A research team led by the Shanghai Astronomical Observatory of the Chinese Academy of Sciences has used the Tianma Radio Telescope to conduct multi-band radio observations of the returning comet 12P/Pons-Brooks (hereafter referred to as '12P'). They measured its water production rate during outburst activities and made the most distant detection of ammonia molecules in a Halley-type comet to date. 

The findings were published in Astronomy & Astrophysics.

Comets contain a rich variety of icy components dating back to the formation of the solar system. As they approach the Sun, these components sublimate due to solar heating, driving diverse cometary activity. The presence of these volatile ices indicates that comets have not undergone significant thermal evolution since their formation. Studying the composition of ices in comets provides crucial insights into the thermal and chemical conditions of the primordial solar system 4.6 billion years ago.

12P is a Halley-type comet with an orbital period of approximately 71 years. Since its discovery in 1812, it has exhibited multiple outbursts during each return—a distinctive characteristic whose cause remains unknown. During its 2024 return, 12P again experienced frequent outbursts, manifesting as short-term surges in overall brightness. As outbursts often involve the release of more gas from the nucleus, the outburst period of 12P provides an excellent opportunity to monitor its gas composition and changes, and to analyze the mechanisms and material sources of the outbursts.

From late 2023 to early 2024, the researchers conducted a series of L-band and K-band radio observations using the Tianma Telescope. In the L-band, they detected the 18-cm hydroxyl (OH) spectral line of 12P, which is a photodissociation product of water vapor. Using radiative transfer modeling, they determined the water production rate and gas expansion velocity of 12P before and after several outbursts. 

Combining these results with previous research, they characterized the short- and long-term activity changes of 12P influenced by outbursts. At a heliocentric distance of 1 AU, 12P can release over 5 tons of water vapor per second, a level exceeding that of most short-period comets and some long-period comets, clearly demonstrating 12P's high activity. During outbursts, 12P's activity (using water as an example) can approximately double.

K-band observations detected ammonia molecules (NH₃) in a Halley-type comet for the first time at radio wavelengths with 3σ confidence, marking the most distant radio-wave ammonia detection in a comet. The researchers measured the NH₃ production rate and relative abundance during one of 12P's outburst periods and found that the relative abundance of NH₃ is at a high level among comets. NH₃ has a relatively low sublimation temperature. 

For short-period comets like 12P, where more volatile substances such as CO and CO₂ may have been depleted, the high abundance of NH₃ and its distribution within the nucleus could explain 12P's frequent outbursts.

This study reveals the evolution of material release during 12P's outbursts and provides new observational evidence for understanding the mechanisms of cometary activity and their internal composition.

This work was supported by the National Natural Science Foundation of China, the National Natural Science Foundation of China, and the Ministry of Science and Technology of China.

Figure 1. Average 18 cm OH lines of 12P at different epochs (scaled to 1667 MHz) (Image by LI Juncen)

Figure 2. (a) Variation of OH and water production rates for 12P with heliocentric distance; (b) The relationship between NH₃ abundance and heliocentric distance for different comets. (Image by LI Juncen)