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.
Every day, Earth sweeps up about 140 tons of cosmic dust. The particles are mostly 100 micrometers to 200 micrometers in size and made of silicate minerals. Most burn up in the atmosphere, although some survive and end up in micrometeorite collections. To figure out how this dust behaves in the inner solar system, planetary dynamicist David Nesvorný of Southwest Research Institute in Boulder, Colorado, and five colleagues set up a computer model. In addition to being subject to the tug of planetary gravity, microscopic particles orbiting the sun are pushed outward by the pressure of sunlight, dragged inward by their own radiative emissions, and worn down by collisions with other particles. Nesvorný and his colleagues followed particles released in their model from various types of comets or from asteroids and compared the particles’ fates with observations of the zodiacal dust cloud.
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 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, and they supply most of the micrometeorites studied on Earth. Massive numbers of comets may even produce the bright debris disks seen around other stars. Now if only light-polluted skies didn’t deny most people a view of comets’ death glow.