Comments on the amplitude-phase relationship of asteroid lightcurves

Effects of topography, surface scattering properties, and obliquity

¹ Instituto de Astrofísica de Andalucía-CSIC, PO Box 3004, 18080 Granada, Spain e-mail: pedroj@iaa.es
² Department of Astronomy and Space Physics, Uppsala University, Box 515, 75120 Uppsala, Sweden

Received: 11 January 2006
Accepted: 6 March 2006

Abstract

Aims.We present a theoretical study on the amplitude-phase relationship (APR) for lightcurves of simulated asteroids.

Methods.In support of the Rosetta (ESA) mission, we developed a numerical model for the investigation of the light reflectance properties of asteroidal bodies. The code is able to deal with irregular and chemically inhomogeneous surfaces, taking shadowing effects into account. Several standard scattering models have been implemented, which govern local reflectance properties, e.g. the Hapke model and the Lumme-Bowell model. From a kinematic standpoint, the body can move in an arbitrary orbit, and it may rotate in either pure or complex mode with an arbitrary orientation of its angular momentum. As an application of the code, we studied the dependence of the APR on several factors, such as the illumination and observational geometries, overall shape, and large-scale topography, as well as the surface characteristics represented by the parameters in the Hapke and Lumme-Bowell models.

Results.In our study, we find that mineralogy, regolith properties, and small-scale surface roughness (i.e., characteristics embodied in the considered surface scattering models), have a negligible effect on the APR. Furthermore, large-scale topography introduces a rather significant dispersion in the APR slope, on the order of . Our simulations suggest that obliquity is the major agent for shaping the APR, causing a dispersion in the APR slope; the larger the obliquity, the smaller the slope of the APR. For intermediate aspect angles, large obliquities could even lead to an amplitude that decreases with the phase angle.