11institutetext:  Université Paris-Saclay, CNRS, Institut d’astrophysique
spatiale, 91405, Orsay, France
11email<EMAIL_ADDRESS>22institutetext: Université Grenoble Alpes, CNRS,
IPAG, Grenoble F-38000, France 33institutetext: Aix-Marseille Université,
Laboratoire de Physique des Interactions Ioniques et Moléculaires (PIIM) UMR-
CNRS 7345, F-13397 Marseille, France

# Irradiation dose affects the composition of organic refractory materials in
space:

results from laboratory analogues

R. G. Urso 11 V. Vuitton 22 G. Danger 33 L. Le Sergeant d’Hendecourt 33 L.
Flandinet 22 Z. Djouadi 11 O. Mivumbi 11 F. R. Orthous-Daunay 22 A. Ruf 33 V.
Vinogradoff 33 C. Wolters 22 R. Brunetto 11

###### Abstract

Context. Near- and mid-infrared observations have revealed the presence of
organic refractory materials in the solar system, in cometary nuclei and on
the surface of centaurs, Kuiper-belt and trans-neptunian objects. In these
astrophysical environments, organic materials can be formed because of the
interaction of frozen volatile compounds with cosmic rays, stellar/solar
particles, and favoured by thermal processing. The analysis of laboratory
analogues of such materials gives information on their properties,
complementary to observations.

Aims. We present new experiments to contribute in the understanding of the
chemical composition of organic refractory materials in space.

Methods. We bombard frozen water, methanol and ammonia mixtures with 40 keV H+
and we warm the by-products up to 300 K. The experiments allow the production
of organic residues that we analyse by means of infrared spectroscopy and by
Very High Resolution Mass Spectrometry to study their chemical composition and
their high molecular diversity, including the presence of
hexamethylenetetramine and its derivatives.

Results. We find that the accumulated irradiation dose plays a role in
determining the residue’s composition.

Conclusions. Based on the laboratory doses, we estimate the astrophysical
timescales to be short enough to induce an efficient formation of organic
refractory materials at the surface of icy bodies in the outer solar system.

###### Key Words.:

Kuiper belt: general; Astrochemistry; Astrobiology; Solid state: refractory;
Methods: laboratory: solid state;

##  1 Introduction

Astronomical observations allowed the detection of various frozen compounds on
the surface of dust grains (icy grain mantles) in the interstellar medium
(e.g. van de Hulst 1949; Tielens & Hagen 1982; Whittet et al. 1996; Caselli &
Ceccarelli 2012; Boogert et al. 2015) as well as on the surface of small
bodies in the solar system, such as comets, centaurs, and Kuiper-belt objects
(e.g. Cruikshank et al. 1998; Barucci et al. 2006; Biver et al. 2006; Altwegg
et al. 2017; Stern et al. 2019). In the early solar system, these bodies
formed thanks to the accretion of the material present in the presolar cloud,
including icy grain mantles in the outer regions. Surviving the sun-formation
process, these objects could have preserved, at least partially, information
on the composition of the presolar cloud (e.g. Biver et al. 2006; Willacy et
al. 2015; Altwegg et al. 2017; Nesvorný 2018; McKinnon et al. 2020).

During their lifetime, frozen volatiles experience both irradiation by UV
photons, cosmic-rays (CR) and stellar or solar particles, as well as heating.
Such processes determine changes in the physical and chemical properties of
ices (e.g. Tielens & Allamandola 1987; Greenberg et al. 1995; Ehrenfreund et
al. 1999; Urso et al. 2019). Frozen small bodies in the outer solar system are
exposed to CR, solar wind (SW) and solar energetic particles (SEP) (e.g.
Johnson 1990; Cooper et al. 2003; Strazzulla et al. 2003; Urso et al. 2020).
Such bodies exhibit red slopes in the visible and near-infrared (IR) spectra
that are related to the presence of a refractory C-rich material, whose
formation is attributed to the irradiation of volatiles on their surfaces
(e.g. Cruikshank et al. 1998; Barucci et al. 2006; Brown et al. 2011; Grundy
et al. 2020) or to the incorporation of red materials present in the presolar
cloud (e.g. Dalle Ore et al. 2011).

Laboratory experiments shed light on the effects induced by irradiation and
heating on solid-phase matter. The irradiation with UV photons or energetic
particles (ions or electrons) of frozen volatiles determines the break of
molecular bonds and the formation of radicals and molecular fragments that
then recombine to form new compounds (e.g. Öberg 2016; Rothard et al. 2017,
and references therein for details on the physico-chemical process). The warm-
up of processed frozen mixtures induces not only the sublimation of volatile
compounds (e.g. Collings et al. 2004; Abou Mrad et al. 2016, 2017) but also an
increase in the molecular diffusion and reactivity (Mispelaer et al. 2013;
Theulé et al. 2013). As a consequence, the chemical complexity further
increases, the initially flat and bright spectra of frozen volatiles show a
reddening in the visible and near-IR spectra (Brunetto et al. 2006; Poston et
al. 2018) and a refractory material, named organic refractory residue, is
eventually formed.

Organic refractory residues are thought to be representative of materials in
comets and at the surface of red frozen bodies in the outer solar system (e.g.
Agarwal et al. 1985; Strazzulla & Johnson 1991; Muñoz Caro & Schutte 2003;
Danger et al. 2013; Baratta et al. 2015; Fresneau et al. 2017; Urso et al.
2017; Accolla et al. 2018; Baratta et al. 2019). Laboratory IR spectra provide
relevant information on the composition of organic refractory residues,
revealing the presence of various functional groups (e.g. Palumbo et al. 2004;
Muñoz Caro et al. 2004; Vinogradoff et al. 2013; Urso et al. 2017; de
Marcellus et al. 2017). The analysis through mass spectrometry (MS) provides a
more detailed characterization of the composition of such samples. MS reveals
a high extent of molecular diversity of residues produced after UV photolysis
and subsequent warm-up of frozen volatiles, with the detection of thousands of
molecules up to 4000 Da (Danger et al. 2013; Fresneau et al. 2017; Gautier et
al. 2020) and the presence of prebiotic compounds, such as amino acids
(Bernstein et al. 2002; Nuevo et al. 2008), sugars, sugar derivatives (Meinert
et al. 2016; Nuevo et al. 2018), and nucleobases (e.g. Nuevo et al. 2009,
2012; Materese et al. 2013, 2017).

The chemistry induced by UV photons shows similarities with that induced by
energetic ions, even if with different by-products formation cross sections
(e.g. Baratta et al. 2002; Muñoz Caro et al. 2014; Rothard et al. 2017). One
of the main differences arises from the fact that whereas the UV photons
penetration is strongly affected by the ice optical constants, ions can
penetrate up to meters according to their energy and independently from the
target optical constants (e.g. Cooper et al. 2003; Strazzulla et al. 2003;
Urso et al. 2020). Previous work pointed out the existence of another
difference among the two types of irradiation, i.e. the production of
hexamethylenetetramine (C6H12N4, hereafter HMT), a precursor of compounds of
prebiotic interest (e.g. Hulett et al. 1971; Bernstein et al. 1995;
Vinogradoff et al. 2018, 2020) and HMT derivatives, i.e. HMT in which a side
group substitutes a peripheral H atom.

On the one hand, in UV residues HMT is detected through both IR spectroscopy
and mass spectrometry (e.g. Bernstein et al. 1995; Muñoz Caro et al. 2004;
Vinogradoff et al. 2013; Danger et al. 2013), up to 50% of the residue mass
(e.g. Bernstein et al. 1995; Muñoz Caro & Schutte 2003). Various works focus
on the production of HMT (e.g. Bernstein et al. 1995; Woon 2001; Muñoz Caro et
al. 2004; Vinogradoff et al. 2013; Zeffiro et al. 2016; Materese et al. 2020).
Vinogradoff et al. (2012) studied the formation of HMT after UV irradiation
and warmup of H2O:CH3OH:NH3 mixtures. In the mechanism they proposed, H2CO and
HCOOH are formed at 25 K, with HCOOH acting as a catalyst throughout the
reaction. During the warm-up, H2CO reacts with NH3 forming aminomethanol,
precursor of methylenimine. At higher temperature, methyleneimine polymerizes
as protonated trimethylenetriamine (TMT), and the further warm-up to 300 K
determines TMT cyclisation to form HMT. A similar mechanism would form HMT
derivatives, with side groups added to intermediate compounds (Materese et al.
2020).

On the other hand, IR spectra of residues produced after ion irradiation of
frozen volatiles do not show evidences of HMT and its derivatives. The missing
detection of HMT in such samples could be attributed to the non-formation of
its precursors after ion bombardment. Taking into account the results obtained
by Baratta et al. (1994) after ion-bombardment of CH3OH-rich ices, Bernstein
et al. (1995) proposed that ion irradiation favours the conversion of methanol
to acetone rather than to H2CO. Thus, the presence of HMT has been proposed as
a probe of UV photolysis of astrophysical methanol-rich ices. However, Hudson
& Moore (2000) have shown that H2CO is formed after ion irradiation of frozen
volatiles containing methanol, and GC-MS analysis revealed the presence of HMT
in residues produced after ion bombardment of H2O:CH3OH:CO:NH3 mixtures with
800 keV H+ of up to a dose of 25 eV molecule-1 (Cottin et al. 2001).

In this work, we use in-situ IR spectroscopy to characterize H2O:CH3OH:NH3
mixtures deposited at 15 K, bombarded with 40 keV H+ and warmed up to room
temperature in order to produce organic refractory residues. Such mixtures and
the source of processing aim to simulate, at our best, the irradiation of
frozen surfaces in the outer solar system by Solar Energetic Particles (SEP),
that have a primary role in the processing of such surfaces (Urso et al.
2020), as well as the heating that surfaces experience during their lifetime.
We use Very High Resolution Mass Spectrometry (VHRMS) to characterize the
composition of residues and to point out eventual variations in the chemical
composition that are related to the irradiation dose given to the original
frozen mixtures. This information is crucial because in space, the dose
accumulated by a frozen surface is linked to the timescale of its exposure to
energetic particles. Thus, to understand how irradiation dose affects the
composition of organic refractory materials can shed light on the timescale of
irradiation of an outer body surface. Finally, we report on dedicated analysis
performed by means of tandem mass spectrometry (MS/HRMS) to reveal the
presence of HMT and its derivatives in organic refractory residues, and we
investigate on the effects of irradiation dose in the production of such
compounds.

##  2 Experimental methods

Experiments were performed with the IrradiatioN de Glaces et Météorites
Analysées par Réflectance VIS-IR (INGMAR Lantz et al. 2017) setup at IAS-
IJCLab (Orsay, France). Water, methanol and ammonia gaseous mixtures are
prepared in a mixing chamber (P$\leq$10-4 mbar) and are then injected in the
main vacuum chamber (P$\sim$5$\times$10-8 mbar) that hosts a ZnSe substrate in
thermal contact with the cold finger of a closed-cycle He cryocooler (CTI,
14-300 K). A 5 cm long copper tube placed on the backside of the sample holder
prevents the deposition on the other side of the substrate. After deposition,
frozen mixtures are bombarded with 40 keV H+ produced by the SIDONIE ion
accelerator (IJCLab, Orsay, Chauvin et al. 2004). The ion beam arrives on
samples at an angle of 10∘ with respect to the surface normal. The rastering
of the beam ensures a homogeneous covering of the sample surfaces. During
bombardment we integrate the ion current to estimate the fluence (ions cm-2).
The ion current density is kept below 800 nA cm-2 to avoid the macroscopic
heating of the samples. We then multiply the fluence by the stopping power in
eV cm2/16 u calculated with the SRIM software (Ziegler et al. 2008) to obtain
the dose, i.e. the energy deposited per unit volume in the sample by incident
radiation, in eV/16 u where u is the unified atomic mass unit (e.g. Strazzulla
& Johnson 1991).

In our experiments, we produce three residues by bombarding frozen H2O, CH3OH,
and NH3 mixtures, varying the irradiation dose. In order to ensure a uniform
irradiation of the whole film thickness, we deposit frozen films whose
thickness (about 400 nm) is lower than the penetration depth of 40 keV H+
111According to calculations with the SRIM software (Ziegler et al. 2008), 40
keV H+ can penetrate up to 800 nm in a H2O:CH3OH:NH3=1:1:1 mixture assuming a
density of 0.8 g cm-3, and H implantation is not negligible above 600 nm.. To
produce thick residues, we perform a multi-deposition and subsequent
irradiation procedure (Urso et al. 2020). After the deposition and irradiation
of the first film, we deposit a second film of the same composition that is in
turn irradiated up to the same dose of the first film. For each sample, this
procedure is repeated three to four times. After irradiation, samples are
warmed up to room temperature with a constant heating rate of 3 K min-1.
Organic refractory residues are obtained using the mixture and irradiation
doses reported in Table 1.

Table 1:  Frozen mixture ratio and irradiation dose of 40 keV H+ used to produce the samples analyzed in this work. Mixture at 15 K | Dose | Sample | Analysis
---|---|---|---
H2O:CH3OH:NH3 | (eV/16 u) | name |
1:0:0 | 32 | blank | FT-IR, VHRMS
1:1:0 | 85 | 1:1:0+85 eV/16 u | FT-IR
1:1:1 | 29 | 1:1:1+29 eV/16 u | FT-IR, VHRMS
3:1:1 | 67 | 1:1:1+67 eV/16 u | FT-IR, VHRMS
1:1:1 | 98 | 1:1:1+98 eV/16 u | FT-IR, VHRMS

Throughout the experiment we perform in-situ Fourier-Transform IR
spectroscopy. The IR beam from the internal source of the spectrometer is
directed to the vacuum chamber thanks to mirrors on an optical bench, and the
IR beam enters the chamber through a ZnSe window. The incident beam arrives on
the sample with an angle of 10∘ with respect to the surface normal and is then
collected by an MCT detector placed on the other side of the vacuum chamber.
IR spectra are acquired with a resolution of 1 cm-1. Further information about
the experimental setup are given in Lantz et al. (2017) and Urso et al.
(2020). The residue on the ZnSe substrate is then removed from the HV chamber
and stored in a dedicated stainless steel sample holder under static vacuum.
Prior to the VHRMS characterization, two residues, the 1:1:1+29 eV/16 u and
the 3:1:1+67 eV/16 u are stored for 14 and 21 days, respectively, while the
1:1:1+98 eV/16 u is stored for 240 days.

VHRMS is performed with a linear trap/orbitrap mass spectrometer (LTQ Orbitrap
XL, ThermoFisher) at Institut de Planétologie et d’Astrophysique de Grenoble
(IPAG, Grenoble, France). Residues are recovered by rinsing five times with 50
$\mu$L of ultrapure methanol. We then collect 50 $\mu$L from the resulting
solution and further dilute it in 250 $\mu$L of ultra-pure methanol. In order
to avoid any degradation of the samples, VHRMS is performed immediately after
the collection of each residue from the substrate. After the rinsing, we
analyse the ZnSe substrates by means of FT-IR, to verify whether all residues
are efficiently collected. In one case only, i.e. the 1:1:1+98 eV/16 u
residue, we observe very weak contributions at about 3200 and 1650 cm-1.
However, the area of the 3200 cm-1 feature is found to be only about the 3% of
the area of the same feature in the residue. This minor contribution could be
due to insoluble compounds or to a minor quantity of solution left on the
substrate during the residue collection, because of the fast evaporation of
methanol. The solution is injected in the electrospray ionization source (ESI)
by means of a Hamilton gastight syringe with a rate of a 3 $\mu$L min-1. ESI
has the advantage to limit the fragmentation of molecules, allowing their
detection as [M+H]+ in positive ionization mode or [M-H]- in negative
ionization mode. In the Orbitrap, charged species are accumulated in a
quadrupolar ion trap and are then transferred into the Orbitrap analyzer.
Here, ions oscillate between two side electrodes and around a central
electrode and produce a periodic signal that is converted into a mass spectrum
using Fourier transform. For each sample, we acquired data in three m/z
ranges: 50-300, 150-400 and 350-950 m/z, with a resolving power $M/\Delta
M$=105 at m/z 400. The instrument parameters are listed in the following:
spray voltage of 3.5 kV, capillary temperature of 275 ∘C, capillary voltage 35
V, tube lens voltage 50 and 70 V and -50 and -70 V in positive and negative
ionization modes, respectively. Spectra are acquired with 4 scans, each
consisting of 128 micro-scans (for details on the choice of the number of
scans and micro-scans, see Wolters et al. 2020). Prior to the samples
analysis, the instrument is calibrated with a mixture of L-methionine-arginyl-
phenylalanyl-alanine, caffeine and ultramark 1621 for positive ESI mode,
sodium dodecyl sulfate, sodium taurocholate and ultramark 1621 for negative
ESI mode. The data reduction and analysis are based on the method reported by
Danger et al. (2013, 2016) and Fresneau et al. (2017), and it allows to get
rid of contamination and to attribute each m/z peak to a stoichiometric
formula CcHhNnOo. The Orbitrap resolution and accuracy allows to precisely
assign stoichiometric formulas up to m/z=400. We also analyse a ZnSe substrate
on which we deposited pure water ice that was then bombarded with 40 keV H+ at
15 K (blank experiment). The ion irradiation of pure water determines the
formation of hydrogen peroxide (e.g. Moore & Hudson 2000; Urso et al. 2018),
that sublimates, together with water, during the further warm-up to room
temperature. Due to the absence of methanol and ammonia, the eventual
detection of C- and N-bearing compounds would be due to contamination on the
substrate or in the experimental setup. Thus, all the molecular ions detected
in this sample are removed from the m/z spectra of residues, and they are not
taken into account for the analysis.

Each molecular ion in the range 50-400 m/z is attributed to the parent
molecule by adding a proton in the analysis in negative ESI and by subtracting
a proton in the analysis in positive ESI mode. After attributions, we also
calculate elemental abundances as unweighted averages, given by ${\sum
x}/{\sum C}$ where x is the number of nitrogen, oxygen or hydrogen and C is
the number of carbon for each stoichiometric formula, and weighted averages,
calculated as ${\sum x\times I}/{\sum C\times I}$ where x is the number of
nitrogen, oxygen or hydrogen, C is the number of carbon and I is the intensity
of the related m/z peak. In fact, even if the intensity of m/z peaks is
affected by variations of the ionization yield (e.g. Danger et al. 2016), due
to the fact that the intensity is proportional to the concentration of the
related species (Hockaday 2009), weighted averages give an estimation of the
elemental abundances in samples. To have information about the structure of
compounds in samples, we calculate the Double Bond Equivalent (DBE), i.e. the
degree of unsaturation within the sample, though the formula
$DBE=C-\frac{H}{2}+\frac{N}{2}+1$, where C, H and N are the number of carbon,
hydrogen and nitrogen atoms in each formula. DBE are then weighted to the
intensity of the m/z peak.

Tandem mass spectrometry (MS/HRMS) is also performed on the residue solutions,
using helium as the activation gas in the LTQ ion trap and final mass analysis
being performed in the Orbitrap. The normalized collision energy is set to 20%
(the absolute collisional energy is not known) with an activation time of 30
ms and an isolation window of 1 Da.

In this work, we use van Krevelen plots (Van Krevelen 1950) to obtain
information on the composition of our samples. van Krevelen plots are atomic
ratio plots that allow a screening of samples with respect to chemical
families using H/C, O/C and N/C ratios. Specific zones in the diagrams are
related to specific chemical functions and structures, and they have been used
to characterize organic matter in meteorites (e.g. Schmitt-Kopplin et al.
2010) as well as organic refractory residues obtained after UV and ion
irradiation of frozen volatiles (e.g. Gautier et al. 2014; Danger et al. 2016;
Ruf et al. 2019; Gautier et al. 2020).

##  3 Results

###  3.1 Infrared spectroscopy

Figure 1: Infrared spectra acquired after irradiation of H2O:CH3OH:NH3
mixtures with 40 keV H+ at 15 K (left panels) and after warm-up to 300 K
(right panels). Panel A and B: H2O:CH3OH:NH3=1:1:1 + 29 eV/16 u; panel C and
D: H2O:CH3OH:NH3=3:1:1 + 67 eV/16; panel E and F: H2O:CH3OH:NH3=1:1:1 + 98
eV/16 u. Spectra in panels B and D are obtained after the smoothing of the raw
spectra (grey lines). Figure 2: IR spectra in the range 1300-900 cm-1 of
residues acquired after 30 minutes at 300 K. The spectra of the 1:1:1+29 eV/16
u and of the 3:1:1+67 eV/16 u residues are obtained after the smoothing of the
acquired spectra (grey lines in the background). For comparison, we show the
spectrum of a 1:1:0+85.2 eV/16 u residue. We do not show spectra between 1253
and 1274 cm-1 because of the presence of spikes due to not well compensated
water vapour. Vertical dashed lines show the positions of pure HMT bands
reported by Bernstein et al. (1995). Black horizontal lines show the range of
HMT and HMT-derivatives bands according to ab-initio calculations by Bera et
al. (2019) and Materese et al. (2020).

The IR spectra acquired during the experiments are shown in Fig. 1, where left
panels show the spectra of the first deposited films after the irradiation
with 40 keV H+ at 15 K. In Table 4 we list the main bands observed in the
three samples, the associated vibrational mode and the possible carriers
identified in similar experiments reported in literature. In the spectra we
identify CO2 (2342 cm-1) and CO (2140 cm-1), whose intensities are found to be
higher with increasing the dose given to samples. Their production, as well as
that of CH4, H2CO, and C2H6 that are also detected in the spectra, is due to
the recombination of unstable by-products obtained after the destruction of
CH3OH. Also N-bearing species are formed, and their production involves the
destruction of NH3 and the formation of NH${}_{2}^{\bullet}$, such as in the
case of OCN-, NH${}_{4}^{+}$, HNCO and NH2HCO (Chen et al. 2007; Jheeta et al.
2013). The presence of HCOO- is attributed to the reaction between formic acid
and ammonia, thus HCOO- could be an indirect evidence of the presence of HCOOH
(Vinogradoff et al. 2011), that in turn forms thanks to the recombination
between HCO and OH radicals (Hudson & Moore 1999). Spectra also reveal the
presence of H2O, CH3OH and NH3, that are not completely destroyed after
irradiation.

During the warm-up of irradiated samples we observe evident changes in the
spectra, and at room temperature organic refractory residues are left on the
substrates. After the arrival at 300 K, we note that spectra show slight
variations, implying that residues undergo further modifications. We thus
analyse the residues spectra acquired after about 30 minutes at 300 K (Fig. 1
panels B, D and F). Due to the low thickness of residues, the signal-to-noise
ratio is low in the spectra of the 1:1:1+29 eV/16 u and of the 3:1:1+67 eV/16
u residues. We thus perform a smoothing procedure to allow a better
visualization of the spectra. The position of the main bands and the
associated vibrational modes are reported in Table 5. In all residues, N-H,
O-H and C-H stretching modes are observed between 3500 and 2500 cm-1 and
several bands are present between 1500 and 1250 cm-1. The analysis of the
spectra reveals the presence of bands associated to various chemical
functions. Among them, alcohols and carboxilic acids ($\sim$3300 cm-1),
nitriles ($\sim$2215 cm-1), esters (1740-1720 cm-1), amides ($\sim$1680 cm-1),
alkenes, imines, and aromatics ($\sim$1590 cm-1). NH${}_{4}^{+}$ signatures
could be present between 2900 and 2600 cm-1. NH${}_{4}^{+}$ could be the
counter-ion of CN-, OCN- whose features are detected at about 2200 cm-1, as
well as of HCOO-. Even if such salts would desorb at lower temperature, their
presence at 300 K could be due to the trapping in the residue complex matrix,
as observed for other volatile species (e.g. Urso et al. 2017). At lower
wavenumbers, residues show bands peaked at 1234-1231 cm-1 and 1078-1056 cm-1,
due to CO and CN stretching modes. These bands are not symmetric, implying the
presence of various subcomponents. The spectra between 1300 and 900 cm-1 are
shown in Fig. 2. In the Figure, we show the position of the CN stretching and
asymmetric CN stretching mode bands of pure HMT in Argon matrix given by
Bernstein et al. (1995) and the range of HMT and HMT derivatives bands
calculated by Materese et al. (2020) and Bera et al. (2019) (black horizontal
lines). The 1:1:1+29 eV/16 u residue clearly shows a shoulder at about 1010
cm-1, where pure HMT exhibits its most intense band. Even if such bands could
be an evidence of HMT in our residues, we note the presence of bands in the
same wavelength range in a NH3-free residue produced after irradiation of a
H2O:CH3OH=1:1 mixture with 40 keV H+ up to a final dose of 85 eV/16 u (green
spectrum in Fig. 2). Ammonia, or at least a source of nitrogen, is needed to
produce HMT (Bernstein et al. 1995; Vinogradoff et al. 2012), thus it cannot
be present in such residue. The presence of these bands in the nitrogen-free
residue hinders any conclusion on the spectroscopic evidence of HMT in our
samples.

###  3.2 Very-High Resolution Mass Spectrometry

Figure 3: Mass spectra in positive (top panels) and negative (bottom panels)
ESI modes of residues synthesised after ion bombardment and warm up of
H2O:CH3OH:NH3 frozen mixtures. Panels A and B: 1:1:1 + 29 eV/16 u residue.
Panel C and D: 3:1:1 + 67 eV/16 u residue. Panel E and F: 1:1:1 + 98 eV/16 u.

In Fig. 3 we show the mass spectra of the three residues in the range 50-950
m/z in both positive and negative ESI modes. Each spectrum is a composite
spectrum obtained by merging spectra acquired in three m/z ranges, 50-300,
150-400 and 350-950 m/z. In the figure, each colour is associated to the m/z
range and to the ESI ionization mode. In positive ESI modes (top panels in
Fig. 3) peaks are observed up to 550-650 m/z. In negative ESI modes (bottom
panels in Fig. 3) peaks are present in the whole investigated range, but the
signals beyond 600-700 m/z could be attributed to instrumental noise.

Table 2:  Elemental abundance ratios in organic refractory residues produced
after ion bombardment and warm-up of H2O:CH3OH:NH3 mixtures analysed in both
positive (p) and negative (n) ESI modes. We estimate Relative Standard
Deviation of 6% for N/C, 12% for O/C, 3% for H/C. Range 50-300 m/z
---
Sample | ESI mode |  | Unweighted average |  |  |  | Weighted average |
|  | N/C | O/C | H/C |  | N/C | O/C | H/C
1:1:1 | p | 0.34 | 0.30 | 1.73 |  | 0.38 | 0.27 | 1.81
+29 eV/16 u | n | 0.24 | 0.60 | 1.59 |  | 0.14 | 0.87 | 1.73
| average | 0.29 | 0.45 | 1.66 |  | 0.26 | 0.57 | 1.77
3:1:1 | p | 0.39 | 0.29 | 1.74 |  | 0.41 | 0.29 | 1.78
+67 eV/16 u | n | 0.30 | 0.61 | 1.57 |  | 0.19 | 0.88 | 1.71
| average | 0.35 | 0.45 | 1.66 |  | 0.30 | 0.58 | 1.74
1:1:1 | p | 0.41 | 0.29 | 1.71 |  | 0.43 | 0.29 | 1.75
+98 eV/16 u | n | 0.32 | 0.57 | 1.47 |  | 0.30 | 0.67 | 1.51
| average | 0.36 | 0.43 | 1.59 |  | 0.37 | 0.48 | 1.63
Range 150-400 m/z
Sample | ESI mode |  | Unweighted average |  |  |  | Weighted average |
|  | N/C | O/C | H/C |  | N/C | O/C | H/C
1:1:1 | p | 0.31 | 0.31 | 1.70 |  | 0.33 | 0.31 | 1.74
+29 eV/16 u | n | 0.27 | 0.50 | 1.63 |  | 0.27 | 0.53 | 1.64
| average | 0.29 | 0.41 | 1.67 |  | 0.30 | 0.42 | 1.69
3:1:1 | p | 0.37 | 0.30 | 1.69 |  | 0.38 | 0.29 | 1.71
+67 eV/16 u | n | 0.35 | 0.49 | 1.62 |  | 0.35 | 0.51 | 1.62
| average | 0.36 | 0.40 | 1.65 |  | 0.36 | 0.40 | 1.66
1:1:1 | p | 0.38 | 0.30 | 1.62 |  | 0.39 | 0.29 | 1.65
+98 eV/16 u | n | 0.35 | 0.48 | 1.48 |  | 0.35 | 0.50 | 1.49
| average | 0.36 | 0.39 | 1.55 |  | 0.37 | 0.40 | 1.57

We calculate the H, C, O and N elemental abundances in each residue. Table 2
reports the N/C, O/C and H/C measured in both positive and negative ESI modes
and their average values. We observe that N/C ratios increase as the
irradiation dose given to the frozen mixtures increases, while H/C ratios
follow the opposite behaviour. The O/C ratio does not show a well defined
trend, even if its values are lower in the sample produced with the highest
irradiation dose (1:1:1+98 eV/16 u). It is interesting to note that in UV
residues produced after irradiation up to about 10 eV/16 u, using the average
of both positive and negative ESI modes in the range 50-400 m/z (analogously
to our analysis), Danger et al. (2016) calculate H/C=1.8 and N/C=0.2. The
higher H/C and lower N/C ratios with respect to our samples are thus explained
by the lower irradiation dose used for their production. In Fig. 4 we plot the
H/C and N/C ratios with respect to the dose given to the frozen mixtures,
including also the values given by Danger et al. (2016) for UV residues. For
both UV residues and our samples, the same warm-up rate has been used, i.e. 3
K min${}^{-}1$.

Figure 4: H/C and N/C ratios in organic refractory residues produced after
irradiation with 40 keV H+. Gray rhombus give the average elemental ratios of
UV residues analysed by means of VHRMS in both positive and negative electron
spry ESI modes in the range 50-400 m/z by Danger et al. (2016). The bars refer
to the relative standard deviation (RSD) given by Fresneau et al. (2017), i.e.
3% for H/C and 6% for N/C values. Top x-axes give the timescale necessary for
a frozen surface (50 $\mu$m thick) to accumulate the dose given in the bottom
x-axes. To calculate timescales, we take into account four cases: bombardment
by Solar Energetic Particles (SEP) on a surface located at 40 AU; bombardment
by cosmic rays (CR) for a surface at 40 AU; bombardment by CR on a surface at
85 AU, bombardment by CR on a surface in the Local Interstellar Medium (LISM),
i.e. beyond 100 AU. Timescales do not take into account the time needed for a
surface to experience the heating necessary for the production of organic
refractory materials.  Figure 5: Weighted abundance of CHO, CHN and CHNO
molecular groups in organic refractory residues. Panel A: 1:1:1+29 eV/16 u
residue; panel B: 3:1:1+67 eV/16 u residue; panel C: 1:1:1+98 eV/16 u residue.
Red and dark-red columns refer to the analysis in positive ESI mode in the
50-300 and 150-400 m/z, respectively, while light blue and blue refer to the
analysis in negative ESI mode in the 50-300 and 150-400 m/z range,
respectively. Figure 6: van Krevelen scatter plots of hydrogen to carbon
versus oxygen to carbon (left panels) and versus nitrogen to carbon (right
panels) in the three residues analysed in positive ESI mode in the 50-300 m/z
range (red dots) and 150-400 m/z range (dark-red dots). Panels A and B:
1:1:1+29 eV/16 u residue; panels C and D: 3:1:1+67 eV/16 u residue; panels E
and F: 1:1:1+98 eV/16 u residue. The size of the dots is given by the
normalized intensity. Gray-dashed lines delimit the zone of highest density of
ions observed in the residues analysed by Danger et al. (2016). Arrows show
the H/C, O/C and N/C weighted averages reported in Table 2 for the range
50-300 m/z (red arrows) and 150-400 m/z (dark-red arrows). Figure 7: van
Krevelen scatter plots of hydrogen to carbon versus oxygen to carbon (left
panels) and versus nitrogen to carbon (right panels) in the three residues
analysed in negative ESI mode in the mass ranges 50-300 (light-blue dots) and
150-400 (blue dots). Panels A and B: 1:1:1+29 eV/16 u residue; panels C and D:
3:1:1+67 eV/16 u residue; panels E and F: 1:1:1+98 eV/16 u residue. The size
of the dots is given by the normalized intensity. Gray-dashed lines delimit
the zone of highest density of ions observed in the residues analysed by
Danger et al. (2016). Arrows show the H/C, O/C and N/C weighted averages
reported in Table 2 for the range 50-300 m/z (cyan arrows) and 150-400 m/z
(blue arrows).

All residues show the presence of three groups of molecules, namely CHO, CHN
and CHNO. We calculate the weighted abundances of each group by dividing the
sum of attribution intensities of a group by the total intensity of the
dataset (Danger et al. 2016), assuming that the ionization yield is constant
for each ion. For each residue, Fig. 5 shows the intensity of each group in
the two ESI modes and in the two m/z ranges we investigate, i.e. 50-300 and
150-400 m/z, ranges where VHRMS allows an exact attribution of stoichiometric
formulas. CHNOs are revealed in both positive and negative ESI mode, while
CHOs and CHNs are detected in negative and positive ESI modes, respectively.
In the 1:1:1+98 eV/16 u residue, the CHNO group reaches the 94% in abundance,
followed by CHO (5.5%) and CHN (0.3%). CHNOs dominate also in the 1:1:1+29
eV/16 u residue, where they account for the 79% of the intensity of the whole
dataset. In this sample, the CHO group has an abundance of about 17%. CHOs are
abundant in the 50-300 m/z range, while CHNs account for the 4% of the dataset
intensity. In the 3:1:1+67 eV/16 u residue, CHNOs account for the 82% in
abundance, followed by CHOs (about 15%) and CHNs (slightly less than 4%).

In Fig. 6 and Fig. 7 we show van Krevelen plots of H/C versus O/C and N/C
ratios. Plots include all the data in the ranges 50-300 and 150-400 m/z. Data
at abscissa zero in the H/C versus O/C and H/C versus N/C diagrams belong to
molecules that do not contain oxygen or nitrogen atoms, thus CHN and CHO,
respectively. In the figure, the size of the dots is given by the intensity of
each peak that is normalized with respect to the most intense peak in each m/z
spectrum. For comparison, grey polygons show the areas with the highest
density of ions observed in residues produced after UV photolysis and warm-up
of H2O:CH3OH:NH3 mixtures reported by Danger et al. (2016), that use VHRSM in
both positive and negative ESI modes to analyse such samples. Plots reveal a
good match between our samples and UV residues. In Fig. 10 and 12 we show the
number of oxygen and nitrogen atoms for each formula with respect to the m/z
ratio. The distribution of data in Van Krevelen plots suggest that samples
show a homogeneous composition, and no difference attributable to the
irradiation dose are displayed.

In Fig. 8 we show the weighted DBE with respect to O/C and N/C ratios
calculated in both positive (p) and negative (n) ESI modes and in the range
150-400 m/z, a similar range to that used by Fresneau et al. (2017) to measure
the DBE of UV residues produced after irradiation of H2O:CH3OH:NH3 mixtures
analysed through VHRMS in both ESI modes, i.e. 200-400 m/z. In the figure, we
also show the average of positive and negative ESI modes. Averaged DBE vary
from 4.1 for the 1:1:1+29 eV/16 u to 4.4 for the 3:1:1+67 eV/16 u and 5.1 for
the 1:1:1+98 eV/16 u. Irradiation dose seems to affect DBE values. In
particular, DBE increases with increasing dose. We also calculate the
unweighted DBE, thus following the same method used by Fresneau et al. (2017)
in the estimation of the DBE of UV residues. Unweighted DBE also increase with
increasing the irradiation dose. In fact, we obtain values of 4.54 for the
1:1:1+29 eV/16 u, 4.81 for the 3:1:1+67 eV/16 u and 5.52 for the 1:1:1+98
eV/16 u. Such values are consistent with those estimated by (Fresneau et al.
2017) in UV residues. For both DBE and elemental ratios, we estimate relative
standard deviation (RSD) taking into account the values reported by Fresneau
et al. (2017), i.e. 6% for N/C, 12% for O/C, 3% for H/C, and 2% for DBE. We
note that we do not reveal differences induced by the higher amount of water
in the 3:1:1 + 67 eV/16 u samples in van Krevelen plots, elemental abundances,
and DBE.

Figure 8: Double Bond Equivalent (DBE) versus O/C (left panel) and N/C (right
panel) measured in the organic refractory residues produced after ion
bombardment and warm-up of H2O:CH3OH:NH3 mixtures in the range 150-400 m/z.
”p” and ”n” indicate the DBE values estimated in positive and negative mode,
respectively, while squares, circles and triangles give the average values.
Gray arrows indicate the evolution of DBE with increasing irradiation dose. We
estimate Relative Standard Deviations of 2% for DBE, 12% for O/C and 6% for
N/C. Figure 9: Fragment pattern of peaks at 141.113 (left panels) and 155.129
m/z (right panels) of: Panels A and B: commercial HMT; panels C and D:
1:1:1+29 eV/16 u residue; panels E and F: 3:1:1+67 eV/16 u residue. For each
fragment, labels indicate the m/z value, the intensity with respect to the
most intense fragment and the stoichiometric formula of the detected molecular
ion M+H+.

Further inspections of the mass spectra acquired in ESI positive mode reveal
the presence of intense peaks at 155.129 m/z in all samples. We also detect
peaks at 141.113 m/z. These peaks are attributed to compounds with the same
stoichiometric formula of protonated HMT-CH3 (Danger et al. 2013) and of
protonated HMT. Our residues also display peaks at 185.140 m/z, also detected
in UV residues and attributed to HMT-C2H5O or its isomers (Muñoz Caro et al.
2004; Danger et al. 2013). The presence of these m/z peaks triggered us to
investigate on the presence of HMT and its derivatives in our samples,
especially because HMT has been revealed only once in residues produced after
ion irradiation thanks to dedicated analysis reported by Cottin et al. (2001)
and no information is given on how the HMT abundance varies with irradiation
dose. We thus performed further analysis by means of tandem MS/HRMS in order
to obtain the fragment pattern of both m/z peaks by collision induced
dissociation (CID) in the 1:1:1 +29 eV/16 u and in the 3:1:1+67 eV/16 u
residues, and we compare the fragment pattern of the 141.113 peaks identified
in residues with the fragment pattern of the 141.113 peak of pure commercial
HMT. Our HMT sample also displays a peak at 155.129 m/z, thus we also have a
fragment pattern of reference for the 155.129 m/z peaks. Furthermore, we
compare our fragment pattern with those reported by Danger et al. (2013) that
used VHRMS and MS/HRMS for the same purpose in UV residues (see Table 2 in
their paper). The results of the analysis are shown in Fig. 9. The intensities
of the fragments of the 141.113 m/z molecular ions are normalized to the
intensities of the 98.071 m/z peaks, while the intensities of the fragments of
the 155.129 m/z molecular ions are normalized to the intensities of the
112.087 m/z peaks (Danger et al. 2013). All the molecular fragments identified
in the MS/HRMS analysis of commercial HMT are revealed in both residues.
Furthermore, the relative intensity of each m/z peaks we identify in residues
is similar to the relative intensity of the same fragment identified in the
commercial HMT and in the residues analyzed by Danger et al. (2013). We note
that in the m/z spectra acquired in ESI positive mode, the intensity of m/z
peaks associated to HMT and its derivatives decreases with irradiation dose as
shown in Fig. 14.

Other peaks in the m/z spectra shown in Fig. 9 are due to the presence of
molecular ions with m/z 141 and 155 selected in the linear ion trap
(resolution of 1 m/z) before the CID and the transfer of the fragments to the
FT-orbitrap. In the two m/z ranges, we find peaks at 141.102 and 155.081 m/z
whose intensity is higher than that of the 141.113 and 155.129 m/z peaks,
respectively. The 141.102 (attributed to C7H13N2O+) and 155.081 (attributed to
C7H11N2O${}_{2}^{+}$) m/z peaks are present also in the mass spectra of the
residues shown in Fig. 3, even if with a lower intensity with respect to the
141.113 and 155.129 m/z peaks.

Table 3: Intensity of m/z peaks of isomers of amino acids and nucleobases identified in residues in the range 50-300 m/z in ESI negative mode. Intensities are normalized to the most intense peak in m/z spectra acquired in the range 50-300 m/z in ESI negative mode, i.e. 105.019 m/z assigned to C3H5O${}_{4}^{-}$. Isomers of (from top to bottom): histidine, glutamine, threonine, proline, aminoisobutanoic acid, alanine, glycine, thymine, cytosine, uracil. Stoichiometric formula | m/z peaks intensity
---|---
| 1:1:1 + 29 eV/16 u | 3:1:1 + 67eV/16 u | 1:1:1 + 98 eV/16 u
C6H9N3O2 | 0.24 | 0.43 | 5.8
C5H10N2O3 | 0.42 | 0.60 | 3.73
C4H9NO3 | 0.41 | 0.52 | 3.27
C5H9NO2 | 0.44 | 0.28 | 2.03
C4H9NO2 | 0.12 | 0.13 | 1.51
C3H7NO2 | 0.22 | 0.27 | 3.16
C2H5NO2 | 0.12 | 0.38 | 2.89
C5H6N2O2 | 1.07 | 1.30 | 14.63
C4H5N3O | 0.11 | 0.35 | 3.31
C4H4N2O2 | 0.23 | 0.57 | 5.91

The analysis of m/z spectra reveals that other prebiotic compounds could be
present in our samples. Danger et al. (2013) give a list of tentatively
detected amino acids in residues produced after UV photolysis of
H2O:13CH3OH:NH3 mixtures. We search for the same stoichiometric formulas in
our residues, and we identify the m/z peaks of molecular ions attributed to
the following amino acids or their isomers: histidine (C6H9N3O2, ESI - 154.062
m/z, ESI + 156.077 m/z), glutamine (C5H10N2O3, ESI - 145.062 m/z, ESI+ 147.076
m/z), threonine (C4H9NO3, ESI- 118.051 m/z, ESI+ 120.065m/z), proline
(C5H9NO2, ESI- 114.056 m/z), amino isobutanoic acid (C4H9NO2, ESI- 102.056
m/z, ESI+ 104.071 m/z), alanine (C3H7NO2, ESI- 88.0404 m/z), and glycine
(C2H5NO2, ESI- 74.0247 m/z). We also identify m/z peaks attributed to
pyrimidine nucleobases or their isomers, i.e. uracil (C4H4N2O2, ESI - 111.020
m/z), cytosine (C4H5N3O, ESI- 110.036, ESI+112.051 m/z) and thymine (C5H6N2O2,
ESI- 125.036 ). None of the residues here analysed show stoichiometric
formulas of purine nucleobases, adenine and guanine. Table 3 reports the
relative intensity with respect to the most intense m/z peak in each spectrum
(i.e. C3H5O${}_{4}^{-}$, 105.019 m/z) of tentatively detected amino acids and
nucleobases stoichiometric formulas. We note that the relative intensity of
these m/z peaks increases with the irradiation dose given to the frozen
mixture. However, despite the procedure we performed to get rid of
contamination described in Sect. 2, we cannot exclude the presence of
contaminants that contribute to the detected signals. Further analysis are
needed to investigate the presence and the dependence on the irradiation dose
of prebiotic compounds using both isotopic labelling and MS/HRMS analysis of
the fragmentation pattern.

##  4 Discussion

###  4.1 The composition of organic refractory residues and the effects of
dose

The analysis here reported gives information on the chemical composition of
organic refractory materials produced after ion irradiation of frozen
volatiles. By means of IR spectroscopy, we identify various bands attributed
to several functional groups, as reported in Table 5. The high molecular
diversity of such samples is better highlighted through VHRMS. This technique
reveals the presence of thousands of molecular ions in each sample and three
diverse group of molecules, namely CHO, CHN and CHNO, with the latter being
the most abundant (up to 94% of all attributions in the 50-400 m/z range).

In Table 2 we calculate the elemental abundances as H/C, N/C and O/C, and we
observe variations that we relate to the irradiation dose given to the
pristine frozen mixture. In Fig. 4, H/C and N/C ratios are also plotted with
respect to the irradiation dose given to the frozen mixtures. We observe that
the H/C ratio decreases while the N/C ratio increases with increasing the
dose. Taking into account the estimation reported by Strazzulla et al. (2003),
we also show the timescales needed to accumulate doses that affect elemental
abundances at different heliocentric distances through irradiation by CR and
SEP within the upper 50 $\mu$m of a frozen surface. Such timescales are given
for SEP and CR at 40 AU and for CR at the termination shock (85 AU) and in the
Local Interstellar Medium (LISM).

The decrease of the H/C with respect to the dose has been reported in the case
of the ion irradiation of benzene by Strazzulla & Baratta (1992), due to the
loss of H2 and a progressive carbonization during irradiation. The loss of
hydrogen has also been observed in hydrogenated amorphous carbon exposed to
irradiation (Maté et al. 2014). N and C elemental abundances of residues
produced after ion irradiation have been reported by Accolla et al. (2018).
Even though they used different initial mixtures (with initial N/C ratios
between 0.25 and 2 with respect to our initial N/C=1), taking into account the
data reported in Table 2 in Accolla et al. (2018), they observe an increase of
the N/C ratio with increasing dose. Together with H/C and N/C ratios, also DBE
show variations related to the dose given to the frozen mixture, and DBE
increases with increasing dose. Elemental abundances, and consequently DBE,
could be also affected by changes in the chemical composition of residues
induced by their temporal evolution or the exposure to atmosphere, during the
removal of residues from the vacuum chamber and during their storage. Accolla
et al. (2018) and Baratta et al. (2019) monitored the evolution of residues
through IR spectroscopy. Their stabilization can require up to 200 days and it
is induced by sublimation of volatiles and reactions within the residue
matrix. Our samples are stored under vacuum, and there is no evidence of
alterations induced by the exposure to atmosphere, as it would determine an
increase in the O/C ratio of residues. In particular, we would expect higher
O/C ratios in the 1:1:1+98 eV/16 u residue, produced 240 days before analysis,
than in the other samples, analyzed only 2 weeks after their production.
However, the 1:1:1+98 eV/16 u residue shows similar O/C ratios (and even
lower, in the range 50-300 m/z) than the other two samples.

A detailed comparison between our samples and UV residues it outside the main
scopes of this work. However, VHRMS in both positive and negative ESI modes
has been used to analyse UV residues, and we now gather the information
obtained on both types of samples. For UV residues, Danger et al. (2016) show
elemental abundances and van Krevelen plots, and Fresneau et al. (2017)
estimate their unweighted DBE. The distribution of data in van Krevelen plots
that we show in Fig. 6 and Fig. 7 resemble the distribution observed for UV
residues, implying a similarity in the chemical composition of the residues,
despite the different source of processing. Also unweighted DBE are found to
be similar in both type of residues. For what concerns elemental abundances,
taking into account the results of this work, the higher H/C and lower N/C
observed in UV residues can be explained by the lower UV irradiation dose
given to the pristine ice, i.e. about 10 eV/16 u.

A common characteristic of UV residues is the presence HMT and its
derivatives, whose main vibrational mode bands are found between 1250-950
cm-1. In the IR spectra shown in Fig. 2 we do not find clear evidence of the
presence of such compounds, because any attribution is hindered by the
presence of intense features in a N-free residue (1:1:0 +85 eV/16 u), where no
HMT can be present. Tandem MS/HRMS allowed the detection of stoichiometric
formulas tentatively attributed to HMT and its derivatives in residues. The
fragment pattern of peaks at 141.113 and 155.129 m/z are similar to those
revealed in commercial HMT and to the fragment pattern reported by Danger et
al. (2013), attributed to HMT and HMT-CH3, respectively. The m/z spectra of
residues also show peaks at 185.140 m/z, attributed to HMT-C2H5O or its
isomers and previously revealed in UV residues (Danger et al. 2013).
Irradiation dose plays a role not only in the changes observed in elemental
abundances and DBE values, but also in the production of HMT. In fact, the
intensity of m/z peaks tentatively attributed to HMT and its derivatives
diminish as the irradiation dose given to the frozen mixtures increases, as
shown in Fig. 14. An explanation could be found in the destruction of the HMT
precursors produced by irradiation in the frozen matrix, i.e. H2CO and HCOOH.
High irradiation doses would partially destroy the newly formed species, and
as a result, lower quantities of HMT could be formed throughout the warm up.
This consideration is supported by the fact that the 1:1:1+29 eV/16 u residue,
the one produced at the lowest irradiation dose, displays the most intense m/z
peaks associated to stoichiometric formulas of HMT and its derivatives, and
its IR spectrum shows a shoulder at about 1010 cm-1 that could be attributed
to HMT.

Finally, in m/z spectra of residues we note the presence of stoichiometric
formulas attributed to isomers of amino acids and nucleobases, and their
intensity seems to be affected by the extent of irradiation dose. In future,
dedicated analysis should be performed to deepen the effects of dose in the
production of prebiotic species.

###  4.2 Astrophysical and Astrobiological implications

Water, methanol and ammonia are widely present in the ISM as well as in the
solar system. In the outer solar system, water ice has been detected on the
surface of various small bodies (e.g. Barucci et al. 2011). Observations and
space mission data also revealed the presence of methanol on the surface of
5145 Pholus (Cruikshank et al. 1998), 55638 2002 VE95 (Barucci et al. 2006)
and (4869580) Arrokoth (Grundy et al. 2020). So far, NH3 has been revealed
through its 2.22 $\mu$m band in localized areas on the surface of Charon
(Grundy et al. 2016), and Delsanti et al. (2010) suggested that the
irradiation of NH3 is responsible for the formation of NH${}_{4}^{+}$ on the
KBO (90482) Orcus, and it could be present in the interior of outer bodies
(Lisse et al. 2020). Recently, Altwegg et al. (2020) and Poch et al. (2020)
suggested the presence of ammonium salts on the comet 67P/Churyumov-
Gerasimenko. According to Poch et al. (2020), these salts would contribute to
the 3.2 $\mu$m feature detected on the comet surface, together with water ice
and organics, and ammonium salts would originate thanks to reactions involving
ammonia. Furthermore, experiments reported by Parent et al. (2009) revealed
that irradiation of ammonia produces up to 12% of N2, the latter being
revealed on Pluto (Owen et al. 1993; Grundy et al. 2016), 90377 Sedna (Emery
et al. 2007), 136199 Eris (Tegler et al. 2012), and on the Neptune satellite
Triton (Cruikshank et al. 1993). Thus, the analysis of organic residues
produced after the irradiation of water, methanol and ammonia mixtures is
necessary to shed light on the physical and chemical properties of cometary
nuclei and of the surfaces of small bodies in the outer solar system, so far
being accessible only by means of observations and data acquired by the New
Horizons space probe.

In this work, we link the composition of organic refractory materials to the
irradiation dose given to the pristine frozen mixture. Recently, Urso et al.
(2020) found that irradiation doses capable to induce detectable changes in
the spectra of frozen surfaces, i.e. about 40 eV/16 u, would be accumulated
within 106-107 years, according to the heliocentric distance. Such short
timescale testify the efficiency of irradiation in modifying the physico-
chemical properties of frozen surfaces in the outer solar system. In
particular, Urso et al. (2020) show the primary role of SEP in affecting
frozen surfaces in the outer solar system. In fact, the timescales reported in
Fig. 4 show that in the Kuiper-belt, at the current orbit of Arrokoth, between
2.1 and 7.4$\times$107 years would be needed to accumulate doses that affect
elemental abundances thanks to the irradiation by SEP. At the same location,
the heliosphere shield CR and to accumulate the same dose CR irradiation would
require up to 8$\times$109 years. However, CR become the main source of
processing beyond the termination shock ($\sim$85 AU), and in the Local
Interstellar Medium the timescale of irradiation by CR shorten to 106-107
years. We note that the timescale of irradiation by solar particles would
further shorten taking into account that the young sun might have been more
active than it is at present times (e.g. Güdel et al. 1997).

The formation of organic refractory materials on frozen surfaces requires both
irradiation by energetic particles and heating. After being exposed to
energetic particles in the outer solar system, small bodies can migrate inward
the solar system and experience heating, allowing the formation of organic
refractory materials. In laboratory, we observe the formation of organic
refractory residues at about 300 K. However, in space both diffusion and
sublimation of volatiles needed to form organic refractory materials can
happen in timescales of billions of years. Thus, such refractory materials
might form even at lower temperatures.

Kuiper-belt objects are thought to have formed in the protoplanetary nebula
(e.g. Grundy et al. 2020). During their formation, small bodies could have
accreted organic molecules present in the presolar disk, possibly produced
thanks to both solid and gas-phase reactions in the interstellar medium.
However, the harsh conditions of the protoplanetary disk could have determined
the destruction of organics and complex compounds. Thus, a possibility is that
only simple species were available to be accreted in the seeds of
planetesimals. In this scenario, our data suggest that the production of
organic molecules through ion bombardment would have required only a few tens
of million years after the formation of a frozen body. The high extent of
molecular diversity revealed in our sample may thus exist on frozen surfaces
in the outer solar system.

Our analysis also point out that ion irradiation favours the formation of
compounds of interest for astrobiology. After ion bombardment, at 15 K we
observe various bands attributed to formamide, a compound revealed in star-
forming regions (e.g. López-Sepulcre et al. 2015) as well as in comets
(Bockelée-Morvan et al. 2000; Biver et al. 2014; Goesmann et al. 2015) that
has a relevant role in the synthesis of various prebiotic compounds, including
nucleobases (e.g. Saladino et al. 2012; Ferus et al. 2015; Kaňuchová et al.
2016; Urso et al. 2017; Botta et al. 2018; López-Sepulcre et al. 2019). In
residues, we detect m/z peaks tentatively attributed to HMT and HMT
derivatives. HMT is a precursor of various organic compounds, including amino
acids (e.g. Hulett et al. 1971; Bernstein et al. 1995; Vinogradoff et al.
2020) and its formation after irradiation and heating of astrophysical ice
analogues suggests that it could be present in solar system outer objects.
Furthermore, in residues we reveal the presence of amino acids and nucleobases
or their isomers. In future, the instruments on board the James Webb Space
Telescope will allow to observe the wavelength range where HMT shows its most
intense bands. In particular, the Mid-infrared Instrument (MIRI) will cover
the range 5-28 $\mu$m (2000-360 cm-1). As reported in this work, HMT shows
intense vibrational mode bands in the same wavelength range where also N-free
residues exhibit intense bands (see Fig. 2). As a result, caution must be
taken when attributing bands to HMT in this wavelength range.

##  5 Conclusions

The frozen surfaces of outer objects in the solar system show the presence of
various volatile molecules as well as refractory red materials, whose presence
may be due to the effect of the bombardment by cosmic rays, solar energetic
particles and solar wind. To date, only limited information is available on
the composition of organic refractory materials on the surfaces of frozen
bodies in the outer solar system, as astronomical observations only show the
presence of red slopes in the visible and near-infrared.

Our experiments simulate the formation of refractory materials on such frozen
surfaces, and shed light on their composition. Analysis reveal the high extent
of molecular diversity of residues. Very High Resolution Mass Spectrometry
data show the presence of CHO, CHN, and CHNO molecular groups. The
distribution of elemental ratios reported in van Krevelen plots suggests a
compositional homogeneity of residues, with distributions similar to those
revealed for UV residues. According to our data, elemental abundances, and in
particular the H/C and N/C ratios are affected by the dose given to the
deposited frozen volatile mixture. Increasing the dose determines a lowering
of the H/C ratio, while the N/C ratio increases. The dose also affects the
Double Bond Equivalent. In fact, the level of unsaturation of residues
increases with the increasing dose.

Although the presence of hexamethylenetetramine in our samples cannot be
confirmed with IR spectroscopy, by means of tandem Mass Spectrometry/High
Resolution Mass Spectrometry we reveal a fragment pattern that corresponds to
that of hexamethylenetetramine, and for the first time, we report the presence
of stoichiometric formulas associated to its derivatives in residues obtained
after ion irradiation of frozen volatiles. Furthermore, data suggest a
dependence of the abundance of hexamethylenetetramine and its derivatives to
irradiation dose. The detection of such compounds in our samples opens to the
possibility that ion irradiation, and not only UV photolysis, induces their
formation, in agreement with the results reported by Cottin et al. (2001).
Thus, hexamethylenetetramine could be present also in astrophysical
environments shielded from UV radiation. Taking into account the effects
induced by solar energetic particles on frozen surfaces in the outer solar
system, we estimate that doses capable to produce organic refractory materials
with a composition similar to that of the samples here analyzed would be
accumulated within 2-7$\times$107 years on a frozen surface located at about
40 AU.

###### Acknowledgements.

We thank the anonymous Referee and the Editor for their comments. We thank D.
Ledu and C.O. Bacri for the access to the SIDONIE facility. INGMAR is a IAS-
IJCLab facility funded by the French Programme National de Planétologie (PNP),
Faculté des Sciences d’Orsay, Université Paris-Sud (Attractivité 2012), 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). This work is supported by the French National Research
Agency in the framework of the Investissements d’Avenir program
(ANR-15-IDEX-02), through the funding of the “Origin of Life” project of the
Université Grenoble Alpes and the French Space Agency (CNES) under their
Exobiology and Solar System programs. We thank D. Baklouti for fruitful
discussions. R.G.U. thanks the CNES postdoctoral program.

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##  Appendix A Vibrational mode bands identified in processed ices and in
organic refractory residues

In the following tables, we list the vibrational mode bands detected after
irradiation with 40 keV H+ of H2O:CH3OH:NH3 mixtures at 15 K (Table 4) and
those in organic refractory residues at 300 K (Table 5). Vibrational mode
bands are attributed taking into account the results of similar irradiation
experiments reported in literature.

Table 4: Bands at 15 K after irradiation with 40 keV H+ of H2O:CH3OH:NH3
mixtures. v int= very intense, int= intense, s=shoulder, w=weak, b=broad, n=
noisy.

1:1:1 | 3:1:1 | 1:1:1 | Mode | Compound | Ref.
---|---|---|---|---|---
+29 eV/16 u | +67 eV/16 u | +98 eV/16 u |  |  |
3378 (v int) | 3373 (v int) | 3375 (v int) |  $\nu$(NH) | NH3 | 1
3261 (v int, b) | 3261 (v int, b) | 3272 (v int, b) |  $\nu$(OH) | H2O, CH3OH | 2
3009 (w) | 3013 (w) | 3012 (w) |  $\nu$(CH) | CH4 | 3
2984 (w) | 2990 (w) | 2980 (w) |  | C2H6 | 4
2958 (w) | 2960 (w) | - |  | CH3OH | 2
2938 (w) | 2938 (w) | 2936 (vw) |  | CH3OH | 2
2908 (w) | 2909(w) | 2900 (vw) |  | H2CO | 5
2830 (int) | 2832 (int) | 2830 (w) |  | CH3OH, C2H6 | 3, 4
2343 (v int) | 2343 (v int) | 2343 (v int) |  $\nu$(CO) | CO2 | 6
2277 (w) | 2278 (w) | 2278 (int) |  $\nu$(CO) |  13CO2 | 7
2260 (vw) | 2261 (vw, s) | 2261 (b) |  $\nu$(N=C=O) | HNCO | 8
2241 (vw) | 2242 (w) | 2242 (b) | $\nu_{3}$ | N2O | 8
2165 (int) | 2168 (int) | 2168 (int) |  $\nu$(CN) | OCN- | 2
2138 (int) | 2139 (int) | 2140 (int) |  $\nu$(C$\equiv$O) | CO | 9
1846 (w) | 1845(w) | 1845 (w) |  $\nu$(CO) | HCO | 1
1718 (int) | 1717 (s) | 1718 (s) |  $\nu$(CO) | H2CO, HCOOH, NH2HCO, |
|  |  |  | (CH3)2CO, HCOOCH3 | 10, 11, 12
1694 (s) | 1690 (v int | 1690 (v int) |  $\nu$(CO) | NH2HCO | 13
1640 (int) | 1643 (int, s) | 1640 (int, s) |  $\delta$(OH), (NH) | H2O | 10, 14
1591 (int) | 1591 (int) | 1591 (int, s) |  $\nu_{asy}$(COO-) | HCOO- | 10
1500 (int) | 1500 (int) | 1499 (int) |  $\delta$(CH2) | H2CO | 10
1479 (int) | 1479 (s) | 1479 (s) |  | NH3, NH${}_{4}^{+}$ | 15
1463 (int) | 1463 (s) | 1460 (s) |  $\delta$(OH), (NH) | CH3OH, NH${}_{4}^{+}$ | 10
1385 (int) | 1385 (int) | 1386 (int) |  $\delta_{sym}$(CH) | HCOO-, NH2HCO | 10, 8
1352 (int) | 1353 (int) | 1353 (int) |  $\delta_{asy}$(CH) | HCOO-, CH3CHO | 10, 4, 15
1305 (int) | 1305 (int) | 1305 (int) |  | CH4 | 4
1250 (w) | 1247 (vw) | 1250 (vw) |  $\rho$(CH2) | H2CO | 10
1221 (vw) | 1222 (vw) | 1218 (w, s) |  | (CH3)2CO | 11
1125 (int) | 1126 (int) | 1124 (int) |  $\omega$(NH), $\rho$(CH) | NH3, CH3OH | 10, 6
1095 (int, s) | 1094 (w, s) | 1094 (w, s) |  | (CH3)2CO | 11
1029 (int) | 1024 (int) | 1025 (w) |  $\nu$(CO) | CH3OH | 10
820 (b) | 820 (b) | 820 (b) | libration | H2O | 10

1 D’Hendecourt et al. (1986), 2 Islam et al. (2014), 3 Ferini et al. (2004), 4
Moore & Hudson (1998), 5 Bossa et al. (2009), 6 Palumbo & Strazzulla (1993), 7
Strazzulla (1999), 8 Kaňuchová et al. (2016), 9 Urso et al. (2016), 10
Vinogradoff et al. (2013), 11 Baratta et al. (1994), 12 Modica & Palumbo
(2010), 13 Urso et al. (2017), 14 Vinogradoff et al. (2011), 15 Raunier et al.
(2004).

Table 5: Vibrational bands observed in the spectra of organic refractory
residues after 30 minutes at 300 K. v int= very intense; int=intense,
s=shoulder; w=weak; b=broad, n=noisy

Organic refractory residues at 300 K
---
1:1:1 | 3:1:1 | 1:1:1 |  |
+29 eV/16 u | +67 eV/16 u | +98 eV/16 u | Mode | Ref.
3306 (int, b) | 3197 (int, b) | 3277 (int, b) |  $\nu$(OH), $\nu$(NH) | 1
2975 (s) | 2970 (s, n) | 2976 (int) |  $\nu$(CH) | 1
2936 (int) | 2936(s) | 2940 (int) |  $\nu$(CH)sym | 2
2878 (int) | 2872 (s, b) | 2879 (int) |  $\nu$(CH)asy | 3,2
2216(w) | 2217(w,b) | 2215 (int) |  $\nu$(C$\equiv$N) | 1
2161 (vw, b) | 2160(w, b) | 2161 (int) |  $\nu$(N$\equiv$C) | 1
1749 (int) | 1739 (int) | 1746 (s) |  $\nu$(C=O) | 4
1720 (s) | 1720 (s) | 1720 (s) |  $\nu$(C=O) | 4, 5
1674 (int) | 1678 (int) | 1674 (v int) |  $\nu$(C=O) | 3
1643 (int, n) | 1650 (int) | 1650 (s) |  $\nu$(C=O), (C=N), $\delta$(NH) | 2
1599 (int) | 1590 (b) | 1593 (int) |  $\nu$(COO-), (C=C), (C=N) | 3, 5, 6
1456 (s) | 1457 (b) | 1458 (b) |  $\delta$(CH, NH) | 3, 4
- | 1376 (w) | 1382 (w) |  $\delta$(CH) | 5
1345 (vw) | 1340 (s, w) | 1343 (w) |  $\nu_{s}$(COO-) | 2
1232 (vw) | 1234 (int) | 1230 (int) |  $\nu$(C-N) | 3
1111 (s) | 1110 (s) | 1110 (s, b) |  $\nu$(C–O) | 5
- | 1074 (int) | 1078 (int) |  | 4
1056 (v int) | 1050 (s) | 1050 (s) |  $\nu$(C–O) | 5
1009(s, w) | 1014 (s, w) | - |  $\nu$(C–N) | 3
925 (vw) | 926 (vw) | 925 (vw) | O–H def. | 4
826 (int, b) | 825 (s, b) | 827 (w, s) |  $\omega$(NH) | 4

1 Accolla et al. (2018), 2 Fresneau et al. (2017), 3 Vinogradoff et al.
(2013), 4 Muñoz Caro & Schutte (2003), 5 Bernstein et al. (1995), 6 de
Marcellus et al. (2017), 7 Briani et al. (2013).

##  Appendix B Oxygen and nitrogen elemental abundances

Figure 10 and Fig. 11 show the elemental abundance of oxygen as the number of
O atoms and the O/C ratio with respect to the m/z. The interpretation of the
different distribution of intensities in the ranges 50-300 and 150-400 m/z is
given in sect. 3.2.

Figure 10: Number of oxygen atoms in stoichiometric formulas with respect to
the m/z ratio in the range 50-300 and 150-400 m/z in both positive ESI mode
(left panels) and negative ESI mode (right panels). Panels A and B: 1:1:1+29
eV/16 u residue; panels C and D: 3:1:1+67 eV/16 u residue; panels E and F:
1:1:1+98 eV/16 u residue. The size of the dots is given by the normalized
intensity. Figure 11: O/C ratios with respect to the m/z ratio in the range
50-300 and 150-400 m/z in positive ESI mode (left panels) and negative ESI
mode (right panels). Panels A and B: 1:1:1+29 eV/16 u residue; panels C and D:
3:1:1+67 eV/16 u residue; panels E and F: residue of the 1:1:1+98 eV/16 u. The
size of the dots is given by the normalized intensity.

Figure 12 and Fig. 13 and Fig. 13 show the elemental abundance of nitrogen as
the number of N atoms and the N/C ratio with respect to the m/z. The
interpretation of the different distribution of intensities in the ranges
50-300 and 150-400 m/z is given in sect. 3.2.

Figure 12: Number of nitrogen atoms in stoichiometric formulas with respect to
the m/z ratio in the range 50-300 and 150-400 m/z in positive ESI mode (left
panels) and negative ESI mode (right panels). Panels A and B: 1:1:1+29 eV/16 u
residue; panels C and D: 3:1:1+67 eV/16 u residue; panels E and F: 1:1:1+98
eV/16 u residue. The size of the dots is given by the normalized intensity.
Figure 13: N/C ratios with respect to the m/z ratio in the range 50-300 and
150-400 m/z in positive ESI mode (left panels) and negative ESI mode (right
panels). Panels A and B: 1:1:1+29 eV/16 u residue; panels C and D: 3:1:1+67
eV/16 u residue; panels E and F: residue of the 1:1:1+98 eV/16 u. The size of
the dots is given by the normalized intensity.

##  Appendix C HMT and its derivatives

Figure 14: Intensity of m/z peaks of molecular ions with the same
stoichiometric formulas of HMTH+, HMT-CH3H+, and HMT-C2H5OH+.

In Fig. 14 we show the intensity of peaks at 141.113, 155.129 and 185.140 m/z
detected in the mass spectra of the residues in positive ESI modes in both the
range 50-300 and 150-400 m/z. The intensity of both the 155.129 and 185.140
m/z peaks is higher than the intensity of the 141.113 m/z peak attributed to
HMT. In VHRMS, the intensity of m/z peaks is associated to the abundance of
the related species within the sample, implying that in ou HMT derivatives
could be more abundant than HMT. The higher intensity of the 155.129 m/z peak
with respect to the 141.113 peak was also observed by Danger et al. (2013),
and it could be attributed to differences in the residue synthesis (mixture,
dose, warm-up rate) or storage as well as to an artefact induced by the
ionization method used in the VHRMS analysis. The intensity of the m/z peaks
shown in Fig. 14 decrease according to the dose given to the pristine ice from
which residues are produced. An anomaly is the high intensity of the 155.129
m/z peak detected in the 1:1:1+98 eV/16 u. In this sample, this molecular ion
is only detected in the mass spectrum acquired between 150-400 m/z, and its
intensity does not follow the trend observed for the 141.113 and 185.140 m/z
peaks.