Grain Size and Density Separation on Asteroids: Comparison of Seismic Shaking

Introduction: One of the most important discoveries of the NEAR Shoemaker mission to Eros was the “ponds”, flat regions (relative to surrounding terrain) inside craters that exhibit a slightly bluer reflectance spectra than most of the asteroid [1]. Ponds have been interpreted as being composed of finer-grained material than the rest of the asteroid surface. Formation processes for ponds include electrostatic levitation, seismic shaking, and fluidization [2,3]. In each case, a force (charging from solar wind, shaking from meteoroid impacts, or gas flow from the interior, respectively) results in a separation of fine-grained material on the surface which, due to down-slope movement, results in formation of accumulation of fine-grained material in depressions. Here, we present results of experiments assessing the effects of shaking and fluidization on chondritic-like analogs. We have previously conducted laboratory and micro-gravity fluidization experiments on chondrite analogs [4,5,6]. Freely moving/flowing particles exhibit behavior intermediate between solids and liquids [7]. One result is that grain-size sorting can occur in shaken or flowing groups of particles, analogous to grain-size sorting that occurs in more dilute aqueous systems such as lakes and rivers. However, particle-based systems exhibit some unusual properties, including “inverse” sizesorting, where large particles can be preferentially sorted to the top of a particle flow (sometimes called the “Brazil nut effect”) [8]. Importantly, grain-size sorting in these systems solely reflects the movement of particles, and thus does not require any pore gas or fluid, and thus could occur on anhydrous bodies.