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Zodiacal light—the faint white glow that stretches across the darkest skies, tracing the same path the sun takes—has mystified scientists for centuries. They've known that it is sunlight reflected from a disk of dust spanning the inner solar system from Mercury to Jupiter. They just didn’t know where the dust came from—until now.
Previous studies had suggested that much of the zodiacal light came from the dust of colliding asteroids, but the only way this model could reproduce the great breadth of the zodiacal cloud above and below the plane of the planets was to have the dust come from the comets that orbit in the vicinity of Jupiter’s orbit. Those comets already range much farther from the plane of the planets than asteroids do, and Jupiter’s gravitational effects would drive their dust even farther afield. To make the modeled zodiacal cloud as dense as the real one, the dust had to come from comets falling apart, not just those shedding dust near the sun, the team reports in the 20 April issue of The Astrophysical Journal.
The dust between the planets, that scatters sunlight our way, is not from the asteroid belt (depicted here in green), but from periodically disrupting comets that spend much of their time near the orbit of Jupiter, according to calculations by Nesvorny and Jenniskens. (Credit: Southwest Research Institute)
Light appearing in the night sky
Nesvorny and Jenniskens, with the help of Harold Levison and William Bottke of the Southwest Research Institute, David Vokrouhlicky of the Institute of Astronomy at Charles University in Prague, and Matthieu Gounelle of the Natural History Museum in Paris, demonstrated that these comet disruptions can account for the observed thickness of the dust layer in the zodiacal cloud.
In doing so, they solved another mystery. It was long known that snow in Antarctica is laced with micro-meteorites, some 80 to 90 percent of which have a peculiar primitive composition, rare among the larger meteorites that we know originated from asteroids. Instead, Nesvorny and Jenniskens suggest that most antarctic micro-meteorites are pieces of comets. According to their calculations, cometary grains dive into Earth's atmosphere at entry speeds low enough for them to survive, reach the ground, and be picked up later by a curious micro-meteorite hunter.
The new modeling has “produced a detailed and convincing case that [90% of] interplanetary dust particles and the hundreds of particles that are now curated on Earth originate from Jupiter family comets,” writes planetary scientist Stanley Dermott of the University of Florida, Gainesville, in an e-mail. Comets throw up the dust veil that creates the zodiacal light
The zodiacal cloud is a thick circumsolar disk of small debris particles produced by asteroid collisions and comets. Their relative contribution and how particles of different sizes dynamically evolve to produce the observed phenomena of light scattering, thermal emission, and meteoroid impacts are unknown. Until now, zodiacal cloud models have been phenomenological in nature, composed of ad hoc components with properties not understood from basic physical processes. Here, we present a zodiacal cloud model based on the orbital properties and lifetimes of comets and asteroids, and on the dynamical evolution of dust after ejection. The model is quantitatively constrained by Infrared Astronomical Satellite (IRAS) observations of thermal emission, but also qualitatively consistent with other zodiacal cloud observations, with meteor observations, with spacecraft impact experiments, and with properties of recovered micrometeorites (MMs). We find that particles produced by Jupiter-family comets (JFCs) are scattered by Jupiter before they are able to orbitally decouple from the planet and drift down to 1 AU. Therefore, the inclination distribution of JFC particles is broader than that of their source comets and leads to good fits to the broad latitudinal distribution of fluxes observed by IRAS. We find that 85%-95% of the observed mid-infrared emission is produced by particles from JFCs and
Although there are meteoroids that are orbiting the sun alone, there are other meteoroids that are traveling in a cloud of meteoroids. This later group is moving within the orbit of a comet or an extinct comet. Astronomers discovered back in the 19th century that comets shed particles every time they passed close to the sun. There are some comets whose orbits are located very close to the orbit of Earth and, at certain times each year, Earth passes through a cloud of particles, which results in a meteor shower.