Thanks to the Atacama Large Millimeter/Submillimeter Array (ALMA), scientists have been provided with a remarkably detailed picture of the distributions of cometary molecules located within the cloudy atmospheres of two bright comets whizzing through the sky. After generating impressive 3D images with the observations, scientists have managed to unearth important clues about how and where comets form new molecules. It is hoped that this information could provide us with novel insights into the birth of our planet. The team has published their work in The Astrophysical Journal.

Comets, or dirty snowballs as they’re sometimes referred to, are icy objects composed of frozen gas, rock and dust. All comets possess a small frozen region called a nucleus that’s sometimes just a few kilometers in diameter. As comets approach the Sun, they start to heat up and form a ghost-like atmosphere called a coma. It is known that these cosmic snowballs are remnants from the birth of our solar system around 4.6 billion years ago. Consequently, understanding their chemistry may provide us with clues about its formation.

To find out more about the distribution and formation of various chemicals in comae, an international team of researchers focused on two comets: ISON and Lemmon. ISON was observed for three days when it was 75 million kms from the Sun, whereas Lemmon was 224 million kms away when observations took place over two consecutive days. The impressive resolution of ALMA allowed the researchers to track the content of the comae on an hourly basis.  

To generate 3D images from the observations, the scientists married high-resolution, 2D images with high-resolution spectra from 3 molecules- hydrogen cyanide (HCN), hydrogen isocyanide (HNC) and formaldehyde (H₂CO). The researchers were then able to precisely map the locations of these molecules and measure their velocities, providing them with information on the depths of the atmospheres. Here's the 3D map of HCN in the inner coma of Lemmon if you're interested:

Credit: Brian Kent (NRAO/AUI/NSF)

The team discovered that while HCN originated from the nucleus and displayed a uniform, spherical outflow, HNC was distributed in a clumpy, asymmetrical manner. Furthermore, their results suggested that H­₂CO and HNC were produced within the coma from unidentified precursors.

These results are important because Earth-based telescopes struggle to obtain sufficient detail of the molecules within comae when they are present in low abundance, as is the case with Lemmon and ISON. Furthermore, previous studies have only looked at brighter comets such as Hale-Bopp, whereas these results demonstrate that ALMA can be used to study dimmer comets and perhaps those that are further away. According to study co-author Stefanie Milam, the findings also indicate that researchers may be able to map more complex molecules that have so far evaded detection.