The tails of comets are never green and we finally know why

Almost 4,000 known comets and probably a lot more. And not one with a green tail! The mystery has long intrigued astronomers. Today, a team experimentally confirms a theory proposed in the 1930s on this subject. It’s all about diatomic carbon.

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Where do they come from? How many are there? Their name comes from the Greek komêtês meaning « hairy ». In Japan, they are nicknamed “broom stars”. So many names that refer to their physiognomies very different from the stars that shine in the sky. Get to know them on video!

For our ancestors, they announced catastrophes. Or at least big changes. Then scientists taught us that, although their destructive potential remains proven – but they are also suspected of having brought to Earth the ingredients of life – comets have nothing supernatural. They find their source, for some, those of short period, in the kuiper belt – a region located beyondorbit from Neptune -, for others, in the Oort cloud, much further still.

The astronomers who have observed them for centuries have noted that these clumps of ice, rocks and dust tend to turn a green color that becomes lighter as they approach our Soleil. But may that green never extend to their tails.

In the 1930s, a physicist German-Canadian, Gerhard Herzberg (1904-1999) – he won the Nobel prize from chemistry in 1971 for his work on the electronic structure and geometry of molecules – suggested an explanation. The light of the Sun would destroy the carbone diatomique (C2) precisely resulting from the interaction between this same light and the organic matter contained in the heads of comets. But the instability of C2 had hitherto prevented this theory from being tested. Until then because of researchers from the University of New South Wales (Australia) just found a way to test this chemical reaction in laboratory.

Diatomic carbon in the head, but not when the tails

Before going into the details of their work, it should be noted that dicarbon only exists in extremely energetic or low-energy environments. oxygen. A star, the interstellar medium or … a comet. But not as long as the latter is far from our Sun. This is his heat which allows to break in C2 the organic material – the kind of molecules that are the ingredients of life – present on the icy nucleus of the comet. The dicarbon then moves towards the coma, that layer of gas and dust that surrounds the nucleus. A coma which then turns green.

What researchers at the University of New South Wales have just proven is that the radiation ultraviolet (UV) from the Sun tends to break the bonds between atoms of carbon that form C2. They evoke a process of photodissociation which destroys the diatomic carbon before it has time to approach the comet tails.

How did they do it? They first had to make C2 which, you imagine, « Not in store ». Researchers obtained it from a larger molecule, perchlorethylene (C2Cl4). Using a high-powered ultraviolet laser, they literally detonated the atoms of chlorine (Cl). Then the remaining diatomic carbon was sent to a vacuum chamber. And other lasers made it possible, one to simulate the sun’s UV radiation, the other to observe what was happening.

However, it was only after nine months of effort that the researchers finally observed the dissociation of C2. They therefore now experimentally confirm the theory proposed by Herzberg almost a century ago. This will allow them in the future to better understand, in particular how much organic matter evaporates from comets.

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