I’m really quite fascinated by singing sand dunes. Also known variously as whistling sand or booming dunes, this is a phenomenon observed in places across the world when the right set of circumstances cause sand to be a little noisier than usual.
What kind of sound does the sand make, you ask? Have a listen…
For a sand dune to be musical, there seem to be a few qualifications needed:
- A dune should be at least ~45m high
- Sand should be loose and dry
- Sand grains should be rounded and 0.1 – 0.5 mm in diameter
- Grains need to contain silica
- There needs to be a harder layer underneath the sand
From what I can find out, the exact mechanism is apparently not well understood. It seems to be to do with a resonant frequency being set up between the surface of the sand and the harder layer beneath (either rock or a wet layer of sand). This effectively makes the entire layer of dry sand resonate, boosting the volume. Though no one’s quite sure if the sound comes from friction as the grains rub together, or air between the sand grains being compressed.
In most places where singing sand exists, it’s less of a deep rumbling noise and more of a squeaking which happens when you walk over it. The most common frequency emitted by squeaky sand is apparently around 450 Hz, which sounds like this. The big dunes, on the other hand, have a much deeper set of frequencies, with the loudest being around 90 – 100 Hz. That’s a deep, bassy sound. Additionally, the dunes have been found to sing notes of E, F, or G. In case you want to sing along.
An interesting thought is whether sand dunes on other planets will also sing this way. Sand, of one form or another, is certainly common. So common that there’s an International Planetary Dunes workshop, which is the source of this handy little document, and the slides from a related talk.
Mars is our friendly neighbourhood desert planet. If I was to look for sand anywhere in the Solar System, Mars is where I’d start. Turns out, Mars doesn’t have as many dunes as you might think. Most are confined to sand traps and crater basins. The exception to this is a region near the North Pole of Mars called Olympia Undae – a dune sea which Star Wars fans would no doubt find quite familiar.
Given our neighbouring world’s lower gravity, sand dunes there can grow quite sizeable. Many are a few tens of metres tall, with the largest being up to 600 m in height! Mars is also different in that it’s a very dusty place. Whether having so much fine dust around would allow martian dunes to sing is unclear. Similarly, without doing some experimentation, it’s hard to say if the lower air pressure and effectively zero humidity would have an effect too.
Of course, there are still other more alien places we could look. Around 15% of Titan’s surface is covered by dunes, making it the most dune covered object in the solar system. I’ll admit, I wouldn’t have guessed that. Being so tiny, Titan’s dunes can also be rather tall, with the tallest identified dunes being in the Belet Sand Sea, reaching 100 – 175 m in height.
Finally, there’s Venus. The only images we have from the surface of Venus are still from the Soviet Venera landers, which showed rocky basalt plains. But there’s more to Venus than volcanic rock. Earth’s tragically underexplored sister planet is home to the Fortuna-Meshknet dunes in a region called the Al-Uzza Undae. It’s thought that these may have heights of 40 – 80 m, meaning that the taller ones may be in the range to produce sounds. Of course, whether or not they do, and what effect the much higher pressure will have is another question.
Or perhaps singing dunes are unique to Earth, and found nowhere else in the Solar System. Currently, it’s impossible to say. Somehow, I doubt it’s a question we’ll be able to answer anytime soon.
Sand Dunes in the Altyn-Emel National Park, Kazakhstan – © Jonas Satkauskas
The El Dorado Ripple Field, photographed by NASA Spirit
Sand dunes in Olympia Undae, from NASA MRO
Martian sand, examined by the Curiosity Rover