The Opposition Effect and the Quasi-fractal Structure of Regolith: I. Theory

Details The Opposition Effect and the Quasi-fractal Structure of Regolith: I. Theory The Opposition Effect and the Quasi-fractal Structure of Regolith: I. Theory

Jul 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001icar..152...96s&link_type=abstract

Icarus, Volume 152, Issue 1, pp. 96-116 (2001).

Computer Science

29

Scientific paper

We present a new heuristic model combining the coherent-backscatter and shadow-hiding opposition effects. The model considers the aggregate structure of regolith in a realistic way and accounts for the fractal architecture of planetary surfaces both on microscopic particulate and macroscopic, texturally faceted size scales. It describes how the shadow-hiding opposition effect is manifested in a fractally arranged surface, the important contribution of incoherent, multiply scattered light among small scatterers within a surface composed of aggregate grains and its amplification by backscatter interference. We provide a general theoretical model as well as a preliminary, simple analytical approximation that is suitable for investigating the opposition behavior of planet, satellite, and asteroid surfaces. The analytical approximation utilizes only four model parameters; ω0, the average, single-scattering albedo of elemental scatterers that are the building blocks of aggregates, h, Hapke's (1986, Icarus 67, 264-280) angular-width parameter for the shadow-hiding opposition effect, L, the diffusion scale length of light through the regolith, and q, a structural parameter that characterizes the relative size and complexity of aggregate particles. Our model explains the simultaneous occurrence of the coherent-backscatter enhancement and shadow-hiding in moderate to low albedo soils in addition to the dominance of coherent-backscatter in bright soils. It predicts that coherent backscatter and shadow-hiding can occur at a variety of size scales within the regolith. Individual regolith particles can exhibit their own opposition effects which are convolved with the contribution that arises from their packing within the planetarysurface. The size domains over which coherent-backscatter and shadow-hiding, respectively, each have their most pronounced effect is strongly controlled by the albedo of internal scatterers and the number of hierarchical generations of aggregates required to characterize the particulate-scale behavior of the regolith surface.

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