Opposition effect

  The opposition effect (also opposition spike, or opposition surge) is the brightening of a rough surface, or an object with many particles, when illuminated from directly behind the observer. It is so named because the reflected light from the Moon and Mars appeared significantly brighter than predicted when at astronomical opposition, giving rise to an opposition spike.

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The effect is particularly pronounced on regolith surfaces of airless bodies in the solar system. The usual major cause of the effect is that a surface's small pores and pits that would otherwise be in shadow at other incidence angles become lit up when the observer is almost in the same line as the source of illumination. The effect is usually only visible for a very small range of phase angles near zero. For bodies whose reflectance properties have been quantitatively studied, details of the opposition effect — its strength and angular extent — are described by two of the Hapke parameters. In the case of planetary rings (such as Saturn's), an opposition effect is due to the covering of shadows on the ring particles.

The opposition effect does not arise on smooth mirror-like surfaces, which give rise to a different anti-solar brightening, nor from completely cloudy planets such as Jupiter.^{[citation needed]}

See also

• Bidirectional reflectance function
• geometric albedo

References

• Drake, J.S. 2004, A Photometric Study of Phocaea Group Asteroid 1584 Fuji, Senior Honors Thesis, Department of Physics and Astronomy, Dickinson College, Carlisle, PA