Is carbon monoxide accumulating underground in Arox? Evidence of the existence of very primitive celestial bodies | sorae website portal to the universe

Outside the solar systemTranssolar system object」 (※1)It is believed that it still retains the volatiles (components that evaporate easily even at low temperatures) that it took up during its formation. However, until now, the details of how volatiles are retained and how they are gradually lost are not well understood.

A research team led by Samuel B. D. Birch of Brown University and Orkan M. O'Murhan of the SETI Institute discovered asteroid 486958, which was closely explored by NASA's (National Aeronautics and Space Administration) Pluto probe New Horizons.ArukuWe conducted a study that modeled the internal structure based on observational data. Results,In small orbs like Arrokoth, vaporized carbon monoxide may remain deep underground, preventing further volatilization.It turns out that. This is it,Creatures like Arrokoth are very primitive and retain materials that can easily be lost.It shows the possibilities.(※2)(※3)

■ The most primitive celestial bodies lie outside the solar system

The Sun and the celestial bodies surrounding it are said to have been born about 4.6 billion years ago. A highly active celestial body like Earth has gone through various processes of change since its birth, and almost no original information remains about it. In contrast, small celestial bodies such as asteroids and comets are estimated to have undergone minimal changes over billions of years. However, when we analyze material from the Itokawa and Ryugu asteroids, from which we have successfully collected samples, we find some traces of change.

So, are there any primitive orbs that have undergone less changes than Itokawa and Ryugu? For example, a planet orbiting the Sun is farther away than Neptune's orbitTranssolar system object“A strong candidate. Because solar system objects have orbited so far away from the Sun from their birth to the present day, they are not thought to have undergone significant changes such as heat.

On rare occasions, some trans-Neptunian objects may change their orbits significantly and pass close to the Sun. The volatiles then evaporate to form a temporary atmosphere or tail. This is a celestial body called a comet. Comets can be studied in detail, but as a result of their proximity to the Sun for such a long time, it is estimated that they have emitted a certain amount of volatiles and lost their primordial materials.

One reason it is assumed that comets are not truly primitive is the amount of carbon monoxide they emit.He is. More primitive orbs contain a large amount of carbon monoxide as their main solid component, so if such an orb became a comet, the resulting atmosphere would likely contain more carbon monoxide than carbon dioxide. On the other hand, carbon monoxide evaporates more easily than carbon dioxide, and is thought to evaporate very quickly.

In fact, the amount of carbon monoxide in a comet's atmosphere is very small compared to carbon dioxide. It is estimated that a small amount of carbon monoxide is present in tiny gaps in the ice, where it is difficult to evaporate, and comes out little by little.

■Contrary to expectations, no carbon monoxide was detected in the Arocs

If the trans-Neptunian object is a very primitive object, it likely contains a large amount of solid carbon monoxide. Although most of the carbon monoxide is retained, it gradually evaporates over billions of years. Therefore, we should be able to observe a small amount of carbon monoxide in the atmosphere and its flux from objects crossing the solar system.

This type of observation has not been possible until now because solar system objects are physically located at the outer edge of the solar system. So far, the only observational record is that the asteroid was spotted by NASA's Pluto probe, New Horizons.Aruku“It is a close observation. After New Horizons completed its first-ever close exploration of Pluto in 2015, it conducted a close exploration of Arrokoth on January 1, 2019.

Due to its small size, Arrokoth is estimated to be a primitive trans-solar system object that has undergone almost no change since its formation. For this reason, it was hoped that carbon monoxide flowing from Arrokoth would be detected during the rare observing opportunity provided by New Horizons' close exploration. But in factNo carbon monoxide was found in Arukus monitoring dataThis was an unexpected discovery.

If we apply this result simply, Arrokoth may not in fact be primitive at all, but rather a celestial body that has undergone significant changes. However, Arrokoth's physical appearance and observations of its surface and orbit indicate that Arrokoth has likely maintained its current orbit for a long time, with almost no changes.

■Building a model that proves the primitiveness that the Aruku could not discover

Birch and O'Morhan's research team modeled the internal structure of small trans-Neptunian objects like Arrokoth to investigate why carbon monoxide was not detected. These small orbs are thought to consist of small, loosely bound rock particles that form a porous structure with many gaps, similar to a sponge. The researchers analyzed the behavior of carbon monoxide evaporating into gas in a porous celestial body containing solid carbon monoxide.

The results showed that while carbon monoxide escapes from areas near the surface, it gradually accumulates in gaps in the porous structure deep inside the Earth, and not much of it escapes into space. It is as if an “underground atmosphere” is forming inside the orb. In such places, further evaporation of carbon monoxide is suppressed. Carbon monoxide, which remains unchanged deep underground, rarely escapes into space.

As far as this model is concerned, Arrokoth has exhausted its carbon monoxide near the surface after a sufficient period of time has passed since its birth.Carbon monoxide remains deep underground and cannot escape.it will be. This may be the reason why New Horizons' careful observations failed to detect carbon monoxide on Arrokoth. In that case, Arrokoth is a truly primitive celestial body, and it is possible that it has retained significant amounts of volatiles, not just carbon monoxide, from the time of its formation.

In order to validate this model, it is necessary to observe multiple celestial bodies with properties similar to Arrokoth. The recently launched James Webb Space Telescope has the ability to monitor the flux of carbon monoxide and carbon dioxide from outer solar system bodies without sending a probe to distant celestial bodies. We may soon discover whether orbs like Arrokoth are truly primitive.

■ Notes

*1… The original paper refers to objects like Arrokoth as “Kuiper Belt Objects (KBO),” but there is controversy over this name. In this article, we will use the less controversial term “solar system object.”

*2…As in this study, the phase transformation of volatiles in a near-vacuum environment is a direct change from solid to gas, and from gas to solid. The transition from the solid state to the gaseous state is called “sublimation,” and the transition from the gaseous state to the solid state is called “sublimation.” Strictly speaking, this is the correct way to express it, but in this article, I will give priority to ease of understanding.It is not used.

*3…In the interior of the orb shown in this paper, the evaporation (sublimation) and solidification (aggregation) of carbon monoxide occur at approximately the same speed, so that the evaporation of carbon monoxide appears to be suppressed. The situation is as follows. Originally, it would be correct to express it in terms of this “equilibrium state”, but this article prioritizes ease of understanding and does not use it.

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Written by Riri Aya