Hayabusa was a Japanese mission to return material from an asteroid, Itokawa. Its long story was a series of major problems, mainly overcome, but until the sample collector was opened, it wasn’t known whether Hayabusa returned any sample at all. Its sample collection mechanism had failed to fire and all the team could hope for was that some grains dislodged from the touch and go by Hayabusa on the asteroid surface had drifted up into the sample collection chambers.
Here at LPSC, a whole morning session was devoted to the Japanese science team who were to report on their preliminary findings. Its a logistical detail that abstracts for talks have to be submitted in January and the Japanese preliminary science team only started work on the samples then. So the abstracts all said “we will report results….” somewhat hopefully perhaps but in the event, almost all teams managed to report results and had conducted remarkably extensive studies considering the very short timescale.
The Japanese team somewhat unusually for scientific talks, also managed to deliver their work with wit and humour and raised a number of laughs. When the sample chambers were opened they looked clean and empty. There was nothing visible >1mm, but fortunately on closer inspection, some smaller grains were observed. The team tried to pick out the grains with a quartz glass probe but this wasn’t too successful at picking up the small particles. They then tried scraping a Teflon spatula across some of the chamber surface and fortunately on subsequent inspection in an SEM, found a number of grains stuck to the edge of the spatula. These grains had to be picked from the Teflon spatula and distinguished and separated from a large number of similarly sized Al-rich particles were also present from scraping the sample chamber surface.
Finally, somewhat in desperation one suspects, they turned the chamber upside down and held it over a quartz disk and hit the back of the chamber 20 times with the handle of a large screwdriver! This raised quite a laugh, possibly in disbelief at the procedure!
So, about 1500 particles have been separated so far for analysis and the second chamber still remains with presumably a similar number of particles inside. The particles were all tiny though. The largest only 180 microns and most of the 1500 were 10 microns or less. Another funny slide showed a huge circular table with lots of seats around with a tiny slice of cake in the middle.
A set of 4 large grains were studied, the largest one 100×30 microns which was given the name Aiko. 3 other slightly smaller ones called Takaoki, Michael and Toshi, after 4 people who had done a great deal of work on the grains. Aiko was covered by fine particulate grains ‘like cosmetics on a woman’ – to more laughter.
Very careful oxygen isotope abundance measurements and the overall petrology and elemental abundances of minerals show pretty conclusively that the Itokawa is an LL5 chondritic body. What this means is that the elemental ratios are broadly similar to the starting material that went to make up the solar system at its formation, it hasn’t been heavily processed like material in the Earth or Mars for example. The LL part means that it is low in iron and low in metals generally. 5 means that it has been quite strongly thermally metamorphosed. That means that most of the minerals have been equilibriated, ie heated to such a high temperature that they have become homogenised between their neighbouring grains. There are some variations with some grains as ‘H’ (ie high iron) and some being a little less altered (LL4) but its clear that most of the body has been extensively heated and metamorphosed.
Perhaps not surprisingly, no organic materials have yet been detected.
The grains that were analysed for noble gas contents showed that they were full of light noble gases that matched the solar wind, so these grains have been at the surface of Itokawa for a long time and become saturated with solar wind atoms.
Possibly the most significant thing that the Itokawa grains can tell us is about a process called ‘space weathering’. When meteorites come to Earth they are ablated in a fireball coming through the atmosphere so we can’t see the original surfaces that were exposed to space. There are however tens of thousands of meteorites in collections around the world and we would like to know where they come from. One way to try and find out the parent asteroids is to compare their optical spectra with different meteorites. The problem however is that the grains on the surfaces of asteroids are altered by solar wind bombardment and micrometeorite impacts. This affects their spectra. The Itokawa grains have been exposed on the surface of the asteroid as shown by the solar wind noble gas contents of the grains so they can studied to see how they have been altered by these processes. Many of the grains showed thin (tens of nanometers) rims of altered materials probably caused by the space weathering process and the preliminary conclusion is that the dominant space weathering process is solar wind sputtering.