Opposition surge
Encyclopedia
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
, moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, or comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
as its phase angle of observation approaches zero. It is so named because the reflected light from the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
and Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
appear significantly brighter than predicted when at astronomical opposition
Opposition (astronomy)
In positional astronomy, two celestial bodies are said to be in opposition when they are on opposite sides of the sky, viewed from a given place . In particular, two planets are in opposition to each other when their ecliptic longitudes differ by 180°.The astronomical symbol for opposition is ☍...
. Two physical mechanisms have been proposed for this observational phenomenon: shadow hiding and coherent backscatter.
Overview
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 situated on Earth.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.
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 oppositionOpposition (astronomy)
In positional astronomy, two celestial bodies are said to be in opposition when they are on opposite sides of the sky, viewed from a given place . In particular, two planets are in opposition to each other when their ecliptic longitudes differ by 180°.The astronomical symbol for opposition is ☍...
from the viewpoint of the observer), this intrinsic brightness is usually close to its maximum. At a phase angle of zero degrees, these shadow areas become negligible. The celestial body in effect becomes an imperfect mirror. 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
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
surfaces of airless bodies in the solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
. 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 astronomical observations 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. In the case of planetary rings (such as Saturn's
Rings of Saturn
The rings of Saturn are the most extensive planetary ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometres to metres, that form clumps that in turn orbit about Saturn...
), an opposition surge is due to the covering of shadows on the ring particles. This explanation was first proposed by Hugo von Seeliger
Hugo von Seeliger
Hugo von Seeliger , also known as Hugo Hans Ritter von Seeliger, was a German astronomer, often considered the most important astronomer of his day....
in 1887.
Coherent backscatter
An alternate theory for the increase in 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.Coherent backscatter phenomena have also been observed with RADAR
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
. 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.
Opposition surge throughout the Solar system
The existence of the opposition surge was first recorded in 1956 by Tom GehrelsTom Gehrels
Tom Gehrels , was an American astronomer, Professor Planetary Sciences, and Astronomer at the University of Arizona, Tucson....
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 gas giant, nor for bodies with pronounced atmospheres.
In the case of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, 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. 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.
See also
- AlbedoAlbedoAlbedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
- Bidirectional reflectance function
- Geometric albedoGeometric albedoThe 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, diffusively scattering disk with the same cross-section....
- HeiligenscheinHeiligenscheinHeiligenschein is an optical phenomenon which creates a bright spot around the shadow of the viewer's head. It is created when the surface on which the shadow falls has special optical characteristics. Dewy grass is known to exhibit these characteristics, and creates a Heiligenschein...
External links
- Hayabusa observes the opposition surge of Asteroid Itokawa
- opposition effect, "Atmospheric optics" website. Includes a picture of the opposition surge on the moon
- opposition effect mechanism, "Atmospheric optics" website. Diagrammatic representation of the opposition surge
- "The-moon wikispaces" opposition surge page
- Opposition surge on Saturn's B Ring as seen by Cassini–Huygens