Over the last century, cold molecular clouds—regions in space containing gas and dust—have been recognized as an integral part in the life cycle of the universe. Cold molecular clouds are relatively dense and cold compared to other regions in the interstellar medium (ISM) such as star forming regions, diffuse molecular colds, and hot cores. Over their lifetime of a few million years, these cosmic nurseries birth stars, planets, and quite possibly the material necessary for life. When astronomers survey these dark regions in space, they observe a higher than expected inventory of complex organic molecules (COMs)—molecules containing six or more atoms—which counterintuitively form in these cold, harsh regions. This enigma poses the fundamental question in astrochemistry, “How could complex organic molecules form in such an unforgiving environment?”

Chemical Processing in Cold Molecular Clouds

Two processes have developed to understand the synthesis of these large molecules. Since the cloud is mostly gas, researchers initially proposed gas-phase reactions; however, these reactions alone have failed to account for observed abundances of COMs in the ISM. Alternatively, researchers have begun considering cosmic dust, also known as interstellar grains, which could provide a surface for chemical reactions. These cold grains freeze gaseous material on their surface where it is bombarded with high-energy irradiation such as galactic cosmic rays. This irradiation produces reactive radicals that can recombine with neighboring molecules, thus generating more complex material. 

Significance

When Dr. George Cooper, currently at NASA Ames, discovered alkylsulfonic acids such as methylsulfonic acid (CH₃SO₃H)¹ on the Murchison meteorite, it provided a possibility that a source of sulfur for early life could have arrived from an extraterrestrial environment. These molecules are critical to life on Earth where they appear in taurine, coenzymes, and even some sulfur-sugars. Recently, this work was solidified by the Hayabusa2 mission that landed on the asteroid Ryugu and collected samples from the surface. Analysis of these samples revealed Ryugu² contains the same alkylsulfonic acids that Cooper had discovered on the Murchison meteorite. If this material is found in so many of these objects, then how did they get there?

We know Ryugu and Murchison contain material from the earliest stages of our solar system, which links them back to their formation from interstellar grains in cold molecular clouds. In order to explain how alkylsulfonic acids formed and became incorporated into Ryugu, which would expand the possibilities for these sulfur-bearing molecules to be used in early life, we adopted a ground-up process of forming these molecules in conditions analogous to cold molecular clouds.

Summary and Implications

By exposing known interstellar ice reagents—sulfur dioxide (SO₂), methane (CH₄), and water (H₂O)—to an environment akin to cold molecular clouds, we identified alkylsulfonic acids in interstellar analog ices for the first time and could explain their presence in Ryugu. This suggests that meteorites like Murchison could be responsible for delivering alkylsulfonic acids to early Earth, kick-starting the first stages of life with a crucial source of sulfur. Although we may be far from understanding the exact chemical processes that have led to life on Earth, as we expand and explore the chemical landscape, we close the gaps in our knowledge that may one day lead to the first experimental formation of life.

References

1          Cooper, G. W., Onwo, W. M. & Cronin, J. R. Alkyl phosphonic acids and sulfonic acids in the Murchison meteorite. Geochim. Cosmochim. Acta 56, 4109-4115, (1992).

2          Yoshimura, T. et al. Chemical evolution of primordial salts and organic sulfur molecules in the asteroid 162173 Ryugu. Nat. Commun. 14, 5284, (2023).