Another fantastic public lecture at Nottingham University ! These really are wonderful FREE events that the local community can take advantage of.
This one was titled "Granular Dynamics : Patterns in the Sand" and was presented by James Clewett, a 3rd yr PhD student in the University of Nottingham Physics Department.
Granular Dynamics is the study of how systems containing large numbers of particles behave. For example :
* Dry Sand (and how it forms sand dunes)
* Wet Sand (and how it can be used to form sandcastles)
* Mixing of powders in foods and medicines (want to ensure even mixing)
An interesting property of granular systems is that they can behave as solids (e.g. damp sand), liquids (e.g. sand in an hour glass) or gases (e.g. sand in a dust storm)
Another aspect of the behaviour of granular systems, especially those that are being agitated in some way, is the "Brazil Nut Effect", so called after the phenomena shown by Brazil nuts rising to the top of a bag of mixed nuts.
James and his colleagues have been looking at asteroids and considering whether they may have a granular structure.
One piece of evidence that at least some asteroids may have a granular structure came with the discovery of the KW4 asteroid (see here), which comprises a main asteroid about 1.3km in diameter, which is orbited by a "moon" that is about 360m in diameter. By measuring the speed of the orbit of the moon, it became clear that KW4 had a low density and must therefore be quite porous.
Another piece of evidence was the finding that, whilst small asteroids have a variety of spin speeds, larger asteroids only spin slowly. This is important because it is consistent with a granular nature. Granular bodies are weak and tend to break up if subjected to larger stresses (i.e. being massive and fast spinning).
A third piece of evidence was found when the Japanese Hayabusa Space Probe visited the Itakawa asteroid (see here and especially here) with the aim of scooping up some of its surface and bringing it back to earth. The probe photographed the asteroid in great detail and it can be seen that it has a very rocky surface (at least in parts, some areas appear very smooth). This kind of surface is consistent with operation of the "Brazil Nut Effect", possibly on timescales of millions of years. James' group has written a paper on the possibility of another asteroid, Eros, also having a similar surface appearance (see here)
NSB has found itself going back to the pictures of Itokawa and gazing at them wondrously and for some considerably time.
As an aside, NSB found a really interesting paper titled "Asteroid Density, Porosity and Structure" (see here) on the Internet whilst preparing this post. About half way through there is a cracking chart showing the different structures of various asteroids, and a lot of other information besides. The suggestion is made that these low density asteroids are basically big piles of rocks and sand - essentially a pile of rubble!
The lecture covered a number of other aspects of granular dynamics. NSB suspects that it would do a pretty poor job of explaining these so is going to respectfully point you, dear reader to the University of Nottingham Granular Dynamics Group Webpage (see here), where you can find out much, much more about the research that goes on there. There is also an interesting video on the behaviour of vibrating granular systems as part of the "60 symbols" series (see here)
Oh, and you can find out more about James, including his past as a World Champion Tetris player, at his website (see here)
Links (same as those embedded in text above)
Asteroid Density, Porosity, and Structure
The Brazil Nut Effect on Eros
Summary of reports in "Science" of Hayabusa imagery
University of Nottingham Granular Dynamics Group :
Granular Dynamics at the 60 Symbols series
James Clewett's Website