Amides from the carbonaceous asteroid (162173) Ryugu: nanoscale spectral and isotopic characterizations

Abstract

C-type asteroids, such as asteroid (162173) Ryugu, may have played a key role in delivering light elements to early Earth. Nitrogen (N)-bearing molecules have been chemically identified in some Ryugu grains, and based on the faint 3.06 um absorption band observed by the hyperspectral microscope MicrOmega, NH-bearing compounds appear to be globally distributed. However, the chemical forms of these NH-bearing compounds - whether organic molecules, ammonium (NH4+) salts, NH4+- or NH-organic-bearing phyllosilicates, or other forms - remain to be clarified. We report the characterization of two Ryugu particles (C0050 and C0052) using infrared spectroscopy at millimeter, micrometer, and nanometer scales, combined with NanoSIMS to constrain the nature and origin of NH-bearing components. Ryugu's C0052 particle contains rare (~1 vol.%) micrometer-sized NH-rich organic compounds with peaks at 1660 cm-1 (C=O stretching, amide I) and 1550 cm-1 (N-H bending, amide II), indicative of amides, absent in C0050. N isotopic analysis shows these amides are depleted in 15N (d15N ~ -200 permil), confirming their indigenous origin. The amides may have formed by hydrothermal alteration of carboxylic acids and amines on Ryugu's parent body or by irradiation of 15N-depleted N-bearing ice in the outer Solar System or interstellar medium. Such amides delivered by primitive small bodies may have contributed to prebiotic chemistry on early Earth.