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Asteroid Phaethon and its comet-like tail reveal new secrets

A new study has shed light on the long-standing mysteries surrounding Asteroid Phaethon, the source of the annual Geminid meteor shower. Researchers from the University of Helsinki have made significant strides in understanding the composition of this enigmatic celestial body, providing answers to questions that have puzzled scientists for years.

Phaethon’s comet-like tail

Asteroid Phaethon, measuring five kilometers in diameter, has intrigued researchers with its unusual comet-like tail, visible when it orbits closest to the Sun.

This phenomenon has been hard to explain, as comet tails are typically formed by vaporizing ice and carbon dioxide. Oddly, these elements not found in sufficient quantities on Phaethon. The asteroid’s tail, according to prevailing theories, should have been visible much farther from the Sun, at distances comparable to Jupiter’s orbit.

Phaethon is known for causing the Geminid meteor shower, a spectacular cosmic event observable from Earth, including in Finland’s skies every mid-December. This shower occurs when Earth crosses the path of Phaethon, and its surface gravel and dust, detached from the asteroid, burn up in our atmosphere.

Unveiling Phaethon’s secrets

The key to understanding Phaethon’s composition lay in the infrared spectrum of the asteroid, previously measured by NASA’s Spitzer Space Telescope. The University of Helsinki team re-analyzed this data, comparing it with the infrared spectra of meteorites studied in laboratories.

The findings were revolutionary, revealing that Phaethon’s spectrum matches that of CY carbonaceous chondrite meteorites, a rare type with only six known specimens.

The CY carbonaceous chondrites are unique among meteorites. They show signs of drying and thermal decomposition, indicative of recent heating, unlike other meteorite types that share a common origin from the early Solar System. These meteorites are characterized by high iron sulfide content, setting them apart from other groups.

Phaethon’s mineral composition

Phaethon’s surface, when near the Sun, heats up to around 800°C. The minerals found on Phaethon — olivine, carbonates, iron sulfides, and oxides — align closely with those in CY meteorites. Particularly, the presence of iron sulfide and carbonates suggests a unique composition altered by water content changes and extreme temperatures.

The study revealed that the high temperatures near the Sun lead to thermal decomposition of Phaethon’s minerals. This process releases gases like carbon dioxide, water vapor, and sulfur gas. This gas expulsion may explain the formation of Phaethon’s tail.

Formation of the Geminid meteors

An intriguing question was whether the gases released near the Sun could lift dust and gravel off Phaethon’s surface, contributing to the Geminid meteor shower. The study’s thermal models, combined with experimental data, suggest that gas release from the mineral structure could indeed cause surface rock to break down and eject dust particles into space.

In summary, the University of Helsinki’s research, led by postdoctoral researcher Eric MacLennan and associate professor Mikael Granvik, has provided a comprehensive explanation for Phaethon’s unique characteristics. Their findings not only explain the asteroid’s behavior near the Sun but also contribute significantly to our understanding of the Geminid meteor shower, one of the most awaited astronomical events of the year.

The full study was published in the journal Nature Astronomy.

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