ALMA confirms comets forge organic molecules in their dusty atmospheres

An international team of scientists using the Atacama Large Millimeter/submillimeter Array (ALMA) has made incredible 3-D images of the ghostly atmospheres surrounding comets ISON and Lemmon. These new observations provide important insights into how and where comets forge new chemicals, including intriguing organic compounds.

Comets contain some of the oldest and most pristine materials in our solar system. Understanding their unique chemistry could reveal much about the birth of our planet and the origin of organic compounds that are the building blocks of life. ALMA’s high-resolution observations provided a tantalizing 3-D perspective of the distribution of the molecules within these two cometary atmospheres, or comas.

“We achieved truly first-of-a-kind mapping of important molecules that help us understand the nature of comets,” said Martin Cordiner from NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

The critical 3-D component of the ALMA observations was made by combining high-resolution 2-D images of the comets with high-resolution spectra obtained from three important organic molecules — hydrogen cyanide (HCN), hydrogen isocyanide (HNC), and formaldehyde (H₂CO). These spectra were taken at every point in each image. They identified not only the molecules present, but also their velocities, which provided the third dimension, indicating the depths of the cometary atmospheres.

The new results revealed that HCN gas flows outward from the nucleus quite evenly in all directions, whereas HNC is concentrated in clumps and jets. ALMA’s exquisite resolution could clearly resolve these clumps moving into different regions of the cometary comas on a day-to-day and even hour-to-hour basis. These distinctive patterns confirm that the HNC and H₂CO molecules actually form within the coma and provide new evidence that HNC may be produced by the breakdown of large molecules or organic dust.

“Understanding organic dust is important because such materials are more resistant to destruction during atmospheric entry, and some could have been delivered intact to the early Earth, thereby fueling the emergence of life,” said Michael Mumma from the Goddard Center for Astrobiology. “These observations open a new window on this poorly known component of cometary organics.”

“So, not only does ALMA let us identify individual molecules in the coma, it also gives us the ability to map their locations with great sensitivity,” said Anthony Remijan from the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia.

The observations were also significant because modest comets like Lemmon and ISON contain relatively low concentrations of these crucial molecules, making them difficult to probe in-depth with Earth-based telescopes. The few comprehensive studies of this kind so far have been conducted on extremely bright comets, such as Hale-Bopp. The present results extend them to comets of only moderate brightness.

Comet ISON was observed with ALMA on November 15–17, 2013, when it was only 46 million miles (75 million kilometers) from the Sun — about half the distance of Earth to the Sun. Comet Lemmon was observed June 1–2, 2013, when it was 139 million miles (224 million kilometers) from the Sun — about 1.5 times the distance of Earth to the Sun.

“The high sensitivity achieved in these studies paves the way for observations of perhaps hundreds of the dimmer or more distant comets,” said Stefanie Milam from Goddard Space Flight Center. “The findings suggest that it should also be possible to map more complex molecules that have so far eluded detection in comets.”

This rotating 3-D ALMA map shows how HCN molecules are released from the nucleus of comet Lemmon and then spread evenly throughout the atmosphere, or coma. Similar maps revealed that HNC and formaldehyde are produced in the coma, rather than originating from the comet’s nucleus. // Visualization by Brian Kent (NRAO/AUI/NSF)