Nitrogen Sublimation as a Driver of Chemistry in ... - ACS Publications

Article Cite This: ACS Earth Space Chem. 2019, 3, 1640−1655

http://pubs.acs.org/journal/aesccq

Nitrogen Sublimation as a Driver of Chemistry in Pluto-Analog Laboratory Ices: Formation of Carbon Suboxide (C3O2) and Various Salts Kamil B. Stelmach,*,†,‡ Yukiko Y. Yarnall,† and Paul D. Cooper†,∥ †

Department of Chemistry and Biochemistry, George Mason University, Fairfax, Virginia 20110, United States Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States

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‡

ABSTRACT: A number of planetary bodies, including Triton and Pluto, and a number of Kuiper Belt objects contain nitrogen ices on their surfaces. Nitrogen ices were also used in laboratory experiments as a matrix isolation material before noble gases could be condensed. Planetary bodies with nitrogen ices then may act as giant matrix isolation experiments, trapping reactive species onto the surface and concentrating them. Upon sublimation, these reactive species are much more likely to encounter each other or another molecule to react with. A pilot study was conducted to test the feasibility of testing the chemistry occurring during the sublimation of nitrogen ices. A high vacuum laboratory setup was used to create ices at ∼6 K (±0.5 K). Ices were deposited under microwave radiation to create radicals to simulate what might be present in the tenuous atmospheres of these planetary bodies. The ices consisted of a mixture of 1:1:100 carbon source + H2O + N2, where the carbon source was either CO or CH4. Reagents and products were primarily identified using FTIR and UV−vis transmission spectroscopy. Once the predominantly N2 ice was characterized with the spectroscopic techniques, the N2 was sublimated to create a H2O ice, and then this ice was characterized using the aforementioned techniques. One completely new product was observed, namely, carbon suboxide (C3O2), and a couple products identified in the nitrogen ice formed various salts. Future work could make use of multiple sublimation steps and other astrochemically relevant matrices and introduce more astrophysically relevant sources of radiation like electron beams or UV irradiation. KEYWORDS: planetary science, solid state astrochemistry, matrix isolation, molecular processes, Kuiper belt objects (KBOs), trans-Neptunian objects (TNOs)

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INTRODUCTION Multiple planetary bodies contain nitrogen (N2) ices on their surfaces. These include Triton,1 Pluto,2 and other Kuiper Belt objects (KBOs).3 Schaller and Brown3 showed a dependence on mass that predicts the presence or absence of N2 and other volatile ices. The recent New Horizons mission confirmed the presence of N2 ice on Pluto’s surface along with the presence of CH4, CO, and H2O.4 Interestingly, N2 was used as a matrix isolation material before the technology that allowed for noble gas condensation existed.5 Pluto then may act as a large matrix isolation © 2019 American Chemical Society

experiment where reactive species are trapped in the N2 ice. Condensation may act to concentrate reactive molecules and trap them in the ice where sublimation then encourages the formation of new chemical compounds without the input of additional energy in the form of ion or UV irradiation. The Received: Revised: Accepted: Published: 1640

January 5, 2019 June 3, 2019 June 6, 2019 June 6, 2019 DOI: 10.1021/acsearthspacechem.9b00005 ACS Earth Space Chem. 2019, 3, 1640−1655

Article

ACS Earth and Space Chemistry

unlikely to find H2O in the gas phase due to the cold temperatures involved. The molecule sublimates at 90 K in the interstellar medium and at about 150 K in the laboratory.22 The surface temperatures on Pluto range from 35 to 50 K.4 That being said, H2O has not been observed in the Pluto− Charon atmosphere, but it has been detected on the surface,4 and radiation sources may lead to the formation of typical H2O radiolysis products, which could then be free to react with other molecules on the surface. Both CO and CH4 themselves are volatile and are found in the atmospheres of these bodies, too.9 While Pluto’s predominantly N2 and tholin-containing atmosphere produces a blue haze,23 a red haze on the surface of Pluto putatively comes from tholins,13 which can be easily created from irradiating CH4.9,11 Furthermore, Pluto’s atmosphere consists of about 20 haze layers composed of particles of an estimated size of >1 μm, which themselves might be made of particles of about 10 nm in radius.23 Tholins are thought to make up the chemical composition of these particles.23 One mechanism by which these particles can form, as described for Saturn’s moon Titan, which has a predominantly N2 atmosphere, is through negatively charged molecules attracting positive ions to form larger complexes.24 Charged species could form through interactions with ionizing radiation. The reactive molecules formed in the atmospheres of these bodies could be exchanged with the surface as the N2 condenses. As mentioned, these N2 ice-containing planetary bodies could then act as giant matrix isolation experiments just as N2 was used as a matrix material before noble gases could be condensed.5 The N2 could then act to trap and concentrate reactive species formed in the atmosphere and bring them closer to reactive species form on the surface. Upon sublimation, these relatively concentrated reactive species could find another species to react with, which were either also previously trapped via condensation in the N2 ice or previously present on the surface of the planetary body due to irradiation of the surface. While direct radiation-induced processes are likely to be the dominant drivers of chemistry in icy bodies, an indirect method such as the described sublimation of reactive molecules from a matrix-isolated type surface (e.g., N2, O2, CO, CH4, etc.) could contribute a small, but non-negligible amount of new chemical species that might be vital in understanding the chemical networks of icy bodies containing such matrices. The objective of this Article then is to highlight the results of a pilot study that was meant to replicate some of the astrophysical and chemical conditions that could be found on Pluto (and similar bodies) and test the hypothesis mentioned above. Carried out in a high vacuum chamber, this study used irradiated gas mixtures of simple molecules to deposit a N2 matrix-isolated ice that contained reactive species. Irradiation was through a Tesla coil or a microwave discharge so the setup is not exactly analogous to Pluto’s or Triton’s environment, but the main goal of this study was to show that new molecules could be formed and detected upon the sublimation of the N2 ice. After the results are presented, a discussion will follow on the molecules that were formed and how future studies can be modified to better represent the astrophysical conditions present at the aforementioned planetary bodies.

main objective of this pilot study was to test this idea, but it is not the first study that is chemically relevant to N2 bodies. N2 containing ices have been studied in both a planetary science and astrochemistry context, but they have also been used in experiments outside the domain of the space sciences to trap and study highly reactive species. Within the space sciences, a recent example of N2 ice experiments includes Lo et al.’s identification of O3, N, NO, NO2, N2O, and c-(NO)2 in 1:500 O2 + N2 ices with the purpose of identifying molecules that could be used as indicators of O2 and N2 ices in the interstellar medium6 though, as noted in a later paper, nitrogen oxides form under a number of conditions.7 Another example includes Fedoseev et al.’s irradiation of N2 ices laced with CH3OH, CO, CH4, H2O, or NH3 with 200 keV H+ and He+ ions, which led to the formation of HNCO, OCN−, HCN, and CN−.8 Recent experiments with N2-containing ices with relevance to Pluto include Materese et al.’s irradiation of 1:1:100 CO + CH4 + N2 ices with 1.2 keV electrons at 15−20 K, which produced N-rich organic molecules. De Barros et al. studied 1:10 H2O + N2 ices with KBOs in mind.9 Reported products included N2O, N3, O3, NO, NO2, and NO3. Hudson also recently commented on the aforementioned work, noting skepticism on some of the conclusions (namely, the use of nitrogen oxides as tracers of N2 + H2O ices) and summarizing the major work done so far on N2-containing ices. 7 Vasconcelos et al. also studied N2-containing ices as analogs to KBOs using 1:19 CH4 + N2 ices irradiated by 6−2000 eV photons and 15.7 MeV 16O5+ ions at 12 and 19 K, respectively, to each irradiation source. The formed species included C2H2, C2H4, C2H6, HCN, HNC, NH3, and N3.10 Furthermore, as mentioned, matrix isolation studies utilized N2 before noble gases could be condensed,5 and some matrix isolation experiments continue to utilize N2.11,12 These might be useful in identifying astrophysically relevant products in the future. As one might expect, the N2 ices residing on the surfaces of planetary bodies are very volatile. In the case of Pluto, their presence largely depends on temperature changes due to seasonal or even daily fluctuations.4,13,14 In areas where the N2 ices have sublimated, other ices can be found in its place. These include CO, CH4, and H2O ices.4 Water ices are a large component of a number of planetary bodies in the outer solar system and include many moons,15 dust grains,15 asteroids,16 and dwarf planets.3,4,17 Of these various environments, only the dwarf planets also contain N2 ices.3 The extent to which these two ice types interact is a matter of debate. However, at a very minimum, one can note that water would act as a reagent in a chemical reaction if a reactive species were condensed right on top of it. A number of primordial molecules exist in these systems that could contribute to the chemistry. These include CO and CH4.3,4 The two can be found on Pluto and putatively on other KBOs.3,4,9 Both can act as carbon sources to build larger organic molecules. For example, the main formation mechanism of methanol (CH3OH) in space is by the continuous addition of H atoms into CO.18 However, CH4 is a basic building block of tholins that can be found on Titan, Triton, and Pluto.19−21 Furthermore, these planetary bodies also have tenuous atmospheres. Water molecules can possibly lace the tenuous atmosphere shared by Charon and Pluto due to the presence of water ice on both bodies.9 Therefore, water may act as a minor reagent in the atmosphere. In an environment like Pluto’s, it is

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METHODS The main apparatus consisted of a 6″ cylindrical apparatus that was maintained at a baseline pressure of ∼10−7 Torr, which 1641

DOI: 10.1021/acsearthspacechem.9b00005 ACS Earth Space Chem. 2019, 3, 1640−1655

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where A(v) is the integrated absorption band area, A is the intrinsic band strength specific to that chemical species, and N is the column density. No new A values were tabulated for molecules if it was not found in the literature. A residual gas analyzer (RGA) was used mainly to monitor the integrity of the vacuum. It sat below the main cylindrical setup, but above the turbomolecular pump. The model used was an ExTorr, Inc. XT 100. This model has a range of 0−100 amu with a resolution of 1 amu. The amount of N2 sublimating off the coldfinger disc was also monitored to help ensure the sublimation steps were removing most of the volatile gases. A gas manifold was used to introduce the gases. A background pressure of ∼0.1 Torr was provided by a rotary pump of the aforementioned model. The gas mixtures consisted of 1:1:100 of a carbon source to water to N2. The carbon source used was either methane (CH4) or carbon monoxide (CO). The gases were mixed according to their partial pressures. The CH4 tank was >99% pure (SigmaAldrich, 295477-56L). The CO tank was >99% pure (SigmaAldrich, 296116-50L). The H2O was >99% pure (Milli-O, Millipore Corporation) and was degassed using a standard freeze−thaw technique. The N2 tank was >99.9% pure (Airgas, NI HP300). The incoming gases were deposited at a rate of ∼1.67 Torr/min (a total of 100 Torr being deposited within an hour) while being irradiated using a microwave discharge at 60 W (McCarroll Cavity, Opthos Instrument Company, LLC) or a Tesla coil set to its maximum setting of 50,000 V at a frequency of 0.5 MHz. The nonirradiated spectra were also collected for comparison. The irradiation methods were not meant to simulate any irradiation process found at Pluto or similar environments. Rather, the irradiation techniques were used to ensure the creation of reactive species that could be trapped in the N2 ice to test the idea that reactive species trapped in a Pluto-analog ice can lead to new chemistry. Sublimation was initiated after the ices were characterized by the FTIR and UV−vis spectrometers. Sublimation was controlled manually using the heating unit installed on the coldfinger. The sublimation procedure was determined through trial-and-error with the final goal being the near total sublimation of the N2 and the creation of a predominantly water containing ice. The sublimation occurred stepwise as follows. First, the ice sample was heated up to ∼25 K (±0.5 K) and then to ∼30 K (±0.5 K). The ice sample remained at each temperature for five minutes. The ice temperature was raised by 5 K increments two more times to a temperature of ∼40 K (±0.5 K), but the temperature was kept constant for 10 min intervals. One more temperature increase to a final temperature of ∼45 K was followed up by a 5 min pause, the heater being shut down, and then allowing the system to go back down to ∼6 K (±0.5 K), at which point the spectrum of the resulting water ice was recorded. Table 1 shows a summary of the sublimation recipe. The Tesla coil experiments did not have a sublimation step. The experiment types are summarized in Table 2. Experiment 3 (from Table 2), which consisted of a CH4laced gas mixture that went through a microwave discharge and was characterized by FTIR spectroscopy, did not follow the sublimation procedure outlined in Table 1. The heating ramp was more aggressive as the first five minute step occurred at ∼30 K. Furthermore, the last heating step at ∼45 K went on for a duration of ∼15 min instead of the usual ∼5 min as summarized in Table 1.

Figure 1. Experimental setup. The main vacuum apparatus was a 6″ cylindrical tube that had utilized transmission FTIR (red) and UV− vis (violet) spectroscopy. An RGA (orange) sat below the main apparatus and largely monitored the quality of the vacuum. Gas mixtures were irradiated prior to deposition and delivered onto a ∼6 K (±0.5 K) cold finger (middle) that was rotatable 360°.

corresponds to a high vacuum (Figure 1). Two pumps were used to maintain this baseline pressure. A rotary pump (Model #DS 102) provided a baseline pressure of ∼0.1 Torr, and a Leybold turbomolecular pump (Model #NT 150/360) brought the pressure down to the system baseline pressure. A rotatable coldfinger that could be brought down to temperatures of ∼6 K (±0.5 K) was centered in the apparatus as shown in Figure 1. The coldfinger was cooled using a helium refrigerator from Advanced Research Systems, Inc. The temperature of the spectra recordings was held at ∼6 K (±0.5 K). FTIR transmission spectroscopy was the most important analytical technique used to characterize the ices and identify any new products that might have formed after sublimation. A Newport Oriel MIR80250 Fourier-transform infrared (FTIR) spectrometer was utilized in these experiments. Both KBr and BaF2 were used as the optical discs, and they measured 25 × 2 mm in size. The latter was only used in experiments that utilized UV−vis spectroscopy. An initial background spectrum was collected before deposition. Another spectrum was recorded after deposition and then another after the sublimation of the volatile ices. The final spectra were created by using the equation A = −log10(I/I0) where A is absorption, I0 is the background spectrum, and I is either the deposition or sublimation spectrum. The resulting spectra were in the 4500− 500 cm−1 portion of the mid-IR. A total of 500 scans at a resolution of 1 cm−1 were used to produce the spectra. UV−vis spectroscopy was used as a supplement to the data provided by the FTIR. This consisted of an Ocean Optics DH2000 ultraviolet−visible−near-infrared (UV−vis−NIR) spectrometer that used a deuterium-halogen light source. However, the fiber optic cables (Model #QP-1000-2-UV−vis from Ocean Optics) chosen meant that the spectrometer really only had UV−vis capabilities. As with the FTIR, background, deposition, and postsublimation spectra were all recorded. A total of 1000 scans at an integration time of 7 ms were taken to produce each spectra. Chemical species within the various spectra could be quantified using the equation N=

2.303∫ A(v)dv A

(1) 1642


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ACS Earth and Space Chemistry Table 1. The Sublimation Recipe for forming H2O Icesa Description of Step

Target Temperature (K)

Interval (Min)

∼6

∼120

∼6 ∼25 ∼30 ∼35 ∼40 ∼45 ∼6

∼60 ∼5 ∼5 ∼10 ∼10 ∼5 ∼60

Deposition Of 1:1:100 C Source + H2O + N2 Collection of N2 Ice Spectrum First Sublimation Step Second Sublimation Step Third Sublimation Step Fourth Sublimation Step Fifth Sublimation Step Collection of Water Ice Spectrum

for water (150 K22). Nevertheless, the temperatures chosen are relevant to Pluto: the surface temperature of Pluto ranges from 35 to 50 K4 with a mean surface temperature of 40 K.25 While the temperatures on planetary bodies are not expected to ramp up in 5 K increments, our main goal was to slow down the heating rate to prevent all the chemical species from being removed as was the case in our initial attempts. A slower heating rate is also more relevant for a body like Pluto as its day is approximately 6.39 Earth days25 and it takes the dwarf planet 248.5 Earth years to revolve around the Sun.25 Figure 2 shows some representative RGA data. The figure shows the partial pressure of N2 observed in the vacuum chamber during the 35 K sublimation step for a CH4-laced ice. The amount of N2 sublimating was much higher than other species during all sublimation steps. The same pattern was observed during each sublimation step: the amount of N2 would increase, level off, and then drop off. Increasing the temperature again would result in even more N2 sublimating. While other m/z values were recorded, the 28 amu line dominated as one might have expected due to the initial gas mixture ratios. Only one run was recorded for each type of experiment as the process easily burnt out the RGA filaments. Future experiments could allow for slower sublimation so other m/z values are more easily observed. Once ∼35 K (±0.5 K) is

a

The method used to create the water ice is summarized below. The C source was either CH4 or CO. The interval for the first step consists of deposition and collection of the FTIR spectrum. The interval for the last step just consists of collection of the FTIR spectrum. The microwave discharge experiment with CH4 and utilizing FTIR as the main analytical technique (expt in Table 2) followed a more aggressive sublimation step schedule, which consisted of missing the first step and remaining at ∼45 K for ∼15 min.

The temperature steps and final temperature were chosen based on the expected sublimation point of N2 in a laboratory setting (20 K22), while remaining under the sublimation point Table 2. List of Experimentsa

analysis Expt

Gas Ratio

Gas Mixtures

1

1:1:100

CH4/H2O/N2

Sublimation

4 cm−1

-

2 3

1:1:100 1:1:100

CH4/H2O/N2 CH4/H2O/N2

4 cm−1 4 cm−1

-

4 5 6

1:1:100 1:1:100 1:1:100

CO/H2O/N2 CO/H2O/N2 CO/H2O/N2

4 cm−1 4 cm−1 4 cm−1

-

7

1:1:100

CO/H2O/N2

4 cm−1

-

8

-

N2

Tesla Coil 60 W Microwave Discharge + Sublimation No Irradiation + Sublimation Tesla Coil 60 W Microwave Discharge + Sublimation 60 W Microwave Discharge + Sublimation Deposition

-

9

-

CH4

Deposition

-

10

1:100

CH4/N2

Sublimation

-

11

1:1:100

CH4/H2O/N2

Sublimation

-

12

1:100

CH4/N2

-

13

1:1:100

CH4/H2O/N2

14

1:1:100

CH4/H2O/N2

15

-

CO

60 W Microwave Discharge + Sublimation 60 W Microwave Discharge + Sublimation 100 W Microwave Discharge + Sublimation Deposition

16

1:100

CO/N2

Sublimation

-

17

1:1:100

CO/H2O/N2

Sublimation

-

18

1:100

CO/N2

-

19

1:1:100

CO/H2O/N2

60 W Microwave Discharge + Sublimation 60 W Microwave Discharge + Sublimation

I = 7 ms; B =3 I = 7 ms; B =3 I = 7 ms; B =3 I = 7 ms; B =3 I = 7 ms; B =3 I = 7 ms; B =3 I = 7 ms; B =3 I = 7 ms; B =3 I = 7 ms; B =3 I = 7 ms; B =3 I = 7 ms; B =3 I = 7 ms; B=3

Experiment Description

FTIR

-

-

UV

Mass Spec 1. Mass Sweeps at 6, 35, and 40 K; 2. Mass Trends at 35 and 40 K; 3. Sublimation Mass Trends (TDS)

a

Experiments (expt) are listed numerically with their corresponding gas ratios, gas mixtures, experiment descriptions, and methods of analysis. All experiments except for expt 3 followed the protocol outlined in Table 1. Please see main text or Table 1 for more details. 1643


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Figure 2. Representative RGA data. The N2 RGA trendline (m/z = 28 amu) at the 35 K sublimation step for a CH4-laced experiment. At each sublimation step, there was an initial increase in the N2 desorbing off the surface before a maximum point was reached and decreased again.

Figure 3. FTIR spectra of 1:1:100 CH4 + H2O + N2 experiments; deposit, 4500−2500 cm−1. Blue represents no irradiation, black represents the Tesla coil experiments, and red represents the 60 W microwave-discharge experiments (expts 1, 2, and 3 from Table 2, respectively). This figure shows the spectrum in the domain of 4500− 2500, and the 2500−500 cm−1 portion is on Figure 4. The spectral lines are numbered here and identified in Table 3.

reached, the system was kept stable at that temperature. Ultimately, the sublimation rate would decline, and it resulted in the pressure of the system decreasing; however, a waterdominated spectrum that was not noisy only resulted with further heating. The initial experiments were conducted using a Tesla coil, and the sublimation steps were not added until after the microwave irradiation experiments were started. Multiple experiments were conducted to come up with a sublimation recipe for forming the water ices. The data presented herein is only from the finalized procedure. FTIR spectroscopy was the main workhorse of this project, and this data is presented first. It is followed up by some UV−vis and RGA data. Finally, results from previous studies that have utilized N2 and H2O ices were used to identify products or identify where spectral lines for products should have appeared if the product was present in only one of the ices.

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RESULTS CH4-Laced Experiments Presublimation. Figures 3 and 4 show the FTIR data from the initial deposition of an irradiated 1:1:100 CH4 + H2O + N2 gas mixture. Figure 3 shows the 4500−2500 cm−1 portion of the spectrum, and Figure 4 shows the 2500−500 cm−1 part. The gas mixtures were irradiated either through a Tesla coil or a 60 W microwave discharge. Multiple spectral lines were characterized though some peaks remained unidentified as shown in Tables 3 and 4. In the microwave discharge experiments, the 4500− 2500 cm−1 range of the spectra was opaque. A reddish substance remained on the optical disc after the experiment was over, but this region of the spectrum is no longer opaque after sublimation. This could possibly be due to the chemical species responsible for those absorptions to have also sublimated along with the N2, but leaving behind nonvolatile species that did not absorb strongly in the 4500−2500 cm−1 region of the spectrum. Tables 3 and 4 summarize the data from the peaks from Figures 3 and 4, respectively. The references column lists the previous studies used to help identify the spectra in this study. While Table 3 does not reveal a large number of products for the irradiation experiments, this is not an entirely accurate

Figure 4. FTIR spectra of 1:1:100 CH4 + H2O + N2 experiments; deposit, 2500−500 cm−1. Blue represents no irradiation, black represents the Tesla coil experiments, and red represents the 60 W microwave-discharge experiments (expts 1, 2, and 3 from Table 2, respectively). This figure shows the spectrum in the domain of 2500− 500 cm−1, and the 4500−2500 cm−1 portion is shown on Figure 3. The spectral lines are numbered here and identified in Table 4.

interpretation of the results. This portion of the spectrum was widely blocked out by what were presumably tholins as the resulting deposits were reddish in color and the irradiation of similar gas mixtures forms these species.19 No effort was made to characterize the tholins that were formed. The only molecule identified in this portion of the spectrum for the irradiated gas mixtures was HCN. Table 4 summarizes the 2500−500 cm−1 portion of the aforementioned experiments for the N2-dominated ice. This portion was not opaque in the mid-infrared and thus made identification of products much easier. Table 4 reveals the formation of CO2, HCNO, N2O, HCN, CH2N2, CN, NO, H2CO, NCN, C2H6, CNN, the HCO radical, O3, and NH3. 1644


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postsublimation water-dominant ice, the sublimation of these products along with N2 is assumed. CO-Laced Experiments Presublimation. Similarly, Figures 6 and 7 show data from the initial deposition of an irradiated 1:1:100 CO + H2O + N2 gas mixture. This data set has two trials with microwave irradiation. Again, multiple spectral lines remain unidentified as shown in Table 5. As with the CH4-laced gas mixture, the 4500−2500 cm−1 range of the spectra was opaque, and a reddish substance remained on the optical disc after the experiment was over, which may represent nonvolatile tholins. Tables 6 and 7 summarize the data from the numbered peaks from Figures 6 and 7, respectively. The 4500−2500 cm−1 range of these experiments did not reveal any identifiable peaks as shown in Table 6, though the formation of tholins is also assumed. Figure 7 and Table 7 reveal the formation of similar products to the CH4-laced experiments: CO2, HCNO, N2O, NO, the N3 radical, CH2N2, O3, and NH3. The CO-laced experiments did not form as many molecules, but still resulted in the formation of unstable species, which was a goal for the presublimation and initial deposition portion of the experiment as in this stage we were mainly concerned of forming an ice that might be similar to a Plutonian ice resulting from the condensation of the atmosphere and/or mixing with some species formed from the irradiation of the surface. Figure 8A,B shows the UV−vis data for the 1:1:100 CO + H2O + N2 gas mixture. As before, the spectra are of the N2dominated ices unless stated otherwise. In the CO-laced gas mixture, several peaks at the 250−275 nm range could be the result of polycyclic aromatic hydrocarbons (PAHs). NH was observed again with the CO-laced experiments, but CN was only observed in the CH4-laced ices. As was the case with the CH4-laced experiments, it is assumed that CN sublimated, while the H2O-dominant ice was formed. Table 5 summarizes the UV−vis data. CH4-Laced Experiments Postsublimation. Figures 9 and 10 continue the FTIR data set for the irradiated 1:1:100 CH4 + H2O + N2 gas mixture. These figures show the waterdominated ice that was created after N2 sublimation for the microwave discharge and nondischarge experiments. The 4500−2500 cm−1 portion of the spectrum cleared up substantially compared to the same region of the initial N2dominated ices though some small peaks remained in the 4000−3500 cm−1 region for the microwave-irradiated gas mixture. The amount of water remaining in the irradiated experiment is much lower (∼4.5x lower) than the nonirradiated experiment as demonstrated by the IR absorbances in Table 8, which revealed a peak that we assigned to 2NH4− H2O and that may have remained hidden if it were not for this serendipitous and accidental occurrence that resulted from increasing the rate of sublimation. As mentioned, the finalized sublimation procedure was not utilized for this set of experiments, and the FTIR spectrometer broke down before the experiment could be replicated with the final sublimation procedure. This proved to be somewhat fortuitous though: the smaller water peak reveals the presence of NH3 and a 2NH3−H2O complex as described in Table 5, which may have remained hidden if the final sublimation procedure was performed during this experiment. Since NH3 was observed in the N2, this cannot be regarded as a completely new product. Furthermore, the literature values for the 2NH3−H2O complex mid-infrared peaks in N2 ices were not found. It is possible the complex was present earlier

Table 3. Identities of Products from Figure 3 ID Number

Molecule

1 2 3 4

CH4 CH4 CH4 H2O

5

H2O

6

H2O

7

H2O

8

H2O

9

H2O

10 11 12 13 14

H2O H2O Unidentified H2O HCN

15 16

Unidentified CH4

17

CH4

18

CH4

19

CH4

Experiment

Peak Position (cm−1)

Deposition Deposition Deposition Deposition Tesla Deposition Tesla Deposition Tesla Deposition Tesla Deposition Tesla Deposition Tesla Deposition Deposition Tesla Deposition Tesla Microwave Tesla Deposition Tesla Microwave Deposition Microwave Deposition Tesla Deposition Tesla

4332.2 4310.8 4220.6 3727.2 3727.1 3716.6 3715.4 3697.7 3703.1 3689.7 3691.4 3635.8 3634.5 3548.8 3548.1 3531.0 3502.7 3417.3 3368.4 3287.1 3288.3 3162.1 3032.2 3029.0 3028.3 3014.7 3016.3 2835.3 2836.1 2822.2 2822.6

References This This This This

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The reader is encouraged to review the cited studies for reaction mechanisms. However, the formation of ozone (O3) will be taken of note here. Although the formation of O3 has been observed in water ices,26 the molecule might have been formed here largely due to the vacuum quality. Experiments were performed in a high vacuum setting, whereas many astrophysical experiments are performed in an ultrahigh vacuum setting, which is lower than ∼10−9 Torr. Furthermore, the gas-mixing apparatus had a baseline pressure of ∼0.1 Torr, so a small amount of O2 would also be present in the initial gas mixture. The formation of O3 in trace amounts is also observed in the irradiation of gas mixtures in similar matrix isolation conditions that have even lower baseline pressures.27 Future studies can utilize an isotopologue to differentiate water’s oxygen from oxygen naturally present in the high vacuum system. Figure 5A,B shows the UV−vis data for the 1:1:100 CH4 + H2O + N2 gas mixture. Table 5 identifies the UV−vis peaks. The presublimation N2-dominant ices are shown unless stated otherwise as the postsublimation H2O-dominant ices did not produce any peaks. The A figure (top) shows an overview of the experiments, whereas the B figure (bottom) shows the close-up of the peaks. The overall shape of the line was fairly consistent between experiments. Absorption features consistent with CN and NH were identified in the microwavedischarged experiments. As these were not seen in the 1645


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ACS Earth and Space Chemistry Table 4. Identities of Products from Figure 4 ID Number

Molecule

1

CO2

2

H2O

3

HCNO

4

N2O

5

Unidentified

6

HCN, CH2N2

7

CN Unidentified Unidentified NO

8 9

10 11 12 13

14

15

HCO Radical Unidentified Unidentified Unidentified H2CO Unidentified H2O N3 Radical H2O H2O

16

H2O

17

H2O

18

Unidentified Unidentified Unidentified Unidentified

Experiment Deposition Tesla Microwave Deposition Tesla Microwave Tesla Microwave Tesla Microwave Tesla Microwave Tesla Microwave Microwave Microwave Tesla Tesla Microwave Microwave Tesla Tesla Tesla Tesla Microwave Microwave Deposition Tesla Microwave Microwave Deposition Tesla Microwave Deposition Tesla Microwave Deposition Tesla Microwave Microwave Tesla Microwave Microwave Microwave

Peak Position (cm−1) 2351.5 2349.2 2350.3 2329.0 2328.1 2328.2 2266.6 2267.1 2235.1 2235.9 2139.5 2139.7 2097.5 2098.1 2043.5 2034.3 1993.9 1874.9 1875.8 1861.5 1830.3 1818.7 1758.6 1740.7 1740.7 1701.7 1653.5 1656.4 1657.4 1652.8 1634.8 1630.7 1630.7 1619.9 1619.2 1622.2 1616.1 1601.2 1600.0 1597.2 1576.3 1566.8 1567.1 1564.0 1558.8

ID Number

References

Molecule

Experiment


19

Unidentified Unidentified H2CO

20

NCN CH4

21

C2H6 CH4 Unidentified CH2N2

22

Unidentified CH4

23 24 25 26

Unidentified CNN Unidentified Unidentified Unidentified

27

Unidentified HCO Radical

28 29 30

Unidentified Unidentified O3

31

Unidentified NH3

Microwave Microwave Tesla Microwave Microwave Deposition Tesla Microwave Microwave Microwave Deposition Tesla Microwave Microwave Deposition Tesla Microwave Microwave Tesla Tesla Tesla Tesla Microwave Microwave Tesla Microwave Tesla Tesla Tesla Microwave Microwave Tesla Microwave Microwave Microwave Microwave Microwave Tesla Microwave Tesla Microwave Tesla Microwave Tesla Microwave

1520.4 1505.9 1500.2 1499.0 1476.8 1458.1 1460.6 1463.8 1459.3 1435.5 1407.1 1407.0 1410.4 1336.1 1305.1 1305.9 1303.9 1290.9 1255.4 1206.1 1143.6 1133.4 1133.3 1118.7 1088.5 1089.7 1070.5 1058.1 1047.7 1046.0 1043.0 970.4 969.4 897.6 611.4 969.4 897.6 746.4 747.6 738.4 736.9 661.8 662.0 608.0 611.4

38

This Study

38 38

37 38

38 38

38

This Study 38 This Study This Study

Unidentified

This Study

This Study

32

Unidentified HCN

33

Unidentified

34

CO2

35

CH3 Radical

References

38 37 This Study 38 This Study 38

This Study

37

38

39

39

H2CO3, and O3. A few of these were not seen in the N2 spectrum. The molecules C3O2, OCN−, HCO3−, and HCOO− were not seen in the original N2, but only C3O2 could be confirmed as a completely new product as the positions of the other three molecules are not known in the N2 ice. It is possible that NH4+ was formed as a peak does appear near 1438.4 cm−1, but that peak also appears as a set of lines present in the nonirradiated experiments. Performing an experiment with an isotopologue could help confirm its presence or absence in the future. Its presence in the irradiated experiments would make sense as a number of anions were formed so presumably other molecules like NH3 could have picked up the H atoms. That transfer could result in the formation of salts

presublimation as many of the spectral lines are unidentified. One product not seen in the original N2 ice was identified: carbon suboxide (C3O2). Furthermore, ammonium cyanate (NH4OCN) seems to have formed after the sublimation process was complete though both ammonia (NH3) and isocyanic acid (HOCN) were observed in the N2 ice. Tables 8 and 9 provide the identities and column densities of the peaks found in Figures 9 and 10, respectively. Figure 10 and Table 9 move on to describe the 2500−500 cm−1 portion of the spectrum for the resulting postsublimation water ice for the CH4-laced experiments. Several molecules were identified and are listed in Table 8: CO2, C3O2, OCN−, HCN, NO, HCO3−, HCOO−, C2H6, N2H4, HCOOH, N2H4, 1646


Article


Figure 5. UV−vis spectra of 1:1:100 CH4 + H2O + N2 experiments. Red represents the microwave-discharge experiments, and blue represents no irradiation (expts 8, 9, 10, 11, 12, 13, and 14 from Table 2). The spectra were captured after the initial deposition (N2 ice). (A) UV−vis spectra in the domain of 250−800 nm. The portion shown in part B of this figure is highlighted in red. (B) UV−vis spectra in the domain of 250−450 nm. The experiment types are labeled in each spectrum, but only experiments with all three chemical reagents yielded peaks in the UV−vis. The spectral peaks are identified in Table 5.

Table 5. UV−vis Peaks of Products from Figures 5 and 8 Peak Position (nm)

Molecule

Electronic Transition

References

378 336

CN NH

(0,0) (0,0)

39 39

Figure 7. FTIR spectra of 1:1:100 CO + H2O + N2 experiments; deposit, 2500−500 cm−1. Blue represents no irradiation, black represents the Tesla coil experiments, and red represents the 60 W microwave discharge experiments (expts 4−5, 6, and 7 from Table 2, respectively). This figure shows the spectrum in the domain of 2500− 500 cm−1, and the 4500−2500 cm−1 portion from this experiment is shown on Figure 6. The spectral lines are numbered here and identified in Table 7.

Figure 6. FTIR spectra of 1:1:100 CO + H2O + N2 experiments; deposit, 4500−2500 cm−1. Blue represents no irradiation, black represents the Tesla coil experiments, and red represents the 60 W microwave discharge experiments (expts 4−5, 6, and 7 from Table 2, respectively). This figure shows the spectrum in the domain of 4500− 2500 cm−1, and the 2500-500 cm−1 portion is shown on Figure 7. The spectral lines are numbered here and identified in Table 6.

production of C3O2 and the formation of the NH4OCN salt. No other new products or transformations were observed. The spectra for all experiment types saw an increase in the noise levels in the 750−500 cm−1 portions of the spectra, and for one microwave irradiated experiment, this extended to the 1250− 750 cm−1 range, too. Tables 10 and 11 provide the identities and column densities of the peaks found in Figures 11 and 12, respectively. Figure 11 and Table 10 show the results from the 4500− 2500 cm−1 portion of the spectrum. Only the water reagent is observed in the portion and, as with the CH4-laced ice this portion of the spectrum is no longer opaque. Figure 12 and Table 11 summarizes the products identified in the 2500−500 cm−1 range: CO2, C3O2, OCN−, CO, NO, HCOOH, HNCO−,

like ammonium cyanate (NH4OCN). These H atoms might have also combined on the surface during sublimation to form H2. Future studies could also look at the overall charge of the ices formed to see if a net loss of H2 results in charged ices. Figures 11 and 12 present the FTIR data set for the waterdominated ice that was created for the irradiated 1:1:100 CO + H2O + N2 gas mixture. One of the microwave discharge experiments appears to be cleaner than the other, though both are no longer opaque in the 4500−2500 cm−1 region. Like the CH4-laced gas mixture, the CO-laced mixture saw the 1647


Article

ACS Earth and Space Chemistry Table 6. Column Densities of Products from Figure 6 ID Number

Molecule

Experiment


1 2

CO H2O

3 4 5 6

H2O H2O H2O H2O

7 8 9

H2O H2O CO

Deposition Deposition Tesla Deposition Deposition Deposition Deposition Tesla Deposition Deposition Deposition

4524.0 3727.0 3727.2 3716.2 3699.4 3689.2 3635.2 3635.7 3550.4 3368.9 3228.9

Table 7. Identities of Products from Figure 7

References

ID Number

This Study This Study This This This This

Study Study Study Study

This Study This Study This Study

NH4+, H2CO, and O3 are observed. Notably, C3O2 and OCN− pop up again, but a positive identification can also be made for HNCO− and NH4+. The latter was suspected to be present in the CH4-laced experiments too and the peak might have been above the noise level if the same sublimation procedure was performed consistently between experiments. The presence of HNCO− represents a new anion not seen with the other carbon source though one can easily see why this anion would more easily form with CO. It is possible that the sublimation rate affects the degree to which molecules deprotonate. Alternatively, the amount of proton acceptors could influence whether certain products will deprotonate. Furthermore, once a certain group of proton acceptors is fully protonated then it is possible that deprotonation ceases. Finally, Table 12 shows a list of the chemical species identified along with a list of their positions in the mid-infrared within N2-dominated ices and in H2O-dominated ices. If the position is known for a molecule in both ices but only appeared in the N2 ice then it was treated as a new molecule that formed during sublimation. Carbon suboxide (C3O2) was the only new molecule formed though several molecules appeared in their deprotonated form in the water-dominated ice.

■

DISCUSSION A number of molecules were formed as shown in the previous figures and their respective tables, but the focus here will be on the new molecules formed during sublimation. Table 12 shows a comparison of the chemical species observed in the N2 ice compared to the postsublimated water ice. Only C3O2, NH4+, and OCN− appear as the new products postsublimation. The charged species suggest a NH4OCN salt. HCO3− and HCOO− could also possibly act as the anions. As NH3 and OCN were both observed in the N2 ice, the thermal changes then only represent a transfer of a hydrogen atom to form a salt. Furthermore, the complex 2NH3−H2O was identified in CH4-laced experiments in the postsublimation water-dominated ice, but not knowing where the complex shows up in the mid-infrared in N2-dominated ices meant that this complex could not be treated as having uniquely formed in the sublimation process. However, this complex most likely formed during the sublimation process as it is improbable to have existed in an N2-majority ice as the ratio between water and N2 was 1:100. The NH3 molecules, which were observed to have formed in the presublimation N2-majority ices, would have then been able to reorient themselves onto a water molecule during sublimation. 1648

Molecule

Experiment

1

CO2

Deposition Tesla Microwave

2

H2O

3 4

HCNO N2O

Deposition Tesla Tesla Tesla Microwave

5

CO


6

CO


7

NO

Tesla Microwave

8

H2O N3 Radical

Deposition Tesla Microwave Microwave Microwave Microwave Deposition Tesla Microwave Deposition

9

H2O N3 Radical H2O H2O

10

H2O

Tesla Microwave Microwave Deposition Tesla Microwave

11

H2O

12

Unidentified

13

CH2N2

14

N2O

15

Unidentified

Microwave

16

O3

Microwave

17

NH3

Tesla Microwave

18

CO2

Tesla Microwave

19

Unidentified

Tesla Microwave

Tesla Microwave Tesla Microwave Tesla Microwave

Peak Position (cm−1) 2349.6 2348.9 2349.4 2349.6 2328.8 2328.6 2266.5 2235.1 2235.8 2235.9 2140.3 2140.1 2139.8 2140.0 2093.1 2092.8 2093.0 2093.4 1875.3 1875.4 1875.0 1653.8 1656.8 1658.0 1652.9 1657.1 1652.2 1628.4 1630.4 1631.1 1620.1 1616.5 1616.7 1616.5 1617.0 1598.1 1599.4 1597.9 1598.2 1568.3 1567.9 1410.4 1410.3 1290.8 1292.1 1291.8 1118.0 1118.6 1042.8 1043.4 969.5 969.8 969.2 661.5 662.7 662.7 588.1 588.0 588.3

References 38

This Study 38 38

This Study

This Study

38

This Study 38 This Study 38

This Study This Study

This Study

This Study

40 40

40 40

38


Article


Figure 8. UV−vis spectra of 1:1:100 CO + H2O + N2 experiments. Red represents the microwave-discharge experiments, and blue represents no irradiation (expts 8, 15, 16, 17, 18, and 19 from Table 2). The spectra were captured after the initial deposition (N2 ice). (A) UV−vis spectra in the domain of 250−800 nm. The portion shown in part B of this figure is highlighted in red. (B) UV−vis spectra in the domain of 250−450 nm. The experiment types are labeled in each spectrum, but only the experiment with all three chemical reagents yielded peaks in the UV−vis. The spectral peaks are identified on Table 5.

Figure 9. FTIR spectra of 1:1:100 CH4 + H2O + N2 experiments; H2O ice, 4500−2500 cm−1. Blue represents no irradiation, and red represents the 60 W microwave discharge experiment (expts 1 and 3 from Table 2, respectively). This figure shows the spectrum in the domain of 4500−2500 cm−1, and the 2500−500 cm−1 portion is shown on Figure 10. The spectral lines are numbered here and identified in Table 8.

Figure 10. FTIR spectra of 1:1:100 CH4 + H2O + N2 experiments; H2O ice, 2500−500 cm−1. Blue represents no irradiation, and red represents the 60 W microwave discharge experiments (expts 1 and 3 from Table 2, respectively). This figure shows the spectrum in the domain of 2500−500 cm−1, and the 4500−2500 cm−1 portion is shown on Figure 9. The spectral lines are numbered here and identified in Table 9.

The objective of the experiments was to see if sublimation can play an important role in forming new molecules from reactive species trapped in matrix-isolation type ices before moving on to more astrophysically relevant conditions. As one might have expected, not a lot of new molecules formed in the sublimation of the N2 ice, but the process did form a number of molecules that did not form initially through irradiation alone. Furthermore, it is possible that more new molecules were formed but were not observed or identified (a number of peaks in the FTIR tables remain unidentified). The rate of sublimation may play a large role in the number of new products that form. Figure 13 shows how the ices interacted based on whether fast or slow sublimation was used when coming up with the sublimation recipe for the described experiments. In Figure 13A, the cartoon shows the initial N2-dominated ice. Figure

13B shows the resulting ice after a relatively quick sublimation step: the N2 molecules leave so fast that they result in an expansive effect that pushes off other molecules that would have normally stayed on the surface because their sublimation temperatures have not been reached. Figure 13C shows that slowly sublimating the original N2 ice allowed molecules to reorient themselves on the surface to form a new ice type. While not quantitatively defined in this study, keeping this in mind for future experiments will help produce more astrophysically relevant scenarios. For example, the Plutonian day is ∼153 h so the sublimation process is slow, which increases the chances of nonvolatiles remaining on the surface. For this study, the condensation of the irradiated gas mixture meant to serve as an analog to a tenuous atmosphere condensing onto a surface. Once temperatures fall to a point 1649


Article


Molecule

Experiment


Peak Area

1

H2O Dangling Bonds

2 3

NH3 H2O

4 5

2NH3−H2O CH4

Deposition Microwave Microwave Deposition Microwave Microwave Deposition Microwave

3668.7 3747.3 3372.7 3272.0 3248.9 3130.2 3011.1 3014.2

0.0408784 0.0465973 0.26964127 59.063393 13.3170032 0.5037546 0.56549697 0.2049495

Integration Value, A (cm molecule−1) 1.30 2.00 2.00 1.90 1.90

Column Density 5.3 6.8 1.5 6.9 2.5

× 10−17 × 10−17 × 10−17 × 10−17 × 10−17

References 41

× 1016 × 1017 × 1017 × 1017 × 1017

22 22 42 43

Table 9. Column Densities of Products from Figure 10 Molecule

Experiment


Peak Area

Integration Value,A (cm molecule−1)

1 2 3 4 5 6

CO2 C3O2 OCN− HCN NO H2O

7 8 9 10

HCO3− HCOO− C2H6 CH4

11 12 13 14 15 16

N2H4 HCOOH N2H4 H2CO3 O3 H2O

Microwave Microwave Microwave Microwave Microwave Deposition Microwave Microwave Microwave Microwave Deposition Microwave Microwave Microwave Microwave Microwave Microwave Deposition Microwave

2347.7 2234.5 2165.2 2101.3 1869.5 1661.9 1688.4 1627.1 1588.5 1463.3 1301.6 1303.5 1287.6 1222.3 1101.1 1082.5 1037.0 797.2 834.1

0.4297227 0.6514317 0.4358288 0.2799571 0.2217865 1.297876 1.3073058 0.0484772 0.5367144 1.524259 0.5311936 0.3363873 0.1111372 0.24894 0.0099013 0.0035324 0.0800907 7.273211 1.914134

2.1 × 10−16 2.1 × 10−16 1.2 × 10−17 1.2 × 10−17 4.16 × 10−18 7.0 × 10−18 7.0 × 10−18 3.0 × 10−18 1.5 × 10−17 2.0 × 10−17 1.5 × 10−17 2.8 × 10−17 2.8 × 10−17

ID Number

Column Density 4.7 1.2 2.5 2.5 8.4 1.7 1.1 8.5 3.8 1.1 1.6 6.0 1.6

× 1015 × 1016

× 1017 × 1017

× × × × × ×

1017 1017 1017 1016 1017 1015

× 1017 × 1017 × 1017

References 18 44 45 46 39 43 47 48 49 43 50 18 50 47 51 22

Figure 11. FTIR spectra of 1:1:100 CO + H2O + N2 experiments; H2O ice, 4500−2500 cm−1. Blue represents no irradiation and red represents the 60 W microwave discharge experiments (expts 4−6 and 7 from Table 2, respectively). This figure shows the spectrum in the domain of 4500-2500 cm−1, whereas Figure 12 shows the 2500500 cm−1 portion. The spectral lines are numbered here and identified in Table 10.

Figure 12. FTIR spectra of 1:1:100 CO + H2O + N2 experiments; H2O ice, 2500−500 cm−1. Blue represents no irradiation, and red represents the 60 W microwave discharge experiments (expts 4− 6 and 7 from Table 2, respectively). This figure shows the spectrum in the domain of 2500-500 cm−1 whereas Figure 11 shows the 45002500 cm−1 portion. The spectral lines are numbered here and identified in Table 11.

where condensation does occurwhether it be on Pluto, another Kuiper Belt object, or a moon like Tritonreactive

species should get trapped in the N2 ice with the planetary body acting as one giant matrix-isolation experiment. 1650


Article


Molecule


Peak Area

Integration Value, A (cm molecule−1)

Column Density

References

1

H2O Dangling Bonds H2O

0.0227136 0.0161899 58.07854 45.14026 44.05468

2.0 × 10−16 2.0 × 10−16 2.0 × 10−16

6.9 × 1017 5.2 × 1017 5.1 × 1017

41

2

3745.3 3744.8 3270.4 3270.8 3267.4

22

Table 11. Column Densities of Products from Figure 12 ID Number

Molecule

1

CO2

2

C3O2

3

OCN−

4

CO

5 6

O3? NO

7

HCOOH

8

HNCO-H2O

9

NH4+

10

CH4

11

H2CO

12

O3

13

H2O


Peak Area

Integration Value,A (cm molecule−1)

2345.6 2345.9 2238.2 2235.4 2168.5 2173.7 2138.5 2137.5 2137.7 2108.1 1866.0 1866.4 1740.5 1739.2 1591.9 1593.9 1436.8 1438.4 1297.2 1298.3 1261.0 1262.3 1038.7 1036.1 790.5 788.0

0.8168616 1.007183 0.9949156 0.7755232 0.2374905 0.3270420 0.153576 0.0788173 0.089473 0.0359813 0.5307958 0.7255366 0.0697788 0.0625454 0.4408795 0.5319421 0.4976224 0.6643888 0.6504616 0.6222915 0.0442189 0.0425924 0.0454777 0.0895588 5.054395 5.534537

2.1 2.1 1.3 1.3 1.7 1.7 1.7 7.0 7.0 1.5 1.5 2.8 2.8

× × × ×

10−16 10−16 10−16 10−16

× 10−16 × 10−16 × 10−16

× 10−18 × 10−18

× × × ×

10−17 10−17 10−17 10−17

Column Density

References

× × × ×

18

9.0 1.1 1.7 1.4 2.1 1.1 1.2 2.1 2.0 7.0 1.4 4.2 4.6

1015 1015 1015 1015

44 45

× 1015 × 1015 × 1015

18

52 40 53 45 46 × 1015 × 1015

43 42

× × × ×

1015 1015 1015 1015

51 22

molecule has previously been believed to be involved in the origin of life due to its polymerizing abilities.31 Interest in the molecule was sparked up again in the 1990s with the appearance of comet Halley, and the molecule was suggested as contributing to the comet’s organic emission features.32 This suggestion inspired a number of follow-up studies relevant to this work. The C3O2 molecule was found to form in solid-state experiments. It appears that the first instance of the molecule forming in an astrophysically relevant setting was in a CO2 + H2O mixture.33 The molecule was formed in CO, CO2, and H2O ices with the aforementioned carbon sources laced in the ice.29 The routes for the molecule’s formation from CO include reactions CO + C2O → C3O2 and C + 2CO → C3O2.28 While there is no confirmed identification of C3O2 in interstellar space, another related carbon oxide, C3O, has been found, which is formed along with C3O2 in CO irradiated ices.34 We are not aware of any evidence that the molecule might be present on Pluto or other similar bodies, but it is a possibility that should be investigated. The molecule’s aforementioned polymerizing abilities make it a potential candidate in the formation of particles in the haze layers of Pluto’s atmosphere. The experiments conducted here only

While the experiments discussed here are really little more than a pilot study, a couple important astrophysical implications can be made that are relevant to Pluto, other KBOs, and Triton. The first revolve around C3O2 formation, which was observed to form with both carbon sources. The molecule can form in one of two reactions: CO + C2O and C + 2CO.28 It did so in our experiments fairly efficiently and was present in levels just under an order of a magnitude of water according to the column density calculations in CH4-laced ices (∼1016 vs 1017 molecules cm−2) and a couple of orders of magnitude in the case of CO-laced ices (∼1015 vs 1017 molecules). A direct comparison cannot be made between the two carbon sources since the heating rate used for the microwave discharged CH4-laced experiment was higher than either one of the CO experiments. The molecule was previously observed to form or believe to have been formed in the irradiation of solid CO and CO2 ices.29,30 The C3O2 to carbon source ratio in the water ice is higher for the CO-laced experiments even though the sublimation temperature of CO is lower than that of CH4.22 This is consistent with the idea that most of the C3O2 is formed from the CO in those experiments utilizing the molecule as its carbon source. The C3O2 molecule actually has some previously established astrochemical and astrobiological significance. First, the 1651


Article

ACS Earth and Space Chemistry Table 12. Products Created in the Sublimation Processa Present in N2 Ice? (Y/ N)

Position in H2O Ice (cm−1)

New Molecule Formed? (Y/N)

Experiment

Molecule

Position in N2 Ice (cm−1)

1:1:100 CH4+H2O +N2

NH3

970

Y

3372.7, 3130.2

N

CO2

Y

2347.7

N

C3O2 OCN− HCN NO HCO3− HCOO− C2H6 N2H4

2350.3, 662 2253 ? 2098.1 1875.8 ? ? 1463.8 ?

N ? Y Y ? ? Y ?

Y ? N N ? ? N ?

HCOOH H2CO3 O3 CO2

? ? 1042.8 2349.4

? ? ? Y

2234.5 2165.2 2101.3 1869.5 1627.1 1588.5 1463.3 1287.6, 1101.1 1222.3 1082.5 1037.0 2345.9

C3O2 OCN− NO HCOOH HNCO NH4+ H2CO O3

2253 ? 1875 ? ? ? ? 1042.8

N ? Y ? ? ? ? Y

2235.4 2173.7 2137.7 1739.2 1593.9 1438.4 1262.3 1036.1

Y ? N ? ? ? ? N

1:1:100 CO +H2O+N2

While other mechanisms have been used to explain tholin hazes in similar bodies like Titan,24 it is possible that alternative mechanisms exist on this body. Furthermore, the formation of charged species like NH4+, OCN−, HCO3−, and HCOO− is reminiscent of current ideas of how haze particles are formed, which make use of charged species to build up particle size.23,24 It is possible that the charged molecules formed in this study might also form on the surface of Pluto or on small solid particles in the Plutonian atmosphere. Other laboratory studies have shown how thermal effects influence NH4OCN formation. One study utilizing formamide ice saw formation of the molecule with energy processing using 200 keV protons.33 The presence of the molecule increased in that study as temperature went up, but decreased once the sublimation temperatures were reached for the components. A mechanistic study regarding the formation of the salt NH4COOHa salt that may have been present in the ices from this study as HCOO− was one of the observed molecules in the water icewas also conducted by another group.35 A similar pattern in the molecule’s abundance was observed with increasing temperatures. Column densities for these salts could not be calculated in this study due to the lack of band strength values in the literature. Bergner et al. also calculated a critical temperature of 3 K for the reaction for NH4COOH salt formation to proceed.35 The authors noted that the reaction should proceed under 10 K. However, above 10 K and in the presence of volatile ices like N2, CO, and CH4, it is possible for those volatile molecules to transfer some of their kinetic energy during sublimation and thus reorienting the molecules into a more favorable formation to form a salt. This idea can be extended out to other reactions and thus might be important in ice and mineral interactions with volatile ices serving to move surrounding molecules onto a surface in a more energetically favored orientation. Future experiments with the RGA could program a steady sublimation step to make the experiments more astrophysically relevant since sublimation occurring in steps in the relevant planetary bodies is unlikely. These experiments could explore the effects that sublimation time, sublimation cycles, and maximum temperature have on the resulting ice. The sublimation procedure used was also efficient in removing more CO off the sample compared to the CH4 ice as demonstrated by the ratio of the column densities between the carbon sources and the amount of what’s leftover on the ices. CH4 is in a roughly 1:1 ratio with water, whereas CO is a couple orders of magnitude lower compared to the abundance

? ? ? N

a

Studying the IR spectra of the aforementioned experiments reveals that some products not present in the original N2 ice did manage to show up as products in the new water ice that was created after sublimation. Comparing what the wavenumbers of products found in the water ice sample should have been in the nitrogen ice allowed us to see what new products were created in the sublimation process.

went up to ∼45 K, but Pluto can reach temperatures of ∼55 K. It is possible that C3O2 makes it to the gas phase at the higher temperature. Furthermore, a high enough sublimation rate of an abundant and volatile compound, like N2, could push C3O2 along to the gas phase too though the molecule might not be stable enough for this to occur. In certain areas of Pluto, the abundant and volatile compound could be CH4 or CO with the latter being the more likely candidate as it is a reagent involved in the formation of C3O2.

Figure 13. Quick vs slow sublimation procedures. (A) Initial deposit. (B) Quick sublimation of the N2 ice. (C) Slow sublimation of the N2 ice. 1652


Article


tenuous atmospheres of various planetary bodies, reactive species might become relatively concentrated with these planetary bodies acting as giant matrix isolation experiments. During the course of sublimation, C3O2 was formed, NH3 was protonated to NH4+, and HOCN was deprotonated to OCN−. Other reactions might be possible as there were many unidentified lines and large portions of the N2 ices were opaque to the mid-infrared region. Microwave discharges were used to create the radical species here to replicate the reactive species that might be found in the tenuous atmospheres of planetary bodies. This and similar experiments could give us an insight on how sublimation from either the surface or icy particles can affect the chemistry of Pluto and other KBOs. The formation of C3O2 may mean that it helps contribute to the reddish color observed on the surface of Pluto, which is characteristic of this molecule and other tholins. Furthermore, it may contribute to the formation of one or more of Pluto’s haze layers due to the molecule’s polymerizing ability. Temperature fluctuations, either due to daily or seasonal changes, could then help contribute to the formation of more C3O2 and ultimately more particles that could be involved in haze layer production. Lastly, other potential charged products that were observed in these experiments could also lead to haze layer production through mechanisms devised for similarly charged species on Titan. Future experiments can use more astrophysically relevant sources of radiation (e.g., electron beam, UV, etc.). These experiments can also explore other astrochemically relevant matrices that might have more relevance to either other areas of the mentioned planetary bodies, other planetary bodies altogether, and/or have relevance to the ISM. Candidates would include CO, CH4, NH3, and O2. Finally, future experiments of this type can be constrained by additional data on the chemistry actually found at these bodies.

of water molecules. This makes sense since the sublimation temperature of pure CH4 in a laboratory vacuum setting is about 30 K compared to 25 K for CO (N2 is 22 K).22 Changing the heating rate could be used to change the amount of other volatiles remaining on the surface (e.g., ramping the heating rate up would result in more CO or CH4 sublimating alongside the N2, whereas ramping it down would allow a greater fraction of those carbon sources to stay on the surface while preferentially removing the N2). Figure 14 shows how future studies can incorporate the sublimation technique in solid state astrochemistry experi-

Figure 14. Sublimation as an analog to evaporation in organic chemistry experiments. Sublimation in solid-state astrochemical experiments performed in vacuum chambers could (i) better replicate the astrophysical environments relevant to comets, KBOs, and ices in the ISM, (ii) be used to prepare a chemical species that could be difficult to introduce otherwise (e.g. the N3 radical), and (iii) be coupled with radiation sources and multiple sublimation steps to devise synthesis steps that could be used to create molecules seen in planetary bodies or in the ISM. In short, in addition to photon or ion radiation, sublimation may provide one more tool in the experimental chemist’s arsenal to create molecules. Future planetary science or astrochemistry experiments could make use of these steps to create reagents that would normally be too reactive to obtain.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kamil B. Stelmach: 0000-0001-5724-045X Paul D. Cooper: 0000-0002-5573-3549

ments. While thermal desorption spectrometry is a common technique used in solid state astrochemistry,36 sublimating ice samples to form new types of ices within any given experiment has not been done. The possible combinations of starting ices are numerous, and astrophysical observations can be used to determine starting ice combinations. Similarly, astrophysical observations can be used to determine the sequence of sublimations and the number of sublimation steps and they can be used to choose the type of radiation directed at a particular ice or sublimation step. The ability to sublimate, condense, and add different types of irradiation sources will make future astrochemistry experiments more similar to classic wet lab chemistry experiments and will better mimic astrophysical processes. The advantage of adding multiple steps within astrochemistry experiments is that one can create reagents that would otherwise be impossible to introduce into the system due to high reactivity.

Present Address ∥

Department of Chemistry, Yale University, New Haven, CT 06520, United States

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.B.S. would like to acknowledge the NASA Earth and Space Science Fellowship (NNX13AO01H) and the Department of Chemistry at George Mason University for helping to fund the research contained within. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. (DGE1356109). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the

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CONCLUSION This Article outlined a pilot study meant to show that important chemistry may happen during the sublimation of N2 ices on various planetary bodies. As N2 condenses from the 1653


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National Science Foundation. K.B.S. would also like to thank Dr. Harold Linnartz from Leiden University for his discussion regarding HNCO. We would also like to acknowledge the two anonymous reviewers of this manuscript who have helped to greatly improve its quality.

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