Zodiacal light

Zodiacal light is a faint glow caused by sunlight scattering off interplanetary dust. When seen before sunrise, it is sometimes called false dawn. The glow is brightest near the Sun and appears in a dark sky as a roughly triangular shape extending from the Sun’s direction along the zodiac. A fainter band of light, called the zodiacal band, runs along the whole ecliptic. The light spans the entire sky and adds to the natural brightness of a clear, moonless night. A related phenomenon is gegenschein, or counterglow, which is sunlight backscattered from the same dust and appears as a faint, slightly brighter oval directly opposite the Sun. Because zodiacal light is so dim, it is usually invisible when moonlight or light pollution is present.

The dust that produces this light forms a thick, pancake-shaped cloud called the zodiacal cloud, which straddles the ecliptic plane. Individual particles range from 10 to 300 micrometres in size, with masses from about one nanogram to tens of micrograms. Observations from the Pioneer 10 and Helios spacecraft in the 1970s confirmed that the light is scattered by this interplanetary dust cloud. Analysis of impact debris images from the Juno spacecraft suggests the dust near Earth extends out to the 4:1 orbital resonance with Jupiter at 2.06 AU and may come from Mars. However, no dedicated dust instruments on Pioneer 10, Pioneer 11, Galileo, Ulysses, or Cassini found evidence that Mars is a significant source; comets and asteroids are the main contributors.

For best viewing at mid-latitudes, look west in spring after evening twilight has fully ended, or east in autumn just before morning twilight begins. The light appears as a column, brighter at the horizon and tilted along the ecliptic. Because the smallest dust particles scatter light strongly forward, the glow is brightest when looking at a small angle from the Sun. This is why it is most visible near sunrise or sunset, when the Sun itself is blocked but the dust near the line of sight is still lit. The dust band is uniform across the whole ecliptic. Dust farther from the ecliptic is almost undetectable except at small angles to the Sun, so the glow appears wider near the horizon.

The origin of the dust has long been debated. It was once thought to come from active comet tails and asteroid collisions. Over 85 percent of the dust is now attributed to occasional fragmentations of nearly dormant Jupiter-family comets, which have orbital periods under 20 years. The first fully dynamical model of the zodiacal cloud showed that only dust released in orbits approaching Jupiter gets stirred enough to explain the cloud’s thickness. Larger meteoroid stream particles, 300 to 10,000 micrometres across, break down over time into smaller zodiacal dust grains. The Poynting–Robertson effect gradually forces dust into more circular orbits while spiraling it into the Sun, so a continuous source of new particles is needed. Cometary dust and dust from asteroid collisions are thought to maintain the cloud. Particles smaller than 10 micrometres are pushed out of the inner Solar System by solar radiation pressure, and comets replenish the supply. Zodiacal dust around other stars is called exozodiacal dust and can be a source of noise when trying to image extrasolar planets; such dust may also indicate the presence of planets, which scatter comets inward. In 2015, results from the COSIMA instrument on the Rosetta orbiter confirmed that the parent bodies of interplanetary dust are most likely Jupiter-family comets like 67P/Churyumov–Gerasimenko. Data from NASA’s Juno mission, first published in 2020, suggests that dust near Earth may come from Mars, but researchers cannot yet explain how it escapes Mars’ gravity.

Zodiacal light is sunlight reflected off cosmic dust, so its spectrum matches the Sun’s. The dust occupies a lens-shaped volume centered on the Sun and extending beyond Earth’s orbit. Most of it lies near the plane of the Solar System, so the light is seen along the ecliptic. The amount of material needed is very small; if it were all in 1 mm particles, the total mass would be tiny.