The composition of outer solar system icy surfaces: hints from the analysis of laboratory analogues

Cosmic rays, solar wind and solar energetic particles induce changes in the physical structure and chemical composition of frozen volatiles on the surface solar system outer objects, such as satellites of giant planets, centaurs and Kuiper-belt objects (e.g., Johnson 1990, Strazzulla et al. 2003, Urso et al. 2020). Such energetic particles can be responsible for the formation of C-rich refractory materials that determine the appearance of red slopes in the visible and near-infrared spectra of small bodies (e.g., Brunetto et al. 2006, Brown et al. 2011).

Laboratory experiments performed to simulate the irradiation of frozen surfaces in space show that energetic ions or electrons (keV-MeV) determine the formation of new species, not present in the original samples (e.g., Rothard et al. 2017). The subsequent warm-up of processed mixtures causes both the sublimation of volatile compounds and an increase in the molecular diffusion and reactivity. As a consequence, the chemical complexity increases and organic refractory residues are formed. 

Residues are thought to be representative of the refractory materials on the surface of outer bodies as well as of cometary materials and a possible precursor of the soluble organic matter (SOM) found in meteorites (e.g., Strazzulla & Johnson 1991, Munoz-Caro & Schutte 2003, Brunetto et al. 2006, Danger et al. 2013, Baratta et al 2015, Urso et al. 2017, Poston et al. 2018}.

We present recent laboratory experiments to produce and characterize organic refractory residues produced after ion irradiation of frozen volatiles. Water, methanol and ammonia mixtures in ratios 1:1:1 and 3:1:1 are deposited at 15 K and exposed to 40 keV H+ by means of the INGMAR setup (Urso et al. 2020). After irradiation, mixtures are warmed up with a constant heating rate and organic refractory residues are formed at 300 K. Throughout the experiment, we monitor the sample evolution by means of in-situ infrared transmission spectroscopy (4000-700 cm-1, 2.5-13.3 µm).

After warmup, organic refractory residues are recovered from the irradiation chamber and are further characterized through ex-situ Very High Resolution Mass Spectrometry. The combination of both techniques allows to shed light on the composition of irradiated frozen volatiles and of residues. We also search for specific molecules by means of tandem Mass Spectrometry/High Resolution Mass Spectrometry. Furthermore, we compare the chemical composition of our samples with that of organic refractory residues produced after UV photolysis of volatile mixtures.

Our results give information on the effects induced by different experimental parameters (dose, mixture ratio) on the composition of organic refractory materials. In particular, we investigate the effects of increasing irradiation dose in the elemental abundance and in the Double Bond Equivalent (DBE) of residues. We also give the timescales necessary to observe in solar system outer objects the effects revealed in laboratory experiments, and we discuss the role of the various sources of processing (cosmic rays, solar wind, solar energetic particles) in determining changes in the chemical composition of frozen surfaces in space.

Our work supports the interpretation of space mission data and astronomical observations of comets and solar system outer objects as well as of star-forming regions in the Interstellar medium, where cosmic rays bombard icy grain mantles and thus play a role in the synthesis of complex organic molecules in the early stage of star-formation.

Acknowledgements

This work is supported by the CNES-France. INGMAR (IAS-CSNSM, Orsay) is funded by the French Programme National de Planétologie (PNP), Faculté des Sciences d'Orsay, Université Paris-Sud (Attractivité 2012), French National Research Agency ANR (contract ANR-11-BS56-0026, OGRESSE), P2IO LabEx (ANR-10-LABX-0038) in the framework Investissements d'Avenir (ANR-11-IDEX-0003-01). We thank the support from RAHIIA SSOM (ANR-16-CE29-0015). R.G.U. thanks the CNES postdoctoral program.

 

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