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Opposition effect
 
 
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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
Moon

The Moon is Earth's only natural satellite and the List of natural satellites by diameter satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is km, about thirty times the diameter of the Earth....
 and Mars
MARS

In cryptography, MARS is a block cipher that was IBM's submission to the Advanced Encryption Standard process. MARS was selected as an AES finalist in August 1999, after the AES2 conference in March 1999, where it was voted as the fifth and last finalist algorithm....
 appeared significantly brighter than predicted when at astronomical opposition
Opposition (astronomy)

Opposition is a term used in positional astronomy and astrology to indicate when one Astronomical object is on the opposite side of the sky when viewed from a particular place ....
, giving rise to an opposition spike.

The effect is particularly pronounced on regolith
Regolith

Regolith is a layer of loose, heterogeneous material covering solid Rock . The term is a combination of two Greek words: Rhegos , which means blanket, and Lithos , which means rock....
 surfaces of airless bodies in the solar system
Solar System

The Solar System consists of the Sun and those Astronomical object bound to it by gravity: the eight planets and five dwarf planets, their 173 known Natural satellite, and billions of Small Solar System body....
.






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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
Moon

The Moon is Earth's only natural satellite and the List of natural satellites by diameter satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is km, about thirty times the diameter of the Earth....
 and Mars
MARS

In cryptography, MARS is a block cipher that was IBM's submission to the Advanced Encryption Standard process. MARS was selected as an AES finalist in August 1999, after the AES2 conference in March 1999, where it was voted as the fifth and last finalist algorithm....
 appeared significantly brighter than predicted when at astronomical opposition
Opposition (astronomy)

Opposition is a term used in positional astronomy and astrology to indicate when one Astronomical object is on the opposite side of the sky when viewed from a particular place ....
, giving rise to an opposition spike.

The effect is particularly pronounced on regolith
Regolith

Regolith is a layer of loose, heterogeneous material covering solid Rock . The term is a combination of two Greek words: Rhegos , which means blanket, and Lithos , which means rock....
 surfaces of airless bodies in the solar system
Solar System

The Solar System consists of the Sun and those Astronomical object bound to it by gravity: the eight planets and five dwarf planets, their 173 known Natural satellite, and billions of Small Solar System body....
. 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
Phase angle (astronomy)

Phase angle in Observational astronomys is the angle between the light incident onto an observed object and the light reflected from the object....
 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
Hapke parameters

The Hapke parameters are a set of parameters for a quasi-experimental model that are commonly used to describe theBidirectional reflectance distribution function of the airless regolith surfaces of bodies in the solar system....
. In the case of planetary rings (such as Saturn's
Rings of Saturn

Saturn has the most extensive planetary ring system of any planet in the Solar System. The rings of Saturn consist of countless small particles, ranging in size from micrometres to metres, that form clumps that in turn orbit about Saturn....
), 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.

See also

  • Bidirectional reflectance function
  • geometric albedo
    Geometric albedo

    The geometric albedo of an astronomical body is the ratio of its actual brightness at zero Phase angle to that of an idealized flat, fully reflecting, diffuse reflection disk with the same cross-section....


External links

  • , "Atmospheric optics" website. Includes a picture of the opposition effect on the moon.
  • , "Atmospheric optics" website. diagrammatic explanation of the mechanism.
  • - as seen by Cassini-Huygens
    Cassini-Huygens

    Cassini?Huygens is a joint NASA/European Space Agency robotic spacecraft mission currently studying the planet Saturn and Saturn's natural satellites....