Brazil Nut Effect Explains Mystery of the Boulder-Strewn Surfaces of Asteroids 58
KentuckyFC (1144503) writes When Japan's Hayabusa spacecraft gently maneuvered into a parking orbit around the asteroid Itokawa in September 2005, it conducted a comprehensive photographic survey, the most detailed ever taken of an asteroid. This survey revealed that Itokawa is covered in large boulders that look like ejecta from craters in other parts of the asteroid. But when astronomers added up the total volume of these boulders, it turned out to be greater than the volume of the craters there were supposed to have come from. Other asteroids also show a similarly skewed distribution of large boulders. That has caused some significant head-scratching among astronomers who are at a loss to explain where the boulders come from.
Now an international team has solved the mystery. They say the boulders float to the surface of asteroids in an astrophysical example of the Brazil nut effect. This is the long observed phenomenon in which shaking a mixture of big and small particles causes the larger ones to rise to the top. That's because the shaking creates gaps beneath the large particles that small particles fall into. The result is that the large particles float. The team simulated the shaking effect that collisions between asteroids would produce and say that these vibrations would cause large boulders to float to the surface in a few hours, finally explaining why asteroids have such boulder-strewn surfaces. Problem solved!
Now an international team has solved the mystery. They say the boulders float to the surface of asteroids in an astrophysical example of the Brazil nut effect. This is the long observed phenomenon in which shaking a mixture of big and small particles causes the larger ones to rise to the top. That's because the shaking creates gaps beneath the large particles that small particles fall into. The result is that the large particles float. The team simulated the shaking effect that collisions between asteroids would produce and say that these vibrations would cause large boulders to float to the surface in a few hours, finally explaining why asteroids have such boulder-strewn surfaces. Problem solved!
Re:That's how I clean my cat's litter box. (Score:5, Informative)
Re:Just buoyancy (Score:5, Informative)
Why is this called the "Brazil Nut Effect?" This is just normal buoyancy, science teachers have been doing demonstrations like this for years. You can do the same thing if you put a golf ball in the bottom of a container full of shredded bark and shake it.
Because in a can of mixed nuts, the Brazil Nuts are almost always on the top. Thus, the Brazil Nut Effect. It actually has nothing to do with buoyancy which involves mass. It's all about size. Put your golf ball in a container and cover it with marbles and then shake it up. The golf ball will rise to the top as the smaller marbles fall beneath it.
Re:Just buoyancy (Score:5, Informative)
"Normal" buoyancy is about less dense objects floating on top of a fluid. This (granular convection) is about larger objects rising to the top of smaller ones.
As your golf ball example shows, the "floating" object can be denser than the "fluid," which would not be the case in "normal" buoyancy. And there's no upward force being exerted on the objects by the "fluid."
Re:Just buoyancy (Score:5, Informative)
Buoyancy is about displacement and density - a buoyant item has less mass than the amount of bulk matter it displaced. Like say, how boats float - the mass of water displaced by a boat is more than the mass of the boat.
The Brazil Nut Effect isn't about buoyancy because it has nothing to do with the Brazil Nut's mass. It's about the observation in a can of mixed nuts, if you give them a shake, the Brazil nut rises to the top. Consistently.
If it was about buoyancy, then it's because the Brazil nut displaces less err, nuts, than the others, but no. You can replicate the effect with many other things - the bigger items rise to the top.