Theopposition surge (sometimes known as theopposition effect,opposition spike orSeeliger effect[1]) is the brightening of a rough surface, or an object with manyparticles, when illuminated from directly behind the observer. The term is most widely used inastronomy, where generally it refers to the sudden noticeable increase in the brightness of acelestial body such as aplanet,moon, orcomet as itsphase angle of observation approaches zero. It is so named because the reflected light from theMoon andMars appear significantly brighter than predicted by simpleLambertian reflectance when atastronomical opposition. Two physical mechanisms have been proposed for this observational phenomenon: shadow hiding and coherent backscatter.
The phase angle is defined as the angle between the observer, the observed object and the source of light. In the case of the Solar System, the light source is the Sun, and the observer is generally on Earth. At zero phase angle, the Sun is directly behind the observer and the object is directly ahead, fully illuminated.
As the phase angle of an object lit by the Sun decreases, the object'sluminous intensity increases. This is partly due to the increased area lit, but is also partly due to the intrinsic brightness (theluminance) of the part that is sunlit. This is affected by theilluminance of the surface, which is strongest right under the sun and goes to zero at the parts of the object that face at right angle to the sun. But the luminance is also affected by the angle at which light reflected from the object is observed. For this reason, moonlight at full moon is much more than at first or third quarter, even though the visible area illuminated is only twice as large.
When the angle of reflection is close to the angle at which the light's rays hit the surface (that is, when the Sun and the object are close toopposition from the viewpoint of the observer), this intrinsic brightness is usually close to its maximum. At a phase angle of zero degrees, all shadows disappear and the object is fully illuminated. When phase angles approach zero, there is a sudden increase in apparent brightness, and this sudden increase is referred to as the opposition surge.
The effect is particularly pronounced onregolith surfaces of airless bodies in theSolar 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 ofphase 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 theHapke parameters. In the case of planetary rings (such asSaturn's), an opposition surge is due to the uncovering of shadows on the ring particles. This explanation was first proposed byHugo von Seeliger in 1887.[2]
A theory for an additional effect that increases brightness during opposition is that of coherent backscatter.[3] In the case of coherent backscatter, the reflected light is enhanced at narrow angles if the size of the scatterers in the surface of the body is comparable to the wavelength of light and the distance between scattering particles is greater than a wavelength. The increase in brightness is due to the reflected light combining coherently with the emitted light.
Coherent backscatter phenomena have also been observed withradar. In particular, recent observations of Titan at 2.2 cm withCassini 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 theantisolar point in various situations. For more details, seeHeiligenschein andGlory (optical phenomenon).
The existence of the opposition surge was described in 1956 byTom Gehrels during his study of the reflected light from anasteroid.[5] Gehrels' later studies showed that the same effect could be shown in the moon's brightness.[6] He coined the term "opposition effect" for the phenomenon, but the more intuitive "opposition surge" is now more widely used.
Since Gehrels' early studies, an opposition surge has been noted for most airless solar system bodies. No such surge has been reported for bodies with significant atmospheres.
In the case of theMoon, B. J. Burattiet al. used observations from theClementine spacecraft at very low phase angle to find that the moon's brightness increases by more than 40% between a phase angle of 4° and one of 0°. (Observation from Earth cannot be at a phase angle less than about half a degree without there being a lunar eclipse. A phase angle of 4° is achieved about eight hours before or after a lunar eclipse.) This increase is greater for the rougher-surfaced highland areas than for the relatively smoothmaria. As for the principal mechanism of the phenomenon, measurements indicate that the opposition effect exhibits only a small wavelength dependence: the surge is 3-4% larger at 0.41 μm than at 1.00 μm. This result suggests that the principal cause of the lunar opposition surge is shadow-hiding rather than coherent backscatter.[7]