Dust - The Gegenschein
An image captured in Iran by Amir Abolfath (site, TWAN) ?At bottom right the Pleiades star cluster. At centre left, fiery Aldebaran. Between them is a faint luminous patch about 5 degrees across. This is the rarely seen gegenshein or counter glow. ?The glow is directly opposite the sun at the antisolar point and moves against the stars as seasons progress. To see it visually needs a locale well away from light pollution. Look also when it is not backed by the Milky Way. November and January are good times for the northern hemisphere.?Below: the same scene processed to bring out the gegenschein and the distant dust clouds of the galaxy. The gegenschein glow is from dust too. Sunlight scattered by the much nearer dust between the planets of the solar system makes the glow.
The gegenshein's dust lies mostly in a vast disk in the plane of the planetary orbits and extending out to the distance of Jupiter and beyond. ?The dust is relatively young and comes from asteroid collisions. Comets add to it also. ?The dust grains are irregular and range in size from 1/1000 to 1mm across. Their sizes and shapes are important because they determine how they scatter sunlight.?
Forward and backwards scattering
Particles smaller than the wavelengths of sunlight scatter the light forwards and backwards equally. Air molecules do this.?Interplanetary dust is larger. Most light is instead scattered sharply forwards. This generates the cones of zodiacal light seen soon after sunset and before sunrise.?A small amount scatters backwards to appear close to the antisolar point as the gegenschein.?An even smaller amount scattered sideways produces the extremely faint zodiacal band that extends around the ecliptic to link the zodiacal light and gegenschein.? ?
Opposition effect - Shadow hiding
The above does not account fully for the gegenschein's small, 5-10 degrees, diameter. There is an additional effect - the opposition effect or shadow hiding operating. The opposition effect causes the moon to be particularly bright when exactly full. It causes Mars and asteroids to be brighter when they are close to the antisolar point. Interplanetary dust grains are similarly brighter.?Interplanetary dust grains are rough and pitted on a microscopic scale. As in the example at right, they can be fluffy assemblages of smaller grains and crystals. ?Protuberances on their surface cast shadows into nearby deeper areas. When we look at them from a great distance and towards the antisolar point these shadows cannot be seen - they are hidden behind the shadow casting protuberance. Dust at the antisolar point therefore looks brighter. On a far larger scale Earth's forests do the same!?There is another effect, coherent backscatter, but that is not thought to contribute much to the gegenschein's brightness.??
A comet spurts dust into interplanetary space. The broad white tail is dust. The straight blue tails are gas and molecular ions.?Hale-Bopp 1997 Copyright Les Cowley?