"Our analysis shows that Phaethon's comet-like activity cannot be explained by any kind of dust."
A recent study reveals that the tail of the comet-like asteroid 3200 Phaethon is made of sodium gas, not dust, shedding new light on the celestial body responsible for the Geminid meteor shower.
Asteroid 3200 Phaethon’s Unusual Behavior
Long known for its comet-like activities, such as brightening and forming a tail near the Sun, asteroid 3200 Phaethon has puzzled scientists. It is the source of the annual Geminid meteor shower, which is typically caused by comets. Researchers previously attributed Phaethon’s behavior to dust escaping from the asteroid when heated by the Sun.
Phaethon: Sodium Gas Tail Discovery
However, a new study using two NASA solar observatories found that Phaethon’s tail is not made of dust but sodium gas. Qicheng Zhang, a Ph.D. student at the California Institute of Technology, said, “Our analysis shows that Phaethon’s comet-like activity cannot be explained by any kind of dust.”
Breaking the Norm: Asteroid vs. Comet
Asteroids, primarily made of rock, usually do not form tails when approaching the Sun. Comets, a mix of ice and rock, often form tails as the Sun vaporizes their ice. This material leaves a trail along the comet’s orbit, which results in meteor showers when Earth passes through it.
Phaethon’s Enigmatic History
Phaethon, discovered in 1983, was identified as the source of the Geminid meteor shower due to its matching orbit. NASA’s STEREO mission observed a short tail on Phaethon during its closest approach to the Sun, which was previously unseen by regular telescopes. This supported the hypothesis that dust was escaping the asteroid’s surface when heated by the Sun.
Challenging Previous Beliefs
However, in 2018, NASA’s Parker Solar Probe observed more material in the Geminid debris trail than Phaethon could possibly shed during its close approaches to the Sun. This prompted Zhang’s team to investigate whether something other than dust was behind Phaethon’s comet-like behavior.
The Implications of Sodium Gas
Using the Solar and Heliospheric Observatory (SOHO) spacecraft, Zhang’s team found that Phaethon’s tail appeared bright in the sodium-detecting filter but not in the dust-detecting filter. The evidence suggests that the tail is made of sodium, not dust. This discovery may cause researchers to reconsider the nature of some comets discovered by SOHO and citizen scientists as part of the Sungrazer Project.
Phaethon: The Mystery of the Geminid Meteor Shower Source
One remaining question is how Phaethon supplies material for the Geminid meteor shower if it doesn’t shed much dust. Zhang’s team suspects a disruptive event a few thousand years ago might have caused the asteroid to eject the material estimated to make up the Geminid debris stream. The upcoming JAXA mission DESTINY+ plans to fly past Phaethon and study its rocky surface and any surrounding dust for more answers.