Presentation #511.06 in the session “Comet Surfaces”.
The surfaces of comets and asteroids undergo impact processes throughout their lifetimes leading to an alteration of their physical and mineralogical states. Observations of these bodies show spectral changes that cannot be explained by the effects of solar wind alone (these include: band alterations, spectral reddening/bluing, and albedo changes). Collisional processing through physical shocks also spectrally changes the grains, but the exact form of the alteration is not well constrained. This motivated laboratory impact studies with emphasis on understanding how lattice dislocations evident in TEM imaging of Stardust grains compare with TEM images of lab-impacted grains, and how the alterations translate into spectral changes in the mid-infrared. We have performed a suite of hypervelocity impact experiments to study the spectral alterations caused by observable shock effects within these lattices, as seen in the TEM images. We varied the velocity of the impacts (2–2.8 km s-1), the temperature of the target (from -100 C to 25 C), and the state of the target (powder, coarse grains, solid) to determine dependencies (if any) of the collisionally shocked grains on these initial conditions. Our initial conditions were selected to represent analog surfaces of bodies from the KB, Trojan asteroids, or cometary surfaces, all of which have been found to contain olivine components in their mineral matrices. Planar dislocations in the crystallographic axes of the lattice within the altered grain itself can be seen in TEM images of our impacted grains, similar to the density of dislocation observed in Stardust returned samples. These results are being compared with in-depth modeling efforts (see Lindsay et al. iPoster) with crystal lattice dislocations specifically along the crystallographic c-plane that are consistent with (1) the orientation of the dislocations in the TEM images, and (2) the spectral shifting of our affected band centers. Specifically, we see a systematic shift in spectral peaks at the 10.2 and 11.3 μm band centers, but with no strong correlation with impact velocity or the temperature of the sample when impacted. This indicates a unique alteration mechanism among the known types of space weathering that may lead to different interpretations of observational data on the surface mineralogy of comets and asteroids. We will present our analysis of our processed spectra and compare to ongoing modeling efforts and results of other space weathering studies. Characterizing these alteration features can also lead to a better understanding of the evolution of the surfaces of small bodies over time.