As someone who was born on a lunar eclipse (explains a lot, no?) the 40th anniversary of man walking on the moon has a special place in my heart. Okay, that sentence makes no sense (I was born on a lunar eclipse however), but anyway everyone is all abuzz about the anniversary of the moon landing so it’s as good as any sentence to let me talk about booming sand dunes.
Take a big sand dune. Kick some of the sand down the face of the dune. Sometimes, if you are lucky, the sand dune will emit a loud 70 Hz to 100 Hz booming sound. I used to have a sealed container of the booming sands along with a sealed container of normal sand. Shake the former and you’d get a nice “thump thump” as opposed the to the latter’s “sh sh.” Why the heck would I have this stuff? Well for many years the mechanism behind booming sand dunes was not known. What actually caused these things to make frequencies in this range? A different effect, squeaking sand, which you may have heard at the beach and having a much higher frequency, does have a good explanation. As an undergraduate one of the projects I worked on was trying to come up with a theory for the booming sand dunes (I recently looked at the paper I wrote and was astounded at how bad my theory was.)
Recently, a lot of work coming out of Caltech has been made on explaining the booming sand dunes:
Recent field research (Vriend et al., 2007) done at Caltech indicates that the dominant booming frequency ranges from 70 – 105 Hz, depending on the desert location and the time of year. Quantitative field research at four additional locations in different seasons invalidate the dependance [sic] of the booming frequency on grain diameter. An internal natural waveguide within the subsurface structure of the dune explains the geographical and seasonal variations in booming frequency. The source of the acoustic emission is the shearing of burping sand, but the waveguide in the subsurface structure sets the booming frequency and amplifies the sound. Field surveys with ground penetrating radar and seismic refraction confirm the existence of the waveguide for booming dunes. Subsurface soil sampling shows that a firm layer exists at a depth of approximately 2 meters with a higher water strata and chemically altered sand. The large dunes in the wintertime are saturated with moisture, loose the velocity contrast across the interface and hence eliminate their natural waveguide. Smaller dune lack the correct subsurface structure even in the summer and are not able to amplify the burping sound. Higher harmonics in the frequency signal are explained by higher mode excitation of resonance in the waveguide. The acoustic characteristics of the waveguide model are consistent with all field measurements.
Which is pretty cool, but if natural waveguides are part of the answer, I’m still curious as to why the containers I had sounded so different.
What in the world does all this have to do with the moon you ask? Well one of the things the Apollo astronauts did was to install seismographs on the moon. They found that the Moon was surprisingly seismically active. In particular four types of moonquakes were identified. Deep ones probably caused by tidal forces, ones caused by the impact of meteors, ones caused effects of the sun heating up the previously dark and cold side of the moon, and shallow earthquakes arising only 20 to 30km below the surface. These later shallow moonquakes actually were quite strong with a record quake of 5.5 magnitude on the Richter scale. They also lasted an amazingly long time (something like tens of minutes) and, to my knowledge, remain unexplained.
What does this have to do with booming sand dunes? Well, way back in the day (read: the seventies) one explanation for some of the earthquakes (the ones activated by the sun) was actually…booming sand dunes! See, for example, Here (typewritten manuscripts!) Wouldn’t that be cool if these crazy booming sand dunes were actually occurring on the moon. Unfortunately, it seems that the energy of the seismic events is not consistent with this conjecture. Sometimes a crazy idea is just a crazy idea I guess.
So too bad: no booming sands on the moon (or at least no ones detected by the Apollo seismographs.) Which is all the more reason we should be going to Mars and listening for martian booming sand dunes!