Electron irradiation of crystalline nitrous oxide ice at low temperatures: Applications to outer Solar System planetary science

The confirmed detection of a number of simple inorganic nitrogen-bearing molecules (e.g., N₂ and NH₃) on the surfaces of several outer Solar System dwarf planets and moons such as Pluto, Triton, Umbriel, and Eris^1–4 has led to an increased interest in the solid-phase astrochemistry of N₂O. This is in part due to the fact that the surfaces of these bodies are routinely exposed to ionizing radiation in the form of galactic cosmic rays and the solar wind, which is known to engender chemical changes in astrophysical ices. Furthermore, laboratory studies have repeatedly demonstrated that the irradiation of ices made up of nitrogen-containing molecules mixed with H₂O or other oxygen-bearing molecules (e.g., O₂, CO, or CO₂) that may also co-exist on the surfaces of these icy bodies leads directly to the formation of N₂O.^5–16 It is to be noted, however, that the presence of N₂O itself has yet to be confirmed on the surfaces of outer Solar System moons and dwarf planets.

Nevertheless, the repeated formation of N₂O in laboratory irradiations of ices representative of the surfaces of outer Solar System planetary-like bodies is suggestive of this molecule being a likely surface component. Accordingly, the radiation physics and chemistry of solid N₂O under conditions relevant to astrochemistry have been considered by a number of previous studies that have shown that several molecules, including nitrogen oxides of the type N_xO_y (where x ≤ y) and O₃, are produced as a result of the radiolytic dissociation of N₂O and subsequent radical recombination reactions.^17–25 However, many of these experimental astrochemistry studies have been performed under temperature conditions more relevant to the cold, dense interstellar medium (i.e., 10–20 K), and the radiolytic physics and chemistry of N₂O in the outer Solar System (which is characterized by higher temperatures in the range of 30–60 K) only have been inferred from the results of these lower temperature studies.

For instance, the study of Almeida et al.¹⁹ was able to quantify the destruction cross-section of solid N₂O during its irradiation at 10 K by 1.5 MeV N⁺ ions, as well as the formation cross-sections of a number of radiolytic products. Using this calculated destruction cross-section together with the known flux of medium-mass ions at the orbit of Pluto,²⁶ Almeida et al.¹⁹ estimated the half-life of solid N₂O in the outer Solar System to be 9⋅10⁶ years. However, such an estimation is critically reliant on the destruction cross-section measured by Almeida et al.¹⁹ at 10 K not varying significantly with increasing temperatures. Despite this requirement, previous astrochemical investigations into the possible temperature dependence of N₂O ice radiolysis (and, by the same token, the possible temperature dependence of the formation of radiolytic product molecules) have been few and far between.

Indeed, to the best of our knowledge, only two recent studies focused on the temperature dependence of the radiolysis of N₂O. The first, conducted by de Barros et al.,¹⁸ investigated the irradiation of solid N₂O at 11 and 75 K using 90 MeV Xe²³⁺ ions and calculated higher destruction cross-sections for the higher temperature experiment. This was attributed to the higher mobility of radiolytically derived radicals at higher temperatures, as well as the relevant reaction activation energy barriers being more easily overcome. The second study was that of Fulvio et al.,²⁰ who investigated the irradiation of N₂O ice at 16 and 50 K using 0.2 MeV protons. Interestingly, the results of this latter study did not detect any noticeable differences between the radiolytic destruction rate of N₂O at these two temperatures.

Although these two studies have made interesting contributions to our understanding of the role of temperature in the radiation-induced dissociation of solid N₂O under conditions relevant to astrochemistry, a number of questions remain unresolved. These primarily relate to the method by which the N₂O ices considered by de Barros et al.¹⁸ and Fulvio et al.²⁰ were prepared, including the resultant solid phase of the ice. In principle, N₂O ices prepared by condensation of a gas onto a cooled substrate at temperatures below circa 30 K should result in an amorphous structure, while those prepared in a similar fashion at higher temperatures should produce crystalline ice.²⁷ This would suggest that each of the studies of de Barros et al.¹⁸ and Fulvio et al.²⁰ would have considered one amorphous ice and one crystalline ice. However, it is well known that the phase adopted by N₂O ices prepared by condensation of the gas on a cooled substrate is heavily dependent on the rate of ice deposition, with higher deposition rates resulting in crystalline structures even at low temperatures (for a more complete discussion of this phenomenon, see the works of Hudson et al.²⁷ and Gerakines and Hudson).²⁸ Since neither of these two previous studies defined the rate at which their ices were grown, there is some ambiguity regarding the phases of the ices that were irradiated in those studies.

Knowledge of the phase of an irradiated astrophysical ice analogue is important, as there has been an increasing body of literature in recent times that has demonstrated that simple molecular ices (including N₂O) are prone to higher radiolytic decay rates if in an amorphous phase compared to a crystalline one.^25,29,30 As such, if the role of temperature on the radiolytic destruction of N₂O is to be accurately quantified, then it is necessary to exclude any potential influence that the phase of the target ice may bear upon the process. The aim of the present study, therefore, was to systematically and quantitatively explore the influence of temperature on the radiation physics and chemistry of solid N₂O, in the absence of any contributing effects from other factors such as the phase adopted by the ice. This has been achieved by preparing five purely crystalline ices under identical conditions and then irradiating them using 2 keV electrons at five different temperatures (i.e., 20, 30, 40, 50, and 60 K) that are relevant to the outer Solar System. Further details on the experimental apparatus and techniques used in this study are provided in Sec. 2, while the experimental results and their implications for astrochemistry in the outer Solar System are discussed in Sec. 3. Finally, concluding remarks are given in Sec. 4.

Experiments were performed using the Ice Chamber for Astrophysics-Astrochemistry (ICA); a laboratory set-up for radiation astrochemistry research located at the HUN-REN Institute for Nuclear Research (Atomki) in Debrecen, Hungary. The set-up has been described in detail in other publications,^31,32 and so only a brief overview of the most salient features will be provided here. The ICA (Fig. 1) is an ultrahigh-vacuum-compatible stainless-steel chamber maintained at a base pressure of a few 10^–9 mbar by the combined action of a scroll pump and a turbomolecular pump. Within the centre of the chamber is a gold-coated oxygen-free high-conductivity copper sample holder into which a number of ZnSe deposition substrates may be installed. The sample holder and deposition substrates may be cooled to a minimum temperature of 20 K using a closed-cycle helium cryostat, although it is possible to accurately control the temperature over a range of 20–300 K.

Once prepared to a satisfactory thickness, the crystalline N₂O ices were subsequently cooled to the desired temperature and individually exposed to a 2 keV electron beam having an average particle flux of 5⋅10¹³ electrons cm^–2 s^–1, with projectile electrons impacting the ices at an angle of 36° to the surface normal (Fig. 1). The electron beam flux, homogeneity, and beam spot size had been defined prior to commencing the irradiations using the method described previously by Mifsud et al.³² Each ice was irradiated for a total of 30 minutes, corresponding to an average delivered fluence of 9⋅10¹⁶ electrons cm^–2 and an average administered dose of 1.38⋅10⁹ Gy.³³ Several mid-infrared absorption spectra were acquired over the course of each irradiation, thus allowing for any possible temperature dependence of the radiation physics and chemistry of N₂O ice to be quantitatively probed. A summary of the experiments performed is given in Table I.

The radiolytic destruction of the N₂O ices considered in this study as a result of their interaction with a 2 keV electron beam has been quantified through the G-value, defined as the change in the number of molecules per 100 eV of energy deposited into the ice. This is calculated by plotting the measured column density of N₂O (in molecule cm^–2) as a function of electron fluence in dose units (i.e., in units of eV cm^–2 rather than electrons cm^–2). The initial slope of the decreasing N₂O column density multiplied by 100 yields the G-value. The radiolytic synthesis of a number of product molecules was also quantified using the G-value, which was determined in an analogous way considering the initial slope of the increasing product molecule column density. Calculations performed using the CASINO software³⁴ revealed that, in all experiments, the projectile electrons were implanted within the N₂O target ice layer and only 6.7% of the incident kinetic energy was lost to backscattering. When considering the G-values of astrophysical ice analogues undergoing radiolytic dissociation or synthesis in this way, it is important to only consider the initial fluence points early on in the irradiation where radiolytic dissociation or synthesis is likely the only process contributing to changes in the measured column density of the molecule of interest. Measurements at these fluence points are likely to be far from the steady-state, where radiolytic destruction and synthesis are balanced through the contribution of “back reactions” to give a near-constant abundance at high fluences.

It is also important to note that the calculation of G-values from mid-infrared absorption spectra assumes that the only mechanism contributing to the loss of N₂O molecules in the irradiated astrophysical ice analogues is radiation-induced molecular dissociation. In fact, it is possible that intact N₂O molecules may be ejected into the gas phase as a result of sputtering. Such an effect has been extensively documented when using large ions as projectiles^35–37 and has also been documented when using electrons,^38,39 albeit to a more limited extent. To the best of our knowledge, the sputtering of N₂O ice at low temperatures by incident electrons has not yet been studied experimentally. Since mid-infrared spectroscopic measurements of N₂O ice column density variations during irradiation cannot discriminate between losses due to true radiolytic destruction and those due to sputtering, it is necessary to emphasize that the G-values calculated in this present study for N₂O radiolysis, therefore, represent upper bounds to these parameters. That being said, it is likely that our calculated values for these parameters are close to the true values since previous experimental studies have determined that electron-induced sputtering, although non-negligible, is not a major contributor to material loss during irradiation.^35–44 

Mid-infrared absorption spectra of solid N₂O, both before and after irradiation using 2 keV electrons, are shown in Fig. 2, and band assignments of the parent and product molecules are provided in Tables II and III, respectively. All ices exhibited the anticipated absorption features associated with crystalline N₂O after deposition at 60 K, which did not vary significantly in their wavenumber position or profile after having been cooled to the desired irradiation temperature (Table II). Moreover, the band assignments of the parent N₂O ices reported herein are in good agreement with those of previous studies.^45,46 Upon the onset of irradiation, the absorption bands attributed to N₂O were observed to decline while new bands attributable to various product molecules emerged against the background continuum.

Among the radiolytic products were several nitrogen oxides of the type N_xO_y (where x ≤ y), including NO, NO₂, NO₃ (tentative detection), N₂O₂, N₂O₃, N₂O₄, and N₂O₅. Additionally, O₃ and N₃ were also detected (Fig. 2). The formation of these products as a result of the irradiation of solid N₂O has been reported by several previous studies,^16–25 and is presumed to proceed primarily as a result of the radiolytic fission of the O–N bond in the N₂O molecule to yield N₂ and a free, supra-thermal oxygen atom (note that N₂ cannot be detected spectroscopically among the radiolytic products due to it being a homonuclear diatomic species, and thus infrared-inactive). The preference for the fission of this bond as opposed to the N≡N bond in the N₂O molecule lies in the lower bond energy of the former (1.7 eV) compared to that of the latter (4.9 eV).^47,48

Once liberated, free supra-thermal oxygen atoms may combine with one another to sequentially yield O₂ (which is not detectable in acquired mid-infrared spectra due to it being infrared-inactive) and O₃.^19,49–51 Liberated oxygen atoms may also react with intact N₂O molecules, resulting in either the formation of N₂O₂^17,52 or, alternatively, in the dissociation of N₂O to N₂ and O₂.^51–54 N₂O₂ itself may undergo unimolecular decomposition to yield two NO molecules.¹⁷ The formation of NO represents the starting point for very rich nitrogen chemistry in the irradiated ice due to the large number of reactions in which this molecule may participate (Fig. 3). Aside from its dimerization to form N₂O₂, NO may also react with an oxygen atom to produce NO₂.^50,51 Presumably, the formation of NO₃ results from the addition of atomic oxygen to NO₂, although it is likely that this reaction occurs in competition with that yielding NO and O₂.^19,50,51,55

An alternative formation pathway towards NO is the radiation-induced dissociation of N₂O to NO and atomic nitrogen, although this dissociation pathway is less likely to occur than that yielding N₂ and atomic oxygen as discussed earlier. Nevertheless, the release of supra-thermal nitrogen atoms throughout the ice allows for them to participate in atom addition reactions and thereby contribute to the diversity of nitrogen-bearing products in the irradiated ice. Perhaps the most evident example of such a reaction is the sequential synthesis of N₂ and N₃,⁵⁶ in an analogous fashion to the synthesis of O₃ as a result of oxygen atom addition reactions. Furthermore, the addition of atomic nitrogen to NO₂ may result in its dissociation to two NO molecules or, alternatively, in the formation of N₂O and atomic oxygen;^19,50,51,55 while its addition to NO results in the formation of N₂ and atomic oxygen.^50,51

Regarding the formation of the nitrogen oxides containing two nitrogen atoms that were detected in our irradiated ices, a step-wise formation via oxygen atom additions, starting from N₂O and leading to the sequential formation of N₂O₂, N₂O₃, N₂O₄, and N₂O₅, is certainly possible. However, previous studies have suggested that other reactions might be more efficient in producing these molecules. This may be related to the fact that each step-wise addition of atomic oxygen occurs in direct competition with the dissociation of the nitrogen oxide to N₂ and O₂.¹⁹ Instead, the formation of N₂O₃ was proposed to result from the reaction between NO and NO₂, or from the reaction between N₂O and O₂.^19,50,51 The dimerization of NO₂ has been suggested to be the principal route toward the formation of N₂O₄, although the reaction between N₂O and O₃ may also contribute to its formation.^19,50,51 Lastly, N₂O₅ is thought to result from one of several reaction processes involving three species, such as that involving N₂O and two O₂ molecules, that involving O₃ and two NO₂ molecules, and that involving N₂O, O₃, and atomic oxygen.^17,19 The complexity of the reactions leading to the formation of N₂O₅ explains why previous studies have reported it to be among the lowest abundance products formed after the irradiation of N₂O ice.^17–20 

Although practically all the mid-infrared absorption bands depicted in the spectra shown in Fig. 2 could be assigned and the chemistry leading to the formation of the molecules with which they are associated elucidated by this and previous studies, we wish to draw attention to one feature in particular that we have not been able to unambiguously assign. Upon inspection of the middle panel in Fig. 2, it is possible to note a broad absorption band ranging over approximately 1460–1360 cm^–1 which we have labeled using a “?”. This feature is present in all our spectra of irradiated N₂O ice, irrespective of the temperature at which the irradiation was performed. There currently exists little consensus in the literature as to the origin of this band and, indeed, not all previous studies have observed it after the irradiation of solid N₂O.

The studies of de Barros et al.¹⁸ and Pereira et al.²¹ both observed this band in their recorded mid-infrared spectra but did not explicitly assign it to any particular molecule or vibrational mode. In their study, Fulvio et al.²⁰ observed this band over a wavenumber range of about 1500–1350 cm^–1 (peaking at approximately 1430 cm^–1) and assigned it to NO₃. However, this assignment is challenging to confirm. Previous gas-phase ^57–59 and matrix-isolation⁶⁰ spectroscopic studies have consistently placed the ν₃ mode of NO₃ at higher wavenumbers of about 1492 cm^–1, and this has been largely corroborated by computational work.⁶¹ Moreover, to the best of our knowledge, no previously published spectroscopic study concerned with NO₃ has described the ν₃ mode as a broad band as suggested by the assignment of Fulvio et al.²⁰ It is thus difficult to assign the broad band observed in our spectra over the 1460–1360 cm^–1 wavenumber range to the ν₃ mode of NO₃ with any certainty. However, a small shoulder is apparent towards the higher wavenumber end of this broad band at about 1457 cm^–1. Given the better (although still somewhat unsatisfactory) match between the position of this band and the literature value of the NO₃ ν₃ mode, we have tentatively assigned this shoulder to NO₃ while leaving the broad band unassigned. Further work clarifying the exact nature of the broad band at approximately 1460–1360 cm^–1 is required.

Nevertheless, our systematic study conducted on pure crystalline ices at temperatures well below the sublimation point has confirmed that the irradiation of N₂O at higher temperatures does indeed result in a higher radiolytic destruction rate, as was concluded by de Barros et al.¹⁸ This is likely due to the increased mobility through the ice matrix of the radicals derived from the radiolysis of N₂O, which thus decreases the likelihood of “back reactions” leading to its reformation. Our results, however, contrast with those of Fulvio et al.,²⁰ who considered the irradiation of N₂O ice at 16 and 50 K using 0.2 MeV protons but did not record any noticeable differences in the rates of radiolytic decay at these two temperatures. The reason for the discrepancy between our results and those of Fulvio et al.²⁰ is not immediately obvious, and thus it is recommended that future studies seeking to further elucidate the temperature dependence of the radiolytic destruction of N₂O ice make use of different charged particles across a wide energy range.

The irradiation of solid N₂O using 2 keV electrons resulted in the synthesis of nine product molecules which could be detected in acquired mid-infrared spectra (Fig. 2, Table III): NO, NO₂, NO₃, N₂O₂, N₂O₃, N₂O₄, N₂O₅, N₃, and O₃. It is logical to assume that N₂ and O₂ were also formed as a result of the irradiation of N₂O, as shown in the reaction scheme in Fig. 3. To complement our analysis of the variation of the N₂O destruction G-value as a function of temperature, we have sought to quantify the temperature dependence of the formation G-value of these products. We have excluded N₂ and O₂ from this analysis due to their being infrared inactive; as well as NO₃ due to the detection of this molecule in our experiments being tentative. Moreover, the absorption bands associated with N₃ were rather small in our acquired spectra, and thus measurement of their integrated absorbances is associated with large uncertainties. For this reason, we have also excluded N₃ from our analysis.

The G-values for the six nitrogen-containing products under investigation (i.e., NO, NO₂, N₂O₂, N₂O₃, N₂O₄, and N₂O₅) as a function of temperature are plotted in Fig. 6. The variation in these G-values reveals that the radiation chemistry of crystalline N₂O induced by 2 keV electrons is significantly impacted by the temperature at which the irradiation takes place. Considering first NO, which should primarily result from the radiolytic dissociation of N₂O (Fig. 3), it is possible to note that higher temperatures correspond to greater yields of this product molecule, with significant increases being noted on raising the temperature beyond 50 K. This aligns with the greater (i.e., more negative) destruction G-values of N₂O at higher temperatures. The reason for this is that, at these temperatures, radiolytically generated radicals become more mobile within the ice matrix. This increased mobility allows them to diffuse away from the site of their formation, reducing the likelihood of the “back reaction” that would lead to the reformation of N₂O.

A consequence of the larger yields of NO at higher irradiation temperatures is the greater abundance of N₂O₂, which results from the dimerization reaction occurring between two NO molecules (Fig. 3). The greater mobility of radicals at these higher temperatures allows for NO molecules to roam the ice matrix and thus increases the probability that they will come into contact with one another and react to yield N₂O₂. It is worthwhile noting that the computed formation G-values for N₂O₂, although lower than those corresponding to NO, are nevertheless fairly high. Indeed, the ratio between G (N₂O₂) and G (NO) increases from about 0.19 to about 0.58 as the irradiation temperature is increased from 20 to 60 K (Table IV). The greater rate of increase of the formation G-value of N₂O₂ with increasing temperature compared to that of NO may indicate that a significant fraction of radiolytically synthesized NO is sequestered into N₂O₂ at higher temperatures.

The relationship between the formation G-value and irradiation temperature is perhaps most stable for the N₂O₃ product, for which the G-value does not vary significantly from 0.15 (although it should be noted that the calculated G-value for the irradiation at 20 K is accompanied by comparatively larger uncertainties). Several reasons could contribute to this relative stability of the G-value, including the lower reaction rate constants at low temperatures, increased rates of product molecule radiolysis at higher temperatures, and even perhaps the inherent instability of N₂O₃ itself.^20,75,76

Interestingly, the relationship between the formation G-value of NO₂ and irradiation temperature is strikingly different, with the G-value sharply decreasing with increasing temperature (Fig. 6) and the greatest decreases being observed at temperatures of 40 K and above. This trend is the opposite of that observed for N₂O₄, for which higher G-values are observed during the irradiation of N₂O ices at higher temperatures (Fig. 6). The most parsimonious explanation for this observation is that, at higher temperatures, the mobility of the nascent NO₂ molecules is sufficiently high that the rate at which they are able to encounter each other and dimerize to yield N₂O₄ (Fig. 3) leads to an accumulation of the latter species at the expense of the former. Indeed, the ratio between G (N₂O₄) and G (NO₂) varies from 0.16 to 3.21 across the 20–60 K temperature range.

Finally, the last nitrogen-containing product for which G-values were computed was N₂O₅. As can be seen from Fig. 6, formation G-values for this product were very low, never exceeding 0.02 at any of the investigated irradiation temperatures. The reason for this is most likely the complexity of the reactions required to synthesize this molecule in the solid phase, which typically require three precursor molecules (Fig. 3). Yields of N₂O₅ increased as the irradiation temperature was increased from 20 to 50 K; however, a slight decrease in the G-value was registered on increasing the temperature further to 60 K. Although it is difficult to pinpoint an exact reason for this observation, it is possible that this was due to the fact that some of the precursors required for its formation, such as O₂ and O₃, undergo increased rates of sublimation from the ice at this higher temperature.^66,73,77 Such a result would also explain the measured relationship between the formation G-value of O₃ and the irradiation temperature (see next subsection for more details).

The evolution of the formation G-value of O₃ as a function of the irradiation temperature of solid N₂O is shown in Fig. 7. It is clear that, on increasing the temperature from 20 to 30 K, the G-value also increases; almost doubling in magnitude. This may be attributed to the greater mobility of oxygen atoms within the ice that were liberated as a result of the radiolytic dissociation of N₂O molecules. These oxygen atoms come together and react to yield sequentially O₂ and O₃^78–80 (Fig. 3). However, on further increasing the irradiation temperature to 40 K and beyond, the O₃ formation G-value was observed to steadily decrease. This is perhaps somewhat counterintuitive, since higher temperatures should, in principle, allow for the oxygen atoms to be even more mobile throughout the ice matrix, thus allowing them to come into contact and yield O₃ more efficiently. However, O₂ (i.e., the direct precursor to O₃) is a particularly volatile species under ultrahigh-vacuum conditions and previous studies have demonstrated that pure O₂ sublimates efficiently at temperatures lower than 40 K, although this may be shifted to higher temperatures if O₂ is entrapped within less volatile ice components.^73,81 As such, it is likely that the radiolytic formation of O₃ in our higher temperature experiments occurred in direct competition with the sublimation of O₂ from the ice, thus limiting the amount of O₃ that could actually be formed. Similar reasons were invoked in recent studies of the implantation of reactive sulphur ions into CO₂ ice, which observed the formation of oxygen-rich products (e.g., O₃ and SO₂) at 20 K but not at 70 K.^82,83

The icy surfaces of many bodies in the outer Solar System are known to be rich in nitrogen-containing species. Pluto’s surface, for instance, is known to be dominated by N₂, with trace quantities of CO also being present.^1,84 The surface of Orcus is known to host H₂O and NH₃ ices,^85,86 which are also thought to be present on the surface of Sedna.⁸⁷ Simple inorganic nitrogen-bearing ices have also been detected on the surfaces of other outer Solar System bodies, such as Triton, Umbriel, and Eris.^2–4 Ices containing both simple inorganic and more complex organic nitrogen-bearing species are also present on the surfaces of outer Solar System bodies in closer orbits of the Sun, such as a number of the icy satellites of Saturn.^88–91 Furthermore, it is now known that ammonium-rich salts are an important constituent of the icy nuclei of comets.^92–94 

Importantly, the surfaces of these outer Solar System bodies are all continuously exposed to ionizing radiation in the form of the solar wind, galactic cosmic rays, and possibly, giant planetary magnetospheric plasma. Thus, although solid N₂O has yet to be detected as a constituent of the icy surfaces of these small bodies, it is likely that it is indeed generated there as a result of the rich radiation chemistry that occurs between nitrogen- and oxygen-bearing molecules, as has been suggested by previous laboratory studies.^5–16 It is further suggested that, given the propensity of solid N₂O to crystallize upon warming to temperatures greater than 30 K or upon its rapid condensation from the gas phase,²⁷ it is probable that should any substantial quantities of N₂O ice exist on outer Solar System bodies then it would largely be crystalline; although it should be noted that ice amorphization may occur to at least some extent in regions of high radiation fluxes.^95,96

The surface temperatures of outer Solar System bodies vary significantly based on their orbital distances to the Sun, and it is also true that the exposure of icy material to fluxes of energetic ions, electrons, and ultraviolet photons can be greatly attenuated by shallow burial beneath other surface material. Our systematic computation of the N₂O destruction G-values at relevant temperatures therefore allows us to effectively constrain the survivability of N₂O on different icy Solar System worlds. Following the example of Zhang et al.,⁹⁷ we shall make the assumption that the survivability of a given solid-phase molecule in the outer Solar System can be approximated by considering the galactic cosmic ray ionizing radiation dose rates in the Kuiper Belt at a distance of 40 AU from the Sun. The 2 keV electrons used in this study are actually very good mimics of galactic cosmic rays impacting the surfaces of outer Solar System bodies since they have similar linear energy transfers to MeV ions within cryogenic molecular solids. However, it is to be noted that the chemistry triggered in ices irradiated by energetic (i.e., keV or MeV) ions and electrons is actually mediated by low-energy (i.e., < 20 eV) secondary electrons released along the track of the primary irradiating particle, as documented by a number of previous studies.^98–100 

We have therefore made use of cosmic ray dose rates of 5.6⋅10^–3 and 1.6⋅10^–8 eV molecule^–1 year^–1 for icy material at the surface of such a Kuiper Belt Object (i.e., within the top 10^–6 cm of the surface material) and for ices buried beneath 10^–3 cm of surface material,^101,102 respectively. Therefore, by dividing our calculated N₂O destruction G-values (Table IV) by 100 and inverting the resultant value, we can express the destruction rate of N₂O at a given temperature in units of eV molecule^–1. Then, by dividing this resultant value by the quoted Kuiper Belt radiation dose rates, we can quantify the survivability of N₂O as the equivalent number of years of exposure to cosmic radiation required to destroy it. This data is depicted in Fig. 8.

A general trend is apparent in Fig. 8 in which the survivability of crystalline N₂O on an icy Kuiper Belt Object (as expressed through the number of years of exposure to cosmic radiation required to destroy it) decreases with increasing temperature. The greatest decrease in survivability appears to occur when increasing the surface temperature from 20 to 30 K, after which the survivability does not seem to change significantly upon further warming to 50 K. However, on warming to 60 K, another noticeable decrease in N₂O survivability is registered. It is also evident that when N₂O is present on the surface of the Kuiper Belt Object [as shown in panel (a) of Fig. 8], its survivability is on the order of just a few thousand years. Conversely, when buried beneath just 10^–3 cm of surface material, this survivability increases to a few billion years. Hence, it is possible to conclude that crystalline N₂O ice may be preserved in the outer Solar System through astronomical timescales if it is shielded from incident ionizing radiation by thin layers of surface material.

Although the results of our study indicate that crystalline N₂O may be preserved on outer Solar System bodies if buried beneath 10^–3 cm of surface material, it is important to note that our study is a somewhat oversimplified representation of processes actually occurring in the outer Solar System. In the first instance, it is likely that the abundance of N₂O on any icy body in the outer Solar System is not only influenced by its destruction via its interaction with ionizing radiation, but rather through a complex chemical network involving photochemical and photophysical processes, low-temperature thermal chemistry, radical combination reactions, sublimation and condensation cycles during surface warming events, and ejection to the gas phase during impacts; none of which has been considered in the present study.

Perhaps more importantly, however, is that solid N₂O in surface and near-surface environments in the outer Solar System is likely a minor constituent in an ice mixture dominated by either N₂ or H₂O. The effect of this mixing on the destruction G-value of N₂O is uncertain since previous studies on the irradiation of nitrogen-bearing species mixed with other molecules have yielded varied results. For instance, Zhang et al.⁹⁷ demonstrated that the radiolytic destruction of pure HOCH₂CH₂NH₂ (i.e., ethanolamine) ice by 1 keV electrons at 20 K results in a more rapid decay compared to the irradiation of a 50:1 mixture of H₂O:HOCH₂CH₂NH₂. Conversely, Bordalo et al.¹⁰ showed that NH₃ is more rapidly destroyed by irradiation with 536 MeV Ni²⁴⁺ ions if it is mixed with H₂O than if it is irradiated as pure ice. As such, it is recommended that future studies quantitatively assess the radiolysis of N₂O in different ice mixtures relevant to the outer Solar System and compare this to the radiolysis of pure N₂O ice. Nevertheless, our present study provides valuable insights into the fundamental radiation chemistry and physics of crystalline N₂O at low temperatures, which will undoubtedly be of use when designing and interpreting the data from more complex experiments performed with the aim of elucidating the radiation chemistry of this molecule in the outer Solar System.

In this study, we systematically irradiated crystalline N₂O ice using 2 keV electrons at five temperatures in the 20–60 K range with the aim of determining the effect of temperature on the radiolytic destruction of N₂O and the formation of different radiolytic products. Our results have conclusively demonstrated that temperature does play a role in this chemistry, with higher temperatures being associated with higher N₂O destruction G-values. Indeed, the relationship between the irradiation temperature and the destruction G-value can be modeled reasonably well by a linear function. The variation in the formation G-value of seven product molecules (i.e., NO, NO₂, N₂O₂, N₂O₃, N₂O₄, N₂O₅, and O₃) with temperature was also assessed. For most product molecules, increased temperatures resulted in increased formation G-values due to the greater mobility of radiolytically derived radicals through the ice matrix; although some exceptions to this trend were noted. Most notably, the formation G-value of NO₂ was found to substantially decrease with increasing temperature, likely as a consequence of its consumption in the synthesis of N₂O₄. More moderate decreases in the formation G-value with increasing temperature were recorded for O₃, probably due to the sublimation of its precursor molecule O₂ at higher temperatures. Finally, we have discussed the applicability of our results to N₂O radiation chemistry on outer Solar System icy bodies, and have provided some tentative evidence for its preservation if buried beneath 10^–3 cm of surface material, although more experimental work is required to validate this.

The authors acknowledge support from the Europlanet 2024 RI which has been funded by the European Union’s Horizon 2020 Research Innovation Programme under Grant Agreement No. 871149. The main components of the ICA set-up at HUN-REN Atomki were purchased using funds obtained from the Royal Society through Grants UF130409, RGF/EA/180306, and URF/R/191018. Further developments of the installation are supported in part by the Eötvös Loránd Research Network through Grants ELKH IF-2/2019 and ELKH IF-5/2020. Support has also been received from the Research, Development, and Innovation Fund of Hungary through Grant K128621. This paper is also based on work from the COST Action CA20129 MultIChem, supported by COST (European Cooperation in Science in Technology). We are also thankful to Gergő Lakatos (University of Debrecen, Hungary) for his helpful comments and suggestions.

Sándor Góbi and Zoltán Juhász are grateful for the support of the Hungarian Academy of Sciences through the János Bolyai Research Scholarship. Sergio Ioppolo acknowledges support from the Danish National Research Foundation through the Centre of Excellence “InterCat” (Grant Agreement No. DNRF150).