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Comets' secrets are unlocked by the power of ALMA

Paul Sutherland, Feature writer
Aug 13, 2014, 15:05 UTC

Sen—While Europe’s Rosetta probe begins to collect unique data about its target comet, 67P/Churyumov-Gerasimenko, astronomers are showing that they can make important discoveries about these cosmic vagabonds from Earth too.

Remarkable work using the world’s highest observatory, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, is helping unlock the secrets of how comets produce new chemicals.

The array of dishes, at an altitude of over 5000 metres, was turned to examine the comas, or ghostly atmospheres, surrounding the heads of two comets, the infamous ISON and another named Lemmon.

From the data collected, an international team of scientists was able to produce 3D images of the comets’ comas which gave them new insight into how they develop. 

Comets are made up of some of the oldest material in the Solar System, left in a fairly pristine state from their time spent in the freezer cabinet that is the depths of space. They contain organic compounds which intrigue scientists, not least because many believe that comets may have helped seed our planet with the ingredients for life.

ALMA made high-resolution observations that showed how the molecules in the comas of ISON and Lemmon were distributed as their heads, or nuclei, were being warmed by the Sun. The 3D structure was mapped by combining high-resolution two-dimensional images of the comets with high-resolution spectra produced from three important organic molecules.


The emission from organic molecules in the atmosphere of Comet ISON, left, and Comet Lemmon, right, as observed with ALMA. Image credit: B. Saxton (NRAO/AUI/NSF); M. Cordiner, NASA, et al.

These molecules—hydrogen cyanide (HCN), hydrogen isocyanide (HNC), and formaldehyde (H2CO)—were examined in each image, allowing the team not only to identify which were present but to measure their velocities too. This measurement gave the scientists their third dimension, and told them how deep the comas were.

Martin Cordiner, an astrobiologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, led the research team. He said: “We achieved truly first-of-a-kind mapping of important molecules that help us understand the nature of comets.”

The ALMA observations showed that HCN gas flows outwards from the nucleus in an evenly manner in all directions, whereas HNC is concentrated in clumps and jets. The telescope’s exquisite resolution was able clearly to resolve these clumps moving into different regions of the comets’ comas over timescales of days or even hours. The results confirm that the HNC and H2CO molecules form within the coma and suggest that HNC may be produced by the breakdown of large molecules or organic dust.

Michael Mumma, Director of the Goddard Center for Astrobiology and a co-author on the study, said in a statement: “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. These observations open a new window on this poorly known component of cometary organics.”

The team’s results are published this week in the Astrophysical Journal Letters.

This rotating 3D 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. Credit: Visualization by Brian Kent (NRAO/AUI/NSF)