Phaethon’s tail revealed to be made of sodium gas, not dust, upending previous theories

Astronomers have discovered that asteroid 3200 Phaethon is even stranger than previously thought. While it has been known for some time that Phaethon behaves like a comet, with a brightening and tail formation as it approaches the sun, a recent study using two NASA solar observatories has revealed that the tail is not made of dust as previously thought, but is actually composed of sodium gas.

Lead author of the study, Qicheng Zhang, a Ph.D. student at the California Institute of Technology, stated that the comet-like activity of Phaethon cannot be attributed to any kind of dust. This finding is surprising, as asteroids, which are primarily rocky, do not typically form tails when they approach the sun, whereas comets, which are made up of ice and rock, do form tails as the sun vaporizes their ice.

Phaethon was discovered in 1983, and its orbit was found to match that of the Geminid meteors, indicating that it was the source of the annual meteor shower. This was unexpected since Phaethon was classified as an asteroid, not a comet.

In 2009, NASA’s Solar Terrestrial Relations Observatory (STEREO) observed a short tail emanating from Phaethon as it approached its closest point to the sun, or “perihelion,” along its orbit of 524 days. Regular telescopes were unable to detect the tail since it only becomes visible when Phaethon is too close to the sun to be observed except by solar observatories.

This two-hour sequence of images from the Solar and Heliospheric Observatory (SOHO) shows Phaethon (circled) moving relative to background stars. The images were taken on May 15, 2022, when the 3.4-mile-wide asteroid ventured close to the Sun, at a distance of 13 million miles. While SOHO routinely observes the Sun, it also observes many objects that pass near the Sun, including comets and asteroids. The random white specks are energetic particles, or cosmic rays, that constantly bombard the SOHO camera. Credit: ESA/NASA/USNRL/Karl Battams

The Solar Terrestrial Relations Observatory (STEREO) had observed Phaethon’s tail in 2009, 2012, and 2016 as it approached the sun. This discovery had led to the idea that dust escaping from the asteroid’s surface was responsible for the comet-like behavior. However, observations from NASA’s Parker Solar Probe in 2018 showed that the amount of material in the Geminid debris trail was too much for Phaethon to have shed.

To investigate further, Qicheng Zhang’s team searched for Phaethon’s tail during its latest perihelion in 2022 using the Solar and Heliospheric Observatory (SOHO) spacecraft. They also analyzed archival images from SOHO and STEREO, finding the tail during 18 of Phaethon’s close solar approaches between 1997 and 2022.

Zhang found that the tail was bright in the sodium filter on SOHO’s color filters but did not appear in the dust filter. Additionally, the shape of the tail and the way it brightened as Phaethon passed the sun matched the behavior expected if the tail were made of sodium and not dust. A previous study had suggested that the sun’s heat during Phaethon’s close solar approaches could vaporize sodium within the asteroid and cause comet-like activity. The latest findings appear to support this idea.

The Large Angle and Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO) imaged asteroid Phaethon through different filters as the asteroid passed near the Sun in May 2022. On the left, the sodium-sensitive orange filter shows the asteroid with a surrounding cloud and small tail, suggesting that sodium atoms from the asteroid’s surface are glowing in response to sunlight. On the right, the dust-sensitive blue filter shows no sign of Phaethon, indicating that the asteroid is not producing any detectable dust. Credit: ESA/NASA/Qicheng Zhang

New evidence from a study using data from NASA’s Solar and Heliospheric Observatory (SOHO) and Solar Terrestrial Relations Observatory (STEREO) has revealed that the tail of asteroid 3200 Phaethon is not made of dust, as previously thought, but rather sodium gas. The findings have overturned 14 years of assumptions about the asteroid’s composition and behavior. The researchers also speculate that some comets discovered by SOHO may actually be rocky asteroids like Phaethon, heated up by the sun.

However, the mystery of how Phaethon supplies the material for the annual Geminid meteor shower remains. The study’s authors believe that a disruptive event, possibly a piece of the asteroid breaking apart a few thousand years ago, caused Phaethon to eject the billion tons of material that make up the debris stream. The upcoming Japan Aerospace Exploration Agency (JAXA) mission called DESTINY+ is expected to fly past Phaethon later this decade and may provide more answers about the enigmatic asteroid.

Source: NASA’s Goddard Space Flight Center

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