Impacts of residual pressure, heating rate and photon to molecular ratio on the molecular composition generated by the processing of astrophysical ices

Introduction

Astrophysical ices were observed in various environments with different compositions : from the icy grains of the interstellar medium (ISM, [1]), to various bodies of the solar system such in icy moons of Jupiter or Saturn systems, or in comets. In such environments, ice is subjected to various alterations ranging from a wide range of UV doses, different physicochemical alterations [2], thermal alterations [3] and various primitive ice composition [4]. As inferred by laboratory experiments, these ice alterations are now known to generate a large molecular diversity ( [5], [6]). This organic matter is well observed in certain meteorites where up to  4 % of the total matter is organic (carbonaceous chondrites [7]), but this number can grow up to 45 % in interstellar dust particles and comets [8]. Laboratory experiments suggest that grain chemistry from primitive ices composed of H2O, NH3, methanol (MeOH) or other molecules is the main source of this astrophysical organic matter ([9], [10]). The reactivity occurring during the alteration of such an ice is a two step mechanism: First, energetic processes form radicals such as H^•CO or ^•CH2OH which directly produces molecules such as CO or CO₂ [14]; Then, thermal reactivity can occur to form more complex organic compounds. During this second phase, radicals diffuse, which allows their recombination forming more complex molecules such as hexamethylenetetramine (HMT), polyethyleneimine (PMI) or polyoxymethylene (POM) [15]. 

The refractory [16] and volatile [17] compositions of the final organic pool is thus highly influenced by the initial ice composition ratio. In the present work, we investigate the influence of the heating rate, the residual pressure or of the photon/molecule ratio on this final organics.

Method

All the experiments were monitored in the MICMOC experiment already described in [11]. It is a cryogenic vacuum chamber (~10-7 mbar) with a finger cooled down up to 77 K using liquid nitrogen. The cold finger contains IR transparent MgF2 windows on which primitive moieties H2O, NH3 and methanol (MeOH) are deposited within a fixed 3:1:2 ratio (H2O:MeOH:NH3). The irradiation at Lyman α (121 nm) is concomitant to the ice formation during 70 h and then warmed up to ~300 K. The following experiments were performed : 

• Various irradiation rates (1,5.103, 141, 5 photon/MeOH), by modifying the flux of matter;
• Fixed irradiation (~80 photon/MeOH) and various heating rates (0.5, 4, 8 K/min) monitored by a heating resistance after the irradiation phase;
• Fixed irradiation and various pressures of Ar added during the heating phase.

All samples were then analyzed using both fourier-transform infrared spectrometry (FT-IR) and electrospray fourier- transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR [18]).

Preliminary results and conclusions

Preliminary results are displayed Figure 1. First, infrared spectroscopy measurements show weak impacts of heating rate and residual pressure during the warming phase: The main IR features present similar intensities implying no significant evolution of the final organic products. The only significant influence observed is the lower FT-IR intensity recorded at lower heating rate (0.5 K/min). Regarding the irradiation rate, it shows the highest evolution on the IR signatures : for low irradiation rates (5 photon/MeOH) ν(OH) at 3242 cm^-1 vs νas(CH) at 2923 cm^-1 ratio is significatively modified and ω(CH₂) at 1458 cm^-1 and νas(COO^-) at 1598 cm^-1 are significatively exhausted. This suggests the presence of salts, polyoxymethylene (POM), hexamethylenetetramine (HMT) and polyethyleneimine (PMI) in higher quantities in this sample. On the contrary, the FT-IR spectrum of the higher irradiated sample shows no features because of a decrease in the yield of organic production implying a lack of sensitivity with FT-IR.

To obtain complementary information on the molecular evolution ESI-FT-ICR analysis was recorded on the same samples. The ESI-FT-ICR confirmed the preliminary results observed with FT-IR with a better insight on the molecular processes impacting the final organic composition. All together the influence of those three parameters are the following :

• Irradiation has a high impact on the molecular content of the sample, mostly on the DBE, but also on the molecular diversity and abundance;
• The increase of the residual pressure during the heating phase has a weak influence, even if it  increases a higher experimental dispersion regarding the molecular abundance while the molecular diversity is not impacted;
• Finally, the heating rate has a very weak influence on the final organic composition. 

Figure 1 Comparison of FT-IR spectra or ESI+ FT-ICR mass spectra of the residue for each sample series (A, B, C). Global shape of both ESI+ and ESI– of FT-ICR analysis are showing similar properties. 

Bibliography :

[1] D. H. Wooden et al., “Composition and Evolution of Interstellar Clouds,” in Comets II, University of Arizona Press, pp. 33–66, 2004.

[2] G. M. Muñoz Caro et al., ACS Earth Space Chem, vol. 3, no. 10, pp. 2138–2157, 2019.

[3] O. Mousis et al., Astrophys. J., vol. 819, no. 2, p. L33, 2016.    

[4] M. J. Mumma and S. B. Charnley, Annu. Rev. Astron. Astrophys., vol. 49, no. 1, pp. 471–524, 2011.

[5] G. Danger et al., GCA, vol. 118, pp. 184–201, 2013. 

[6] A. Ruf et al., Life, vol. 9, no. 2, p. 35, 2019.    

[7] S. Pizzarello and E. Shock, Cold Spring Harb. Perspect. Biol., vol. 2, no. 3, pp. a002105–a002105, 2010.

[8] J.-L. Bertaux and R. Lallement, MNRAS, vol. 469, no. Suppl_2, pp. S646–S660, 2017.

[9] A. Gutiérrez-Quintanilla et al., MNRAS, vol. 506, no. 3, pp. 3734–3750, 2021.    

[10] G. Fedoseev et al., MNRAS, vol. 448, no. 2, pp. 1288–1297, 2015.    

[11] L. d’Hendecourt and E. Dartois, Spectrochim. Acta A Mol. Biomol. Spectrosc., vol. 57, no. 4, pp. 669–684, Mar. 2001.    

[12] P. Schmitt-Kopplin et al., PNAS, vol. 107, no. 7, pp. 2763–2768, 2010.

[13] F. Duvernay et al., ApJ, vol. 791, no. 2, p. 75, 2014.    

[14] K. I. Öberg, Chem. Rev., vol. 116, no. 17, pp. 9631–9663, 2016.    

[15] V. Vinogradoff et al., A&A, vol. 551, p. A128, 2013.

[16] A. Fresneau et al., ApJ, vol. 837, no. 2, p. 168, 2017.

[17] N. Abou Mrad et al., ApJ, vol. 846, no. 2, p. 124, 2017.   

[18]  P. Schmitt-Kopplin et al., Anal. Chem., vol. 82, no. 19, pp. 8017–8026, 2010.