Why does a comet have a green head but not a green tail?

The evaporating head of a comet approaching the Sun often turns bright green, but the comet’s tail, oddly enough, is never colored by it. They are white and blue. Scientists now know why: the green particles on the head are broken by light particles from the sun before they reach the tail. Scientists from Sydney published results Last week in the magazine PNAS. They thus confirm the theory proposed nearly a century ago by the German physicist Gerhard Herzberg.

Comets are balls of ice, rock, and dust. It is a remnant of the formation of the solar system and is usually about 4.6 billion years old. The size of a comet ranges from a few kilometers to tens of kilometers. The solar system is surrounded by two regions filled with comets: the Kuiper belt and the Oort cloud. The Kuiper Belt is a region, a kind of cake around the solar system, and it begins behind the orbit of the planet Neptune. The Oort cloud is an envelope around the solar system at a distance of about ten thousand times the distance from the Earth to the Sun. Every now and then, a comet from one of those regions will invade the solar system and then make an elliptical orbit around the sun. There are now thousands of comets in the solar system. Sometimes it can be seen from the ground.

Dust tail and gas tail

As a comet approaches the sun, it heats up and releases gas and dust. This results in two tails, one of dust particles and a gas tail of electrically charged particles. Their tails can reach millions of miles in length. When the head, the ice ball itself, approaches the sun, it can turn bright green.

diatomic carbon molecule (C₂) causes this green color. These two carbon atoms are stuck together. On Earth, this is a rare molecule. Here it sometimes happens, for example, in fire and this gives a green color, but it reacts almost instantly with other particles in the air. The molecule can continue to exist in the vacuum of space. On the comet’s head, diatomic carbon is formed from interactions between the Sun and the organic molecules on the head. Diatomic carbon surrounds the head like a green cloud. The dust tail of the comet is white-yellow due to the reflection of sunlight. Near the head there is still a small amount of green diatomic carbon in the gas tail, but other than that there is mainly bluish carbon monoxide. The question of why green diatomic carbon can always be seen only in the head, and never increased in the gas tail, has puzzled astronomers for nearly a century.

There was already a suspicion. In 1930, Herzberg came up with the idea that light particles emitted by the Sun rapidly break green diatomic carbon into two separate carbon atoms, even before it reaches the gas tail. However, his theory is difficult to prove in the laboratory, because diatomic carbon is unstable on Earth.

laser on gas

Now, for the first time, it is possible to simulate the cracking of diatomic carbon with particles of light in the laboratory. Scientists led by the University of New South Wales in Australia first released chlorine away from a large molecule, tetrachloroethylene (C) using a laser.₂Cl₄). This left them with a gas of diatomic carbon. Then they quickly pumped this gas into a two-meter vacuum chamber. Then they directed ultraviolet lasers at the gas to see how many particles of light would be needed to destroy a given amount of diatomic carbon into two separate carbon atoms. In this way, the scientists calculated that the particles of light from the Sun were already enough to destroy the diatomic carbon of the comet’s head before it reached the tail.

“The work was done cleverly,” says Ignas Snellen, an astronomer at Leiden University. “The fact that it took nearly a century to recreate the conditions around a comet in the lab shows how difficult that can be.”

Now, through early January, Comet Leonard is close enough to be viewed from Earth with binoculars or telescopes. The comet is about one kilometer wide and traveling through the solar system at a speed of about 47 kilometers per second. The week before Christmas, the closest was about 35 million kilometers away. Sneelen: “Leonard has also been seen with such a bright green in his head.”