New Mystery Discovered Regarding Active Near-Earth Asteroid Phaethon

Surface of near-Earth asteroid Phaethon may reflect less light than previously thought, according to a new study.

This is an exciting mystery for the recently approved DESTINY+ mission to investigate when it flies past Phaethon.

Artist’s conception of polarized light reflected by near-Earth asteroid Phaethon. The DESTINY+ mission is scheduled to investigate Phaethon

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 1.6-m Pirka Telescope at Nayoro Observatory in Hokkaido Japan to observe the near-Earth asteroid (3200) Phaethon.

They studied the changes in the polarization of the light it reflected at many different illumination angles. The results show that at some angles, the light reflected from Phaethon is the most polarized light ever observed among small bodies in the Solar System.

Discovered in 1983, Phaethon has been shown to be the parent body of the Geminid meteor shower. Most meteor-shower parent bodies are comets, but Phaethon doesn’t show typical cometary activity. Instead it is an active asteroid with confirmed dust ejections. It also has a surprisingly blue color. The fact that its reflected light is strongly polarized is one more mystery surrounding this curious asteroid.

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 again before being reflected towards the observer, these multiple scatterings randomize the polarization.

“If the albedo is lower than previously thought, that would reduce the effectiveness of multiple scatterings; so that strongly polarized light that has only been reflected a single time would dominate,” Dr. Ito from NAOJ, a leader of the research team explains.

Other possibilities that could reduce the effectiveness of multiple scatterings are that the rubble covering Phaethon’s surface might be composed of larger grains, or the material may be more porous than expected. A possible mechanism to produces large grains is sintering. The surface of Phaethon can be heated up to 1000 degrees Celsius during its closest passage to the Sun. Such extreme heating can cause sintering on an asteroid’s surface, resulting in coarser grains.

The Japan Aerospace Exploration Agency’s DESTINY+ probe, scheduled to launch in 2022, will take pictures as it flies by Phaethon to help astronomers better characterize its surface geology.

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