Extended Data Fig. 7: Laser ionization mass spectrum of benzoic acid and benzyl alcohol dissolved in water. | Nature

Extended Data Fig. 7: Laser ionization mass spectrum of benzoic acid and benzyl alcohol dissolved in water.

From: Macromolecular organic compounds from the depths of Enceladus

Extended Data Fig. 7

Analogue TOF mass spectrum recorded with the liquid microbeam ionization setup (see Methods, ‘Laser dispersion analogue experiments for icy dust impacts’ and Extended Data Fig. 9) to simulate the formation of tropylium and benzene cations and their fragmentation ions at impact speeds78 of the order of 10 km s−1. The concentrations of benzoic acid and benzyl alcohol are 3 g l−1 and 0.2 g l−1, respectively. Water ions are marked in blue, aromatic ions and ions from aromatic fragmentation are marked in orange and mixed organic–water species are yellow. To yield both benzene cations (77 u–79 u) and tropylium ions (91 u), two different aromatic structures are required (Fig. 2). The predominant aromatic fragments of benzoic acid are at 77 u and 79 u, whereas benzyl alcohol almost exclusively forms tropylium ions at 91 u. The peak at 95 u is a water cluster of the phenyl cation, which is much more pronounced than in the HMOC spectra. Although the strong phenyl–water cluster signature here illustrates the intimate mixing of organics with water, the much lower 95 u signature in HMOC spectra argues for less efficient mixing of organics with water there, probably due to a core–shell structure that physically separates organics from ice in the grain. Cations from the fragmented ring can be seen at 39 u, 51 u–53 u and 63 u–65 u and agree with the CDA observations (Fig. 1b). In contrast to the CDA spectra, however, saturated C3 fragments (41 u–43 u) are depleted, and C2 (27 u–29 u) and C1 (15 u) fragment cations are entirely absent, confirming the presence of an abundance of aliphatic cations in HMOC grains. The ratios of benzene and tropylium ions and the water ions match the HMOC spectra well. The total concentration of organic species used here (~0.32% by weight) can be used to estimate a lower limit for the concentration of organics in CDA HMOC grains for two reasons. First, in the analogue experiment we selected substances that most efficiently yield the desired aromatic species and other, less efficient precursors would yield even lower signals at 77 u and 91 u. Second, to account for both the low- and high-mass fragments between 100 u and 2,000 u, which are absent in the laboratory spectrum, additional organic substances or larger molecules would be needed to further increase the organic concentration. Therefore, the concentration in Enceladean HMOC ice grains in many cases can be estimated to be near or even above the per cent level.