Opposition surge

The term is most widely used in astronomy, where generally it refers to the sudden noticeable increase in the brightness of a celestial body such as a planet, moon, or comet as its phase angle of observation approaches zero.

Two physical mechanisms have been proposed for this observational phenomenon: shadow hiding and coherent backscatter.

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.

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 particular, recent observations of Titan at 2.2 cm with Cassini have shown that a strong coherent backscatter effect is required to explain the high albedos at radar wavelengths.

[4] On Earth, water droplets can also create bright spots around the antisolar point in various situations.

The existence of the opposition surge was described in 1956 by Tom Gehrels during his study of the reflected light from an asteroid.

Since Gehrels' early studies, an opposition surge has been noted for most airless solar system bodies.