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Was wondering if anyone with physics knowledge could tell me something about desert worlds.

If the surface of a planet was mostly a fine sandy material (e.g. Sand) then could tidal forces cause the whole desert to flow like a slower, dustier ocean?

The idea came when watching the huge waves on a planet in the movie "Interstellar" and it was because of the tidal forces from a black hole. I figured, a planet in orbit around a small, high density, stellar object, would develop tides in anything fluid that covered the whole surface. Sand, I have read, can be described mathematically as being an incredibly viscous fluid, meaning it flows more slowly, but still flows. Could you have a desert with winds and currents and sand sailing on the high desert? I think it would be cool to have sand pirates in a world with flowing deserts.

PS: This is my first post. Sorry for wordiness.

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    $\begingroup$ Waves from Interstellar was totally unscientific. Just so you know. $\endgroup$ – Mołot Feb 27 '17 at 20:40
  • $\begingroup$ Is there any truth in it at all then? $\endgroup$ – Douglas Feb 27 '17 at 20:42
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    $\begingroup$ Hello and welcome. I edited your title to make it more informative, so the people who potentially could answer would have easier time noticing your question. If that's not exactly what you meant, you can edit again. $\endgroup$ – Mołot Feb 27 '17 at 20:43
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    $\begingroup$ Can you specify an atmosphere and how much hand-waving we can have? There's currently no way to even get water to behave the way the movie described, not to mention very fine dust, and not to mention sand. +1 very good question, though. Some allowance for hand-waving or speculative science would help answers. $\endgroup$ – Mikey Feb 28 '17 at 7:01
  • $\begingroup$ Have a look at Earth tide it may be slightly useful to you. $\endgroup$ – Ash Sep 10 '17 at 11:00
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Sand is too viscous to flow like that...

The resistance of a fluid to forming waves is measured by the fluid's viscosity. Viscosity measures to ability of motion in one part of a fluid to impart motion to another part of the fluid. In the case of shear viscosity, it is the ratio of the shear stress to rate of shear deformation, or velocity of deformation. The viscosity of water is around 1 mPa-s at 20 Celcius.

Sand is not a fluid, so it doesn't properly have a viscosity. However, in situations like earthquakes, it can effectively flow like a fluid; indeed it can flow like a fluid in an hourglass too. So there are measurements of its effective viscosity.

Water molecules can each move around each other at the molecular level, sand grains are much much bigger. So the effective viscosity of sand is much more variable, since sand grains are different sizes and compositions, while water molecules do not differ (much). This book on earthquake engineering suggests viscosity values of around 100 to 1000 kPa-s for sand; this is 100 million to a billion times higher than water.

So the force needed to perturb a bucket of sand is a around a billion times higher than the force needed to perturb a bucket of water; similarly, the force needed to make a 10 foot tide in an ocean of sand is around a billion times higher than the force needed to make a 10 foot tide in the Ocean.

...anywhere on a planet

Can we imagine sand forming tides? Then we have to imagine a world with at least 6 orders of magnitude more tide-causing gravitational variation.

We can estimate the magnitude of tidal forcing from the force of gravity the tide-inducing object exerts on the planet in question. For example, the moon's gravitational effect on an object on Earth is $$\frac{F}{m_{\text{obj}}} = \frac{GM_{\text{moon}}}{r^2}$$ where Gm, the standard gravitational parameter, for the moon is $4.905\times10^{13} \text{ m}^3\text{s}^{-2}$, and r is the distance of the moon at 384399 km. We then get $F/m = 0.0003319 \text{ N/kg}$.

What if the tide causing object was much closer? The force of Jupiter's gravity on the moon Io is $$\frac{GM_{\text{Jupiter}}}{r^2} = \frac{1.267\times10^{17}}{421700000^2} = 0.712 \text{ N/kg},$$ which is about 3 orders of magnitude higher than seen on Earth. Much more powerful, but not quite there.

Lets see if we can get even better. The Roche limit (the limit to how close a satellite can be to the object it orbits) for the Earth and the Sun is 556,000 km, and the gravitational parameter for the sun is a hefty $1.327\times10^{20}$. If we set the Earth's distance from the sun to the Roche limit (Note: this is within the outer surface of the sun, so this is just theoretical demonstration), then we get $$\frac{1.327\times10^{20}}{556000000^2} = 429 \text{ N/kg}.$$

These tidal forces are now a million times stronger than those on Earth. Unfortunately, we have reached a limit. If the Earth got any closer to the sun, those powerful tidal forces would rip the planet apart. And they are still not powerful enough to make sand dune flow like water.

Conclusion

Tidal forces strong enough to make sand dunes flow in tides, would also be powerful enough to rip the planet they were on apart. Therefore, tides in sand oceans are impossible.

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  • $\begingroup$ If I wrote that the sand on the planet wasn't earth sand but some ridiculously fine powder, could you theoretically have slow-moving waves? $\endgroup$ – Douglas Feb 28 '17 at 13:54
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    $\begingroup$ I updated my answer with some more math. The problem with ridiculously fine powder is two fold. First, I don't have any data on effective viscosity of that powder, so I can't really say. Second, the magnitude of viscosity difference between water and sand (8 or 9 orders of magnitude) is so large, that all these things you are asking about, tides, wind-forced waves, etc., are very unlikely, even if very fine sand brought down the viscosity significantly. There is still an enormous difference between the smallest sand particle and a single molecule of water. $\endgroup$ – kingledion Feb 28 '17 at 14:38
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    $\begingroup$ @Douglas Sand is mostly silicon dioxide and even if you grind it down to individual molecules (which would not be possible to do because it would clump together and remain at larger grains however much you grind), it would still be too much friction between the molecule sized grains as it would be solid in reasonable temperatures for life. If you, on the other hand, heat it up to melting, then you'd get a liquid which would be somewhat susceptible of tidal forces (it would still have very high viscosity), but then your problem would be that you'd have lakes of molten glass instead of sand. $\endgroup$ – Mrkvička Feb 28 '17 at 21:16
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I think the friction between grains of sand is too high to allow it to flow like a liquid; you need an external force such as the wind. Dunes are known to move under the pressure of the wind, with grains of sand effectively "rolling" from the upwind side to the downwind side. But those are hardly "tides", and they couldn't be sailed anyway.

A sufficient outgassing from below would fluidify the sand and let it flow exactly like a liquid, and be subject to tides. But it would be exceedingly difficult to explain a constant outgassing on a very large area.

Electric charge flux can cause fluidization in some substances and some types of sand could qualify (there's actually a short story by Hal Clement, "Dust Rag", that deals with this phenomenon), but to fluidize enough sand, you would need a flux intense enough to threaten life, and your sand pirates might end up like the Ly-Cilph in their ninth year.

However, you could have sand ships able to sail over the sand on some kind of skates; for this you only need wind enough, and this would give you much of the same framework as marine navy. Moving dunes could uncover rocky, unpassable stretches or smooth over difficult wadis, so you could have a kind of "sand monsoons" to play with.

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  • $\begingroup$ So all I need to have sand currents is wind currents? $\endgroup$ – Douglas Feb 27 '17 at 21:05
  • $\begingroup$ No special gravity or anything? $\endgroup$ – Douglas Feb 27 '17 at 21:05
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    $\begingroup$ They would be extremely slow currents. A dune may take as little as one hour to cross a road, or as long as a week, depending on sand type and wind strength. $\endgroup$ – LSerni Feb 27 '17 at 21:10
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    $\begingroup$ @Douglas Sand dunes, like in the Sahara desert, move as the wind blow very slowly. This already happens here on earth with our deserts. If you want to add special gravity or "effects" you certainly can to make it cooler or more more sci-fi. To at least get the dunes moving, yes, all you need is wind. $\endgroup$ – ggiaquin16 Feb 27 '17 at 21:39
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I am not in a position to fully answer this question but it is an interesting one. The equations for granular dynamics have really only recently been described. I haven't really studied them, though they have similarities to the Naiver-Stokes equations. Then we are asking about tides, which as others have mentioned do exist in solid planets, besides just oceans. So obviously the sand will move via tidal forces, and it would certainly seem that given the right grain size and friction the sand will flow in tides in more than just the deformation of planet as a whole.

To have currents on a long time scale would seem to require changing the sand to be a suspension, or fluidization as seen in Sand boils and mud volcanoes. Also to be considered is ionization as with lunar dust.

Of course, as already pointed out having seasonal "tides" and slow currents via wind moving the sand is something that happens on earth. And land sailing is something that also happens, with wheels as sand is abrasive and can be used to cut steel.

Hope this helps.

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  • $\begingroup$ Yes; it seems to me that the description of anything as a fluid is clunky. A mass of sand grains will, given certain conditions, exhibit one of the properties of a fluid: flow to establish a uniform level. Just pile sand in a cylindrical container and remove the walls. It doesn't have viscosity, per se, which is measured on the molecular scale — and grains of sand are much larger than the hetrogenous mixture of mineral crystals chunks and organic molecules which comprise most sand. $\endgroup$ – can-ned_food Feb 28 '17 at 7:46
  • $\begingroup$ I really shouldn't have written "tides" in the question as it's giving a lot of people the wrong impression. I was mostly looking to have slow moving waves of sand. Something that would exert a force on something designed to sail on it in a similar manner to a boat on water. $\endgroup$ – Douglas Feb 28 '17 at 13:56
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Sand tides? You mean like a cloud of dust moving across the land as if it were an ocean wave coming up to the shore? If this is the case, I would say yes. It may not necessarily be a fluid wave, but there certainly could be a cloud of sand or dust that flows back and forth onto shores.
Edit: The post was a bit confusing. An entire desert cannot be moved in this way. Sand is quite heavy in large quantities, and I couldn't possibly fathom an entire desert being moved like a fluid by tides. Depends on the power of the waves, really; If these waves were like tsunamis, then you'd blow the desert-like surrounding area away altogether.

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