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Space Science

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!
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Brazil Nut Effect Explains Mystery of the Boulder-Strewn Surfaces of Asteroids

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  • by Captain Hook ( 923766 ) on Tuesday July 15, 2014 @11:19AM (#47457405)

    I would have intuitively said the other way around.

    Since the gravity is so small I would have expected the motion of the smallest particles to be close to random, perhaps close to Brownian motion if you looked at the system over a long enough period of time.

    I guess, even though there isn't much to pull the material together, once a small particle is in a crack or void it is very unlikely to ever escape and so the crack does eventually fill in, it seems to me that the process should exist but be much slower than when compared to the effect in a strong gravity field.

    As you said, "Intuitively, which we all know is probably wrong"

  • by dunkindave ( 1801608 ) on Tuesday July 15, 2014 @11:46AM (#47457619)
    My problem with these kind of articles is how they state it as 'case closed'. All this is is a theory of what is happening. Maybe it has a lot of solid science behind it, maybe it is even right, but right now it is still just a theory for us to explain what is happening. Using words like "Now an international team has solved the mystery" makes it sound like there is no debate, this is the answer, and anyone who says otherwise is an idiot. While I am not a scientist, I come close enough, and this fails the scientific method, at least in how the reporting represents it.

    OK, I feel better now.

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