Capture In Aerogel

A problem with studying grains of particles found in space is that their encounter speed with an observer (or collecting spacecraft) is typically many kilometers per second. Even though the grains are typically small (100mm diameter is already quite large!), at such speeds the violence of the impacts is such that the projectile vaporises leaving just a crater in the target (plus a residue that is detected only with sensitive scanning electron microscope work). In such circumstances determining the nature of the impactor is understandably difficult.
The solution is to use aerogel as the target (capture medium). Aerogel is a dried silica gel that is underdense. Underdense means it has a density less than expected for a solid (i.e. less than that of water).

The trick is that although it appears solid, it is in fact highly porous. It is also transparent. Thus is a particle is fired into aerogel, even at speeds of kilometers per second, instead of vaporising and producing the normal crater, it tunnels in and is captured, relatively intact at the end of a carrot shaped track. The figures below are from impacts of typically 100mm diameter glass beads at 5kms-1 onto aerogel with a density of approx. 100kgm-3.
A block of Aerogel material
A block of Aerogel
Thus it is possible to capture particles and then study them intact in the laboratory. Aerogel can be flown on spacecraft (and has indeed been flown by several groups world wide, including Kent) and impacts can also be reproduced in the laboratory using a two-stage light gas gun (just like the one we have here at Kent). Our work here at Kent on aerogel has provided several useful insights into how aerogel performs as a medium to capture particles and analyse them.
Entrance hole on the face of an aerogel target
Entrance hole in face of aerogel target
A trapped particle inside the aerogel
Particle (dark circle) at end of a track (entered the aerogel from the bottom, travelling upward)
A few years ago we showed that it was possible to closely correlate the impact direction with the direction of the track in the aerogel. Thus you can “see” the incoming track direction.
Tracks of particles trapped inside aerogel
Image of tracks from particles which hit the aerogel from above at an inclined angle

Data obtained over a range of angles of incidence (0 degrees was normal incidence, 90 degrees glancing incidence)
The data shown above is from: Capture of hypervelocity particles in aerogel: in ground laboratory and low earth orbit, by M.J. Burchell, R. Thomson and H. Yano, in Planetary and Space Science 47, 189-204 (1999).We have also carried out other work to understand the capture process. One key aspect of capture is the relationship between track length in the aerogel and impact speed. We were the first to establish this, and to show that track length doesn’t increase indefinitely as speed increase, but reaches a maximum and then decreases. The data below illustrates this.
The figure above is from: Capture of particles in hypervelocity impacts in aerogel, by M.J. Burchell et al., in Meteoritics and Planetary Science 36, 209-211 (2001). The next task we set ourselves was to find a good method of identifying the nature of the particles captured in aerogel. The method had to be simple, work without having to remove the particles from the aerogel and work with small particles (less than 100 microns diameter).The most appropriate technique seemed to be Raman scattering. But did it work on particles fired into aerogel? With a Raman expert (Alan Creighton from the Chemistry Laboratory at the University of Kent) we found we could obtain Raman spectra from minerals captured in aerogel. In Capture of particles in hypervelocity impacts in aerogel, by M.J. Burchell et al., in Meteoritics and Planetary Science 36, 209-211 (2001) we gave spectra from olivine and enstatite. These were the first spectra ever obtained from Raman for particles fired into aerogel. Since that work we have continued with more minerals and below we show spectra from raw and captured grains of pyroxene.

Raman spectra for pyroxene (lower is a raw grain, upper is a grain captured in aerogel)