Laboratory experiments to investigate the origin and composition of organic refractory materials on frozen bodies in the solar system

Abstract:

Frozen volatile compounds are observed at the surface of small bodies in the outer solar system, such as centaurs, satellites of giant planets, and Kuiper-belt objects. These surfaces are exposed to cosmic rays, solar wind and solar energetic particles, that are thought to contribute in the formation of a C-rich refractory material that may be responsible for the red slopes in the visible and near-infrared spectra of various small bodies (Barucci et al. 2006). However, such materials could have also been inherited from the presolar cloud or accreted in the protoplanetary disk (e.g. Dalle Ore et al. 2011).

Laboratory experiments to simulate the interaction between energetic particles (keV-MeV) and frozen volatiles have shown that various new species, not present in the original samples, are formed (e.g., Rothard et al. 2017). Furthermore, the heating of processed samples up to 300 K determines the formation of C-rich materials, named organic refractory residues, that are thought to be representative of the red refractory materials on frozen bodies (e.g., Brunetto et al. 2006).

We report here on new irradiation experiments of frozen mixtures containing water, methanol and ammonia. During the experiments, samples are monitored through mid-infrared spectroscopy. After the warm-up to 300 K we obtain organic refractory residues that are further analyzed by means of Very High Resolution Mass Spectrometry.

Our results inform on the composition of organic refractory residues and reveal the high extent of molecular diversity of such samples. We investigate the effect of the irradiation dose on the residues composition, and we estimate timescales necessary to observe on frozen surfaces in space the effects revealed in our laboratory experiments. We also discuss the role of various sources of processing (cosmic rays, solar particles) in determining changes in the properties of frozen surfaces in space. Finally, we investigate the presence of specific compounds through tandem Mass Spectrometry/High Resolution Mass Spectrometry.

Our results support the interpretation of space mission data and astronomical observations of solar system small bodies as well as of star-forming regions in the Interstellar medium and thus do provide clues about the possible origin and composition of complex organics in space.