Weathering Asteroids

It sounds like a novel . . . But

The new study appears to answer an old question about why small meteorites, which are chips off asteroids, are often a different color than the typical asteroid. . . . .

The surfaces of asteroids are reddened over billions by space weathering effects, the study concludes.

So what could cause weathering?

• Solar wind protons (1-3 MeV/c momentum) burying themselves in the outer layers has to cause some dislocations in the structure of the minerals making it up. (They should penetrate somewhere between 10 and 60 microns.) It also changes the chemistry of the outer layer. I wonder if adding extra protons tends to burn away the oxygen. Charged particles moving through matter ionize and excite nearby molecules. If you blast a silicate with protons, I'd guess that sometimes one of those ionized oxygens will combine with the proton when it comes to rest. If so, then you should see some absorbtion signatures for hydrosilicates, or exotics left behind after H2O or O2 diffuse out (the surface layer is thin, remember). X-O-SiO2 goes to X-O-SiO-H where X is the base part of the mineral (iron or magnesium or calcium or whatever).

  I don't know if these exotics are more red. Not enough chemistry. However, the dislocations should make the outer layer absorb shorter wavelengths better than an untreated surface--and make it darker. Over time you might be able to make the surface pretty black. If the asteroid is close enough to the sun to get hot, I'd expect the dislocation effect to go away thanks to annealing.

  According to today's 7-day solar wind report the average speed was about 300 km/sec and the density was about 5/cm3, which translates to a rate of 1.5 E8/(cm2 sec). 150 million protons per second per square centimeter is more than I expected.

• Hard UV and X-rays would tend to split up compounds too, but since the pieces of the original molecule are still in the same general area I'd expect them to recombine at some rate. So there'd still be some exotics, but not a lot. The very first few molecular layers could be depleted in oxygen, which stands a better chance of diffusing out than in deeper layers.

  They also can, by ionizing molecules, make exotic bonding combinations; but most of these are probably not stable enough to worry about. If you had carbon instead of silicon, that would be a different story.

• Gamma rays and penetrating cosmic rays will dump most of their energy deep enough into the rock that it should not effect surface color. I've not heard of embrittlement in things like olivine, but I suppose that might be possible. Still, meteor samples aren't famous for bulk changes due to radiation damage, so I think I can rule out any effect from high energy particles.

This could be interesting. I'll have to poke around and see what the NASA results were from the probe they launched years ago to measure solar wind and micrometeorite damage.