Amino Acid Glycine Observed within Sodium Chloride Precipitates: Laboratory Analysis of Ceres Analogues

Introduction: Here, we report laboratory experiments to synthesize sodium chloride (NaCl) salts under equilibrium conditions with the addition of the amino acid glycine (C₂H₅NO₂). We performed microscopy and spectroscopy of samples to assess how the presence of glycine changes the chemical, physical, and optical properties of the salts. These experiments are designed to investigate carbon incorporation within NaCl crystals, likely occurring on Ceres’ surface. Such work on Ceres analog minerals is crucial for the design of a robust sample collection system for sample return at the Occator site, as suggested for a future New Frontiers class mission in the recently released Origins, Worlds, Life — Decadal Strategy for Planetary Science and Astrobiology 2023-2032.

Results: Pure NaCl crystals appear predominantly white in the reflected light images (Fig. 1). The distinct boundaries or growth layers appear dark. The outer edges of the crystals are the thickest, and it steps down in height going towards the center of the crystal in a staircase pattern. The NaCl crystals with glycine show 2 separate features in reflected light, namely (a) blebs of dark material in the NaCl matrix scattered heterogeneously (Fig. 2) and (b) a dark hue over the entire crystal (Fig. 3). Distinct layers and a staircase pattern are also present in these crystals. However, the pattern becomes less geometric/angular, as observed in Fig. 3.

IMS 6f measurements on the NaCl crystal without glycine (blank) show an average ¹²C/²³Na ratio of 4.59×10^-8. The blebs in the NaCl crystals show (Fig. 2) show a ¹²C/²³Na ratio that is >50 times the ratio in the blank. The glycine-bearing NaCl crystals show a high cathodoluminescence signal and are more luminescent at the layer boundaries compared to the surrounding NaCl. This luminescent, localized component has a lower mass contrast than NaCl.

Conclusions: These results confirm the inclusion of carbon-bearing materials during the growth of NaCl. How glycine is bound in the NaCl lattice, whether glycine itself can act as independent nucleation sites, or if glycine can survive fragmentation, and be detected as an individual molecule needs further exploration.

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Reflected light images of (1) NaCl crystal and (2) & (3) glycine-bearing NaCl