Amino Acid Precursors from a Simulated Lower Atmosphere of Titan: Experiments of Cosmic Ray Energy Source with ¹³C- and ¹⁸O-Stable Isotope Probing Mass Spectrometry

Abstract

The organic haze of aerosols that shrouds the Saturnian moon Titan has previously been studied by both observations and laboratory simulation experiments. Here we report the abiotic formation of amino acid precursors in complex organic molecules during experimental simulation of the environment near Titan’s surface with proton irradiation. Pyrolysis of the organic molecules formed in the simulated Titan atmosphere by proton irradiation at 600^°C yielded compounds that contained HCN and NH3 (m/z = 27 and 17). These experimental results are consistent with the molecular information obtained by pyrolysis gas chromatography/mass spectrometry (pyrolysis GC/MS) of samples collected by the Huygens probe to Titan. Scanning electron microscopy (SEM) and three-dimensional atomic force microscopy (AFM) images of the irradiation products reveal nanometer-scale filaments and globules in complex amorphous structures (approximately 1000 Da). Isotope probing experiments by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) show that oxygen atoms were incorporated into the racemic amino acids by hydrolysis of ¹⁸O-labeled water. We suggest that the amino acid precursors possibly formed after water hydrolysis, as suggested in a previous observational study (C. A. Griffith, T. Owen, T. R. Geballe, J. Rayner, and P. Rannou, Science, 2003, 300, 628). We propose that cosmic rays are a significant and effective energy source for producing complex organics and amino acid precursors in Titan’s atmospheric haze.