Low-albedo asteroids: analogues with a high polarization at large phase angles

Abstract

ABSTRACT While remote observations of the linear polarization of five low-albedo near-Earth asteroids are available at large phase angles, space missions have collected materials from the surface of two of them and one of them is already back to Earth. The structure of the regolith on the surface may be different from that encountered on larger objects, because of their low gravity and thermal stress cycling. Dust particles crushed from low-albedo meteorites (i.e. Orgueil and Allende) are tentatively used as analogues to provide a better approach of such regoliths. The PROGRA2 experiment studies the light-scattering properties of dust particles of various size distributions under Earth’s gravity either deposited or with clouds lifted by an air-draught, as well as under μ-gravity conditions. Similar maximum in polarization (i.e. Pmax) values are obtained experimentally for dark particles deposited or in levitation, showing that multiple scattering is negligible. The increased sensitivity of PROGRA2 instruments, together with imaging techniques, makes it possible to study Pmax with increasing sizes of the lifted particles, up to mm-sizes and above. Our results confirm that particles constituting the regolith are mm-sized and may reach polarization values up to 50 per cent in good agreement with remote observations. Some materials are also suggested to be present on the surface of the particles. Also, huge agglomerates made by random ballistic deposition may be considered as relevant analogues e.g. for pebbles and boulders.