Ion irradiation triggers the formation of the precursors of complex organics in space The case of formaldehyde and acetaldehyde

Context. Cosmic rays and solar energetic particles induce changes in the composition of compounds frozen onto dust grains in the interstellar medium (ISM), in comets, and on the surfaces of atmosphere-less small bodies in the outer Solar System. This induces the destruction of pristine compounds and triggers the formation of various species, including the precursors of complex organics. Aims. We investigate the role of energetic ions in the formation of formaldehyde (H 2 CO) and acetaldehyde (CH 3 CHO), which are observed in the ISM and in comets, and which are thought to be the precursors of more complex compounds such as hexamethylenete-tramine (HMT), which is found in carbonaceous chondrites and in laboratory samples produced after the irradiation and warm-up of astrophysical ices. Methods. We performed ion irradiation of water, methanol, and ammonia mixtures at 14–18 K. We bombarded frozen films with 40–200 keV H + that simulate solar energetic particles and low-energy cosmic rays. Samples were analysed by infrared transmission spectroscopy. Results. Among other molecules, we observe the formation of H 2 CO and CH 3 CHO, and we find that their abundance depends on the dose and on the stoichiometry of the mixtures. We find that the H 2 CO abundance reaches the highest value after a dose of 10 eV/16u and then it decreases as the dose increases. Conclusions. The data suggest that surfaces exposed to high doses are depleted in H 2 CO. This explains why the amount of HMT in organic residues and that formed after irradiation of ices depends on the dose deposited in the ice. Because the H 2 CO abundance decreases at doses higher than 10 eV/16u, a lower quantity of H 2 CO is available to form HMT during the subsequent warm-up. The H 2 CO abundances caused by ion bombardment are insufficient to explain the ISM abundances, but ion bombardment can account for the abundance of CH 3 CHO towards the ISM and comets.


Introduction
Frozen compounds are present on dust grains in the interstellar medium (ISM; e.g. Boogert et al. 2015) and on the surface of small bodies in the Solar System, such as comets (e.g. Bockelée-Morvan et al. 2000;Altwegg et al. 2017), satellites of giant planets (e.g. Johnson 2005;Spencer & Nimmo 2013;Dougherty & Spilker 2018), Kuiper-belt (KBOs), and transneptunian objects (TNOs; e.g. Cruikshank et al. 1998;Brown et al. 2011;Grundy et al. 2020; Barucci & Merlin 2020). Various KBOs and TNOs exhibit frozen compounds together with red slopes in the visible and near-infrared (NIR) spectra that could be attributed to complex C-rich materials (Brunetto et al. 2006;Brown et al. 2011;Dalle Ore et al. 2011). Complex organics are also found in materials sampled from asteroids (e.g. Chan et al. 2021;Pilorget et al. 2021;Parker et al. 2022), in interplanetary dust particles (IDPs; e.g. Chan et al. 2020) and carbonaceous chondrites (CC; e.g. Sephton 2002;Busemann et al. 2006;Schmitt-Kopplin et al. 2010;Oba et al. 2020Oba et al. , 2022. Laboratory experiments allow us to shed light on the physical and chemical properties of small bodies and to improve our understanding of the processes that contribute to the formation of complex organics. Experiments allowed studying how energetically charged particles in space, such as cosmic rays (CR) and solar particles, affect the spectral properties of frozen surfaces in the ISM and in the Solar System, such as the appearance of red slopes in irradiated ices (e.g. Strazzulla et al. 1983;Moore et al. 1996;Brunetto et al. 2006;Palumbo 2006;Dartois et al. 2015;Rothard et al. 2017;Poston et al. 2018). Changes primarily depend on the initial composition of the ice, the source of processing, and the irradiation dose (e.g. Rothard et al. 2017). Simple and complex compounds are formed after irradiation of mixtures that simulate the pristine composition of astrophysical ices (e.g. Moore et al. 1996;Palumbo et al. 1997;Kaňuchová et al. 2016;Urso et al. 2019). Among them, vibrational mode bands attributed to aldehydes are found in the mid-IR spectra of samples exposed to UV photons and ion irradiation. In particular, formaldehyde (H 2 CO) and acetaldehyde (CH 3 CHO) have been reported among the byproducts of irradiation of frozen mixtures containing H-, C-, and O-bearing compounds (e.g. Khare et al. 1989;Moore & Hudson 1998;Palumbo et al. 1999). In space, solid-phase H 2 CO is likely identified toward massive A&A 668, A169 (2022) young stellar objects (MYSOs) and low-mass YSOs (LYSOs), with abundances of 0.06-6.00 relative to CH 3 OH (Bottinelli et al. 2007;Boogert et al. 2015). In the Solar System, H 2 CO is detected in comets (e.g. Mitchell et al. 1987;Altwegg et al. 2019) with abundances that span between 0.02 and 5 with respect to CH 3 OH (e.g. le Roy et al. 2015;Boogert et al. 2015;Schuhmann et al. 2019). Solid-phase CH 3 CHO is only tentatively detected in the ISM and in comets, while it is observed in the gas-phase in both environments (e.g. Cazaux et al. 2003;Crovisier et al. 2004;Öberg et al. 2010;Jaber et al. 2014;Codella et al. 2015;Schuhmann et al. 2019). Information on solid-phase CH 3 CHO is lacking, possibly because its main vibrational mode bands overlap with those of other species. In particular, HCOO − shows a vibrational feature at the same wavelength and can contribute to the 1350 cm −1 feature (e.g. Schutte et al. 1999).
In space, aldehydes are among the frozen compounds that can react to form more complex species. Reactions are also favoured by thermal processing that increases the diffusion and reactivity of molecules (e.g. Mispelaer et al. 2013;Theulé et al. 2013;Vinogradoff et al. 2013). When irradiation and thermal processing of astrophysical ices is simulated in the laboratory, a complex organic refractory material forms. Organic refractory residues are laboratory analogues of cometary refractory organics (e.g. Strazzulla & Johnson 1991;Baratta et al. 2019), of the organics in extraterrestral samples (e.g. Nuevo et al. 2008;de Marcellus et al. 2017), and of organic materials that can form on the surface of KBOs and TNOs (e.g. Strazzulla et al. 2003;Urso et al. 2020b). The characterization of residues reveals various astrobiologically relevant compounds, including amino acids, nucleobases, and sugars Fresneau et al. 2017;Meinert et al. 2016;Nuevo et al. 2018;Urso et al. 2020b). According to Vinogradoff et al. (2012b), together with NH 3 and HCOOH, which act as a catalyst, H 2 CO is one of the precursor of hexamethylenetetramine (HMT). This is one of the most abundant organics detected in residues and forms at relatively high temperatures (>280 K) during the warm-up of ices. HMT was recently found in the carbonaceous chondrites Murchison, Murray, and Tagish Lake (Oba et al. 2020), and in the laboratory, it forms after UV and ion irradiation at 10-20 K and subsequent warm-up to room temperature of mixtures containing water (H 2 O), methanol (CH 3 OH), and ammonia (NH 3 , Hulett et al. 1971;Bernstein et al. 1995;Cottin et al. 2001;Vinogradoff et al. 2013;Danger et al. 2016;Urso et al. 2020b). Urso et al. (2020b) reported that the composition of organic refractory residues depends on the extent of irradiation experienced by ices. They also found that the HMT abundance decreased when the dose was increased.
CH 3 CHO is also invoked as a precursor of various complex compounds. Laboratory experiments that simulated the thermal processing of CH 3 CHO-rich ices showed that CH 3 CHO reacts with NH 3 to form α-aminoethanol (NH 2 CH(CH 3 )OH) above 100 K (Duvernay et al. 2010). Furthermore, above 70 K and in presence of HCOOH, the reaction between CH 3 CHO and NH 3 leads to the CH 3 CHO-NH 3 trimer after the formation of the reaction intermediates α-aminoethanol and ethanimine (Vinogradoff et al. 2012a).
We here report new experiments that we performed to investigate the role of ion irradiation in the production of H 2 CO and CH 3 CHO. We study how their abundance is affected by the dose and we discuss how the formation of HMT can be affected by the destruction of H 2 CO, its precursor. For this study, we deposited H 2 O:CH 3 OH:NH 3 mixtures at 14-18 K and exposed them to 40 and 200 keV H + . H 2 O, CH 3 OH, and NH 3 mixtures were chosen because they are among the most abundant solid-phase compounds observed in YSOs (e.g. Caselli & Ceccarelli 2012;Boogert et al. 2015) and in small bodies in the outer Solar System (e.g. Cruikshank et al. 1998;Delsanti et al. 2010;Grundy et al. 2016;Barucci & Merlin 2020). Furthermore, organic refractory residues produced after irradiation of H 2 O:CH 3 OH:NH 3 mixtures are well characterized and show complex organics, including HMT (e.g. Bernstein et al. 1995;Danger et al. 2016;Urso et al. 2020b). We can thus interpret our results by taking into account similar experiments reported in literature. The irradiation with 40 and 200 keV H + allows us to simulate the effects of solar charged particles and CR on icy surfaces in the Solar System and on icy grain mantles. As discussed by Mewaldt et al. (2007) and Urso et al. (2020a), protons in the range of tens of keV are abundant in the spectrum of solar energetic particles (SEP) impinging on frozen surfaces in the outer Solar System. Furthermore, several previous studies used hundreds ok keV H + to simulate the effects of low-energy CR (e.g. Palumbo 2006;Islam et al. 2014;Urso et al. 2019).

Experimental methods
The ion irradiation experiments were performed at the Intitut d'Astrophysique Spatiale, Irène Joliot-Curie Laboratoire (IAS-IJCLab Orsay, France) and at the Laboratory of Experimental Astrophysics (LASp) at the Istituto Nazionale di Astrofisica INAF-Osservatorio Astrofisico di Catania (Catania, Italy). We used the IrradiatioN de Glaces et Météorites Analysées par Réflectance VIS-IR (INGMAR) setup at IAS-IJCLab to irradiate ices with 40 keV H + by means of the SIDONIE accelerator (Chauvin et al. 2004). The setup at the LASp allows accelerating ions up to 200-400 keV (Fulvio et al. 2009;Urso et al. 2016). For this work, we used 200 keV H + . Although the two experimental setups show distinct characteristics, they both consist of high vacuum chambers (P ∼ 10 −8 -10 −9 mbar), where He cryocoolers allow cooling substrates down to ∼15 K. H 2 O:CH 3 OH:NH 3 gaseous mixtures were injected in the vacuum chambers, where they condensed on the cold substrates. The resulting frozen films were then exposed to H + beams that were rastered on samples, in order to ensure a homogeneous covering of their surfaces.
The energy of impinging ions determines their penetration depth in the frozen mixture. According to simulations performed with the SRIM software (Ziegler et al. 2008), 40 keV and 200 keV H + implant in H 2 O:CH 3 OH:NH 3 mixtures within 1 µm and 3.3 µm, respectively. We therefore deposited frozen films with thicknesses that were lower than the H + implantation depth to ensure that the impinging particles released their energy homogeneously within the ice, and to limit their implantation. In particular, for the experiments with 40 keV H + , we deposited films between 0.4 and 0.6 µm, while in the case of irradiation with 200 keV, we deposited 0.5 and 1.2 µm thick films. During ion irradiation, we integrated the ion current to estimate the fluence, that is, the number of impinging ions cm −2 . We then estimated the dose, that is, the energy deposited per molecule in the sample by incident particles, by multiplying the fluence by the stopping power (eV cm 2 /16u) calculated with SRIM. We give the dose in eV/16u, where 16u is the atomic mass unit of a CH 4 molecule that we take as a reference (Strazzulla & Johnson 1991).
To properly simulate the ion irradiation of frozen surfaces, we kept the ion current in the range of some hundred nA and below 1 µA. For an ion current density of 1 µA cm −2 , the flux is equal to 6.25×10 12 ions cm −2 s −1 . This value prevents the macroscopic heating of the samples. Furthermore, the low current is required to ensure that enough time passes between A169, page 2 of 10 R. G. Urso et al.: Ion irradiation triggers the formation of the precursors of complex organics in space the arrival of two ions on the same target area. Ions travelling through the target cause ionizations and excitations of molecules and the formation of molecular fragments and radicals, whose recombination forms species that were not present in the original sample (e.g. Rothard et al. 2017). The whole process, from the excitation to the recombination and thus relaxation, requires about 10 −12 s or less. The mixtures we deposited at 14-18 K had a density of about 0.9 g cm −3 . In these samples, keV protons create a radial struggle of r = 1.7×10 −5 cm as calculated by means of the SRIM software. The ion track within the ice can be approximated to a cylinder with a section area a = πr 2 , equal to 9.1×10 −10 cm 2 , which we can approximate to 10 −9 cm 2 . Thus, it is necessary to reach a fluence of 10 9 ions cm −2 to fully cover the surface of a sample of 1 cm 2 . Taking into account a constant flux value as that used in our experiment, i.e. about 6.25×10 12 ions cm −2 s −1 , a fluence of 10 9 ions cm −2 is reached after 1.6×10 −2 s. This is the time required for an impinging ion to hit the same target area a already excited by a former ion in our experiments. This time is about 10 10 times longer than the relaxation time along the ion track (about 10 −12 s). As a consequence, impinging ions always travel through volumes of ices that are stable and relaxed. This allows our laboratory experiments to be representative of the particle irradiation taking place in astrophysical ices exposed to cosmic and solar charged particles. Samples are analysed in situ by means of transmission Fourier-Transform infrared spectroscopy (FT-IR) with a resolution of 1 cm −1 . In the INGMAR setup, the IR beam arrives on the sample with an angle of 10 • with respect to the surface normal, while at LASp the IR beam arrives with an incident angle of 45 • with respect to the surface normal.
H 2 O, CH 3 OH, and NH 3 are deposited at 14-18 K on IRtransparent substrates (ZnSe or KBr) and bombarded with 40 and 200 keV H + . Spectra are acquired after the deposition and after each irradiation step, in order to monitor the sample spectral evolution as a function of irradiation dose. Spectra are then analysed to obtain qualitative and quantitative estimations on the newly-formed compounds. We give the abundance of the newlyformed compounds N(X) by estimating the column density, i.e.
is the optical depth (-ln T , transmittance) and A is the band strength (cm molecule −1 ) of the vibrational mode band taken into account.
The comparison between experimental results obtained in different laboratories is not trivial. We thus carefully plan experiments and we use the same procedures to acquire, reduce, and analyse data so to minimise the differences that arise when comparing different experiments. This allow us to obtain qualitative and quantitative information that show consistent trends, thus confirming the reliability of our results.

Results
Table 1 groups all the experiments taken into account in this work. Experiments are labelled according to (i) the amount of H 2 O with respect to CH 3 OH and NH 3 in the mixture and (ii) the energy of the H + beam used to bombard the samples. In the case of the experiments performed with 200 keV H + , we deposit the same mixture ratio twice, in the same experimental conditions, and we perform irradiation in two ranges of dose that overlap with each other. These experiments are labelled according to the dose range we investigate (L for the low dose range and H for the high dose range). between about 2980 and 2830 cm −1 , two broad features between 1720 and 1370 cm −1 attributed to bending modes in water and NH 3 molecules, a feature at 1125 attributed to the wagging mode in NH 3 and to the rocking mode in CH 3 OH, the C-O stretching mode of CH 3 OH at 1025 cm −1 , and the water libration mode band between about 990 and 500 cm −1 (e.g. Urso et al. 2020b). Irradiation determines evident changes in the spectra. The features at about 3300 and 1025 cm −1 decrease and new bands appear. In particular, the CH 3 OH band at 1025 cm −1 decreases with increasing dose, following an exponential decay, because of the ion-induced destruction of CH 3 OH. Figure 2 shows the variation in the CH 3 OH abundance as a function of the dose. We fit the data with an exponential curve (Eq. (1)),

Methanol destruction and methane formation
where σ is the cross section, which is found to be 0.34 16u/eV, and D is the dose in eV/16u. About 50% of CH 3 OH is destroyed after a dose of about 20 eV/16u, and only 10% of CH 3 OH is left at about 70 eV/16u. The analysis also shows that about 3% of methanol survives the highest irradiation dose. The most intense new features are those attributed to CO 2 (2345 cm −1 ), OCN − (2165 cm −1 ), and CO (2139 cm −1 ). Furthermore, Fig. 3 shows a zoom in the 1600-1200 cm −1 range before (panel A) and after irradiation with 40 and 200 keV H + (panel B). In this range, several new features appear after irradiation. The band centred at 1305 cm −1 is attributed to CH 4 that formed after irradiation. Figure 4 shows the CH 4 abundance as a function of irradiation dose. For our estimations, we used the CH 4 1300 cm −1 band strength given by Mulas et al. (1998). The highest column density (4.6%) is reached after about 50 eV/16u. Figure 4 also shows the CH 4 formation σ that we estimate by means of Eq. (2), where y ∞ is the asymptotic value of the N(CH 4 )/N(CH 3 OH) i ratio. Table 2 reports the values of y ∞ and σ for each experiment. The analysis of the CH 3 OH destruction and of the CH 4 formation does not show differences attributable to the H + energy used to irradiate samples.

Formaldehyde and acetaldehyde formation
Various vibrational mode bands attributed to H 2 CO (1720, 1500, 1250 cm −1 ) and CH 3 CHO (1720, 1382, 1350 cm −1 , e.g. Moore & Hudson 1998;Urso et al. 2020b) are found in both 3:1:1 and 1:1:1 mixtures after irradiation. We perform quantitative analysis to study the evolution of their abundance with increasing irradiation dose. For the analysis, we do not consider the 1720 cm −1 band as it contains contributions from all the -C=O bearing species in the samples. Furthermore, cautions has to be taken because of other features in the same spectral range (NH 2 CHO at 1385 cm −1 , NH + 4 at 1480 and 1460 cm −1 , HCOO − at 1385 and 1350 cm −1 ; e.g. Brucato et al. 2006;Raunier et al. 2004;Schutte et al. 1999).
H 2 CO column densities were estimated by analysing the CH 2 rocking mode band at 1250 cm −1 using the band strength value given by Bouilloud et al. (2015), that is, 1.5×10 −18 cm molecule −1 . Panel A of Fig. 5 shows the H 2 CO abundance with respect to the initial abundance of CH 3 OH as a function of the dose. In all the experiments, the H 2 CO abundance increases rapidly at low doses and the highest abundance is observed at about 10 eV/16u. At this dose, the irradiation of 3:1:1 mixtures produces 6.2% of H 2 CO, and the irradiation of 1:1:1 mixtures produces 2.4%. At higher irradiation doses the H 2 CO abundance starts to decrease, following an exponential decay. At about 30 eV/16u, the 1:1:1 and 3:1:1 mixtures show comparable amounts of H 2 CO. At the highest irradiation dose we investigated, the H 2 CO band is at the detection limit and the abundance is about 1% with respect to the initial amount of CH 3 OH. In Fig. 5 we also show the abundance of H 2 CO with respect to the amount of CH 3 OH at the same dose. Again, the ratio increases at low doses, with higher abundances in the case of the 3:1:1 mixtures. At doses higher than 50 eV/16u both H 2 CO and CH 3 OH are efficiently destroyed and the observed trend is based on features that are at the detection limits.
We estimated the CH 3 CHO abundance by analysing the 1350 cm −1 band. Various features overlap with this band, in particular the νC-O of HCOO − (Schutte et al. 1999). We therefore also analysed the 1580 cm −1 band, which is attributed to HCOO − alone (Schutte et al. 1999), and we calculated the 1580/1350 band area ratio in our experiments and in the spectrum of a H 2 CO:NH 3 :HCOOH mixture reported by Vinogradoff et al. (2011), in order to check the relative intensity of the two bands and to verify that the 1350 cm −1 band is not mainly due to HCOO − . Vinogradoff et al. (2011) attributed both bands to HCOO − because their mixtures did not contain CH 3 CHO. When we use their 1580/1350 ratio as a reference, we would find a similar 1580/1350 band ratio if our mixtures were also to contain manly HCOO − . However, our mixtures show averages of the   Fig. 4. CH 4 abundance relative to the initial amount of methanol, (CH 3 OH)i with increasing irradiation dose. The CH 4 column density is estimated from the analysis of the 1305 cm −1 band; the CH 3 OH column density is estimated from the 1025 cm −1 band.
1580/1350 band ratios that are as low as about 50% of the ratio in Vinogradoff et al. (2011). This preliminary analysis gives us confidence in attributing the 1350 cm −1 band to CH 3 CHO, although dedicated studies with more sensitive techniques are needed to estimate the relative amounts of HCOO − and CH 3 CHO in irradiated mixtures, such as in situ mass spectrometry currently under implementation at LASp. For our estimations of the CH 3 CHO abundance, we took into account a 1350 cm −1 band strength of 7.11×10 −18 cm molecule −1 (Hudson & Ferrante 2020). This value is higher than the one given by Schutte et al. (1999), that is, 1.5×10 −18 cm molecule −1 , and thus allows a more conservative estimation of the CH 3 CHO abundance. We used two methods to estimate the 1350 cm −1 band area, as shown in Fig. B.2. We first subtracted a linear continuum between 1330 and 1360 cm −1 and then integrated the band intensity (Fig. B.2, panel A). This method tends to underestimate the band area because a relevant portion of it is found beneath the linear baseline that is subtracted to exclude other components. Panel A of Fig. 6 shows the results of the estimations. As in the case of H 2 CO, the irradiation of more diluted mixtures results in a higher abundance of CH 3 CHO. The irradiation of 3:1:1 mixtures up to 50 eV/16u determines the production Notes.
[gauss] refer to cross section and asymptotic values estimated from the analysis of the 1350 cm −1 band area through gaussian fit; (linear) refers to cross section and asymptotic values estimated from the analysis of the 1350 cm −1 band area after the subtraction of the linear continuum below the band (see Fig. B.2).
of about the 4% of CH 3 CHO. In 1:1:1 mixtures, the highest abundance is about 3%. We also performed multi-Gaussian fits between 1500 and 1300 cm −1 by simulating a Gaussian curve to fit the 1350 cm −1 band (Fig. B.2, panel B). This method estimates the total area of the 1350 cm −1 band from the baseline at optical depth zero. The estimated of the CH 3 CHO abundances we obtained through the multi-Gaussian fits are reported in Fig. 6, panel B. CH 3 CHO abundances reach 12% in 3:1:1 mixtures and 6% in 1:1:1 mixtures. We fitted the data in Fig. 6 with Eq.
(2) to obtain the cross sections that are given in Table 3. The values of y ∞ vary according to the method we used to estimate the 1350 cm −1 band area. Figure 6, panel C, shows the ratio of the CH 3 CHO abundance and the CH 3 OH abundance at the same dose. The exponential trend of the data has to be attributed to the formation of CH 3 CHO and the destruction of CH 3 OH.

Discussion
The reactions induced by ion irradiation in ices are due to the transfer of kinetic energy between ions and the target molecules, resulting in their excitation and ionization along the ion track (e.g. Rothard et al. 2017). Figure

Chemical changes induced by ion bombardment
The irradiation of H 2 O:CH 3 OH:NH 3 mixtures at 14-18 K determines the formation of CH 4 (Fig. 4), a compound observed in various star-forming regions and on frozen surfaces in the outer Solar System. The quantitative analyses show that cross sections and asymptotic values reported in Table 2 are comparable to those reported in literature (Islam et al. 2014). The CH 4 abundance reaches a plateau above 50 eV/16u, and at these doses, we find 4.5% of CH 4 with respect to the initial CH 3 OH. This abundance represents the highest amount of CH 4 that can form on a CH 3 OH-rich body exposed to irradiation. However, other processes could contribute to form CH 4 and thus increase its abundance with respect to CH 3 OH. In the ISM, hydrogenation of C atoms on the dust grains forms CH 4 (e.g. Allen & Robinson 1977). On frozen small bodies, higher CH 4 abundances could be attributed to the accretion of pristine CH 4 from the presolar cloud, during the bodies formation, to the irradiation of other CH 4 precursors, or to the more efficient migration of CH 4 towards the surface as it is more volatile than CH 3 OH (e.g. Fray & Schmitt 2009). We also detect H 2 CO and CH 3 CHO. The highest amount of H 2 CO with respect to CH 3 OH, that is, 6%, is observed after irradiation of 3:1:1 mixtures up to about 10 eV/16u, while only 2% of H 2 CO is found in 1:1:1 mixtures. In all mixtures, the H 2 CO abundance decreases at doses higher than 10 eV/16u, revealing that ices exposed to a higher extent of irradiation will be depleted in H 2 CO. Taking into account the most conservative approach, we find up to 4% of CH 3 CHO with respect to the initial CH 3 OH abundance. The CH 3 CHO abundance could be as high as 12% by considering the data obtained through the multi-Gaussian fit of the 1350 cm −1 band. In Fig. 7 we show the ratio of the two aldehydes, H 2 CO/CH 3 CHO as a function of the dose. Ices exposed to a low extent of irradiation show a high ratio because of the efficient formation of H 2 CO at low dose. At higher dose, the ratio decreases because H 2 CO is destroyed, while the amount of CH 3 CHO reaches a plateau. In our experiments, the formation of H 2 CO could be explained by taking the dehydrogenation of CH 3 OH induced by the impinging H + into account, in a mechanism similar to that of secondary electrons in CH 3 OH-rich mixtures (Ciaravella et al. 2010). CH 3 CHO could be formed through the reaction path reported by Bennett et al. (2005), which involves solid-phase CO and CH 4 , which are both abundant byproducts of CH 3 OH irradiation. The cleavage of a C-H bond in CH 4 would form CH 3 and a suprathermal H that would then react with CO, forming the formyl radical HCO. Neighbouring CH 3 and HCO would then recombine to form CH 3 CHO. This mechanism involving CH 4 also explains why more quantities of CH 3 CHO form during irradiation, as the CH 4 abundance also increases with the dose. We also find that both H 2 CO and CH 3 CHO are formed more efficiently in mixtures richer in H 2 O. Although dedicated studies are needed to explain the role of H 2 O in the production of aldehydes, it is possible that on one hand, the aldehyde formation is independent of the abundance of H 2 O and during irradiation aldehydes would destroy more efficiently simply because they are more highly concentrated in the samples; on the other hand, H 2 O could take part in the reactions leading to aldehydes, favouring their formation through a reaction network that includes OH radicals and H.
Aldehydes are invoked as precursors of complex organics that can form in ices exposed to ion irradiation and thermal processing in space. In particular, H 2 CO is a precursor of HMT. Due to the decreasing H 2 CO abundance that we observe at doses higher than 10 eV/16u, a lower quantity of H 2 CO would be available in the ice to form HMT during the subsequent warm-up, explaining why the dose affects the final amount of HMT in residues, as reported by Urso et al. (2020b). By warming up the frozen mixtures with a constant heating rate of 3 K min −1 , as reported in the literature for similar experiments (e.g. Urso et al. 2020b), after their irradiation up to doses well above this 10 eV/16u value, we do not detect any feature that would be attributable to HMT in the IR spectra. We thus expect astrophysical ices exposed to doses well above the threshold value of 10 eV/16u to be depleted in H 2 CO and thus they would be less likely to lead to the formation of HMT in space. estimates obtained from the integration of Gaussian curves centred at about 1350 cm −1 obtained after a multi-Gaussian fit performed in the range 1500-1300 cm −1 . Panel C: CH 3 CHO with respect to the amount of CH 3 OH during irradiation. The ratio is calculated using the abundances of CH 3 CHO obtained with the method of the integration from linear baseline.

Astrophysical implications and timescales of irradiation in space
In this section, we estimate the timescales necessary to form H 2 CO and CH 3 CHO in ices exposed to energetic H + on icy grain mantles and on frozen surfaces in the outer Solar System. To obtain the timescale of irradiation on icy grain mantles, we followed the estimations reported by Kaňuchová et al. (2016) and the approximation for the energy of cosmic ions reported by Mennella et al. (2003). Timescales of irradiation in dense molecular clouds, protostars, and protoplanetary disks can be evaluated from the measured cosmic-ray ionization rate ζ (s −1 ). For our calculation, we considered the lowest and highest values of ζ estimated in different sources in the ISM reported by Mennella et al. (2003), Podio et al. (2014), Woods et al. (2015) and Kaňuchová et al. (2016). These values vary between 1.3×10 −15 s −1 and 1.3×10 −17 s −1 . With this ζ, icy grain mantles would accumulate 10 eV/16u within 1.1×10 6 and 1.1×10 8 yr, respectively. At this dose, we find the highest abundance of H 2 CO (Fig. 5), while the CH 3 OH abundance decreases to about 40% of its initial value. Higher doses determine the destruction of H 2 CO and at 60 eV/16u its abundance drops to 1% with respect to CH 3 OH. Considering our most conservative estimates, up to 4% of CH 3 CHO relative to the initial CH 3 OH can form within ices exposed to irradiation. This abundance is reached at about 50 eV/16u, which corresponds to 5.5×10 6 -5.5×10 8 yr. These timescales are comparable to the expected lifetimes of icy mantles in dense molecular clouds and in protoplanetary disks (e.g. Greenberg 1982;Larson 2003). However, when we take the data in Fig. 5 into account, the column density of H 2 CO detected in icy grain mantles (e.g. Boogert et al. 2015) is higher than the amount that can form after CR irradiation of CH 3 OH-rich ices, suggesting that other processes contribute to the formation of solid H 2 CO in icy grain mantles, such as hydrogenation of solid CO (e.g. Watanabe & Kouchi 2002). So far, CH 3 CHO is only tentatively detected in the line of sight of the high-mass YSO W33A (Schutte et al. 1999). In the near future, this tentative detection could be confirmed by James Webb Space Telescope (JWST) observations that may also include information on the CH 3 CHO relative abundances and in turn contribute to constrain its formation route (e.g. McClure et al. 2017). During the star-formation process, icy mantles sublimate and thus inject volatile species into the gas phase (e.g. Caselli & Ceccarelli 2012). Before sublimation, frozen species can react and form new compounds, however. In particular, the reaction between H 2 CO and NH 3 is the first step of a more complex reaction leading to HMT (e.g. Bernstein et al. 1995;Vinogradoff et al. 2012b). The IR spectrum of HMT shows two intense and narrow bands at 1007 and 1234 cm −1 (Bernstein et al. 1994). Although the detection of the 1005 band would be hindered by the silicate feature at the same wavelength, the 1234 cm −1 band might be detected. However, even if the temperature would increase enough to allow the HMT formation, we estimate that the 1234 cm −1 HMT band would show an optical depth peak of about 2×10 −2 -4×10 −3 if we were to assume that 10-40% of NH 3 observed in interstellar ices would react to form HMT and if we used the spectral parameters reported by Bernstein et al. (1994Bernstein et al. ( , 1995. This value is below the detection limits of the instruments on board the JWST A169, page 7 of 10 A&A 668, A169 (2022) (Wells et al. 2015). However, the presence of HMT in the early Solar System and its incorporation in the meteoritic parent bodies could explain the presence of such compounds in carbonaceous chondrites, although also hydrothermal aqueous alteration in asteroids may contribute to the formation of HMT Oba et al. (2020).
Since their formation, frozen bodies in the outer Solar System are exposed to SEP and to CR, although the shielding of the heliosphere strongly limits the flux of CR with energies lower than a few hundred MeV (e.g. Langner et al. 2003). CH 3 OH is detected on 5145 Pholus (semimajor axis, S.A. 20.36 au), 2002 VE95 (S.A. 39.18 au), and 486958 Arrokoth (S.A. 44.58 au), but neither solid-phase H 2 CO nor CH 3 CHO are detected on frozen surfaces in the Solar System. By scaling the values reported by Strazzulla et al. (2003), who estimated the timescales of irradiation at various locations in the outer Solar System, we find that a frozen body located at 40 au accumulated about 65 eV/16u at surface depth of 1 µm and about 46 eV/16u at surface depth of 1 cm since its formation 4.6 billion years ago. Deeper layers, however, experience decreasing dose values.
The missing detection of HMT in CH 3 OH-rich bodies could be explained by the fact that IR photons only probe limited thicknesses of the surface and HMT would only form in deeper layers, where the lower dose allows its precursors to form and survive. In the outer Solar System, we estimate that energetic ions can induce the formation of the highest abundance of H 2 CO and CH 3 CHO on timescales of about 10 6 yr (Urso et al. 2020b). These timescales imply a fast and efficient conversion of CH 3 OH into its byproducts, including CH 4 and aldehydes. However, bodies in the Kuiper belt are thought to have remained largely undisturbed since their formation (e.g. Grundy et al. 2020). During their lifetime, these bodies might have accumulated doses up to 40-70 eV/16u by CR and much higher doses at smaller surface depth by SEP. These doses would destroy not only the complex organic precursors, but also volatiles species that are observed on frozen surfaces, however, including CH 3 OH (e.g. Barucci et al. 2011). Nevertheless, our analysis on the irradiation of CH 3 OH shows that its destruction reaches a plateau and that about 3% of CH 3 OH survives at 100 eV/16u. Thus, lower quantities of CH 3 OH could survive the high extent of irradiation on small bodies, and aldehydes could still form on highly irradiated surfaces. Another consideration regards the fact that IR photons that are detected by remote-sensing observations and that inform on the composition of frozen surfaces can probe larger thicknesses than the penetration depth of SEP (Urso et al. 2020a). Low-energy SEP alter a lower thickness than high-energy SEP and CR. IR photons might travel through both layers strongly altered by SEP and underlying layers that are only affected by CR and high-energy SEP. These buried layers, exposed to much lower doses, can still contain high quantities of CH 3 OH and its irradiation byproducts, such as aldehydes, which might then take part in reactions to form complex organics. Furthermore, rejuvenating processes have to be taken into account as they cause the exposure of fresh and less altered materials buried below frozen surfaces. These unprocessed ices would then undergo ion bombardment and form byproducts within the short timescales reported above.
Cometary comae also show H 2 O, CH 3 OH, NH 3 , H 2 CO, and CH 3 CHO along with various other compounds (e.g. Bockelée-Morvan et al. 2000;le Roy et al. 2015, and references therein). Oort cloud comets are exposed to high fluxes of CR that induce the formation of organic refractory materials, and Strazzulla & Johnson (1991) estimated that organic refractory materials form in the upper 1-2 m of cometary surfaces within the age of the Solar System. Deeper layers are less strongly irradiated, but about 2 eV/16u can cumulate because radioactive elements decay within the nucleus (e.g. Draganic & Draganic 1984;Modica et al. 2012).
Observational studies showed that the abundance of complex molecules relative to water in cometary comae is comparable (with differences smaller than one order of magnitude) to the abundances observed towards YSOs, supporting the hypothesis that comets inherited ices from the presolar cloud or that cometary ices were processed in the same physical and chemical ways as interstellar ices (e.g. Bockelée-Morvan et al. 2019, and references therein). Various comets show an abundance of H 2 CO that is comparable to that of CH 3 OH (e.g. le Roy et al. 2015;Schuhmann et al. 2019). Thus, other processes might contribute to the presence of H 2 CO in addition to its formation from the CH 3 OH-rich ices irradiation. On the other hand, the abundance ratio CH 3 CHO/CH 3 OH ranges between 0.01 and 0.2, which means that CH 3 CHO might form after cosmic ion irradiation of CH 3 OH-rich ices. However, CH 3 CHO has been detected only towards a limited number of comets (e.g. le Roy et al. 2015;Bockelée-Morvan et al. 2019). The capabilities of the JWST will enable future observations to contribute to better constrain the origin of complex molecules in comets as well as on other frozen bodies located in the outer Solar System.

Conclusions
We exposed H 2 O, CH 3 OH, and NH 3 to 40 and 200 keV H + to simulate the irradiation of frozen surfaces in both the ISM and in the Solar System. We followed the destruction of CH 3 OH and the formation of its byproducts CH 4 , H 2 CO, and CH 3 CHO. We find that the composition of ices depends on the irradiation dose and that the abundance of aldehydes is affected by the amount of water in mixtures. In all mixtures, the H 2 CO abundance decreased at doses higher than 10 eV/16u, thus ices exposed to a higher dose would be depleted in such a compound. In turn, lower abundances of HMT might form in the refractory organic material that is produced after irradiation and warm-up of ices due to the role of H 2 CO as a precursor of HMT.
Frozen volatile compounds such as CH 3 OH, aldehydes, and complex organics might be buried below the surface that is probed by astronomical observations. Volatile compounds might survive below surfaces and the irradiation by CR would induce the partial destruction of methanol and the formation of its byproducts, including aldehydes, which might then take part in reactions to form more complex compounds.
Further data are needed to better constrain the fate of organics after their formation, to understand how radiation affects their chemical properties, and to determine whether they can survive the harsh conditions on atmosphere-less surfaces. In this regard, exposure platforms on board space facilities allow the exposure to the full solar spectrum as well as to higher fluxes of galactic CR and solar particles (e.g. Baratta et al. 2015Baratta et al. , 2019Cottin et al. 2017;Bryson et al. 2015). The Organics Exposure in Orbit (OREOcube; Elsaesser et al. 2014) and Exocube (Sgambati et al. 2020) facilities will allow exposing various organic compounds of relevance for astrobiology as pure and in contact with mineral surfaces and gaseous compounds, allowing us to simulate the physico-chemical conditions of various astrophysical scenarios. Both platforms will allow the in situ analysis by means of UV-Visible (OREOcube) or mid-IR (Exocube) spectroscopy, allowing to monitor the sample evolution during flight. At the end of the mission, samples will be brought back to Earth allowing their further characterization. Both experiments will thus provide information about the survival of organics and about the possibility that complex molecules formed after the processing of solid-phase simple volatiles can be trapped in small bodies and delivered on planetary surfaces.