Chemical reactions in the nitrogen-acetone ice induced by cosmic ray analogs: relevance for the Solar System

TL;DR

This study investigates how cosmic ray analogs induce chemical reactions in nitrogen-acetone ice at 11 K, revealing molecule formation pathways relevant to icy bodies in the Solar System.

Contribution

It provides new data on destruction and formation cross sections of molecules in nitrogen-acetone ice under cosmic ray irradiation, highlighting differences from pure acetone ice.

Findings

Nitrogen quickly leaves porous ice under irradiation.

Formation of complex molecules like HNCO, acetic acid, and possibly glycine.

Porosity and chemical pathways differ from pure acetone ice.

Abstract

The radiolysis of a nitrogen-acetone mixture, condensed at 11 K, by 40 MeV $^{58}$Ni$^{11+}$ ions is studied. These results are representative of studies concerning solar system objects exposed to cosmic rays. In the Kuiper Belt, region of Trans-Neptunian Objects (TNOs), acetone, molecular nitrogen and other small molecules were detected and may be present on icy surfaces. Bombardment by cosmic rays triggers chemical reactions leading to synthesis of larger molecules. In this work, destruction cross sections of acetone and nitrogen in solid phase are determined from a sequence of infrared spectra obtained at increasing ion beam fluence. The results are analyzed and compared with those of previous experiments performed with pure acetone. It is observed that the molecular nitrogen column density decrease very fast, suggesting that nitrogen quickly leaves a porous sample under irradiation. The behavior of acetone in the mixture confirms that the ice formed by deposition of vapor mixture is more porous than that of pure acetone ice. The most abundant molecular species formed from the mixture during irradiation are: C$_3$H$_6$, C$_2$H$_6$, N$_3$, CO, CH$_4$ and CO$_2$. Some N-bearing species are also formed, but with low production yield. Comparing with pure acetone results, it is seen that dissolving acetone in nitrogen affects the formation cross sections of the new species: CH$_4$, CO$_2$ and H$_2$CO, for example, have formation cross section smaller than the respective values for irradiated pure acetone, while those for CO and C$_2$H$_6$ species are higher. This fact can explain the presence of C$_2$H$_6$ even in regions on Pluto where CH$_4$ is not pure, but diluted in a N$_2$ matrix together with less abundant species. These results also show the formation of more complex molecules, such as HNCO, acetic acid and, possibility, glycine.