Comet 12P/Pons-Brooks: The Importance of Halley-Type Comets to Understanding the Early Solar System

Comets are remnants from the early solar system and retain volatiles from the time and place of their formation in the outer protoplanetary disk. Gravitational interactions with the young giant planets scattered many of them into one of two main dynamical reservoirs where they have been preserved since their formation: the Kuiper Belt and the Oort Cloud. Subsequent perturbations have modified some cometary orbits towards the inner solar system where they can be observed. Comets from the Kuiper Belt may be perturbed into short period (<20 year) orbits that are primarily affected by Jupiter’s gravity. These are known as Jupiter Family comets (JFCs). Oort cloud comets (OCCs) can be perturbed, primarily by galactic tides, into very long period (>200 year) orbits.  The long residences of cometary nuclei in the cold, outer solar system has allowed them to preserve the volatiles they formed with, giving them the potential to provide a window on the chemical and physical processes that occurred during planet formation.

 

A subset of OCCs have dynamically evolved into shorter (decades long) orbital periods. These Halley-type comets (HTCs) are the least well studied class because there are relatively few of them compared to other OCCs. In addition, their relatively longer orbital periods, compared to JFCs, results in a very small number of bright apparitions since the advent of sensitive high-resolution spectrographs. For these reasons, only two HTCs (1P/Halley and 8P/Tuttle) have had their parent volatile compositions well characterized to date, which is insufficient to make a meaningful compositional comparison with the other dynamical families. However, HTCs are of particular interest because they have the potential to help resolve long-standing questions in comet science.

 

This work focuses on HTC 12P/Pons-Brooks (hereafter 12P), which was discovered in 1812 and observed again in 1883-84 and 1953-54. Its 2024 apparition was particularly favorable for infrared daytime observations. We performed high-resolution (λ/Δλ~50,000), near-infrared spectroscopic measurements with iSHELL at the NASA Infrared Telescope Facility (IRTF) on eight dates between 2024 February 15 (pre-perihelion, R_h = 1.4 au) and 2024 May 1 (post-perihelion, R_h = 0.80 au). With these observations we address four key questions in comet science:

 

• Does comet volatile composition depend on the comet’s dynamical history? In particular, is the observed composition of comets with shorter periods affected by thermal processing during multiple close approaches to the Sun? Volatile compositions have been measured in both JFCs and OCCs and vary within each dynamical class. Of particular interest are hypervolatiles, which have been suggested to be depleted in JFCs relative to OCCs. One possible explanation is that multiple (and frequent) close perihelion passages cause preferential loss of more hypervolatile ices from the nuclei of JFCs. Another is that JFCs and OCCs may have formed in different (or not entirely overlapping) regions of the protoplanetary disk. One way to distinguish between these two scenarios is to study HTCs, which come from the Oort cloud, but have dynamically evolved to much shorter orbital periods than other OCCs.
• Does small heliocentric distance affect the composition measured in the coma of a comet? Past observations have suggested a systematic enrichment of H₂CO, NH₃, and C₂H₂ for comets observed within approximately 0.8-1 au from the Sun, thought to be caused by more complex (less volatile) species that disintegrate at small heliocentric distances once a thermal threshold is reached. 12P was observable between 1.4-0.78 au, enabling investigation of the heliocentric dependences of these key volatiles. 
• How is the composition of volatiles subliming from the comet’s surface connected to that of the interior? 12P underwent repeated outbursts during its two most recent apparitions and was observed to outburst repeatedly in 2024. Such outbursts have been observed in other comets (such as 67P/Churyumov-Gerasimenko) and are often attributed to the collapse of surface features, such as cliff walls, exposing fresh material to sublimation. Hence 12P offered the possibility to sample more pristine subsurface material that had not been subject to surface heating and processing.
• How do seasons affect observed compositions? Depending on nucleus size, shape, orientation, and rotational properties, different areas of the surface may be exposed to solar irradiation and sublimate as a comet orbits the Sun. The Rosetta mission demonstrated that the high obliquity (52⁰) of 67P led to variations in coma composition owing to both diurnal and seasonal effects. 12P was observed before, near, and after perihelion in search of coma compositional variations owing to seasonal changes on its large (33 km) nucleus, if such occur on this comet.

 

We report production rates (molecules s^-1), rotational temperatures, and relative abundances among measured volatiles (C₂H₆, CH₃OH, H₂CO, HCN, C₂H₂, NH₃, CO, OCS, and H₂O) in 12P. We will compare the composition of 12P to the few other HTCs observed to date (see also the DiSanti et al. and Saki et al. presentations at this meeting), as well as to comets of other dynamical types, and discuss implications for addressing the four science questions above. 

 

This work was supported by the NSF AARG program (awards 2009910 and 2009398) and NASA SSO (80NSSC22K1401). We also thank the IRTF staff for helping to make these challenging daytime observations successful.