ALMA maps organic molecules in the ‘coma’ of comet Wirtanen
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ALMA maps organic molecules in the ‘coma’ of comet Wirtanen

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A bright comet passed by the Earth in December, and ALMA has studied it in detail.

On December 16, comet Wirtanen came as close as 11.4 million kilometers. That’s about thirty times as far as the moon – pretty close for a comet. Around that time, the comet was visible as a hazy patch of light in the sky.

Comets are small bodies that traverse the solar system. In general, they are just a few kilometers across. They consist of ice and dirt. As a comet comes close to the sun, part of the ice evaporates. As a result, the comet’s icy nucleus becomes surrounded by a large cloud of gas and dust particles. This cloud is called the coma (the ‘head’) of the comet.

Two weeks before its closest approach, on December 2, ALMA was trained at the inner parts of the coma of comet Wirtanen. ALMA detected the millimeter radiation emitted by hydrogen cyanide molecules (HCN). Most of the hydrogen cyanide gas was found close to the comet’s nucleus, as can be seen in the accompanying image. Surrounding this small, central cloud is a larger, less symmetrical area that contains less HCN gas.

At the time of the ALMA observations, comet Wirtanen was still 16.5 million kilometers away. On December 9, when the comet’s distance had shortened to 13.6 million kilometers, new ALMA osbervations were carried out, to study other molecules.

Hydrogen cyanide consists of one atom of hydrogen (H), one atom of carbon (C) and one atom of nitrogen (N). It is a simple ‘organic’ molecule – one of the building blocks of life. The fact that hydrogen cyanide is so prevalent in the coma of comet Wirtanen implies that comets may have brought the earliest building blocks of life to Earth, billions of years ago.


Comet Wirtanen (officially known as 46P/Wirtanen) was discovered in 1948 by American astronomer Carl Wirtanen. Most comets are in very elongated orbits, and they visit the sun at most once during a human lifetime. But comet Wirtanen has an orbital period of just 5.5 years. It has been studied during many earlier apparitions, but never before by ALMA. The comet was the original target of the European spacecraft Rosetta, but due to delays in the project, Rosetta was eventually sent to comet 67P/Churyumov-Gerasimenko. The nucleus of comet Wirtanen is probably just 1.4 kilometers across.


The ALMA observations of comet Wirtanen were carried out by a team of astronomers led by Martin Cordiner. Martin is an astrochemist at the Catholic University of America in Washington D.C. and at NASA's Goddard Space Flight Center in Greenbelt, Maryland. He has also used ALMA to study comet C/2012 S1 (ISON), as well as the atmosphere of the large Saturnian satellite Titan. The ALMA observations of comet Wirtanen have not yet been published in a scientific journal.