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

Abstract

Hypervolatile species such as carbon monoxide (CO) and molecular nitrogen (N2) have been detected in comets, and could be used to constrain comet formation temperature conditions if their presence is due to freeze-out and/or entrapment. Here we instead explore another plausible origin of cometary hypervolatiles: photodissociation of less volatile species. We characterize CO and N2 formation following ultraviolet (UV) irradiation and electron bombardment of carbon dioxide (CO2), ammonia (NH3), H2O:CO2, H2O:NH3, and H2O:CO2:NH3 cometary ice analogs. We find that CO and N2 form in all photoprocessed ices at temperatures between 10 K and 100 K, resulting in 0.4-0.9 % CO and 0.03-0.7 % N2 relative to water, and CO/CO2 and N2/NH3 mixing ratios of 2.5-62 % and 0.7-9 %, respectively, across the experiments. Because our initial ices are reasonably well-matched to interstellar ices and we use UV exposure similar to a dark cloud, we can compare the resulting ratios directly to cometary abundances. Such a comparison shows that while only a few of CO observations in comets are readily explained by photodissociation, almost all observed cometary N2 can be accounted for by photodissociation of NH3 embedded in water ice. The latter result is also consistent with observed similarly elevated isotopic ratios of N2 and NH3 in 67P. Taken together, our results suggest that N2/H2O ratios less than 1 % should be used cautiously when inferring a comet's formation location, while the more substantial CO abundances seen in many comets do likely imply entrapment at low ice temperatures.

Figures (17)

• Figure 1: The infrared spectra of various ices throughout UV photolysis at 10 K. Left:CO$_{2}$; Middle:CO. Right:NH$_{3}$. Top Row: Pure Ices; Middle Row: Water-rich Binary Ices; Bottom Row: Water-rich ternary ices.
• Figure 2: The temporal behavior of CO$_{2}$ (top panel) CO (middle panel) and NH$_{3}$ (bottom panel) following UV irradiation of various ice compositions at 10 K. The error bars in this plot only reflect the spectral uncertainty and are smaller than the markers.
• Figure 3: The TPD of $^{13}$CO (m/z=29 top row) and $^{15}$N$_{2}$ (m/z=30 bottom row) following UV irradiation of various ices at 10 K. Left column: primary ices; Middle column: water-rich binary ices; Right column: water-rich ternary ices. Note that the high-temperature peak around 200 K in some TPD traces (bottom left panel) are presumed to correspond to either codesorption of hypervolatiles with ammonium salts or to desorption from another part of the chamber. These peaks are not included in the abundance calculation to avoid over-reporting. Some of the TPD curves have been scaled for readability, which is listed on the individual panel.
• Figure 4: A summary of the CO and N$_{2}$ formed following irradiation of various ices at 10 K. The initial reactant for CO is the number of CO$_{2}$ monolayers before irradiation, and the initial reactant for N$_{2}$ is the number of NH$_{3}$ monolayers before irradiation. The panels show the hypervolatile yield relative to the initial CO$_{2}$ and NH$_{3}$ abundance (the top panel), to the consumed CO$_{2}$ and NH$_{3}$ (the second panel), to the final CO$_{2}$ and NH$_{3}$ abundance (the third panel) and to the abundance of water in the ice (the bottom panel).
• Figure 5: The temporal behaviour of CO$_{2}$ (top panel) CO (middle panel) and NH$_{3}$ (bottom panel) following UV irradiation of ternary ices at various temperatures. The error bars in this plot only reflect the spectral uncertainty and are smaller than the markers.