Asteroid Phaeton does not have a dust tail

Asteroid Phaeton does not have a dust tail

Asteroid (3200) Phaeton near the sun (illustration). © NASA/JPL-Caltech/IPAC

Unlike most meteor showers, the Geminids are not caused by a comet, but by the asteroid (3200) Phaeton. When it passes through the sun, it is said to give off the dust from which shooting stars are formed. But now observations of the asteroid reveal that while it does briefly form a tail near the Sun, the tail is not made of dust. Instead, the color and shape suggest that Phaeton’s tail is composed primarily of outgassed sodium. Such an emission of sodium has often been observed from comets close to the Sun, but never from an asteroid. In addition, this now raises the question of where the dust of the Geminid cloud comes from.

Every year in mid-December, the Geminid meteor shower reaches its peak. But what caused this cloud of tiny dust grains in the Earth’s orbit remained unclear for more than 150 years. Because unlike most falling stars, there was no comet whose dust tail these dust particles could have come from. In 1983, astronomers discovered a possible originator: 3200 Phaethon, an asteroid crossing the Earth’s orbit nearly six kilometers across. Its eccentric 524-day orbit takes it from outside the orbit of Mars to very close to the Sun, and aligns well with the location of the Geminid dust cloud. However, it remained unclear whether and how this asteroid was able to leave behind so much dust, because unlike the ice-rich comets, asteroids normally emit hardly any material into their surroundings, even when they are close to the sun.

Phaeton has a tail – but from what?

Because Phaeton is almost always close to the sun from our point of view, it is difficult to observe it closely through terrestrial telescopes. In 2009, however, NASA’s space-based solar observatory STEREO managed to catch a glimpse of Phaeton as it passed the closest point in its orbit to the sun. In the resulting images, astronomers detected for the first time a striking flare from the asteroid and a short tail. “This was attributed to the emission of dust grains from Phaeton’s surface,” explain Qicheng Zhang of the California Institute of Technology in Pasadena and his colleagues. “This indicated that the asteroid was indeed losing material, albeit too little to sustain the Geminid meteor shower in the long term.” Further observations in 2012 and 2016 confirmed that Phaeton forms such a tail with each solar passage. However, what this tail consists of and how much material is in it remained unclear.

That’s why Zhang and his team have now targeted the asteroid again. For their study, they evaluated archive data from 18 sun passages from 3200 Phaetons from the period 1997 to 2022. They also used the LASCO camera at NASA and ESA’s Solar and Heliospheric Observatory (SOHO) to study Phaeton and its tail more closely at its perihelion on May 15, 2022. The LASCO coronagraph has special color filters through which an object can be imaged in different wavelength ranges. The color of light emitted by the tail can provide first indications of its chemical composition.

Sodium instead of dust

The observation with the LASCO filters showed that the tail of the asteroid does not shine equally brightly in all colors. “The activity of Phaeton appears much brighter in the orange-filtered LASCO images than in unfiltered filters or filters adapted to the wavelengths of the hydrogen,” report Zhang and his team. In blue-filtered images, the asteroid’s tail could not be seen at all. The astronomers see the striking orange glow as an indication that the tail is primarily gaseous sodium. “Even comets close to the sun often glow intensely due to sodium emission,” says Zhang. The shape of the tail provided another indication: Its curvature and the timing of the formation of the tail match sodium, but not the classic material of a dust tail, as the astronomers report. In their view, everything suggests that the asteroid Phaeton near the sun emits sodium into space, but hardly any dust. “Our analyzes show that Phaeton’s comet-like activity cannot be explained by any type of dust,” says Zhang.

However, these results and conclusions raise several questions. For one thing, such a sodium tail has never been observed from an asteroid before. According to calculations by Zhang and his team, the asteroid loses at least 550 trillion (5.5 x 1023) sodium atoms per orbit. Based on the typical composition of a chondritic asteroid, Phaeton would have to lose around one million kilograms of rock material into space each time. However, because a dust tail is missing, it is still unclear where this material is and how it is composed. In addition, even the amounts calculated by Zhang and his team are insufficient to generate or fill up the Geminid dust cloud. The astronomers suspect that this cloud of dust is not fed by the asteroid’s normal particle emission, but dates back to a one-time event, possibly a few thousand years ago. At that time, a piece of the asteroid could have broken off and disintegrated, throwing an unusual amount of dust and rocks into space – but that’s just speculation for now. The astronomers are hoping for more clarity from the Japanese space probe Destiny+, which will be launched in 2024 to the asteroid Phaeton and examine its surface and emissions.

Source: Qicheng Zhang (California Institute of Technology, Pasadena) et al., The Planetary Science Journal, doi: 10.3847/PSJ/acc866

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