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Space Science

Brazil Nut Effect Explains Mystery of the Boulder-Strewn Surfaces of Asteroids 58

Posted by Unknown Lamer
from the asteroids-go-well-with-moon-cheese dept.
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 BoRegardless (721219) on Tuesday July 15, 2014 @10:10AM (#47456751)

    Where the biggest nuts rise to the top.

    • by jeffb (2.718) (1189693) on Tuesday July 15, 2014 @10:23AM (#47456859)

      Still does nothing to explain the similar sorting of Slashdot comments, as illustrated in this thread.

    • by SQLGuru (980662)

      Where the biggest nuts rise to the top.

      Politicians, too......

    • by Anonymous Coward

      The large boulders rise due to a density disparity. If you take the smallest box that would hold a large boulder, you will find a lot of airspace in it. The same sized box filled with smaller rocks would have less airspace. The larger box is therefore lighter than the smaller box.

      This can be experimentally verified by making the large boulders out of a material that is much more dense than the small rocks, such that the cubic density would still be higher for the large boulders, even including the airspa

      • If a collection of boulders/rocks have the same shape and density (individually), then they will have the same space between them regardless of their size. This is ignoring the conflicts with the walls of your hypothetical box. However, asteroids in space have no walls. Your experiment should not show what you think.

        It might be more accurate to say that smaller rocks settle down more readily than larger boulders (in the spaces between), and the effect perceptively "floats" the boulders to the surface.

  • I thought this was going to relate to nuts with high amounts of radioactivity.
  • I shake 'er up. The big pieces float to the top. I scoop them out. Brazil nut effect. Asteroid problem solved. It took an international team to sort this out? Come on! :D
    • by gstoddart (321705) on Tuesday July 15, 2014 @10:31AM (#47456937) Homepage

      It took an international team to sort this out? Come on!

      That's kind of what I was thinking.

      I heard about this in Geology class 20+ years ago. It's why farmers fields keep producing rocks, because the bigger pieces move up and work their way to the surface.

      I thought this was pretty well understood for quite some time.

      • by Captain Hook (923766) on Tuesday July 15, 2014 @10:42AM (#47457027)
        But does the process work when the gravity field is tiny? That is what needed to be found out before saying that that is definitely the process at work.
        • by gstoddart (321705)

          Intuitively, which we all know is probably wrong, you'd think a smaller gravity would magnify it, no?

          The bigger pieces can move more, the smaller pieces still settle.

          The mechanism is that the smaller pieces fall down through the cracks between the bigger pieces, and eventually push the bigger pieces upwards.

          Yes, it's space, so it's all new ... but I should think the mechanism is pretty universal, and was already widely understood.

          • 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"

          • TFA describes the situation on a dry asteroid.

            An asteroid or comet that contains water as well as stone is likely to behave differently. If its ice is melted by impact or increased exposure to sunlight, then frost heaves might cause a faster migration of big stones to the surface than would happen by granular convection. But if the ice is acting as a concrete binding agent, then there will be no frost heaves and no granular convection. Probably on a lot of asteroids both processes will be active.

            I'm think

        • by OglinTatas (710589) on Tuesday July 15, 2014 @11:39AM (#47457565)

          Easy enough to check. Send up a can of mixed nuts to the space station and put it in a slow centrifuge. Spin it for a few days then open it up and... shit, someone ate all the cashews and almonds, and reluctantly the peanuts, leaving only the brazil nuts. FAIL.

      • There is possibly some confusion in parent post between granular convection and frost heaves.

        In New England and other climes that have a winter freeze and spring thaw, the winter freeze pushes rocks upward as the water in the soil expands into ice. In the spring thaw, the ice under the rocks melts from the periphery inward, and slurries of ice water mud fill the voids. As a result, the rocks stay in their higher place as the soil settles back to its spring level.

        One of my chores when I was growing up was

    • I shake 'er up. The big pieces float to the top. I scoop them out. Brazil nut effect. Asteroid problem solved. It took an international team to sort this out? Come on! :D

      It's the same in an organization -- shake it up -- the management rises to the top.

      Any similarity to a litter box is purely coincidental

    • by GTRacer (234395)
      I really hope you're doing this in a well-ventilated area and/or using low-dust litter! But points for ingenuity!
  • by jellomizer (103300) on Tuesday July 15, 2014 @10:34AM (#47456955)

    I never stopped to think if the Brazil nut is on top, but more how the heck do I crack open the shell. A triangle is one of the strongest shapes. The Brazil nut shell is in a nice triangle. Thus my approach is smashing it to pieces, and eating the nut in little fragments.

  • Wow, the World Cup of Brazil really left a mark all the way into space! Now all I need for today is a scientific article, which explain complex physics by using the Mexican Wave!!

  • 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.
    • Re:Just buoyancy (Score:5, Informative)

      by Dcnjoe60 (682885) on Tuesday July 15, 2014 @10:58AM (#47457169)

      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)

      by wonkey_monkey (2592601) on Tuesday July 15, 2014 @11:03AM (#47457227) Homepage

      "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)

      by tlhIngan (30335) <slashdot@ w o r f.net> on Tuesday July 15, 2014 @11:22AM (#47457443)

      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.

      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.

      • by sdack (601542)

        I believe you are wrong. The mass of objects will certainly play a role in both effects. The Brazil Nut Effect will likely have something to do with the inertia of objects.

        The difference when compared to buoyancy will be that with buoyancy you have a calm system, without any energy coming in from the outside, where objects come to a rest. Here the objects will order themselves merely based on their density - their size-to-mass ratio.

        While in the Brazil Nuts Effect its the opposite and objects will find them

    • by Nyder (754090)

      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.

      I don't think you want to call it what my grandma would call Brazilian Nuts. Not very PC.

    • by Livius (318358)

      Well, not *normal* buoyancy, but Brazil nut is certainly a specialized (I would say obscure) metaphor to choose.

  • 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.
    • by sdack (601542)

      You now make it sound like there is still a need for a debate, but where is your explanation for it? Where is the debate? I do not see you coming up with an explanation as to why there is still a need for a debate. All you really seem to be doing is to express your fear over being called an idiot, but I am fairly certain you will be clever enough to know that insults can never replace a good explanation. The only explanation you are giving is in fact that you are not a scientist.

      And yes, it does not only "s

    • Mod parent up.

      Too many people fall into the trap of mistaking scientific authority for scientific method. A scientist may be an excellent authority in his specialty, but he is still just an authority and is prone to all the kinds of errors of that any man can make. The scientific method with its hypotheses and experiments is the gold standard, and even its results should be rigorously questioned.

      A scientist who is not a profound skeptic about just about everything is not really a scientist, but merely an

  • Odd; My first thought would be that those surface boulders were just the more recent micro-asteroids that were drawn to the larger asteroids gravity. I'm not sure why a more complicated explanation is necessary? But I'm open to learning.

  • by Anonymous Coward

    Another explanation is that the asteroid got showered by a significant amount of other material that was not part of the primary body of the crater making asteroid. Perhaps when a nearby moon or planet exploded like the fictitious planet of Alderan in the popular Star Wars movie A New Hope.

    This type of astronomical event would also explain why one side of Mars is the flattest place in the solar system and has a crust thickness of only 1km and the other side hosts the tallest mountains in the solar system

  • This also explains why all the big nuts and bolts rise to the top of my bolt bucket!
  • The apparent discrepancy of the total volume of large boulders being greater than that of the visible craters they have supposedly come from is not resolved by the BNE. In the paper, this paradox is only mentioned in passing, and no definite resolution is offered. No-one seems to have ruled out the possibility that there are additional craters beneath the rubble, or that the excess includes remnants of the impactors. Perhaps there is an assumption that, absent the BNE, the boulders formed by early impacts s

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