Task Group for Albedo - Polarimetry Relation

Members:

R. Gil-Hutton (Chair), E. F. Tedesco, A. Cellino, E. L. G. Bowell, I. N. Belskaya, K. Muinonen, and A. -C. Levasseur-Regourd.


Charter:

The Task Group is charged with recalibration of the albedo-polarization relation for asteroids. Specifically this means to define improved procedures to determine the relation between the albedo and the polarization minimum (Pmin) or the slope of the phase-polarization curve. The Task Group should prepare progress reports to the Organizing Committee of Commission 15.


Justification:

The albedo is a fundamental parameter to characterize small solar system bodies, being strictly related to their mineralogic composition, and to the properties of their surface regolith. Systematic differences in albedo also distinguish asteroids that are believed to be primitive from those that have likely experienced some kind of thermal metamorphism during their lifetime. The derivation of asteroid albedos is a difficult task, and only a few observing techniques can be used for this purpose. Given the relation between albedo, size, and absolute magnitude, albedos have been in many cases derived from radiometric observations in the thermal IR, resulting in good estimates for the sizes of the objects, but relying on nominal, and often very inaccurate, values of the absolute magnitude at visible wavelengths.

An alternative method, which is in principle much better, is based on observations of the polarimetric properties of sunlight scattered by the asteroid surface. The degree of linear polarization of the light received from asteroids changes with changing circumstances of illumination. In particular, the so-called polarization-phase curves describe how the linear polarization changes as a function of the phase angle (the angle between the directions of the Earth and the Sun as seen from an asteroid). These curves are characterized by trends that can be described by a small number of parameters, including the so called slope parameter and the Pmin parameter. Empirical relations exist between the albedo and both the polarimetric slope and Pmin. As a consequence, polarimetry is in principle the best technique to derive the albedo of asteroids, being directly derived from the results of polarimetric observations without any other input from other physical parameters.

A problem exists in that different calibrations of the slope and Pmin albedo relation have been published in the literature, and different authors are using different calibrations in deriving their albedos from polarimetric observations. This creates confusion, making it desirable to update the calibration and better define of the slope-Pmin-albedo relation. An updated analysis of the data at our disposal would be very timely, also to check some possible limits of the slope-Pmin-albedo relation caused by possible saturation effects at very low values of the albedo.