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A weird asteroid just got a little weirder.
We’ve known for some time that asteroid 3200 Phaethon behaves like a comet. It glows and forms a tail when near the sun, and is the source of the annual Geminids meteor shower, although comets are responsible for most of the meteor showers. Scientists had attributed Phaethon’s comet-like behavior to dust spewing from the asteroid as it is scorched by the sun. However, a new study using two NASA solar observatories reveals that Phaethon’s tail isn’t dusty at all, but is actually made up of sodium gas.
“Our analysis shows that Phaethon’s comet-like activity cannot be explained by any type of dust,” said California Institute of Technology Ph.D. student Qicheng Zhang, who is the lead author of a paper published in Planetary science journal reporting the results.
Asteroids, which are mostly rocky, usually don’t form tails as they approach the sun. Comets, however, are a mix of ice and rock, and typically form tails as the sun vaporizes their ice, blasting material off their surfaces and leaving a trail along their orbits. When Earth passes through a trail of debris, those comet fragments burn up in our atmosphere and produce a swarm of shooting stars, a meteor shower.
After astronomers discovered Phaethon in 1983, they realized that the asteroid’s orbit matched that of the Geminid meteors. This pointed to Phaethon as the source of the annual meteor shower, even though Phaethon was an asteroid and not a comet.
In 2009, NASA’s Solar Terrestrial Relations Observatory (STEREO) spotted a short tail extending from Phaethon as the asteroid reached its closest point to the sun (or “perihelion”) along its 524-day orbit. Ordinary telescopes had never seen the tail because it only forms when Phaethon is too close to the sun to be observed, except with solar observatories.
STEREO also saw Phaethon’s tail develop on subsequent solar approaches in 2012 and 2016. The tail’s appearance supported the idea that dust was escaping from the asteroid’s surface when heated by the sun.
However, in 2018, another solar mission imaged part of the Geminid debris trail and found a surprise. Observations by NASA’s Parker Solar Probe showed that the trail contained much more material than Phaethon could possibly lose during its close approaches to the sun.
Zhang’s team wondered if something else besides dust was behind Phaeton’s comet-like behavior. “Comets often shine brightly with sodium emission when they are very close to the sun, so we suspected that sodium may also play a key role in Phaethon’s illumination,” Zhang said.
An earlier study, based on models and laboratory tests, suggested that the sun’s intense heat during Phaethon’s close solar approaches could actually vaporize the sodium within the asteroid and drive the comet-like activity.
Hoping to find out what the tail is really made of, Zhang searched for it again during Phaethon’s last perihelion in 2022. He used the Solar and Heliospheric Observatory (SOHO) spacecraft, a joint mission between NASA and the European Space Agency (ESA), which has color filters capable of detecting sodium and dust. Zhang’s team also searched archival images from STEREO and SOHO, finding the tail during 18 of Phaethon’s close solar approaches between 1997 and 2022.
In the SOHO observations, the asteroid’s tail appeared bright in the sodium-detecting filter, but did not appear in the dust-detecting filter. Additionally, the shape of the tail and the way it glowed as Phaethon passed in front of the sun matched exactly what scientists would expect if it were made of sodium, but not if it was made of dust.
This evidence indicates that Phaeton’s tail is made of sodium, not dust.
“Not only do we have a really interesting result that overturns 14 years of thinking about a well-examined object,” said team member Karl Battams of the Naval Research Laboratory, “but we also did it using data from two SOHO heliophysics spacecraft and STEREO that were not at all intended to study phenomena like this.”
Zhang and his colleagues now wonder whether some comets discovered by SOHO and citizen scientists studying SOHO images as part of the sungrazer project aren’t comets at all.
“Many of those other ‘comets’ that skirt the sun may not even be ‘comets’ in the usual icy body sense, but may instead be rocky asteroids like Phaethon warmed by the sun,” Zhang explained.
However, one important question remains: if Phaethon doesn’t release much dust, how does the asteroid provide the material for the Geminids meteor shower we see every December?
Zhang’s team suspects that some sort of disruptive event a few thousand years ago, perhaps a piece of the asteroid that broke apart under the stresses of Phaethon’s rotation, caused Phaethon to eject the billion tons of material estimated to form the Geminid debris flow. But what that event was remains a mystery.
Other answers may come from an upcoming Japan Aerospace Exploration Agency (JAXA) mission called DESTINY+ (short for Demonstration and Experiment of Space Technology for Interplanetary voyage Phaethon fLyby and dUst Science). By the end of this decade, the DESTINY+ spacecraft is expected to fly past Phaethon, photograph its rocky surface and study the dust that may exist around this enigmatic asteroid.
Qicheng Zhang et al, Sodium Brightening of (3200) Phaethon near Perielion, The planetary science journal (2023). DOI: 10.3847/PSJ/acc866