make sure the equator is very warm and moist; lets make it an ocean! This works will with both ensuring that we have ocean flow along the entire magnetic equator, and also allows us to have a 'small' salty ocean by leaving only this part of the planet covered in water.

In the Hadley flow, heat and moisture is transported equator-ward on the surface. Thus, let's make the edge of the continents as dark and wet as possible, by covering them with rainforest. Lets also make the Hadley cell more stable by leaving it in the same place all year. Due to Earth's 23 degree inclination, the center of the Hadley cell, called the Intertropical Convergence Zone, moves from the Tropic of Cancer to the Tropic of Capricorn all year. Instead, let's have it stay stationary over the equator by giving the planet near-zero orbital inclination.

With the Hadley cell as strong as possible, it will be generating equator-ward air flow, towards the already rapidly moving equatorial ocean. Now we need to bend that air flow in a westerly direction to get it to add to our wind speeds. Enter the Coriolis effect! Trade winds on Earth are bend westerly in the tropics, so now we need to spin the planet as fast as we can to direct as much of that energy as possible in the westerly direction, as seen on the graphic below:

# Conclusion

We have a planet with:

• A very strong magnetic field, lined up closely with the magnetic north and south poles.
• Two continents covering most of the northern and southern hemisphere
• A very salty ocean restricted to a band within ~10 degrees of the planetary and magnetic equator.
• Magnetically induced, high velocity, west to east ocean current.
• A low inclination leaving the sun and ITCZ over the equatorial ocean all year long.
• A fast rotation and short day, so the Coriolis effect helps most of the power of the Hadley cell reinforce the equatorial winds.
• Thick jungle from ~10 to ~30 degrees north and south, absorbing as much solar radiation as possible.

In the end you get constant, extremely powerful trade winds in the desert regions mentioned above. In particular, 'off-shore' wind farms in the equatorial ocean would be able to harvest a lot of energy. Enjoy your hurricane-force planet!

• You said a lot of the things I was going to mention. May build off of your answer later. – Clay Deitas Sep 7 '18 at 18:35
• I don't understand why there is an ocean current. Could you explain me a bit better? Sure, a magnetic field would affect positive charges in a way, but also negative charges in the opposite way. And about salty ocean, there is charge conservation, so net charge is null. There would be an electric current yes, but there won't be a mass (or fluid) current because motion of positive and negative ions would be opposite to one another. From guiding center equations we need more complicated fields to achieve a drift velocity (and thus current). – Physicist137 Sep 7 '18 at 21:54
• @Physicist137 I'm using $J$ instead of $qV$ in the Lorentz Force equation, because $J$ represents current flow, not ionic particles. You can pass an electric current through seawater, especially if it is extra salty. Assume a current in the ocean, because....I'm sure there is a good reason to assume bottom to surface electric current flow in the ocean (assuming the magnetic north pole is at the true north pole to get westerly oceanic currents). Return current can go near the shore or bottom, where MHD motive force is reduced by friction/turbulence. – kingledion Sep 7 '18 at 23:45
• I think now I got it what you meant. Well, if you're assuming there exists a vertical electric current in the ocean, I think it would be nice to identify why it would exist in the first place (if it is not too much to ask :D). I for one can't think in any reason for it to exist. (Also.: I don't know what you meant to say in the beginning, but, just to be clear, J represents electric current flow, not a fluid current (of mass)). – Physicist137 Sep 8 '18 at 0:11
• @Physicist137 Yes, electric current. I based the induced flow on the principles of an MHD drive. I'm disappointed that someone picked out the handwavey bits so quickly, I thought they would be better hidden :) – kingledion Sep 8 '18 at 0:14

I wonder if tidal forces from a nearby large moon/planet could cause wind the way we get tides in the ocean from our smaller moon? Normally the smaller body (if this planet is a moon of a larger planet) would become tidally locked to the large planet and no longer have tides. However, this takes time, and your base might be on a recently formed or captured moon.

In researching, "little sunlight" is a great deal more than no sunlight. For example read about Saturn's moon Titan. Titan is tidally locked to Saturn, so I can't get my gaseous tide question answered, but there are some other possibilities. While there seems to be little wind on the surface in general, there is quite a bit in the upper atmosphere. Also, when the seasons change from summer to winter, a large portion of the atmosphere moves from one pole to the other. While this is not a consistent wind, it could be harvested twice a "year", however long a year is.