CO and N2 Produced from H2O, CO2, and NH3 Cometary Ice Analogs

This study demonstrates that while photodissociation of ammonia in cometary ice analogs can account for most observed molecular nitrogen, the substantial carbon monoxide abundances in many comets likely require low-temperature entrapment rather than in-situ chemical processing.

The Gist

The Cosmic Ice Lab: How Comets Might Be Cooking Up Their Own Secrets

Imagine a comet not as a dirty snowball, but as a frozen time capsule. For decades, astronomers have looked at these icy wanderers to figure out where and how they formed in our solar system's infancy. The logic was simple: if you find a super-cold gas like Carbon Monoxide (CO) or Nitrogen (N2) frozen inside a comet, it must have formed in a very cold place, far from the Sun, where those gases could freeze.

But this new study asks a tricky question: What if the comet didn't just freeze those gases from the air? What if it cooked them up inside the ice itself?

Think of it like this: You walk into a kitchen and see a bowl of flour and a bowl of sugar. You assume the baker just mixed them together. But what if the baker actually baked a cake, and the flour and sugar were just the ingredients that became the cake?

This paper is about the "baking" process inside comets.

1. The Ingredients: The Cosmic Kitchen

The researchers started with "ice analogs"—fake cometary ice made in a lab. They mixed the most common ingredients found in space:

• Water (H2O): The main ingredient, like the flour in our cake.
• Carbon Dioxide (CO2): A common gas, like the sugar.
• Ammonia (NH3): Another common gas, like the baking powder.

They put these mixtures in a vacuum chamber and cooled them down to 10 Kelvin (that's -263°C, colder than almost anywhere in the universe). Then, they "zapped" the ice with two things:

1. UV Light: Simulating the radiation from nearby stars.
2. Electrons: Simulating cosmic rays hitting the ice.

2. The Reaction: Breaking and Making

When you zap these ices, you break the big molecules apart.

• The CO2 Experiment: When they broke apart the Carbon Dioxide, it didn't just disappear. It reassembled into Carbon Monoxide (CO).
• The Ammonia Experiment: When they broke apart the Ammonia, it reassembled into Nitrogen gas (N2).

It's like taking a Lego castle (CO2) and smashing it with a hammer, only to find that the bricks naturally snap back together to form a different shape (CO).

3. The Results: How Much Did They Make?

The team measured how much "new" gas was created compared to the original ingredients.

• In Pure Ice (No Water): The reaction was very efficient. About half of the CO2 turned into CO, and a good chunk of Ammonia turned into Nitrogen.
• In Water-Rich Ice (Realistic Comets): This is where it gets interesting. When they added a lot of water (which is what real comets are mostly made of), the reaction slowed down. The water molecules acted like a cage, trapping the broken pieces and making it harder for them to find each other to rebuild.
  • CO Production: They made a little bit of CO (about 0.4% to 0.9% of the total ice).
  • N2 Production: They made a tiny bit of N2 (about 0.03% to 0.7% of the total ice).

4. The Big Reveal: Solving the Comet Mystery

Now, the researchers compared their "kitchen" results to real comets, specifically the famous Comet 67P (visited by the Rosetta spacecraft).

The Nitrogen Mystery (N2):

• The Old Theory: We thought the Nitrogen in comets was frozen gas from the very cold outer solar system.
• The New Theory: The lab results show that the amount of Nitrogen found in Comet 67P can be completely explained by the "cooking" process (photodissociation) of Ammonia inside the ice.
• The Smoking Gun: The Nitrogen in 67P has a specific "fingerprint" (isotopic ratio) that matches the Ammonia in the same comet, but not the Nitrogen gas in the solar system. This strongly suggests the Nitrogen was made inside the comet from Ammonia, not frozen from the air.

The Carbon Monoxide Mystery (CO):

• The Old Theory: Same as above.
• The New Theory: The lab only produced a tiny amount of CO. But Comet 67P has a lot of CO (much more than the lab could make).
• The Conclusion: For CO, the "cooking" theory doesn't work well enough. The high amounts of CO in many comets likely do mean they formed in extremely cold places where CO gas froze directly onto the ice.

5. The Takeaway

This study is like a detective story with two suspects: Freezing (trapping gas from space) and Cooking (making gas inside the ice).

• For Nitrogen: The evidence points to Cooking. The nitrogen we see in comets was likely made right there in the ice from ammonia, meaning we can't use Nitrogen levels to prove how cold the comet's birthplace was.
• For Carbon Monoxide: The evidence points to Freezing. There's too much of it to have been made inside the ice, so it probably froze from the gas, telling us the comet did indeed form in a very cold spot.

In short: Comets are complex chefs. They didn't just freeze the ingredients they found; they also cooked up some new dishes inside their frozen hearts. By understanding this, we can better read the history of our solar system.

Technical Summary

1. Problem Statement

Comets contain hypervolatile species such as carbon monoxide (CO) and molecular nitrogen (N2). Traditionally, the presence of these species has been interpreted as evidence that cometary building blocks formed at extremely low temperatures (below 20–30 K), allowing these gases to freeze out or become trapped (entrapment) within amorphous water ice mantles. However, this interpretation relies on the assumption that these hypervolatiles were inherited directly from the gas phase.

The authors propose an alternative hypothesis: that a significant portion of cometary CO and N2 could be generated in situ through the photodissociation (photolysis) or radiolysis of less volatile parent molecules (specifically CO2 and NH3) within icy grain mantles during the protostellar or protoplanetary disk phases. If this mechanism is efficient, it challenges the use of CO and N2 abundances as strict thermometers for comet formation locations.

2. Methodology

The study utilized laboratory experiments to simulate the processing of interstellar ice analogs under conditions mimicking dark molecular clouds and protoplanetary disks.

• Experimental Setup: Experiments were conducted using two ultra-high vacuum (UHV) chambers:
  • SPACECAT: Used for UV irradiation experiments using an H2D2 lamp (peaking at ~161 nm) with a photon flux of $\sim 1 \times 10^{14} \text{ cm}^{-2} \text{ s}^{-1}$.
  • SPACETIGER: Used for electron bombardment experiments (2 keV electrons) to simulate cosmic ray interactions.
• Ice Analogs: Ices were deposited on CsI (UV) or Cu (electron) substrates at temperatures ranging from 10 K to 100 K. The compositions included:
  • Pure CO2 and Pure NH3.
  • Binary mixtures: H2O:CO2 and H2O:NH3.
  • Ternary mixtures: H2O:CO2:NH3 (mimicking typical protostellar ratios of ~100:28:6).
• Isotopic Labeling: To avoid interference from residual background gases (specifically $^{12}$CO and $^{14}$N2 at mass-to-charge ratio $m/z=28$), the experiments utilized $^{13}$CO2 and $^{15}$NH3.
• Diagnostics:
  • FTIR Spectroscopy: Monitored the depletion of parent molecules (CO2, NH3) and the formation of products (CO) in real-time.
  • Temperature Programmed Desorption (TPD) with Quadrupole Mass Spectrometry (QMS): Used to quantify the production of N2 (detected as $^{15}$N2 at $m/z=30$) and CO after irradiation.
• Fluence: Ices were irradiated until steady state was approached, delivering fluences of $\sim 1 \times 10^{18} \text{ photons cm}^{-2}$ (UV) or equivalent electron doses, comparable to exposure in molecular clouds or disk upper layers.

3. Key Contributions

• Quantification of Yields: The study provides the first comprehensive quantitative yields of CO and N2 production from CO2 and NH3 photolysis/radiolysis across a range of ice compositions (pure vs. water-rich) and temperatures (10–100 K).
• Matrix Effects: It characterizes how the presence of water ice (the "cage effect") influences the dissociation efficiency and product branching ratios of CO2 and NH3.
• Temperature Dependence: It investigates how irradiation temperature affects the retention and formation of hypervolatiles, specifically looking at the 10–100 K range relevant to disk evolution.
• Source Comparison: It compares UV photolysis with electron bombardment (radiolysis) to determine if the energy source significantly alters the production efficiency of these species.

4. Key Results

A. Production Efficiencies and Mixing Ratios

• Pure Ices:
  • CO: Up to ~50% of consumed CO2 was converted to CO, resulting in a final CO/CO2 mixing ratio of 51–62%.
  • N2: ~12% of destroyed NH3 formed N2, resulting in a final N2/NH3 mixing ratio of 15–21%.
• Water-Rich Ices (Binary and Ternary):
  • The presence of water significantly suppressed yields due to the cage effect and recombination.
  • CO: CO/CO2 ratios dropped to 3–10%; CO/H2O ratios ranged from 0.4% to 0.9%.
  • N2: N2/NH3 ratios dropped to 4–19%; N2/H2O ratios ranged from 0.03% to 0.7%.
  • Ternary Effect: Adding NH3 to H2O:CO2 ice slightly increased CO2 destruction (softening the matrix), while adding CO2 to H2O:NH3 slightly suppressed N2 yield (promoting ammonium salt formation).

B. Temperature Dependence

• Irradiation at higher temperatures (30–100 K) generally increased destruction rates and production yields due to reduced cage effects (easier diffusion of fragments).
• However, at 100 K, outgassing of CO and CO2 became significant, complicating retention.
• A local minimum in yield was observed at 30 K, likely due to the sublimation of newly formed hypervolatiles before they could be trapped.

C. Photolysis vs. Radiolysis

• CO yields were similar between UV and electron experiments.
• N2 yields were significantly higher (up to 5x) in electron bombardment experiments, possibly due to more efficient fragmentation of NH3 into smaller radicals (e.g., NH) that recombine into N2.

D. Comparison with Cometary Observations

• N2: The experimentally produced N2/H2O ratios (0.03–0.7%) cover the range of observed N2 abundances in most comets (e.g., 67P at ~0.09%). Furthermore, the isotopic ratio $^{14}$N/$^{15}$N in 67P's N2 matches that of its NH3, strongly supporting an in-situ photolytic origin rather than gas-phase entrapment.
• CO: While the experiments can explain the low CO abundances of a few comets (e.g., 103P, 73P), they cannot account for the high CO/H2O ratios (>1%) observed in most comets, including 67P (3%). The CO/CO2 ratios in 67P are also higher than what water-rich photolysis produces.

5. Significance and Conclusions

The study fundamentally shifts the interpretation of cometary hypervolatiles:

1. N2 Origin: The majority of cometary N2 is likely not a tracer of ultra-cold formation temperatures via gas-phase entrapment. Instead, it is plausibly produced by the photodissociation of NH3 embedded in water ice. Consequently, using N2/H2O ratios to infer a comet's formation location (specifically the N2 snowline) should be done with extreme caution.
2. CO Origin: While photolysis contributes to the CO reservoir, it is insufficient to explain the high CO abundances seen in most comets. Therefore, the substantial CO observed in comets (like 67P) likely does require entrapment of gas-phase CO at low temperatures, implying that the CO snowline remains a valid constraint for the formation of CO-rich cometary material.
3. Isotopic Evidence: The consistency between N2 and NH3 isotopic ratios in Comet 67P provides strong independent evidence for the photolytic origin of N2, whereas CO and CO2 isotopic ratios are less constraining due to potential inheritance pathways.

In summary, the authors conclude that while photolysis can explain the bulk of cometary N2, the high CO abundances in many comets still point to formation in very cold environments where gas-phase entrapment occurred.