Why The Phaethon Asteroid Is Peculiarly Polarized

The Phaethon asteroid reflects less light than previously thought, suggesting that its surface may be made of coarse rubble.

AsianScientist (Jul. 17, 2018) – In a study published in Nature Communications, an international team of scientists has found that the surface of the near-Earth asteroid Phaethon reflects less light than previously thought.

Scientifically, light is referred to as electromagnetic waves. The waves interfere with electric and magnetic fields, and the directions of these interferences can either be random or aligned. When the electromagnetic effects of light are aligned, the light is said to be polarized. Scientists are interested in how the polarization changes when sunlight reflects off the surface of an asteroid.

In this study, an international team, including astronomers from the National Astronomical Observatory of Japan (NAOJ), Seoul National University, Chiba Institute of Technology and other institutes, used the Pirka Telescope at Nayoro Observatory in Hokkaido, Japan, to observe the near-Earth asteroid Phaethon. They studied the changes in the polarization of light reflecting off Phaethon at many different illumination angles.

Discovered in 1983, Phaethon has been shown to be the parent body of the Geminid meteor shower. It is an active asteroid with confirmed dust ejection and has a surprisingly blue color. The researchers found that at some angles, the light reflected from Phaethon is the most polarized light ever observed among small bodies in the solar system.

One possible explanation for the strong polarization is that the surface of Phaethon might be darker than expected. Asteroid surfaces are covered with loose rubble. When light reflected by the rough surface strikes another part of the surface and is reflected once more before being picked up by the observer, these multiple scatterings randomize the polarization.

“If the albedo—the percentage of light reflected by Phaethon—is lower than previously thought, that would reduce the effectiveness of multiple scatterings. Hence, strongly polarized light that has only been reflected a single time would dominate,” said Dr. Takashi Ito from NAOJ, who led the research team.

The rubble covering Phaethon’s surface might be composed of larger grains, or may be more porous than expected, which could also reduce the effectiveness of multiple scatterings, said the researchers. They suggested sintering—the compacting of solid material under high heat or pressure—as a possible mechanism for the production of these large grains of rubble. Because the surface of Phaethon can be heated up to 1,000 degrees Celsius during its closest passage to the Sun, the extreme heat could cause sintering on the asteroid’s surface, resulting in coarser grains.

To help astronomers better characterize the surface geology of Phaetheon, the Japan Aerospace Exploration Agency’s DESTINY+ probe, scheduled to launch in 2022, will take pictures as it flies by the asteroid.

The article can be found at: Ito et al. (2018) Extremely Strong Polarization of an Active Asteroid (3200) Phaethon.


Source: National Astronomical Observatory of Japan.
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