![Armstrong heiligenschein](https://upload.wikimedia.org/wikipedia/commons/b/b4/Armstrong_heiligenschein.jpg)
The opposition surge (sometimes known as the opposition effect, opposition spike or Seeliger effect
) is the brightening of a rough surface, or an object with many
particle
In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass.
They vary greatly in size or quantity, from ...
s, when illuminated from directly behind the observer. The term is most widely used in
astronomy
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
, where generally it refers to the sudden noticeable increase in the brightness of a
celestial body such as a
planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
,
moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, or
comet
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ...
as its
phase angle of observation approaches zero. It is so named because the reflected light from the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
and
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
appear significantly brighter than predicted by simple
Lambertian reflectance
Lambertian reflectance is the property that defines an ideal "matte" or diffusely reflecting surface. The apparent brightness of a Lambertian surface to an observer is the same regardless of the observer's angle of view. More technically, the su ...
when at
astronomical opposition
In positional astronomy, two astronomical objects are said to be in opposition when they are on opposite sides of the celestial sphere, as observed from a given body (usually Earth).
A planet (or asteroid or comet) is said to be "in opposit ...
. Two physical mechanisms have been proposed for this observational phenomenon: shadow hiding and coherent backscatter.
Overview
![Ceres opposition effect](https://upload.wikimedia.org/wikipedia/commons/0/03/Ceres_opposition_effect.png)
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's brightness rapidly increases. This is mainly due to the increased area lit, but is also partly due to the intrinsic brightness of the part that is sunlit. This is affected by such factors as the angle at which light reflected from the object is observed. For this reason, a full moon is more than twice as bright as the moon at first or third quarter, even though the visible area illuminated appears to be exactly twice as large.
Physical mechanisms
Shadow hiding
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 to
opposition
Opposition may refer to:
Arts and media
* ''Opposition'' (Altars EP), 2011 EP by Christian metalcore band Altars
* The Opposition (band), a London post-punk band
* '' The Opposition with Jordan Klepper'', a late-night television series on Com ...
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 on
regolith
Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestr ...
surfaces of airless bodies in the
Solar System
The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
. 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 The Hapke parameters are a set of parameters for an empirical model that is commonly used to describe the directional reflectance properties of the airless regolith surfaces of bodies in the Solar System. The model has been developed by astronomer ...
. In the case of planetary rings (such as
Saturn's), an opposition surge is due to the uncovering of shadows on the ring particles. This explanation was first proposed by
Hugo von Seeliger
Hugo von Seeliger (23 September 1849 – 2 December 1924), also known as Hugo Hans Ritter von Seeliger, was a German astronomer, often considered the most important astronomer of his day.
Biography
He was born in Biala, completed high school in ...
in 1887.
Coherent backscatter
A theory for an additional effect that increases brightness during opposition is that of coherent backscatter. 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 with
radar
Radar is a detection system that uses radio waves to determine the distance ('' ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, we ...
. 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.
Water droplets
On Earth, water droplets can also create bright spots around the
antisolar point
The antisolar point is the abstract point on the celestial sphere directly opposite the Sun from an observer's perspective. This means that the antisolar point lies above the horizon when the Sun is below it, and vice versa. On a sunny day, ...
in various situations. For more details, see
Heiligenschein
(; ) is an optical phenomenon in which a bright spot appears around the shadow of the viewer's head in the presence of dew. In photogrammetry and remote sensing, it is more commonly known as the hotspot. It is also occasionally known as Cellin ...
and
Glory (optical phenomenon).
Throughout the Solar System
The existence of the opposition surge was described in 1956 by
Tom Gehrels
Anton M.J. "Tom" Gehrels (February 21, 1925 – July 11, 2011) was a Dutch–American astronomer, Professor of Planetary Sciences, and Astronomer at the University of Arizona, Tucson.
Biography
Youth and education
Gehrels was born at Haa ...
during his study of the reflected light from an
asteroid. Gehrels' later studies showed that the same effect could be shown in the moon's brightness. 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 the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, B. J. Buratti ''et al.'' have suggested that its brightness increases by some 40% between a phase angle of 4° and one of 0°, and that this increase is greater for the rougher-surfaced highland areas than for the relatively smooth
maria
Maria may refer to:
People
* Mary, mother of Jesus
* Maria (given name), a popular given name in many languages
Place names Extraterrestrial
* 170 Maria, a Main belt S-type asteroid discovered in 1877
* Lunar maria (plural of ''mare''), large, ...
. 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.
[Burrati, B. J.; Hillier, J. K.; & Wang, M. (1996)]
The Lunar Opposition Surge: Observations by Clementine
. ''Icarus'' 124: 490-499.
See also
*
Albedo
Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refl ...
*
Bidirectional reflectance function
The bidirectional reflectance distribution function (BRDF; f_(\omega_,\, \omega_) ) is a function of four real variables that defines how light is reflected at an opaque surface. It is employed in the optics of real-world light, in computer ...
*
Brocken spectre
A Brocken spectre (British English; American spelling Brocken specter; german: Brockengespenst), also called Brocken bow, mountain spectre, or spectre of the Brocken is the magnified (and apparently enormous) shadow of an observer cast in mid ai ...
, the apparently enormous and magnified shadow of an observer cast upon the upper surfaces of clouds opposite the Sun
*
Gegenschein
Gegenschein (; ; ) or counterglow is a faintly bright spot in the night sky centered at the antisolar point. The backscatter of sunlight by interplanetary dust causes this optical phenomenon.
Explanation
Like zodiacal light, gegenschein is ...
*
Geometric albedo
In astronomy, the geometric albedo of a celestial body is the ratio of its actual brightness as seen from the light source (i.e. at zero phase angle) to that of an ''idealized'' flat, fully reflecting, diffusively scattering ( Lambertian) disk wi ...
References
{{reflist
External links
Hayabusa observes the opposition surge of Asteroid Itokawa "Atmospheric optics" website. Includes a picture of the opposition surge on the moon
"Atmospheric optics" website. Diagrammatic representation of the opposition surge
"The-moon wikispaces" opposition surge pageOpposition surge on Saturn's B Ringas seen by
Cassini–Huygens
Astronomical events
Lunar science
Optical phenomena
Observational astronomy
Radiometry
Scattering, absorption and radiative transfer (optics)