About nucleobases and amino acids on comet 67P/Churyumov-Gerasimenko

Terrestrial (carbon-based) biochemistry relies on chemical functionality introduced by heteroatoms. Among them, the nitrogen atom (N) defines the (bio)chemical properties of crucial building blocks of life such as amino acids (AAs) and nucleobases (NBs). However, it has not been clear until today whether these building blocks of life on Earth were synthesized from simple prebiotic molecules on the young planet itself or rather delivered by impacting material. Comets not only carry some of the most pristine and original material in our Solar System, but also may have delivered substantial amounts of organics to the early Earth through impacts (Marti et al. 2017, Rubin et al. 2019). From studying comets, we can thus learn about the prevalence of such complex organic molecules (COMs) in space.

An unprecedented milestone in cometary science was the European Space Agency’s Rosetta mission that rendezvoused with comet 67P/Churyumov-Gerasimenko mid-2014. Rosetta studied this comet up close for two years, sending a huge amount of invaluable data back to Earth. One of the key instruments to study the chemical composition of the cometary outgassing was the high-resolution Double Focusing Mass Spectrometer (DFMS) – part of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA; Balsiger et al. 2007). It unveiled a surprising organic diversity and complexity. In the past, we applied an Occam’s razor-based spectra deconvolution approach to identify as many cometary COMs as possible and we found strong evidence for the presence of O- and N-bearing heterocycles (Hänni et al. 2022, 2023, in prep.). However, the ambiguity introduced by structural isomerism often hampers the assignment of a detected signal (chemical sum formula) to a specific molecular structure as different isomers usually have very similar mass-spectrometric fingerprints. Moreover, under the assumption of a bottom-up chemistry, Occam’s razor may not be capable of capturing the emergent chemical diversity. Here, we want to highlight another perspective on the cometary data: If a specific molecule yields a strong molecular ion signal (i.e., a signal of the unfragmented parent molecule) according to its reference mass spectrum, and if this molecular ion signal is not detected, then this molecule’s presence can be ruled out, even if the analyte is as complex as a cometary coma. We therefore investigate the detectability (by electron-ionization mass spectrometry) and the presence/absence of the N-bearing building blocks of life, which are the biogenic nucleobases and amino acids. First, preliminary results suggest that the presence of bionic nucleobases can be ruled out within error margins. However, for amino acids, which do not normally yield strong molecular ion signals, the case is less clear cut. We argue that a typical amine fragment is limiting and can be used to constrain the total abundance of amines. We will compare our findings with asteroidal carbonaceous matter.

 

Marti et al. Science (2017) 356, 6342, 1069-1072.

Rubin et al. ACS Earth Space Chem. (2019) 3, 1792−1811.

Balsiger et al. Space Sci. Rev. (2007) 128, 745-801.

Hänni et al. Nat. Commun. (2022) 13, 3639.

Hänni et al. Astron. Astroph. (2023) 678, A22.

Hänni et al. in prep. for Astron. Astroph.