For a long time, researchers have been perplexed by the five-kilometer-diameter asteroid Phaethon. When the asteroid passes closest to the Sun during its orbit, a comet-like tail appears for a few days. However, comet tails are typically formed by vaporizing ice and carbon dioxide, which cannot account for this tail. At Jupiter’s current distance from the Sun, the tail should be visible.
The comet-like tail of the asteroid that causes the Geminid shooting star swarm has also perplexed researchers. The infrared spectrum of rare meteorites assisted in determining the asteroid’s composition.
For a long time, researchers have been perplexed by the five-kilometer-diameter asteroid Phaethon. When the asteroid passes closest to the Sun during its orbit, a comet-like tail appears for a few days. However, comet tails are typically formed by vaporizing ice and carbon dioxide, which cannot account for this tail. At Jupiter’s distance from the Sun, the tail should be visible.
When an asteroid’s surface layer breaks up, the detached gravel and dust continue to travel in the same orbit and give birth to a cluster of shooting stars when they collide with the Earth. The Geminid meteor shower, which appears in the skies of Finland every year around mid-December, is caused by Phaethon. At least according to the prevailing hypothesis because that’s when the Earth crosses the asteroid’s path.
Until now, theories about what happens on Phaethon’s surface near the Sun have remained purely hypothetical. What comes off the asteroid? How? The answer to the riddle was found by understanding the composition of Phaethon.
Sodium emission can explain the weak tail we observe near the Sun, and thermal decomposition can explain how dust and gravel are released from Phaethon. It was amazing to see how each of the discovered minerals seemed to fall into place and also explain the behavior of the asteroid.
Eric MacLennan
A rare meteorite group consisting of six known meteorites
In a recent study published in the journal Nature Astronomy by researchers from the University of Helsinki, the infrared spectrum of Phaethon previously measured by NASA’s Spitzer space telescope is re-analyzed and compared to infrared spectra of meteorites measured in laboratories.
The researchers discovered that Phaethon’s spectrum corresponds exactly to a specific type of meteorite, known as a CY carbonaceous chondrite. It is a very rare type of meteorite, with only six specimens known.
Asteroids can also be studied by retrieving samples from space, but meteorites can be studied without the need for costly space missions. Asteroids Ryugu and Bennu, the targets of recent JAXA and NASA sample-return missions, are CI and CM meteorites, respectively. All three types of meteorites originate from the birth of the Solar System and partially resemble each other, but only the CY group shows signs of drying and thermal decomposition due to recent heating.
All three groups show signs of a change that occurred during the early evolution of the Solar System, where water combines with other molecules to form phyllosilicate and carbonate minerals. However, CY-type meteorites differ from others due to their high iron sulfide content, which suggests their own origin.
Phaethon’s spectrum match the spectra of CY carbonaceous chondrites
Analysis of Phaethon’s infrared spectrum showed that the asteroid was composed of at least olivine, carbonates, iron sulfides, and oxide minerals. All of these minerals supported the connection to the CY meteorites, especially iron sulfide. The carbonates suggested changes in water content that fit the primitive composition, while the olivine is a product of thermal decomposition of phyllosilicates at extreme temperatures.
Thermal modeling was used in the study to show what temperatures exist on the asteroid’s surface and when certain minerals break down and release gases. When Phaethon gets close to the Sun, the temperature on its surface rises to around 800°C. This is well suited to the CY meteorite group. Carbonates emit carbon dioxide at similar temperatures, phyllosilicates emit water vapor, and sulfides emit sulfur gas.
According to the study, all of the minerals found on Phaethon appear to be minerals found in CY-type meteorites. The oxides portlandite and brucite were the only exceptions, as they were not found in the meteorites. However, when carbonates are heated and destroyed in the presence of water vapor, these minerals can form.
The tail and the meteor shower get an explanation
The composition and temperature of asteroids explained the formation of gas near the Sun, but do they also explain the dust and gravel that formed the Geminid meteors? Did the asteroid have enough pressure to lift dust and rock from its surface?
The researchers used experimental data from other studies in conjunction with their thermal models, and it was estimated that when the asteroid passes closest to the Sun, gas is released from the asteroid’s mineral structure, which can cause the rock to break down. Furthermore, the pressure created by carbon dioxide and water vapor is strong enough to lift small dust particles from the asteroid’s surface.
“Sodium emission can explain the weak tail we observe near the Sun, and thermal decomposition can explain how dust and gravel are released from Phaethon,” says the study’s lead author, University of Helsinki postdoctoral researcher Eric MacLennan.
“It was amazing to see how each of the discovered minerals seemed to fall into place and also explain the behavior of the asteroid,” says University of Helsinki associate professor Mikael Granvik.
