High mobility of asteroid particles revealed by measured cohesive force of meteorite fragments.

The cohesive force of asteroid particles is a crucial parameter in microgravity. The cohesive force was evaluated under the assumptions of lunar regolith and proportionality to particle size; however, it is sensitive to particle shape. In this study, cohesive-force measurements of meteorite fragments and aggregates consisting of silica microspheres revealed that the cohesive force is independent of the sizes of the fragments and aggregates as well as of the fragment preparation methods. The cohesive forces of the asteroid particles were found to be orders of magnitude smaller than previously predicted, explaining the high mobility of asteroid surface particles identified by space exploration.

Packing fraction of clusters formed in free-falling granular streams based on flash x-ray radiography.

We study the packing fraction of clusters in free-falling streams of spherical and irregularly shaped particles using flash x-ray radiography. The estimated packing fraction of clusters is low enough to correspond to coordination numbers less than 6. Such coordination numbers in numerical simulations correspond to aggregates that collide and grow without bouncing. Moreover, the streams of irregular particles evolved faster and formed clusters of larger sizes with lower packing fraction. This result on granular streams suggests that particle shape has a significant effect on the agglomeration process of granular materials.

Hypervelocity impacts as a source of deceiving surface signatures on iron-rich asteroids.

Several arguments point to a larger proportion of metal-rich asteroids than that derived from spectral observations, as remnants of collisional disruptions of differentiated bodies. We show experimentally that this apparent deficit may result from the coating of metallic surfaces by silicate melts produced during impacts of hydrated or dry projectiles at typical asteroid impact speeds. Spectral analysis of steel and iron meteorite targets after impact shows a profoundly modified optical signature. Furthermore, hydrated projectiles leave a 3-µm absorption hydration feature. This feature is thus consistent with a metallic surface and does not require an unusual low-speed impact. Unless systematizing radar measurements, ground-based spectral observations can be deceptive in identifying iron-rich bodies. The NASA Psyche mission rendezvous with Psyche will offer the unique opportunity both to measure the relative abundances of regolith and glassy coated surfaces and to substantially increase our understanding of impact processes and signatures on a metal-rich asteroid.

Regolith migration and sorting on asteroid Itokawa.

High-resolution images of the surface of asteroid Itokawa from the Hayabusa mission reveal it to be covered with unconsolidated millimeter-sized and larger gravels. Locations and morphologic characteristics of this gravel indicate that Itokawa has experienced considerable vibrations, which have triggered global-scale granular processes in its dry, vacuum, microgravity environment. These processes likely include granular convection, landslide-like granular migrations, and particle sorting, resulting in the segregation of the fine gravels into areas of potential lows. Granular processes become major resurfacing processes because of Itokawa's small size, implying that they can occur on other small asteroids should those have regolith.