Sodium sparkling on asteroid Phaeton

It was in 1983 that NASA’s IRAS satellite spotted (3200) Phaeton, an asteroid that grazes Earth’s orbit based on its path at only eight times the Earth-Moon distance. It is also he, and the dust he has lost in space, that are at the core of the Geminids meteor shower. It happens every year in mid-December. Phaeton has long attracted the attention of astronomers, particularly because its brightness increases near the Sun and then it presents an aspect resembling a comet.

Phaeton is a strange asteroid

It is its trajectory that brings it closer to our star that furthermore gave Phaeton his name: he is the son of Helios, an incarnation of the Greek god the Sun. (3200) Phaeton resembles a large cannon ball 5.8 km in diameter. One of its poles is marked by a deep depression that extends to the equator. The star is composed mainly of rocks and not ice like a comet, and this particle is enveloped in a cloud of dust. Upon reaching Earth’s atmosphere, these are consumed and form the shooting stars of the Geminids. Researchers from Caltech and NASA’s Jet Propulsion Laboratory wanted to understand why the asteroid falls on this cometary aspect near the Sun. Without the snow being sublimated it should not begin to shine unless some other element plays the role of this snow.

A trace emerges thanks to the Geminids’ observation. Before disintegrating, fragments of asteroids heat the air around them, which then reaches temperatures of several thousand degrees. A phenomenon that produces light whose color is representative of the elements taken from the asteroid. For example, sodium creates an orange color. But geminides have low sodium content. Until now, scientists have assumed that these tiny pieces of rock lose their sodium after leaving the asteroid. This new study published in planetary science magazineTurns out that sodium may actually play an important role in their removal.

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These radar images of asteroid 3200 Phaethon were generated by astronomers at Arecibo Observatory on December 17, 2017. Credits: Arecibo / NASA / NSF Observatory.

sodium that shines

The authors suggest that as (3200) Phaeton approaches the Sun, its sodium heats up and sublimation occurs. This process would have long erased any traces of sodium on the surface, but not the ones contained in the star’s rocks. Near our star, once transformed into gas, it jets through cracks in the outer crust powerful enough to carry rock debris with it. The phenomenon isn’t very violent: the star’s low gravity doesn’t necessarily provide much energy to tear off smaller parts. Furthermore, the researchers believe that it is not the explosion that allows the sodium to escape, but rather a kind of constant fizzing. This glowing sodium could explain not only the asteroid’s gain in comet-like brightness, but also how the meteorites that make up the Geminids would have been ejected from the asteroid and why they contain little sodium.

To find out whether the sodium on phaeton actually turns into vapor, experiments were carried out on samples from the Allende meteorite, which fell on Mexico in 1969, at a JPL laboratory. The meteorite may have come from an asteroid smaller than Phaeton and belongs to a class of stars that formed in the early days of the Solar System. The samples were heated to the highest temperatures the asteroid would experience as it approached the Sun and for 3 hours, a period that covered a period of one day on Phaeton. Result: Sodium disappeared from the samples after heating. This study therefore supports this theory of sodium as a motor to activate the asteroid. According to the researchers, other asteroids may react in the same way as the Sun approaches, or even eject other elements from their rocks that are at higher temperatures, even at higher temperatures.

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